SARS‐CoV‐2, periodontal pathogens, and host factors: The trinity of oral post‐acute sequelae of COVID‐19

COVID‐19 as a pan‐epidemic is waning but there it is imperative to understand virus interaction with oral tissues and oral inflammatory diseases. We review periodontal disease (PD), a common inflammatory oral disease, as a driver of COVID‐19 and oral post‐acute‐sequelae conditions (PASC). Oral PASC identifies with PD, loss of teeth, dysgeusia, xerostomia, sialolitis‐sialolith, and mucositis. We contend that PD‐associated oral microbial dysbiosis involving higher burden of periodontopathic bacteria provide an optimal microenvironment for severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) infection. These pathogens interact with oral epithelial cells activate molecular or biochemical pathways that promote viral adherence, entry, and persistence in the oral cavity. A repertoire of diverse molecules identifies this relationship including lipids, carbohydrates and enzymes. The S protein of SARS‐CoV‐2 binds to the ACE2 receptor and is activated by protease activity of host furin or TRMPSS2 that cleave S protein subunits to promote viral entry. However, PD pathogens provide additional enzymatic assistance mimicking furin and augment SARS‐CoV‐2 adherence by inducing viral entry receptors ACE2/TRMPSS, which are poorly expressed on oral epithelial cells. We discuss the mechanisms involving periodontopathogens and host factors that facilitate SARS‐CoV‐2 infection and immune resistance resulting in incomplete clearance and risk for ‘long‐haul’ oral health issues characterising PASC. Finally, we suggest potential diagnostic markers and treatment avenues to mitigate oral PASC.

hours for Reverse Transcription-quantitative Polymerase Chain Reaction) antigen-based assays were developed for Point-of-Care or athome testing.Marin et al. detected seven (spike, nucleocapsid, membrane, replicase polyproteins 1ab (R1AB), R1A/R1AB, ORF3a, and ORF9b) out of 18 SARS-CoV-2 proteins in the saliva of COVID-19 subjects (n = 42).Of these proteins, R1AB (100%), R1A (91.3%), and nucleocapsid (45.2%) were detected in the greatest number of samples. 47Another study by Isho et al. reported stable IgG antibody titers to the SARS-CoV-2 full-length spike protein and its receptorbinding domain (RBD) in the saliva samples of COVID-19 patients (n = 90) up to 15 weeks post-infection. 480][51] Moreover, SARS-CoV-2 is present in the periodontal pocket, dental caries lesion, and gingival sulcus.These sites serve as potential viral reservoirs where SARS-CoV-2 can continue to produce and release viral proteins into circulation, consequently modulating host immune responses and contributing to PASC. 35,52,53Recent studies from our lab also showed S protein detection in salivary, predominantly in epithelial cells, in post-acute (>8 weeks post-infection) COVID-19 patients suggesting SCHWARTZ ET AL. a viral persistence in the oral cavity for a prolonged time period (Naqvi et al., unpublished results).Thus, viral endurance is potentially enhanced in individuals with pre-existing oral conditions, such as PD, caries, and mucositis, which can further compromise the oral mucosa and diminish mucosal immunity.
In general, the initial adhesion and subsequent entry of viruses and bacteria into oral epithelial cells initiates endocytosis.Here, a morphologic transformation of oral epithelial cells results following the establishment of a physical connection between microbe, vesicle, and cytoplasmic enzymes, allowing the release of microbe genomic material intracellularly.However, the degree of permissibility in the oral epithelial cell may vary and be dependent upon two mechanisms: cell membrane activating adhesion receptors and endosomal membrane receptors linked to lysosomal degradation activity.Cathepsins serve as markers for the latter mechanism, being the most abundant lysosomal degradative enzymes. 54In both instances, after initial virus attachment to adhesion receptors, additional activations occur through tyrosine and serine/threonine receptors, calcium regulatory/ dependent receptors, and signals from the pathogen pattern recognition system (PRR: damage-associated molecular patterns [DAMPs], pathogen-associated molecular patterns [PAMPs], AMPs).Toll like receptors (TLRs; e.g., TLR3, TLR7, and TLR9), NOD-NLR (nucleotide oligomerisation domain), RIG-1 (retinoic acid-inducible gene-I), and MDA5 (melanoma differentiation antigen 5)/RIG-I like receptor 3 viral sensors are recognized triggers.Through endosomal attachment, there is uncoating of endocytosed viral particles, triggering serine/threonine kinase TBK1 (tank-binding kinase-1) and phosphorylation of IFN regulatory factors (IRF) 3 and 7, inducing IFN-α/β production in a paracrine and autocrine fashion.This cascade leads to the production of an inflammasome, which assists in stabilising DNArepair and controlling oral pathogen infections. 55 important consequence from these events is the enhanced number and volume of sites allowing different viruses to persist in the oral cavity, establishing viral reservoirs that may promote reactivation or continual persistent low levels of viruses (e.g., HPV, HHV, SARS-CoV-2, etc.).Even in low titers, viral replication can over time dampen oral mucosal immune resistance, which supports viral persistence and promotes reactivation (e.g., HPV, HHV, SARS-CoV-2, etc.).Especially in individuals with pre-existing diseases-PD, rampant caries, uncontrolled mucositis, keratoses, and neoplasiathat damage the oral mucosa and oral mucosal immunity, viruses can persist.[58][59] Even when viral titers are low, for example, when Polymerase Chain Reaction detection falls below <1000 copies, viral replication is likely to proceed. 60As infection persists, even lowly pathogenic microbes can overcome mucosal immune resistance, causing a shift from immune cell effects to immunosuppressive molecules (e.g., IL-10, TGF-β, adenosine, IL-35 (IL-12)).This slow but persistent immunosuppressive immunity offers an opportunity for sequestration of bacterial periopathogens and oral viruses in phagocytes.Eventually, these microbes can exit dormancy and release in high numbers into a microenvironment that is already weakened by inflammation.This scenario upregulates apoptosis, and at times, facilitates antigen processing for immune recognition. 61en pathogens, like those causing oral infections, enters our cells, it can lead to a morphological change in the targeted cells.
These pathogens might directly enter the cell's cytoplasm or use endocytosis, which involves the formation of Clatherin-coated vesicles.An array of RNAases guide Clatherin coated vacuolation-vesicle induction; this activity often coincides with the release of exosomes. 62There is a need to study how these vesicles form, considering factors like pathogen size and the stages they go through, such as initiation, assembly, scission, and uncoating.
As the oral pathogens enter the cell through endocytosis, the cell's outer layer expands and inverts, forming what is known as an 'early arriving complex'.This complex is expected to connect with various cellular components, including membrane adapter proteins DAB2 and AP-2, and phosphoinositide-Catherin coating sites.These interactions lead to the creation of a supportive lattice around the cargo pit vacuole.Once the vacuole reaches an optimal size, a series of proteins and enzymes, including GPTase, dynamin, endophilin, and nexins, come into play to carefully control its movement.This process involves the interaction of endophilins with dynamin, acting like cellular traffic controllers.In response to innate inflammatory activity, enzymes like LPAATs (lysophosphatidic acid acyltransferase) are released, fostering a chain of biochemical reactions (Kennedy pathway) that culminate in the production of phosphatidic acid-a key player in cellular responses.This is an organised lipid cell response derived from hydrophobic domains of bacterial and viral pathogenic membrane guiding autophagic signals derived from cell membrane response to external or internal organelle membrane issuing from mitochondrion, or endosomal membranes. 63is section shows shared adhesion and entry mechanisms among SARS-CoV-2 and PD pathogens, suggesting a supportive overlapping or opportunistic redundancy.This section reviews (1)   how bacterial and viral periopathogens prime the oral cavity for SARS-CoV-2 infection and (2) the parallel adhesion-entry mechanisms between periopathogens and SARS-CoV-2.

| Microbial and oral epithelial adhesion molecules
Below are mechanisms by which lipid or carbohydrate cell surface display by periopathogens promotes SARS-CoV-2 infection by (1)   providing parallel adhesion molecules for oral virus and (2) modulating host immunity.Bacterial pathogens require adherence for replication, thereby primarily targeting the oral epithelium, rendering these epithelial tissues vulnerable to damage and inflammation.The localization of F. nucleatum and other intracellular periopathogens is a critical factor causing inflammation at gingival oral sites.For example, the small area of the sulcus formed by non-keratinised or parakeratinised gingiva is a site of aggregation for periopathogens. 64F. nucleatum and other PD pathogens release a variety of enzymes, such as proteases and metalloproteinases, to degrade adhesion proteins while also releasing metabolites and other endopeptidases that function as virulence factors in the gingival sulcus. 65Different adhesins are found on the oral cell surface, for example, gene-encoding adhesins that include RadD, Aid1, and FomA.These factors promote co-aggregation of periopathogens and assist with reforming a biofilm in contact with the oral epithelium.This physical proximity provides an opportunity for continual inflammatory signals and bacterial entry into the oral epithelium.These inflammatory reactions manifest as clinical presentations of mucositis, ulceration, atrophy-thinness, and desquamation of oral mucosa.Other oral epithelium, such as salivary ductal cells, also shows clinical oral pathology, including sialoadenitis, mucocele, and salivary cysts.Adhesion molecular arrangements regulate interactions with the host oral mucosal epithelium, influencing immune reactivity.

| Oral pathogen surface molecule displays regulate adherence
Adhesion to oral epithelial cells by bacterial and viral periopathogens is possible due to surface molecules, including phospholipids phosphatidylserine (PS).Under physiological conditions, as a constituent of cell membranes, PS is found on the cytosolic side of the membrane.However, cells can sometimes externalise PS in a process dependent on caspase-mediated cleavage of the phospholipid flippase ATP11C. 66,67Once displayed on the cell surface, PS functions as an eat-me signal for apoptotic recognition.Specifically, PS receptors bind to PS ligands located on the cell surface to mediate the engulfment of apoptotic cells. 68,69Subsequent steps include the engulfment of the apoptotic bodies via endocytosis and intracellular trafficking of the phagosome for degradation.Periopathogens, including bacteria and enveloped viruses (e.g., HPV and HHV), can exploit this PS externalisation in a process referred to as apoptotic mimicry. 70During classic apoptotic mimicry, microbes acquire host cell PS and incorporate it into their membrane, masquerading as an apoptotic body.Through this ploy, microbes can enter host cells via the PS/PS receptor mechanism (Figure 1a).Apoptotic mimicry by microbes, therefore, functions as an immune evasion tactic. 71In fact, Bohan et al. demonstrated that SARS-CoV-2 can uptake PS to adhere and internalise into an endosomal compartment of cells through the PS/PS receptor mechanism (Figure 1b). 724][75] The mechanism underlying the viral uptake of PS is poorly understood.However, it is surmised that viruses acquire PS when they bud from the plasma membrane or intracellular organelles. 66,76Alternatively, when a membrane degrades due to oral epithelial stress, the formation of blebbing, generation of multilamellar vesicles, or release of exosomes results in a shower of soluble PS.The soluble PS may then function as an independent molecule that can promote viral or bacterial adhesion.
PS display by oral enveloped viruses may aid in SARS-CoV-2 infectivity and worsen COVID-19 outcomes through multiple mechanisms.The first mechanism relies on the fact that apoptotic mimicry ultimately results in the induction of an anti-inflammatory response and a dampened production of pro-inflammatory cytokines.8][79] Moreover, soluble PS may interfere with the replication capacity of commensal bacteria, increasing dysbiosis and accelerating the risk for persistent inflammation-a significant contributing factor to PASC.Furthermore, apoptotic mimicry promotes the selective removal of dying cells.This decrease in dying cell population provides more opportunities for ssRNA viruses to infect viable host cells, replicate, and release mature ssRNA virions. 66other mechanism by which PS perpetuates COVID-19 is through PS-driven ADAM17 (ADAM metallopeptidase domain 17) activity (Figure 1b).PS surface exposure is essential for ADAM17 sheddase activity. 80As a shedding protease, ADAM17 cleaves and subsequently releases the extracellular domain (ECD) of the host receptor ACE2 upon the initial binding between SARS-CoV-2 spike protein and ACE2. 81The ACE2 ECD reportedly upregulates inflammation. 81,824][85][86][87][88][89] As the main endogenous pyrogen, IL-1 causes fever, while IL-6 stimulates B lymphocyte proliferation and antibody production, possibly increasing autoantibody synthesis.Increased presence of IL-6 associates with PD, IBD, and autoimmune disorders.Furthermore, ADAM17 is the main enzyme responsible for the proteolysis, subsequent activation of pro-TNFα, and release of TNFα from the transmembrane (Table 1). 90- 93TNFα contributes to the onset and progression of inflammation in both periodontitis and COVID-19. 94Because hyperinflammation is implicated in SARS-CoV-2-driven organ damage, PS-driven ACE2 ECD and pro-inflammatory cytokine release via ADAM17 may worsen clinical outcomes.
One of the PS receptors is referred to as TIM-1 (T cell immunoglobulin and mucin domain-1), which mediates the entry of enveloped viruses.PS attachment to TIM-1 may induce ACE2 cleavage by ADAM17, which aids SARS-CoV-2 adherence and entry while dysregulating the innate and cell-mediated immunity.In brief, lipid derivatives (e.g., free fatty acids, membrane ceramides, sphingomyelin, phosphatidylcholine, phosphatidyl ethanolamine) binds to host epithelial cell via CD47-lectin-glucan receptors (Siglec-SIRP-LIRB) causing impaired immune recognition and altering intercellular interaction, macrophage-driven phagocytosis, and possibly CD8þ functions. 95Continual activation of TIM receptors triggers calcium mobilisation to the cytoplasmic domain of TIM-3, thereby functioning as a hallmark for cytotoxicity blockers (e.g., PD-1).1][102][103][104][105] This cascade is a product of bacterial-viral periopathogen exposure and bears essential implications: oral epithelial cell damage, loss of cell-to-cell interactions, and reduced molecular integrity.Consequently, oral mucosal immunity is depressed, delaying damaged oral mucosa cell removal and slowing the rate of epithelial wound repair required to maintain the mucosal protective barrier likely causing clinical manifestations including atrophy, erosive mucosa, or mucosa ulceration.Nonetheless, further study is needed to establish the role of TIM-1 on SARS-CoV-2 attachment, particularly in the context of the oral cavity. 106Of note, a recent study by Mori et al. identifies TIM-1 as a receptor for SARS-CoV-2 in lung and kidney epithelial cells. 107

| Heparan sulphate proteoglycans and glycosaminoglycan adherence contribute towards SARS-CoV-2 oral infection
Heparan sulphate proteoglycan (HSPG) is a class of glycosaminoglycans nestled within the extracellular matrix and facilitates the entry of multiple viruses. 106Such viruses include both DNA (dsDNA; F I G U R E 1 Surface lipid or carbohydrate display by SARS-CoV-2 and host cells is modulated by periodontal bacteria to promote viral tropism, suppress host immune responses, and provide parallel adhesion molecules for the virus (a) PS display by a periopathogen (shown is an enveloped oral virus) allows the pathogen to bind to PS ligands on the surface of cells and enter host cells through apoptotic mimicry.After the PS/PS receptor interaction, the pathogen is engulfed and endocytosed into the host cell, initiating the production of anti-inflammatory cytokines such as IL-10 and TGFβ.The enveloped virus binds to bridging molecules (e.g., GAS6), promoting the activation of tyrosine protein kinase receptor 3 (TYRO3)-AXL-MER (TAM) family receptors.In turn, the TAM receptors heterodimerises with type 1 interferon receptor (IFNAR), promoting SOCS1 (suppressor of cytokine signalling 1) and SOCS3 expression.SOCS1 and SOCS3 then inhibit IFNAR and toll-like receptor signalling, reducing the innate immune response.(b) Similarly, SARS-CoV-2 can uptake and externalise PS.This PS display allows SARS-CoV-2 to enter host cells.Additionally, PS exposure can activate ADAM17, which can then cleave and release the extracellular domain (ECD) of ACE2 into the extracellular environment.The precise function of the ACE2 ECD is unknown but reportedly upregulates inflammation.(c) Parallel adhesion molecules between periopathogens and SARS-CoV-2.Created with Biorender.com.PS, phosphatidylserine; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. e.g., herpesvirus, and human papilloma virus) and RNA (ssRNA; e.g., influenza virus and picornavirus) viruses. 108Notably, HSPG also serves as an attachment receptor for SARS-CoV-2 and is required for epithelial cell entry.0][111][112][113] Similarly, HSPG plays a crucial role in endocytosis and vesicular trafficking, thus regulating the movement of molecules between intracellular and extracellular compartments while promoting internalisation of macromolecules such as cationic polymers, liposomes, DNA, RNases, cancer cell exosomes, cell-penetrating peptides, protein aggregates, and pathogens 114,115 HSPG is also a known entry receptor for herpes simplex virus 1 (HSV-1), a virus found in abundance in the oral cavity and implicated in periodontitis.7][118] Indeed, the anti-HS peptide, G1, significantly decreased HSV-1 infection in vitro and inhibited HSV-1 infection in the mouse cornea in vivo. 116Furthermore, we reported Streptococcus gordonii, a periodontal pathogen, releases a (zinc) metallo-serine endopeptidase, that mimics endogenous, serine endopeptidase, proprotein convertase, furin required for degradation and opening of spike protein for adhesion of SARS-CoV-2 but also assistance for other oral viruses such as HPV through activity of the HSPG receptor, indicating a series of enzymatic activities are required for the efficiency and affinity of SARS-CoV-2 attachment. 119nsequently, HSPG's role as an attachment for SARS-CoV-2 makes it a promising therapeutic target against COVID-19.
HSPG signalling has important implications for the immune response to infection and dysbiosis.For example, various viruses and bacteria use HSPG to generate extracellular signal-regulated kinase (ERK) activity, a particular branch of the mitogen-activated protein kinases (MAPK). 120The association of MAPK-related biomarkers with COVID-19 clinical features may be relevant for identifying COVID-19 positive participants at risk of PASC-related complications.Notably, MAPK is related to damages in airway epithelial cells decreasing ventilation, acute lung injury, and Acute Respiratory Distress Syndrome.Moreover, MAPK increases pro-inflammatory cytokine production by activating intracellular pathways used by periodontopathogens. 120,121 MAPKs respond to extracellular microbial displays by transmitting transcriptional signals (G-protein) to intracellular nuclear sites. 122This signal transduction further triggers cellular growth cascade responses, often through epidermal growth factor receptor (EGFR) and its downstream signalling pathways: PI3K/Akt/PTEN/mTOR and RAS/RAF/MAPK/ERK kinase (MEK)/ ERK. 123Furthermore, downstream EGFR-ERK expression pathways initiate elevated levels of hypoxia-inducible factors (HIF-1α) with ribosomal protein phosphorylation of S6-(K1) expression, inducing cell proliferation. 124,125HIF-1α competes and interacts with cytochrome P450 (CYP1A1, CYP1A3), helping to regulate functional transcription for the small-molecule activity of aryl-hydrocarbon receptor (AHR). 126We will describe in greater detail later, but bacterial metabolites can abnormally upregulate AHR, ultimately contributing to immunosuppression. 127Additional control for these activities comes from the expression of a stress chaperone HSP90.This chaperone produces successful translocation of Arnt and other transcription factors to promote site expression of xenobiotic responsive elements (XREs) responding to a xenobiotic event, such as periopathogen generated dysbiosis. 128P90 binds to HSPG receptor expression cooperating with surface glycoproteins such as glycans, glucan, and sialylated complex proteins.Sialic acids and heparan sulphates are present on the outermost part of the cell membrane and the extracellular matrix of oral epithelial cells.0][131][132][133] Heparan sulphate can interact with a variety of ligands mediating additional receptor activations. 134Some membrane receptors are also regulatory for cell-to-cell communication, tissue integrity, oral mucosal infection, inflammatory and neoplasia responses associated with pathogen pattern recognition (e.g., pattern recognition receptor, PRR) producing an amplification of transcriptional inflammatory signalling through NF-kB leading to cytokine expressions. 135,136PG receptor expression also cooperates with surface glycoproteins such as glycans, which are supportive attachment domains in the PS family of receptors.Their co-dependent activity is unexplored and needs attention related to SARS-CoV-2.However, SARS-CoV-2 displays glycan and PS, increasing the possible attachment involvement of HSPG receptors as an accessory attachment avenue for ACE2 attachment. 72,137Taken together, additional consideration is needed to understand the roles between lectin-glycoproteins, sialylated proteins, and glycosaminoglycans activation of tyrosine kinase receptors (e.g., TAM-Axl-CDN-STING), which mediate inflammation through lipid metabolism. 138Employing tyrosine-kinase inhibitors in conjunction with targeting glycoprotein-glycosaminoglycan activity needs consideration for addressing continual risk for infection from SARS-CoV-2 and other similar viruses. 139,1401.4| Neuropilin-1: Neurologic and immunologic adhesion receptor system for SARS-CoV-2 and an oral to brain inflammation axis Oral clinical manifestations suggest that the neurologic inflammation following COVID-19 is potentiated by retrograde neural inflammation.This may occur through haematologic and cranial nerve avenues extending to the brain and central nervous system where ACE2 receptor expression is documented to facilitate SARS-CoV-2 infection. 141Similarly, in ACE2-expressing oral epithelial cells (e.g., tongue > salivary gland > gingiva > palate), there is a risk for SARS-CoV-2 infection to produce a variety of conditions associated with oral neurologic function.This risk is due to the reported link of neurotransmitter interactions (e.g., serotonin) and metabolism by oral epithelial cells. 142Resulting neural dysfunctions include loss of taste, glossitis, burning mouth syndrome, dry mouth and pain responses in teeth and mucosa.Beyond neural dysfunctions, cardiovascular symptoms may also persist past initial SARS-CoV-2 infection.Select neural related proteins, neuropilin-1 (Nrp1) and neuropilin-2 (Nrp2), contribute to neurologic and vascular inflammation.Specifically, Nrp1 and Nrp2 interact with a variety of signal pathways that control neural and vascular responses, possibly triggering cerebrovascular disorders such as epilepsy. 143The activities of plexin, a protein that serves as a receptor for semaphorin family signalling proteins, mediate neuropilin functions and aid axonal guidance and neural development.In turn, Nrp1 and Nrp2 act as co-receptors for the ligand semaphorins, 144,145 which may cause inappropriate immune checkpoint control of T memory cells function, B lymphocyte activities, and T regulatory cell (CD4þ/Foxp3þ/CD25) differentiation.The shift towards Treg stimulates the release of immunosuppressive cytokines, IL-10 and TGF-β, depleting mucosal resistance to SARS-CoV-2. 146Nrp1 may also act as a co-receptor for several extracellular ligands, which includes angiogenesis-enhancing vascular endothelial growth factor (VEGF). 147us, neuropilin receptors and semaphorin ligands amplify neural and vascular inflammation and increase PASC risk.Langerhan cells). 162,163This lectin-driven mechanism not only promotes viral persistence in the oral cavity, enabling rapid replication, but also offers additional adherence opportunities independent of ACE2 expression.Thus, SARS-CoV-2 and periopathogens utilise glycoproteins for attachment, increasing their chances of evading the immune system and persisting in the oral cavity.
For SARS-CoV-2, glycoprotein adherence sites are immediately available on the S protein, which contains 22 N-glycosylation sites and 3 O-glycosylation sites. 163,164For both periopathogens and SARS-CoV-2, heavy glycosylation is assumed to facilitate adherence to oral epithelial cells and immune effectors, providing ample opportunity to suppress antigen recognition and increase persistence in the oral cavity.A consistent finding for the type of glycosylation present in SARS-CoV-2 includes mannose or other complex Nglycans.The presence of mannose increases activation of complement by an alternative pathway. 165The persistence of SARS-CoV-2 and the possible reduction in antigen recognition is thought to stem from the host glycosylation machinery and incorporation of self 'glycans'. 166scussed above, SARS-CoV-2's interaction with HSPG and sialic acid-lectin chemistry from an N-terminus location creates an open conformation. 167 Notably, interaction of DC-SIGN, L-SIGN, and mannose with SARS-CoV-2 is Ca 2þ -dependent and S glycoprotein glycosylationdependent. 162 It is important to recognise the calcium ion dependence for sialic acid-lectin receptors.Moreover, in the oral mucosa, there is an array of calcium regulatory receptors (e.g., GPCR, CRAC, TRPA1, calcineurin-calcitonin-calmodulin receptors, etc), which also likely mediate the adhesion of SARS-CoV-2 and periopathogens.
Clinically, mannose and sialic acid-lectin receptors increased with and LTα) or inhibit (BTLA and CD160) T cell function when the virus is presented to immune effectors.However, Herpesviruses also use related immunoglobulin superfamilies (Siglec-H, TIM1-3) to attach to heparan sulphate by specific 3-O-sulfotransferases in concert with glycosidases (sialidases) hydrolysing sialic acid acting as a marker for activity. 170,171Following fusion between the viral envelope and cell membrane, liberation of viral nucleocapsid and tegument into the cytoplasm is expected.[127] Conceptually, lectins initially stimulate pro-inflammatory reactions towards pathogens but drive immunosuppressive functions in oral epithelial cells and myeloid factors as the infection persists. 172- 174Such is the case when mannan binding lectins (MBLs) (mannan binding lectins) bind to periodontal pathogens, resulting in complement pathway activation and cytokine expression. 175MBL reportedly modulates LPS/TLR4 signalling pathways, initially promoting proinflammatory cytokine synthesis then producing depressed signals for cytokine transcription, blocking an array of possible targets: adapter molecules (e.g., TIRAP/Mal-TRAM-MYD88, TRIF), Janus activator kinase, STATs (signal transducers and activators of transcription), and NFATs (nuclear factor of activated T cells). 176,177ese depressive signals, such as SOCS1 (suppressor of cytokine signalling-1), reduce NFκB cytokine transcription in oral epithelial cells and phagocytes, suppressing the innate immune response and reducing antigen presentation to adaptive immune cells. 178Immunosuppression further amplifies when the ligand CD47 binds to SIRPα (signal regulatory protein alpha; CD172α or SHPS-1), which is a cell surface receptor expressed predominantly in monocytes, granulocytes, dendritic cells, and haematopoietic stem cells and interacts with lectins to regulate epithelial cell receptors.The CD47-SIRPα interaction initiates a signalling cascade that prevents the phagocytic function of macrophages. 179Exposure to PD pathogens triggers CD47 expression through PS and protease (e.g., ADAM-17)   activity in concert with SIRPα.Magrolimab, a monoclonal inhibitor for CD47, also blocks SIRPα, restoring phagocytosis, maintaining tissue homoeostasis, and reducing loss of tolerance and production of autoantibody. 180This approach may provide an avenue for blocking other PD pathogen-associated lectin responses contributing to SARS-CoV-2 infection.Furthermore, these signalling cascades would affect cell antigen recognition, cell-to-cell interactions, virally related growth factor receptors, and cell cycle checkpoint control expression.
Altogether, these contribute to co-stimulatory expression of immunoglobulin-like super family of receptors from oral epithelial cells, impacting persistence of oral infection ultimately facilitating a risk for SARS-CoV-2 related 'long-haul' problems. 170

| Cellular responses to viral adhesion
Once adhesion is successful, periodontal pathogens and SARS-CoV-2 may utilise a process known as vacuolisation-clathrin-coated vesicle endocytosis to enter host cells.This entry method is significant in the context of oral epithelial cells because it not only allows pathogens to gain access but also potentially shields them from T cell immune effectors and phagocytes.Moreover, this entry process can trigger oxidative stress, characterised by an accumulation of stress granules and the presence of oxidative substances like reactive oxygen species (ROS) radicals and oxidised proteins.This periopathogen-or SARS-CoV-2-induced oxidative stress can pose a threat to the stability of the host's genome.
Oral tissues pore-forming proteins, Orai (Orai1-3), and the calcium sensor, STIM1-2 (stromal interaction molecules) involve crucial cell signalling for growth and assist in neurotransmitter-oral tissue interactions.Calcium mobilisation induces the interaction of protein tyrosine kinase-coupled receptors and G protein-coupled receptors, such as TRPA1-TRPV1 receptors (detected in tongue, dental pulp, and gingiva), resulting in cytokine release.Moreover, the TAM receptor complex, comprising Tyro3, Axl, and Mertk receptors, plays a role in tyrosine kinase signalling that ultimately leads to cytokine production. 181The TAM receptor complex and tyrosine kinase activity results in the formation of the Annexin A2-S100 (AnxA2/ S100A) complex, supporting binding and entry of a variety of oral viruses (e.g., HPV, HHV, SARS-CoV-2) into oral epithelial cells. 182ructurally, AnxA2 exists as a monomer, but it can complex to form the AnxA2/S100A10 heterotetramer (A2t), a set of calcium regulated receptors that promotes STAT3 signalling.AnxA2 and A2t participate 10 of 30 - in an array of cellular functions, such as endocytosis, exocytosis, membrane domain organization, and translational regulation through RNA binding. 182The AnxA2/A2t complex is particularly important in the context of oral viruses' entry into oral epithelial cells, as it promotes vesicle formation, actin polymerisation, and polarization of various epithelial and immune effector cells.This function becomes crucial when considering the entry of periodontal pathogens and viruses like SARS-CoV-2. 183gnals derived from the AnxA2/A2t complex also contributes to the regulation of two sets of functions within cells.First, it oversees processes related to cell growth, differentiation, proliferation, and cell survival or apoptosis.Second, it takes charge of Ca 2þ homoeostasis, metabolic energy management, and the inflammatory response.
Importantly, this second set of functions is largely orchestrated by AnxA2/a2t, coordinating various steps in the viral lifecycle, including adherence, entry, replication, and release.It also guides the formation of endocytic vesicles and helps with membrane organization, as well as the binding of RNA for translational processes. 184ditionally, AnxA2/S100A provides essential support in maintaining the structural integrity of cells during periodontopathogen and viral infection.It also facilitates the migration and infiltration of immune cells into the infected sites. 185Particularly intriguing is the role of AnxA2/A2t in SARS-CoV-2's affinity for specific mucosal sites.
SARS-CoV-2 tropism is closely tied to the expression of ACE2/ TMPRSS expression, with the highest expression observed in the tongue, followed by salivary tissues, gingiva, and then the palate.This suggests that Anx2/A2t activity coincides with SARS-CoV-2 entry via ACE2/TMPRSS, with additional support from proteases released by periodontal pathogens, such as granzyme and gingipains. 186M receptors interact with additional transcriptional complex signals related to the cGAS-STING pathway, which responds to both DNA and RNA viruses by regulating the type I interferon (IFN-I) response.We noted below in more detail, however, the importance of regulating IFN-I, a key pathway in the antiviral defenses of oral epithelial cells. 187However, this antiviral response depends on context and can also lead to IFN-I responses that promote autoimmunity, requiring differentiation in the T cell lineage to produce Th17 cells. 188IFN-I induction activates tyrosine kinase complex that regulates the GAS6 (growth arrest-specific 6).This activation, in turn, triggers the activation of the AXL ligand for the TAM receptor

| Bacterial and viral oral pathogen persistence in oral epithelial cells
Interactions between periopathogenic bacteria and viruses (ssRNA and dsDNA viruses) influence bacterial metabolism.These interactions lead to the production of metabolites that can alter the microbiome.In this context, metabolites produce nutrients that sustain and support the survival of some bacterial pathogens while acting as a bactericide to some non-pathogenic bacteria.Notably, these altered bacterial metabolic processes can result in the synthesis of sulfides or ROS, including nitrite, nitric oxide, or nitrosyl radicals, with implications for pathogen competition and potential dysbiosis. 189,190ce pathogens have adhered to receptors on oral epithelial cells or membrane adhesion molecules, such as glycosaminoglycans, phospholipids, metalloproteins, they initiate processes that involve the synthesis of specific proteins, such as FAD-1.FAD-1 associates with F. nucleatum and is linked to alterations in phospholipidphospholipase-diacylglycerol dynamics, as well as the activation of protein kinase C.This intracellular molecular reorganization is coordinated with the pathogen's inward movement, propelling it around the oral epithelial cell while simultaneously releasing defensin.
Defensins are small cysteine rich cationic proteins (β (hBD−2) that complex with an array of molecules, including proteins, nucleic acids, and carbohydrates.They exhibit antiviral activity by targeting various viral components, including envelopes, glycoproteins, capsids, and interfering with viral fusion and post-entry neutralisation. 191On the other hand, defensins' antimicrobial activity and chemokine function, which regulates recognition and immune signalling, primarily stem from activated immune effectors, such as polymorphonuclear leucocytes and macrophages).This helps maintain a mucosal barrier of keratinocytes against pathogens within the microbiome. 192al pathogens, as previously mentioned, gain entry into cells through the binding of surface molecules like PS or lectins (C-typedectrin-1-glycoproteins), which identify various microbe recognition pathways and induce inflammatory responses. 193It is worth  196,197 Moreover, the presence of periopathogens facilitates the release of proteases (e.g., ADAM17), enabling SARS-CoV-2 S-protein binding to ACE2, even at low cell membrane densities of 1-2 receptors μm −2 .
As periopathogens undergo vacuolation, ADAM17 induces pathogen entry into the cell through autophagy, causing stress and membrane damage resulting in DAMP signalling. 198 Zinc deficiency may be a derivative of SARS-CoV-2 and long COVID-19 as manifested by a loss of taste, dysgeusia.Zinc deficiency is a result of a redistribution of zinc to enzymes forming metallomoieties forming stable metallopeptidases (e.g., cysteine carboxypeptidase).Normally, 50% of zinc is found in the cell cytoplasm (e.g., Golgi, endoplasmic reticulum), about 30%-40% in the nucleus, and 10% in the plasma membrane indicating the need for zinc to maintain normal cell activities.Zinc can be found bound to proteins, with about 10% of all human proteins and more than 300 enzymes requiring zinc for catalytic and cell structural activities. 201Zinc homoeostasis is associated with Zn2þ transport and buffering, with at least 10 members belonging to the ZnT (Zn2þ Transporter) family and 15 members of the ZIP (i.e., Zn2þ-regulated metal transporter, Iron-regulated metal transporter-like protein) family with 3 distinct isoforms of metallothionein. 202,203Zinc ion is part of a charge elemental ion mix balancing micro and cell environments.Zinc functions with calcium and iron enhancing site specific activities regulating plasma membrane and endosome receptors, effecting virus entry such as SARS-CoV-2. 204Metallopeptidases can either be derived from microbes or host cells.Human zinc metallopeptidases support immune resistance, controlling inflammation including antigen recognition, processing and ultimately clearance of SARS-CoV-2.
Deficiency or imbalance in zinc promotes additional ion imbalance when there is a microbial dysbiosis.Periopathogens require zinc, with concentration ranging 0.1-1.0mm range for survival and aggregating.For example, P. gingivalis requires zinc, which we theorise comes from the stores of cellular zinc cation.The array of metallopeptidases released as endopeptidases by oral pathogens can block the zinc-sodium transporters or the zinc extruder, Naþ/Zn2þ exchanger. 205,206imulation of taste bud cells involves the stimulation of synaptic sites.These sites can be overstimulated during inflammation resulting in an overload of Na influx and concomitant Zinc efflux.
Released zinc then becomes available for complexing with peptidases derived from accumulating oral pathogens (e.g., SARS-CoV-2) adjacent to tongue taste buds resulting in cellular zinc deficiency in taste buds while there is an increasing concentration of metallopeptidases that can further trigger inflammation (e.g., CD47, B-7(CD80/CD86, CD26, CD147)) reducing antigenic recognition, and SARS-CoV-2 clearance.Extracellular zinc concentrations are under control by the sodium/zinc exchanger and a ZnR.8][209] Thus, microbial dysbiosis induced Zn deficiency can contribute to hyperinflammation and loss of taste buds function.

| Bacterial periodontopathogen metabolic interaction with oral epithelium
Bacterial metabolites tryptamine-indolamines can lead to immunosuppression by acting as agonists for the AHR ( , thus suppressing immune cell activation. 210The differentiation into iT reg accompanies the activation of the transcription factor c-Maf, which is followed by the release of immunosuppressive cytokines (e.g., IL-10, IL-12, and TGF-β). 211Subsequently, there is differentiation into Th17 þ (CD4 þ ) cells and release of IL-17, resulting in a loss of self-tolerance and increased potential for autoantibody expression.Th17 and particularly T cell receptor γ/δ activations are common in the oral mucosa compared to other immunologic peripheral sites, potentially enhancing the continual expression of AHR. 211This phenomenon is significant because the AHR transcriptional complex can increase the risk for doublestranded breaks, resulting in malignant transformation.
The interaction between periodontopathogens can also influence bacterial metabolism and drive selective oxidative damage.For SCHWARTZ ET AL.
-13 of 30 triggers the transcription of β-glucanase to degrade β-glycan into glucose and permits sialidase production of sialic acid derived from the metabolism of salivary mucins.Glucose and sialic acid cooperate with methylglycol synthase to produce AGE, binding to RAGE receptors.This AGE-RAGE signalling synthesises nitric oxide and induces NFκB transcription, producing many cytokines with a variety of functions (Figure 3).One set of cytokines-IL-4, IL-6, and IL-8-influences vascular flow, coagulation, and accumulation of innate immune effectors, such as polymorphonuclear cells and monocytes with production of endothelin-1, ICAM-1, E-selectin, VCA-1, tissue factor, VEGF, IL-6, and TNFα. 212her amino acid syntheses from an umbrella of enzymes or precursor metabolites from oral pathogens.Remethylation, transulfuration, and nitration (nitric oxide formation) are all associated with arginine, aspartate, and citrulline synthesis.In the latter pathway, S-adenosyl-L-methionine synthesis catalysed by methionine adenosyltransferase expressed by PD pathogens. 213,214This biochemistry depletes the non-essential amino acid cysteine in the microenvironment while producing nitric oxide and Z DNA, a marker for DNA instability, thereby enhancing the risk for autoantibody induction and loss of tolerance. 215Accompanying metabolic and enzy- the regulation of polyamine synthesis, causing the depletion of vitamin K-related derivatives (biogenic amine-purine synthesis: folate, thiamine, putrescence-spermidine, and vitamin K).Not only are these critical for cell differentiation but also physiologically affect oxygen carrying capacity and possibly increase ischaemia of oral tissues. 217ting the activity of these biochemical interactions discloses PD pathogen foundational activities to precondition oral mucosa as initiation sites for developing PASC following SARS-CoV-2 infection.

| Bacteriophages and immunosuppression
The oral microbiome consists of approximately 10 8 bacteria ml −1 and 10 the type IX secretion system (T9SS) with complex translocons.Functions of this system include pathogenesis of various oral and non-oral diseases, enhancing bacterial motility and consequently increasing interactions with the oral mucosa. 218T9SS is unique to the Bacteroidetes phylum, which includes keystone periopathogen P. gingivalis.
T9SS allows P. gingivalis to synthesise at least 30 proteins, especially major virulence factors, disrupting normal coagulation, creating a thrombosis risk, and activating the complement response. 218In this manner, phage-laden periopathogens create the pathogenesis of PD but also hypercoagulation that may expand from oral to non-oral PASC. 219The phage-infected periopathogens may support the dispersal of pathogens from adjoining biofilm to gingival tissues-a system that SARS-CoV-2 may hijack to support its own infectivity.
Most human oral viruses are bacteriophages, which through their lysogeny and integration potential can mediate microbial diversity ultimately influencing the opportunity for SARS-CoV-2 to infect oral host epithelial cells through their release of selective endopeptidases. 220For instance, phage inclusion in periopathogens may increase virulence activity, exemplified by A. actinomycetemcomitans phage linked to rapidly destructive periodontitis, but other studies have not found a connection between phages and PD, and this will be influenced by the lytic capability of the phage and elimination of virulent pathogens from the microenvironment. 221,222reover, these bacteria provide an opportunity for the phage to enter and affect oral epithelial cell and immune effector cell function.
Here, the phage's closer proximity to nuclear structures of RNA and DNA may translate into cell stress. 223Thus, assuming that incorporation of bacteriophage occurs during dysbiosis, it is plausible that phage interactions may be a foundational factor for the metabolism and physiology of oral epithelial mucosa interactions contributing to depressed oral mucosal immunity, increasing vulnerability to SARS-CoV-2 infection. 129,224,225Moreover, dispersion of periodontopathogens creates competition between uninfected and phage-infected bacteria.A decreased population of uninfected bacteria can alter the enzyme microenvironment composition and influence metabolitesubstrate concentrations.Change in enzymatic activity involves the release of matrix metalloproteinases (MMPs), which alters oral mucosal immunity and oral epithelial integrity.How MMP release may contribute to SARS-CoV-2 adherence and entry to oral epithelial cells is described in the subsequent sections.
One prime target of bacteriophages is F. nucleatum, which aggregates with other periopathogens, such as P. gingivalis or T. forsythia.
Bacteriophage-F.7][228] Entrance of bacteriophage into F. nucleatum and other periopathogens can result in bacterial-cell lysis-releasing nucleic acids, proteins, and lipids-and initiating host-pathogen PRR. 229,230cteriophages infecting and replicating in periopathogens can affect the synthesis of metabolites by PD pathogens. 227For example, in response to bacteriophages, F. nucleatum releases indole, an agonist ligand for AHR, through the bacterial tryptamine pathway.As mentioned above, AHR can promote immunosuppression.
Bacterial adaptation driven by bacteriophage exemplifies indirect regulation of selective metabolites by periopathogens, affecting substrate-nutrient availability and selective bacterial survival resulting in changes to microbiome diversity, richness, and evenness.For instance, synthesis of butyrate by F. nucleatum provides an important beneficial regulator of inflammation and inducer of apoptosis of cancer cells but can also promote proliferation of butyrate-degrading bacteria, which may release bactericidal toxins to inhibit butyrateproducing beneficial bacteria, thereby shifting the balance away from mucosal protection. 231Another example of bacterial metabolism producing selective bacterial survival is the release of taurine or glycan.These metabolites synthesise sulfides and reactive nitrogen species (RNS) (e.g., nitric oxide) which promotes lysis of sensitive bacteria possibly reinforcing dysbiosis and interactions with host mucosa.
Phage behaviour in F. nucleatum, Porphyromonas ginigvalis, or other periopathogens could also regulate bacterial resistance to immune recognition and phagocytosis.These periopathogens have important surface molecule displays of PS and glycoprotein-lectin that interfere with ligand-receptor signalling (e.g., TLR, NOD, TIMs), which alters microbial recognition by PRRs and MHCs (major histocompatibility complexes). 232Therefore, phage presence raises the possibility for the overexpression and continual release of endopeptidases and proteases, such as ADAM-17, which are crucial regulator of SARS-CoV-2 adhesion and entry.These direct and indirect associations between bacteriophage, oral pathogens and oral mucosa causes an array of possible inflammatory activities in oral mucosa.Some these of activities may encourage long-term damage and continual susceptibility to SARS-CoV-2 enhancing 'long-haul' PASC.
Specifically, we will discuss (zinc) metallo-dipeptidyl-peptidases, cysteine-serine proteases, and isomerases.These enzymes regulate the metabolism and physiology between bacteria and host oral mucosa.A MMP (MMP) is any protease or endopeptidase with a metallodomain, which contributes to the catalytic activity of the enzyme.
MMPs degrade or change the adhesion characteristics of cells to the extracellular matrix (proteoglycan-collagen, integrins-adhesins). 233 In oral epithelial cells, MMPs result in the loss of the basement membrane and hemidesmosome attachments, increasing the potential for vesicle formation (e.g., mucous membrane pemphigoid). 234e action of MMPs on the cell membrane signals through MAPK-ERK pathway.This pattern of expression also identifies with epithelial cell proliferation, EGFR expression, and PRR cytokine expression. 235or MMPs, zinc is an important electrolyte and a functional element. 236The taste disorder, dysgeusia, is an early sign of SARS-CoV-2 and identifies with PASC.Importantly, dysgeusia is related to zinc deficiency. 237Ten percent of all human proteins and more than 300 enzymes require zinc for catalytic and cell structural activities.Even periopathogens need zinc, with concentrations ranging from 0.1 to 1.0 mm, for survival and aggregation between bacterial species, enabling virulence activities.The mechanism to achieve proper balance of zinc, extracellularly and intracellularly in oral epithelial cells, is unclear but may involve storage in the mitochondrion, Golgi, and endoplasmic reticulum, which would facilitate zinc availability during post-translation and functional rearrangements for MMPs. 43Taken together, we suggest that zinc deficiency associated with loss of taste is a possible indication of microbiome imbalance and improper metalloproteinase activity.

MAPK-ERK
Some MMPs function as serine endopeptidases, which can compete with host enzymes at receptor sites and may promote SARS-CoV-2 adherence to oral epithelial cells.Specifically, oral bacterial pathogen-synthesized endopeptidases mimic furin at the oral epithelium.Furin activates many proprotein substrates, one of which is the SARS-CoV-2 S-protein.Cleavage of the S-protein enhances ACE2 attachment and influences a variety of immune functions, such as inducing the expression of cathepsins-lysosomal cysteine proteases that provide additional support for S-protein degradation.
Cleavage of the SARS-CoV-2 spike occurs at two sites: S1 and S2.The S1 subunit is responsible for receptor binding, while the S2 subunit is involved in membrane fusion.The S1 domain contains an N-terminal domain and a RBD used for binding to ACE2 and heparan sulfate (HSPG) receptors. 111,240Precursor ADAM17, together with serine endopeptidase-proprotein convertase furin, cleaves at the S1/S2 domains of the SARS-CoV-2 spike.Here, furin recognizes a polybasic amino acid motif at the spike protein PRRAR 685 ↓.Interestingly, phylogenetic analysis of SARS-CoV-2 reveals the absence of the RRAR motif at the S1/S2 site in other SARS-CoV and SARS-related coronaviruses (SARSr-CoVs), particularly RaTG13, which has a 96% genomic sequence identity to SARS-CoV-2. 240Because periopathogens synthesize MMPs with furin-like activity site suggests either coincident random cooperation or a parallel adaptation between periopathogen and SARS-CoV-2. 112,241Target-cleavage site motifs similar to that of SARS-CoV-2 also adhere and originate from S. gordonaii-derived substilisin, F. nucleatum-derived karyolysin, and T. forsythia-derived mirolase. 119,242Altogether, these studies explain how MMP synthesis or induction by periopathogens may assist SARS-CoV-2 infectivity.Incretins stimulate the secretion of insulin and inhibit the release of glucagon, thereby lowering blood glucose levels. 243 and worsen immune dysregulation. 247Mechanistically, hyperglycemia promotes the interaction of AGEs with their receptors (RAGE).

P. gingivalis, T. forsythia, and
The AGE-RAGE pathway is linked to immune dysregulation by interfering with normal neutrophil function, leukocyte recruitment, and cytokine production. 248,249Furthermore, soluble DPPIV enhances NFκB signalling, promoting the secretion of pro-inflammatory cytokines (Figure 4). 243[252][253] Of note, while DPPIV can maintain GLP-1, GIP, and insulin, these three hormones can eventually decline as periodontal inflammation damages intestinal enteroendocrine cells (L and K cells), producing a diabetogenic process that is linked to both PD and PASC.Thus, the periopathogen-DPPIV-diabetes mellitus axis has important consequences in COVID-19 pathology.DPP inhibitors such as metformin and gliptins should be pursued to counter periodontopathogen mediated immune suppression and enhanced tissue damage associated with diabetes mellitus, COVID-19, and PASC.oral epithelium. 254,255[258][259][260] As explained in greater detail below, the induction of PPIase activity during periodontitis can support SARS-CoV-2 replication in host cells to exacerbate the development of PASC.

Cyclophilin A and FK506-binding proteins
Environmental stress, including thermal stress, UV radiation, and changes in pH, can disrupt the proper folding of microbial intracellular proteins.In response to stress, periodontopathogens, such as F.
nucleatum, release PPIases.Here, PPIases exhibit chaperone-like activity and ensure proper protein folding. 261For example, Hspchaperone activity in oral epithelial cells can influence microbial enzyme folding and function. 262Moreover, P. gingivalis expresses the glycoprotein PGN0743, which contains a sequence motif characteristic of the FKBP-type cis-trans isomerase, FkpA. 263FkpA has wellcharacterised chaperone functions. 263,264This chaperone-like activity extends to cyclophilins. 262,265The most abundant cyclophilin in the human body is cyclophilin A (CypA).In PD, there is a demonstrated interaction between CypA and CD147 receptors and cell CypA and FKBP can exert immunosuppressive and immunomodulatory effects.This occurs predominantly when CypA and FKBP bind to their respective ligands, Cyclosporin A (CysA) and FK506 (Tacrolimus). 271,272Both these ligands are immunosuppressants due to their shared link with calcineurin, a calcium and calmodulin-dependent serine-threonine protein phosphatase.Specifically, the CypA-CysA and FKBP-FK506 complexes commonly target and block calcineurin. 273Inhibition of calcineurin blocks NF-AT transduction, subsequently inhibiting IL-2 production and the activation of T-cells. 256ocking NF-AT impairs adept activation of pro-inflammatory cytokines (IL-2, IL-4, IL-6, and IL-8), resulting in a weaker inflammatory response. 274Other immunosuppressive functions of FKBPs include indirectly regulating AHR transcription and complexing with immunosuppressive drugs FKBP12 and the mTOR inhibitor rapamycin.
Despite the immunosuppressive effects of CypA-CysA and FKBP-FK506, both are promising targets against SARS-CoV-2 infection.Protein-protein interaction studies demonstrate that cyclophilins directly interact with SARS-CoV nonstructural protein 1 (NSP1), whereas FKBP family members (FKBP10 and FKBP7) and FKBP ligand (FK506) bind to SARS-CoV-2 ORF8-indicating the importance of cyclophilins and FKBPs as antiviral targets. 259,275,276ditionally, CypA strongly interacts with the SARS-CoV-2 RBD within the spike protein.Sekhon et al. found that the CypA-RBD complex prevented the binding between the RBD and the ACE2 receptor, interfering with viral attachment to host cells. 277th SARS-CoV-2 and periopathogens can enter epithelial cells and become intracellular.Even in the case of successful viral entry into host cells, CysA can impede viral replication during the late stage of infection by binding to CypA and blocking the formation of the CypAnucleocapsid complex. 117,278,279In fact, CysA has demonstrated antiviral activity against several viruses, including coronaviruses, in vitro. 257 Of the parvulins, we highlight PIN1 (peptidyl-prolyl cis/trans isomerase NIMA-interacting 1) because of its reported roles in periodontitis and viral infection.In general, PIN1 is involved in cellular processes, which include regulating the cell cycle, folding newly synthesised proteins, assisting the cellular response to DNA damage, and eliciting DAMP activity after pathogen recognition. 285 chronically inflamed PDLCs (periodontal ligament cells) from periodontitis patients, LPS-and nicotine-exposed PDLCs, PIN1 expression is upregulated compared to healthy and untreated con- PIN1 expression also identifies with the replication of oral dsDNA viruses HHV (e.g., HHV-4), 289,290 and HPV (subtype 16), 291 enhances HIV-1 replication, and contributes to SARS-CoV-2 infection. 292Inhibition or knockdown of PIN1 reportedly suppresses the proliferation of SARS-CoV-2, decreasing viral mRNA transcription and protein synthesis in vitro. 2933][294][295][296] The first mechanism results in the increased production of inflammatory or oncogenic proteins in host cells through the association of PIN1 with cyclin D1, NFκB, and Tax. 293,296The second mechanism requires PIN1 and thus plays a direct role in the different viral life cycles (e.g., genome integration, RNA or DNA replication, or core exuviation). 292,293For example, PIN1 directly binds to the BALF5 subunit of DNA polymerase in EBVs, enhancing replication. 260,291In the case of SARS-CoV-2, PIN-1 likely contributes to viral gene transcription or plays a role in an earlier step after the virus enters the host cell, although this requires further investigation.
Overall, PPIases not only serve as promising therapeutic targets against periodontitis and SARS-CoV-2 infection but also provide a link between the two diseases.In fact, PIN1 expression in obese or diabetic patients may explain the increased severity of COVID-19 in such individuals. 260In a related topic, it is interesting to note that the inhibition of DPP IV (dipeptidyl peptidase IV) by the antidiabetic drug metformin can reverse the immunosuppressive activity of PPIases, such as PIN1. 297Combinatorial use of metformin and PPIase inhibitors may prove a powerful therapeutic strategy against COVID-19.

| Disease severity and viral clearance from oral microenvironments
Relationships between severity of COVID-19 and capability to clear This manifests as a depressed clearance of virus such as SARS-COV-2.
[304] In addition, a reduction in number through autophagy or apoptosis, exhaustion, or anergic may occur if viral infection persists.
9][310] In addition, quantity of viral particles exposed, virulence of virus variant, susceptibility of host because of prior comorbidities compromising oral mucosal and systemic immunity and as noted above, dysbiosis further impacts oral mucosal resistance and clearance capacities. 311

| CONCLUSION
The interactions between periodontopathogenic bacteria and viruses in the oral cavity represent a complex and multifaceted relationship with significant implications for oral and systemic health. 131,312ove Further microbiome, metabolome and immunological studies will establish this dynamic and evolving relationship between 'old' and 'emerging' oral pathogens advancing our understanding of chronic oral diseases and oral sequelae of COVID-19.
Other signals from the microenvironment and microbiome contribute to neuropilin immune suppressant function, lessening resistance to pathogens and subsequently promoting oral reservoirs of bacterial and viral pathogens.For example, the display of lectins, a glycoprotein, by PD pathogens further enhances Nrp1 function.Here, lectins stimulate the release of the semaphorin 3A, which in turn binds to and activates the receptor Nrp1.Following this logic, Nrp1 is unsurprisingly upregulated in the Gingival Crevicular Fluid of periodontitis patients.148As a transmembrane receptor, Nrp1 binds viruses, including the periodontitis-associated Epstein-Barr virus (EBV).Because Nrp1 binds furin-cleaved substrates, the transmembrane also facilitates SARS-CoV-2 adhesion and entry.149,150Host cell protease, furin, cleaves spike into S1 and S2 polypeptides, exposing the Cend (C-end terminal rule) motif in S1.CendR subsequently binds to the b1 domain of Nrp1 and increases virus infectivity.146,151Thus, Nrp1 potentiates the entry of SARS-CoV-2 and other viruses into oral epithelial cells and other mucosal epithelial cells in the intestines.Indeed, Daly et al. found that pharmacological inhibition of Cend-Nrp1 interaction reduced viral infection in vitro.151Specific to COVID-19, Nrp1 serves as an alternative receptor for SARS-CoV-2, allowing the virus to infect cells with low levels of ACE2, such as astrocytes and neurons.152Thus, Nrp1-dependent attachment and entry of SARS-CoV-2 can increase neurologic inflammation.As mentioned above, COVID-19-associated neurological abnormalities include dysgeusia (loss of sense of smell and taste), one of the persisting symptoms in oral PASC patients.Upregulated Nrp1 expression may confer an increased risk for the development of dysgeusia and mucosal inflammation.Nrp1 support of SARS-CoV-2 entry and PASC development may involve: (1) the binding of neuropilin co-receptors to sempahorin, allowing furin-mediated cleavage of the SARS-CoV-2 spike protein for host-virus membrane fusion and subsequent virus host-cell entry, irrespective of ACE2 expression in tongue and gut epithelial cells (e.g., enterocytes, gustatory cells, and tufts cells).(2) Neuropilin receptors and semaphorins functioning in a calciumdependent microenvironment, in which CRGP (calcitonin generelated peptide activation) supports non-selective calcium channel TRPA1 (transient potential receptor ankyrin type 1) activation, regulating taste bud, oral, and gut tissue integrity functions.(3) neuropilins assisting in the management of neuropeptides, which have immune and neurologic activities at oral and gut mucosal sites, resulting in metabolic changes that depress tryptophan and serotonin concentrations; depressed serotonin reuptake transporter in the mucosal epithelium causes increased interstitial serotonin and mucosal inflammation as the microbiome grows increasingly dysbiotic, as shown in experimental animals. 153-156(4) neuropilin regulation of VEGF ligand binding, affecting vascular flow, immune effector distribution, and immune effector localization.Taken together, these neuropilin activities aid in the persistence of oral viruses, such as SARS-CoV-2, and contribute to the development of PASC by dysregulating immunity, altering vascular flow, and increasing neural inflammation.2.1.5| Periopathogens display of lectins and glycans: Accessory oral mucosal adhesion molecules Oral viruses, such as SARS-CoV-2, and bacterial periopathogens display specific molecules on their cell surface, often including lectins and glycans, some of which are glycans-glucans.These glycoproteins facilitate microbial adherence and entry into oral epithelial cells and immune effectors.Lectins and glycans, on the other hand, facilitate microbial adhesion and biofilm formation, as exemplified in the T.denticola-P.gingivalis-F.nucleatum periopathogen aggregate.103This aggregation by F. nucleatum and T. forsythia triggers the release of sialidase, an enzyme that degrades salivary mucins, which facilitate the aggregation of periopathogens while promoting pathogen clearance through phagocytic immune response.Sialylation involves posttranscriptionally adding sialic acid (negatively charged) to glycoproteins, including mucins and its glycan branches.This charge distribution enhances the interaction with mono-glycosaccharides and potentially increases their capacity to activate TIM receptors.In turn, TIM receptors will affect cell-to-cell communication and possible differentiation of immune effectors.157Notably, P. gingivalis releases sialidase, degrading sialic acid linked to glycans, depressing TIM-3 and subsequently promotes high iron concentrations.This ion accumulation enhances ferrous ion oxidation, impairing the clearance activity of macrophages and other phagocytes while inhibiting the differentiation of naïve CD4 þ T cells, causing a differential depression of Th1 to Th2.158 Furthermore, synthesised branched sialylated glycan clusters interact with Siglec-H (sialic acid-binding immunoglobulin-like lectin H), which is involved in the adhesion of microbes to each other and to epithelial cells.For example, SIGLEC5, an inhibitory transmembrane receptor, binds to sialic acids and sialic acid-containing glycan ligands on the surfaces of oral epithelial cells when they contact periopathogens.These interactions fine-tune cell-to-cell communications and the recognition of periopathogen antigen presentation.159The lectin or glycan-mediated F. nucleatum-T.forsythia aggregation may enhance SARS-CoV-2 adhesion to oral epithelial cells.The concept of bacterial aggregation facilitating SARS-CoV-2 adhesion requires further study.However, even without microbial biofilms, lectins can serve as attachment receptors for SARS-CoV-2.This is because lectins can exist as both independent soluble molecules and transmembrane proteins.As transmembrane proteins, lectins serve as attachment receptors, connecting to microbial surface molecule display or oral epithelial surface membrane domains.SARS-CoV-2 can use these glycoprotein-interconnected bridges, such as those provided by CTL receptors (C-type lectin receptors)-DC-SIGN, L-SIGN, and SIGLECS-as alternative attachment receptors, particularly in cells that poorly express ACE2.160CTL receptors are used by enveloped viruses (e.g., HIV-1) as a vehicle to produce a viral-host membrane fusion, typically in antigenpresenting immune cells (e.g., macrophage, dendritic cells, Langerhan cells) in the oral mucosa.This creates an opportunity for endocytosis and sequestration from immune cells via entry of viral and/or bacterial periopathogen into vesicles.An example of a CTL receptor is langerin, used by Langerhan cells to activate antigen processing and promote cell-to-cell adhesion between oral epithelial cells and SARS-CoV-2 glycoprotein-glycans, responding to mannose and glycans with an affinity similar to DC-SIGN, L-SIGN, and mannose receptors.161Lectin surface molecule displays by SARS-CoV-2 proteins are heavily glycosylated, enabling host cell-virus interactions and subsequent viral entry.Studies report that the SARS-CoV-2 S glycoprotein readily interacts with other cell receptors, mainly C-type lectins, found on antigen presenting cells (e.g., macrophage, DC, Previous studies indicate several glycosylation sites for interaction with sialic acid receptors: DC-SIGN and liver/lymph node-specific intercellular adhesion molecule-3-grabbing nonintegrin (L-SIGN).ACE2, DC-SIGN, L-SIGN, and mannose receptors all bind to SARS-CoV-2 S glycoprotein in a dose-dependent manner, with ACE2 showing the highest affinity; DC-SIGN, mannose strong affinity, and L-SIGN moderate affinity.

(comprising GAS 5 , 6 ,
and AXL).The transcriptional processes driven by this tyrosine kinase complex have profound effects.They inhibit apoptosis and activate the MET gene (MNNG HOS transforming gene).This dual effect leads to the persistence of already damaged or improperly repaired epithelial cells, increasing the risk of mucosal instability and a compromised protective barrier function.It will be interesting to monitor the expression of Anx2/TAM transcription signalling receptors, which are utilised by a variety of oral viruses, as markers of 'long-haul' post-acute sequelae of SARS-CoV-2 (PASC).Activation of these receptors exhibit characteristics such as poor clearance and the reactivation of a range of oral viruses, including HHV-4 (EBV).Taken together these relationships suggest PD pathogens or SARS-CoV-2 can hijack lectin associated auxiliary receptors to evade immune response and clearance.Thus, adhesion mechanisms for bacterial/viral periodontopathogens and SARS-CoV-2 overlap, suggesting anoverlapping or opportunistic redundancy in the oral cavity (Figure1c).Furthermore, the simultaneous presence of multiple adhesion molecules may indicate permissive entry by more than one pathogen into a single epithelial cell.However, there is a knowledge gap about efficacy and time requirements for different types of pathogens to attain replication and overcome clearance.Further, basic questions about simultaneous or multiple pathogens targeting associated with permissive activity for an oral epithelial cell need to be studied to understand the biological consequence of these interactions.

Figure 2 )
. Exposure to bacterial toxins or bacterial degradation through CYP1A1 results in the dysregulation of AHR.Specifically, bacterial toxins induce AHR signalling, resulting in higher expression levels of AHR.Dysregulation of AHR signifies an imbalance in the antioxidant and reductive capacity, leading to the generation of reactive oxygen metabolites through multiple pathways for example, glutathione pathway.This disruption extends to nuclear and cellular stress, affecting the capacity of antioxidant responsive elements/XRE.The consequence is a decrease in NRF2-Keap1 corrective expressions, limiting the accumulation of damaged DNA sequences and nucleotides.Notably, AHR activation modifies T cell differentiation and antigen-presenting cell function, specifically promoting the conversion of inflammatory T cells example, the coaggregation of F. nucleatum with T. forsythia produces glucose-producing glycation end products, such as nitric oxide.A shift in the type and numbers of periodontopathogens may also create a continual accumulation of reactive substances.Specifically, oral plaque studies show bacteria from the genus Fusobacterium bind to bacteria from the genus Bacteroidota (e.g., T. forsythia) that are housed in the gingival crevicular sulcus.This bacterial interaction F I G U R E 2 Bacterial metabolite tryptamine activates AHR signalling and contributes to immunosuppression against SARS-CoV-2.As an agonist for the aryl-hydrocarbon receptor (AHR), tryptamine binds to and activates AHR.AHR then forms a complex with the AHR nuclear translocator (ARNT).The AHR-ARNT heterodimer binds to the xenobiotic responsive element (xenobiotic response element) sequence in the promoter region of AHR target genes, inducing the transcription and subsequent release of immunosuppressive cytokines.Under hypoxic conditions, HIF-1α translocates to the nucleus and competes with AHR for binding to ARNT.HIF-1α and ARNT interactions induce the transcription of hyoxia response element-containing genes.Created with Biorender.com.SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

F I G U R E 3
matic DNA instability due to bacterial and viral pathogens may lead to DNA replication fork stress and DNA damage with double stranded breaks.For instance, SARS-CoV-2 nsp13 (nonstructural protein13)   interacts with host DNA polymerase δ.216 Furthermore, with methionine reduction, there are tyrosine receptor activations that depress Aggregation and proliferation of intracellular periopathogens in dental biofilms foster metabolic activities that target oral mucosal epithelium receptors to facilitate SARS-CoV-2 adhesion and entry.Members of Bacteroidaceae, Tannellaceae, and Lactobacillaceae form a 'Red Complex' of periodontal disease (PD) causing-pathogens, such as Porphyromonas gingivalis (FimA II and FimA IV genotype (Pg), Actinobacillus actinomycetemcomitans (Aa), Tannerella forsythia (Tf), Fusobacterium nucleatum (Fn), and Treponema denticola (Td)).Various families of oral viruses will also present as oral disease such as vesicles, atrophy or ulceration of oral epithelial mucosa and may contribute to periodontitis.Fn and Tf aggregate and cooperate to synthesise β-glucan, mucins, sialic acid-sialylated clusters, glucose, and AGE (advanced glycation end-products).Specifically, Fn triggers the release of β-glucanase from Tf by prompting the activation of an ECF σ-factor (extracytoplasmic function sigma factor) that triggers the glcA operon in Tf.Consequently, the secreted β-glucanase plays a role in breaking down dietary β-glucans into glucose fragments.In turn, glucose serves as a carbon source for Fn and a precursor for generating methylglyoxal (MGO) within Tf.MGO then covalently modifies host proteins, resulting in the formation of AGE.Additionally, Tf-secreted sialidase enzyme may liberate sialic acid from salivary mucins, which Fn could employ to adorn its cell surface with this sugar.Altogether, β-glucan, mucins, sialic acid-sialylated clusters, glucose, and AGE products can combine with proteases and metalloproteinases to target oral mucosal epithelium receptors.This interaction facilitates SARS-CoV-2 adhesion and entry by supporting the expression of TMPRSS2, ADAM17, and Furin or expand inflammatory receptor expressions (TIMs, Protease activated receptor, CD47, CD27, CD147).Additionally, AGE can bind to RAGE and produce reactive oxygen species (ROS) nitric oxide.Accumulation of reactive oxygen species and overt inflammation result in the loss of viability, integrity, and inflammation of oral epithelial mucosal cells.Figure adapted from https://www.researchgate.net/figure/Model-of-Tforsythia-F-nucleatum-interbacterial-interactions-in-dental-biofilm-and_fig1_338899965.Created with Biorender.com.AGE, advanced glycation end-product; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.
pathway signals are characterised by Raf, MAPK/ERK kinase (MEK), and ERK drivers that facilitate cellular responses due to extracellular stimuli.In oral epithelial cells, the MAPK-ERK pathway results in the activation of Casitas B-lineage lymphoma-b (Cbl-b), a RING finger E3 ubiquitin ligase.Cbl-b promotes iTreg differentiation (by Foxp3 phosphorylation), and suppress Th2 proliferation, and expression of EGFR/HIF-1α/AP-1, which regulates AHR nuclear translocation capacity, and manages DNA damage.
Porphyromonas intermedia (Pi) produce and release serine protease DPPIV (dipeptidyl-peptidase (IV); also known as T-cell activation CD26), which has a specific capacity to cleave dipeptides.This action mimics the host DPPIV mucosal function of breaking down endogenous incretin hormones, such as GLP-1 (glucagon-like-peptide-1) and GIP (gastric inhibitory peptide).

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Peptidyl-prolyl cis-trans isomerases Peptidyl-proline cis-trans isomerases (PPIases) are enzymes that catalyse the interconversion of cis-trans isomers of proline peptide bonds.Focusing on the oral cavity, PPIases attack proline-rich nucleocapsid proteins of oral viruses or periopathogens.The PPIases superfamily of enzymes consists of three structurally distinct families with representatives in every species of prokaryote, eukaryote, and viruses: the cyclophilins, FK506-binding proteins (FKBPs), and the parvulin family.PPIase functions encompass gene expression, signal transduction, protein secretion, development, tissue regeneration, and regulation of viral adherence and entry into

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DPP produced by periodontopathogens may contribute to oral PASC by increasing inflammation and exacerbating diabetes mellitus.Periodontopathogen (e.g., Pg, Tf, and Pi) derived DPPIV cleaves and inactivates incretins GLP-1 and GIP.As a result, the insulinotropic effects of these incretins are shortened, decreasing insulin production, and increasing glucose production.Glucose metabolism influences glycation/AGE-RAGE, producing NO.Thus, the increase in glucose levels during diabetes, accelerates NO production, causing mucosal epithelial cell stress.Pharmacological inhibition can help prevent DPPIV-mediated inactivation of incretins.Soluble DPPIV also activates NFκB signalling through PAR2 (protease-activated receptor 2), consequently promoting the secretion of pro-inflammatory cytokines (e.g., TNFα, IL-1, IL-6, IL-8).Created with Biorender.com.AGE, advanced glycation end-product; DPPIV, dipeptidyl peptidase IV; PAR2, Protease activated receptorPASC, post-acute sequelae of SARS-CoV-2 infection; RAGE, receptor for advanced glycation end-products.surfaceheparans.The CypA-CD147 interaction elicits downstream signalling, resulting in the activation of ERK1/2 and NFκB signalling cascades, increased expression of MMPs, and other mediators of inflammation.266In fact, MMP7, CD147, and CypA protein expression correlate with worsened clinical manifestations of PD.267 CypA/CD147-driven inflammatory reactions likely contribute to the pathogenesis of PD, although this principle requires further investigation.Studies suggest that the SARS-CoV-2 nucleocapsid protein binds to CypA.In turn, CypA recognises the host cells' CD147-glycoprotein (Basigin, EMMPRIN) receptors, regulating cell proliferation and apoptosis.268SARS-CoV-2 infection, as noted above, is connected to changes in vascular flow, possibly enhancing thrombosis and abnormal coagulation.This is expected to produce ischaemia, which can lead to hypoxia.It has been hypothesised that the hypoxic conditions during COVID-19 infection activate HIF-1α, which in turn increases the expression of CD147 and CypA in infected cells.269Increased CD147 expression is consequential because SARS-CoV-2 can invade host cells via the CD147-S protein route, an alternative entry mechanism for SARS-CoV-2 through endocytosis, especially in ACE-2 deficient cells.In fact, blockade or loss of CD147 in Vero ET and BEAS-2B cell lines inhibits SARS-CoV-2 amplification.Contrastingly, overexpression of CD147 in BEAS-2B cells increased virus expression.270Hence, PPIase production due to continual infection by periopathogens may render individuals more vulnerable to SARS-CoV-2 infection.
trols.Conversely, PIN1 inhibition and silencing promoted antiinflammatory effects and blocked osteoclastic differentiation in LPSand nicotine-treated PDLCs.286PIN1 inhibition may also support periodontal tissue regeneration because it promotes osteoblast differentiation, although the underlying molecular mechanism remains poorly studied.287Inhibition of PIN1 reportedly stimulates odontogenic differentiation of human dental pulp cells by activating the bone morphogenetic protein/Wnt signalling pathway/B-catenin/extracellular ERKs and c-jun N-terminal kinase/NFκB pathway.288Focusing on periodontopathogens, PIN1 is of particular interest because its amino terminus is a tryptophan core, allowing participation in the tryptophan or bacterial tryptamine pathway.Through the tryptamine pathway, periodontopathogens produce indole agonists, activating the AHR transcription complex, inducing iTreg differentiation, enhancing oxidative cell stress and DNA damage.Altogether, these periodontitis-driven events lower mucosal resistance, increasing the risk for SARS-CoV-2 infection and PASC development.
the virus to maintain a symbiotic microbiome and healthy oral mucosal tissue is unclear.It is assumed that with increased severity the host clearance response would be decreased as demonstrated by problems of antigen recognition and processing.For example, a severe-virulent SARS-CoV-2 is expected to heighten pro-inflammatory Th1 response (e.g., TNF-a, Type I INF responses, IL-2, IL-3/4, IL-6, IL-22/IL-23, etc…) balanced by a Th2 activity.However, we also expect a gradual switch from Th1 to Th2, reducing CD4/CD8þ antiviral control as Th17 and iTreg begin to produce immunosuppressive factors (e.g., TGF-b.IL-10, adenosine, ROS, etc…) further depressing B-7(CD80/CD86) antigen recognition and processing aligned with MHC regional display (I/II).
which in turn, will amplify inflammatory signals.This crosstalk between inflammation and viral entry pathways may contribute to poor clearance suggesting that patients with severe PD SCHWARTZ ET AL. are not only more susceptible to viral infection but also at risk of dysfunctional oral mucosal immunity and subsequently tissue homoeostasis and function.Additionally, other immune cells such as CD8þ T lymphocytes kill virus through defensin release and associated activation of endosomal receptor sites and adapter molecules, such as RIG-1 like receptors and signals.These can be blocked by SARS-CoV-2 reducing potent antiviral immune activity.Surprisingly, multiple viral proteins including specifically RIG-1 and the upstream as well as downstream effector molecules.For instance, ORF3b, ORF6, ORF8 inhibit RIG-1 signalling-mediated type I IFN, ISREs and ISGs expression, while compromised by a depletion of antibodies, or complement, allowing viruses to increase virion concentrations.We note one SARS-CoV-2 immune evasion strategy is mitochondrial sabotage: by causing ROS production, mitochondrial physiology is impaired, and the interferon antiviral response (e.g., IRF) is suppressed.Oxidative stress and overt inflammation are a double-edged sword that can have a deleterious impact on tissue integrity and activity.Hyperinflammation can stimulate oxidative stress and vice versa causing a vicious cycle of unresolved inflammation.While oral pathogens are shown to enhance oxidative stress, which can aid in viral immune evasion, SARS-CoV-2 also expresses its own weaponry to modulate ROS/RNS.305Mitochondria localised SARS-CoV-2 ORF3c exert potent oxidative stress by increased ROS production and blockade of autophagy pathways leading to accumulation of autophagosomes by reducing lysosome acidification while Non-structural protein 4 interacts with Lon peptidase 1, a mitochondrial chaperone, and potentially contribute to dysfunctional oxidative phosphorylation.306,307When the host immune system and tissue responses are insufficient to clear SARS-CoV-2, a persistent dysbiosis would occur along with an insufficient oral mucosa immune response characterised by mucosal degradation and loss of integrity.Furthermore, the clearance response is not all or none but likely to be in phases where the balance between symbiosis and dysbiosis and tissue integrity shifts as the microenvironment and host tissues attempt to reset to a state of health.In relation to exposures to other environmental pathogen exposures or to the dietary contributions of substrates such as simple sugars (e.g., glucose, fructose) or fat compounds (e.g., taurine, butyrate) can initiate metabolic responses that select some bacteria for survival through release of sulfides and removal of microbes susceptible to these compounds.This activity modifies microbiome diversity (alpha, beta) ultimately influencing the concentrations of metabolites and enzymes (described in sections above) aiding SARS-CoV-2 persistence and depressing oral mucosa immunity and clearance.In addition, as microbiome dysbiosis worsens adherence and entry mechanisms such as endocytosis (e.g., Catherin coated) vacuolation and intracellular movement may become stalled providing an opportunity for sequestration of SARS-CoV-2 like ssRNA virus, HIV-1.

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, we have reviewed and provided a variety of mechanisms on how these bacterial and viral interactions in the oral cavity can induce dysbiosis and have profound effects on the oral mucosal resistance against SARS-CoV-2.Periopathogens contribute, at the very least, to the initial risk for contracting COVID-19.For example, their surface molecules may prime the oral cavity for SARS-CoV-2 function by depressing host immunity and providing parallel adhesion molecules for SARS-CoV-2.Oral epithelial cell reactivity is also dependent on receptors responding to calcium-influencing transcriptional expressions (e.g., TAM-AXL-GAS-STING) and conducting an immune-generating signal (e.g., STATs/JNK/NFATs), which induces NFκB transcription and subsequent synthesis of a variety of pro-inflammatory cytokines (e.g., TNF, IFN, IL-6, IL-4, and IL-2) and immunosuppressive cytokines (e.g., IL-10).During this process and expression of cytokines, we expect a shift in innate and cell-mediated differentiation and immune reactivity (e.g., recognition, cytokine expression for innate and cell mediators (e.g., complement: (CD147), macrophage (CD47), T lymphocytes (iTreg/Th17)).An altered microbiome that reflects the predominance of periopathogens further perpetuates immunosuppression.This dysbiotic microbiome often accompanies a shift in the bacterial metabolome.As periopathogens continue to secrete metabolites that support their replication, this action contributes to the genomic interchange between pathogen and oral epithelial cells, supporting microbial adaptation to host immunity.Periopathogenic bacterial metabolic derivatives may also act as agonists for AHR and influence the dif-ferentiation of CD4 þ /CD8 þ to immunosuppressive iT reg (CD25,Fox3pþ, CD45Roþ).Other contributors to this dysbiosis-altered metabolome axis include bacteriophages and periopathogens directly producing or inducing the release of enzymes including metalloproteinases, DPPIV mimics, peptidases (dipeptidyl-peptidases (serine endopeptidase-like furin enzymes e.g., Subtilisin, Karolysin, Mirolase)), and propyl-cis-trans isomerases.This array of enzymes can directly interact with coronavirus proteins and direct host-oral epithelial cell interaction with metabolic substrates such as glucose, leading to diabetes risk.Thus, the influence of periopathogens goes beyond the initial infection, exacerbating an individual's risk for oral and non-oral PASC.Further, there remains a need to confirm proteinprotein interactions between SARS-CoV-2 proteins and PPIases; some of the studies cited above regarding protein-protein interactions 20 SCHWARTZ ET AL.pertain to SARS-CoV (e.g., SARS-CoV Nsp1-FKBP interactions).Given the generally significant sequence similarity between SARS-CoV and SARS-CoV-2 proteins, previously identified SARS-CoV protein and PPIase interactions can serve as guidelines for future studies.Understanding the complex interactions between periodontopathogens and SARS-CoV-2 is crucial from a public health perspective.Because periodontitis affects more than one billion people around the world, the mechanisms above emphasise the significance of maintaining good oral hygiene and managing PD as essential components of overall health.For instance, elucidating SARS-CoV-2 adherence and entry into oral epithelial cells may provide novel detection methods and treatments for PASC and other oral pathologies, including xerostomia, mucositis, and sialadenitis.Identification of specific oral microbes, microbial metabolites, host metabolites, immune cell phenotype and immune mediators (described in this review) that associate with long Covid symptoms may allow development of novel diagnostics to study how oral comorbidities functionally correlate with COVID-19.Furthermore, the relationships discussed in this review between periopathogens and SARS-CoV-2 offer an opportunity for mitigating the risks associated with PD and viral infections.As an example, whether periodontal bacteria encoded proteases can be therapeutically targeted to inhibit SARS-CoV-2 adherence, entry or replication in oral tissues thereby preventing virus-associated oral manifestations.While our knowledge on SARS-CoV-2 and its interaction with oral disease and the causal etiological microbes is very limited, we contend that pathogenic bacterial (P.gingivalis, T. forsythia, T. denticola, A. actinomycetemcomitans, etc) and viral (EBV, Cytomegalovirus, HSV-1) species that share similarity in their ability to adhere, infect and compromise oral mucosal epithelial barrier and immunity could synergistically interact with SARS-CoV-2 by providing a conducive microenvironment for viral tropism and long-term persistence.

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Substrates of ADAM-17 that promote inflammation.