Skin innate immune response against fungal infections and the potential role of trained immunity

Fungal skin infections are distributed worldwide and can be associated with economic and social traits. The immune response related to skin cells is complex and its understanding is essential to the comprehension of each cell's role and the discovery of treatment alternatives. The first studies of trained immunity (TI) described the ability of monocytes, macrophages and natural killer (NK) cells to develop a memory‐like response. However, the duration of TI does not reflect the shorter lifespan of these cells. These conclusions supported later studies showing that TI can be observed in stem and haematopoietic cells and, more recently, also in non‐immune skin cells such as fibroblasts, highlighting the importance of resident cells in response to skin disorders. Besides, the participation of less studied proinflammatory cytokines in the skin immune response, such as IL‐36γ, shed light into a new possibility of inflammatory pathway blockade by drugs. In this review, we will discuss the skin immune response associated with fungal infections, the role of TI in skin and clinical evidence supporting opportunities and challenges of TI and other inflammatory responses in the pathogenesis of fungal skin infections.


| FUNG AL S K IN INFEC TI ON S AND TRE ATMENT CHALLENG E S
The frequency of fungal skin diseases can vary by geographic region and socioeconomic conditions but it is estimated that 20%-25% of the world's population has skin mycoses, making this one of the most frequent forms of infection. 1Normally, fungi thrive in tropical countries, or in population groups that live in rural areas in close proximity to animals, crops and poor hygienic conditions. 2 In 2016, 2.1 billion cases involving fungal skin diseases were estimated to occur. 3 addition, a retrospective study observed that between 2012 and 2017, fungal skin diseases were more prevalent than other diseases such as atopic dermatitis, psoriasis and melanoma with a prevalence of 10.09%, 2.79%, 0.88% and 0.03%, respectively. 4ngal infections that primarily affect the skin can be classified as superficial or cutaneous mycoses.Among the superficial mycoses Malassezia, dermatophytosis caused by Microsporum and Trichophyton, and dermatomycosis caused by Candida species are most common.In general superficial mycoses result in the appearance of hypo-or hyperpigmented areas, scaling or, in cases of Candida, in red skin rashes with intense itching. 5,6In contrast, subcutaneous mycosis agents such as chromoblastomycosis, mycetoma and sporotrichosis are the result of traumatic inoculation of the infectious agent resulting in more severe clinical manifestations of the disease, in which complete tissue destruction is observed. 7Among the subcutaneous mycoses, chromoblastomycosis and mycetoma are the rarest infections.Sporotrichosis, on the contrary, is the most widely distributed subcutaneous mycoses and is becoming a public health problem, especially after the emergence of Sporothrix brasiliensis, a more virulent strain that has been spreading across Latin America. 8perficial mycoses and dermatophytosis can be treated topically and/or systemically, with the most recommended classes of antifungals being azoles, polyenes and allylamines. 9Long-term use with sustained efficacy is relatively safe for topical antifungals; however, their unsupervised use can lead to the potential development of resistance. 10,11Despite this, clinical response to antifungal therapy is hampered by the lack of clinical breakpoints for dermatophytes, increased global resistance of Candida and Trichophyton species, and mixed infections. 12romoblastomycosis shows cure rates ranging from 15% to 80% with oral administration of itraconazole for periods of 8-10 months.Surgery is strongly recommended in cases of small and delimited skin lesions, as well as cryosurgery can be used to remove the remaining lesions. 13For sporotrichosis, treatment with potassium iodide is initially recommended according to tolerance levels.
It shows good results acting as an immunostimulant through neutrophil chemotaxis, phagocytosis fungi cells, breakdown of granulomas and inhibition of biofilm formation.Because it has fewer adverse effects, administration of itraconazole for 3-6 months is the first-line treatment recommended in most treatment guidelines. 14,15ngal infections are well known for being recalcitrant to treatment. 16The most commonly used antimycotic targets are cell wall/ membrane components and fungal RNA synthesis, but finding new targets is challenging because fungal cells are eukaryotes with a close evolutionary relationship to humans.This fact associated with long periods of treatment exposes the patient to a substantial risk of toxicity. 10Another treatment challenge is matching species identification with optimal therapy; for example, in cases of skin trauma, where environmental fungi can be inoculated into the tissue, multiple species can be detected, but only one can invade the tissue.
In addition, fungi isolates can exhibit variable susceptibility profiles, making susceptibility testing necessary to choose the optimal antifungal treatment. 17 importance, the current rate of emergence of antifungalresistant fungi is unprecedented. 18Mechanisms of antifungal resistance can be intrinsic as Candida species to fluconazole 10 and Trichophyton species to terbinafine 19 ; acquired resistance due to insufficient treatment duration, lower drug dosage and poor adherence to treatment or even; acquired resistance by organism-related factors as overexpression of efflux pump proteins, biofilm formation and mutations causing conformational alterations at the drug target site. 18In addition, multidrug-resistant strains are a reality, such as Trichophyton mentagrophytes/interdigitale, causing severe infections that require new treatment approaches. 10This whole context has created a scenario where the search for new antifungals or treatment strategies is the major challenge in the mycology field.
A timely diagnosis is essential to ensure successful treatment; otherwise, various complications caused by the disease or treatment will affect the patient's quality of life.In addition, targeted treatment helps control antifungal resistance.The application of molecular biology techniques combined with mutation analysis and gene expression can provide insights into the mechanisms of fungal versus antifungal pathogens.A different approach to prevent and treat fungal diseases is the use of nanoparticles as carriers of antifungal drugs, or even immunotherapy, such as the use of vaccines that activate the immune response and induce the production of antimicrobial molecules in the host. 20,21

| S K IN IMMUNE RE S P ON S E TO FUNG AL INFEC TIONS
The skin comprises the epidermis, dermis, cutaneous appendages and subcutaneous tissue. 22The epidermis mainly comprises melanocytes, keratinocytes and Langerhans cells (LCs). 23The dermis supports a larger population of specialized immune cells such as macrophages, DCs, NKs, mast cells, lymphocytes CD4 + T, lymphocytes γδ T and non-immune cells such as fibroblasts.In addition, blood and lymphatic vessels are located within the dermis, which are responsible for maintaining the flow of these cells between blood and skin. 24,25The skin is the first physical barrier for the host's defence and its disruption through wounds and trauma exposes the host to a wide range of fungal infections that can primarily reach the skin through the injured site. 26However, the skin has many active defence mechanisms that are part of the skin-associated lymphoid tissue (SALT), which comprises both skin cells and professional immune cells. 24The proper functioning of this immune defence is essential to ensure homeostasis and not allow the individual to develop the infection.Patients who have limited immune function due to inborn immunodeficiencies or immunosuppression are more susceptible to fungal diseases. 27Below, we describe the main cells of the innate immune response and their role in fungal skin infections.

| Keratinocytes: The initiator of the immune response
Keratinocytes represent 90% of epidermal cells, which are able to distinguish between normal microbiota and pathogens. 23These specialized cells play a crucial role in homeostasis and in the initiation of immune response after skin injury. 28Keratinocytes, as well as fibroblasts, express on their surface toll-like receptors (TLRs), a class of pattern recognition receptors (PRRs) capable of binding to evolutionarily conserved microbial molecules present on the fungal surface, known as molecular patterns, associated with pathogens (PAMPs). 28TLR activation is crucial for triggering dermal immune responses. 29Specifically, TLR2 and TLR4 play a role in the recognition of Sporothrix spp.by human keratinocytes. 30Kischkel et al. 31 demonstrated that peptidorhamnomannan from the cell wall of S. schenckii and S. brasiliensis is the molecule responsible for the recognition of these fungi by macrophages and human PBMCs, besides possibly being involved in the keratinocyte-mediated immune response.
TLR2 was described in C. albicans, playing a role in the phospholipomannan recognition through p38MAPK and NF-κB signalling pathways. 32Activation of these receptors results in the production of proinflammatory cytokines by activating NF-κB signalling pathway to produce IL-6, TNF and IL-8, activation of the inflammasome and caspase-1 to produce IL-1 and IL-18, 23,30 as shown in Figure 1.

| Fibroblasts: The sentinels
The main function of fibroblasts is the production of structural proteins of cutaneous tissue, with an important role in wound healing.
Beyond that, fibroblasts are also capable to recognize pathogens, induce the recruitment of inflammatory cells via cytokines and growth factors and release antimicrobial peptides. 24Like keratinocytes, fibroblasts can produce cytokines such as TGFβ, FGFβ, TNFα, INFγ, IL-6, IL-12p70 and IL-10, which can modulate the local immune response. 33,34Kühbacher and collaborators 7 demonstrated through the 3D human skin model that the immune response orchestrated by dermal fibroblasts against C. albicans invasion was dependent on the activation of TLR2 (Figure 1).Of note, IL-1β expression was induced after infection, but only secreted in the presence of CD4 + T cells.The authors reasoned that caspase-1 may have been activated by an alternative mechanism such as interferon-inducible guanylate binding protein 2 (GBP2), as already reported for bacterial infections. 35,36Furthermore, fibroblasts can modulate the induced immune response to the invading pathogen through upregulation of functions such as antigen presentation and chemotaxis of diverse cell types in fibroblasts. 36

| Macrophages
Skin macrophages are key effector cells that function in cicatrization, homeostasis and antifungal defence.These macrophages are F I G U R E 1 Skin immune and non-immune cells that participate in the response to fungal infections.TLR, toll-like receptor; ROS, reactive oxygen species; NETs, neutrophils extracellular traps; NK cell, natural killer cell; IL, Interleukin; NO, nitric oxide, M1: macrophages phenotype 1; M2, macrophages phenotype 2.
able to produce inflammatory cytokines and chemokines that induce the development of suppressive or proinflammatory responses at the site of infection. 37Once within tissues, macrophages further develop into a distinct functional phenotype, which is determined by the cytokine milieu.Proinflammatory cytokines, particularly IFNγ, drive a classically activated M1 phenotype, whereas antiinflammatory cytokines, for example, TGFβ, drive alternatively activated M2 macrophages. 38Candida albicans can increase its survival through this mechanism, inducing macrophages to switch from a more inflammatory M1 phenotype to a less inflammatory M2 macrophage phenotype, as illustrated in Figure 1. 39Phospholipomannan and glucuronoxylomannan from Candida spp.and Cryptococcus spp., respectively, have been reported to interact with TLR2/TLR6 and TLR2/TLR1 leading to the production of proinflammatory mediators, including TNFα and nitric oxide (Figure 1).In addition, O-linked mannosyl residues as well as rhamnomannans from Candida spp.and Scedosporium spp., respectively, can be recognized by TLR4 leading to the induction of TNFα and IL-6. 40Meanwhile, TLR2, TLR4/CR3 was described as important for phagocytosis of the yeast form of S. schenckii and S. brasiliensis by macrophages through interaction with peptidorhamnomannans. 41,42 Macrophages along with other innate and adaptive cells play a crucial role in the formation of granuloma.The granuloma is formed when these cells accumulate at the site of infection to prevent the spread of the pathogen.In its interior, caseation and necrosis are usually observed to limit the nutritional factors of the pathogen.
The integrity of the granuloma is maintained by the secretion of proinflammatory cytokines such as IFNγ, IL-17 and TNFα by macrophages and T cells. 43

| Langerhans cells (LCs)
Dendritic cells (DCs) recognize highly conserved structures in pathogens, and while in the presence of fungal pathogens, they increase proinflammatory cytokines expression, causing migration to local lymph nodes, where they stimulate T cells or induce tolerance. 44ngerhans cells (LCs), found in the epidermal layer, are the bestdefined DC population resident in the skin.They express large amounts of the C-type lectin Langerin, which is the major antifungal receptor in human LCs by binding to both mannose and ®-glucan within cell walls. 45Igyártó et al. 46 47 demonstrated that yeast cells in the presence of dectin-1 produce IL-6 cytokines through the Th17 response, while filamentous cells induced Th1 responses due to the absence of dectin-1 ligation (Figure 1).In S. schenckii, DCs were able to phagocytize the fungi and induce the proliferation of T lymphocytes. 48In addition to controlling antifungal T-cell responses, LC have also been shown to modulate the behaviour of NK cells (Figure 1).

| Neutrophils
Neutrophils present oxidative and nonoxidative killing mechanisms against fungal pathogens.Oxidative mechanisms include ROS production (Figure 1), mediated by the enzymes NADPH oxidase and myeloperoxidase, while nonoxidative mechanisms include the release of granules containing antimicrobial and degradative properties, including different enzymes. 49Despite their crucial role, excessive neutrophil activation and/or recruitment may cause concurrent host tissue damage. 50Neutrophils also produce neutrophil extracellular traps (NETs-extracellular DNA fibres comprised of histone and cytoplasmic granule proteins) against fungal pathogens followed by phagocytosis as larger hyphal forms, as shown in Figure 1. 51For example, neutrophils can phagocytosis C. albicans recognized via dectin-1 and release of NETs. 52Moreover, neutrophils were described being Candida species-specific after different degree of activation was observed for C. albicans and C. glabrata. 53milarly, human neutrophils were found to lack effective activity against C. auris and failed to phagocytose the C. auris or release NETs.While presented with both C. albicans and C. auris, human neutrophil exhibited a strong preference for engaging and killing C. albicans. 54Neutrophils can secrete and utilize IL-17 in an autocrine manner to enhance the ROS production and antifungal skin activity (Figure 1).IL-17 is a potent mediator of neutrophil recruitment in a dectin-1-, IL-23-and IL-6-dependent manner. 55In superficial dermatophytosis, neutrophils are activated by keratinocytes via IL-8 production, a potent chemoattractant for neutrophils, that can kill dermatophytes. 56RD9, a downstream adaptor molecule associated with dectin-1 and CR3 activation, is essential against systemic fungal infection as well as mucocutaneous candidiasis.Mutations in CARD9 that result in loss of function are related to a poor immune response that leads to a reduction in Th17 cells and consequently, increases patient susceptibility to infection.Mucocutaneous candidiasis, an autosomal recessive form of the disease, is associated with homozygous mutations in CARD9.In neutrophils, CARD9 is essential for killing the unopsonized fungi and mediating the innate and Th17-related adaptive immune responses against dematiaceous fungal infections at the early stage of infection. 57,58

| Natural killer cells
NKs contribute to the rapid innate immune response by inducing cell death through proinflammatory cytokines production, especially IFNγ and factors that regulate the functions of other immune cells. 59,60In addition, NKT cells (cells with characteristics of NK and T cells) are involved in the activation of macrophages and in the maintenance of epithelial barrier integrity, which stimulates the generation of antimicrobial peptides and production of chemokines in the skin through releasing cytokines such as IFNγ, IL-22 and IL-17 (Figure 1). 24NKp30 receptor has been described as crucial in the recognition of C. albicans and C. neoformans and consequently involved in killing through perforin release. 61Furthermore, usually NK cells are not phagocytic cells, but in vitro experiments demonstrated that they could phagocytose C. albicans. 62Activation of a potent innate immune response by epithelial cells and phagocytic cells, possibly aided by NK cells, is in most cases enough to counteract displacement of Candida species from surface colonization to tissue invasion and to prevent disseminated infection. 57While in dermatophyte infections, the authors reported that NK cells activated monocyte-derived dendritic cells (Figure 1) and induced an even stronger activation in presence of Malassezia spp.Upregulation of CD83 and CD86 indicates that NKs can mature DCs and improve their co-stimulatory capacity. 56

| γδ T cells
γδ T cells perform a key role in the innate immune response against fungal infections.In candidiasis, for example, γδ T cells are the main IL-17-producing cells in murine models during the initial phase of infection.Afterwards, the majority of these cells become tissueresident memory CD4+ T cells. 63While Eyerich et al. 64

| S E VERE S K IN INFEC TI ON: THE PERPE TUATION OF TISSUE INFL AMMATION
In addition to microbial stimuli, specific cytokines can induce the production of other cytokines by keratinocytes and amplify the inflammatory response. 65,66Members of the extended IL-1 cytokine family (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β and IL-36γ) normally play a significant role in both innate and acquired immunity by promoting the resolution of infection, but also represent key initiators of necrosis-initiated inflammation, as well as amplifiers of inflammation in response to infection-associated tissue injury. 65Inflammation is initiated by molecules released from necrotic cells, called damageassociated molecular patterns (DAMPs).Members of the extended IL-1 cytokine family are important DAMPs and require limited proteolytic processing for activation. 67ratinocytes express high levels of IL-36, which has been shown to be associated with inducing a robust inflammatory response in skin disorders. 68Furthermore, IL-36γ is involved in endothelial cells/ keratinocytes crosstalk in inflammatory skin conditions and defence against and clearance of fungal skin infections. 69Braegelmann et al. 70 showed C. albicans and T. mentagrophytes inducing a significantly IL-36γ expression compared with uninfected controls, especially in the upper epidermal layers.However, recent studies have shown evidence that IL-36γ collaborates to perpetuate the cytokine loop that results in the persistence of inflammation in tissues. 66,68,71l these findings together suggest that an unbalanced host immune response can lead to more severe manifestations of skin infectious diseases.

| TR AINED IMMUNIT Y D ISCOVERY AND ITS ROLE IN NON -IMMUNE S K IN CELL S
The first evidence of trained immunity (TI) was acknowledged in plants, as a mechanism called priming.This term is associated with the hyperresponsive state found in a plant cell that previously received a stimulus, improving further immunological responses. 72e systemic acquired resistance (SAR) is a type of priming and a mechanism that resulted from DNA methylation changes, and it was related to the protection against biotrophic pathogens, intriguing whether it was possible that a simple organism, without adaptive immune system, could have an immune memory. 73Still, a similar mechanism was found in invertebrates.It was discovered that an immunity memory could be stimulated in Macrocyclops albidus, a copepod, enhancing protection against similar parasites after previous exposure to Schistocephalus solidus. 74Also, a Pseudomonas fluorescens, Paenibacillus alvei and P. larvae sublethal inoculum were able to protect Bombus terrestris insects from their respective lethal inoculum. 75 humans, one of the most studied cases of TI is related to Bacillus Calmette-Guerin (BCG) vaccine, which contains an attenuated strain of live tuberculosis bacteria.Previous studies already demonstrated that BCG vaccine is related to nonspecific protection against infant mortality 76 and against various diseases, such as acute lower respiratory tract infections 77 and COVID-19. 78These protection traits were identified regarding not only the infection acquisition, but also the severity of the disease.These findings were essential to highlight the ability of nonspecific immunity stimulation in the search for protective approaches.

| Acquisition of trained immunity
The TI is, therefore, described as a nonspecific, fast and robust response of innate immune mechanisms after a primary exposure to a pathogen or a stimulant molecule.For example, after the recognition of fungal PAMPs such as β-glucans, mannans and chitins, cells initiated an inflammatory response through the release of cytokines. 79,80e initial priming of these cells results in epigenetic and metabolic changes that allow the cell to respond more efficiently and nonspecifically to a secondary presentation of the same or different pathogens. 81Although its duration is shorter than the adaptive immune memory, considered to last from 3 months to 1 year, during this period, TI is capable of enhancing the innate immune response, contributing to host protection. 82e epigenetic changes caused by these stimuli include DNA methylation, histone methylation and acetylation.The BCG vaccine can lead to neutrophil-trained immunity by increasing the expression of long non-coding RNAs (lncRNAs), resulting in the accumulation of trimethylation at the fourth lysine residue of the histone H3 protein, termed H3K4me3, specifically in genes that encode antimicrobial molecules. 83β-glucans are able to induce TI through stable epigenetic reprogramming in macrophages and monocytes, altering the metabolic state of the cell, in this case, induction of aerobic glycolysis by the Akt-mTOR-HIF-1α pathway 79 (Figure 2).The phosphorylation of Akt leads to the upregulation of mTOR, which controls the metabolic environment and activates HIF1α. 84HIF1α has an important role in stimulating glycolytic enzymes, leading to glycolysis activation over oxidative phosphorylation and, consequently, in its products. 79,85The increased production of glycolysis-derived acetyl-CoA stimulates the tricarboxylic acid (TCA) cycle, enhancing its metabolites' concentrations, such as fumarate and succinate. 86Previous studies evaluating the β-glucan metabolic modifications in monocytes already demonstrated that the inhibition of mTOR led to a reduction in glycolysis and, consequently, in the training ability.The same effect was observed with HIF1α inhibition. 79e glutamine pathway also modulates the cell metabolism, even though it is still not clear how it is activated by training-inducing molecules.Its increase leads to a higher production of glutamate and α-ketoglutarate, feeding the TCA cycle and increasing fumarate concentration.Fumarate has an important role in stimulating the histone trimethylation in H3K4 and acetylation in H3K27, and it also works in parallel with succinate for the inhibition of KDM5 family demethylases, enzymes acting in the demethylation of H3K4.The inhibition of HIF1α-degradation proteases is also performed by fumarate, being the most important metabolite modulating epigenetic changes in trained monocytes and macrophages. 86e mevalonate (MVA) pathway is related to the TCA cycle through acetyl-CoA, a common metabolite in both pathways.
Although the mechanism regarding IGF-1 receptor (IGF1-1R) stimulation by mevalonate is still unknown, its role was demonstrated in training immunity.The IGF-1R is related to AKT phosphorylation, and consequently, mTOR increased activation.Previous results showed that monocytes pre-incubated with mevalonate presented similar changes to β-glucan-stimulated monocytes, indicating that the mevalonate is the main metabolite produced in the cholesterol synthesis contributing to a trained cell phenotype. 87though TI may be a promising strategy in therapeutic and prophylactic purposes regarding a diverse range of diseases, the activation of innate immune cells is nonspecific, and therefore, it may have damaging effects in autoimmune disease cases.This mechanism was already related to atherosclerosis, since macrophages may go through histone methylation in response to modified LDL stimulus and become constantly active, increasing the known vascular F I G U R E 2 Representation of metabolic and epigenetic alterations in a trained monocyte.After recognition of β-glucan (presented in Candida albicans cell wall) or muramyl peptide (presented in the BCG vaccine) by dectin-1 or NOD2, respectively, internal signalling leads to the phosphorylation of AKT, upregulating mTOR (mechanistic target of rapamycin) and, thus, acting in glycolysis stimulation through HIF1α (hypoxia-inducible factor 1α) activation.The overstimulation of glycolysis reflects in other metabolic pathways, such as TCA cycle and MVA pathway, increasing histone trimethylation in H3K4 and acetylation in H3K27, decreasing the action of demethylases and proteases and, also, positively feedbacking the AKT-mTOR-HIF1α pathway.The Glutamine pathway also plays a role in feeding TCA cycle, although less explored in literature.inflammation related to this disease. 88Also, this could explain the high rates of cardiovascular diseases in patients with diabetes and controlled glucose blood levels, considering the glucose TI-inducing ability in hyperglycaemia occurrences. 89Hence, TI is an innate immune cell mechanism that should be carefully evaluated in each case, considering its possibility to have beneficial or deleterious outcomes.

| Epithelial stem cells (EpSCs)-Induction of immunity trained in tissue repair
Recently, TI was observed in epithelial stem cells and non-immune cells such as fibroblasts and therefore likely play a role in the pathogenesis of skin infections. 90SCs are found in the basal layer of the epithelium and are crucial for tissue maintenance, differentiation and generating a constant flow of new cells that form the epithelium. 91Recently, Naik et al. 92 demonstrated that EpSCs are also able to respond to inflammatory stimuli and remember their experience when directly exposed to the stressor (Figure 3).In addition, exposure to different inflammatory stimuli, such as topical application of immunomodulators such as Imiquimod and MC903, purposeful epidermal abrasion and C. albicans infection, triggered increased skin sensitivity.These results suggest that regardless of the stimulus, if the skin was exposed to inflammation, EpSCs will be sensitized and will be able to respond better to a secondary stimulus.Specifically, prior exposure of mice to Imiquimod resulted in increased ability to heal wounds up to 2.5 times faster than controls.This phenomenon has been shown to be regulated by the inflammasome pathway, AIM2, CASP1 and IL-1β. 92rsen et al. 93

| Trained immunity and its contribution to pathogenesis
The discovery that skin cells can be trained by stimuli capable of inducing inflammation can be extrapolated to help understand the mechanisms of pathogenesis of infectious diseases that attack the skin and the contribution of TI in the development of the disease.The skin is in constant contact with microorganisms that can make cells alert and more responsive to infections.In addition to pathogens and other exogenous immunogenic compounds, recent research has shown that TI can also be induced indirectly through the production of specific cytokines. 94,95mour necrosis factor alpha (TNFα) therapy results in longterm transcriptomic and epigenetic reprogramming in fibroblast-like synoviocytes that contribute to chronic inflammation in rheumatoid arthritis.The authors observed that the sustained activated genes are NF-kappaB, STAT and AP-1 signalling cascades.The sustained repressed genes included critical mediators and targets of the bone morphogenetic protein (BMP) signalling pathway. 94glucan is capable of inducing TI in stem and haematopoietic stem progenitor cells (HSPCs), even though these cells lack the dectin-1 receptor.Mitroulis et al. 95  Since IL-1β is capable of epigenetically reprogramming monocytes in a similar way to β-glucan, the induction of TI via IL-1β may be involved in the pathogenesis of sporotrichosis and linked to severe tissue destruction by S. brasiliensis.Furthermore, the immune response induced by S. brasiliensis is strongly dependent on IL-1, which offers the possibility of controlling the infection by regulating the IL-1 signalling pathway. 31 first glance, some fungal skin infections can resemble autoimmune diseases such as psoriasis and atopic dermatitis due to the clinical manifestations of the disease, such as the appearance of red spots and scaly patches on the skin.It is important to highlight that the first is the result of an infection and the second of a dysregulation of the immune system, but in both cases, the result is tissue inflammation.Although, in fungal infections and psoriasis, the immune response is majorly mediated by IL-17 and IL-22, IL-36 is crucial in inducing inflammation in psoriatic skin.Recently, IL-36γ has been reported to play a role in the induction of TI in primary human monocytes. 97An investigation of IL-36γ expression in the context of psoriasis and fungal infections led the authors to conclude that the inflammation observed in psoriatic skin may be a misdirected reaction of IL-36γ originally produced by the presence of a fungal pathogen. 71Furthermore, Macleod et al. 98 demonstrated that intracellular IL-36γ is upregulated in the presence of fungal and bacterial pathogens.The release of IL-36γ in the medium only occurs after cell damage and its processing into the mature form by the action of proteases from the pathogens.Therefore, it was proposed that the presence of IL-36γ can act as a discriminator between harmless and pathogenic microbes in tissues. 98All this suggests that IL-36γ may have implications for TI induction in the skin as well.However, specific studies need to be conducted to determine its involvement in the induction of trained immunity in the skin during infections caused by fungal pathogens.

| CON CLUDING REMARK S
The study of TI and its relationship with the skin is still poorly understood and further studies need to be conducted to determine whether the innate immune memory induced after the first contact with the infecting agent is restricted to the site of injury.
Furthermore, the induction of TI in the context of infectious diseases needs to be tightly regulated to prevent chronic inflammation and tissue damage.Thus, understanding the role of each cytokine in the course of the development of a skin infection by a given pathogen is crucial since they are also responsible for signalling that leads not only to inflammation, but also to defence, tissue repair, fibrosis, angiogenesis and restriction of neoplastic growth. 28owledge of the cytokine profile induced by fungal pathogens during infection is also essential for selecting the best immunotherapeutic strategy.For example, sporotrichosis caused by S. brasiliensis is strongly dependent on IL-1; therefore, antagonistic members of the family, such as IL-1Ra or IL-1 inhibitor drugs, may be an alternative for treatment. 99However, complete inhibition of a cytokine signalling pathway may or may not result in susceptibility to other infectious diseases.Therefore, it remains to be studied how long inhibition of a cytokine signalling pathway would be beneficial for the successful treatment of a fungal skin infection.In the case of skin diseases, targeted inhibition through topical use of creams offers more benefits than systemic inhibition of the inflammatory response.Furthermore, it should be highlighted that these treatment regimens must be combined with antifungals to achieve a potential treatment strategy.
demonstrated LCs being sufficient for Th17 cell generation by C. albicans, while for the generation of antigen-specific cytotoxic lymphocytes and Th1 cells, the Langerin dermal DCs are required.Moreover, different fungal cell forms can induce distinct responses.In C. albicans, Kashem et al.
demonstrated in vitro infection model of innate immunity induction, that keratinocytes stimulated with Th22 supernatants or recombinant IL-22 plus TNFα, effectively inhibit the growth of C. albicans and maintain survival of epithelia.
analysed chromatin changes, histone modification and transcription factor binding in murine epidermal stem cells at three different stages, namely during inflammation, postresolution and exposure to the second stimulus.In this study, they demonstrated that the DNA binding proteins, Fos and Jun proteins, cooperate with STAT3 for the establishment of memory.Jun accessed open memory domains and remained bound to them after inflammation.In this way, through the secondary stimulus Jun was able to quickly recruit Fos and reactivate the open memory domains.
attributed this TI to bone marrow-derived production of IL-1β and GM-CSF in response to β-glucan with modifications in glucose metabolism and cholesterol biosynthesis.Subsequently, the contribution of the MyD88 adapter protein downstream of IL-1Rs to the b-glucan-induced trained phenotype was demonstrated, in which no changes were observed in leukocyte subsets in MyD88-deficient animals. 96Sporotrichosis causes ulcerative skin lesions and infections caused by S. brasiliensis are often reported as the most serious.Recently, we demonstrated that S. brasiliensis is capable of inducing high concentrations of IL-1β and that the recognition of peptidorhamnomannan in the fungal wall leads to the production of proinflammatory cytokines such as TNFα, IL-6 and IL-1β and T-helper cytokines IFNγ, IL-17 and IL-22.
Investigation; writing -original draft; visualization.Julia Marcondes Figueiredo: Investigation; visualization; writing -original draft.Carlos Pelleschi Taborda: Writing -review and editing; resources.Leo A. B. Joosten: Writing -review and editing; funding acquisition; supervision; validation.Vania Aparecida Vicente: Resources; writing -review and editing.Flavio Queiroz-Telles: Writing -review and editing; resources.Jacques F. Meis: F I G U R E 3 Comparison between the mechanism of trained immunity proposed to immune cells and EpSC. A. Trained immunity in immune cells proposed by Ref.71 B. Trained immunity in non-immune epithelial cells proposed by Naik et al. (2017).Mφ, macrophage; EpSC, epithelial stem cell.