Potential role of inflammaging mediated by the complement system in enlarged facial pores

Enlarged facial pores are a common cosmetic concern of the skin, rather than a disease, and have not received much attention from dermatologists in recent years. Consequently, progress in understanding their pathogenesis has been limited, and current cosmetic solutions have limitations. Given that the complement system has regained interest as a key player in chronic inflammatory skin conditions, various mechanisms involving this system are being investigated.


| INTRODUC TI ON
Enlarged facial pores are a common cosmetic skin problem and a prominent feature of skin aging, characterized by an uneven skin surface. 1 In Asia, a study conducted by Katsuta et al. 2 investigated the prevalence of enlarged facial pores among a cohort of 1781 women aged 20-30.Surprisingly, more than half of the participants expressed dissatisfaction with this particular aesthetic concern.
Moreover, a separate investigation led by Miyamoto et al. 3 involving a substantial sample size of 250 995 women aged 20-35 corroborated these findings, emphasizing that enlarged pores ranked among the top facial issues of concern for young women.The significance of this matter extends beyond mere cosmetic implications, as it has been observed that individuals who place a premium on beauty may experience psychological distress due to the presence of enlarged facial pores.However, there is no consensus on the definition of enlarged pores in the medical community.Kim et al. 4 have defined the size range of enlarged pores (0.3-0.6 mm 2 ), while skin analyzers define pores as any round hair follicle opening larger than 0.02mm 2 .
Studies have suggested that the formation of enlarged facial pores is primarily associated with excessive sebum secretion, reduced elasticity of supporting structures around pores, and increased volume of hair follicles. 5Our research highlights the role of inflammaging (the low-grade, asymptomatic, and persistent chronic inflammation), a process closely linked to the innate immune system's complement system, as the underlying cause of enlarged pores.
During the 1970s and 1980s, the complement system was recognized as a significant contributor to skin diseases.However, in the following decades, research on innate immunity waned, and research on adaptive immunity became more prevalent. 6In recent years, studies have revealed that the complement system functions as a bridge between innate and adaptive immunity, leading to the development of related complement inhibitors that are now effectively used in clinical practice.Consequently, the complement system has regained attention in the field of skin diseases. 7spite the prevalence of enlarged facial pores and their impact on skin appearance and self-esteem, they have not received significant attention from researchers.The underlying pathogenesis of enlarged pores remains poorly understood.Our study fills this gap by investigating the potential role of the complement system in the development of enlarged facial pores.We provide mechanistic insights into the association between inflammaging and complement activation in the formation of enlarged facial pores, laying a theoretical foundation for the development of targeted cosmetics.

| Structural features
It is widely accepted that enlarged facial pores are characterized by an uneven skin surface, abnormal dermal-epidermal junction (DEJ), and extracellular matrix (ECM) within the skin.found that the degree of damage to the SL structure was closely related to pore size and age.Similarly, Nkengne et al. 10 also identified the SL structure and collagen degradation by studying the microscopic structure of enlarged pores.Interestingly, the SL structure appears to be unique to enlarged facial pores, as it was not found in abdomen pores by Mizukoshi et al. 11 Although the mechanism for the formation of abnormal DEJ and ECM associated with enlarged pores remains unclear, studies have shown that age spots, another important feature of skin aging, also exhibit SL structure.Mizukoshi et al. 11 studied the structural changes of dermal papilla caused by skin aging and found that both age spots and enlarged pores had SL structures at DEJ.While they did not explore abnormal SL structure, this discovery provides a promising direction for future research on the SL structure of enlarged facial pores.

| Cause and mechanism
Enlarged facial pores are commonly caused by excessive sebum secretion, decreased elasticity of supporting structures, and increased volume of hair follicles. 5Previous studies have shown that sebum production is regulated by hormones such as insulin-like growth factor-1 (IGF-1), which activates the phospholipid inositol 3-kinase/ Akt pathway. 12Roh et al. 13 found a positive correlation between pore size and sebum output, and further studies have explored the composition of sebum in people with enlarged facial pores, revealing higher levels of unsaturated fatty acids that can induce abnormal differentiation of keratinocytes and production of inflammatory factors, such as tumor necrosis factorα (TNFα) and interleukin-1α (IL-1α).The activation of N-methyl-D-aspartate (NMDA) receptors by sebum may further aggravate the situation, contributing to the development of SL structures. 2,14,15 structures, which are characterized by stalagmite-like features at the dermal-epidermal junction, have been shown to be an important internal feature of enlarged pores.Sugiyama-Nakagiri et al. 16 found a positive correlation between the increase of serum IGF-1 levels and the aggravation of SL structures, suggesting a potential link between sebum secretion and SL structure formation.
However, the exact mechanism is not yet fully understood.
In addition, reduced elasticity of the structures surrounding the pores, such as the extracellular matrix protein microfibrilassociated glycoprotein-1 (MAGP-1), has been implicated in the development of enlarged pores. 17Loss of MAGP-1 has been observed around hair follicles/pores, which is essential for the assembly of elastic fibers, resulting in decreased elasticity of the structures around pores. 18Collagen degradation has also been observed in the tissues around enlarged pores, which may affect the skin surface tension around hair follicles, leading to the depression of pore tissue.Hair follicle volume has been shown to influence the visual assessment of skin pores and is positively correlated with the number of dermal papilla cells and the volume of ECM. 10 Androgen receptors expressed on dermal papilla cells also contribute to the appearance of conspicuous pores under androgen stimulation. 4 conclusion, the formation of SL structures may be influenced by inflammaging triggered by sebum-induced complement system activation.Further research is needed to fully understand the relationship between sebum secretion and SL structure formation.

| Complement system in skin
The complement system is a crucial component of the non-specific immune response, playing a vital role in defending against microbial infections and removing damaged cells. 19In its inactive state, complement in the skin is activated through a series of sequential reactions in response to invading pathogens or damaged cells. 20ese reactions involve the activation of three pathways: the lectin pathway (LP), the classical pathway (CP), and the alternative pathway (AP). 21The LP is activated by the polysaccharides on the surface of microorganisms, the CP is activated by antibody binding to the pathogen, and the AP is maintained in a state of low activation (tick-over) due to the spontaneous hydrolysis of C3.In all three pathways, the splitting of C3 into C3a and C3b by C3 convertase triggers subsequent immune reactions, culminating in the formation of the membrane attack complex (MAC) to remove damaged cells or pathogens. 22Importantly, the complement system also acts as a vital link between innate and adaptive immunity. 19For instance, C1q, a major component of the CP, indirectly triggers adaptive immune responses by activating the complement cascade upon binding to antibodyantigen complexes. 23Additionally, complement can activate antigenpresenting cells (APCs) expressing complement receptors to enhance adaptive immune responses. 24These findings highlight the critical role of the complement system in both innate and adaptive immunity.
As a significant immune and physical barrier, the skin contains various immunocompetent cells, including abundant keratinocytes. 257][28] Among these, Langerhans cells and dendritic cells (DCs) belong to APCs that can initiate primary immune responses.Furthermore, the majority of complement is produced by DCs, and their biological function depends on the production and activation of complement. 29Giang et al. 6 reported that keratinocytes produce various complement proteins, such as C3, Factor B, and C4, which can be regulated and promoted by TNFα, IL-1α, and interferonγ (IFNγ).Moreover, soluble and cell-bound regulators, such as C1-esterase inhibitor (C1-INH), membrane cofactor protein (MCP/CD46), decay-accelerating factor (DAF/CD55), complement receptor-1 (CR1), and C4b binding protein (C4BP), expressed by complement-producing cells can prevent complement from attacking itself. 30Notably, the C5aR1 signaling pathway of the complement system has a significant impact on the skin microecology.Chehoud et al. 31 found that inhibiting the C5aR1 signaling pathway would lead to a reduction in skin microbial diversity, expression of cytokines, chemokines, pattern recognition receptors, and antimicrobial peptides, ultimately weakening the skin's resistance to pathogenic microorganisms.Thus, it is critical to recognize the vital role of the complement system in maintaining skin health.

| Complement system mediates inflammaging
The term "inflamm-aging" was coined over 20 years ago to describe the low-grade, asymptomatic, and persistent chronic inflammation that occurs during aging. 22,32Researchers describe inflammaging as a "silent killer" because it is difficult to diagnose early and accumulates over time, causing internal tissue damage. 33Nowadays, inflammaging is considered an important factor in aging, including skin aging. 34o-inflammatory cytokines and the innate immune system are key drivers of aging. 34Two essential components of the innate immune system, the complement system and macrophages, play vital roles in the inflammatory process. 22With aging, the complement in the skin increases and activates, which may be the main driving force of the inflammaging process. 35,36The accumulation of damaged macromolecules and cells (self-debris) in the skin with age may be a possible cause of inflammaging.The endogenous "damage"-related molecular patterns, including the complement system, of innate immunity are activated for a prolonged time due to insufficient skin self-clearance capacity. 37rthermore, activated complement plays a crucial role in UVinduced aging (photoaging).Ultraviolet light induces epidermal keratinocytes to produce C3 and completion factor B (CFB) through alternative pathways. 38Complement C3 is transformed into iC3b upon activation, which may be relevant to ligate monocytes' β2integrin receptors.Subsequently, CD11b + monocytes differentiate into activated macrophages, inducing matrix metalloproteinase (MMP) release and reactive oxygen species (ROS) generation. 39cumulation of ROS and inflammaging are recognized as major factors in skin aging. 40Therefore, the relationship between the complement system and inflammaging is well established.We believe that the inflammaging mediated by the complement system is not only related to skin aging but also may be inseparable from other features of skin aging, such as enlarged facial pores.

| Inflammaging induced SL structure
The abnormality in the structure of the DEJ of age spots offers a valuable insight into the formation of SL structures in enlarged facial pores.Age spots are skin lesions linked to exposure to ultraviolet (UV) radiation, which triggers an overactivation of the complement system. 41Zhuang et al. 22 proposed a mechanism for the abnormal structure of the dermal papillae in age spots, positing that UVinduced complement system overactivation causes a deposition of complement complexes in the DEJ region, leading to the activation and recruitment of macrophages.Subsequently, macrophages release MMPs that degrade the ECM and pro-inflammatory cytokines, ultimately leading to the breakdown of dermal papillae via inflammaging.Notably, this conclusion is supported by another interesting study.Yoshida et al. 42 discovered that UV irradiation resulted in the deposition of a significant amount of complement iC3b in the DEJ region, with macrophages regularly observed in close proximity to the fine granular deposits of iC3b along the DEJ.Hence, hyperactivity of the complement system leads to DEJ damage, where the complement complex is deposited.

| SL structure shows enlarged facial pores
While it is widely accepted that sebum output and the structure of SL formations are positively correlated with pore size, the underlying mechanism by which sebum output contributes to the formation of SL structure remains unresolved.Our investigation into the complement system and inflammaging has led us to hypothesize that sebum-induced inflammaging, mediated by the complement system, may be a crucial factor in the formation of SL structure.
Although UV radiation has been shown to contribute to the formation of enlarged pores, it is important to also examine why sebum output is positively correlated with pore size.We emphasize that unsaturated fatty acids in sebum can stimulate the production of inflammatory factors in keratinocytes through NMDA receptors, leading to the upregulation of complement protein production and activation of the complement system (Figure 1B). 6,14peractivity of the complement system results in continuous inflammation in the hair follicle microenvironment, with more complement complexes deposited on the DEJ, leading to immune cell infiltration and gradual damage to the DEJ, ultimately resulting in the formation of the unique SL structure.It is worth noting that the activation of alternative pathways may contribute to complement deposition on the DEJ, since the alternative pathway acts as a "surveillance mechanism" that is constantly in a state of hypoactivation (tickover). 43Activation of alternative pathways has also been linked to the initiation of inflammatory diseases of hair follicles.Studies have shown that deposition of complement C3b at the DEJ around the hair follicle can be observed in the early stages of acne lesions, prior to mononuclear cell intervention and without concurrent deposition of immunoglobulin or C1q, indicating that the complement system may be one of the first factors involved in the onset of acne. 6,44While bacteria in the hair follicle may also activate the complement system, recent studies have demonstrated that microorganisms have developed strategies to avoid complement activation.For instance, Staphylococcus aureus can secrete specific proteins to interfere with the function of C3, thereby circumventing complement system activation. 45n summary, the hair follicle microenvironment creates conditions that lead to hyperactivity of the complement system, resulting in persistent inflammaging, complement fragment deposition on the DEJ, and long-term immune cell infiltration, which may be important factors in the formation of the SL structure.

| CON CLUS ION
Enlarged facial pores are primarily an aesthetic skin concern rather than a skin disease and therefore have received limited attention from dermatologists as a main research subject.This lack of research has resulted in minimal advancements in understanding the correlation between sebum output, SL structure, and enlarged pores, which remains a challenge in treating this condition. 8,9,13 the complement system has returned to the research spotlight as a causative factor of chronic inflammatory skin diseases, researchers are also beginning to investigate its role in skin aging. 46in aging and inflammaging are closely linked, as the skin is constantly exposed to damage from UV radiation and oxidative stress. 47larged pores, an important feature of skin aging, need to be further explored to better understand the relationship between sebum production and SL structure.Our analysis suggests that the complement system may potentially play a role in the mechanism of enlarged pores, with unsaturated fatty acids in sebum involved in SL structure and pore formation.Notably, although complement system activation is an inevitable part of skin aging, inflammaging induced by sebum oversecretion and bacterial colonization can exacerbate damage to the DEJ in the hair follicle, leading to more severe tissue structure damage.
Further studies are needed to confirm the potential role of the complement system in enlarged pores and to develop novel therapeutic approaches.For example, future research should investigate the amount of complement deposition in the DEJ when enlarged pores occur, and whether regulating the complement system and reducing complement deposition could be potential treatments for enlarged pores.If we can confirm that the complement system is a potential driver of enlarged pores, it will provide a new approach to improving this condition.This could involve regulating complement deposition or promoting clearance of complement complexes on the DEJ, which can reduce the damage caused by immune cells processing complement complexes.For instance, Dovezenski et al. 48ported that complement receptor 1 (CR1), a membrane protein located in the basal layer of the epidermis, participates in the process of clearance and phagocytosis of immune complexes, preventing random deposition of C3b, which is a potential inhibitor of complement activation.Additionally, complement factor H (CFH) acts as a complement inhibitor, effectively blocking activation of the alternative pathway. 22These may be potential targets for regulating complement to improve enlarged facial pores.
In conclusion, while inflammaging mediated by the complement system may have important pathophysiological implications in the occurrence of enlarged facial pores, there may be other pathways that affect the formation of SL structure.Therefore, a more detailed understanding of these factors is needed.
[8][9][10] Sugata et al.8 first observed the structural features of enlarged pores by confocal laser scanning microscopy (CLSM), and they found a strong undulation of DEJ around the pore in the cross-sectional image.Subsequently, the stalagmite-like (SL) structure (shown in Figure 1A) and its relationship with enlarged pores has been widely studied.Sugiyama-Nakagiri et al. 9 investigated 138 volunteers of different ages and F I G U R E 1 The correlation between sebum output, SL structure, and enlarged pores.Sebum-mediated inflammaging contributes to the formation of SL structure, resulting in enlarged pores.(A) Enlarged pores structure features -stalagmite-like (SL) structure.(B) Unsaturated fatty acid-induced abnormal dermal papillary structures.