Acne is recognized as a disease with multifactorial pathogenesis which involves the sebaceous gland, with highest prevalence among adolescents. A further understanding of the pathogenesis of this disease could provide new ideas for the mechanism-targeted treatments(33).
Inflammation has been proved to be strongly associated with the pathogenesis of acne, additionally, previous researches illustrated that immune cell infiltration is a key factor involved in the development of acne(13, 17). The activation of TLRs, the increased productions of chemokines for neutrophils, T cells, monocytes and dendritic cells, cytokines (especially Th1/Th17 related cytokines), and other inflammatory markers such as MMPs have been confirmed participating in the acne inflammation(12, 34, 35).
Based on the GSEA and GSVA analyses, we identified that the JAK-STAT signaling pathway, NOD-like receptor signaling pathway, and Toll-like receptor signaling pathway were enriched in acne lesions. The JAK-STAT signaling pathway stimulated by cytokines, which is a signal transduction pathway(36). It plays roles in the processes of proliferation, differentiation, immunoregulation and apoptosis. According to the N-terminal domain, NOD-like receptors (NLRs) are classified into four subfamilies: NLRA, NLRB, NLRC, and NLRP(37). NLRs play a vital role in inflammasomes, C. acnes could powerfully trigger the activation of the NLRP3 inflammasome and the downstream caspase-1 and IL-1β(38). IL-1β which produce the inflammatory papules of acne(39). It has been observed that NOD-like receptors could regulate the MAPK and NF-κB pathways leading to proinflammatory cytokines release such as TNF-α, IL-6(40) when it acts in synergy with TLRs. Hanna-Leena Kelhälä et.al(9) observed that significant elevation of the TNF-α in acne skin compared to normal control. IL-6 family is regarded as the key trigger of JAK-STAT pathway. J Ji et.al found that IL-6 was increased in acne patients’ skin lesion tissues(41).
In our current study, we explored the crucial genes in the pathogenesis of acne, including FPR1, C3AR1, CXCL1, CXCL8, FPR2, C3, CCR7 and ITGB2. However, there was no significant difference in the expression of CCR7 between the acne group and the non-lesional group in our validation dataset. FPR1 and FPR2, the peptides belong to the formyl peptide receptor family, C3AR1, the receptor for C3a, members of the CXC chemokine family (such as CXCL1, CXCL8), C-C motif chemokine receptor (such as CCR2) have been identified the crucial genes in acne lesions(42, 43). FPR1, FPR2 and FPR3 play roles in inflammatory and antibacterial processes, which are primarily expressed in leukocytes, belonged to Gi-protein-coupled receptors. C3, C3AR1, and C5AR1 belong to the core members of complement family, participate in cellular response to inflammation(44). CXCL1, CXCL8, CXCL2, CXCL3, CXCL5 are involved in guiding the neutrophils to the site of infection and promote inflammation. CXCL9, CXCL11 and CXCL13 are chemoattractants for lymphocytes, CCR2 specifically mediates monocytes chemotaxis, CCR5 mediates monocytes, macrophages and T cells chemotaxis, and CCR7 has been proved to stimulate dendritic cell maturation as well as memory T cells’ migration to inflamed tissues. ITGB2 which encodes integrin that is involved in leukocytes adhesion and cell-surface mediated signaling.
Our study not only identified a range of key genes and significant pathways in the pathogenesis of acne, but also analyzed the immune cell infiltration patterns of this disease. CIBERSORT was used to evaluate immune cell infiltration in acne. We discovered that an increased infiltration of neutrophils, activated mast cells, monocytes, activated dendritic cells, M0 macrophages, resting memory CD4 + T cells, activated memory CD4 + T cells, and a decreased infiltration of plasma cells, memory B cells, Tregs, follicular helper T cells, resting mast cells, resting dendritic cells may participate in the immune pathogenesis of acne.
It has been illustrated that the number of mast cells was observed to be increased in adjacent areas to the sebaceous glands in the acne lesion(45). Substance P (SP), a kind of neuropeptide associated with stress-induced acne, could stimulate mast cells degranulation and macrophage infiltration(15) as well as stimulate the release of a series of pro-inflammatory cytokines such as IL-6, IL-1, and TNF-α(46)from sebocytes. Hao Jiang et.al found that in acne lesions, a massive uptake of oxidized lipid could lead to the generation of neutrophils (foam cells) and lipid-loaded macrophages(47). C. acnes has been proved to activate dendritic cells and enhance the release of cytokines and adhesive molecules, leading to antigen-presenting cells that tend to activate Th17 cells(48). Min Qin et.al showed that C. acnes stimulate monocytes to NLRP3-mediated pathway partially dependent IL-1β secretion(39). George W Agak et.al(49) suggested that C. acnes strains may play different roles in skin disease and health, leading to different responses of CD4 + T cells and either acne pathogenesis or homeostasis contributed by the generation of Th17 cells. It is well known that Th17 cells and FOXP3 + Treg cells modulate the immune balance in acne(14). I Carlavan et.al(50) found that 48-h-old papules in acne, neutrophils, T cells and macrophages infiltration increased. however, long-lived acne papules (3-week-old) in scar-prone patients are characterized by B-cell infiltration increased. Our results are consistent with the previous studies that activated mast cells, monocytes, activated memory CD4 + T cells, resting memory CD4 + T cells, Tregs, activated dendritic cells, M0 macrophages, plasma cells and memory B cells may participate in acne development and further research is need. Follicular helper T cells’ function in acne has not been focused on, and further experimental data are required. The specific mechanisms of the correlation among immune cells infiltration in acne require further experimental evidence.
We speculated that neutrophils, monocytes, activated mast cells were the most positively, and resting mast cells, Tregs were the most negatively correlated with the hub genes, as well as the significant correlations between these immune cells with JAK-STAT signaling pathway, Toll-like receptor signaling pathway and NOD-like receptor signaling pathway. These results suggested that neutrophils, monocytes, resting mast cells, activated mast cells, Tregs may play critical roles in the occurrence and progress of acne. Further research is required to elucidate the association between genes, pathways and immune infiltration in the pathogenesis of acne.
We used CIBERSORT algorithm to identify acne lesions’ immune infiltration for the first time. Our study has also some limitations. CIBERSORT analysis may deviate from phenotypic plasticity, heterotypic interactions of cells or disease-induced disorders with limited genetic data. Besides, although some of our analysis data is consistent with the previous view, and because of the difficulty in collecting skin samples from acne patients, we used GSE6475 as a validation dataset and got similar findings, which may make the conclusions of our study more convincing, using another dataset to verify the research conclusions is not as reliable as the validation in patient samples, candidate genes and pathways should be confirmed at the level of protein with the identification of the cell types expressing/producing those.
Conclusions
In the pathogenesis of acne, FPR1, C3AR1, CXCL1, CXCL8, FPR2, C3, CCR7, ITGB2 may serve as potential biomarkers in acne. However, there was no significant difference in the expression of CCR7 between the acne group and the non-lesional group in our validation dataset. Whether CCR7 can be regarded as a biomarker of acne remains to be verified. We found that neutrophils, activated mast cells, monocytes, activated dendritic cells, M0 macrophages, resting memory CD4 + T cells, activated memory CD4 + T cells, Tregs, plasma cells, memory B cells may participate in acne development and further research is need. The key genes FPR1, C3AR1, CXCL1, CXCL8, FPR2, C3, CCR7, ITGB2 and pathways such as JAK-STAT signaling pathway, Toll-like receptor signaling pathway, and NOD-like receptor signaling pathway were the most significantly associated with raising neutrophils, monocytes, activated mast cells, and reducing resting mast cells, Tregs. Further clinical and experimental research are needed to investigate the immunopathogenesis of acne and explore novel targets for acne immunotherapeutic.
Declarations