Single Cell Transcriptomics Identifies A Potential Role For Arg1+Macrophages In Alopecia Areata Pathogenesis

Alopecia Areata (AA) is an autoimmune disease that attacks the hair follicle and leads to hair loss ranging from small patches to total scalp and body hair loss. We previously showed that AA pathogenesis is predominantly mediated by CD8+ T cells, however, the role of other immune cells in AA has not been widely studied. We performed single cell RNA-seq on CD45+ immune cells harvested from the dorsal skin of AA-affected and control C3H/HeJ mice. Unbiased clustering revealed three distinct subsets of macrophages, two of which were shared between AA and control mice. The third cluster was comprised of cells that predominantly originated from AA mice, suggesting that these cells may be involved in AA pathogenesis. Interestingly, this cluster of macrophages exhibited significant upregulation of genes involved in arginine metabolism, including Arginase 1 (Arg1), the main enzyme involved in arginine catabolism. Although Arg1 was previously associated with suppressive M2 macrophages, the AA-enriched Arg1+ macrophages also showed upregulation of pro-inflammatory cytokines as well as multiple glycolytic genes that were associated with an inflammatory phenotype in macrophages. Flow cytometry and immunohistochemistry validation showed an increased frequency of Arg1+ macrophages in AA skin, localized near the hair follicle bulb, the site of autoimmune attack in AA. Further, treating C3H/HeJ mice prior to disease onset with nor-NOHA, an arginase inhibitor, reliably resulted in a delay in disease onset, consistent with recent studies showing that arginase inhibitors demonstrated therapeutic efficacy in pre-clinical models of other inflammatory disorders such as systemic lupus erythematosus and inflammatory bowel disease. The identification of a unique population of Arg1+ macrophages delivers novel insight into metabolic control of macrophages in AA, and reveals potential novel therapeutic targets in metabolic pathways.