UCLA

Studies of the human skin microbiome suggest that Propionibacterium acnes strains may contribute differently to skin health and disease. However, the immune phenotype and functions of T helper type 17 (Th17) cells induced by healthy (P_H) versus acne (P_A) skin-associated P. acnes strains are currently unknown. We stimulated peripheral blood mononuclear cells from healthy donors and observed that P_A strains induce higher IL-17 levels than P_H strains. We next generated P_H and P_A strain-specific Th17 clones and show that P. acnes strains induce Th17 cells of varied phenotype and function that are stable in the presence of IL-2 and IL-23. Although P_H- and P_A-specific clones expressed similar levels of LL-37 and DEFB4, only P_H-specific clones secreted molecules sufficient to kill P. acnes. Furthermore, electron microscopic studies showed that supernatants derived from activated P_H and not P_A-specific clones exhibited robust bactericidal activity against P. acnes, and complete breaches in the bacterial cell envelope were observed. This antimicrobial activity was independent of IL-26, because both natural IL-26 released by Th17 clones and rhIL-26 lacked antimicrobial potency against P. acnes. Overall, our data suggest that P. acnes strains may differentially modulate the CD4⁺ T-cell responses, leading to the generation of Th17 cells that may contribute to either homeostasis or acne pathogenesis.