Keeping the peace: commensal Cutibacterium acnes trains CD4+ TH17 cells to trap and kill.

Commensal or pathogenic bacterial communities of the skin interact with the host immune system to preserve homeostasis or sustain disease. In this issue of the JCI, Agak et al. substantially advance our conceptual understanding of TH17 cell biology. The researchers identified IL-26-independent mechanisms by which CD4+ TH17 clones directly kill bacteria. These CD4+ TH17 clones share antimicrobial properties with cytotoxic T cells and granulocytes as evidenced by secretion of granulysin, granzyme B, and histone-laden DNA extracellular traps. Interestingly, these clones emerged following monocyte education by Cutibacterium acnes strains associated with healthy skin, but not those associated with acne. Overall, the antimicrobial mechanisms employed by these TH17 subsets suggest a unique link between innate and adaptive immune responses.