Contextual control of skin immunity and inflammation by Corynebacterium.

How defined microbes influence the skin immune system remains poorly understood. Here we demonstrate that Corynebacteria, dominant members of the skin microbiota, promote a dramatic increase in the number and activation of a defined subset of γδ T cells. This effect is long-lasting, occurs independently of other microbes, and is, in part, mediated by interleukin (IL)-23. Under steady-state conditions, the impact of Corynebacterium is discrete and noninflammatory. However, when applied to the skin of a host fed a high-fat diet, Corynebacterium accolens alone promotes inflammation in an IL-23-dependent manner. Such effect is highly conserved among species of Corynebacterium and dependent on the expression of a dominant component of the cell envelope, mycolic acid. Our data uncover a mode of communication between the immune system and a dominant genus of the skin microbiota and reveal that the functional impact of canonical skin microbial determinants is contextually controlled by the inflammatory and metabolic state of the host.