Commensal microbes colonize mucosal surfaces throughout the body, but whether there is a resident population of microbes present on the ocular surface has been controversial. In Immunity, Caspi and colleagues establish that Cornyebacterium mastitidis is a de facto resident microbe present on mouse conjunctiva that 'tunes' the local immune response. Several other bacteria tested, including other Cornyebacterium species and Staphylococcus epidermidis, fail to colonize the conjunctiva after inoculation. The presence of C. mastitidis results mainly in the activation and population expansion of ocular IL-17A+ γδ T cells, which in turn supports the induction of anti-microbial peptides and the recruitment of neutrophils to the conjunctiva. Mice lacking C. mastitidis have fewer conjunctival IL-17A+ γδ T cells and greater vulnerability to infection of the eye with Candida albicans or Pseudomonas aeruginosa. C. mastitidis is therefore a specialized commensal resident in the conjunctiva that can mediate localized protection through the induction of IL-17A production.

Immunity 47, 148–159 (2017)