Endogenous retrovirus control of homeostatic and inflammatory responses to the microbiota [scRNA-seq]

How the homeostatic dialogue between the host and its microbiota is initiated remains largely unknown. Here, we show that immune response to the skin microbiota is dependent on activity of endogenous retroviruses. Notably, response to the microbiota promotes a discrete transcriptional induction of retroelements. As such, keratinocyte intrinsic sensing of endogenous retrovirus (ERV) derived DNA via cGAS/STING promotes the induction of commensal specific T cells including CD8+, CD4+ and MAIT cells. Consequently, inhibition of reverse transcriptase activity impairs both homeostatic immunity to the microbiota and associated tissue repair function. On the other hand, altered diet and in particular increase in dietary lipids primed the skin for aberrant ERV expression in the context of microbiota exposure leading to tissue inflammation. Together our results support the idea that the host may have coopted its endogenous virome as a means to communicate with exogenous microbiota resulting in a multikingdom dialogue that controls both tissue homeostasis and inflammation.