Macrophage MR1 antigen presentation promotes MAIT cell immunity and lung microbiota modulation.

Mucosal-associated invariant T cells (MAIT cells) mediate tissue homeostasis and antimicrobial immunity. However, the cells that express major histocompatibility complex (MHC) class I-related protein 1 (MR1) and present microbial vitamin B-derived antigens (VitBAg) to MAIT cells remain unknown. We found that MR1 expression varied across tissues and cell types. Macrophages from the lung and peritoneal cavity expressed the highest levels of MR1 and were the most efficient at capturing and presenting VitBAg to MAIT cells. Expression of MR1 in macrophages was regulated transcriptionally and induced by the tissue environment and microbiota. Depletion of MR1 in macrophages, dendritic cells, and monocytes changed the composition of the microbiota and impaired MAIT cell responses against bacterial infection. We concluded that macrophages are key for MR1 antigen presentation and MAIT cell immunity.