The immune system initiates appropriate responses against pathogens by detecting pathogen-specific antigens. A novel class of antigen is derived from Vitamin B2 synthesis, which occurs in diverse microbes, but not mammals. These metabolites, termed Vitamin B-related Antigens (VitBAg), are captured by major histocompatibility complex class I-related protein 1 (MR1) and are then presented to mucosal-associated invariant T (MAIT) cells. The MR1-MAIT cell axis is open to therapeutic intervention, however MR1 has a low expression and is difficult to detect and thus, the cells that express MR1, capture VitBAg and present it to MAIT cells are unknown. This hampers research in this field.
To address this knowledge gap, we generated a MR1-reporter mouse line, where the expression of MR1 is revealed by a fluorescent reporter protein, tdTomato. This revealed that the highest MR1-expressing cells in mice are tissue resident macrophages including lung alveolar macrophages (AMs) and large peritoneal macrophages (LPMs). Our functional verification demonstrated that these types of macrophages had the greatest capacity to activate MAIT cells ex vivo. Using a fluorescently labelled VitBAg analogue we demonstrated that AMs and LPMs have the highest capacity for antigen uptake in vivo, which also correlated with their MR1 expression level, and detection of the fluorescent VitBAg analogue presented on the surface of these cells. Therefore, macrophages are key MR1 antigen presenting cells in vivo. Moreover, we found a decreased MR1 expression level in LPMs from mice housed under germ-free conditions, indicating that the presence of microflora is important for normal MR1 expression in LPMs.
Together, these findings suggest that tissue-resident macrophages are the major MR1-expressing and antigen-presenting cells in vivo. This indicates that MAIT cell-directed therapies could be targeted to these innate presenting cells to drive or suppress this arm of immunity.