Synaptotagmin 1 and Synaptotagmin 7 promote MR1-mediated presentation of antigens.

(Mtb) is an intracellular pathogen that can be sensed by T cells, which are essential for the control of infection. In comparison to viral infections, Mtb antigens are relatively rare and hence, challenging to sample. Specialized antigen presentation pathways enable the presentation of such scarce antigens to CD8 T cells, which are, thus, uniquely poised to survey intracellular environments. A subset of CD8 T cells prevalent in the airways, known as mucosal associated invariant T (MAIT) cells, can be activated through the presentation of Mtb antigens via the MHC class I-related protein 1 (MR1) molecule. Prior work demonstrates that endosomal calcium signaling is critical for MR1-mediated presentation of Mtb-derived antigens. Here, we show that the calcium-sensing trafficking proteins Synaptotagmin (Syt) 1 and Syt7 specifically promote MAIT cell activation in response to Mtb-infected cells. In bronchial epithelial cells, Syt1 and Syt7 localize to late endo-lysosomes and MR1 vesicles. Loss of Syt1 and Syt7 results in enlarged MR1 vesicles and an increased number of MR1 vesicles in close proximity to Mtb-containing vacuoles during infection. This study identifies a novel pathway in which Syt1 and Syt7 facilitate the translocation of MR1 from Mtb-containing vacuoles, potentially to the cell surface for antigen presentation.