Endosomal MR1 Trafficking Plays a Key Role in Presentation of Mycobacterium tuberculosis Ligands to MAIT Cells

• Published in: PLOS Pathogens
• Main Authors: Harriff, Melanie J., Karamooz, Elham, Burr, Ansen, Grant, Wilmon F., Canfield, Elizabeth T., Sorensen, Michelle L., Moita, Luis F., Lewinsohn, David M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: PLOS 2016
• Subjects: Mycobacterium tuberculosis; T cells; Small interfering RNAs; Antigen-presenting cells; Enzyme-linked immunoassays; Cloning; Intracellular pathogens; Antigen presentation; DML
• Online Access: http://hdl.handle.net/10400.7/577; https://doi.org/10.1371/journal.ppat.1005524

Mucosal-Associated Invariant T (MAIT) cells, present in high frequency in airway and other mucosal tissues, have Th1 effector capacity positioning them to play a critical role in the early immune response to intracellular pathogens, including Mycobacterium tuberculosis (Mtb). MR1 is a highly conserved Class I-like molecule that presents vitamin B metabolites to MAIT cells. The mechanisms for loading these ubiquitous small molecules are likely to be tightly regulated to prevent inappropriate MAIT cell activation. To define the intracellular localization of MR1, we analyzed the distribution of an MR1-GFP fusion protein in antigen presenting cells. We found that MR1 localized to endosomes and was translocated to the cell surface upon addition of 6-formyl pterin (6-FP). To understand the mechanisms by which MR1 antigens are presented, we used a lentiviral shRNA screen to identify trafficking molecules that are required for the presentation of Mtb antigen to HLA-diverse T cells. We identified Stx18, VAMP4, and Rab6 as trafficking molecules regulating MR1-dependent MAIT cell recognition of Mtb-infected cells. Stx18 but not VAMP4 or Rab6 knockdown also resulted in decreased 6-FP-dependent surface translocation of MR1 suggesting distinct pathways for loading of exogenous ligands and intracellular mycobacterially-derived ligands. We postulate that endosome-mediated trafficking of MR1 allows for selective sampling of the intracellular environment. Career Development Award: (#IK2 CX000538); U.S. Department of Veterans Affairs Clinical Sciences Research and Development Program (MJH); U.S.Department of Veterans Affairs Biomedical Laboratory Research and Development Program (DML) Merit Award: (#I01 BX000533); American Lung Association: (RT-350058).