Date Published: March 31, 2016
Publisher: Public Library of Science
Author(s): Melanie J. Harriff, Elham Karamooz, Ansen Burr, Wilmon F. Grant, Elizabeth T. Canfield, Michelle L. Sorensen, Luis F. Moita, David M. Lewinsohn, Padmini Salgame.
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.
Mucosal-Associated Invariant T (MAIT) cells are a class of CD8+ T cells that are unique in their use of a semi-invariant TCR, restriction by the highly conserved major histocompatibility complex, class I-related protein 1 (MR1), and their recognition of small molecule metabolites. In support of a role for these cells in host-defense to mucosal infection, human MAIT cells are present in high numbers in mucosal tissues and blood [1, 2] and secrete pro-inflammatory molecules including TNFα and IFN-γ [3, 4]. MAIT cells have the capacity to respond to intracellular pathogens such as Mycobacterium tuberculosis (Mtb)  and Francisella tularensis  and animal models demonstrate a requirement for MR1, and by inference, MAIT cells in early control of certain pathogens [5–7]. Although MR1 is ubiquitously expressed in all mammalian tissues examined, surface expression is very low or undetectable in both phagocytic professional antigen presenting cells and non-hematopoetic cells [8, 9]. Previous work demonstrated that MR1 surface expression in mouse cells overexpressing MR1 requires both the MHC-II chaperone, invariant chain (Ii), and trafficking through late endosomal compartments . This is in contrast to the constitutive surface expression of other Class I molecules, and may be key to understanding the regulation of MR1-restricted MAIT cell activation.
MAIT cells are a unique class of MR1-restricted T cells capable of recognizing small molecule vitamin B metabolites. There is a growing appreciation for the importance of these cells in the mucosal response to intracellular microbial infection. At present, little is known about the mechanisms by which this novel class of antigens are processed and presented. As we have demonstrated previously, MAIT cells recognize Mtb-infected dendritic and epithelial cells and are present in high numbers in lung tissues [3, 14, 24]. We postulate that the circumstances by which MAIT cells become activated is tightly regulated to allow for the recognition of intracellular infection while avoiding tissue damage. Here we have demonstrated that MR1 is primarily localized to an endosomal compartment, and have identified trafficking molecules responsible for presentation of Mtb antigens on MR1. These results provide a framework with which to understand the mechanisms regulating activation of MAIT cells.