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

Abstract

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.

The Toll‐like receptor 9 signalling pathway regulates MR1‐mediated bacterial antigen presentation in B cells

“…To the best of our knowledge, our study is the first to directly show that the early endosomal TLR9 signalling pathway is important for antigen presentation by MR1. Consistent with our data, two groups have recently shown that brefeldin A suppressed the functional expression of MR1 in their individual experimental systems . The SNARE proteins Stx18 and VAMP4, as well as the small GTPase Rab6, were identified as important regulators of MR1‐dependent antigen presentation .…”

“…Indeed, Chen et al ., have demonstrated that co‐stimulatory signals, such as TLR ligands, are as essential as microbial antigen in MAIT cell activation and accumulation in vivo . It has been demonstrated by several laboratories that endocytic trafficking is important for MR1‐mediated antigen presentation . To the best of our knowledge, our study is the first to directly show that the early endosomal TLR9 signalling pathway is important for antigen presentation by MR1.…”

MAIT cells: new guardians of the liver

“…Recently, however, it was demonstrated that MR1 accumulates in the endoplasmic reticulum in an incompletely folded form, and in the absence of bound ligands only a few MR1 molecules traffic to the surface 24 . Increased ligand availability leads to the association of MR1 with β2‐microglobulin and egress of the MR1–β2‐microglobulin–ligand complex, 24 inducing rapid MR1 surface expression 17 , 24 , 25 . MR1 surface expression also increases with the nuclear factor‐κB‐dependent activation of antigen‐presenting cells 26 .…”

“…24 Increased ligand availability leads to the association of MR1 with β2-microglobulin and egress of the MR1-β2-microglobulin-ligand complex, 24 inducing rapid MR1 surface expression. 17,24,25 MR1 surface expression also increases with the nuclear factor-κB-dependent activation of antigen-presenting cells. 26 This contrasts with MHC class II and CD1, which capture their exogenous ligands in endosomal compartments and are highly expressed even in the absence of infection.…”

Mucosal‐associated invariant T cell receptor recognition of small molecules presented by MR1

“…The MR1 protein is ubiquitously expressed in all cells, but is presented on the cell surface at very low or undetectable levels that is strictly dependent on antigen availability. [44][45][46] In the early 2000s, it was shown that MAIT cells are restricted to MR1 and activated by a broad range of bacteria and yeast. 9,22,47 This suggested conserved Ag(s) common to these activating microbes, however, the nature of these Ags remained unknown.…”