Alternative Splicing of MR1 regulates antigen presentation to MAIT cells

Abstract

Mucosal Associated Invariant T (MAIT) cells can sense intracellular infection by a broad array of pathogens. These cells are activated upon encountering microbial antigen(s) displayed by MR1 on the surface of an infected cell. Human MR1 undergoes alternative splicing. The full length isoform, MR1A, can activate MAIT cells, while the function of the isoforms, MR1B and MR1C, are not well characterized.In this report, we sought to characterize these splice variants. Using a transcriptomic analysis in conjunction with qPCR, we find that that MR1A and MR1B transcripts are widely expressed. Despite the widespread expression of MR1A and MR1B, only MR1A can present mycobacterial antigen to MAIT cells. Coexpression of MR1B with MR1A serves to decrease MAIT cell activation following bacterial infection.However, expression of MR1B prior to MR1A lowers total MR1A abundance, suggesting competition between MR1A and MR1B for either ligands or chaperones required for folding and/or trafficking. Finally, we evaluated CD4/CD8 double positive thymocytes expressing surface MR1. Relative MR1A/MR1B expression in MR1-expressing thymocytes is associated with their prevalence.Our results suggest alternative splicing of MR1 represents a means of regulating MAIT activation in response to microbial ligand.

Ligand-dependent downregulation of MR1 cell surface expression

“…Staining with a polyclonal anti-MR1 antibody revealed the total MR1 content was unaffected, thus ruling out degradation of MR1 molecules (Fig. 6A); this was further confirmed with the epithelial cell line, BEAS2B, expressing a tetracycline-inducible MR1 construct tagged with GFP (34,35) or constitutively expressing GFP-tagged MR1 molecules. In both cell lines, in the presence of DB28, we observed down-regulation of MR1 from the cell surface ( Fig.…”

“…4D). To avoid potential off-target effects of cycloheximide, we tested the epithelial cell line BEAS2B expressing a tetracycline-inducible MR1 construct tagged with GFP (34,35). Upon doxycycline treatment, more than 90% of cells became GFP positive and 10% to 15% of cells expressed cell surface MR1 (SI Appendix, Fig.…”

“…BEAS2B WT cells were grown in DMEM supplemented with 10% heatinactivated FBS (Gemini Bio-products) and 3.5 mM L-glutamine (Gibco). BEAS2B MR1 KO expressing doxycycline-inducible MR1 was previously described (34,35).…”