Mucosal-associated invariant T (MAIT) cells are a subset of T-cells restricted by the MHC-related protein 1 (MR1). It has been established in the past that MR1 is alternatively spliced into species-specific isoform variants. In humans, three isoforms; MR1A, MR1B and MR1C are ubiquitously transcribed. MR1A, the most well characterised isoform, encodes for a heavy chain with three extracellular domains (α1-α3), a transmembrane domain and a cytoplasmic domain. In contrast, MR1B encodes for a truncated membrane bound protein lacking the α3-domain, and MR1C for a soluble MR1B variant. It has recently been shown that MR1A has the capacity to present riboflavin (vitamin B2) and folate (vitamin B9) derivatives as antigens. In contrast, less is known about the function of the other isoforms, although two separate groups have provided evidence that these are functional. To better understand the function of these isoforms in MAIT cell biology, we first verified the existence of and quantifed alternative splice forms in various human cell types and tissues at the transcriptional level. Strikingly MR1 isoforms are differentially expressed in a tissue and cell-type-specific-manner. Furthermore, MR1B/D transcripts were the dominant isoforms in both human thymus and blood.

To test the function of MR1B we generated MR1 KO cell-lines, overexpressed either MR1A or MR1B in these cells and tested them in MAIT activation assays. We found that, despite earlier findings, MR1B did not activate MAIT cells in the presence of antigen. Interestingly, when MR1B was overexpressed into cells containing endogenous MR1A, MAIT cell activation was mildly enhanced. Indeed, in cells overexpressing MR1B, overexpressed MR1A trafficked faster to the cell surface than controls. These results suggest that MR1B plays a moderate role in MAIT cell activation by modulating MR1A dependent antigen presentation.