Synthesis, biological evaluation and molecular modelling of new potent clickable analogues of 5-OP-RU for their use as chemical probes for the study of MAIT cell biology.

MAIT cells are preset αβ T lymphocytes that recognize a series of microbial antigens exclusively derived from the riboflavin biosynthesis pathway, which is present in most bacteria. The most active known antigen is unstable 5-(2-oxopropylideneamino)-6-(d-ribitylamino)uracil (5-OP-RU) which is stabilized when bound and presented to MAIT cells by MHC-related protein 1 (MR1). Here we describe the chemical synthesis and biological evaluation of new chemical probes for the study of MAIT cell biology. The two probes were ethinyl functionalized analogues of 5-OP-RU able to react through CuAAC also called "click chemistry". The molecules up-regulated more MR1 than 5-OP-RU and they efficiently activated iVα19 Vβ8 TCR transgenic murine MAIT cells but not iVα19 TCRα transgenic MAIT cells indicating a surprisingly strong impact of the TRCβ chain. Moreover, the use of these molecules as chemical probes was validated in vitro by efficient and selective binding to MR1 revealed via fluorescence microscopy. This study was also complemented by molecular modelling investigation of the probes and the binary/ternary complexes they form with MR1 and the TCR. These new probes will be crucial to delineate the dynamics of 5-OP-RU at the cellular or whole organism level and to identify the cells presenting 5-OP-RU to MAIT cells in vivo.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.