Inhibitory TIGIT signalling is dependent on activating T cell receptor signalling

TIGIT is an immune checkpoint receptor that can signal via its intracellular inhibitory motifs to inhibit T cells, though the mechanisms remain poorly defined. To elucidate inhibitory signalling interactors of TIGIT, proximity proteomics was employed in model Jurkat T cells expressing TIGIT fused to APEX2. This identified several ligation-specific TIGIT interactors, including proteins involved in signalling (Grb2 and SOS1), cytoskeletal (CD2AP and SdcBP), and endocytic processes (IST1 and SNX3). A TIGIT mutant incapable of signalling via its inhibitory motifs prevented the recruitment of these proteins, indicating inhibitory signalling-specific engagement of these pathways. Furthermore, TIGIT engagement with these pathways, in addition to its phosphorylation and internalisation, was dependent on T cell activation as well as TIGIT ligation. Collectively, these findings demonstrate that TIGIT inhibitory signalling is restricted to activated T cells, indicating a regulatory mechanism that limits inhibition to when functionally required.