PAG supports T cell immune synapse function by binding to actin.

Immunotherapies affect T cell function by modifying the immune synapse, which forms at the interface between T cells and their targets. Phosphoprotein associated with glycosphingolipid-rich microdomains 1 (PAG) is a T cell adaptor protein within the immune synapse that mediates signaling by the inhibitory co-receptor PD-1. Because PAG undergoes palmitoylation, which positions it within plasma membrane lipid rafts, and contains a carboxyl-terminal PDZ-binding motif that connects to the actin cytoskeleton, PAG is a potential target for modulating immune function. Here, we investigated how PAG functioned with actin to regulate T cell immune synapse organization and function. We found that the dynamics of PAG and actin were closely synchronized during immune synapse maturation. Mutation of the PDZ-binding motif of PAG disrupted the PAG-actin interaction, impairing immune synapse formation, stability, and function. To assess the function of the PDZ-binding motif of PAG in vivo, we used mouse models of type IV hypersensitivity and highly immunogenic cancer. In both cases, mice with T cells expressing mutant PAG lacking this motif showed reduced immune responses, particularly in terms of cytotoxicity. These results highlight the importance of the PAG-actin connection for optimal formation and function of the T cell immune synapse. Our research indicates that targeting PAG may be a promising approach to improving immunotherapy in those patients who do not respond or experience immune-related adverse events.