Unphosphorylated tyrosines mediate PD-1 inhibition of T cell signaling condensate formation.

T cell receptor (TCR) engagement stimulates formation of a biomolecular condensate, including the scaffold protein linker of activated T cells (LAT). We investigated whether the immune-inhibitory receptor programmed cell death protein 1 (PD-1) inhibits by impeding TCR-induced LAT condensation. In vitro reconstitutions revealed that the cytoplasmic domain of PD-1 directly inhibited LAT condensation. This inhibitory interaction required tyrosine residues in the two inhibitory motifs of PD-1. Contrary to current working models, tyrosine phosphorylation eliminated PD-1 inhibition. Replacing tyrosine with tryptophan reconstituted PD-1 inhibition in T cells, confirming that phosphorylation does not mediate this effect. These findings support the concept that hydrogen bond-donating aromatic residues within inhibitory motifs provide the primary mechanism that explains how inhibitory receptors disrupt signaling networks and block immune activation.