Molecular Features Underlying Shp1/Shp2 Discrimination by Immune Checkpoint Receptors

Numerous inhibitory immunoreceptors operate by recruiting phosphatase effectors Shp1 and Shp2 through conserved motifs ITIM and ITSM. Despite the similarity, these receptors exhibit distinct effector binding specificities, as exemplified by PD-1 and BTLA, which preferentially recruit Shp2 and Shp1 respectively. The molecular basis of Shp1/Shp2 discrimination is unclear. Here, we provide evidence that optimal PD-1 and BTLA binding to both Shp1 and Shp2 occurs via a bivalent, parallel mode that involves both SH2 domains of Shp1/Shp2. Moreover, PD-1 mainly uses its ITSM to discriminate Shp2 from Shp1 via their C-terminal SH2 domains. Supportive of this model, swapping the Shp1-cSH2 with Shp2-cSH2 enabled PD-1:Shp1 association in T cells. In contrast, BTLA primarily utilizes its ITIM to discriminate Shp1 from Shp2 via their N-terminal SH2 domains. Substitution of glycine at pY+1 position of the PD-1-ITIM with alanine, a residue conserved in several Shp1-recruiting receptors, was sufficient to induce PD-1:Shp1 interaction in T cells. Finally, mutagenesis screening shows that Shp1 recruitment exhibits a bell-shaped dependence on the side chain volume of the pY+1 residue of ITIM. Collectively, we provide a molecular interpretation of the Shp1/Shp2-binding specificities of PD-1 and BTLA, with general implications for the mechanism of effector discrimination by inhibitory receptors.