Affinity purification mass spectrometry analysis of PD-1 uncovers SAP as a new checkpoint inhibitor

Significance Antibodies targeting PD-1 have elicited clinical responses in multiple tumors. Nevertheless, response to anti–PD-1 interventions is limited to a fraction of patients, and a comprehensive understanding of the signaling downstream of PD-1 could provide biomarkers for tumor response. We used affinity purification-mass spectrometry to uncover proteins associated with PD-1 and found that the adaptor protein SAP inhibits PD-1 functions by blocking the enzymatic interaction between the phosphatase SHP2 and the subset of its substrates that also bind to SAP. Signaling downstream of PD-1 in T cell subsets did not correlate with PD-1 expression but was inversely correlated with SAP levels. Thus, SAP is a modulator of PD-1 function and a potential biomarker for reduced responses to PD-1–based interventions.