SHP2 Targets ITK Downstream of PD-1 to Inhibit T Cell Function

PD-1 is a critical therapeutic target in Cancer Immunotherapy and Antibodies blocking PD-1 are approved for multiple types of malignancies. The Phosphatase SHP2 is the main effector mediating PD-1 downstream signaling and accordingly attempts have been made to target this Enzyme as an alternative approach to treat immunogenic tumors. Unfortunately, small molecule inhibitors of SHP2 do not work as expected, suggesting that the role of SHP2 in T cells is more complex than initially hypothesized. To better understand the perplexing role of SHP2 in T cells, we performed interactome mapping of SAP, an adapter protein that is associated with SHP2 downstream signaling. Using genetic and pharmacological approaches, we discovered that SHP2 dephosphorylates Itk specifically downstream of PD-1 and that this event was associated with PD-1 inhibitory cellular functions. This study suggests that Itk is a unique target in this pathway, and since Itk is a SHP2-dependent specific mediator of PD-1 signaling, the combination of Itk inhibitors with PD-1 blockade may improve upon PD-1 monotherapy in the treatment of Cancer.