PD-1 inhibits the TCR signaling cascade by sequestering SHP-2 phosphatase, preventing its translocation to lipid rafts and facilitating Csk-mediated inhibitory phosphorylation of Lck.

Abstract PD-1 inhibits T cell responses but the biochemical mechanisms are poorly understood. PD-1 cytoplasmic tail contains an ITIM and an ITSM motif. SHP-2 tyrosine phosphatase interacts with the ITSM and has a critical role in PD-1-mediated inhibition. Although phosphatases are considered negative regulators of activation, SHP-2 deficient T cells display impaired activation of MAPK in response to TCR/CD3 stimulation. Conversely, SHP-2 mutations leading to gain of function induce activation of Ras-MAPK pathway in AML. Thus, SHP-2 may activate some signaling pathways. We investigated how PD-1: SHP-2 interaction inhibits T cell activation. A key event required for activation of the TCR/CD3 signaling cascade is the downregulation of Csk-mediated inactivating phosphorylation of Lck on Tyr-505. Growth factor stimulation of epithelial cells recruits SHP-2 to the plasma membrane and abrogates Csk-mediated inactivating phosphorylation of Src kinase. We determined that TCR/CD3 stimulation downregulated Lck phosphorylation on Tyr-505 but this was prevented by PD-1 ligation. Because Lck is localized in membrane lipid rafts, we examined whether SHP-2 might be recruited to the rafts. TCR/CD3 stimulation resulted in translocation of SHP-2 to the lipid rafts. In contrast, PD-1 ligation recruited SHP-2 to the PD-1 ITSM abrogating translocation of SHP-2 to the rafts and resulted in enhanced Lck phosphorylation on Tyr-505. Our results unravel a previously unidentified mechanism via which PD-1 inhibits T cell activation by sequestering SHP-2 and preventing its translocation to the site on the plasma membrane where SHP-2 plays a key role in activating the TCR signaling cascade by reversing Csk-mediated inactivating phosphorylation of Lck.