PD1/PD-L1 is a well-known immune checkpoint that when activated, inhibits proliferation of T-cells, release of cytokines, and cytotoxicity. Cancer cells expressing the ligand PD-L1 can escape the immune system by binding to and activating PD1 on tumor specific T-cells leading to exhaustion and apoptosis of the T-cells [1, 2]. PD-L1 is used as a biomarker in IHC to identify patients eligible for immune checkpoint inhibitor treatments including anti-PD1 and anti PD-L1 therapies [3]. However, unfortunately not all patients respond successfully to these therapies, therefore it is crucial to advance biomarkers and the tools to study the underlying mechanisms of this immune checkpoint axis [1, 4].​

Protein-protein interaction PD1/SHP-2

The activation of PD1 and the subsequent inhibiting signaling pathway requires PD1 to be bound to PD-L1, as well as being phosphorylated. The phosphorylation of PD1 occurs via the T-cell receptor (TCR) that, upon binding to MHC, activates TCR proximal src-kinases that in turn phosphorylate PD1 intracellular tyrosine residues ITIM and ITSM. When PD1 is phosphorylated and bound to PD-L1, the phosphatase SHP-2 can, via its two SH2 domains, bind to the phosphorylated sites of PD1. When bound to PD1, SHP-2 is active and de-phosphorylates its targets [5, 6] (Figure 1).

The targets of SHP-2 in this event are downstream regulators of the CD28 and TCR pathways, leading to a downregulation of PI3K/Akt and RAS/ERK, respectively. Eventually this results in the T-cell becoming inhibited, enabling the cancer cell to escape the tumor specific T-cells [7]. Of the interactions within the PD1-PD-L1 axis, the PD1-SHP2 interaction can be regarded as one of the most biologically relevant, since it determines whether or not downregulation of the T-cells can occur.

Learn more about PD1/PD-L1 signaling here

Studies of PD1/SHP-2 interactions

Both inhibitors and activators targeting SHP2 have been developed for the treatment of different diseases. The SHP2 allosteric inhibitor SHP099 together with a PD-1 inhibitor showed that tumor growth in xenograft models of colon cancer was better controlled than it was in response to each inhibitor alone (8, 9). We have used the classic Naveni in situ proximity ligation assay to visualize the specific interaction between PD1 and SHP2 in tissues (Figure 2). This optimized application has high specificity and can be used in basic research as well as clinical research to investigate the PD1-SHP2 signaling pathway for the development of molecular antagonists for cancer therapies. The PD1/SHP2 can be used as a biomarker application for clinical use to identify patients eligible for treatment with PD1/PD-L1 checkpoint inhibitors.​