Post-translational regulations of PD-L1 and PD-1: Mechanisms and opportunities for combined immunotherapy

Abstract

Antibodies targeting programmed cell death protein 1 (PD-1) or its ligand programmed death-ligand 1 (PD-L1) are profoundly changing the methods to treat cancers with long-term clinical benefits. Unlike conventional methods that directly target tumor cells, PD-1/PD-L1 blockade exerts anti-tumor effects largely through reactivating or normalizing cytotoxic T lymphocyte in the tumor microenvironment to combat cancer cells. However, only a small fraction of cancer patients responds well to PD-1/PD-L1 blockade and clinical outcomes have reached a bottleneck without substantial advances. Therefore, better understanding the molecular mechanisms underlying how PD-1/PD-L1 expression is regulated will provide new insights to improve the efficacy of current anti-PD-1/PD-L1 therapy. Here, we provide an update of current progress of PD-L1 and PD-1 post-translational regulations and highlight the mechanism-based combination therapy strategies for a better treatment of human cancer.

Keywords: Acetylation; Glycosylation; Immunotherapy; PD-1; PD-L1; PROTAC; Palmitoylation; Phosphorylation; Post-translational modification; Ubiquitination.

Figure 1.. Mechanism of PD-1/PD-L1 blockade.

The binding of TCR and MHC activates adaptive immune response. PD-L1 expressed on tumor cells interacts with its physiological receptor, PD-1 on T cell surface, preventing the signaling transduction of T cells to inhibit the immune response. Anti-PD-1 or anti-PD-L1 antibodies block the interaction of PD-1 and PD-L1, and abolish the inhibition of CD8⁺ T cell, thus enhancing the antitumor activity. TCR, T cell receptor; MHC, major histocompatibility complex.

Fig. 2.. Regulations of PD-L1 by polyubiquitination, deubiquitination and PROTACs.

A. PD-L1 can be polyubiquitinated by E3 ubiquitin ligases β-TRCP, SPOP, STUB1 and HRD1 and deubiquitinated by deubiquitinases CSN5, USP22, OTUB1 and USP9X. B. PROTACs of PD-L1. AbTAC of PD-L1 is a bispecific antibody which can target both PD-L1 and the E3 ligase RNF43 to induce the lysosomal degradation of PD-L1. LYTAC of PD-L1 is composed of an antibody specific to PD-L1 conjugated to a synthetic oligoglycopeptide ligand that binds CI-M6PR, a transmembrane glycoprotein responsible for trafficking proteins to lysosomes for degradation. PROTAC of PD-L1 is a novel resorcinol diphenyl ether-based PROTAC molecule which consists of BMS1198, a linker region and Pomalidomide.

Fig 3.. Regulation of PD-L1 by glycosylation.

STT3 and B3GNT3 can glycosylate PD-L1, protecting PD-L1 from degradation. Chaperone Sigma1 and FKBP51s have also been reported to promote PD-L1 glycosylation.

Fig. 4.. Regulations of PD-L1 by phosphorylation, acetylation and palmitoylation.

PD-L1 can be phosphorylated by GSK3β, AMPK and JAK1. PD-L1 is acetylated at K263 by the acetyltransferase p300 and deacetylated by HDAC2. Moreover, ZDHHC3/9 promote the palmitoylation of PD-L1 at the C272 residue.

Fig 5.. Regulation of PD-1 by polyubiquitination.

PD-1 can be polyubiquitinated by E3 ligases FBXO38, KLHL22, and c-Cbl, leading to PD-1 degradation through the 26S proteasome.