Exosomal PD-L1: New Insights Into Tumor Immune Escape Mechanisms and Therapeutic Strategies

Abstract

As a classical immune checkpoint molecule, PD-L1 on the surface of tumor cells plays a pivotal role in tumor immunosuppression, primarily by inhibiting the antitumor activities of T cells by binding to its receptor PD-1. PD-1/PD-L1 inhibitors have demonstrated unprecedented promise in treating various human cancers with impressive efficacy. However, a significant portion of cancer patients remains less responsive. Therefore, a better understanding of PD-L1-mediated immune escape is imperative. PD-L1 can be expressed on the surface of tumor cells, but it is also found to exist in extracellular forms, such as on exosomes. Recent studies have revealed the importance of exosomal PD-L1 (ExoPD-L1). As an alternative to membrane-bound PD-L1, ExoPD-L1 produced by tumor cells also plays an important regulatory role in the antitumor immune response. We review the recent remarkable findings on the biological functions of ExoPD-L1, including the inhibition of lymphocyte activities, migration to PD-L1-negative tumor cells and immune cells, induction of both local and systemic immunosuppression, and promotion of tumor growth. We also discuss the potential implications of ExoPD-L1 as a predictor for disease progression and treatment response, sensitive methods for detection of circulating ExoPD-L1, and the novel therapeutic strategies combining the inhibition of exosome biogenesis with PD-L1 blockade in the clinic.

Keywords: abscopal effect; antitumor immune memory; biomarker; detection method; exosomal PD-L1; immune escape; immunotherapy; tumor microenvironment.

FIGURE 1

The mechanisms by which tumor ExoPD-L1 induce immunosuppression. (A) ExoPD-L1 originating from tumor cells induces T cell dysfunction in the tumor microenvironment by directly ligating to PD-1 on T cells as well as stationing on PD-L1-negative tumor cells, while its immunoregulatory effect remains intact. ExoPD-L1 also migrates to macrophages and dendritic cells, but the potential effects remain unknown. (B) ExoPD-L1 is able to leave tumor foci and enter the draining lymph node to mediate T cell suppression. (C) ExoPD-L1 can enter and circulate in the blood. (D) ExoPD-L1 inhibits the immune response in spleen and decreases spleen size.

FIGURE 2

Potential targets for antitumor therapy in ExoPD-L1 biogenesis pathways. Multiple molecules, including Rab27a, nSMase2, ALIX, and HRS, participate in the complex processes of ExoPD-L1 biogenesis, which originates from the cell surface rather than from the ER or Golgi apparatus. Deletion of Rab27a decreases ExoPD-L1, but does not alter cell-surface PD-L1 levels. Deletion of nSMase2 reduces the levels of both cellular PD-L1 and ExoPD-L1 protein. Rab27a deletion causes a greater inhibition in exosome production compared with nSMase2 deletion, while nSMase2 deletion leads to a greater inhibition of ExoPD-L1 production compared with Rab27a deletion. GW4869, an inhibitor of nSMase2, inhibits ExoPD-L1 generation, but does not increase cellular PD-L1 levels. Knockdown of ALIX, which redistributes PD-L1 between the cell-surface and exosomes, results in a reduction of ExoPD-L1 production but an increase in cell-surface PD-L1. Blockade of Rab27a or nSMase2 results in suppression, whereas ALIX knockdown promotes tumor growth. Knockdown of HRS, an ESCRT-0 subunit, confers a decrease in ExoPD-L1 levels but an increase in cellular PD-L1 levels. The effects of HRS knockdown on the cell-surface PD-L1 levels and tumor growth remain unknown.

FIGURE 3

The abscopal effect and antitumor immune memory induced by ExoPD-L1-deficient tumor cells. Tumor cells with ExoPD-L1 deletion are generated by genetic mutation of Rab27a, nSMase2, or PD-L1. The growth of PD-L1-positive tumors at a distant site is inhibited when ExoPD-L1-deleted tumor cells are coinjected simultaneously. In addition, the growth of PD-L1-positive tumors injected secondarily 92 days later is also suppressed.