At the bench: preclinical rationale for CTLA-4 and PD-1 blockade as cancer immunotherapy

Abstract

Tumors can avoid immune surveillance by stimulating immune inhibitory receptors that function to turn off established immune responses. By blocking the ability of tumors to stimulate inhibitory receptors on T cells, sustained, anti-tumor immune responses can be generated in animals. Thus, therapeutic blockade of immune inhibitory checkpoints provides a potential method to boost anti-tumor immunity. The CTLA-4 and PD-1Rs represent two T cell-inhibitory receptors with independent mechanisms of action. Preclinical investigations revealed that CTLA-4 enforces an activation threshold and attenuates proliferation of tumor-specific T lymphocytes. In contrast, PD-1 functions primarily as a stop signal that limits T cell effector function within a tumor. The unique mechanisms and sites of action of CTLA-4 and PD-1 suggest that although blockade of either has the potential to promote anti-tumor immune responses, combined blockade of both might offer even more potent anti-tumor activity. See related review At the Bedside: CTLA-4 and PD-1 blocking antibodies in cancer immunotherapy.

Keywords: PD-L1; coinhibitory; costimulation; costimulatory; inhibitory; ipilimumab; receptor; tumor.

Figure 1.. Unique spatiotemporal regulation of CTLA-4 and PD-1.

Activation of a naïve T cell requires TCR-mediated signals and costimulatory signals, generated by CD28:B7 ligand interactions. Upon activation, T cells induce expression of the inhibitory receptors CTLA-4 and PD-1, and the relative balance of stimulatory and inhibitory signaling can dictate the outcome of the T cell response. When CTLA-4- and PD-1-mediated inhibitory signals dominate, T cell activation is aborted, resulting in an unresponsive anergic state. Tregs can tip the balance toward inhibitory signals by removing B7 ligands from the APC surface via transendocytosis, thus favoring B7 ligand sequestration by the higher-affinity CTLA-4 receptor. When TCR- and CD28-mediated stimulatory signals dominate, T cells undergo clonal expansion, acquisition of effector function, and trafficking through nonlymphoid tissues. Effector T cell function can be limited by PD-1 interaction, with PD-L1 expressed on the surface of nonhematopoietic cells, including many different tumors. Moreover, PD-1:PD-L1 interactions can enhance Treg function, resulting in an additional layer of effector T cell inhibition.

Figure 2.. Distinct mechanisms of intracellular signaling by CTLA-4 and PD-1.

Upon activation, T cells synthesize CTLA-4, and intracellular vesicles containing CTLA-4 undergo transport to the immune synapse. Phosphorylation of CTLA-4 at the YKVM motif (red box) promotes sustained expression of CTLA-4 at the cell surface by preventing AP2- and clathrin-mediated endocytosis. At the cell surface, CTLA-4 competes with CD28 for access to B7 ligands. CTLA-4-mediated transendocytosis of B7 ligands can further limit availability of B7 ligands for CD28. Ligation of CTLA-4 by B7-1 or B7-2 blocks TCR/CD28-mediated activation of Akt, in part, through CTLA-4-mediated activation of the serine/threonine phosphatase PP2A. However, CTLA-4 ligation does not interfere with PI3K-mediated induction of the antiapoptotic molecule Bcl-xL. In contrast, ligation of PD-1 by PD-L1 or PD-L2 (not shown) results in phosphorylation of ITSM (purple box) and ITIM (blue box) motifs within the cytoplasmic tail of PD-1. The phosphorylated ITSM motif recruits the tyrosine phosphatases SHP-1 and SHP-2, which in turn, block TCR/CD28-mediated activation of PI3K. Signals delivered via B7-1:PD-L1 interactions may also inhibit T cell function, although the signaling pathways have not been elucidated.