Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy

Abstract

The expression of immune checkpoint molecules on T cells represents an important mechanism that the immune system uses to regulate responses to self-proteins. Checkpoint molecules include cytotoxic T lymphocyte antigen-4, programmed death-1, lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, and several others. Previous studies have identified individual roles for each of these molecules, but more recent data show that coexpression of checkpoint molecules occurs frequently on cancer-specific T cells as well as on pathogen-specific T cells in chronic infections. As the signaling pathways associated with each checkpoint molecule have not been fully elucidated, blocking multiple checkpoints with specific monoclonal antibodies results in improved outcomes in several chronic viral infections as well as in a wide array of preclinical models of cancer. Recent clinical data suggest similar effects in patients with metastatic melanoma. These findings support the concept that individual immune checkpoint molecules may function through nonoverlapping molecular mechanisms. Here, we review current data regarding immune checkpoint molecule signaling and coexpression, both in cancer and infectious disease, as well as the results of preclinical and clinical manipulations of checkpoint proteins.

Figure 1. Known Signaling Pathways of Selected Checkpoint Molecules and Current Therapeutics

Upon binding B7-1 or B7-2, CTLA-4 recruits the phosphatases SHP2 and PP2A via the YVKM motif in its cytoplasmic domain. SHP2 recruitment results in attenuation of TCR signaling by dephosphorylating the CD3ζ chain. PP2A recruitment results in downstream dephosphorylation of AKT, further dampening the T cell activation pathway. PD-1 ligation by PD-L1 or PD-L2 also recruits SHP2 to the ITIM domain, resulting in membrane proximal decreases in TCR signaling. LAG-3 signaling is dependent on interaction with its ligand, MHC II, as well as its intracellular KIEELE domain. TIM-3 binds to Galectin-9, as well as other ligands. In the absence of ligand binding, TIM-3 is associated with Bat3, protecting the cell from TIM-3 mediated inhibition and allowing for greater activation. However, once TIM-3 binds to Galectin-9, Y265 is phosphorylated and the interaction with Bat3 is disrupted, allowing TIM-3 to deliver inhibitory signals to the T cell. BTLA and CD160 bind to Herpes Virus Entry Mediator (HVEM). BTLA contains an intracellular ITIM domain that may be important in signaling. 2B4 binds to CD48, but further signaling mechanisms are poorly understood. Ig domains are depicted in orange, mucin domains in green, cysteine rich domains in brown, and GPI anchors are depicted as bolded black lines. Current therapeutics to block checkpoint signaling molecules include both monoclonal antibodies and Ig fusion proteins. 1) Anti-CTLA-4: Ipilimumab (BMS-734016), Tremelimumab (CP-675,206) 2) Anti-PD-1: Nivolumab (BMS-936558, MDX1106), Lambrolizumab (MK-3475), CT-011 3) Anti PD-L1: BMS-936559 (MDX1105), MEDI4736 4) PD-L2 Ig: AMP224 5) LAG-3 Ig: IMP321