Molecular Pathways: Co-Expression of Immune Checkpoint Molecules: Signaling Pathways and Implications for Cancer Immunotherapy

“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 CTLA-4 (Cytotoxic T Lymphocyte Antigen-4), PD-1 (Programmed Death-1), LAG-3 (Lymphocyte Activation Gene-3), TIM-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 co-expression of checkpoint molecules occurs frequently on cancer-specific T cells, as well as on pathogen-specific T cells in chronic infections. While 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 pre-clinical 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 non-overlapping molecular mechanisms. Here we review current data regarding immune checkpoint molecule signaling and co-expression, both in cancer and infectious disease, as well as the results of preclinical and clinical manipulations of checkpoint proteins.”