Extended co-expression of inhibitory receptors by human CD8 T-cells depending on differentiation, antigen-specificity and anatomical localization

Present and future of immune checkpoint blockade: Monotherapy to adjuvant approaches

Immune regulation of aggressive tumor growth is often outpaced by tumor up-regulation of ligands that inhibit effector immune responses through the activation of immune checkpoints. A few of such checkpoints include programmed death-1 (PD-1), cytotoxic T lymphocyte associated antigen-4 (CTLA-4), lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, Glucocorticoid-induced TNFR family-related receptor (GITR), and killer cell immunoglobulin like receptor. With the exception of GITR, after binding to their respective ligands these checkpoints induce down-modulation of immune responses to prevent autoimmunity. However, such immune mechanisms are co-opted by tumors to allow rapid tumor cell proliferation. Pre-clinical studies in antibody blockade of PD-1 and CTLA-4 have led to promising augmentation of effector immune responses in murine tumor models, and human antibodies against PD-1 and CTLA-4 alone or in combination have demonstrated tumor regression in clinical trials. The development of immune checkpoint blockade as a potential future immunotherapy has led to increasing interest in combining treatment modalities. Combination checkpoint blockade with chemotherapy and radiation therapy has shown synergistic effects in pre-clinical and clinical studies, and combination checkpoint blockade with bacterial vaccine vectors have produced increased effector immune responses in pre-clinical models. The future of immune checkpoint blockade may be as a powerful adjuvant alongside the current standard of care.