Immune Checkpoint Blockade in Hematological Malignancies: Current Status and Future Directions

Immune checkpoint proteins including PD-1, CTLA-4, LAG-3, TIM-3, and TIGIT regulate T-cell functions, which are essential for anti-tumor immunity. Over-expression of these immune checkpoint proteins leads to T-cell exhaustion and a significant impairment of anti-tumor immunity. Rejuvenation of effector T-cell function with immune checkpoint inhibitors (ICI) restores anti-tumor immunity, which translates into clinical efficacy in the frontline and salvage treatment of various hematological malignancies. Efficacy of ICIs is highest in classical Hodgkin lymphoma, primary mediastinal large B-cell lymphoma, and NK/T-cell lymphomas, and modest in immune-privileged-site lymphomas and cutaneous T-cell lymphoma. However, in myeloid malignancies and multiple myeloma, the efficacy of ICIs remains doubtful. In addition to being used as single agents, ICIs have also been combined with other ICIs; as well as chemotherapy, antibody drug conjugates, and epigenetic agents (histone deacetylase inhibitors and hypomethylating agents). More innovative strategies include the use of ICIs in the context of allogeneic haematopoietic stem cell transplantation and chimeric antigen receptor T-cell therapy. This review synthesizes current evidence for the use of ICI in different haematological malignancies, and highlights future directions toward biomarker-driven, rationally designed therapeutic combinations.