Mechanisms of resistance to cancer immunotherapy
Cancer immunotherapy uses the host′s immune system to mobilize immune cells to recognize and eventually eliminate cancer cells. At present, studies in terms of cancer immunotherapy mainly focus on programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) antibody, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibody, chimeric antigen receptor T-cell immunotherapy (CAR-T), T cell receptor Immunotherapy (TCR-T), etc. Despite the fact that cancer immunotherapies elicit unprecedented durable responses in clinical therapy, they appear to be ineffective to some patients. In addition, some responders relapse and show resistance to immunotherapies even if their symptoms are relieved for a time. Resistance to cancer immunotherapy can be categorized into primary, adaptive and acquired, which can occur in every stage during the process of anti-tumor response. In this review, we discuss the known mechanisms of resistance and provide a rationale for the use of combination therapy to overcome resistance. 
 
 
Key words: 
Immunotherapy; Immune checkpoint; Resistance mechanism