Resistance Mechanisms to Immunotherapy
Immunotherapy has improved outcomes for many cancer patients. Checkpoint inhibitors that target PD-1, PD-L1, and CTLA-4 help T cells attack tumors. However, many patients do not respond at all, and others respond at first but later progress. Researchers divide these patterns into primary resistance and acquired resistance.
Primary resistance means the tumor never responds to treatment. Acquired resistance means the tumor shrinks initially but then grows again. Both types arise from tumor-intrinsic changes and changes in the surrounding immune environment.
Tumor cells use several strategies to escape immune attack. Some tumors carry a low number of mutations. As a result, they produce few neoantigens that T cells can recognize. Others lose the ability to present antigens. Mutations in β2-microglobulin or components of the MHC class I pathway hide the tumor from cytotoxic T cells. Defects in the interferon-gamma pathway also play a major role. Loss of JAK1 or JAK2 makes tumor cells less sensitive to immune signals and reduces PD-L1 expression that checkpoint inhibitors need to work.
Oncogenic pathways further support resistance. Loss of PTEN or activation of the PI3K-AKT and WNT-β-catenin pathways can limit T-cell infiltration. Epigenetic changes can silence genes needed for antigen presentation. These intrinsic alterations make the tumor less visible or less vulnerable to the immune system.
The tumor microenvironment also drives resistance. Many tumors exclude T cells and remain “cold.” Regulatory T cells, myeloid-derived suppressor cells, and certain macrophages release immunosuppressive cytokines such as TGF-β and IL-10. These cells create a barrier that weakens effector T-cell function. T cells that do enter the tumor often become exhausted. They express multiple inhibitory receptors, including TIM-3, LAG-3, and TIGIT, in addition to PD-1. Blocking only one pathway is often not enough.
Metabolic factors add another layer of resistance. Hypoxia, high lactate levels, and nutrient depletion impair T-cell activity. Abnormal blood vessels limit immune cell entry. In some cases, the gut microbiome influences response, and unfavorable microbial communities are linked to poorer outcomes.
Acquired resistance often involves selection of resistant clones. Under immune pressure, tumors can lose neoantigens, downregulate MHC molecules, or upregulate alternative checkpoints. Chronic interferon signaling can also paradoxically promote immune escape over time.
Understanding these mechanisms guides new strategies. Combinations that block multiple checkpoints, reprogram the microenvironment, restore antigen presentation, or pair immunotherapy with targeted agents are under active study. Researchers continue to map resistance pathways so that more patients can benefit from immunotherapy.