Immune Stress-induced Tumor Mutation Burden and Neoantigen Expression in 4T1 Mammary Cancer Cells: A Potential Mechanism for Long-term Survival in Patients Treated With Immune Checkpoint Inhibitors.

Background/aim: The Kaplan-Meier curves for patients treated with immune checkpoint inhibitors (ICIs) display a small group of potentially-cured patients with long-term survival, creating a 'kangaroo-tail' shape of the survival curve. However, the mechanistic basis of this phenomenon and what occurs in patients whose cancer is resistant to ICIs remain unclear. The present study aimed to answer these questions.

Materials and methods: We analyzed mutations in mouse 4T1 mammary-gland-derived cancer cells expressing the hemagglutinin antigen (4T1-HA), which were grown in either wild-type mice or cytotoxic T-lymphocyte (CTL)-loaded immunocompromised mice (RAG-/- + ACT) under immune stress. These mutations were compared to those in 4T1-HA cells grown in RAG-/- mice without immune stress as a control.

Results: The number of gene mutations, the tumor mutation burden (TMB) and microsatellite instability (MSI) scores were increased in the cancer cells under immune stress. The mutations in the antigen protein were such that the protein retained its immunogenicity and could still function as a neoantigen. Repeated immune recognition of additional neoantigens may lead to the kangaroo-tail survival phenomenon. The common genetic mutations of the analyzed 4T1-HA cells under immune stress included genes related to immune response. Analysis of alternative splicing of genes showed that are accumulated gene alterations under immune stress related to cancer-cell proliferation. Copy-number variation (CNV) analysis indicated that normal-antigen presentation and immune responses may be impaired under immune stress.

Conclusion: Cancer cells, under immune stress, may acquire both immune escape capabilities and increased immunogenicity. This dual effect could lead to either resistance or response to ICIs, respectively.