Adoptive T-cell therapy (ACT) has emerged as a powerful treatment option in patients with metastatic melanoma. However, tumor cells frequently relapse from therapy by acquired resistance mechanisms such as loss of target antigen expression. Currently, it is not completely understood how the choice of target antigen influences resistance mechanisms to antigen-specific immunotherapies.
Therefore, we established CRISPR-assisted insertion of epitopes (CRISPitope), a technique that fuses a defined T-cell epitope to endogenous gene products. We applied CRISPitope to murine melanoma cells and tagged endogenous melanosomal TYRP1 and oncogenic CDK4R24C with the human gp10025-33 epitope, which rendered them targetable by gp100-specific pmel-1 TCR-transgenic T cells. This enabled us to investigate melanoma escape mechanisms to ACT targeting non-essential melanosomal and essential oncogenic antigens in direct comparison.
Using experimental mouse models, we could identify different escape mechanisms to gp100-specific immunotherapy in TYRP1 versus CDK4R24C melanomas. Resistance to ACT targeting TYRP1 was mainly caused by hardwired loss of antigen accompanied by a non-inflamed microenvironment or reversible downregulation of the antigen associated with an enforced melanoma phenotype switching. In contrast, CDK4R24C melanomas escaping ACT displayed antigen persistence and were associated with an IFN-rich inflamed tumor microenvironment. In CDK4R24C melanomas IFN-driven feedback inhibition by negative immune-checkpoint molecules promotes resistance to ACT despite persistent antigen expression.
Applying CRISPitope to syngeneic mouse models, we could show that target antigen choice can influence ACT resistance mechanisms, phenotype and immune contexture of melanomas in response to antigen-specific immunotherapies. Thus, our work could help to better understand acquired resistance and optimize personalized cancer immunotherapy.