105 Assessing CEACAM1 blockade as a therapeutic strategy using a CRISPR-Cas9-based mouse melanoma model

Abstract

Current standard of care for advanced-stage melanoma includes immune checkpoint blockade (ICB) and kinase inhibitors. Unfortunately, melanomas treated with kinase inhibitors typically develop resistance; therefore, ICB has become first line therapy due to more durable responses. However, currently available ICBs, such as anti-CTLA-4 and anti-PD-1/PD-L1, engender durable responses in only a subset of patients, demanding a better understanding of other immune checkpoints as potential therapeutic targets. Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), which is expressed on the surface of activated (but not naïve) T cells and on many types of tumor cells, has been found to contribute to tumor progression and immune evasion. We examine the relevance of CEACAM1 in a genetically engineered mouse melanoma model harboring BRAFV600E mutation, which represents 40-60% of human melanomas. In this model, melanomas are induced by activation of mutant BRAF and neonatal exposure to UV irradiation. We have detected high expression of CEACAM1 at RNA and protein levels in UV-BRAF melanomas. We hypothesize CEACAM1 is involved in immune evasion, and its inhibition alone or in combination with PD-1/PD-L1 and CTLA-4 blockade could be a new strategy to treat melanoma. To test our hypothesis, we developed an inducible CRISPR-Cas9 system to knockout CEACAM1 expression in UV-BRAF melanoma cells. We are injecting these cells into syngeneic immunocompetent mice to evaluate the effects of CEACAM1 knockdown on tumor growth, and will use immunodeficient mice to determine the implication of the adaptive immune system in our findings. Future preclinical studies will include the combination of anti-PD-L1 and anti-CTLA-4 treatments with CEACAM1 inhibition to elucidate potential synergistic effects of these ICBs. Overall, we have validated CEACAM1 expression in a mouse model representing a large cohort of human melanoma and created CEACAM1 knockout mouse melanoma cells for mechanistic studies. Amid current immunotherapies, blockade of CEACAM1 may offer a promising alternative treatment strategy for advanced melanoma.