Abstract 6362: Overcoming resistance to immunotherapy due to loss of antigen presentation

Abstract Resistance to immune checkpoint inhibitors represents a major therapeutic challenge, as only 40% of patients with melanoma (and less with other tumor types) have a long-term response to anti-PD-1 therapy. Resistance can arise because of somatic mutations in cancer cells that allow evasion of T cell-mediated killing. One commonly acquired resistance mutation in melanoma, loss of beta-2 microglobulin (B2m), prevents T cell killing by hiding cancer cells from CD8+ T cell recognition. To understand the failed immune response against resistant tumors, we used single-cell RNA-seq to characterize tumor-infiltrating immune cells in antigen presentation-deficient human melanoma biopsies and CRISPR-modified mouse melanoma tumors. Our data demonstrate an increase in immunosuppressive M2-like macrophages and absence of CD8+ T cells in B2m-null tumors. To overcome this resistance, we treated tumor-bearing mice with CD40 agonist antibody, which promotes differentiation of macrophages towards a pro-inflammatory phenotype and increases dendritic cell priming of CD8+ T cells. Treatment with CD40 agonist reduced tumor growth and improved tumor clearance in B2m-null melanoma and colorectal cancer models. To determine how CD40 agonist treatment works, we depleted different immune populations from the tumor microenvironment. We hypothesized that by depleting M2 macrophages, CD40 agonist treatment would remove an immunosuppressive brake to allow natural killer (NK) cells to kill tumor cells lacking MHC expression. To our surprise, NK cells were not required for the efficacy of CD40 agonist. Instead CD8+ T cells were required, even though the CD8+ T cells cannot directly recognize the tumor cells. scRNA-seq identified a transcriptionally unique state of CD8+ T cells that is recruited to the tumor microenvironment after CD40 agonist treatment. These CD8+ T cells produce IFNγ, which is required for the efficacy of CD40 agonist treatment. These data demonstrate that CD8+ T cells, a key mediator of anti-tumor immunity, can still be recruited to control tumors deficient in antigen presentation. More broadly, they suggest that strategies to activate CD8+ T cells may be effective even in the context of acquired resistance to checkpoint inhibitor therapy. Citation Format: Brian C. Miller, Yacine Choutri, Rose Al Abosy, Amy Huang, Emily K. Cox, Matthew P. Zimmerman, Wan Lin Chong, Katherine J. Vietor, Jenna Collier, Sarah A. Weiss, Debattama Sen, W. Nicholas Haining, Arlene H. Sharpe. Overcoming resistance to immunotherapy due to loss of antigen presentation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6362.