Abstract C011: Spatiotemporal requirements of immune checkpoint blockade-mediated control of MHC-deficient melanoma

Abstract Blockade of T-cell inhibitory receptors, PD1 and CTLA4, leads to durable remissions in over 50% of advanced melanoma patients. Downregulation or loss of MHC molecules is a common mechanism of tumor escape, but immune checkpoint inhibition (ICI) can, in some cases, overcome MHC deficiency. The exact nature of ICI-mediated control of MHC-deficient tumors remains uncertain. In this study, we investigated the spatiotemporal requirements for immunological control of MHC-deficient tumors following ICI, in a mouse melanoma model. Y3.3UVRc34 melanoma variants lacking MHCI and/or MHCII expression (MHCI+MHCII-, MHCI-MHCII+ or MHCI-MHCII-) were subcutaneously inoculated into naïve C57BL/6 mice. Animals with established tumors were started on isotype, anti-PD1 (aPD1) or combination aPD1 plus aCTLA4 (combi-ICI). Tumor growth, animal survival and immune responses were evaluated. Local and systemic T-cell activation and tissue immune contexture were analyzed with flow cytometry and multiplex immunofluorescence. Animals that achieved complete response (CR) on ICI were re-challenged with the three tumor variants to evaluate immunological memory. In the naïve setting, mice bearing MHCI+ tumors responded to aPD1 (70% CR) and combi-ICI (95% CR). In contrast, animals bearing MHCI-deficient tumors were resistant to both treatments, regardless of MHCII expression. Early T-cell proliferative burst observed in systemic circulation after combi-ICI but not aPD1 treatment, was consistent with efficient T-cell priming in all three models. Early on-treatment tissue analyses demonstrated reduced immune (CD45+) infiltration into MHCI-deficient tumors. All tumor microenvironments showed abundant macrophages/monocytes, while activated CD8+ T cells infiltrated MHCI+ but not MHCI-deficient tumors. To test immunological memory, animals that cleared MHCI+ tumors on ICI, were rested for 80 days and rechallenged with one of the three tumor variants. aPD1-induced immunological memory was 50% protective against parental tumors (MHCI+) but afforded no protection against MHCI-deficient tumors. In contrast, combi-ICI induced memory protected against all three variants, with efficacy ranging from 70% (MHCI-MHCII-) to 100% (MHCI+ and MHCII+). Protection was associated with a dramatic accumulation of activated effector CD4+ T cells (CD44+CD62L-ki67+PD1+Tbet+Tcf1-) in blood, lymph nodes and tumors, 6 days post rechallenge. At this time, tumor microenvironments of all three models contained abundant granulocytes and dendritic cells, but few macrophages. Immune subset depletion experiments confirmed that protection against MHCI-deficient tumors was critically dependent on CD4+ T cells. Our findings highlight a unique role of combi-ICI in the induction of CD4+ T-cell memory capable of efficiently controlling MHCI-deficient tumor variants. Combi-ICI induced immunological memory relied on the rapid expansion of tumor-specific effector CD4+ T cells capable of trafficking into tumors and remodelling their microenvironments. The specific cellular targets are currently under investigation. Citation Format: Elena Shklovskaya, Bernadette Pedersen, Ashleigh Stewart, Mal Irvine, Jesse R. Brown, Su Yin Lim, Helen Rizos. Spatiotemporal requirements of immune checkpoint blockade-mediated control of MHC-deficient melanoma abstract. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C011.