Abstract 1378: CD8 T cell mediated killing of MHC class 1 negative tumors requires antigen presenting myeloid cells and interferon gamma

Abstract Major Histocompatibility Complex (MHC) Class I downregulation is a well described mechanism of tumor immune escape, posing a challenge for T cell based immunotherapies including immune checkpoint blockade (ICB). Recent studies, however, have demonstrated mixed roles of MHC Class 1 and the critical component beta-2-microglobulin (β2m) expression in cancer progression and ICB response, with some studies showing inactivation of antigen presentation to be associated with resistance to ICB and others showing low β2m expression to be associated with favorable prognosis. Glioblastoma (GBM) in particular expresses little or no MHC Class 1 and patients remain unresponsive to ICB. We thus sought to evaluate the role of MHC Class 1 in ICB, given that we have previously demonstrated that combination ICB with anti-PD-1 and co-stimulation with 4-1BB agonism has marked efficacy against intracranial murine glioma tumors in a CD8 T cell dependent manner. Surprisingly, in a CT2A murine glioma tumor line expressing the antigen TRP2 and lacking cell surface MHC I (CT2A-TRP2-β2mKO), the efficacy of combination 4-1BB and PD-1 therapy (ICB) was re-demonstrated in a CD8 dependent manner, independent of NK cells, CD4 T cells, and B cells. Furthermore, the efficacy of immunotherapy against intracranial CT2A-TRP2-β2mKO was demonstrated to be antigen dependent, with an adoptive lymphocyte transfer (ALT) of TRP2 TCR transduced T cells (TRP2 T cells) into a CD8KO mouse sufficient to eliminate CT2A-TRP2-β2mKO in the setting of ICB. Additionally, an ALT of TRP2 T cells did not kill CT2A-β2mKO tumors in the setting of ICB, while OT-1 mice whose CD8+ T cells primarily recognize OVA peptide with CT2A-TRP2-β2mKO tumors did not respond to ICB. In vitro studies revealed that TRP2 T cells demonstrated anti-tumor cytotoxicity against MHC Class I negative CT2A-TRP2-β2mKO tumor cells in the presence of TRP2 loaded bone marrow derived macrophages (TRP2 Mφ), but neither cell type was individually sufficient to induce tumor cell death, while the combination of TRP2 T cells and TRP2 Mφ demonstrated no cytotoxicity against CT2A-β2mKO tumors. Transwell experiments in which TRP2 Mφ and CT2A-TRP2-β2mKO tumor cells were separated by a 0.5µm membrane permeable to T cells but not Mφ or tumor cells revealed that contact between TRP2 Mφ and tumor cells was not necessary to induce T cell dependent killing. Indeed, tumor-bearing β2mKO bone marrow chimeras lacking MHC class 1 on hematopoeitically derived cells did not respond to ICB, highlighting the importance of antigen presentation from myeloid cells. The mechanism of killing was found to be dependent on interferon gamma (IFNγ), as IFNγKO mice did not respond to ICB. These findings demonstrate that tumors with low MHC Class 1 expression may still be targeted by T cell dependent immunotherapies such as ICB when antigen presentation can occur from myeloid cells. Citation Format: Emily Lerner, Vincent D'Anniballe, William Tomaszewski, Jonathan Perera, Xiuyu Cui, Jessica Waibl-Polania, Daniel Wilkinson, Michael D. Gunn, Peter E. Fecci, Karolina Woroniecka. CD8 T cell mediated killing of MHC class 1 negative tumors requires antigen presenting myeloid cells and interferon gamma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1378.