IMMU-13. DEVELOPMENT OF ADAPTIVE IMMUNE RESISTANCE IN A MURINE GLIOMA MODEL TREATED WITH DENDRITIC CELL VACCINATION AND PD-1 CHECKPOINT BLOCKADE

Abstract

Abstract Immunotherapies such as immune checkpoint inhibitors and active vaccination have shown great potential in unleashing antitumor immunity for cancer treatment. However, their effectiveness in treating glioblastoma (GBM) is limited. In a preclinical glioma model, administration of both autologous tumor lysate-pulsed dendritic cell vaccination (ATL-DC) and anti-PD-1 checkpoint blockade (aPD1) significantly increased survival and enhanced intratumoral T cell activation. We hypothesize that the combination of these two treatments will substantially increase the abundance and activity of tumor-reactive cytotoxic T cells. As such, we recently started a phase I trial administering ATL-DC and neoadjuvant aPD1 to recurrent GBM patients (NCT ID: NCT04201873). To understand the local antitumor immune response in greater depth, we performed single-cell RNA sequencing analysis of the CD45+ immune compartment of explanted gliomas from twelve mice, four of which were treated with ATL-DC and aPD1 while the other eight received no treatment. We found that the combination therapy increased tumor-infiltrating T cells, monocytes and DCs. We also observed the expansion and differentiation of CD8+ effector T cells under the effect of combination therapy. These T cells exhibited a differentiation trajectory from a Tcf7+Il2ra+ early activated phenotype to a terminally exhausted state marked by high expression of Pdcd1, Tox and Tigit. Interestingly, combination therapy specifically increased a macrophage subset that overexpressed Ch25h, an interferon-stimulated gene encoding for the oxysterol lipid 25-hydroxycholesterol (25-HC). Preliminary results from in vitro studies demonstrated that 25-HC restricted T cell proliferation and tumor cell cytolysis, suggesting its immunosuppressive function. In summary, our study reveals the impact of ATL-DC + aPD1 combination therapy on the immune microenvironment of GBM and a potential maladaptive mechanism mediated by 25-HC. Future studies will seek to exploit these targets therapeutically to develop more effective GBM treatment regimens.