TMIC-21. Dual blockade of Siglec-15 and PD-1, combined with oncolytic Zika virus therapy, confers protection against immune-resistant gliomas

Abstract The immunosuppressive tumor microenvironment is a major challenge in GBM. We are developing oncolytic Zika virus (ZIKV) as a treatment that targets treatment-resistant glioma stem cells and ignites CD8+ T-cell dependent anti-tumor immune response. We previously demonstrated that ZIKV infection significantly improves survival in multiple mouse models of GBM (Nair et al, JCI insight, 2021). However, myeloid driven immunosuppression remains a critical barrier. While Siglec-15 has recently been described as a myeloid immune checkpoint in other cancers (Wang et al, Nat Med, 2019), it is relatively unexplored in GBM. An antibody against Siglec-15 is in a phase II clinical trial for metastatic solid tumors (NCT04699123), but not tumors of the central nervous system. In human GBM specimens, we observed that Siglec-15 is highly expressed on myeloid (16-23%) and tumor cells (18-19%). Here, we use therapeutic and genetic approaches to target Siglec-15, as well as immunotherapy-resistant mouse models of GBM. In CT2A-bearing mice, long-term survival (90 days) increased from 40% (only ZIKV) to 60% (anti-Siglec-15+ZIKV). Adding antibody against PD-1 further improves the long-term survival (83%). In the immunotherapy-resistant SB28 model, triple therapy (anti-Siglec-15+ZIKV+anti-PD-1) led to 76% long-term survivors and greater CD8+ T-cell activation (1.7-fold). Upon re-challenge of cured mice, we detected an increase in CD8+ brain resident memory T-cells (11-fold) and CD8+ effector memory T-cells (11-fold), and 80% of mice survived. These findings with antibody against Siglec-15 were further confirmed using Siglec-15 knockout mice. Mechanistically, using bone marrow derived myeloid cells and T-cell proliferation assays, we determined that depletion of Siglec-15 improves phagocytosis of tumor cells (CT2A; 25.2% and SB28; 6.9%) and significantly improves T-cell activation (81%) and T-cell proliferation (86.8%), and these findings may explain why we observed improved outcomes. These results suggest the potential of this triple therapy in overcoming the problem of the immunosuppressive tumor microenvironment in GBM.