TMIC-38. INTEGRIN BLOCKING PEPTIDE REPROGRAMS THE IMMUNOSUPPRESSIVE MICROENVIRONMENT OF EXPERIMENTAL GLIOMAS IMPROVING ANTI-PD-1 THERAPY OUTCOME

Abstract Clinical trials with immune checkpoint inhibitors (ICI) have benefited many cancer patients but failed in a number of tumors, including glioblastoma (GBM), the most common and aggressive primary brain tumor in adults. The main obstacle for ICI efficacy in GBM is the continuously evolving tumor microenvironment (TME), in which antitumor immunity is inhibited or eluded by tumor-secreted factors. Accumulation and reprogramming of myeloid cells (GAMs) creates a “cold” immunosuppressive TME with a poor infiltration and exhaustion of effector T cells. We employ Cellular Indexing of Transcriptomes and Epitopes by sequencing (CITE-seq) with 40 protein markers along with spatial transcriptomics and immunophenotyping to dissect identities and functionalities of immune cells (CD45+) in experimental GL261 gliomas. The results revealed identities of immune cells instrumental for creating the immunosuppressive milieu and cell-cell communication networks. We have developed a designer RGD peptide that blocks the reprogramming of GAMs by targeting tumor-GAMs interactions. We demonstrate that RGD efficiently blocks microglia-dependent invasion of murine and human glioma cells in vitro. We explored if the intratumorally delivered RGD can revert tumor-induced changes in the TME of experimental gliomas and improve anti-PD-1 immunotherapy. While RGD alone did not reduce tumor growth in vivo, it prevented reprogramming of myeloid cells into protumoral GAMs and led to the normalization of peritumoral blood vessels. Combining RGD with anti-PD-1 antibody resulted in reduced tumor growth, increase in percentages of proliferating, interferon-ɣ producing CD8+T cells and depletion of regulatory T cells. Transcriptomic profiles of GAMs were altered by the combined treatment, consistently with the restored “hot” inflammatory TME which resulted in boosting immunotherapy responses. Intratumorally delivered RGD modified in the same way functionalities of GAMs in the TME of human U87MG gliomas in nude mice. Our results demonstrate that the integrin blockade combined with immune checkpoint inhibitors reinvigorates both innate and adaptive antitumor immunity. The results pave the way to improve responses to immunotherapy in GBM and other cancers.