7aaRGD - a novel SPP1/integrin signaling-blocking peptide reverses immunosuppression and improves anti-PD-1 immunotherapy outcomes in experimental gliomas

Clinical trials with immune checkpoint inhibitors have benefited many cancer patients but were unsuccessful in a number of tumors, including glioblastoma (GBM), the most common and aggressive primary brain tumor in adults. The main obstacle is the immunosuppressive and continuously evolving tumor microenvironment (TME) in which antitumor immunity is inhibited or eluded by tumor-secreted factors. Reprogramming of glioma associated myeloid cells (GAMs) creates a “cold” immunosuppressive TME resulting in a poor infiltration, exhaustion or death of effector T cells. We have developed a synthetic RGD peptide that blocks the reprogramming of GAMs by targeting tumor-GAMs interactions. Here, we explored if the RGD can revert tumor induced changes in the TME of experimental gliomas and improve anti-PD-1 immunotherapy. We demonstrate that RGD efficiently blocks microglia-dependent invasion of murine and human glioma cells in vitro. While RGD alone did not reduce tumor growth in vivo, it prevented tumor-induced transcriptomic changes in GAMs, augmenting the expression of immune response genes. Interestingly, intratumoral RGD delivery led to the normalization of peritumoral blood vessels. Combining RGD with anti-PD-1 antibody reduced tumor growth and induced profound changes in the immune composition of TME as demonstrated using multiparametric flow cytometry. Interferon-ɣ producing capacity of effector T cells and its intratumoral levels increased in mice treated with RGD and anti-PD-1 antibody. Transcriptomic profiles of GAMs were altered by the combined treatment, consistently with the restored inflammatory TME and boosted immunotherapy responses. Intratumoral RGD similarly modified the myeloid cells in TME of human U87MG gliomas in immunocompromised mice.