Abstract
Single-agent immunotherapy, including with immune checkpoint inhibition with anti-PD-1 antibody, has not extended survival in patients with malignant glioma. However, PD-1 inhibition may still play a role in combination immunotherapy with multiple agents. In this study, we evaluated anti-PD-1 antibody treatment in combination with multiple approaches, including vaccination and agonist anti-OX40 immunotherapy, as well as triple combination immunotherapy with each of the above agents in a murine glioma model. Treatments were delivered on days 3,6, and 9 after intracranial implantation of glioma cells in the right frontal lobes of the mice. Vaccination consisted of subcutaneous implantation of irradiated GL261 cells engineered to express GM-CSF. We harvested splenocytes and brain tissue 18 days after glioma implantation and analyzed them by ELISPOT and flow cytometry, respectively. Treated mice surviving for 120 days were challenged with implantation of large numbers of GL261 cells and either followed for survival or sacrificed for study of the memory response. Survival was assessed by the Kaplan-Meier method and the log-rank test. Means were compared by the 2-tailed student's t-test. We report that combining anti-PD-1 immunotherapy with either vaccination or agonist anti-OX40 immunotherapy improves survival in GL261-bearing mice compared with any of the above as monotherapy. Triple combination immunotherapy with vaccination, anti-PD-1 antibody, and agonist anti-OX40 antibody results in long-term survival in all mice. Triple combination immunotherapy resulted in an elevated CD4+/CD8 + T lymphocyte ratio amongst tumor-infiltrating lymphocytes as well as a diminished fraction of regulatory T lymphocytes, likely reflective of a more vigorous Th1 antitumor response.