Oncolytic virotherapy potentiates chemo-PD-1 immunotherapy by engaging chemo-resistant bystander CD8⁺ T cells

The efficacy of combined chemotherapy and programmed cell death protein-1 (PD-1) immune checkpoint blockade (ICB) is constrained by the collateral cytotoxicity of chemotherapy toward proliferating tumor-specific CD8⁺ T (T_TST) cells, a population indispensable for antitumor immunity. This study aimed to overcome this limitation by targeting a potential chemotherapy-resistant immune cell reservoir.

By using the murine acutely resolved lymphocytic choriomeningitis virus (LCMV) infection and tumor models (colorectal cancer and melanoma), we characterized the susceptibility of CD8⁺ T_TST and virus-specific bystander memory CD8⁺ T (T_BYS) cells to platinum-based chemotherapy-induced cytotoxicity. We next evaluated the antitumor efficacy and underlying mechanisms of combined chemotherapy and T_BYS cell-targeted oncolytic virus therapy (OV-BYTE) in tumor-bearing mice with prior LCMV infection or SARS-CoV-2 vaccination. Finally, we assessed the antitumor efficacy of the triple combination regimen (including OV-BYTE, chemotherapy, and PD-1 ICB) in both murine colorectal cancer model and patient-derived colorectal cancer organoid.

We first demonstrated that within the tumor microenvironment, CD8⁺ T_TST cells are highly susceptible to platinum-based chemotherapy, whereas CD8⁺ T_BYS cells constitute a quiescent, chemo-resistant population. Leveraging this, CD8⁺ T_BYS-targeted OV-BYTE therapy synergized with chemotherapy to control tumorigenesis in multiple murine models. Mechanistically, this dual combination directly engaged the CD8⁺ T_BYS cell reservoir for tumor killing, which was accompanied by the restoration of CD8⁺ T_TST cell function via reduced apoptotic susceptibility and acquisition of a polyfunctional, effector-like state. Consequently, integrating OV-BYTE into the standard chemo-PD-1 ICB regimen resulted in improved antitumor efficacy in both preclinical and patient-derived tumor models.

Our study establishes the chemotherapy-resistant CD8⁺ T_BYS cell niche as a pivotal therapeutic target. By engaging this target, OV-BYTE emerges as a potent combinatorial agent that successfully circumvents a core limitation of standard chemo-immunotherapy, thus offering a rationally designed and translatable strategy to advance combination cancer therapy.