Intratumoral virus-like particles containing a TLR9-agonist combined with systemic αPD-1 activate tumor-specific CD8+ T cells.

One strategy for enhancing the anti-cancer immune response is to inject tumors with immunostimulatory agents that modify the tumor microenvironment (TME) to induce a stronger anti-tumor T cell response. In situ immunization with Vidutolimod, a virus-like particle containing a CpG-A TLR9 agonist, has demonstrated anti-tumor activity in pre-clinical and early phase clinical studies, however its effect on tumor-specific CD8⁺ T cells remain poorly defined. Using the OT-1 model, we assessed how Vidu impacts activation, cytotoxicity, and anti-tumor activity of tumor-specific CD8⁺ T cells in vitro and in vivo. In vitro, Vidu reduced proliferation but increased expression of both activation and exhaustion markers. In vivo, repeated intratumoral Vidu injections induced a transient increase in the frequency of intratumoral tumor-specific CD8+ T cells and enhanced anti-tumor activity. The addition of αPD-1 to Vidu led to a persistent increase in intratumoral tumor-specific CD8+ T cells and sustained tumor control. Vidu treatment increased expression of markers associated with terminal exhaustion on intratumoral tumor-specific CD8⁺ T cells. Such treatment also increased the number of circulating tumor-specific CD8⁺ T cells that expressed high PD-1 but lacked co-expression of other exhaustion markers. Together, these findings demonstrate Vidu treatment expands the number of intratumoral and circulating tumor-specific CD8⁺ T cells, and that the number of tumor specific CD8+ T cells and the anti-tumor response is sustained by the addition of αPD-1. These results support continued evaluation of Vidu as a cancer immunotherapeutic agent, including in combination with immune checkpoint blockade.