Intratumoral Virus-Like Particles Containing a TLR9 Agonist Combined with Systemic αPD-1 Activate Tumor-Specific CD8+ T Cells

One strategy for enhancing the anticancer immune response is to inject tumors with immunostimulatory agents that modify the tumor microenvironment (TME) to induce a stronger antitumor T-cell response. In situ immunization with vidutolimod (Vidu), a virus-like particle containing a CpG-A TLR9 agonist, has demonstrated antitumor activity in preclinical 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 affects the activation, cytotoxicity, and antitumor 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 antitumor 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 coexpression of other exhaustion markers. Together, these findings demonstrate that 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 antitumor 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. SIGNIFICANCE: In situ immunization with Vidu combined with αPD-1 therapy enhances the antitumor response by tumor-specific CD8+ T cells by expanding the systemic and intratumoral populations. These findings demonstrate how Vidu alters T-cell biology and supports its continued development, particularly in combination with checkpoint blockade to optimize antitumor immunity.