ABSTRACT: To protect viral particles from neutralization, sequestration, nonspecific adhesion, and mislocalization following systemic delivery, we have previously exploited the natural tumor-homing properties of antigen-specific CD8+ T cells. Thus, OT-I T cells, preloaded in vitro with the oncolytic vesicular stomatitis virus (VSV), can deliver virus to established B16ova tumors to generate significantly better therapy than that achievable with OT-I T cells, or systemically delivered VSV, alone. Here, we demonstrate that preconditioning immune-competent mice with Treg depletion and interleukin-2 (IL-2), before adoptive T-cell therapy with OT-I T cells loaded with VSV, leads to further highly significant increases in antitumor therapy. Therapy was associated with antitumor immune memory, but with no detectable toxicities associated with IL-2, Treg depletion, or systemic dissemination of the oncolytic virus. Efficacy was contributed by multiple factors, including improved persistence of T cells; enhanced delivery of VSV to tumors; increased persistence of OT-I cells in vivo resulting from tumor oncolysis; and activation of NK cells, which acquire potent antitumor and proviral activities. By controlling the levels of virus loaded onto the OT-I cells, adoptive therapy was still effective in mice preimmune to the virus, indicating that therapy with virus-loaded T cells may be useful even in virus-immune patients. Taken together, our data show that it is possible to combine adoptive T-cell therapy, with biological therapy (Treg depletion+IL-2), and VSV virotherapy, to treat established tumors under conditions where none of the individual modalities alone is successful.
