Transient activation of tumor-associated T-effector/memory cells promotes tumor eradication via NK-cell recruitment: minimal role for long-term T-cell immunity in cure of metastatic disease

Local delivery of IL-12 and GM-CSF to advanced primary tumors results in T- and NK-cell-dependent cure of disseminated disease in a murine spontaneous lung metastasis model. Post-therapy functional dynamics of cytotoxic T- and NK-cells were analyzed in primary and metastatic tumors to determine the specific roles of each subset in tumor eradication. Time-dependent depletion of CD8+ T and NK-cells demonstrated that CD8+ T-cells were critical to eradication of metastatic tumors within 3 days of treatment, but not later. In contrast, NK-cells were found to be essential to tumor regression for at least 10 days after cytokine delivery. Analysis of tumor-infiltrating lymphocyte populations in post-therapy primary tumors demonstrated that treatment resulted in the activation of tumor-associated CD8+ T-cells within 24 h as determined by IFNγ and perforin production. T-cell activity peaked between days 1 and 3 and subsided rapidly thereafter. Activation was not accompanied with an increase in cell numbers suggesting that treatment mobilized pre-existing T-effector/memory cells without inducing proliferation. In contrast, therapy resulted in a ≥3-fold enhancement of both the quantity and the cytotoxic activity of NK-cells in primary and metastatic tumors on day 3 post-therapy. NK-cell activity was also transient and subsided to pre-therapy levels by day 5. Depletion of CD4+ and CD8+ T-cells prior to treatment completely abrogated NK-cell infiltration into primary and metastatic tumors demonstrating the strict dependence of NK-cell recruitment on pre-existing T-effector/memory cells. Treatment failed to induce significant NK-cell infiltration in IFNγ-knockout mice establishing the central role of IFNγ in NK-cell chemotaxis to tumors. These data show that transient activation of tumor-associated T-effector/memory and NK-cells, but not long-term CD8+ T-cell responses, are critical to suppression of metastatic disease in this model; and reveal a novel role for pre-existing adaptive T-cell immunity in the recruitment of innate effectors to tumors.