Cellular Mechanisms of Interleukin-12-mediated Neuroblastoma Regression

Background/purpose: Interleukin-12 (IL-12) is a proinflammatory cytokine with potent antitumor effects. Previous studies from the authors laboratory showed regression of established neuroblastoma in mice vaccinated with IL-12 transduced dendritic cells (DC). Although regression was associated with intense T cell infiltration, the precise role of T cells is unknown. The purpose of this work is to study the cellular mechanisms in IL-12-mediated tumor regression.

Methods: Three groups of mice (n = 12) received subcutaneous inoculation with 1 x 10(6) murine neuroblastoma cells (TBJ). Anti-CD4 (T helper), anti-CD8 (T cytotoxic), or antiasialo-GM1 (natural killer) antibodies were injected intravenously at 3-day intervals to deplete various immune effector cell populations. Mice in each depletion group and the control (nondepleted) group were injected intratumorally on day 7 with 1 x 10(6) DC IL-12-transduced DC. Tumors were harvested for morphometry and immunohistochemistry at 21 days.

Results: CD4 depletion had no effect on tumor growth in either control or IL-12-vaccinated animals. In contrast, CD8-depleted animals treated with IL-12-transduced DC underwent initial regression followed by progressive tumor growth (P <.01). These tumors were smaller in size at the same time-point. However, NK cell depletion (antiasialo GM1) completely abrogated the antitumor effects of IL-12-transduced DC, leading to progressive tumor growth from the outset. There was no difference between the control and treated animals in this group.

Conclusions: Contrary to our hypothesis that IL-12 DC primarily function to stimulate a T cell-mediated response, these data suggest that NK cells are essential for the initial antitumor response of animals treated with IL-12-transduced DC. CD8+ T cells appear to be necessary effector cells for complete rejection of tumor and possibly memory. NK cells are responsible for the early immune response. Furthermore, CD4+ (T helper) cells did not play any role in IL-12-induced regression. These results imply that for DC to generate an effective antitumor response against neuroblastoma both acquired and innate effector cells are required.