Adenoviral mediated IL−15 and IL−15Ra immunotherapy of a Murine Breast Cancer

Interleukin-15 (IL-15) induces the differentiation and proliferation of T, B and NK cells, enhances activity of CD8+ cytolytic T cells, helps maintain CD44hiCD8+ memory T cells, and stimulates immunoglobulin synthesis by B cells, making IL-15 a potentially ideal candidate for cancer immunotherapy. IL-15 functions through its receptor, IL-15Rα expressed on dendritic cells and monocytes where it is trans-presented to effector T cells. The efficacy of exogenously administered IL-15 may be limited by the availability of IL-15Rα, therefore a genetic immunotherapy strategy where both IL-15 and its receptor are expressed may be advantageous. In this study we examined the effectiveness of adenoviral-mediated gene transfer of both IL-15 and IL-15Rα in a mouse breast cancer model. We constructed E1, E3 deleted adenoviruses expressing murine IL-15 (Ad.mIL-15) or IL-15Rα (Ad.mIL-15Rα) and showed that both vectors were able to efficiently transduce TS/A breast cancer cells. We examined whether the expression of IL-15, or the combination of IL-15 and IL-15Rα by TS/A cells would lead to reductions in tumor growth in BALB/c mice. Mice were subcutaneously implanted with 1 × 106 TS/A cells transduced ex vivo with Ad.mIL-15, Ad.mIL-15 + Ad.mIL-15Rα or Ad.null (a control vector) at an MOI of 30. IL-15, or IL-15 and IL-15Rα transduced tumors had significantly slower growth rates than that of the Ad.null control. At day 20, Ad.null tranduced tumors were on average 1653 ±131 mm3, whereas tumors tranduced with IL-15, or both IL-15 and IL-15Rα were significantly smaller at 185 ±61 mm3 and 100 ±55 mm3, respectively. By day 30, tumors transduced with Ad.mIL-15 alone were larger (953 ±183 mm3) than those treated by the combination Ad.mIL-15 + Ad.mIL-15Rα (658 ±95 mm3). This effect was also demonstrated using direct adenoviral transduction of established tumors. Tumors were treated with 3 intratumoral injections (1 × 109 PFU) of Ad.mIL-15, Ad.mIL-15 + Ad.mIL-15Rα or Ad.null control, 8 days after tumor implantation (tumor size: 100-150 mm3). At day 20, Ad.null treated animals had average tumor sizes of 1703 ±130 mm3 while animals treated with Ad.mIL-15 or both IL-15 and IL-15Rα had tumors 520 ±55 mm3 and 321 ±26 mm3, respectively. By day 28, tumors treated with Ad.mIL-15 alone were significantly larger (1396 ±134 mm3) than those treated by the combination of Ad.mIL-15 and Ad.mIL-15Rα (561 ±67 mm3) indicating a benefit of combining IL-15 with IL-15Rα. We depleted specific immune cell populations to determine which cells were critical for the anti-tumor activity. We found that in animals which had been treated with Ad.mIL-15 alone, the depletion of NK cells abrogated the anti-tumor effects while in animals treated with the combination of Ad.mIL-15 and Ad.mIL-15Rα, the depletion of NK cells and CD8 cells affected the growth rates indicating that in addition to innate immune responses, an adaptive response may also be responsible for the anti-tumor action when using this combination. Conclusion: Gene transfer of IL-15 and its receptor, IL-15Rα, inhibited tumor growth significantly better than IL-15 alone, and response to this treatment appears to involve both adaptive and innate immunity.