Mechanisms of IL-21 Enhancement of Rituximab Efficacy in a Lymphoma Xenograft Model.

Interleukin 21 (IL-21) is an IL-2 family cytokine produced by activated CD4+ T cells. Potent effects of IL-21 have been observed on the growth, survival, and functional activation of T cells, B cells, and natural killer (NK) cells. A Phase I clinical trial of IL-21 in metastatic melanoma and renal cell carcinoma is currently in progress. We recently reported that IL-21 significantly enhanced rituximab mediated clearance of CD20+ lymphoma cell lines both in vitro and in vivo, and that these effects were potentially mediated through IL-21 enhancement of NK cell capacity to effect antibody dependent cellular cytotoxicity (ADCC). Specifically, NK cells treated with IL-21 showed increased cytotoxicity, granzyme B and IFNg production. Current studies aim to further evaluate the mechanisms by which IL-21 enhances ADCC. A number of observations suggest a multi-factorial basis for IL-21 synergy with rituximab. In a xenograft tumor model, SCID mice were injected IV with HS Sultan cells on day 0. Treatment with recombinant murine IL-21 (mIL-21; starting day 1) combined with rituximab (starting day 3) resulted in significantly increased survival (70% vs. 20% on day 100), compared to rituximab alone. In separate studies, the spleens of mice treated with mIL-21 showed increased numbers of activated macrophages and granulocytes. As macrophages and granulocytes can participate in ADCC, IL-21 synergy with rituximab in vivo may be partly dependent on its activation of these cell types. We have also evaluated whether direct effects of IL-21 on lymphoma cells contribute to enhancement of rituximab efficacy. The xenogeneic B lymphoma models in which IL-21 plus rituximab exhibited enhanced survival are highly aggressive and these models were not shown to respond to treatment with mIL-21 alone. In vitro studies were performed to determine if IL-21 could potentiate the growth inhibitory and pro-apoptotic effects of rituximab. In the absence of effector cells synergistic interaction was not observed. In addition, we tested the ability of IL-21 to enhance cytotoxicity when combined with antibodies targeting non-hematopoietic tumor cells (e.g. trastuzumab). Human NK cells treated with IL-21 displayed significantly increased cytotoxicity in ADCC assays using trastuzumab to target breast cancer cells expressing varying levels of HER-2 antigen. In summary, the current evidence suggests that IL-21 can enhance antibody-mediated tumor cell lysis through activation of multiple effectors of ADCC. Thus IL-21 may prove to be broadly applicable to monoclonal antibody therapy of cancer.