Abstract B95: MUC16 protects ovarian cancer cells from the cytolytic responses of human and murine natural killer cells and macrophages.

MUC16 is a membrane-spanning mucin that frequently is overexpressed by ovarian cancer cells. Knockdown of MUC16 is known to increase apoptosis in both ovarian and breast cancer cells. MUC16 also plays an important role in facilitating metastasis and protecting cancer cells from innate immune responses. Here, we demonstrate that MUC16 protects ovarian cancer cells from cytolysis by NK cells and macrophages. Immunodeficient SCID and SCID/beige mice implanted with MUC16-expressing and MUC16 knockdown tumors have a median survival of 78 and 198 days, respectively. In vitro assays demonstrate that better immune recognition of the MUC16 knockdown ovarian cancer cells contributes to the higher median survival of the tumor-bearing immunocompromised animals. NK cells which were unstimulated, activated in vivo with anti-CD40 antibody, and/or stimulated with lipopolysaccharides preferentially lyse MUC16-knockdown cancer cells in standard chromium release assays. Thus the reduction of MUC16 expression leads to a significant increase of the natural cytotoxic responses of NK cells. In addition, MUC16 protects the cancer cells from Antibody-Dependent Cell-mediated Cytotoxicity (ADCC). ADCC of the MUC16 knockdown cancer cells was studied using the anti-EpCAM antibody KS1/4 and its corresponding immunocytokine huKS-IL2. NK cells from immunocompetent C57BL/6 mice exhibited an increase in the cytolysis of MUC16-knockdown ovarian cancer cells in the presence of either KS1/4 or huKS-IL2. Similar effect was also observed when the ADCC assays were conducted using human NK cells isolated from healthy donors. The immune protection provided by MUC16 not only reduces the effectiveness of NK cells, but also murine macrophages, explaining the longer survival observed in the NK cell deficient SCID-beige mice transplanted with the MUC16-knockdown cancer cells. Immunohistochemical analysis of the tumors from the SCID-beige mice showed increased infiltration of macrophages in the tumor nests of MUC16-knockdown tumors. In contrast, macrophages were primarily detected in the peritumoral stroma of MUC16pos tumors. Murine macrophages from unstimulated and anti-CD40 antibody stimulated animals showed a 3-fold increase in tumoristatic activity against the MUC16-knockdown versus the MUC16pos ovarian cancer cells. The results of our study also demonstrate that siRNA-based strategies to knockdown MUC16 can be coupled with immunotherapeutic approaches, such as the use of activated NK cells or specific immunocytokines, to render ovarian tumors especially vulnerable to innate immune responses. Therefore, our results not only indicate that immune protection is a major biological role of MUC16, but also demonstrate that attenuation of MUC16 expression will likely serve as an important strategy to boost the anticancer efficacy of immunotherapeutic agents. Citation Format: Mildred Felder, Arvinder Kapur, Alexander Rakhmilevich, Joseph Connor, Manish S. Patankar. MUC16 protects ovarian cancer cells from the cytolytic responses of human and murine natural killer cells and macrophages. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr B95.