Immune evasion by melanoma and ovarian tumor cells through upregulation of the immunosuppressive molecule CD200

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Background: Immune escape by tumors can occur by multiple mechanisms, each a significant barrier to immunotherapy. Upregulation of the immunosuppressive molecule CD200 on chronic lymphocytic leukemia cells inhibits Th1 cytokine production required for an effective cytotoxic T cell response. CD200 expression on human tumor cells in animal models prevents human lymphocytes from rejecting the tumor; treatment with an antagonistic anti-CD200 antibody restored lymphocyte-mediated tumor growth inhibition. This study evaluated CD200 expression on other cancers, and its effect on immune response. Methods: CD200 levels in ovarian adenocarcinoma and metastatic melanoma samples were evaluated by RT-QPCR and immunohistochemistry. Cell-surface CD200 on melanoma and ovarian cancer cell lines was assessed by flow cytometry. The effect of CD200 on cytokine production in mixed lymphocyte reactions (MLR) was assessed by adding the cells to cultures containing human monocyte-derived dendritic cells and allogeneic T cells. Th1 and Th2 cytokines in culture supernatants were detected by ELISA. Results: RT-QPCR showed CD200 expression levels upregulated in serous ovarian adenocarcinoma compared to normal samples. In malignant melanoma, CD200 expression in jejunum metastases was significantly higher than in normal samples, and 2 of 6 lung metastases showed CD200 upregulation. IHC showed strong, membrane-associated CD200 staining on malignant cells of two melanoma patients. Three ovarian cancer patients showed varying levels of CD200 tumor staining; all showed strong stromal staining. CD200 was highly expressed on the cell surface of SK-MEL-24 and SK-MEL-28 melanoma and OV-CAR-3 ovarian cancer cell lines and moderately expressed on the melanoma cell line SK-MEL-5. Addition of these cell lines to MLRs downregulated the production of Th1 cytokines; addition of CD200-negative cell lines did not. Inclusion of an antagonistic anti-CD200 antibody during the culture restored Th1 cytokine responses. Conclusion: Melanoma and ovarian tumor cells can upregulate CD200, thereby potentially suppressing anti-tumor immune responses. Therapy with an antagonistic anti-CD200 antibody may permit an effective cytotoxic immune response against the tumor cells.

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