5097

The murine monoclonal antibody AR20.5 recognizes a hypoglycosylated epitope on the tumor-associated antigen MUC1 (sequence DTRPAP) within the MUC1 tandem repeat peptide sequence that is not accessible on normal epithelial cells. In vitro studies have shown that immune complexes (IC) of AR20.5 and MUC1 enhanced processing of MUC1, including cross-presentation on class I. We have further investigated the activation of MUC1 specific immune responses in MUC1-transgenic (tg) mice, which are tolerant to MUC1 on the CD4 and CD8 T cell level. Mice were immunized with MUC1 alone, AR20.5 alone, IC containing MAb-AR20.5 and MUC1, and appropriate controls. Subcutaneous (s.c.) and intravenous (i.v.) routes were compared at various MUC1 concentrations. MUC1 was affinity purified from the pancreatic cancer cell CFPAC-1. The murine IgG1 AR20.5 antibody was also compared to a human IgG derivative. Mice were monitored for antibodies to MUC1 by ELISA and for T cell responses by IFN-γ intracellular cytokine staining, chromium release assay and T cell proliferation. Once the immunization protocol was optimized, tumor control studies with Panc02.MUC1 s.c. transplants were conducted. We showed that MUC1 in the form of IC with AR20.5 was more efficiently processed than MUC1 alone. MUC1-specific antibodies could only be induced with the IC and MUC1-KLH. T helper (p<0.05) and particularly CTL (p<0.001) responses were superior in mice immunized with IC compared to mice immunized with MUC1 alone with the s.c. route of immunization being more effective than the i.v. route. Relatively low MUC1 concentrations (10-20 U/mouse) in combination with 10-50 µg/mouse of AR20.5 were able to induce T cell responses. Tumor growth was delayed or prevented in mice immunized with IC compared to controls (p<0.01) and was more effective using s.c. immunizations. IC containing the human IgG derivative of AR20.5, which is more immunogenic in mice, showed better tumor control than IC with the murine MAb. We were able to detect MUC1-specific CTL in mice with small or no tumors and immunized with IC or AR20.5 in the presence of circulating MUC1, but not in mice with large tumor burden. Further, tumor size correlated with presence of regulatory T cells in circulation; however, the slope was decreased in groups immunized with the immune complexes. These results indicate that antigen-complexed MUC1-specific antibodies can reverse unresponsiveness to MUC1 in MUC1-tolerized hosts including priming of T helper cells, CTL and antibodies, which translated into significant tumor control.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA