Induction of helper and cytolytic T cell responses in MUC1-tg mice immunized with MUC1-anti-MUC1 immune complexes

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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. This antibody has been tested in human in vitro studies and shown to facilitate processing of MUC1 as an immune complex (IC). Both, enhanced induction of CD4 and CD8 T cell responses occured, indicating cross-presentation on class I. We have further investigated the activation of MUC1 specific T cells in MUC1-transgenic mice, which are tolerant to MUC1 on the CD4 and CD8 T cell level, for MUC1 presented alone or as an IC with MAb-AR20.5. Mice were immunized with MUC1 peptide (31-mer comprising the tandem repeat sequence of the antigen), whole pancreatic cancer cell derived MUC1, the AR20.5 antibody alone or IC consisting of MAb-AR20.5 and the two forms of MUC1 at various concentrations as well as appropriate controls. Mice were monitored for antibodies to MUC1 by ELISA and for T cell responses by IFN-γ ELISPOT and chromium release assay. We showed that MUC1 in the form of IC with MAb-AR20.5 was more efficiently processed than MUC1 alone. In the absence of circulating MUC1 to form endogenous complexes with MAb-AR20.5, MUC1-specific antibodies could only be induced with the IC. T helper (p<0.05) and particularly CTL (p<0.001) responses were superior in mice immunized with IC than in mice immunized with MUC1 alone. These results indicate that antigen-specific antibodies can reverse unresponsiveness to MUC1 in MUC1-tolerized hosts, facilitate HLA class I processing of exogenous antigens and can be an efficient tool to prime T helper cells and CTL against tumor antigens.