MUC1 was first defined as a tumor antigen in the late 1980s, yet little is known about the types of immune responses that mediate rejection of MUC1+ tumors in vivo. MUC1‐specific antibodies, Th cells and cytotoxic T cells can be detected in patients with different adenocarcinomas, yet these tumors usually progress. Thus, there is a need to better understand the in vivo mechanisms of antigen‐specific tumor rejection. To characterize the nature of MUC1‐specific immune responses in vivo, rejection of a MUC1‐expressing melanoma tumor line (B16.MUC1) was evaluated in mice lacking specific T cell subsets, cytokines, co‐stimulatory molecules or molecular effectors of cytolytic pathways. Results demonstrated that rejection of the B16.MUC1 tumor cell line was primarily mediated by CD4+ T cells, and required Fas ligand, lymphotoxin‐α, CD40, CD40 ligand and CD28, but not perforin, γδ T cells, IL‐4, IL‐10, IL‐12 or tumor necrosis factor receptor‐1. Depletion of NK cells demonstrated that NK cells might also contribute to MUC1 immunity in the B16.MUC1 tumor model. These results demonstrated that the immune response generated against MUC1 does not fit the type 1 or 2 model described for many immune responses. Additionally, multiple cytolytic mechanisms are required for B16.MUC1 rejection.
A C57BL/6 mouse transgenic for human MUC1 (MUC1.Tg) was developed to evaluate MUC1-specific tumor immunity in an animal that expresses MUC1 as a normal self protein. Previous studies showed that MUC1.Tg mice, challenged with syngeneic tumors expressing MUC1 (B16.MUC1), developed progressively growing MUC1-positive tumors, whereas wild-type C57BL/6 (wt) mice developed MUC1-negative tumors at a significantly slower rate. The results of a limiting dilution CTL frequency assay were not informative, in that similar numbers of MUC1-specific CTL precursors (CTL) were detected in MUC1.Tg and wt mice. Tumor immunity in vivo was characterized by an adoptive transfer method to evaluate the degree of MUC1 or non-MUC1 tumor immunity in wt or MUC1.Tg mice. The results revealed that wt mice developed protective tumor immunity mediated by MUC1-specific CD4+ lymphocytes, while MUC1.Tg mice were functionally tolerant to MUC1 in vivo. The potential of adoptive immunotherapy to provide immunity to tumors expressing MUC1 and to produce undesirable autoimmunity in recipient MUC1.Tg mice expressing MUC1 as a self Ag was evaluated. Adoptive transfer of immune cells from wt mice primed in vivo with B16.MUC1 tumor cells into MUC1.Tg recipients resulted in significant increases in the survival of MUC1.Tg recipients compared with unmanipulated control MUC .Tg mice challenged with B16.MUC1 tumor cells. This response was specific for MUC1 since control tumors developed at equivalent rates in recipient or control MUC1.Tg mice. No gross or histologic evidence of autoimmunity was observed in recipient MUC1.Tg mice, indicating that tumor immune responses mediated by MUC1-specific CD4+ lymphocytes spare nontransformed epithelia-expressing MUC1.
The purpose of this chapter is to describe inflammatory and autoimmune reactions that are secondary to the development of cancer. In some cases, it is believed that inflammatory and autoimmune reactions result from interactions between substances and cells of the immune responses that are directed against tumor cells but cross react with normal tissues.
The human epithelial mucin, MUC1, is a large transmembrane glycoprotein that is expressed on most simple epithelia. It is overexpressed and aberrantly glycosylated on many human epithelial tumors, including more than 90% of human breast cancers. MUC1 is of interest as an immunotherapy target because patients with breast, ovarian, and pancreatic cancers have T lymphocytes in their tumor-draining lymph nodes that can be induced to recognize and lyse MUC1-expressing tumor cells. We have produced a transgenic mouse model that expresses the human MUC1 molecule on an inbred C57Bl/6 background to investigate the effect of endogenous expression of MUC1 on the ability of mice to generate antitumor immunity to MUC1-expressing tumors. Transgenic mice expressed the human transgene in a pattern and level consistent with that observed in humans. Transgenic mice were tolerant to stimulation by MUC1 as evidenced by the ability of MUC1-expressing tumor cells to grow in these mice, whereas MUC1-expressing cells were eliminated from wild-type mice. Moreover, transgenic mice immunized with MUC1 peptides failed to exhibit immunoglobulin class switching to the IgG subtypes. These data suggest that endogenous expression of MUC1 protein by MUC1 transgenic mice induces T-cell tolerance to stimulation by MUC1. The transgenic mice will provide a useful model to investigate the mechanisms that regulate immunological tolerance to tumor antigens and will facilitate the investigation of anti-MUC1 immunotherapy formulations.
