Naturally processed class II epitope from the tumor antigen MUC1 primes human CD4⁺ T cells

Epithelial cell mucin MUC1 is expressed on adenocarcinomas in an underglycosylated form that serves as a tumor antigen in breast, pancreatic, ovarian, and other tumors. Two predominant MUC1-specific immune responses are found in patients: CD8⁺ CTLs, which recognize tandemly repeated epitopes on the MUC1 protein core, and IgM antibodies. There have been no reports to date of MUC1-specific CD4⁺ T-helper cells in cancer patients. We show here that MUC1-specific CD4⁺ T cells are neither deleted nor tolerized and that CD4⁺ T cell responses can be generated when an appropriate soluble form of MUC1 is used. Naive CD4⁺ T cells from healthy donors were primed in vitro to a synthetic MUC1 peptide of 100 amino acids, representing five unglycosylated tandem repeats, presented by dendritic cells. They produced IFN-γ and had moderate cytolytic activity. We identified one core peptide sequence, PGSTAPPAHGVT, that elicits; this response when it is presented by HLA-DR3.