Abstract 2403: Glycosylated (Tn) and Anchor-Modified MUC1 Peptide Vaccines Induce Effective Anti tumor Immunity and Break Tolerance in MUC1 Transgenic Mice

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC MUC1 is an excellent target for immunotherapeutic strategies for cancers expressing Mucin 1 (breast, pancreas etc). MUC1 was recently selected by the NCI working group as the number 2 cancer vaccine target antigen, based on its immunogenicity, oncogenicity and wide-spread cell-surface expression. More than 90% of breast cancers over express MUC1, it stimulates both humoral and cellular immunity, its normal apical distribution is lost in cancer cells and aberrant glycosylation exposes peptide epitopes and novel carbohydrate antigens. Clinical trials using unglycosylated MUC1 epitopes have resulted in MUC1 specific cytotoxic T lymphocytes (CTLs) with limited effects on clinical response. Patients with breast tumors have exhibited spontaneous immune responses to MUC1 with the presence of T cells and antibodies specific for glycosylated MUC1. Because these natural immune responses are often not efficient in eradicating the tumor, it is necessary to specifically modulate these immune responses. Hence it is possible that better immunogenicity would be obtained using peptides more representative of the novel (glycosylated) forms of MUC1 as seen in cancer to which individuals may be less tolerant. The presentation of antigenic peptides by MHC class I molecules is crucial for the development of vaccines. MUC1 peptides from the VNTR region, which has five potential glycosylation sites, can be presented by a variety of class I molecules and can generate MUC1 specific CTLs. But these epitopes lack definite anchors and bind with low affinity compared to high affinity peptides. Hence, we developed several peptides from the MUC1 H-2Kb peptide (SAPDTRPAP) epitope with all possible modifications to improve their MHC and TCR binding efficiency. Preclinical studies were performed using the modified H-2Kb-restricted peptides along with the DC stimulants (CpG-ODN and GM-CSF) and foreign helper epitope (hepatitis B virus optimized I-Ab T helper peptide) in incomplete Freund's adjuvant in C57BL/6 mice transgenic for human MUC1. Peptides that were glycosylated at threonine and had methionine or leucine as the terminal amino acid induced a strong CTL response and were able to overcome tolerance in MUC1.Tg.mice. In the breast cancer model with the MUC1- expressing MMT cell line, the onset of tumor was delayed with peptide immunization and there was significant reduction in the tumor weight. Increased lytic activity of CD8+ CTLs and infiltration of T lymphocytes to the tumor site were observed with modified peptides as compared to the effect of MHC class I Kb and Db restricted native MUC1 peptides. Interestingly, the CTLs specific for glycosylated peptides were able to recognize both the Tn (GalNAc) and peptide epitopes. Thus, MUC1 peptides with carbohydrate at the appropriate position may be an optimal vaccine candidate because they can generate CTLs of dual specificity and high affinity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2403.