Sixteen research groups participated in the ISOBM TD-4 Workshop in which the reactivity and specificity of 56 monoclonal antibodies against the MUC1 mucin was investigated using a diverse panel of target antigens and MUC1 mucin- related synthetic peptides and glycopeptides. The majority of antibodies (34/56) defined epitopes located within the 20-amino acid tandem repeat sequence of the MUC1 mucin protein core. Of the remaining 22 antibodies, there was evidence for the involvement of carbohydrate residues in the epitopes for 16 antibodies. There was no obvious relationship between the type of immunogen and the specificity of each antibody. Synthetic peptides and glycopeptides were analyzed for their reactivity with each antibody either by assay of direct binding (e.g. by ELISA or BiaCore) or by determining the capacity of synthetic ligands to inhibit antibody binding interactions. There was good concordance between the research groups in identifying antibodies reactive with peptide epitopes within the MUC1 protein core. Epitope mapping tests were performed using the Pepscan analysis for antibody reactivity against overlapping synthetic peptides, and results were largely consistent between research groups. The dominant feature of epitopes within the MUC1 protein core was the presence, in full or part, of the hydrophilic sequence of PDTRPAP. Carbohydrate epitopes were less easily characterized and the most useful reagents in this respect were defined oligosaccharides, rather than purified mucin preparations enriched in particular carbohydrate moieties. It was evident that carbohydrate residues were involved in many epitopes, by regulating epitope accessibility or masking determinants, or by stabilizing preferred conformations of peptide epitopes within the MUC1 protein core. Overall, the studies highlight concordance between groups rather than exposing inconsistencies which gives added confidence to the results of analyses of the specificity of anti-mucin monoclonal antibodies.

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Key words: MUC1; breast cancer; ADCC; NK cells; vaccination; immunotherapyMUC1 is a transmembrane O-linked glycoprotein present on the apical surface of normal secretory epithelial cells. 1 The extracellular domain of MUC1 consists mainly of a variable number of tandem repeats 2 and it has a cytoplasmatic tail of 69 amino acids. 3 In the vast majority of human adenocarcinomas this protein is over-expressed and hypo-glycosylated 4 exposing an immunodominant repetitive amino acid sequence. The overproduction and secretion of MUC1 is correlated with the progression of breast, 5 ovarian 6,7 and colon 8 carcinoma. In breast cancer patients MUC1 serum levels are used to monitor therapy and for early detection of recurrences. 9,10 Proliferative responses to MUC1 and its tandem-repeat peptides have been demonstrated with peripheral blood mononuclear cells (PBMC) of patients with ovarian adenocarcinoma 11 and MUC1 specific cytotoxic T lymphocytes (CTL) have been isolated from tumor-draining lymph nodes of breast 12 and ovarian 13 cancer patients. In patients with ovarian, breast and pancreatic adenocarcinomas cytotoxic T cells can be induced that are specific for the MUC1 tandem repeat. 14,15 Recognition of MUC1 by T lymphocytes may be due to exposure of the immunogenic PDTRP region of the tandem repeat that is normally not exposed due to extensive O-glycosylation of the peptide core. The cytotoxic action of these T cells has also been described to be MHC-nonrestricted, due to crosslinkage to poorly glycosylated MUC1 tandem repeats on tumor cells. 16 Not only cellular but also humoral responses to MUC1 are found in breast, ovarian, colon and pancreatic carcinomas. [17][18][19] Moreover, the presence of immune-complexed MUC1 in breast cancer patients is related to a favorable disease outcome 20 and survival in early breast cancer patients is favorably influenced by a natural humoral immune response to MUC1. 21 These findings point to MUC1 and its repeat peptide as a target for immunotherapy of carcinomas. One possible approach is the use of MAbs against tumor-associated antigens, 22,23 another the use of tumorassociated antigens such as MUC1 as a vaccine and target for cellular and humoral immune responses.MUC1 MAbs have already been used in a number of vaccination trials as carrier molecule of radioactive elements 24,25 and recently a clinical trial has been set up with unlabeled humanized (Hu)HMFG-1 MAb for adjuvant therapy of breast cancer patients (Dr. D.W. Miles, Guy's Hospital, London). Also active vaccination trials with MUC1 tandem-repeat peptides 26 have been initiated. Recently, in 3 phase I vaccination trials executed by Dr. P.O. Livingston in the Memorial Sloan-Kettering Cancer Center, NY, breast cancer patients with no evidence of disease were repeatedly injected with a 30-mer 27,28 a 33-mer or a 106-mer MUC1 tandem repeat peptide conjugated to KLH plus adjuvant QS-21. In these patients high MUC1 IgG and IgM could be induced. 21 MAbs specific for tumor-associated antigens (TAA) can induce a complement-depende...

Introduction: About one-third of breast and ovarian carcinoma patients have circulating antibodies reactive with polymorphic epithelial mucin (MUC1), either free or bound to immune complexes. While the presence of these immune complexes has prognostic significance in breast cancer patients, the significance of free MUC1 antibodies is less clear. The objective of this study was to develop a reliable assay for the accurate determination of circulating free antibodies to MUC1. Material and Methods: We developed an enzyme-linked immunosorbent assay (ELISA) (PEM.CIg) employing a 60 mer peptide (a triple tandem repeat sequence of the MUC1 peptide core) conjugated to bovine serum albumin and peroxidase-labeled antihuman immunoglobulin G or M antibodies. The assay was standardized and its analytical performance evaluated. A total of 492 serum samples were obtained from 40 healthy men, from 201 healthy women (including 55 women without a history of pregnancy and 45 pregnant women), and (before primary treatment) from 62 benign breast tumor patients and 190 breast cancer patients. MUC1 serum levels were determined with commercial CA 15-3 tests. Results: Circulating antibodies to MUC1 are present both in healthy subjects and in breast cancer patients. The within- and between-assay coefficients of variation were, respectively, 2 and 12% for the IgG determinations and 1.2 and 3% for the IgM determinations. Correlation coefficients for serially diluted serum samples ranged from 0.9998 to 0.9920 for IgG and from 0.9996 to 0.9818 for IgM determinations. The reactivity of serum samples was partially blocked by the addition of various MUC1 peptides and by MUC1 mucin. The inhibiting effect of modified 60 mer peptides suggests the presence of antibodies directed to more than one epitope. Conclusions: The PEM. CIg assay is a reliable ELISA for measuring free MUC1 antibodies in serum. We are in the process of relating the results obtained in the breast cancer group to disease outcome to evaluate its prognostic significance. In addition, the assay may become a useful tool for vaccine therapy monitoring.