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...