Tumor-antigen heterogeneity of disseminated breast cancer cells: Implications for immunotherapy of minimal residual disease

Single micrometastatic tumor cells encased in mesenchymal tissues, such as bone marrow (BM), are regarded as suitable targets for adjuvant immunothera py since they are easily accessible for both immunoglobulins and immune eff ector cells. However, the antigen profile of such cells, to which antibody therapy might be targeted, cannot be deduced from the antigen pattern of th e primary tumor. To evaluate the antigen profile of disseminated cells foun d in BM aspirates from 20 breast cancer patients, we applied a quantitative immuno-cytochemical double-marker assay and typed for 4 common tumor-assoc iated cell-surface antigens (c-erbB-2, CO17-IA, MUG-I, Lewis(Y)). Individua l breast cancer cells were identified by F-ab fragments of the pan-cytokera tin (CK) monoclonal antibody (MAb) A45-B/B3, directly conjugated with alkal ine phosphatase, which identified cancer cells as sensitively as the standa rd APAAP procedure (r = 0.998; p < 0.0001). CK+ cells co-expressed c-erbB-2 , CO17-IA, MUC-I and Lewis(Y) in 87%, 78%, 79% and 79% of patients, respect ively; however, the frequency of double-positive cells per sample varied co nsiderably. The mean percentage of double-positive cells per total number o f CK+ cells was 41% for c-erbB-2 (range 0-92%), 47% for CO17-IA (range 0-75 %), 49% for MUC-I (range 0-67%) and 32% for Lewis(Y) (range 0-59%). In 14 o f these patients, we used an antibody cocktail to type CK+ cells for the co mbined expression of all 4 antigens. The antibody cocktail labeled signific antly more CK+ cells than each of the single MAbs alone, resulting in a mea n of 71% double-positive tumor cells (34-100%). We conclude that expression of tumor-associated cell-surface antigens on micrometastatic cancer cells in BM is heterogeneous, which may limit the efficacy of monovalent immunoth erapeutic strategies directed against only one particular antigen. Thus, de fining target antigens expressed by the actual target cells emerges as a cr ucial first step in selecting appropriate therapeutic targets. (C) 1999 Wil ey-Liss, Inc.