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Two novel mast cell phenotypic markers, monoclonal antibodies Ki‐MC1 and Ki‐M1P, identify distinct mast cell subtypes

Summary In order to identify more specific or selective mast cell markers, the reactivity of two monoclonal antibodies, Ki‐MC1 and Ki‐M1P, was studied by immunohistochemistry in two human cell lines (mast cell line HMC‐1, basophilic cell line KU812), in mast cells cultured from blood precursors, in adherent mononuclear cells from peripheral blood, and in mast cells of tissue sections from 1 3 urticaria pigmentosa lesions, live mastocytomas and live normal skin specimens. Toluidine blue staining, fluorescence staining with FITC‐conjugated avidin, and immunohistochemical staining (APAAP) with other mast cell reactive monoclonal antibodies, was performed for comparison. Double staining with the APAAP method, using the Ki‐antibodies and toluidine blue, was also carried out. Both Ki‐antibodies showed reactivity for skin mast cells, but with a different staining pattern. In addition, the Ki‐MC1 antibody did not react with the cell lines, and reacted only with a few peripheral blood mononuclear cells and cultured mast cells. In contrast, the Ki‐M1P antibody reacted with almost all cultured mast cells and blood mononuclear cells, but stained only about one‐half of lesional and one‐fifth of normal skin mast cells. Ki‐M1P also reacted with many toluidine blue‐negative dermal cells, particularly in urticaria pigmentosa. Ki‐MC1 antibody can thus be considered as a useful additional marker for normal skin mast cells. In contrast, the Ki‐M1 P antibody primarily identifies immature mast cells and monocytes/macrophages, suggesting that these cell types probably originate from the same bone marrow precursor.