Spleen cells from rats bearing syngeneic metastatic 13762NF mammary adenocarcinoma clone MTLn3 tumors were fused with the rat myeloma Y3 Ag1.2.3 to generate a panel of monoclonal antibodies (MAbs). The MAbs could be divided into three groups: those crossreactive with all 13762NF cells; those reactive with cloned MTLn3 and MTC cells; and those predominantly reactive with the highly metastatic MTLn3 cells. One of these MAbs, MT10:21 (an immunoglobulin G2a), binds predominantly to highly metastatic MTLn3 cells and has a high tumor-cell affinity as determined by its saturation kinetics. MAb MT10:21 has a 6-h half-life on the MTLn3 cell surface and a 24-h half-life in the blood of syngeneic rats. Immunoblotting experiments using lysates from the cloned 13762NF sublines revealed that MAb MT10:21 binds to several proteins having relative molecular weights of 72,000, 73,000, and 120,000. Using an immunohistochemical procedure with frozen tissue sections, MAb MT10:21 shows little reactivity with normal rat mammary tissue, irrespective of the stage of the estrous cycle, and it failed to react with a number of other normal fetal and adult tissues. Furthermore, MAb MT10:21 is heterogeneous in its reactivity to cloned sublines of the 13762NF mammary adenocarcinoma, on both tissue cultured cells and tissue sections prepared from tumors growing in situ in the mammary fat pads of syngeneic rats. MAb MT10:21 reacted with certain human breast cancer cell lines and with a subpopulation of metastatic human breast cancer cells in frozen tissue sections from biopsies and autopsies. Metastases from breast cancers reacted more intensely than the primary tumors from which they were derived.

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Supported by USPHS Grant R01 CA28844 from the National Cancer Institute and by the Susan G. Koman Foundation (to G. L. N.).

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