Isolation and characterization of a monoclonal antibody, K1, reactive with ovarian cancers and normal mesothelium.
We have isolated a new monoclonal antibody (MAb), K1, that reacts with an epitope on the surface of human ovarian carcinoma cells. This antibody was generated by immunization of mice with periodate-treated human ovarian carcinoma (OVCAR-3) cells. These mice had been previously made tolerant with normal human kidney membranes. Spleen lymphocytes from these mice were selected prior to fusion using a panning purification method on living OVCAR-3 cells. Initial screening of surface-reactive clones was performed in a single day using immunofluorescence on living OVCAR-3 cells, and secondary screening was performed using immunoperoxidase histochemistry on cryostat sections of normal human tissues and human tumors. The K1 clone was subcloned and identified as an IgM isotype, but was subsequently isotype-switched to IgG1K using a panning selection method. When evaluated by immunohistochemistry, the antigen reactive with K1 was found in many ovarian non-mucinous tumors, as well as in squamous tumors of the esophagus, and cervical cancer. The only normal adult human tissues showing uniform reactivity with K1 were the mesothelia of the peritoneal, pleural and pericardial cavities. There was also limited reactivity with epithelia of the trachea, tonsil and Fallopian tube. A similar tissue reactivity for K1 was found in tissues from cynomolgus monkeys. K1 reacted with many of the same tissues and tumors as the previously identified antibody OC125, but several lines of evidence indicate that K1 reacts with a different epitope and probably a different molecule, when compared to OC125. This evidence included assays employing immunofluorescence competition, double-label immunofluorescence, and solid-phase and live-cell radioimmunoassays. Since our data indicate that the antigen reactive with the K1 antibody is a new molecular species, we have named the antigen CAK1. Unlike the shed antigen CA125, CAK1 was only cell-associated and was not found in the supernatant of cultured OVCAR-3 cells or in the blood of ovarian cancer patients. The K1 antibody may be useful as a targeting agent for therapy and in the diagnosis of ovarian carcinoma, as well as some other human cancers.