Difference in cell binding patterns of two monoclonal antibodies recognizing distinct epitopes on a human melanoma-associated oncofetal antigen.

Two monoclonal antibodies (MAbs), 140.240 and 96.5, generated independently in different laboratories, have been shown to detect the target structures of 87,000 (gp87) and 97,000 (p97) glycoproteins, respectively, both strongly expressed by melanoma cells and fetal small intestine. To determine whether MAb 140.240 and MAb 696.5 recognized a same target structure, they were tested in immunoprecipitation/SDS-PAGE using NP-40 lysates of melanoma cells labelled with [35S]methionine for 18 hr. Both antibodies precipitated a single band with Mr = 87,000. Reciprocal immunodepletion studies showed that neither of the two antibodies detected the 87,000 band in the lysate immuno depleted by either antibody, suggesting that these two antibodies recognize the same or extremely similar molecules. Two-dimensional tryptic peptide mapping analysis showed that the two identified molecules shared the same finger-printing pattern. A 40,000 fragment of the 87,000 molecule produced by protease digestion was precipitated by MAb 96.5 but not MAb 140.240, indicating that the epitopes recognized by the two antibodies are localized at discrete sites on the molecule. Serological studies on these two antibodies revealed slightly different binding patterns in the MAb 140.240 exhibited a more melanoma-restricted specificity, while MAb 96.5 had a specificity to melanoma and to some other cell types. The observed difference in epitope specificity may be important in the clinical applications of these antibodies.