On the nature of crossreactions observed with antibodies directed to defined epitopes.

Antibodies directed against a synthetic peptide (src-c) containing the six carboxyl-terminal amino acids of p60src, the transforming protein of Rous sarcoma virus, recognize p60src. However, when used at sufficiently high concentrations they also react with a number of constituents of untransformed cells. These reactions can be completely inhibited by src-c peptide. Crossreactivities are to different components in cells from different species and cannot be attributed to p60c-src, the ubiquitous cellular homologue of p60src. By indirect immunofluorescence microscopy and immunochemical techniques we have identified three cytoskeletal proteins, myosin, tubulin, and vimentin, as well as an unknown intranuclear antigen, as major targets of anti-src-c antibodies in different untransformed cells. These crossreactivities probably reflect identities or similarities in the amino acid sequence of the immunogenic peptide and segments of the otherwise unrelated crossreactive proteins. These findings are discussed with respect to the interpretation of crossreactivities that are occasionally observed with anti-peptide sera and with monoclonal antibodies.