Antigenic and immunogenic characteristics of nuclear and membrane-associated simian virus 40 tumor antigen

Antisera were prepared in syngeneic hosts against subcellular fractions of simian virus 40 (SV40)-transformed cells (MoαPM, MoαNuc), glutaraldehyde-fixed SV40-transformed cells (HaαH-50-G, MoαVLM-G), and electrophoretically purified denatured SV40 tumor antigen (T-ag) (RaαT). Immune sera were also collected from animals bearing tumors induced by SV40-transformed cells (HaαT, MoαT, HAF) and from SV40-immunized animals that had rejected a transplant of SV40-transformed cells (HaαS, MoαS). Immunological reagents prepared against cell surface (MoαPM, HaαS, MoαS, HaαH-50-G, MoαVLM-G) reacted exclusively with the surface of SV40-transformed cells by indirect immunofluorescence or protein A surface antigen radioimmunoassay. Immunological reagents prepared against the nuclear fraction (MoαNuc) or whole-cell determinants (HaαT, MoαT, HAF, RaαT) reacted with both the nuclei and surface of SV40-transformed or -infected cells. All reagents were capable of immunoprecipitating 96,000-molecular weight large T-ag from solubilized whole cell extracts of SV40-transformed cells. The exclusive surface reactivity of HaαS exhibited in immunofluorescence tests was abolished by solubilization of subcellular fractions, which then allowed immunoprecipitation of T-ag by HaαS from both nuclear and plasma membrane preparations. Specificity was established by the fact that all T-reactive reagents failed to react in serological tests against chemically transformed mouse cells, and sera from mice bearing transplants chemically transformed mouse cells (MoαDMBA-2) failed to react with SV40-transformed mouse or hamster cells. Reagents demonstrating positive surface immunofluorescence and protein A radioimmunoassay reactions against SV40-transformed cells were capable of blocking the surface binding of RaαT to SV40-transformed cells in a double-antibody surface antigen radioimmunoassay. This blocking ability demonstrated directly that a component specificity of each surface-reactive reagent is directed against SV40 T-ag. A model is presented which postulates that the differential detection of T-ag by the various serological reagents is a reflection of immunogenic and antigenic differences between T-ag polypeptides localized in nuclei and plasma membranes.