Cell surface binding affinity of Simian virus 40 T-antigen

Abstract

Simian virus 40-transformed cells are characterized by a virus-induced tumor transplantation antigen (SV40 TSTA) defined in vivo by the rejection of tumorigenic SV40-transformed cells in SV40-immunized mice and in vitro by SV40 tumor cell-specific cytotoxic T cells. Several experimental findings support the notion that SV40-infected and -transformed cells express SV40 large tumor antigen (TAg) or closely related antigens on the cell surface (surface T). In this report, evidence is presented for a cell surface binding affinity of SV40 TAg solubilized and extracted by disruption of SV40-transformed and SV40-infected cells in growth medium. Incubation of various transformed and nontransformed living monolayer cells in situ with these extracts led to a significant uptake of TAg to the cell surface (called “externally bound TAg”) up to two to five times higher amounts in comparison to native surface T on SV40-transformed cells. This was demonstrated by the highly sensitive 125I-protein A assay using rabbit antisera directed against purified SV40 TAg. Serological analysis of TAg in cellular extracts and of externally bound TAg revealed no apparent differences suggesting the cell surface binding affinity as a new property of SV40 TAg. We interpret our results as an indication that this property enables purified TAg to initiate the cellular immune response necessary for the SV40-tumor rejection in mice.