Estimation of mouse mammary tumor virus gp52 abundance by protein A assay: a comparison of viral antigen expression on C3H and GR mammary tumor cells.

An adherent cell isotopic staphylococcal protein A test (ISPAT) was used to estimate the abundance of mouse mammary tumor virus (MMTV) gp52 cell surface antigen (CSA)on mammary tumor cells. Protein A assays were first utilized, not strictly as a means of antigen detection, but as a means to determine the kinetics of dexamethasone-mediated changes in gp52 cell surface expression. Results indicated increases in gp52 CSA within 4 h of dexamethasone treatment and maximal levels of antigen expression within 12-24 h after treatment. Comparison of gp52 determinants on dexamethasone-stimulated mammary tumor cells demonstrated a greater abundance on C3H Mm5mt/cl than on GR-MMTV cultures. Parallel antigen assays with gp52 and Rauscher murine leukemia virus (R-MuLV) antisera demonstrated that GR-MMTV cells expressed fewer C-type determinants and thus a more preferential expression of gp52 determinants than other cell lines tested. The gp52/R-MuLV binding ratio was only 2.4:1 for C3H cultures in contrast to 5.7:1 for GR cultures. In addition, a comparison of gp52 expression on viral producer and nonproducer cells provided a quantitative estimate of the extent of expression which occurred in a retrovirus nonproducer culture. Results of ISPAT demonstrated that 75% of the gp52 detected on producer cells was present on nonproducer cultures. Comparison of the expression of gp52 CSA and the release of gp 52 into culture fluids during hormone treatments demonstrated that both assays (ISPAT and gp 52 radioimmunoassay) detect and quantitate coordinate changes in the expression and release of MMTV gp52. In antibody excess, protein A assays provided quantitative estimates of CSA abundance not offered by alternative methods of CSA detection.