BALB/c3T3 cells transformed in vitro with polyoma virus were cloned and passaged once in syngeneic mice. Resulting tumors from each clone were explanted and recultured. Expression of receptor for Fc of IgG (FcγRII) in the original in vitro maintained clones and in cells derived from tumors elicited by the respective cells was measured at the protein level as well as at the mRNA level. Clones were assayed in pairs. The ancestor in vitro maintained clones [designated cultured cells (C)] were compared with cells derived from the same clones after a single passage in vivo followed by explanation and reculturing [designated cultured-tumor-cultured cells (CTC)]. C cells of any of the tested clones did not express FcγRII. On the other hand, certain CTC cells were positive. The FcγRII-positive cells were derived from tumors appearing after a long precancer latency period (>140 days). CTC cells derived from tumors that appeared after shorter latency periods (<80 days) were FcγRII negative. These results were obtained both by using radioimmunoassay and monoclonal antibodies against mouse FcγRII as well as by Northern blot analysis using the FcγRII complementary DNA probe. The involvement of macrophages as the FcγRII-expressing cells in CTC cells was excluded.

FcγRII expression was down-regulated in CTC cells as a function of time following their explantation into culture. FcγRII expression could be up-regulated in these cells and induced on C cells by maintaining the cultured cells in the presence of normal mouse serum or recombinant interferon. We also tested the expression of FcγRII on CTC cells following their inoculation into syngeneic mice for a second time (CTCx2 cells). The results showed a positive correlation between FcγRII expression in the inoculated ancestor CTC cells and on the CTCx2 cell progeny.