Antigenic similarity between plasma membrane proteins of fetal hamster cells and simian virus 40 tumor surface antigens.

Both simian virus 40 (SV40)-induced viral oncogenesis and the transplantation of SV40-transformed tumor cells in ham sters can be prevented by the immunization of animals with intact fetal cells from primiparous females. The fetal determi nants responsible for inducing transplantation resistance are temporally expressed on the surface of intact embryonic cells through the 11 th day of gestation and are no longer detectable thereafter by direct immunization techniques. We now report that crude plasma membrane preparations from both 10-dayold and 14-day-old fetal cells and 3 M KCI extracts of these membranes are capable of inducing similar protection in ham sters against a challenge with SV40-transformed hamster sar coma cells. In addition, spleen cells from adult hamsters im munized against SV40 tumor surface antigens will undergo a proliferative response in vitro in the presence of homologous tumor cells and 10-day-old fetal cells but not 14-day-old fetal cells. These results confirm the observation that midgestation hamster embryo cells express antigen(s) that is cross-reactive with the tumor-associated transplantation antigen(s) on SV40transformed hamster sarcoma cells. Splenocytes sensitized to SV40 tumor surface antigens will proliferate in the presence of the plasma membranes from hamster sarcoma cells, 10-dayold and 14-day-old fetal cells, or 3 M KCI extracts of these membranes. Such sensitized splenocytes will not proliferate in the presence of the intact heterologous chemically transformed tumor cells, their plasma membranes, or 3 M KCI extracts or their plasma membranes. These findings suggest that such fetal determinants reside cryptically within the membranes of term embryo tissue even though their antigenicity and immunogenicity is restricted.