Virus-dependent cytostatic activity to mammary tumor cells of lymphocytes from normal mice

A cytostasis assay has been used to study the natural immunity of mice to murine mammary tumor virus (MTV). Spleen cells from adults of all strains tested were found to be cytostatic to a variety of MTV-positive mammary tumor cell lines. Newborn spleen cells were unreactive in the same cytostasis assay. The degree of reactivity to the target cells was greater in spleen cell preparations from low MTV expressors than from syngeneic, high MTV expressors. The cytostasis was specific, since MTV antigens prepared from gradient-purified whole MT virions significantly blocked the reaction. In addition, spleen cells were totally nonreactive to MTV-negative cell lines. Other types of lymphoid cells, such as lymph node cells as well as peritoneal macrophages, were highly cytostatic under similar conditions. Spleen cells from nude athymic donors were not cytostatic. Since depletion of splenic T-lymphocytes by use of anti-theta serum also did not significantly affect cytostasis, it was concluded that T-cells were required for initiation of immunity to MTV but that the effector cells were not theta positive. Blocking factors were found to exist in the sera of mammary tumor-bearing animals that prevented cytostasis by reactive spleen cells.