Cytotoxic Thy 1.2-Positive Blasts with NK-Like Target Selectivity in Murine Mixed Lymphocyte Cultures

Abstract

CBA × A/Sn spleen cells kept in H-2^b allosensitized and control cultures (all containing 10% FCS) were assayed for cytotoxic activity in the ⁵¹Cr-release assay against YAC-1, an A/Sn-derived lymphoma that is highly sensitive to murine NK cells. After a decline during the first 24 hr of culture, anti-YAC-1 activity reappeared with a peak on day 3, thereafter it decreased to baseline levels as the allospecific activity (against RBL-5 H-2^b lymphoma cells) peaked on day 5. This early, transient, anti-YAC-1 activity, termed “anomalous killing (AK)”, also occurred in control cultures, but was weaker and less reproducible there. With effectors from day 3 MLC, there was no cross-competition between the AK and the allospecific component in cold target inhibition experiments. The same effectors lysed human and murine semisyngeneic tumors in the same preferential order as fresh NK cells. The cells responsible for AK were nonadherent, nonphagocytic, of large size (sedimentation velocity >6 mm/hr), and sensitive to treatment with monoclonal anti-Thy 1.2 antibodies + complement. In contrast, NK activity of fresh spleen cells was mediated by small lymphocytes and was not decreased by anti-Thy 1.2 treatment in parallel experiments. The AK may represent further differentiation of NK cells or activation of a mature T cell subset. The mechanism of activation in relation to the in vivo immune response and in vitro induced NK-like activity in human systems is discussed.