Virus-dependent cellular cytotoxicity in vitro. Mechanisms of induction and effector cell characterization.

When human peripheral blood lymphocytes were incubated with 51Cr-labelled tissue culture cells (T24 bladder carcinoma cells or Chang liver cells), their natural cytotoxicity (NK) usually stopped after 8 h of incubation. The 51Cr release induced by lymphocytes treated with small amounts of live or ultraviolet-inactivated mumps virus was strongly enhanced and lasted longer. When the lymphocytes were fractionated by Percoll gradient centrifugation, the highest NK activity was found in the low-density fraction enriched in large granular lymphocytes, whereas that of the T-cell-enriched high-density fractions was low. In contrast, the virus-dependent cellular cytotoxic (VDCC) activity was more evenly distributed between these fractions. However, there was a difference between the target cells in that the T24 cells were more susceptible to the cytotoxicity of lymphocytes in the high-density fractions than the Chang cells. Studies of Percoll fractions in the single-cell agarose assay showed that virus treatment increased the proportion of both target binding cells and killer cells in all fractions. Moreover, in the high-density fractions the increase in the number of killer cells was greater than that in binding cells, suggesting that the enhanced target cell killing induced by the virions reflected both increased binding and effector cell activation. Surface marker analysis of unfractionated lymphocytes indicated that the number of T3+ effector cells was greater than that of the HNK-1+ effector cells, regardless of whether the lymphocytes were treated with virus or not. However, for both NK and VDCC, the T3 to HNK-1 distribution ratio on the effector cells was 5-8:1 for T24 and 2:1 for Chang. Taken together, the results indicate that both NK and VDCC effector cells are phenotypically heterogeneous and that the target cells may play an active role in the recruitment of those effector cells that are most efficient in that system. The enhancement of lymphocyte cytotoxicity primarily reflects effector cell recruitment.