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Human thymocytes cultured for 5 days in interleukin 2 containing supernatants (IL 2 Sup) virtually become a population of mature T cells (T3+, HTA-) that acquires strong cytotoxic activity against NK-sensitive and NK-resistant target cells. The addition of different lectins to the cultures abrogated the expression of the cytotoxic activity and enhanced thymocyte proliferation. The modulation of this cytotoxic activity by lectins has the following properties: a) inhibition of cytotoxicity is related to the concentration of lectins added, but does not correlate with their mitogenic properties, because either strong mitogens such as PHA or weak mitogens such as wheat germ agglutinin (WGA) are both able to strongly decrease cytotoxicity; b) lectin presence is not required at the onset of the culture but is required during the last 24 hr of the 5-day incubation period; c) reversion of inhibition with full expression of cytotoxic activity can be obtained after removal of the lectin and subsequent culture in lectin-free conditions for at least an 18 to 24 hr period; d) lack of cytotoxicity is observed regardless of target cell specificities and cannot be overcome in a lectin-dependent cytotoxicity assay (LDCC); and e) abrogation of cytotoxicity is not restricted to thymocyte cultures because it can also be observed in peripheral lymphocytes. These results cannot be explained by a simple steric blockade, the overgrowth of a distinctive noncytotoxic lymphocyte population, autologous killing, or a failure in the recognition phase of the lytic event. Modulation by lectins of function-associated cell surface structures implicated in cytolysis is discussed as an alternative hypothesis that might account for the observed phenomenon. |
