The human LT system. V. A comparison of the relative lytic effectiveness of various MW human LT classes on 51Cr-labeled allogeneic target cells in vitro: enhanced lysis by LT complexes associated with Ig-like receptor(s).

The present studies examine the in vitro cell-lytic capacity of various molecular weight (MW) human lymphotoxin (LT) classes obtained from lectin-activated normal or immune lymphocytes on allogeneic target cells. The findings reveal that the high-MW complex class of LT is up to 100 times more effective than the smaller MW LT forms (α, β, and γ) in causing lysis of various allogeneic cell types including lymphoid cells in vitro. Moreover, the data suggest that lectin-stimulated alloimmune cells (MLC sensitized) release complex LT forms in association with a specific antigen-binding receptor(s), and that these complexes are from 3 to 10 times more effective on the sensitizing target cell than complexes obtained from lectin-stimulated nonimmune cells. Positive evidence that complex-induced lysis involved LT was indicated by the finding that lysis was completely neutralized by incubation with heterologous antisera directed against a refined human α₂-LT subclass (anti-α₂) and partially neutralized with anti-human Fab₂′ serum. These findings support the concept that LT molecules may represent a system of related cell-lytic molecules. While the smaller MW forms are only weakly lytic by themselves, they can be assembled into highly lytic complexes which may be focused or directed by an antigen-binding receptor(s).