Definition of a "trigger" stage in the NK cytolytic reaction sequence by a monoclonal antibody to the glycoprotein T-200.

The monoclonal antibody 13.1 recognizes an epitope on the T-200 glycoprotein and blocks natural killer (NK) lysis of the erythroleukemia target K562, but not of the acute lymphoblastic leukemia T cell target Molt-4. The inhibitory effect is at the killer cell level and not the target cell level, which suggests that 13.1 may react with a receptor on NK cells. This hypothesis was tested in assays to delineate precisely where in the NK cytolytic reaction sequence 13.1 interferes with lysis; 13.1 did not block initial NK-target cell interaction as measured in a target binding cell assay. With the use of a Ca++ pulse technique, 13.1 did not block any events occurring during Ca++-dependent programming. If the antibody was added after conjugate formation but before the addition of CaCl2 to initiate programming, however, full inhibition of NK lysis occurred. Therefore, 13.1 antibody defines a distinct stage in the NK reaction sequence that links target binding to the initiation of calcium-dependent programming events. NK cell binding alone is not sufficient to trigger lytic events, and the presence of a second structure or a distinct portion on a single structure is required to trigger lysis. We show that 13.1 blocks the ability of K562 to inhibit NK killing of Molt-4 in a cold target inhibition assay. Therefore, despite the fact that 13.1 does not disrupt conjugate formation, NK specificity may exist at a post-binding site rather than at the initial NK-target binding interaction. Our data suggest that the T-200 glycoprotein on NK cells triggers the initiation of the lytic events.