Antibody-dependent lymphocytotoxicity (ADL) was studied with human peripheral blood lymphocytes as effector cells and P815 mouse mastocytoma cells, sensitized with rabbit IgG antibodies, as target cells. Enzymesubstrate-like kinetics were used to describe ADL inhibition induced by two types of inhibitors. Human IgG, both native and heat-aggregated, proved to be a competitive inhibitor of ADL at the target cell level. Human peripheral blood monocytes inhibited ADL in an apparently irreversible fashion, without appreciable evidence for competition. The data obtained provide strong support for the validity of an enzyme-substrate-like mechanism of ADL. Moreover, our results indicate that in order to measure the lytic capacity properly, cell populations, well defined as to their composition, should be used.

In the second part of the study, it was investigated whether the two types of cells capable of lysing sensitized target cells, i.e., null cells and T cells, differed with respect to their affinity for the target cells. Application of enzyme-like kinetics revealed considerable differences in maximal killing rate between the two subsets of K cells. However, the parameter related to the affinity toward the target cells was found to be of the same magnitude for the two types of effector cells.

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