Complement and cell-mediated lysis of haptenated erythrocytes sensitized with oligomers of rabbit igg antibody

Trinitrophenylated (TNP)‡ chicken erythrocytes were coated with various amounts of radiolabeled, affinity cross-linked oligomers of rabbit anti-TNP IgG. These cells were used as targets for lysis by complement or by several types of effector cells, normal splenocytes, cells of a macrophage line (P388D₁) or cells of two lymphoma lines (PL-1 and ABLS-5). With complement as mediator, lysis increased with the number and size of target-bound molecules. In contrast, antibody-dependent, cell-mediated cytolysis (ADCC) depended only on the number of sub-units bound, and was independent of oligomer size. Moreover, complement-mediated lysis required a threshold level of bound antibody before lysis occurred, whereas at low levels of sensitization ADCC increased linearly with increasing antibody. These results suggest that IgG clusters on the target cell are required for complement-mediated lysis but not for cell-mediated lysis. Furthermore, the data suggest that the crosslinking of neighboring Fc receptors on the surface of effector cells is not required for initiating ADCC. Non-immune IgM and small oligomers of non-immune IgG were used to block lysis mediated by each type of effector. ADCC was readily inhibited by small oligomers but not by IgM, while complement was inhibited by only larger IgG oligomers and by IgM. These results suggest that ADCC is not blocked by IgM, but is much more susceptible to inhibition by small IgG immune complexes than complement-mediated lysis.