Serum factors influencing antibody-directed cell-mediated cytotoxicity (ADCC) and their effects on the detection of immune complexes by inhibition of ADCC.

An imporved assay system, using inhibition of ADCC, for the detection of 0.6 microgram/ml. of aggregated IgG in medium is described. Under these conditions, normal human serum is extremely inhibitory, this effect being attributed mainly to the weak binding of monomeric IgG. Enhancement of ADCC by albumin was also observed. Using this assay system without further modification, immune complexes may be detected in gel-fractionated sera by the distribution of the inhibitory material relative to the IgG peak. However, for routine use in the assay of untreated sera, a modification of the method is required. Preincubation of the spleen cells with the serum in the presence of EDTA, followed by a washing stage, reduces the background inhibition by normal sera to a low level. Effects of serum factors in producing variable amounts of background inhibition or stimulation are also avoided. The modified assay is sensitive to 6 microgram/ml of aggregated IgG in neat serum.