Reactions of immunoglobulin G-binding ligands with platelets and platelet-associated immunoglobulin G

Immunoglobulin G (IgG) bound to platelets is usually detected by one of two general methods: binding of labeled anti-IgG or consumption of anti-IgG. The latter method gives, in general, values 5-10-fold greater than the former under the same conditions. To investigate these discrepancies, we have compared the detection of platelet-bound IgG by a labeled anti-IgG binding assay and by a quantitative antiglobulin consumption test using the same antibodies. The interaction of 125I-labeled monoclonal anti-IgG or polyclonal anti-IgG with washed and IgG-coated platelets was studied. The binding of these ligands to washed normal platelets was largely (50-80%) nonspecific; the binding was not saturable and was only partially inhibitable by excess unlabeled anti-IgG. The binding of anti-IgG to platelets coated with anti-PIA1, a platelet-specific IgG antibody, appeared to be saturable and inhibitable; the dissociation constant (KD) of this IgG-anti-IgG reaction was 4.9 X 10(-9) for monoclonal and 1.4 X 10(-7) for polyclonal anti-IgG. The ratio of sites present on the membrane (determined by 131I-labeled anti-PIA1) to the number of binding sites for anti-IgG determined by Scatchard analysis was 0.53 for monoclonal anti-IgG and 1.3 for polyclonal anti-IgG. The binding of monoclonal anti-IgG to platelet-bound immune complexes or IgG aggregates appeared to be complex. 131I-Labeled IgG was affixed to platelets and was detected by three tests: direct binding of radiolabeled monoclonal anti-IgG and quantitative antiglobulin consumption (QAC) tests, which were quantitated either by measuring directly the amount of radiolabeled anti-IgG consumed from fluid phase (direct QAC), or indirectly by reference to a calibration curve relating the consumption of anti-IgG by known amounts of fluid-phase, non-immune IgG (indirect QAC). The amount of platelet-bound IgG detected by the direct binding of 125I-labeled monoclonal anti-IgG and by the direct QAC approximated that known to be bound to the platelet. The results of the indirect QAC test were 10-fold greater. The discrepancy appears to be due to the fact that there is a difference between the IgG-anti-IgG interaction when IgG is bound to a platelet and when it is in solution or bound to plastic nonspecifically or specifically. This difference results in a falsely high value for platelet-bound IgG when fluid-phase or plastic-bound IgG is used to calibrate the antiglobulin consumption test.