Radioreceptor assay for IgG-Fc receptors on human peripheral blood lymphocytes: Studies of the mechanism by which interferon increases the number of lymphocytes bearing receptors for IgG

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In order to resolve the mechanism of increase in number of Fcγ-receptor bearing lymphocytes by interferon (IFN), the number of Fcγ-receptor (FcγR) on human peripheral blood lymphocytes (human PBL) was calculated by competitive radioimmunoassay using 125I-labeled IgG-Fc (Fcγ) fragments.Fcγ-fragments from a patient with γ-chain disease were radiolabeled with solid phase lactoperoxidase method of David (Specific activity was 3, 200 Ci/mmol).Binding of 125I-labeled Fcγ-fragments to Fcγ-R was inhibited by a large excess of unlabeled Fcγ-fragments, but not IgM protein. Scatchard plots in the binding assay were almost linear, suggesting that FcγR on the cells have similar affinity for the Fcγ-fragments. In the analysis, it was also found that the average number of FcγR per cell (_??_) was about 34, 000/cell, and the association constant (Ka) was 9.8×106/M.Human PBL, after incubation with human IFN (500 u/ml) at 37°C for 3 hr, contained 72, 000 receptors per cell, which were approximately twice of the control cells. However, IFN preparation of the lymphocytes had not an effect on the association constant of the reaction, namely not an effect on affinity of binding of Fcγ-fragment for its specific receptor.It means that increase in number of FcγR bearing lymphocytes by IFN was not due to enhancement in the affinity of FcγR, but an actual increase in number of FcγR.
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