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

Abstract

In order to resolve the mechanism of increase in number of Fc_γ-receptor bearing lymphocytes by interferon (IFN), the number of Fc_γ-receptor (Fc_γR) on human peripheral blood lymphocytes (human PBL) was calculated by competitive radioimmunoassay using ¹²⁵I-labeled IgG-Fc (Fc_γ) fragments.
Fc_γ-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 Fc_γ-fragments to Fc_γ-R was inhibited by a large excess of unlabeled Fc_γ-fragments, but not IgM protein. Scatchard plots in the binding assay were almost linear, suggesting that Fc_γR on the cells have similar affinity for the Fc_γ-fragments. In the analysis, it was also found that the average number of Fc_γR 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 Fc_γ-fragment for its specific receptor.
It means that increase in number of Fc_γR bearing lymphocytes by IFN was not due to enhancement in the affinity of Fc_γR, but an actual increase in number of Fc_γR.