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 9.8×10
6/M.
Human PBL, after incubation with human IFN (500 u/m
l) 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.
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