Binding of soluble 125I-human C3b, the third component of complement, to specific receptors in human cultured B lymphoblastoids cells: characterization of a low affinity interaction.

The interaction of human C3b with human B and T lymphoblastoid cells was studied by using 125I-human C3b and 125I-human C3. No specific binding could be detected on the two T cell lines tested. However, on the two B cell lines, the specific binding of labeled C3b was rapid and a steady state was observed in 15 min at 37°C. Bound labeled C3b was dissociated rapidly by addition of excess unlabeled human C3b. Analysis of steady state data revealed a single order of binding sites with an affinity constant of 106 M−1 and about 5 × 104 binding sites per cell. When a Hill plot analysis was conducted, the Hill coefficient was 1.05, consistent with noncooper-ativity. Under the conditions of incubation, no change in TCA precipitability of labeled C3b or in its ability to bind to cells could be detected. The specific binding of labeled C3b on human B lymphoblastoid cells and the competitive effect of unlabeled C3b on this specific binding were unchanged in the absence or presence of protease inhibitor, whereas the specific binding of labeled C3 on human B lymphoblastoid cells and the competitive effect of unlabeled C3 on the specific binding of labeled C3b were abolished in the presence of protease inhibitor. This suggests that a protease activity takes place in the specific binding of C3 on B cell surface exposing an interaction site on the native C3 molecule that could react with C3b receptor with the same affinity as the trypsin-activated form C3b.