Histidine-rich glycoprotein regulates the binding of monomeric IgG and immune complexes to monocytes

Histidine-rich glycoprotein (HRG) is a relatively abundant plasma protein which we have shown previously inhibits the formation of insoluble immune complexes (IC). In this study we examined the ability of HRG to regulate the binding of monomeric IgG and IC to monocytes. Initial studies demonstrated that HRG interacts with FcγRI on the monocytic cell line THP1 and blocks the binding of monomeric IgG to these cells. However, despite totally blocking the binding of monomeric IgG to FcγRI, pre-incubation of THP1 cells with HRG had no effect on the binding of IC to these cells. In contrast, depending on the HRG:IgG molar ratio, pre-incubation of monomeric IgG with HRG resulted in either enhanced or reduced IgG binding to FcγRI. Similarly, under certain highly defined conditions, incorporation of HRG in IgG-containing IC potentiated the binding of IC to THP1 cells. The key conditions involved incorporating approximately equimolar concentrations of HRG and IgG in the IC, the IC being formed at a near equivalence antigen:antibody ratio and usually physiological concentration (20 μM) of Zn²⁺ being present. Collectively these observations indicate that HRG is an important regulator of IC uptake by monocytes. Thus HRG can interact with FcγRI on monocytes and block monomeric IgG binding, whereas when incorporated in IgG containing IC, HRG can enhance the uptake of IC by monocytes, probably via its heparan sulfate binding domain.