International Immunology, Vol. 11, No. 8, 1275-1282,
August 1999
© 1999 Japanese Society for Immunology
Histidine-rich glycoprotein regulates the binding of monomeric IgG and immune complexes to monocytes
Division of Immunology and Cell Biology, The John Curtin School of Medical Research, and
1 Division of Biochemistry and Molecular Biology, School of Life Sciences, Faculty of Science, Australian National University, Canberra, ACT 0200, Australia
Correspondence to: C. R. Parish
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 Fc
RI 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 FcRI, 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 FcRI. 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 FcRI 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.
Keywords: FcRI, histidine-rich glycoprotein, IgG, immune complex, monocytes/macrophages
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