Gene copy number variation (CNV) and single nucleotide polymorphisms (SNPs) count as important sources for interindividual differences, including differential responsiveness to infection or predisposition to autoimmune disease as a result of unbalanced immunity. By developing an FCGR-specific multiplex ligationdependent probe amplification assay, we were able to study a notoriously complex and highly homologous region in the hu-
Human Fcγ receptors (FcγRs) are glycoproteins that bind the Fc region of IgG. The genes encoding the low-affinity FcγRs are located on chromosome 1q23-24. Beside single nucleotide polymorphisms (SNPs), gene copy number variation (CNV) is now being recognized as an important indicator for inter-individual differences. Recent studies on identifying CNV in the human genome suggest large areas at chromosome 1q23-24 to be involved, and CNV in this region has been associated with manifestations of systemic autoimmune disease. To study both SNPs and CNV of the low-affinity FcγRs in one assay, we have developed a Multiplex Ligation-dependent Probe Amplification (MLPA) assay. A novel CNV for FCGR3A was observed. Similar to FCGR3B and FCGR2C, a gene-dosage effect of FCGR3A was found, that seemed to correlate nicely with the FcγRIIIa expression on NK cells. Next, we delineated the approximate boundaries of CNV at the FCGR locus. Variation in co-segregation of neighboring FCGR genes was limited to four variants, with patterns of Mendelian inheritance. No CNV of the FCGR2A and FCGR2B genes was observed in over 600 individuals. In conclusion, we report a novel CNV of the FCGR3A gene that correlates with FcγRIIIa expression and function on NK cells. Only FCGR3A, FCGR2C and FCGR3B show CNV, in contrast to FCGR2A and FCGR2B.
Intravenous Ig preparations (IVIg), originally developed as a substitution therapy for patients with low plasma IgG, are nowadays frequently used in the treatment of various immune diseases. However, the mechanism of action of IVIg in these diseases remains elusive and is often referred to as “immunomodulatory.” We hypothesized that monomeric IgG may act as a low-affinity FcγR antagonist and sought experimental evidence for this hypothesis. Human neutrophils as well FcγRIIa-transfected IIA1.6 cells were used as FcγR-positive cells and aggregated IgG (aIgG) or stable dimeric IgG as FcγR-specific agonists for these cells. We found that monomeric IgG purified from IVIg at concentrations similar to that of IgG in plasma, diminished the binding of stable dimeric IgG to FcγRIIa transfectants, reduced aIgG-induced influx of Ca2+ ions into the cytosol of neutrophils, and attenuated the aIgG-induced release of elastase. Notably, monomeric IgG by itself did not elicit these responses, nor did it affect these processes in response to fMLP. Absorption of IgG from normal plasma revealed that plasma IgG exerted similar effects as monomeric IgG in IVIg. In addition, adding monomeric IgG to blood of healthy volunteers showed a dose-dependent decrease of aIgG-induced elastase release. Finally, we observed decreased aIgG-induced polymorphonuclear neutrophil responses in two hypogammaglobulinemic patients upon treatment with IVIg. We conclude that monomeric IgG at physiological levels acts as a low-affinity FcγR antagonist. Moreover, FcγR antagonism constitutes an immunomodulatory effect of IVIg.
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