Factors affecting IgG4-mediated complement activation

Abstract

Of the four human immunoglobulin G (IgG) subclasses, IgG4 is considered the least inflammatory, in part because it poorly activates the complement system. Regardless, in IgG4 related disease (IgG4-RD) and in autoimmune disorders with high levels of IgG4 autoantibodies, the presence of these antibodies has been linked to consumption and deposition of complement components. This apparent paradox suggests that conditions may exist, potentially reminiscent of in vivo deposits, that allow for complement activation by IgG4. Furthermore, it is currently unclear how variable glycosylation and Fab arm exchange may influence the ability of IgG4 to activate complement. Here, we used well-defined, glyco-engineered monoclonal preparations of IgG4 and determined their ability to activate complement in a controlled system. We show that IgG4 can activate complement only at high antigen and antibody concentrations, via the classical pathway. Moreover, elevated or reduced Fc galactosylation enhanced or diminished complement activation, respectively, with no apparent contribution from the lectin pathway. Fab glycans slightly reduced complement activation. Lastly, we show that bispecific, monovalent IgG4 resulting from Fab arm exchange is a less potent activator of complement than monospecific IgG4. Taken together, these results imply that involvement of IgG4-mediated complement activation in pathology is possible but unlikely.

The unique properties of IgG4 and its roles in health and disease

“…This is in contrast to increased levels of galactosylation promoting IgG1 hexamerization and complement activation by the classical route 37 , and other studies suggest a pathogenic role of IgG4 autoantibodies to PLA2R1 independent of complement 38 . Complement activation could be shown in vitro for glyco-engineered recombinant IgG4 antibodies, but only at high antigen density and high antibody concentration, and no contribution of the lectin pathway was observed 39 .…”

“…2 ). Furthermore, Fab-arm exchange seems to further decrease the limited potential of IgG4 for complement activation 39 . Therefore, Fab-arm exchange together with the overall reduced ability of IgG4 to activate Fcγ receptors and complement means that IgG4 is often regarded as a natural type of ‘blocking’ antibody — a high-affinity monovalent binder with limited potential to induce inflammatory responses 4 .…”

“…This eliminates the potential of the antibody to cross-link antigen, further minimizing the immune-activating potential of IgG4. Complement activation also seems to be further impaired by Fab-arm exchange 39 . Fc, fragment crystallizable.…”

Optimized Enzyme-Linked Immunosorbent Assay for Anti-PEG Antibody Detection in Healthy Donors and Patients Treated with PEGylated Liposomal Doxorubicin

“…signal for IgG and IgM in every plate for normalization. Furthermore, the serum contains different clones and isoforms of anti-PEG antibodies with varying degrees of glycosylation and various affinities and specificities toward different PEGylation types, densities, architectures, and different complement fixation efficiencies. ,− This raises the question of whether the absolute quantification of antibody concentration using standards is necessary. Notably, taking an example from measuring antiphospholipid antibodies in the antiphospholipid syndrome, there is a known variability between laboratories despite the availability of commercial kits and standard chimeric antiphospholipid antibodies. , The current guidelines for APL antibodies as a diagnostic biomarker consider >99th percentile of historical O.D.…”

A public resource of single cell transcriptomes and multiscale networks from persons with and without Alzheimer’s disease

“…IgG4 is one of four main IgG immunoglobulin subclasses, and the least prevalent among healthy adults 69 (approximately 5% of IgG) with important structural differences driving functional differences including reduced triggering of antibody-dependent cell-mediated phagocytosis and complement activation 70 . Although IgG4 is generally regarded as exerting a relatively anti-inflammatory effect relative to other IgG subclasses, it is still capable of driving phagocytosis of opsonized microbial antigens 71 and mediating complement activation at higher antigen and antibody concentrations 72 . Despite the emergence of IGHG4 as the most differentially abundant protein, we did also observe a significant upregulation of IGHG3 and IGHG2 (which would be expected to induce more aggressive antipathogen responses), which is overall suggestive of a complex, adaptive immune response present in the transverse colon of subjects with CD83(+) microglia in the SFG.…”