A novel heterophilic antibody interaction involves IgG4

Area of Science:

• Immunology
• Biochemistry

Background:

• Immunoglobulin G4 (IgG4) is linked to various pathologies like autoimmune pancreatitis and idiopathic membranous nephropathy.
• IgG4 possesses unique characteristics, including absent classical complement pathway activation and Fab-arm exchange.
• Previous research identified IgG4's rheumatoid factor (RF)-like activity via Fc-Fc interaction, termed novel RF (NRF), contrasting with classical RF (CRF)'s Fab-Fc interaction.

Purpose of the Study:

• To investigate heterophilic interactions of human IgG4 with animal immunoglobulins.
• To compare the binding mechanisms of novel RF (NRF) with classical RF (CRF).
• To explore the diagnostic implications and potential interferences related to IgG4's heterophilic binding.

Main Methods:

• Testing human IgG4 reactivity against various animal IgGs.
• Characterizing the binding site (Fab vs. Fc) of IgG4 to rabbit IgG.
• Correlating IgG4 binding with its concentration in biological samples.

Main Results:

• Human IgG4 demonstrated variable heterophilic reactivity, strongly binding to rabbit and mouse IgGs.
• IgG4 binding to rabbit IgG was mediated by the Fc region, not the Fab region, consistent with NRF.
• The observed binding correlated with IgG4 concentration, suggesting potential diagnostic utility and explaining assay interferences.

Conclusions:

• A novel heterophilic antibody interaction involving IgG4 has been defined.
• The universality of IgG4's unique Fc-Fc binding mechanism is established.
• This Fc-Fc interaction has implications for understanding IgG4-related diseases and improving biological assay accuracy.