C3 Plays a Role in Formation of Gd-IgA1-Containing Immune Complexes in IgAN

In IgA nephropathy (IgAN), circulating immune complexes (IC) contain IgA1 with some O-glycans that are deficient in galactose (galactose-deficient IgA1 [Gd-IgA1]) bound by IgG autoantibodies specific to Gd-IgA1. Other serum components, such as complement proteins, may also be associated with these IC. Some Gd-IgA1-containing IC deposit in the glomeruli, causing kidney injury.

In a poster presented at ASN Kidney Week 2024, Stacy D. Hall and colleagues examined the role complement C3 plays in the pathogenicity of Gd-IgA1-IgG IC. Their poster was titled Complement C3 Contributes to the Pathogenicity of Galactose-Deficient IgA1-Containing Immune Complexes in IgA Nephropathy.

The researchers utilized size-exclusion chromatography to isolate IC from the native or IgA1-depleted sera of IgAN patients. Using recombinant polymeric Gd-IgA1 and recombinant IgG autoantibodies specific for Gd-IgA1 in C3-depleted or C3-repleted serum, engineered IC were developed.

The research team determined the biological activity of the isolated IC or engineered IC based on their ability to prompt cellular proliferation of cultured primary human mesangial cells. Enzyme-linked immunosorbent assay was used to determine IgA, Gd-IgA1, IgA-IgG, and IgA-C3 complexes. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis immunoblotting under nonreducing conditions was used to determine the covalent association of C3 with IgA or IgG. Reducing conditions were used to determine C3 processing, namely the presence of alpha-chain or its fragments indicative of C3, C3b, and iC3b.

The analysis found that IC >700 kDa from IgAN sera increased cellular proliferation of quiescent mesangial cells twofold to fourfold. These IC included IgA, IgG, and C3, the latter of which was covalently associated with IgA and IgG. Molecular forms of C3 included C3, C3b, and iC3b. Removing IgA1 from the sera eliminated the stimulatory IC, and subsequent preparations were free of IgA, IgG, and C3.

To confirm C3’s role in Gd-IgA1-IgG IC formation, the researchers used engineered IC formed in C3-depleted or repleted serum. They determined that C3 was necessary for the formation of large-molecular-mass engineered IC that stimulated mesangial cells to proliferate. In addition, IgA and IgG established covalent complexes with C3 in C3-repleted serum. The engineered IC contained C3, C3b, and iC3b.

In summary, the researchers determined that C3 plays a key role in the formation of Gd-IgA1-containing IC with a nephritogenic capacity for IgAN.

Source: Hall SD, Gurganus G, Huang ZQ, et al. Complement C3 contributes to the pathogenicity of galactose-deficient IgA1-containing immune complexes in IgA nephropathy. FR-PO809. Abstract of a poster presented at the American Society of Nephrology Kidney Week 2024; October 25, 2024; San Diego, California.