The Combined Role of Galactose-Deficient IgA1 and Streptococcal IgA–Binding M Protein in Inducing IL-6 and C3 Secretion from Human Mesangial Cells: Implications for IgA Nephropathy

Abstract

IgA nephropathy is characterized by mesangial cell proliferation and extracellular matrix expansion associated with immune deposits consisting of galactose-deficient polymeric IgA1 and C3. We have previously shown that IgA-binding regions of streptococcal M proteins co-localize with IgA in mesangial immune deposits in patients with IgA nephropathy. In the current study, the IgA-binding M4 protein from group A streptococcus was found to bind to galactose-deficient polymeric IgA1 with higher affinity than to other forms of IgA1, as shown by surface plasmon resonance and solid-phase immunoassay. The M4 protein was demonstrated to bind to mesangial cells not via the IgA-binding region but rather via the C-terminal region, as demonstrated by flow cytometry. IgA1 enhanced binding of M4 to mesangial cells, but not vice versa. Co-stimulation of human mesangial cells with M4 and galactose-deficient polymeric IgA1 resulted in a significant increase in IL-6 secretion compared to each stimulant alone. Galactose-deficient polymeric IgA1 alone, but not M4, induced C3 secretion from the cells and co-stimulation enhanced this effect. In addition, co-stimulation enhanced mesangial cell proliferation compared to each stimulant alone. These results indicate that IgA-binding M4 protein binds preferentially to galactose-deficient polymeric IgA1 and that these proteins together induce excessive pro-inflammatory responses and proliferation of human mesangial cells. Thus, tissue deposition of streptococcal IgA-binding M proteins may contribute to the pathogenesis of IgA nephropathy.

Current Understanding of the Role of Complement in IgA Nephropathy

“…87 Furthermore, the autoantigen in IgAN, polymeric Gd-IgA1, can itself stimulate mesangial cells to produce and secrete C3. 88 Activation products C3a and C5a can induce cultured human mesangial cells to produce DAF, a potent membrane-bound regulator of the alternative pathway. 89 The expression of DAF and C3 mRNA by mesangial cells in IgAN was confirmed by in situ hybridization as relatively specific for the disease.…”

Biomarkers and targeted new therapies for IgA nephropathy

“…The presence of glomerular C4d deposition has been proposed to be an early biomarker for progressive IgAN [44]. It is of interest that mesangial cells exposed to Gd-IgA1 or aggregated IgA1 secrete C3 and matrix components via interaction with mesangial cell C3a receptors [45]. A recent GWAS [46] reported, among the genetic risk factors for IgAN, a single nucleotide polymorphism that showed a protective effect for the development of IgAN.…”

C4d deposits in IgA nephropathy: where does complement activation come from?

“…Several studies suggest that C3 bound to IgA confers a particular nephrotoxicity to the IgA1-containing immune material, leading to the hypothesis that this step is the crucial hit in the pathogenetic event cascade of IgAN [8,9,11]. A different efficacy of complement activation may modulate the glomerular inflammatory reaction and the final tissue damage, driving different renal outcomes.…”

“…Gd-IgA1 or aggregated IgA1 can bind to mesangial cells and directly trigger the secretion of C3, C3a formation and activation of C3a receptors on mesangial cells [11]. However, in addition to this in situ activation, a systemic complement activation has been suggested by several studies, including a negative correlation between circulating C3 and mesangial C3 deposition [12].…”

“…In the event of complement activated by IgA containing immune material on the surface of mesangial cells, a defective cell-surface complement regulation by locally acting factors, including CD55, CD46 and CD59, might be implicated [6,7]. These regulators act on activating surfaces of bacterial cell-wall glycans, but also on IgA macromolecular immune aggregates [11]. Without their inhibitory effect, an amplification loop develops in the presence of Factor D and properdin, leading to the accumulation of C3bBb, the alternative pathway C3 convertase.…”