Glycosylation of circulating IgA in patients with IgA nephropathy modulates proliferation and apoptosis of mesangial cells

Abnormalities in circulating IgA 1 have been demonstrated in patients with IgA nephropathy (IgAN). This study addresses the question of the functional significance of this alteration in creating mesangial injury. Biologic eff ects of selected IgA glycoforms isolated from serum of IgAN patients and co ntrols and in vitro deglycosylated normal IgA were tested on cultured human mesangial cells (MC). IgA glycoforms, ranging from 250 to 500 kD molecular weight, were isolated by lectin affinity chromatography followed by HPLC. IgA and IgG content was measured by enzyme-linked immunosorbent assay. HPLC fractions were incubated with MC to evaluate proliferation and apoptosis r ates and nitric oxide synthesis. Moreover, MC were conditioned with in vitr o desialylated and degalactosylated normal IgA. Patients with IgAN displaye d increased levels of IgA glycoforms exposing sialic acid in alpha2.6 linka ge with N-acetvlgalactosamine (Neu5Ac alpha2,6GalNAc) (P<0.02) and GalNAc ( P<0.05), indicating truncation of O-linked glycans of IgAl. Moreover, IgA g lycoforms with increased exposure of mannose were observed (P<0.03), sugges ting a defective N-linked glycosylation. No modification in IgG glycosylati on was detected. When incubated with MC. the IgA glycoforms isolated from p atients with increased exposure of GalNAc, Neu5Ac<alpha>2,6GalNAc, or manno se, significantly depressed the proliferation and increased the apoptotic r ate and nitric oxide synthesis activity of cultured MC, in comparison with fractions isolated from controls. Similarly, in vitro desialylated and dega lactosylated IgAs significantly depressed the proliferation and enhanced th e apoptosis rates of MC. In conclusion, a significant modulation of several human MC functions exerted by serum IgA with increased exposure of GalNAc, Neu5Ac alpha2,6GalNAc, and mannose residues isolated from IgAN patients is reported for the first time.