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Relationship between mesangial deposition of IgA1 and IgA2 subclasses and complement activation in IgA glomerulonephritis

Our immunohistological study demonstrated that mesangial IgAl deposition activated the alternative pathway in IgA glomerulonephritis (IgAGN), and mesangial IgA2 deposition was associated with the activation of the lectin pathway.' The present study aims to reconfirm the relationship between IgA2 subclass and the activation of the lectin pathway and to elucidate whether the association is transient or continuous. Immunohistological staining was performed on frozen renal specimens from 26 patients with IgAGN, who received serial biopsies, using polyclonal antibodies of IgG, IgA, IgM, Clq, C ~ C , C4, factor B and monoclonal antibodies of IgAl, IgA2, mannose-binding lectin (MBL) and MBL-associated serine protease1 (MASP1). On the first biopsied sections, mesangial deposition of IgA1, IgA2, C ~ C , C4, factor B and MBL/MASP-1 was found in 11 of 26 patients, and IgA2 and MBL/MASP1 were colocalized in the mesangium. Mesangial deposition of IgAl alone as well as factor B was evident in the remaining 15 patients, and mesangial deposition of C3c in 13 of these 15 patients without C4 and MBL/MASP1. In 11 patients with both IgAl and IgA2 deposits in the first biopsy, mesangial deposition of IgA1, IgA2, C ~ C , factor B and MBL/MASP-1 was detected in all patients in the second biopsy, but C4 in nine. Four of 15 patients with IgAl deposits alone in the first biopsy showed mesangial deposition of IgA1, IgA2, C4 and MBL/MASP1 in the second biopsy. In the remaining 1 1 patients, immunohistological findings were not different in between the first and the second biopsy. There was no correlation in the clinical outcome and pathological indicators between IgA2-positive and IgA2-negative patients. In conclusion, the relationship between mesangial IgAl and IgA2 deposition and the activation of the complement pathway is reconfirmed and the association is continuous. In patients with mesangial deposition of both IgAl and IgA2, IgA2 is subsequently deposited in the mesangium after IgAl is initially deposited.