Patients with IgA nephropathy have circulating immune complexes containing IgA, IgG, and C3. We have mixed human IgG and IgA1 and heated them to form mixed aggregate. On sucrose density gradients IgG aggregates were 11 to 19S whereas IgA aggregates were either 11S or greater than 19S. Mixed aggregates had both an 19 and 11 S peak. The isoelectric point of aggregates with only IgG was 7 to 9 and of only IgA 4.5 to 5.5. The isoelectric point of mixed aggregates decreased as the percent IgA increased. IgG aggregates mixed with normal human serum caused 30% C3 activation (20 min, 37 degrees C) whereas IgA aggregates causes no activation. There was a linear decrease in C3 activation as the percent IgA increased. Mixed aggregates that contained either radiolabeled IgG or IgA were mixed with normal human serum (1 h, 37 degrees C) and then solubilized, reduced, and separated by 10% SDS-PAGE. Heavy m.w. bands, consistent with covalent bonding of C3b and C3bi to Ig H chain were only seen in lanes with labeled IgG. This was confirmed by Western blot analysis. A human dimeric IgA1 myeloma protein with rheumatoid factor activity was also studied. It caused 15% alternative pathway C3 activation but did not fix C3 to its H chain. Binding of aggregates (+/- C3) to E was tested. Aggregates with IgG C3 bound but IgA (+/- C3) did not. Addition of greater than 10% IgA to an IgG-C3 aggregate inhibited E binding. We conclude that IgG in mixed aggregates is the site of C3 fixation. In contrast, IgA does not fix C3 but instead lowers the isoelectric point, increases the size and inhibits binding to E. These properties would inhibit clearance and promote mesangial deposition and local C activation.

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