Direct binding of antibodies to fixed glomerular antigens is the principal mechanism of subepithelial immune deposit formation in experimental membranous nephropathy induced by injection of heterologous antibody to rat Fx1A (passive Heymann nephritis, PHN). In actively induced Heymann nephritis (autologous immune complex nephritis, AICN) the role of this in situ mechanism as opposed to circulating immune complex deposition has been debated. To examine this question we studied the deposit-forming characteristic of rat IgG eluted from AICN kidneys and compared the results to those obtained with heterologous (sheep) anti-Fx1A IgG. We made the following observations: (1) AICN eluate IgG binds in vitro to proximal tubular brush borders and capillary walls of isolated glomeruli by immunofluorescence. Furthermore, 125I AICN eluate IgG binds specifically to isolated glomeruli (AICN eluate > 8 x control IgG). (2) After a single intravenous injection of 125I AICN eluate IgG, antibody continued to accumulate slowly in glomeruli over 48 hr. This pattern of antibody uptake closely resembles that previously described by us as unique for heterologous anti-TxIA. (3) Heterologous anti-Fx1A appeared to competitively inhibit the glomerular binding of AICN eluate IgG. When glomeruli containing increasing amounts of in vivo deposited 125I heterologous antibody were isolated and incubated with a fixed quantity of 131I AICN eluate IgG, an inverse relationship between the binding of the two antibodies was observed (r = -0.573, P <0.01). (4) Specific glomerular binding of AICN eluate IgG was observed in paired-label studies using the isolated perfused kidney (IPK). This confirms previous experiments in the IPK using heterologous antisera and demonstrates the capacity of AICN eluate IgG to form glomerular deposits in situ under relatively physiological conditions. We conclude that antibodies eluted from AICN glomeruli demonstrate deposit-forming characteristics very similar to those of heterologous anti-Fx1A antibodies in vitro and in vivo. Both antibodies produce subepithelial deposits in vivo and appear to bind to similar antigenic sites in the glomerular capillary wall. These results support the contention that in AICN, subepithelial immune deposit formation probably occurs by the same in situ mechanism previously demonstrated in PHN.
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