All four putative ligand-binding domains in megalin contain pathogenic epitopes capable of inducing passive Heymann nephritis.

Megalin (gp330) is the main target antigen involved in the induction of Heymann nephritis (HN), a rat model of human membranous nephropathy. Its large extracellular region contains four putative ligand-binding domains separated by spacer regions. Previously, it was reported that the second ligand-binding domain (LBD II) of megalin is involved in the pathogenesis of passive HN because it is capable of binding antibodies in vivo and initiating formation of immune deposits (ID). This study explores the possibility that pathogenic epitopes might also be present in the other putative ligand-binding domains. Recombinant fragments of ligand-binding domains (LBD) I through IV expressed in a baculovirus system were used to generate polyclonal domain-specific antibodies. Antibodies raised against each of the recombinant megalin fragments reacted preferentially with its respective antigen and with whole megalin by immunoblotting. Each of the antibodies also gave a characteristic brush-border staining for megalin by indirect immunofluorescence on rat kidney. When rats were injected with the domain-specific antibodies to test their ability to produce passive HN, glomerular ID were present in kidneys of all injected animals. The staining pattern in glomeruli of rats injected with LBD I, III, or IV was similar to that obtained with antibodies to LBD II. It is concluded that passive HN can be induced with antibodies against LBD I, III, and IV, as well as LBD II, and that each of the ligand-binding domains contains a pathogenic epitope. These findings provide further evidence for the multiple epitope model of HN.