MEGALIN (GP330) POSSESSES AN ANTIGENIC EPITOPE CAPABLE OF INDUCING PASSIVE HEYMANN NEPHRITIS INDEPENDENT OF THE NEPHRITOGENIC EPITOPE IN RECEPTOR-ASSOCIATED PROTEIN

The Heymann nephritis antigenic complex (HNAC) consists of two glycopr oteins, megalin (gp330), and the receptor-associated protein (RAP), HN AC is expressed on the surface of the glomerular epithelium where it p lays a primary role in the pathogenesis of Heymann nephritis (HN), Sev eral models were previously proposed describing how antibody binding e pitopes in HNAC may contribute to the initiation and progression of HN . Although these models suggest that nephritogenic epitopes capable of initiating HN are present in both megalin and RAP, the structural rel ationship between these epitopes has not been established, Previously a nephritogenic epitope was identified and characterized in RAP that i nitiates immune complex formation in HN. In this report, the immunolog ic relationship between nephritogenic epitopes in megalin and RAP were examined to determine whether these epitopes are immunologically dist inct or antigenically related, To this end, a polyclonal antibody to m egalin was generated that does not recognize RAP by immunoblotting or immunoprecipitation and whether this antibody is capable of inducing p assive HN was determined, It was found that antimegalin antibodies dev oid of RAP cross-reactivity induced the formation of subepithelial imm une deposits (passive HN) when injected into rats, Antibodies eluted f rom glomeruli of the injected rats recognized only megalin by immunobl otting a cortical extract and did not recognize a RAP fusion protein o r any other renal protein. In addition, the eluted antibodies immunopr ecipitated two proteolytic fragments of megalin (140 and 75 kd) identi fying a pathogenic epitope within a smaller fragment of megalin. These results indicate that at least one nephritogenic epitope is present i n megalin that can bind circulating antibodies and initiate immune com plex formation independent of the pathogenic epitope in RAP, The data presented here support the multiple epitope model, proposing that both megalin and RAP possess nephritogenic epitopes that are not antigenic ally related and can independently induce HN.