Induction of anti-GBM nephritis in rats by recombinant α3(IV)NC1 and α4(IV)NC1 of type IV collagen. The capability of the noncollagenous (NC1) domains of the six α chains of human type IV collagen to induce anti-glomerular basement membrane (GBM) nephritis in WKY rats was determined. This was accomplished by using recombinant technology to express the six NC1 domains in mammalian 293 cells and to purify the proteins using an anti-Flag affinity column. All rats injected with α3(IV)NC1 and α4(IV)NC1 developed proteinuria and hematuria. Rats injected with α5(IV)NC1 developed mild hematuria, whereas rats injected with the α1(IV)NC1, α2(IV)NC1 and α6(IV)NC1 domains developed neither proteinuria nor hematuria. The renal lesions induced by α3(IV)NC1 and α4(IV)NC1 domains were characteristic of those in patients with anti-GBM nephritis and Goodpasture syndrome. The experimental nephritis is mediated by anti-basement membrane antibodies that are targeted to α3(IV)NC1 and α4(IV)NC1 domains and which bind to the glomerular basement membrane. The uniqueness of the α3(IV)NC1 and α4(IV)NC1 domains, among the six NC1 domains, to induce severe anti-GBM disease may relate to the accessibility of epitopes in the GBM for binding of antibody. The pathogenicity of the α4(IV)NC1 antibodies establishes a conundrum because the pathogenic antibodies in patients are not targeted to the α4(IV)NC1, but are targeted to the α3(IV)NC1 domain in anti-GBM nephritis and to the α3(IV)NC1 and α5(IV)NC1 domains in Alport post-transplant anti-GBM nephritis.