Distinct Epitopes for Antia€“Glomerular Basement Membrane Alport Alloantibodies and Goodpasture Autoantibodies within the Noncollagenous Domain of ?±3(IV) Collagen: A Janus-Faced Antigen

摘要

Alport posttransplantation antia€“glomerular basement membrane (GBM) nephritis is mediated by alloantibodies against the noncollagenous (NC1) domains of the ?±3?±4?±5(IV) collagen network, which is present in the GBM of the allograft but absent from Alport kidneys. The specificity of kidney-bound anti-GBM alloantibodies from a patient who had autosomal recessive Alport syndrome (ARAS) and developed posttransplantation nephritis was compared with that of Goodpasture autoantibodies from patients with autoimmune anti-GBM disease. Allograft-eluted alloantibodies reacted specifically with ?±3?±4?±5 NC1 hexamers, targeting their ?±3NC1 and ?±4NC1 subunits, and recognized a noncontiguous alloepitope formed jointly by the EA and EB regions of ?±3NC1 domain. In contrast, human Goodpasture autoantibodies recognized the separate EA and EB autoepitopes of ?±3NC1 but not the composite alloepitope. Molecular modeling of ?±3NC1 revealed that the alloepitope is more accessible within the NC1 hexamers than the partially sequestered Goodpasture autoepitopes. Overall, the specificity of alloantibodies indicated a selective lack of immune tolerance toward the ?±3 and ?±4(IV) collagen chains not expressed in patients with ARAS. Using COL4A3 knockout mice, a model of ARAS, it was shown further that acid-dissociated rather than native ?±3?±4?±5 NC1 hexamers elicited murine anti-GBM antibodies most closely resembling human ARAS alloantibodies. In contrast, ?±3NC1 monomers elicited Goodpasture-like murine antibodies, targeting the EA and EB autoepitopes. Thus, the identity of ?±3NC1 epitopes targeted by anti-GBM antibodies is strongly influenced by the molecular organization of the immunogen. These findings suggest that different isoforms of ?±3(IV) collagen may be implicated in the pathogenesis of ARAS posttransplantation anti-GBM nephritis and Goodpasture disease.