Structural differences between the human and mouse 52‐kD Ro autoantigens associated with poorly conserved autoantibody activity across species

Anti‐nuclear autoantibodies found in human autoimmune diseases frequently cross‐react with homologous autoantigens in distant species, supporting the notion that autoantibodies target conserved functional domains. However, the 52‐kD Ro(SS‐A) protein is an exception, in that human autoantibodies are not known to recognize any equivalent antigen in the cells of rodents and other non‐primate species. To understand this lack of cross‐reactivity we have isolated cDNAs encoding the mouse 52‐kD Ro molecule. The cDNA encoding mouse 52‐kD Ro revealed an open reading frame of 470 amino acids, with 70% sequence identity to the human 52‐kD Ro antigen. The putative leucine‐zipper and zinc‐finger motifs present in human Ro52 were conserved in the mouse protein. Recombinant mouse 52‐kD Ro protein reacted with human autoantibodies by ELISA and immunoblot, but with approximately 10‐fold lower reactivity than recombinant human 52‐kD Ro protein under the same conditions. Detection of both human and mouse 52‐kD Ro by immunoblot was dependent on antigen concentration which was limiting in the cell equivalents generally used in immunoblot assays. Differential chaotropic disruption of antibody binding suggested a lower avidity of human autoantibody binding to the mouse 52‐kD Ro protein compared with the human antigen. Thus the poor reactivity of native mouse 52‐kD Ro with human autoantibodies is associated with species divergence diffusely distributed throughout the primary structure of the 52‐kD Ro molecule.