Extraordinary antigenicity of the human Ro52 autoantigen.

Autoantibody levels to the SSA complex, composed of Ro52 and Ro60 proteins, are commonly measured in the diagnoses of Sjögren's Syndrome (SjS), as well as other rheumatological diseases. One of these proteins, Ro52, is an interferon-inducible member of the tripartite motif family bearing a RING motif functioning as an E3 ligase that ubiquitinates interferon regulatory factor 8 and other proteins. Using Luciferase Immunoprecipitation Systems (LIPS) we explored the antigenicity of Ro52 in detail. Analysis of antibody responses against Ro52 and 20 other established antigens revealed that Ro52 had the highest antibody titers and most likely represents one of the most immunogenic human proteins. While the antibody titers in many of the SjS patients were significantly and substantially higher than the controls, all healthy individuals had anti-Ro52 autoantibodies. N- and C-terminal fragments of Ro52 showed immunoreactivity in these serum samples, but the sums of these antibody titers were significantly lower than the antibody titers directed against the full-length Ro52. Antibody profiling of controls and SjS patients with three different N-terminal fragments of Ro52 revealed that the coiled-coil region was the most useful diagnostic (66% sensitivity), followed by the B-box (31% sensitivity), and then the RING-finger (24% sensitivity). The C-terminal region of Ro52, containing the B30.2 domain, showed higher antibody titers in SjS patients compared to controls and this region was responsible for the high level of Ro52 immunoreactivity in healthy individuals. Analysis of immunoreactivity to TRIM5, a Ro52-related protein, and the B30.2 domain from BTN1 and pyrin, failed to show significant antibody titers with the control or SjS patient serum. These results highlight the unusually high level of Ro52 antigenicity and demonstrate that autoantibodies are directed at both linear and conformational epitopes spanning the entire molecule.