Anti-Mi2 autoantibodies target the PHD fingers of SP140L and TIF1γ, while anti-TIF1γ autoantibodies primarily recognize epitopes outside the PHD region of TIF1γ.

ObjectivesPlant homeodomain (PHD) fingers are present in many chromatin-binding proteins. We recently discovered that anti-Mi2 autoantibodies recognize PHD fingers in Mi2 and AIRE. The purpose of this study was to characterize anti-Mi2 autoantibody recognition of PHD fingers in SP140L and TIF1{gamma} as well as to explore recognition of TIF1{gamma} by both anti-TIF1{gamma} and anti-Mi2 autoantibodies. MethodsLuciferase immunoprecipitation system (LIPS) assays were performed to detect autoantibodies against full-length and protein fragments of SP140L and TIF1{gamma} in serum samples from myositis patients, disease controls, and healthy controls. ResultsAnti-Mi2 autoantibodies recognized SP140L. When a 49 amino acid fragment of the PHD finger of SP140L was used as the target, the specificity for selectively detecting anti-Mi2 autoantibodies increased. Additionally, anti-Mi2 autoantibodies weakly bound TIF1{gamma} compared to anti-TIF1{gamma} autoantibodies. Excluding the TIF1{gamma} PHD finger from the TIF1{gamma} target autoantigen eliminated cross-reactivity with anti-Mi2 autoantibodies, confirming that anti-Mi2 autoantibodies specifically target the PHD finger of TIF1{gamma}. Switching two amino acids in the TIF1{gamma} PHD finger to resemble those in AIRE markedly enhanced anti-Mi2 autoantibody immunoreactivity. Anti-TIF1{gamma} autoantibodies primarily recognized the N-terminal fragment outside of the PHD finger, indicating this region contains the immunodominant epitopes. ConclusionsAnti-Mi2 autoantibodies recognize the PHD fingers of SP140L and TIF1{gamma}. TIF1{gamma} is recognized by two different myositis-specific autoantibodies: anti-Mi2 autoantibodies bind the C-terminal PHD domain and anti-TIF1{gamma} autoantibodies predominantly bind the N-terminal region. Removing the PHD finger from the anti-TIF1{gamma} target autoantigen can improve the specificity of anti-TIF1{gamma} autoantibody assays by reducing cross-reactivity with anti-Mi2 autoantibodies.