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Anti-Ro/SSA antibodies (ab) are directed to two proteins, Ro60 and Ro52. While anti-Ro60 aab are predominantly found in patients with SLE or primary Sjögren's syndrome, anti-Ro52 ab can be detected also in sera of healthy subjects. These antibody escape detection by ELISA or immunoblotting and can be found only in immunoprecipitation assays. Recently, an unexpected interaction of Ro52 with IgG has been reported which appeared to occur independently of the antigen binding site. To investigate this unusual interaction, serum IgG was covalently bound to protein A sepharose (PAS) or an anti-IgG argarose column was used and incubated with HeLa cytoplasmic extracts and various Ig fractions as competitors (IgG1-4, IgM, Fab, F(ab)2, Fc, pFc', Fc', C1q). Proteins bound to immobilized IgG were detected by immunoblotting using anti-Ro/La positive patients sera and a monospecific anti-Ro52 antibody for identification of bound proteins.
In these assays Ro52 only was bound to the column-IgG complex. Binding of Ro52 was inhibited by total IgG and Fc, but not Fab, F(ab)2 or C1q.
In another experiment IgG anti-IgM was complexed with PAS and Ro52 binding was demonstrated. This binding was partly inhibited by IgM, suggesting an unspecific interaction of Ro52 with the IgG-Fc.
This antigen/antibody reaction is seen in all IgG subclasses binding to the PAS, namely IgG1, IgG2 and IgG4, with IgG2 reacting very strongly with the Ro52.
In conclusion, these data show an interaction of Ro52 with all types of IgG of healthy persons. This interaction appears therefore to be substantially different from normal antigen-antibody interaction. The biological function of this interaction is unclear.