Is Citrullinated Vimentin a Potential Antigenic Target for RA Specific Antibodies?

Citrullination is the process of post-translational protein modification in which argenine residues are enzymatically deiminated to citrulline, a non-standard amino acid. Antibodies directed against citrullinated proteins (anti-CCP antibodies) are found in patients with RA and predict severity of disease and radiographic progression (See EULAR highlights). Because these antibodies can be detected years to decades before the onset of symptomatic RA, a role in the pathogenesis of disease has been postulated, but unproved (See EULAR highlights). Many auto-antigenic targets for anti-CCP antibodies have been proposed, with indefinite results. Antibodies directed against Sa antigen are highly specific for RA. Here, Vossenaar et al (Arthritis Research & Therapy 2004:6(2); R142) seek to prove that Sa antigen is actually cirullinated vimentin, a protein component of the cellular cytoskeleton, and that anti-Sa antibodies comprise a portion of the anti-CCP antibody system.

Methods:

1. Anti-Sa and anti-CCP antibodies were assayed in serum samples from 61 patients with RA and 26 patients without RA attending rheumatology clinics at the Universite de Sherbrooke, Quebec.
2. Anti-Sa reference serum was immunoblotted against semipurified placental Sa antigen, human recombinant vimentin, and human recombinant vimentin that had been citrullinated in vitro.
3. Binding of antibodies to modified citrullines and anti-Sa reference serum was assayed against immunoprecipitants of a semipurified placental Sa preparation treated with anti-vimentin antibodies.

Results:

1. 46 of the 61 RA patients were anti-Sa antibody positive. None of the 26 patients without RA were anti-Sa antibody positive. 96% of the anti-Sa antibody positive RA patients had detectable anti-CCP antibodies. However, 60% of anti-Sa negative RA patients also had detectable anti-CCP antibodies.
2. Anti-Sa serum recognized both semipurified placental Sa antigen and citrullinated vimentin, but did not recognize unmodified vimentin- suggesting that Sa antigen is citrullinated vimentin.
3. Vimentin, immunoprecipitated with anti-vimentin antibodies from the semipurified placental Sa preparation, was recognized by both anti-Sa reference serum and by antibodies to modified citrullines suggesting, once more, that Sa antigen is citrullinated vimentin.

Conclusions: The antigenic target of the antibodies detected in RA patients positive for anti-Sa antibodies is citrullinated vimentin. These antibodies make up a portion, but not all, of the anti-CCP antibody system.

Editorial Comments: This series of experiments efficiently identifies citrullinated vimentin as a potential antigenic target for RA specific antibodies. Other citrullinated antigens that have been identified in the anti-CCP antibody system include fillaggrin, another structural fiber found in epidermal tissues, and fibrin. The identification of these antigenic targets raises as many questions as they answer 1) Do anti-CCP antibodies play a pathogenic role in the clinical manifestations of RA, or are they merely a marker of disease? 2) If so, how is the regulation of citrullination of these antigens different in RA to contribute to the development and/or progression of disease? Recent investigation suggests that the interaction of citrullinated peptides with the RA shared epitope may contribute to abnormal peptide presentation to auto-reactive T-cells. Why certain cytoskeletal elements are implicated is another question raised by these studies. One proposed mechanism involves accelerated citrullination of vimentin residues during macrophage apoptosis. Further work is currently underway to determine the links between these processes and the pathogenic processes that make up RA.