Objective: To clarify the possible roles of protein-bound homocitrulline in causing an antibody response to citrulline and as a confounding factor in citrulline detection assays.
Methods: Native, carbamoylated, and citrullinated albumins were used for testing the specificity of commercial antibodies against modified citrulline by Western blot. Rabbits were immunized with human albumin and/or bone type I collagen, both of which were treated with cyanate to produce homocitrulline, or with citrullinated synthetic telopeptide of type I collagen. These antisera were tested for binding to cyclic citrullinated peptide 2 (CCP-2), mutated citrullinated vimentin, and type I or II collagen telopeptides containing either citrulline or homocitrulline.
Results: Commercial antibodies that had been considered to be specific for chemically modified citrulline were found to recognize both homocitrulline- and citrulline-containing albumins. The rabbits immunized with proteins containing homocitrulline produced high-affinity antibodies against CCP-2 and, to a lesser extent, against mutated citrullinated vimentin. The antisera also bound homocitrulline-containing collagen telopeptides and, less strongly, citrulline-containing telopeptides.
Conclusion: Our findings demonstrate that homocitrulline, which is a structural analog of citrulline (longer by 1 carbon, and readily formed in the body), can be involved in raising citrulline-recognizing antibodies in experimental animals and can cause false-positive reactions in assays for citrulline.
Copyright © 2010 by the American College of Rheumatology.