Antibodies in rheumatoid arthritis react specifically with the glycine alanine repeat sequence of Epstein-Barr nuclear antigen-1.

Antibodies to rheumatoid arthritis nuclear antigen (RANA) are four- to sixfold increased in sera from patients with rheumatoid arthritis (RA), whereas levels of antibodies to other EBV encoded antigens are slightly elevated or normal. We have demonstrated that the major epitopes recognised by anti-RANA antibodies are represented by a synthetic peptide, P62, corresponding to part of the internal repeat sequence which contains only the amino acids glycine and alanine. In an enzyme-linked immunosorbent assay, anti-P62 antibodies in rheumatoid arthritis sera were four fold higher than healthy and disease controls. By contrast, levels of antibodies to a cloned fusion protein, representing the C-terminus of EBNA-1 and excluding the IR3 region, were normal in RA, but elevated fivefold in nasopharyngeal carcinoma (NPC). Affinity purified anti-P62 antibodies reacted with EBNA-1 and RANA but also with a 60 kD protein present in tissue extracts which has been tentatively identified as cytokeratin. This suggests that the specific increase of anti-P62 antibodies in RA may be due to cross-reactions with autoantibodies to structural proteins with repeat sequences containing glycine. Such sequences are found in cytokeratin and proteoglycans, suggesting that anti-P62 (and hence anti-RANA) antibodies may be cross-reactive antibodies of pathogenic significance in RA, though not necessarily indicating an aetiological role for EBV.