Antibodies from patients with rheumatoid arthritis and systemic lupus erythematosus recognize different epitopes of a single heterogeneous nuclear RNP core protein : possible role of cross-reacting antikeratin antibodies

Antibodies to recombinant heterogeneous nuclear RNP core protein A1 were detected in sera from 27 of 58 patients with rheumatoid arthritis (RA) and from 7 of 31 patients with systemic lupus erythematosus, by immunoblotting and enzyme‐linked immunosorbent assay. Protein A1 consists of 2 distinct domains: The N‐terminal sequence is identical to a single‐stranded DNA binding protein termed UP1, and the Ckerminal domain shows a partial homology with keratin. All 7 Al‐positive systemic lupus erythematosus sera reacted with UP1, whereas 9 of the 27 Al‐positive RA sera did not. In RA, anti‐A1 activity was significantly associated with antikeratin antibodies (AKA); these antibodies were present in 23 of 27 Al‐positive sera and 10 of 31 Al‐negative sera (P > 0.01). Immunoabsorption with recombinant protein A1 resulted in a significant reduction of AKA titers in 6 of 10 RA sera tested, suggesting that AKA from RA patients may cross‐react with the C‐terminal portion of the heterogeneous nuclear RNP protein Al.