Identification of a linear epitope recognized by a monoclonal antibody directed to the heterogeneous nucleoriboprotein A2.

Rheumatoid arthritis (RA) is a chronic autoimmune disorder, characterized by progressive joint destruction and disability. Classical autoantibodies of RA are rheumatoid factors and citrulline antibodies. Patients positive for these autoantibodies are usually associated with a progressive disease course. A subgroup of RA patients does not express citrulline antibodies, instead are approximately 35% of these anti-citrulline-negative patients reported to express autoantibodies to the heterogeneous nucleoriboprotein A2, a ribonucleoprotein involved in RNA transport and processing also referred to as RA33. In the absence of citrulline antibodies, RA33 antibodies have been suggested to be associated with a milder disease course. In this study we screened the reactivity of a monoclonal antibody to RA33-derived peptides by modified enzyme-linked immunosorbent assays (ELISA). Terminally truncated resin-bound peptides were applied for determination of the functional epitope necessary for antibody recognition. In addition, screening of substituted peptides by modified ELISA identified amino acids necessary for antibody reactivity. A potential epitope was identified in the region 71-79 (PHSIDGRVV), where the amino acids Ser, Ile and Asp were found to be essential for antibody reactivity. These amino acids were found to contribute to the antibody-antigen interface through side-chain interactions, possibly in combination with a positively charged amino acid in position 77. Moreover, the amino acids in the N-terminal end (Pro and His) were found to contribute to the interface through backbone contributions. No notable reactivity was found with RA-positive patient sera, thus screening of RA33 antibodies does not seem to be a supplementary for the diagnosis of RA.