A heterogeneous immune response to an SmD-like epitope by SLE patients

Two mouse cDNA clones were isolated by immunoscreening with an SLE serum. These clones encode an epitope which consists of a di-peptide repeat (Gly-Arg)n (n = 9 and 19 in isolated clones). These sequences, when expressed as fusion proteins, inhibit the binding of antibodies from one patient's serum to the SmD autoantigen. This cross-reactivity is based on the sequence identity with the carboxyterminal end of the human SmD [(Gly-Arg)g]. An Exonuclease III deletion analysis demonstrates that the minimal number of Gly-Arg repeats necessary for immune recognition on the Western blot is patient-specific, and varies from nine to three. The defined epitope is recognized by 35% of sera from patients with SLE as well as with other autoimmune diseases (rheumatoid arthritis, scleroderma, Sjogren's syndrome), in contrast to SLE-specificity of anti-Sm antibodies. Affinity-purified antibodies of the identified epitope cross-react with EBNA1 protein in EBV-infected B-cell lines.