A single-stranded DNA-cross-reactive immunogenic epitope of human homocysteine-inducible endoplasmic reticulum protein

The mechanism involved in generating anti-DNA antibodies (Abs) remains unclear, as DNA is poorly immunogenic. Molecular mimicry between DNA and non-DNA substances has been implicated as a possible mechanism. We previously reported that homocysteine-inducible endoplasmic reticulum protein (Herp), which is induced by endoplasmic reticulum stress, is recognized by anti-double-stranded DNA (dsDNA) IgG from patients with systemic lupus erythematosus and that immunization with Herp elicits anti-dsDNA Abs in BALB/c mice. In this study, we observed that anti-single-stranded DNA (ssDNA) Abs were also generated in Herp-immunized BALB/c mice and established an anti-Herp monoclonal antibody (mAb), HT4, which specifically cross-reacted with ssDNA. The epitope of the HT4 mAb on Herp, 'EPAGSNR', was identified by screening a synthetic peptide library. The binding of the HT4 mAb to the peptide was competitively inhibited by ssDNA. Immunization of the epitope peptide elicited anti-ssDNA Abs in BALB/c mice. These results indicate that the epitope exists in a human self-protein, mimics ssDNA and shows antigenicity for anti-ssDNA Abs in normal mice. Anti-ssDNA Abs are often found in patients with drug-induced lupus erythematosus. Treatment with representative drugs that cause drug-induced lupus (chlorpromazine, procainamide and hydralazine) induced Herp expression and apoptosis in HeLa cells. These findings suggest that molecular mimicry between Herp and ssDNA is involved in anti-ssDNA Ab production in drug-induced lupus.