Background  Antidesmoglein (anti-Dsg) 3 serum antibody titres are usually correlated with the disease activity of pemphigus vulgaris (PV), but some patients retain high titres even in remission.

Objectives  The aim of our study was to determine whether anti-Dsg3 antibodies in PV sera recognized calcium (Ca2+)-dependent or non-Ca2+-dependent epitopes, and to evaluate their pathogenicity.

Methods  Dsg3 baculoprotein-coated enzyme-linked immunosorbent assay (ELISA) plates were treated with 0·5 mmol L−1 ethylenediaminetetraacetic acid (EDTA). The binding ability of anti-Dsg3 monoclonal antibodies (mAbs) was analysed. Eight of the 83 patients with PV who were screened had elevated Dsg3 ELISA index values > 100 in remission. The binding ability of these PV sera was analysed. We evaluated the pathogenicity of anti-Dsg3 serum antibodies against the non-Ca2+-dependent epitopes using a dissociation assay.

Results  The reactivity of pathogenic anti-Dsg3 mAbs against the Ca2+-dependent epitopes diminished markedly in the EDTA-treated ELISA, whereas no such reduction was observed in mAbs against the non-Ca2+-dependent epitopes. The sera of all the patients contained antibodies against both Ca2+-dependent and non-Ca2+-dependent epitopes. In six out of the eight patients, the ratio of antibodies against Ca2+-dependent to non-Ca2+-dependent epitopes decreased in remission. EDTA-treated Dsg3 baculoproteins adsorbed anti-Dsg3 serum antibodies against the non-Ca2+-dependent epitopes, but the remnant PV antibodies retained the ability to induce acantholysis in the dissociation assay.

Conclusions  We have established an assay to measure indirectly the titres of anti-Dsg3 serum antibodies against the Ca2+-dependent epitopes, based on the differences between EDTA-untreated and EDTA-treated ELISA index values, as a routine laboratory test to reflect the pathogenic anti-Dsg3 serum antibody titres more accurately.