Follow-up of avidity and titer of anti-myeloperoxidase antibodies in sera from patients with primary ANCA-associated vasculitis

Antineutrophil cytoplasmic antibodies (ANCA) are important serologic markers for ANCA-associated vasculitis (AAV). Our previous studies in propylthiouracil (PTU)-induced AAV demonstrated that withdrawal of PTU resulted in clinical remission and significant decrease of avidity of PTU-induced myeloperoxidase (MPO)-ANCA. This study investigated the changes in avidity and titer of MPO-ANCA in sequential sera from some patients with primary AAV with different disease activities. Sequential sera samples of seven patients with MPO-ANCA-positive vasculitis at their initial onset, remission and relapse were collected. The avidity of MPO-ANCA was assessed by antigen-inhibition enzyme-linked immunosorbent assay (ELISA). The titer of MPO-ANCA was determined by a two-fold dilution of sera in MPO specific ELISA. The titer of MPO-ANCA was not significantly different between initial onset and remission. The avidity constant (aK) of MPO-ANCA in active phase is not significantly different from that in remission (724.9 ± 828.4 l/mol vs. 353.4 ± 551.7 l/mol, p = 0.303). No significant correlation could be found between aK and the level of Birmingham Vasculitis Activity Score, times of relapse, the number of organ involvement, serum creatinine, or CRP. Avidity and titer of MPO-ANCA did not decreased significantly during remission in AAV, indicating the chronic repeated antigen stimulation was not removed, which might be the reason for recurrent relapses.