ANTIBODY REACTIVITY AGAINST THYROID PEROXIDASE AND MYELOPEROXIDASE INAUTOIMMUNE-THYROIDITIS AND SYSTEMIC VASCULITIS

Potential cross-reactivity between thyroid peroxidase (TPO) and myelop eroxidase (MPO) molecules was evaluated by analysing the binding of 19 9 TPO antibody- and 145 MPO antibody- positive sera to TPO and MPO mol ecules. Sera from six patients with autoimmune thyroiditis (AITD) and four patients with systemic vasculitis (SV) with different TPO-MPO ant ibody findings were then chosen for further analyses. All six patients with AITD had TPO antibodies in enzyme immunoassay (EIA) and lour of them had simultaneously MPO antibodies in EIA. In AITD patients antibo dy binding to TPO could not be inhibited by adding native MPO to the s erum diluent, suggesting that the possible cross-reactive epitopes wer e exposed in the denaturated MPO molecule. Similarly, the MPO ab react ivity of patients with systemic vasculitis could not be inhibited by n ative TPO. To study whether TPO and MPO antibodies recognize linear ep itopes, the binding of antibodies to synthetic TPO and MPO peptides wa s analysed, Several TPO and MPO peptides were reactive, including pept ides reacting with both TPO and MPO antibody-positive sera. One of the most crossreactive peptides contained AA 586-601 in TPO, showing also particularly high AA homology (88%) with MPO (AA 594-609). The result s suggest that TPO and MPO molecules contain cross-reactive epitopes t hat are exposed in denaturated molecules and may thus cause false posi tive antibody findings in solid phase EIA assays.