SOME ANTITHYROPEROXIDASE ANTIBODIES POSITIVE SERA GIVE A PANCA PATTERN ON ETHANOL-FIXED HUMAN NEUTROPHILS - CROSS-REACTIVITY OR FALSE POSITIVES

Fifty-six sera from patients with autoimmune thyroiditis and 33 sera f rom patients with MPO-ANCA were examined in order to ascertain whether a cross reactivity between MPO-ANCA and anti-thyroperoxidase (aTPO) w as present, Sera from 20 healthy donors aTPO and aMPO negative were us ed as control, About 95% of heat inactivated sera from patients with a utoimmune thyroiditis and from controls gave positive results (atypica l pANCA pattern) on ethanol-fixed neutrophils, The prevalence of posit ive results was significantly lower when unheated aTPO positive sera w ere used (17.8%), On the other hand, only 9% of sera with MPO-ANCA wer e positive on cryostatic sections of human thyroid, Indirect immunoflu orescence tests (IF) on human neutrophils with MPO defect were negativ e with sera from patients with MPO-ANCA, but uninactivated sera with a TPO and positive for pANCA on normal neutrophils showed a very high pr evalence of positive results (90%), According to our data only few ser a positive for aTPO recognize ''normal'' MPO, but the majority of sera from patients with autoimmune thyroiditis and positive for pANCA on n ormal neutrophils recognize also an ''abnormal'' MPO. On the other han d MPO-ANCA usually recognize epitopes present only on the normal enzym e, a small proportion of these autoantibodies can react with TPO, Heat inactivated sera give false positive results for pANCA on ethanol fix ed human neutrophils.