Case Report: Two atypical cases of M6-like antimitochondrial antibody pattern unlinked to iproniazid

Rare M6-subtype antimitochondrial antibody (AMA-M6) has been described in patients treated with iproniazid, an antidepressant drug. Diagnosis can be made by identifying its highly characteristic indirect immunofluorescence pattern on rat organ substrates. In the existing literature, the presence of AMA-M6 has been associated with hepatitis cytolysis, with a favorable outcome following the discontinuation of treatment. This report presents two cases of AMA-M6-like pattern in patients who had never taken iproniazid. In the first case, the patient was receiving pembrolizumab treatment for lung adenocarcinoma. The presence of this pattern was identified 20 days after introducing immunotherapy, with a diagnosis of liver cytolysis made 10 days later, associated with the presence of nodular regenerative hyperplasia. Following the suspension of the injection, liver enzyme levels decreased and the pembrolizumab treatment was resumed with no recurrence of liver damage. In the second case, the same AMA-M6-like pattern was discovered fortuitously in a patient presenting with a complaint of dry mouth only, and no evidence of liver damage. In contrast to previous reports, our findings do not support the presence of MAO-B-specific autoantibodies in these patients, and the target antigen remains to be identified. These two cases demonstrate that AMA-M6-like pattern can develop in patients who have never been exposed to iproniazid. While liver damage was present in one case, the patient remained asymptomatic in the second case. Immunologists must thus watch for uncommon patterns in indirect immunofluorescence to ensure appropriate medical care and patient follow-up.