Infliximab treatment for refractory COVID-19-associated multisystem inflammatory syndrome in a Japanese child.

Patients with multisystem inflammatory syndrome in children (MIS-C) can develop clinical features resembling Kawasaki disease (KD). A full picture of MIS-C in East Asia which has higher incidence of KD than other regions remains unclear. We report on a 15-year-old Japanese boy with refractory MIS-C who was successfully treated with infliximab. A Japanese boy who was diagnosed with coronavirus disease 2019 (COVID-19) before a month developed MIS-C with fulfilling six principal symptoms of KD. Laboratory data showed extreme hyperferritinemia (11,404 ng/mL), besides lymphopenia and thrombocytopenia. The patient was refractory to initial therapy with intravenous immunoglobulin (IVIG; 2 g/kg), aspirin, and prednisolone. He was therefore administered a second IVIG (2 g/kg) and infliximab (5 mg/kg) on days 7 and 8 from the onset of fever, respectively, which resulted in an improvement of clinical symptoms. Only four Japanese cases with MIS-C were reported and all of them were responsive to IVIG. The hyperferritinemia in this case was distinctive from previously reported MIS-C cases in Japan and other cohorts and may be associated with refractoriness to IVIG therapy. Marked elevation of circulating ferritin levels is known to be induced by tumor necrosis factor-α, which plays a key role in the pathogenesis of both KD and MIS-C. Thus, for MIS-C patients with hyperferritinemia, early intervention with adjunctive infliximab may induce a more rapid resolution of inflammation and improve outcome. Because MIS-C may be heterogeneous with respect to immunopathology, genetic background, clinical phenotypes and response to therapies, optimized treatment strategies according to immunopathogenesis are required.