Common bone marrow signature in COVID‐19‐associated multisystem inflammatory syndrome in children: A first‐wave small case series experience

The hyper‐inflammatory response, also known as multisystem inflammatory syndrome in children (MIS‐C), represents a major concern in children with SARS‐CoV‐2 infection. We report bone marrow features of three patients with MIS‐C who were diagnosed during the first wave of the SARS‐CoV‐2 pandemic. A bone marrow evaluation was performed at onset of the inflammatory condition in order to exclude secondary hemophagocytic lymphohistiocytosis (sHLH). The bone marrows of the patients presented common features: the erythroid and megakaryocytic lineages were prominently affected and hemophagocytosis was moderately increased, differently than observed in sHLH. Megakaryocytopoiesis was increased, representing a peculiar feature of MIS‐C differing from sHLH. SARS‐CoV‐2 RT‐PCR and viral panel were studied in bone marrow aspiration samples. MIS‐C is a rare complication of SARS‐CoV‐2 infections in children. An immuno‐dysregulation considering both innate and adaptive immunity together with vascular inflammation and endothelial dysfunction play a major role. Our observations, although limited due to the small sample size, suggest that there are unique features in the bone marrow of patients with MIS‐C that are likely secondary to immuno‐dysregulation, and there are notable differences in bone marrow features compared to those reported in sHLH.