Case Report: Hemophagocytic lymphohistiocytosis secondary to Escherichia coli infiltration of bone marrow in a patient with seronegative rheumatoid arthritis treated with low-dose methotrexate.

Hemophagocytic lymphohistiocytosis (HLH) is a disorder characterized by dysregulated immune activation triggered by various etiologies, leading to excessive inflammatory responses, cytokine storm, and subsequent organ dysfunction. While the presence of hemophagocytic cells in septic shock is generally considered a physiological response that seldom requires specific intervention, the finding of abundant hemophagocytes along with Escherichia coli(E. coli) on a bone marrow smear warrants careful assessment for possible HLH. We herein present a fatal case of HLH secondary to an E. coli urinary tract infection in a seronegative rheumatoid arthritis patient on low-dose methotrexate. The bone marrow smear not only demonstrated typical hemophagocytes but also revealed phagocytes containing ingested E. coli. In contrast to previously reported cases, E. coli in this patient not only caused severe bacteremia but was also demonstrated on the bone marrow aspirate, resulting in profound immunosuppression.