Association of Monoclonal Expansion of Epstein-Barr Virus-Negative CD158a(+) NK Cells Secreting Large Amounts of Gamma Interferon with Hemophagocytic Lymphohistiocytosis

We report the first case of hemophagocytic lymphohistiocytosis (HLH) induced by the monoclonal expansion of Epstein-Barr virus (EBV)-negative NK cells. Consanguinity of the patient's parents made it necessary to discard familial HLH in the patient and her sister with identical HLA markers and demonstrate that no cause other than the expansion of NK cells, which secrete high levels of gamma interferon, was inducing HLH in this patient.