Bone marrow granulomas associated with chronic natural killer cell lymphocytosis

Two patients with chronic natural killer cell lymphocytosis (CNKL) and associated bone marrow granulomas (BMGs) are described. The increases in the peripheral blood of both the proportion (64 and 74%) and absolute number (2,000/μL and 4,700/μL) of NK cells have persisted for more than 1 and 3 years. One patient was asymptomatic with thrombocytopenia and neutropenia, and the other presented with fever of unidentified origin, which has since responded to a nonsteroidal anti-inflammatory agent. Bone marrow examination in both patients showed loose aggregates of epithelioid histiocytes similar to the noncaseating granulomas seen in other diseases. An unusual feature, however, was the presence of many scattered, large mononuclear histiocytes that were morphologically similar to atypical megakaryocytes. Extensive clinical and laboratory studies did not reveal an established cause for the BMGs. BMGs may be associated with CNKL, with the unusual feature of scattered, atypical mononuclear histiocytes.