Absence of circulating natural killer (NK) cells in a child with erythrophagocytic lymphohistiocytosis lacking NK cell activity

A 5‐year‐old girl who was diagnosed as having erythrophagocytic lymphohistiocytosis died at age 9 years. Peripheral lymphocytes from the patient persistently lacked natural killer (NK) cell activity during the 4‐year observation period: the percent lysis values as measured by a 4‐hr 51 Cr release assay at a 40:1 effector:target ratio were below 1.0% against K562 and Molt‐4 cells as compared with the normal lymphocyte value (mean ± SD) of 46.2% ± 5.8% and 43.9% ± 6.7%, respectively. The patient's lymphocytes never developed NK cell activity by their incubation with target cells for longer time periods or by their stimulation with interferon‐α, interleukin‐2, or polyinosinic‐polycytidilic acid. Single cell‐in‐agarose assay showed the absence of target‐binding cells (TBCs): TBC numbers were below 0.3% as compared with the normal lymphocyte value of 8.1% ± 1.3% (mean ± SD). Flow cytometry showed a marked decrease in Leu‐7+ cells (1.7%) and the absence of Leu‐11+cells (0.4%) in the peripheral blood. These results first demonstrate a case of erythrophagocytic lymphohistiocytosis in which there is the lack of NK cell activity due to the absence of circulating NK cells.