Severe aplastic anemia associated with chronic natural killer cell lymphocytosis.

Chronic natural killer (NK) lymphocytosis involves a persistent increase in CD56+ large granular lymphocytes (LGLs) that is sometimes associated with immune-mediated complications, such as anemia and neutropenia. However, aplastic anemia (AA) is a rare complication. Here we describe 2 patients with severe AA who presented with persistent increases in NK cells. Their LGLs were positive for CD56, CD16, and intracellular interferon (IFN)-gamma but negative for CD3, Fas-ligand, and T-cell receptor rearrangement, findings that are compatible with NK cells. Not only the number of NK cells, but NK activity as well, was increased in both patients. The number of NK cells changed according to hematologic recovery and relapse in 1 case. Thus, there seemed to be a close relationship between NK cells and the progression of AA, at least in this instance. Further investigation of the clinical course of similar cases and the characteristics of NK cells is necessary.