Analysis of natural killer cells in patients with aplastic anemia.

We have analyzed natural killer (NK) cells in 43 patients with severe aplastic anemia, using cytotoxicity assays and microfluorometry with monoclonal antibodies, prior to and after treatment with antithymocyte globulin (ATG). Before treatment, natural killer cell activity (NKa) in both peripheral blood and bone marrow was markedly decreased in 76% of patients as compared with normal controls. Although we have measured low NKa in patients receiving large numbers of blood transfusions (means = 150 U of RBCs), six aplastic patients had low NKa in the absence of transfusions, and the average number of transfusions in the total population was low (means = 24). Purification of larger granular lymphocytes (LGLs) from peripheral blood of aplastic anemia patients failed to recover significant NKa. Most of these large granular lymphocytes showed few azurophilic granules. NKa was appropriately enhanced in these patients samples by exposure of mononuclear cells to either interleukin 2 (IL-2) or interferon (IFN). Analysis of peripheral blood phenotypic markers showed that cells bearing Leu 7 antigen were in the normal range in aplastic anemia (means = 12% +/- 2%; normal = 16% +/- 2%), but there was a deficiency of Leu 11+ cells (means = 8% +/- 2%; normal = 15% +/- 2%). The number of Leu 11+ cells was well correlated with NKa. In 13 of 22 patients treated with ATG, NKa returned to the normal range, and recovery of NKa was correlated to hematopoietic recovery. Our results suggest that deficient NKa is an intrinsic feature of aplastic anemia, and that the circulating cells in this disease are of the pre-NK cell stage.