Decreased natural killer (NK) activity in patients with myeloproliferative disorders

Natural killer (NK) cell number and activity were measured in 26 patients with myeloproliferative disorders and the results were compared with 16 age‐matched control patients. The percent of Leu‐11b‐positive cells was 11% ± 3% in the patients, compared with 12% ± 4% in the control patients. Ten of 26 patients, however, had NK activity lower than all of the control values at three different effector to target cell ratios (E:T) (P < 0.005). The values of those patients with low unstimulated NK activity remained low despite stimulation with interleukin‐2 (IL‐2) or alpha‐interferon (α‐IFN), whereas the values of those patients with normal unstimulated activity responded to IL‐2 and α‐IFN like the control patients. Three of the ten patients with low NK activity had a history of malignant neoplasms. None of the 16 patients with normal NK activity had a history of malignant neoplasms (P < 0.05). We conclude that patients with myeloproliferative disorders frequently have low endogenous NK cell activity in vitro. The dysfunction of the NK system appears to be intrinsic because the relative number of NK cells was similar to control values and the response to stimulation with IL‐2 and α‐IFN was suboptimal. There may be a relationship between low NK activity and the development of malignant disease in such patients.