Immunological recovery and dose evaluation in IFN‐α treatment of hairy cell leukemia: Analysis of leukocyte differentiation antigens, NK and 2‘,5’‐oligoadenylate synthetase activity

A low‐dose interferon (IFN)‐α regimen for the treatment of hairy cell leukemia (HCL) was evaluated by following changes in leukocyte differentiation antigens (LDA), natural killer cell (NK) and 2′,5′‐oligoadenylate (2‐5A) synthetase activities. Due to hairy cells' (HC) weak expression of several antigens positive for T cells, B cells, NK cells and monocytes, the use of a double marker specific for hairy cells was needed to distinguish the different subpopulations. Analysis of LDA in peripheral blood (PB) showed a total normalization of the T cell and monocyte numbers within 90 days, the number of NK cells normalized in 90 to 180 d, whereas normalization of B cell number was seen only after 180 to 360 d of treatment. Mean pretreatment 2‐5A synthetase activity was normal or low, but upon treatment the levels rose immediately to higher than normal values and remained high throughout the study. Pretreatment NK activity was low, but normalized after between 90 to 360 d, except in 2 patients with severe splenomegaly. In vitro incubation of peripheral blood mononuclear cells (PBMNC) with IFN‐α induced activation of the NK and 2‐5A synthetase activity in untreated patients, but with treatment these effects were gradually abolished, indicating an increasing effect of IFN‐α in vivo with time. These results shows that the different PBMNC subpopulations and important immunological functions normalize with treatment. This normalization is, however, not seen until at least after 1 year of treatment, indicating that the treatment schedule should be longer. As no exhaustion to the effect of IFN was seen, as measured by the 2‐5A synthetase activity, a continuing beneficial effect of treatment is anticipated. The increasing effect of IFN‐α after the first signs of clinical effect suggests that the doses used in the present study were higher than necessary.