Flow cytometry in hairy cell leukemia before and during interferon alfa‐2b therapy

Mononuclear cells from 15 patients with hairy cell leukemia were studied before and during therapy with interferon alfa‐2b (IFN) by regular peripheral blood differential counts and flow cytometry, using a panel of monoclonal antibodies (Moab). Seven leukemic phase patients (Group 1) had a mean leukocyte count of 48,870/μl at entry with a mean absolute hairy cell (HC) count of 40,100/μl. After 3 months on IFN, both parameters decreased significantly (WBC 3,500/μl; HC count 130/μl). In eight patients with a cytopenic form of the disease (Group 2) the mean leukocyte count rose from 2950/μl to 3890/μl while the mean absolute HC count decreased from 300/μl to 120/μl. The morphologic shifts correlated well with changes in the Moab reaction pattern. In Group 1 the activity of all Moab decreased significantly. In Group 2, only cells expressing Leu 3a and Leu 11a (a marker of natural killer cells) showed a significant shift, the latter increasing from 170/μl to 360/μl. This increase in natural killer cell antigen expression was not obvious based on routine morphologic observations alone. We show that flow cytometry may be a useful adjunct in monitoring the response of HCL to therapy. Changes in populations of cells that may be difficult to discriminate on morphologic grounds alone may be observed.