Changes in hairy cells after alpha-interferon treatment as measured by flow cytometry

Patients with hairy cell leukemia who responded to alpha-interferon (IFN-alpha) were compared to those who did not by examining their hairy cells throughout treatment. The examination of the cells by cytofluorography included the expression of surface antigens (such as B1, HLA, transferrin receptors, surface immunoglobulins, TAC), and cytofluorometric measurement of light scatter including forward (which correlates with cytoplasmic size and shape) and perpendicular (which correlates with nuclear size and shape) directions. Although the entire lymphocyte population from these patients was examined initially to look for changes in phenotype with treatment, in later studies an enriched leukemic population was examined and alterations in response to interferon treatment became more apparent. The monoclonal antibody M5 was used to enrich the hairy cell population via a panning technique. The enriched population contained greater than 90% tartrate-resistant acid phosphatase-positive hairy lymphocytes. By cytofluorographic analysis of these cells serially in four responding patients and one refractory, all of whom had been followed longer than 11 months, several observations were made: 1) leukemic cells from responding patients showed a rapid decline in the forward and perpendicular light scatter indicative of cell differentiation; 2) coexisting normal B cells showed no changes in the above features; and 3) leukemic cells exhibited a definite increase in the HLA antigen density and a slight decrease in transferrin receptors in responding patients but not in the refractory patient. The increase in HLA antigen expression by hairy cells indicates progression toward differentiation, and decrease in transferrin receptors correlates with growth reduction. These data indicate that the administration of IFN-alpha is associated with cellular changes toward differentiation. We can not exclude, however, the possibility of other actions of IFN-alpha in HCL as proposed by other investigators.