The beneficial effects of alpha-interferon in hairy cell leukemia are not attributable to NK cell-mediated cytotoxicity.

The natural killer (NK) activity of peripheral blood mononuclear cells against K562 targets was variable in 10 untreated patients with hairy cell leukemia and was inversely related to the number of hairy cells (HCs) present. After therapy with alpha-interferon (IFN-alpha) NK activity in vitro was equivalent to that of normal controls. It is suggested that the low activity often seen before treatment is attributable to dilution of NK cells by large numbers of inactive HCs and that this diluting effect is reduced as HCs disappear from the blood during IFN-alpha treatment. HCs was consistently resistant to NK lysis by normal or hairy cell leukemia allogeneic and autologous mononuclear cells, despite whether effector or target cells had been pretreated with IFN-alpha. Cold-target inhibition and direct binding experiments showed that HCs do not bind to NK effectors. It is therefore concluded that NK cells play no direct role in the progressive disappearance of HCs seen in patients receiving IFN-alpha.