Hairy cell leukemia cells are relatively NK-insensitive targets

The hypothesis that interferon alpha (IFN alpha) has its beneficial effects in hairy cell leukemia by activating natural killer cells against hairy cells was examined. Leukemic cells from patients with hairy cell leukemia were tested for their susceptibility to lysis by fresh and IFN alpha activated peripheral blood mononuclear (PBMC) cells from normal donors. All hairy cells tested were relatively insensitive to cytolysis by PBMC and IFN alpha activated PBMC. The low levels of 51Cr release obtained with a few donors was due to lysis of leukemic cells, not residual normal cells, and was mediated by a natural killer cell (T cell receptor independent) mechanism. Chronic lymphatic leukemic cells before and after treatment with phorbol ester were also resistant to cytolysis. Hairy cells were not susceptible to lymphokine activated killer (LAK) cells but were sensitive to lysis by antibody and complement. The insensitivity to cell mediated cytolysis against hairy cells was shown by cold target inhibition to be a lack of target recognition by NK cells.