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NK cell‐mediated killing of AML blasts: role of histamine, monocytes and reactive oxygen metabolites

Abstract:  Blasts recovered from patients with acute myelogenous leukaemia (AML) were lysed by heterologous natural killer (NK) cells treated with NK cell‐activating cytokines such as interleukin‐2 (IL‐2) or interferon‐α (IFN‐α). The cytokine‐induced killing of AML blasts was inhibited by monocytes, recovered from peripheral blood by counterflow centrifugal elutriation. Histamine, at concentrations exceeding 0.1 μM, abrogated the monocyte‐induced inhibition of NK cells; thereby, histamine and IL‐2 or histamine and IFN‐α synergistically induced NK cell‐mediated destruction of AML blasts. The effect of histamine was completely blocked by the histamine H2‐receptor (H2R) antagonist ranitidine but not by its chemical control AH20399AA. Catalase, a scavenger of reactive oxygen metabolites (ROM), reversed the monocyte‐induced inhibition of NK cell‐mediated killing of blast cells, indicating that the inhibitory signal was mediated by products of the respiratory burst of monocytes. It is concluded that (i) monocytes inhibit anti‐leukemic properties of NK cells, (ii) the inhibition is conveyed by monocyte‐derived ROM, and (iii) histamine reverses the inhibitory signal and, thereby, synergizes with NK cell‐activating cytokines to induce killing of AML blasts.