Monocytes/Macrophages Stimulate Hairy-Cell Proliferation

During a study of the effect of B-cell growth factors on the growth of hairy cells (HCs), proliferation of a sub-population of HCs was observed in the absence of added growth factors in 1 (WB) of 9 patients with hairy-cell leukaemia (HCL). Double staining analysis using monoclonal antibodies detecting the Ki-67 nuclear proliferation antigen and a range of surface/cytoplasmic antigens, confirmed that the HCs were proliferating and that these cells broadly resembled the majority non-proliferative population. The HC proliferation of patient WB was shown to be dependent on the presence of autologous monocytes, but not of T cells. Investigation of other cases of HCL showed that allogeneic monocytes/macrophages from various sources stimulated HC proliferation in 3/4 cases. Neither supernatants from HC-monocyte/macrophage cultures nor tumour necrosis factor (TNF-α) could replace the proliferative effect induced by monocytes/macrophages. The proliferative effect was dependent on monocyte viability and on cell contact between monocytes/macrophages and HCs. This proliferation was markedly inhibited by alpha interferon (α-IFN). In view of the known tissue association in the bone marrow between HCs and macrophages, it is suggested that these observations are of pathophysiological significance in vivo.