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Abstract.In Plasmodium-infected mammals, phagocytosis and production of tumor necrosis factor (TNF) by monocytes and macrophages are prominent features. The present work aimed at clarifying the relationship between the maturation of human monocytes to macrophages and their TNF productivity and phagocytic ability in the presence of Plasmodium falciparumâ€"infected erythrocytes. Fresh monocytes produced a significantly higher quantity of TNF in the presence of schizont-infected erythrocytes than macrophages obtained by in vitro monocyte maturation on autobogous serum, whereas phagocytic activity of macrophages was much higher than that of fresh monocytes. This indicated that the TNF-inducing factors from P. falciparum-infected erythrocytes could stimulate fresh monocytes, but not macrophages, to release TNF, regardless of their development of phagocytosis. Activation of macrophages by interferon--1. could not recover their TNF productivity in the presence of P. falciparum-infectcd erythrocytcs, but it enhanced their TNF productivity in the presence of bipopolysaccharide(s).The TNF-inducing factors were contained mainly in erythrocytes infected with mature schizonts but not in crythrocytes infected with the younger stages of the parasites. Fractionation of infected crythrocytes revealed that both soluble and insoluble components almost equally contained those factors.

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