Human monocytes cultured with and without interferon-gamma inhibit Plasmodium falciparum parasite growth in vitro via secretion of reactive nitrogen intermediates.

Adherent cells from human peripheral blood were studied for their interaction with asexual blood forms of Plasmodium falciparum in vitro. Freshly isolated monocytes only showed weak anti-parasitic effects. However, an enhancement of this anti-parasitic activity was apparent when monocytes were allowed to mature in vitro. Monocytes activated with IFN-gamma for two or three days had an enhanced anti-parasitic effect. In contrast, the inhibition mediated by cells incubated for five days was the same with or without IFN-gamma treatment. There was no evidence of toxicity when IFN-gamma at high concentrations was added directly to P. falciparum cultures. The anti-parasitic activity of the activated cells seemed to be due to nitric oxide since incubation of macrophages with L-NMMA reduced the level of inhibition. However, inhibition was only partial suggesting that other factors also were involved in inhibition of parasite growth.