The heme moiety of malaria pigment (β-Hematin) mediates the inhibition of nitric oxide and tumor necrosis factor-α production by lipopolysaccharide-stimulated macrophages

To investigate the effect of the heme moiety of malaria pigment, hemozoin, on phagocyte functions, mouse macrophages were fed with insoluble β-hematin, the synthetic heme-polymer chemically identical to the native pigment, or the soluble monomer, hematin. Production of inflammatory cytokines, interleukin 1 (IL1), tumor necrosis factor alpha (TNFα), and nitric oxide (NO) was assayed in tile supernatants after stimulation with lipopolysaccharide. The results indicate that both β-hematin and hematin induce a dose-dependent inhibition of macrophage production of TNFα and NO, but not of IL1. One-hour pretreatment with soluble hematin inhibited production of cytotoxic mediators by more than 50% compared to controls, while 6-hr exposure was necessary for insoluble β-hematin to induce the same level of inhibition. However, the same treatment did not modify the production of TNFα and NO by mouse microglia cell lines. The inhibition was partially counterbalanced by adding sulphydryl group donors such as 2-mercaptoethanol, glutathione, or N-acetyl-cysteine during the preincubation time. The results of the present study confirm the inhibitory role of malaria pigment and show that such effect is due to the heme moiety and may be selective for the production of cytotoxic mediators by specific phagocytes. The implications of these findings in tile control of malaria infection and disease and in the pathogenesis of severe malaria are discussed.