Demonstration of a lipopolysaccharide-induced cytostatic effect on malarial parasites.

In an in vitro assay, malarial parasites (Plasmodium vinckei petteri) taken from mice 7 to 8 h after the injection of bacterial lipopolysaccharide (LPS) incorporated significantly less hypoxanthine into nucleic acid than did parasites from saline-treated controls. In contrast, incorporation was normal in parasites taken from mice within minutes of the injection of LPS and in parasites cultured with LPS. These results implied that the injection of LPS induced the release of mediators with a cytostatic effect on the parasite. This suggested an explanation for the protective action of LPS in malarial infections as well as for the "crisis forms" seen both in naturally resolving infections and in animals pretreated with such agents as BCG.