Detoxified exoantigens and phosphatidylinositol derivatives inhibit tumor necrosis factor induction by malarial exoantigens

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Abstract
We have previously shown that malaria parasites liberate exoantigens which, through a phospholipid component, stimulate mouse macrophages to secrete tumor necrosis factor (TNF), which are toxic to D-galactosamine-sensitized mice, and which therefore might be involved in pathology. Plasmodium yoelii exoantigens detoxified by dephosphorylation or digestion with lipases do not induce TNF production. However, these partial structures inhibited its production in response to the exoantigens, although not to bacterial lipopolysaccharide (LPS). When pure phospholipids were tested in a macrophage assay, none stimulated the production of TNF, but phosphatidylinositol (PI) inhibited TNF induction by P. yoelii exoantigens. Moreover, inositol monophosphate (IMP) was the only one of a number of monophosphate saccharides tested which was inhibitory; inositol was not. Macrophages pretreated with PI, IMP, or detoxified exoantigens and then incubated with parasite exoantigens also yielded much less TNF. PI, IMP, and lipase-digested exoantigens of P. yoelii similarly inhibited the TNF-inducing activity of exoantigens of the human parasites Plasmodium falciparum and Plasmodium vivax. Neither PI nor IMP diminished TNF production in response to LPS, in contrast to a platelet-activating factor antagonist [1-O-hexadecyl-2-acetyl- sn-glycero-3-phospho(N,N,N-trimethyl hexanolamine)] which inhibited both exoantigen- and LPS-induced production of TNF. We conclude that at least two different parts of the molecule are involved in the induction of TNF secretion by parasite exoantigens: one requires the presence of a phosphate bound to inositol, and, since dephosphorylated exoantigens were also inhibitory, one does not. It would seem that both affect interactions between parasite-derived exoantigens and the macrophage receptors.