Vaccines, volcanoes and local lore on the Web
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Biomedical subjects
Publications and source records attributed to J Taverne.
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While a steady flow of questions were put to the general parasitology newsgroup during November, none provoked discussion. Other discussion lists came to life:
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by Kevin O'Donnell and Larry Winger, Harwood Academic Publishers, 1997. pound14.50 (pbk)/ pound30.00 (hbk) (xi+309 pages) ISBN 90 5702 221 4.
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Hypoglycaemia in falciparum malaria is associated with a poor prognosis and is correlated with mortality. High levels of serum TNF are also correlated with disease severity and mortality, and it has been suggested that TNF may cause the hypoglycaemia. However hypoglycaemia in mice infected with Plasmodium chabaudi or the lethal strain of P. yoelii YM is related to hyperinsulinaemia. Its development was not prevented by treatments which diminished TNF activity or production without affecting levels of plasma insulin. Conversely, it was inhibited by diazoxide, which inhibited insulin secretion but did not affect TNF production. Furthermore, in mice exhibiting neurological symptoms during infection with P. berghei, blood glucose concentrations were significantly raised when TNF levels were high, and TNF levels in the spleen were highest of all in non-lethal P. yoelii infections in which hypoglycaemia does not occur. Administration of human rTNF to normal animals caused an increase rather than a drop in blood glucose levels. Mice transgenic for human TNF did not develop hypoglycaemia when infected with P. yoelii YM, but showed signs of insulin resistance. In line with current views on the role of TNF in obesity and the control of glucose homeostasis, we conclude that the hypoglycaemia of malaria is not caused by increased levels of TNF, which may in fact be beneficial, but is secondary to a hyperinsulinaemia that is probably stimulated directly by products of the parasite.
Malaria toxin causes hypoglycemia and induction of tumor necrosis factor. Extracts of parasitized erythrocytes which were coeluted and copurified with one of the two subtypes of mammalian insulin-mimetic inositolphosphoglycans similarly induced fibroblast proliferation in the absence of serum. In addition, induction of tumor necrosis factor in macrophages by malaria toxin and by lipopolysaccharide from Escherichia coli was enhanced by pretreatment of these toxins with alpha-galactosidase. Thus, parasitized erythrocytes contain both soluble inositolphosphoglycan-like insulin second messengers and endotoxin-like lipidic molecules.
The excessive production of tumour necrosis factor (TNF) is associated with the pathology of blood-stage malaria and phosphatidylinositol-containing phospholipid antigens from parasitized erythrocytes stimulate its secretion by macrophages, thus acting as toxins. This brief report describes some properties of an inhibitor present in lysates from erythrocytes infected with malarial parasites that blocked the detection of recombinant TNF in an enzyme-linked immunosorbent assay and diminished or abolished the cytotoxicity of TNF. It was not found in control lysates of normal erythrocytes. Its addition to macrophage cultures stimulated by toxic malarial preparations or by bacterial lipopolysaccharide also blocked the detection of TNF. These findings may explain the contradictory results obtained from different assays for TNF, and emphasize the need for caution when interpreting the results of a single assay system. If released when parasitized erythrocytes rupture in vivo, the inhibitor could help protect both parasite and host from the damaging effects of TNF.