Malarial toxic antigens synergistically enhance insulin signalling.

Hypoglycaemia is a major complication of severe malaria [(1990) Trans. Roy. Soc. Trop. Med. 84 (suppl. 2) 1-65], especially cerebral malaria, in which it is associated with increased mortality [(1990) Lancet 336, 1039-1043; (1989) Quart. J. Med. (New series) 71, 441-459]; however, the mechanisms responsible have not been fully explained. Preparations containing toxic malaria antigens (TMA) released by blood stage Plasmodium yoelii malaria parasites have been shown to induce hypoglycaemia in mice lasting at least 8 h [(1992) Clin. Exp. Immunol. (in press)]. Here we report that TMAs can act synergistically with insulin in both stimulating lipogenesis and inhibiting lipolysis in rat adipocytes in vitro, and, furthermore, that they act synergistically with insulin in the induction of hypoglycaemia in vivo.

Inhibition of intraerythrocytic development of Plasmodium falciparum by proteinase inhibitors.

A group of inactivators of cysteinyl proteinases which function by covalent bond formation have been examined for their ability to inhibit the development of Plasmodium falciparum within red blood cells. The most effective of these caused inactivation of the parasite near 10(-8) M concentration. The range of inhibitory action varied with peptide structure in a manner characteristic of affinity labels for proteinases suggesting that the target of inhibition was an unidentified proteinase, probably of the cysteinyl type, but different from cathepsins B and L.

Macrophages as effector cells in immunity to malaria.

Experiments with malaria in mice suggest that protective immunity depends not only on antibody but also on activation of macrophages. Activated macrophages may cause intra-erythrocytic death of parasites by releasing reactive oxygen intermediates and/or tumour necrosis factor. Macrophage activation for both types of product correlates well with the timing of recovery in a range of different malaria infections and in mice protected by vaccination.

Malaria exoantigens induce TNF, are toxic and are blocked by T-independent antibody.

The production of cytokines, including tumour necrosis factor (TNF), may be involved in the pathology of malaria, as well as in protection against the parasite. We have shown that parasite exoantigens induce the secretion of TNF in vitro and in vivo and kill mice made hypersensitive to TNF. They elicit T-independent antibody that inhibits their capacity to stimulate TNF production and protects against toxicity in vivo, and those of human and rodent parasites are serologically related. Their active component does not appear to be protein. Here we review their properties and consider the epidemiological significance of our findings and their possible contribution to the development of an "anti-disease" vaccine.

Reversal of type 2 diabetes in mice by products of malaria parasites: I. Effect of inactivated parasites.

C57BL/KsJ-db/db and C57BL/KsJ-ob/ob mice are good models for studies on human obesity and type 2 diabetes. We have previously shown that infection with blood-stage malaria or injection of extracts from malaria-parasitized red blood cells induces hypoglycemia in normal mice and normalizes hyperglycemia in mice made moderately diabetic by streptozotocin. In the present study, we show that a single intravenous (IV) injection of Formalin-fixed Plasmodium yoelii YM (FFYM) preparation decreases blood glucose in db/db mice from an initial value of 19 mmol/L to a normal value of 7 mmol/L (P < .0001) for at least 24 hours and reduces food intake. Plasma insulin concentrations in db/db mice were not altered. FFYM was also active in normal and ob/ob mice, an effect associated with an increase in plasma insulin. Although the rate of weight gain in lean ob/+ and lean db/+ was not altered by this treatment, there was a significant reduction in weight gain in db/db and ob/ob mice (P < .001). We suggest that malaria-derived molecules, when fully characterized, may provide structural information for the development of new agents for the management of type 2 diabetes.

Improvement of glucose homeostasis in obese diabetic db/db mice given Plasmodium yoelii glycosylphosphatidylinositols.

We have previously reported that infection with Plasmodium yoelii, Plasmodium chabaudi, or injection of extracts from malaria-parasitized red blood cells induces hypoglycemia in normal mice and normalizes the hyperglycemia in streptozotocin (STZ)-diabetic mice. P yoelii glycosylphosphatidylinositols (GPIs) were extracted in chloroform:methanol:water (CMW) (10:10:3), purified by high-performance thin layer chromatography (HPTLC) and tested for their insulin-mimetic activities. The effects of P yoelii GPIs on blood glucose were investigated in insulin-resistant C57BL/ks-db/db diabetic mice. A single intravenous injection of GPIs (9 and 30 nmol/mouse) induced a significant dose-related decrease in blood glucose (P < .001), but insignificantly increased plasma insulin concentrations. A single oral dose of 2.7 micromol GPIs per db/db mouse significantly lowered blood glucose (P < .01). P yoelii GPIs in vitro (0.062 to 1 micromol/L) significantly stimulated lipogenesis in rat adipocytes in a dose-dependent manner both in the presence and absence of 10(-8) mol/L insulin (P < .01). P yoelii GPIs stimulated pyruvate dehydrogenase phosphatase (PDH-Pase) and inhibited both cyclic adenosine monophosphate (cAMP)-dependent protein kinase A and glucose-6-phosphatase (G6Pase). P yoelii GPIs had no effect on the activity of the gluconeogenic enzymes fructose-1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK). This is the first report of the hypoglycemic effect of P yoelii GPIs in murine models of type 2 diabetes. In conclusion, P yoelii GPIs demonstrated acute antidiabetic effects in db/db mice and in vitro. We suggest that P yoelii GPIs, when fully characterized, may provide structural information for the synthesis of new drugs for the management of diabetes.

Lethal Plasmodium yoelii malaria: the role of macrophages in normal and immunized mice.

Mice were injected with silica or Corynebacterium parvum, which, respectively, inhibit and stimulate macrophages in vivo, in an attempt to study the role of macrophages in lethal Plasmodium yoelii infection and in mice protected by immunization. In the normal infection, macrophages were able to control parasitaemia for up to 1 week, whereas in immunized mice they appeared to inhibit the sterilizing immune response. A model is proposed in which this dual role of activated macrophages may account for the chronic non-sterilizing course of natural malaria infections.

Hypoglycemia and hyperinsulinemia in rodent models of severe malaria infection.

Severe hypoglycemia developed during nonlethal Plasmodium chabaudi and lethal P. yoelii blood stage malaria infection in mice, always in association with hyperinsulinemia. Supernatants of lethal P. yoelii incubated overnight induced hypoglycemia and hyperinsulinemia in normal mice. In murine malaria, hypoglycemia may be largely secondary to increased insulin secretion.

The role of T cells in regulating immunoglobulin class and subclass.

A plaque-forming cell assay was used to follow the changes in Ig class and subclass during the response of mice to repeated injections of sheep red cells. The proportion of IgG1 rose, and that of IgG2b fell, as the response proceeded. In cell-transfer experiments both B and T cells displayed this tendency, and when asynchronously primed cells co-operated together, the pattern was characteristic of the longer-primed population. Thus T cells can advance, but cannot reverse, the progress of isotype changes in B cells. Evidence is presented for two physically separable T-cell subpopulations which need to interact in order the help B cells produce certain isotypes.

Two distinct types of non-specific immunosuppression in murine malaria.

A comparative study of non-specific immunosuppression by malaria has been carried out in five situations: in both unvaccinated and vaccinated mice infected with the lethal Plasmodium yoelii or the lethal Plasmodium berghei, and in the unvaccinated non-lethal P. yoelii infection. Spleen cells showed a suppressive effect on the normal blastogenic response to mitogens. This suppression was strongest in the mice vaccinated before infection with the lethal P. yoelii and in those infected with non-lethal P. yoelii, suggesting that the suppressive effect did not interfere with recovery. Silica, anti-Thy-1, and indomethacin treatment suggested that this suppression was caused by macrophages. However, the plaque-forming cell response to sheep RBC in vivo was suppressed equally in every case at the peak of the parasitaemia, whereas the suppression of contact sensitivity to oxazolone was strongest in mice with fatal infections. We suggest that different suppressor mechanisms operate in malaria, some being harmful to the host and others possibly beneficial.