Antisense oligonucleotides targeting malarial aldolase inhibit the asexual erythrocytic stages of Plasmodium falciparum.

A major obstacle in the global effort to control malaria is the paucity of anti-malarial drugs. This is compounded by the continuing emergence and spread of resistance to old and new anti-malarial drugs in the malarial parasites. Here we describe the anti-malarial effect of phosphorothioate antisense (AS) oligodeoxynucleotides (ODNs) targeting the aldolase enzyme of Plasmodium falciparum, using the asexual blood stages of the parasite grown in vitro. The blood stages of P. falciparum depend almost entirely on the energy produced by their own glycolysis. Aldolase, the fourth enzyme of the glycolytic pathway, is highly upregulated during the malarial 48-h life cycle. We found that the mRNA of this enzyme can be inhibited, in a sequence specific manner, using AS-ODN to the splice sites on the pre-mRNA of malarial aldolase. At the enzyme level, both specific AS-ODNs for the splice sites, as well as for the translation initiation site on mature mRNA, can inhibit aldolase enzyme activity within the trophozoites of P. falciparum. Furthermore, this downregulation of the malarial aldolase results in a reduction in the production of ATP within the parasite. Finally, the treatment reduces parasitemia. In summary, AS-ODNs targeting the aldolase gene of P. falciparum can interfere with the blood-stage life cycle of this parasite in vitro by inhibiting the expression of the enzyme aldolase which results in decreased malarial glycolysis and energy production. Thus, we conclude that blockade of the expression of malarial glycolytic enzymes using specific AS-ODNs has the potential of a new anti-malarial strategy.

Correlation of increased expression of intercellular adhesion molecule-1, but not high levels of tumor necrosis factor-alpha, with lethality of Plasmodium yoelii 17XL, a rodent model of cerebral malaria.

Previous studies demonstrated that Plasmodium yoelii 17XL, a lethal strain of rodent malaria, causes a syndrome in SW mice that resembles human cerebral malaria. The mouse brain pathology is characterized by cytoadherence of parasitized erythrocytes. Here, the possible mechanisms mediating cerebral malaria in this model were studied and the results were compared with a nonlethal strain of this parasite, P. yoelii 17XNL (nonlethal), which does not cause cerebral malaria. Immunostaining for intercellular adhesion molecule-1 (ICAM-1) revealed an increase in expression of this protein in the small venules and capillaries of the brains of infected mice that increased with time after infection. Staining was more pronounced during the lethal infection than the nonlethal infection. Some staining with monoclonal antibody to vascular cell adhesion molecule-1 was also observed, but it was quantitatively less than ICAM-1 staining and was limited to larger venules. During the lethal infection, levels of tumor necrosis factor-alpha (TNF-alpha) increased rapidly, peaking on day 4. In contrast, mice infected with nonlethal P. yoelii had a slower serum TNF-alpha response that peaked on day 10, prior to the maximum parasitemia. In addition, mice with a targeted disruption of the TNF-alpha gene (TNF-alpha-/- mice) were infected with the lethal and nonlethal strains of P. yoelii 17X. The TNF-alpha-/- mice infected with the nonlethal parasite had significantly higher levels of parasitemia than controls, whereas TNF-alpha-/- mice infected with the lethal strain had slightly higher levels of infected erythrocytes but were equally susceptible to death from this infection. Thus, TNF-alpha does not appear to be essential in mediating death. These results demonstrate that P. yoelii 17XL infection has features in common with human cerebral malaria and suggest that this model may be useful in testing strategies to alleviate this syndrome.

Predominant role of tumor necrosis factor-alpha in human monocyte IL-10 synthesis.

In previous studies it has been shown that the bacterial endotoxin LPS induces an initial burst of inflammatory cytokine synthesis in human monocytes, which is followed by substantial IL-10 production; the IL-10 then down-regulates the inflammatory cytokine production as well as IL-10 production itself. Herein we tested the hypothesis that IL-10 production in human monocytes is under control of one of the cytokines induced by LPS. Accordingly, we cocultured purified human peripheral blood monocytes with a panel of cytokines including TNF-alpha, IL-1 alpha, IL-1 beta, IL-6, granulocyte macrophage-CSF, transforming growth factor-beta, and IFN-alpha and then measured IL-10 mRNA production using a semiquantitative reverse transcription-polymerase chain reaction technique. We found that TNF-alpha had a major effect on IL-10 mRNA production, inducing a 20- to 120-fold increase over baseline production. In contrast, IL-1 alpha, IL-1 beta, IL-6, granulocyte macrophage-CSF, transforming growth factor-beta, and IFN-alpha had little effect (< 3-fold). The induction of IL-10 mRNA by TNF-alpha in monocytes was dose dependent and began between 8 and 24 h after the addition of TNF-alpha; this suggests that the increased IL-10 mRNA level was due to de novo mRNA synthesis rather than mRNA stabilization; this latter finding was corroborated by actinomycin-D time course studies, which showed that the half-life of IL-10 was less than 1 h and was not significantly altered by TNF-alpha. These studies concerning IL-10 mRNA induction by TNF-alpha were corroborated by studies of IL-10 protein secretion: TNF-alpha alone, but not IL-1 alpha, IL-1 beta, or IL-6 induces substantial IL-10 secretion. Furthermore, LPS induces a large amount of IL-10 secretion that is largely inhibited (50 to 75%) by anti-TNF-alpha but not by antibodies to other inflammatory cytokines. Finally, TNF-alpha augments LPS-induced IL-10 secretion. Taken together, these findings suggest that TNF-alpha is unique among the inflammatory cytokines in its role as an inducer of IL-10 in human monocytes, as such, it induces a molecule that provides negative feedback to its own production.

Cross-reacting antigens to Pc96, a protective antigen of Plasmodium chabaudi, in P. falciparum, P. vivax, and P. cynomolgi.

Mice can be partially protected against Plasmodium chabaudi by immunization with the antigen Pc96, isolated from the erythrocyte membranes of infected mice. We used a Pc96 specific monoclonal antibody to identify antigens which cross-react with Pc96 in P. falciparum, P. vivax, and P. cynomologi. The cross-reactive molecules are antigens of Mr 155,000 in P. falciparum, Mr 220,000 in P. cynomologi. They are located in the surface membranes of infected erythrocytes. Pc96 is characterized by immunoelectron microscopy and epitope mapping.

Protective antigen in the membranes of mouse erythrocytes infected with Plasmodium chabaudi.

A malarial antigen, Pc96, in the plasma membrane of erythrocytes infected with Plasmodium chabaudi has been identified. It is synthesized by the parasite and present during most of the growth stages of the intra-erythrocytic cycle as demonstrated by immunofluorescence. The antigen has a molecular weight of approximately 96,000. Monoclonal antibodies raised against this antigen were used to isolate the protein by affinity chromatography. Mice immunized with affinity-purified Pc96 were partially protected against blood induced-P. chabaudi infection. This result indicates the existence of a protective antigen in the membranes of erythrocytes parasitized by a rodent malaria and encourages the search for analogous antigens in human malaria parasites as possible candidate molecules for malaria vaccination.