Inhibition of Plasmodium falciparum oocyst production by membrane-permeant cysteine protease inhibitor E64d.

During asexual intraerythrocytic growth, Plasmodium falciparum utilizes hemoglobin obtained from the host red blood cell (RBC) as a nutrient source. Papain-like cysteine proteases, falcipains 2 and 3, have been reported to be involved in hemoglobin digestion and are targets of current antimalarial drug development efforts. However, their expression during gametocytogenesis, which is required for malaria parasite transmission, has not been studied. Many of the available antimalarials do not inhibit development of sexual stage parasites, and therefore, the persistence of gametocytes after drug treatment allows continued transmission of the disease. In the work reported here, incubation of stage V gametocytes with membrane-permeant cysteine protease inhibitor E64d significantly inhibited oocyst production (80 to 100%). The same conditions inhibited processing of gametocyte-surface antigen Pfs230 during gametogenesis but did not alter the morphology of the food vacuole in gametocytes, inhibit emergence, or block male exflagellation. E64d reduced the level of oocyst production more effectively than that reported previously for falcipain 1-knockout parasites, suggesting that falcipains 2 and 3 may also be involved in malaria parasite transmission. However, in this study only falcipain 3 and not falcipain 2 was found to be expressed in stage V gametocytes. Interestingly, during gametocytogenesis falcipain 3 was transported into the red blood cell and by stage V was localized in vesicles along the RBC surface, consistent with a role during gamete emergence. The ability of a membrane-permeant cysteine protease inhibitor to significantly reduce malaria parasite transmission suggests that future drug design should include evaluation of gametogenesis and sporogonic development.

Gametocytocidal activity and synergistic interactions of riboflavin with standard antimalarial drugs against growth of Plasmodium falciparum in vitro.

Our previous studies have shown that riboflavin has activity against Plasmodium falciparum asexual-stage parasites in vitro. In the present study we examine the gametocytocidal activity of riboflavin and the interaction of riboflavin with some commonly used antimalarial drugs against the asexual forms of P. falciparum in vitro. The addition of riboflavin to P. falciparum cultures killed gametocytes at all stages, even those at late stages (III to V), which are not affected by many of the commonly used antimalarials. Combinations of riboflavin with mefloquine, pyrimethamine, and quinine showed a marked potentiation of the activities of these drugs against asexual-stage parasites in vitro. The combination of riboflavin with artemisinin was additive, while that with chloroquine was mildly antagonistic. High doses of riboflavin are used clinically to treat several inborn errors of metabolism with no adverse side effects. Its efficacy in combination with standard antimalarial drugs in treating and preventing the transmission of P. falciparum malaria can therefore be evaluated in humans.

Host genetics and resistance to acute Trypanosoma cruzi infection in mice: profiles and compartmentalization of IL-2-, -4-, -5-, -10-, and IFN-gamma-producing cells.

Survival of acute Trypanosoma cruzi infection by mice is influenced by genes inside and outside the major histocompatibility complex (MHC) and genes associated with resistance must be expressed in both the MHC and the genetic background or the host will die within a few weeks of infection. Both the levels and the kinetics of cytokine production have also been implicated as important factors for resistance. Antigen-stimulated spleen cells from mice that express the resistant H-2q MHC haplotype produced significantly more interferon (IFN)-gamma than did cells from mice that share the susceptible H-2k haplotype. But, spleen cells from susceptible and resistant mice produce similar levels of IFN-gamma when stimulated with concanavalin A. The kinetics of interleukin (IL)- 10 production by ConA (ConA)-stimulated spleen cell were inversely correlated with IFN-gamma levels throughout the course of acute infection in all mouse strains. Levels of IL-2 produced by ConA-stimulated spleen cells were also initially high (day 0) then decreased as acute infection progressed. Conversely, IL-4 production by ConA-stimulated spleen cells increased during infection, and mice that express the susceptible C3H background produced significantly more IL-4 than those that share the resistant B10 background. IL-2 production by lymph node cells from mice that express the susceptible C3H genetic background also declined during infection, while lymph node cells from B10 background mice showed a moderate increase in IL-2 secretion. These data suggest that both the levels and the kinetics of cytokine production may be genetically regulated and that cytokine responses are compartmentalized in the T. cruzi-infected host.