Targeting the Parasite to Suppress Malaria Transmission

This article attempts to draw together current knowledge on the biology of Plasmodium and experience gained from past control campaigns to interpret and guide current efforts to discover and develop exciting new strategies targeting the parasite with the objective of interrupting transmission. Particular note is made of the advantages of targeting often unappreciated small, yet vital, bottleneck populations to enhance both the impact and the useful lifetime of hard-won interventions. A case is made for the standardization of methods to measure transmission blockade to permit the rational comparison of how diverse interventions (drugs, vaccines, insecticides, Genetically Modified technologies) targeting disparate aspects of parasite biology may impact upon the commonly used parameter of parasite prevalence in the human population.

Transmission blocking potency and immunogenicity of a plant-produced Pvs25-based subunit vaccine against Plasmodium vivax

Malaria transmission blocking (TB) vaccines (TBVs) directed against proteins expressed on the sexual stages of Plasmodium parasites are a potentially effective means to reduce transmission. Antibodies induced by TBVs block parasite development in the mosquito, and thus inhibit transmission to further human hosts. The ookinete surface protein P25 is a primary target for TBV development. Recently, transient expression in plants using hybrid viral vectors has demonstrated potential as a strategy for cost-effective and scalable production of recombinant vaccines. Using a plant virus-based expression system, we produced recombinant P25 protein of Plasmodium vivax (Pvs25) in Nicotiana benthamiana fused to a modified lichenase carrier protein. This candidate vaccine, Pvs25-FhCMB, was purified, characterized and evaluated for immunogenicity and efficacy using multiple adjuvants in a transgenic rodent model. An in vivo TB effect of up to a 65% reduction in intensity and 54% reduction in prevalence was observed using Abisco-100 adjuvant. The ability of this immunogen to induce a TB response was additionally combined with heterologous prime-boost vaccination with viral vectors expressing Pvs25. Significant blockade was observed when combining both platforms, achieving a 74% and 68% reduction in intensity and prevalence, respectively. This observation was confirmed by direct membrane feeding on field P. vivax samples, resulting in reductions in intensity/prevalence of 85.3% and 25.5%. These data demonstrate the potential of this vaccine candidate and support the feasibility of expressing Plasmodium antigens in a plant-based system for the production of TBVs, while demonstrating the potential advantages of combining multiple vaccine delivery systems to maximize efficacy.

The cell biology of malaria infection of mosquito: advances and opportunities

Recent reviews (Feachem et al.; Alonso et al.) have concluded that in order to have a sustainable impact on the global burden of malaria, it is essential that we knowingly reduce the global incidence of infected persons. To achieve this we must reduce the basic reproductive rate of the parasites to < 1 in diverse epidemiological settings. This can be achieved by impacting combinations of the following parameters: the number of mosquitoes relative to the number of persons, the mosquito/human biting rate, the proportion of mosquitoes carrying infectious sporozoites, the daily survival rate of the infectious mosquito and the ability of malaria-infected persons to infect mosquito vectors.

This paper focuses on our understanding of parasite biology underpinning the last of these terms: infection of the mosquito. The article attempts to highlight central issues that require further study to assist in the discovery of useful transmission-blocking measures.

The Plasmodium berghei sexual stage antigen PSOP12 induces anti-malarial transmission blocking immunity both in vivo and in vitro

Anti-malarial transmission-blocking vaccines (TBVs) aim to inhibit the transmission of Plasmodium from humans to mosquitoes by targeting the sexual/ookinete stages of the parasite. Successful use of such interventions will subsequently result in reduced cases of malarial infection within a human population, leading to local elimination. There are currently only five lead TBV candidates under examination. There is a consequent need to identify novel antigens to allow the formulation of new potent TBVs. Here we describe the design and evaluation of a potential TBV (BDES-PbPSOP12) targeting Plasmodium berghei PSOP12 based on the baculovirus dual expression system (BDES), enabling expression of antigens on the surface of viral particles and within infected mammalian cells. In silico studies have previously suggested that PSOP12 (Putative Secreted Ookinete Protein 12) is expressed within the sexual stages of the parasite (gametocytes, gametes and ookinetes), and is a member of the previously characterized 6-Cys family of plasmodial proteins. We demonstrate that PSOP12 is expressed within the sexual/ookinete forms of the parasite, and that sera obtained from mice immunized with BDES-PbPSOP12 can recognize the surface of the male and female gametes, and the ookinete stages of the parasite. Immunization of mice with BDES-PbPSOP12 confers modest but significant transmission-blocking activity in vivo by active immunization (53.1% reduction in oocyst intensity, 10.9% reduction in oocyst prevalence). Further assessment of transmission-blocking potency ex vivo shows a dose-dependent response, with up to a 76.4% reduction in intensity and a 47.2% reduction in prevalence observed. Our data indicates that PSOP12 in Plasmodium spp. could be a potential new TBV target candidate, and that further experimentation to examine the protein within human malaria parasites would be logical.

The flagellum in malarial parasites

The malarial parasites assemble flagella exclusively during the formation of the male gamete in the midgut of the female mosquito vector. The observation of gamete formation ex vivo reported by Laveran (Laveran MA: De la nature parasitaire des accidents de l'impaludisme. Comptes Rendues De La Societe de Biologie. Paris 1881, 93:627-630) was seminal to the discovery of the parasite itself. Following ingestion of malaria-infected blood by the mosquito, microgamete formation from the terminally arrested gametocytes is exceptionally rapid, completing three mitotic divisions in just a few minutes, and is precisely regulated. This review attempts to draw together the diverse original observations with subsequent electron microscopic studies, and recent work on the signalling pathways regulating sexual development, together with transcriptomic and proteomic studies that are paving the way to new understandings of the molecular mechanisms involved and the potential they offer for effective interventions to block the transmission of the parasites in natural communities.

A DNA prime-modified vaccinia virus ankara boost vaccine encoding thrombospondin-related adhesion protein but not circumsporozoite protein partially protects healthy malaria-naive adults against Plasmodium falciparum sporozoite challenge

The safety, immunogenicity, and efficacy of DNA and modified vaccinia virus Ankara (MVA) prime-boost regimes were assessed by using either thrombospondin-related adhesion protein (TRAP) with a multiple-epitope string ME (ME-TRAP) or the circumsporozoite protein (CS) of Plasmodium falciparum. Sixteen healthy subjects who never had malaria (malaria-naive subjects) received two priming vaccinations with DNA, followed by one boosting immunization with MVA, with either ME-TRAP or CS as the antigen. Immunogenicity was assessed by ex vivo gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) and antibody assay. Two weeks after the final vaccination, the subjects underwent P. falciparum sporozoite challenge, with six unvaccinated controls. The vaccines were well tolerated and immunogenic, with the DDM-ME TRAP regimen producing stronger ex vivo IFN-gamma ELISPOT responses than DDM-CS. One of eight subjects receiving the DDM-ME TRAP regimen was completely protected against malaria challenge, with this group as a whole showing significant delay to parasitemia compared to controls (P = 0.045). The peak ex vivo IFN-gamma ELISPOT response in this group correlated strongly with the number of days to parasitemia (P = 0.033). No protection was observed in the DDM-CS group. Prime-boost vaccination with DNA and MVA encoding ME-TRAP but not CS resulted in partial protection against P. falciparum sporozoite challenge in the present study.

A proteomic analysis of malaria biology: integration of old literature and new technologies

The genomic revolution has brought a new vitality into research on Plasmodium, its insect and vertebrate hosts. At the cellular level nowhere is the impact greater than in the analysis of protein expression and the 'assembly' of the supramolecular machines that together comprise the functional cell. The repetitive phases of invasion and replication that typify the malaria life cycle, together with the unique phase of sexual differentiation provide a powerful platform on which to investigate the 'molecular machines' that underpin parasite strategy and stage-specific functions. This approach is illustrated here in an analysis of the ookinete of Plasmodium berghei. Such analyses are useful only if conducted with a secure understanding of parasite biology. The importance of carefully searching the older literature to reach this understanding cannot be over-emphasised. When viewed together, the old and new data can give rapid and penetrating insights into what some might now term the 'Systems-Biology' of Plasmodium.

Mosquito--malaria interactions: a reappraisal of the concepts of susceptibility and refractoriness

This paper considers the available literature on the transmission of malaria by insects and concludes that, in contrast to the commonly held view (that implies mosquitoes are naturally vectors of malaria), it is more useful to consider that mosquitoes, like plants, normally express a variety of gene products, which together render the host resistant to infection. The consequences of this hypothesis upon current research are that when studying the passage of the malarial parasite through a competent vector it is relevant to ask either 'How have the natural innate defences of the insect failed?' or 'What mechanisms has the parasite used to overcome these defences?' At the population level, the hypothesis is consistent with the conclusions of Koella et al. that the evolutionary cost of maintaining defence mechanisms that can render the mosquito refractory (e.g. melanization) has prevented fixation of the necessary gene(s) in the insect population. We simply extend that concept by stating the innate and genetic defences that confer the natural (and sometimes incomplete) resistance to infection are of sustainable cost, with the consequence that the encoding genes may become highly prevalent or fixed in a population.

The dynamics of interactions between Plasmodium and the mosquito: a study of the infectivity of Plasmodium berghei and Plasmodium gallinaceum, and their transmission by Anopheles stephensi, Anopheles gambiae and Aedes aegypti

Knowledge of parasite-mosquito interactions is essential to develop strategies that will reduce malaria transmission through the mosquito vector. In this study we investigated the development of two model malaria parasites, Plasmodium berghei and Plasmodium gallinaceum, in three mosquito species Anopheles stephensi, Anopheles gambiae and Aedes aegypti. New methods to study gamete production in vivo in combination with GFP-expressing ookinetes were employed to measure the large losses incurred by the parasites during infection of mosquitoes. All three mosquito species transmitted P. gallinaceum; P. berghei was only transmitted by Anopheles spp. Plasmodium gallinaceum initiates gamete production with high efficiency equally in the three mosquito species. By contrast P. berghei is less efficiently activated to produce gametes, and in Ae. aegypti microgamete formation is almost totally suppressed. In all parasite/vector combinations ookinete development is inefficient, 500-100,000-fold losses were encountered. Losses during ookinete-to-oocyst transformation range from fivefold in compatible vector parasite combinations (P. berghei/An. stephensi), through >100-fold in poor vector/parasite combinations (P. gallinaceum/An. stephensi), to complete blockade (>1,500 fold) in others (P. berghei/Ae. aegypti). Plasmodium berghei ookinetes survive poorly in the bloodmeal of Ae. aegypti and are unable to invade the midgut epithelium. Cultured mature ookinetes of P. berghei injected directly into the mosquito haemocoele produced salivary gland sporozoites in An. stephensi, but not in Ae. aegypti, suggesting that further species-specific incompatibilities occur downstream of the midgut epithelium in Ae. aegypti. These results show that in these parasite-mosquito combinations the susceptibility to malarial infection is regulated at multiple steps during the development of the parasites. Understanding these at the molecular level may contribute to the development of rational strategies to reduce the vector competence of malarial vectors.

Molecular interactions between Plasmodium and its insect vectors

Our understanding of the intricate interactions between the malarial parasite and the mosquito vector is complicated both by the number and diversity of parasite and vector species, and by the experimental inaccessibility of phenomena under investigation. Steady developments in techniques to study the parasite in the mosquito have recently been augmented by methods to culture in their entirety the sporogonic stages of some parasite species. These, together with the new saturation technologies, and genetic transformation of both parasite and vector will permit penetrating studies into an exciting and largely unknown area of parasite-host interactions, an understanding of which must result in the development of new intervention strategies. This microreview highlights key areas of current basic molecular interest, and identifies numerous lacunae in our knowledge that must be filled if we are to make rational decisions for future control strategies. It will conclude by trying to explain why in the opinion of this reviewer understanding malaria-mosquito interactions may be critical to our future attempts to limit a disease of growing global importance.

Interaction between host complement and mosquito-midgut-stage Plasmodium berghei

After ingestion by mosquitoes, gametocytes of malaria parasites become activated and form extracellular gametes that are no longer protected by the red blood cell membrane against immune effectors of host blood. We have studied the action of complement on Plasmodium developmental stages in the mosquito blood meal using the rodent malaria parasite Plasmodium berghei and rat complement as a model. We have shown that in the mosquito midgut, rat complement components necessary to initiate the alternative pathway (factor B, factor D, and C3) as well as C5 are present for several hours following ingestion of P. berghei-infected rat blood. In culture, 30 to 50% of mosquito midgut stages of P. berghei survived complement exposure during the first 3 h of development. Subsequently, parasites became increasingly sensitive to complement lysis. To investigate the mechanisms involved in their protection, we tested for C3 deposition on parasite surfaces and whether host CD59 (a potent inhibitor of the complement membrane attack complex present on red blood cells) was taken up by gametes while emerging from the host cell. Between 0.5 and 22 h, 90% of Pbs21-positive parasites were positive for C3. While rat red and white blood cells stained positive for CD59, Pbs21-positive parasites were negative for CD59. In addition, exposure of parasites to rat complement in the presence of anti-rat CD59 antibodies did not increase lysis. These data suggest that parasite or host molecules other than CD59 are responsible for the protection of malaria parasites against complement-mediated lysis. Ongoing research aims to identify these molecules.

Both mosquito-derived xanthurenic acid and a host blood-derived factor regulate gametogenesis of Plasmodium in the midgut of the mosquito

Gametogenesis of Plasmodium in vitro can be induced by the combined stimulus of a 5 degrees C fall in temperature and the presence of xanthurenic acid (XA). In-vitro experiments showed that P. gallinaceum (EC(50)=80 nM) is much more sensitive to XA than P. berghei (9 microM), P. yoelii (8 microM), and P. falciparum (2 microM). However, in the mosquito vector, we do not know whether the temperature shift and XA are the only gametocyte-activating factors (GAF), nor do we know with certainty the true source(s) of XA in the mosquito blood meal. Previous studies indicate that XA is the only source of GAF in the mosquito. By defining, and then contrasting, the ability of an XA-deficient mutant of Aedes aegypti, with the wild-type mosquito to support exflagellation and ookinete formation in vivo, we determined the roles of parasite-, mosquito- and host blood-derived GAF in the regulation of gametogenesis of P. gallinaceum. Removal of both host and vector sources of GAF totally inhibited both exflagellation and ookinete production, whilst the lack of either single source resulted in only a partial reduction of exflagellation and ookinete formation in the mosquito gut. Both sources can be effectively replaced/substituted by synthetic XA. This suggests (1) both mosquito- and vertebrate-derived factors act as GAF in the mosquito gut in vivo; (2) the parasite itself is unable to produce any significant GAF activity. Studies are underway to determine whether vertebrate-derived GAF is XA. These data may form the basis of further studies of the development of new methods of interrupting malarial transmission.

Cryofracture electron microscopy of the ookinete pellicle of Plasmodium gallinaceum reveals the existence of novel pores in the alveolar membranes

The malaria parasite invades the midgut tissue of its mosquito host as a motile form called the ookinete. We have examined the pellicle of the ookinete of Plasmodium gallinaceum by freeze-fracture and quick-freeze, deep-etch electron microscopy. The general organization is analogous to that of invasive stages of other members of Apicomplexa. The pellicle is composed of three membranes: the plasma membrane, and the two linked intermediate and inner membranes, which in the ookinete form one flattened vacuole that is located beneath the plasma membrane. The edges of this vacuole form a longitudinal suture. Beneath the vacuole is found an array of microtubules that are connected to the inner membrane by intramembranous particles. During freeze-fracture, the membranes can split along their hydrophobic planes, thus yielding six fracture faces, each of which displays a characteristic pattern of intramembranous particles. Additionally, we find that the ookinete pellicle differs from all other apicomplexan motile stages by the presence of large pores. These pores are of unknown function, but clearly might constitute a novel pathway for the transport of molecules to and from the cortex, which is independent of the well-described route through the apical micronemal/rhoptry complex. The pores may be the route by which motor proteins or other non micronemal surface proteins are trafficked, such as P25/P28 and SOAP, some of which are implicated in transmission blocking immunity.

Knockout of the rodent malaria parasite chitinase pbCHT1 reduces infectivity to mosquitoes

During mosquito transmission, malaria ookinetes must cross a chitin-containing structure known as the peritrophic matrix (PM), which surrounds the infected blood meal in the mosquito midgut. In turn, ookinetes produce multiple chitinase activities presumably aimed at disrupting this physical barrier to allow ookinete invasion of the midgut epithelium. Plasmodium chitinase activities are demonstrated targets for human and avian malaria transmission blockade with the chitinase inhibitor allosamidin. Here, we identify and characterize the first chitinase gene of a rodent malaria parasite, Plasmodium berghei. We show that the gene, named PbCHT1, is a structural ortholog of PgCHT1 of the avian malaria parasite Plasmodium gallinaceum and a paralog of PfCHT1 of the human malaria parasite Plasmodium falciparum. Targeted disruption of PbCHT1 reduced parasite infectivity in Anopheles stephensi mosquitoes by up to 90%. Reductions in infectivity were also observed in ookinete feeds-an artificial situation where midgut invasion occurs before PM formation-suggesting that PbCHT1 plays a role other than PM disruption. PbCHT1 null mutants had no residual ookinete-derived chitinase activity in vitro, suggesting that P. berghei ookinetes express only one chitinase gene. Moreover, PbCHT1 activity appeared insensitive to allosamidin inhibition, an observation that raises questions about the use of allosamidin and components like it as potential malaria transmission-blocking drugs. Taken together, these findings suggest a fundamental divergence among rodent, avian, and human malaria parasite chitinases, with implications for the evolution of Plasmodium-mosquito interactions.

Anopheles gambiae laminin interacts with the P25 surface protein of Plasmodium berghei ookinetes

Laminin is a major constituent of the basal lamina surrounding the midgut of the malaria vectors that has been implicated in the development of the Plasmodium oocyst. In this report we describe the cloning of the Anopheles gambiae gene encoding the laminin gamma 1 polypeptide and follow its expression during mosquito development. To further investigate the putative role of laminin in the transmission of the malaria parasite we studied the potential binding of the P25 surface protein of Plasmodium berghei using a yeast two-hybrid system. Heterodimer formation was observed and does not require any additional protein factors since purified fusion proteins can also bind each other in vitro. Laminin gamma 1 also interacts with the paralogue of P25, namely P28, albeit more weakly, possibly explaining why the two parasite proteins can substitute for each other in deletion mutants. This represents the first direct evidence for molecular interactions between a surface protein of the Plasmodium parasite with an Anopheles protein; the strong interplay between laminin gamma 1 and P25 suggests that this pair of proteins may function as a receptor/ligand complex regulating parasite development in the mosquito vector.

Distinct roles for pbs21 and pbs25 in the in vitro ookinete to oocyst transformation of Plasmodium berghei

We have developed an in vitro culture system for early sporogonic stages of Plasmodium berghei, which can be used to study developmental events normally taking place in the midgut of an infected mosquito. These include penetration of insect cells by the mature ookinete, transformation into oocysts and the early development of the latter, sustained through several rounds of nuclear division. The system, based upon co-culture of enriched ookinetes with several established insect cell lines, was used to study the development of mutant ookinetes lacking both the Pbs21 and Pbs25 surface proteins. Motility and entry of double knockout and Pbs21 single knockout ookinetes into the insect cells are normal, but the number of ookinetes successfully transforming into oocysts expressing the CSP protein are substantially reduced. Finally, using the yeast two-hybrid system we also show that Pbs25 has the capacity to homodimerise as well as to form heterodimers with Pbs21.

The roles of the glycosylphosphatidylinositol anchor on the production and immunogenicity of recombinant ookinete surface antigen Pbs21 of Plasmodium berghei when prepared in a baculovirus expression system

Malarial ookinetes express an immunodominant surface protein (P28) that is a priority candidate for the development of transmission-blocking vaccines. The full length P28 gene from Plasmodium berghei [Pbs21(1-213)] and a deletion construct [Pbs21(1-188)] encoding a protein that lacks the 25 C-terminal amino acids, including the glycosylphosphatidylinositol (GPI) anchor signal, were expressed in insect cells using baculovirus vectors. Pbs21(1-213) protein is strongly hydrophobic, found in the cytoplasm and on the surface of Spodoptera Sf21 cells, and in the culture medium. Pbs21(1-188) protein was largely found in the aqueous phase of the medium and in the cytoplasm of Sf21 cells, but was not detected on the cell surface. The presence of 25 C-terminal amino acids is therefore critical to the attachment of recombinant Pbs21 to the parasite plasma membrane. Mice were immunized subcutaneously or intramuscularly with affinity purified recombinant Pbs21(1-213), Pbs21(1-188) or native Pbs21 proteins. Following two immunizations, native Pbs21 induces higher titres when administered by either route, than the recombinant protein bearing an insect GPI anchor, which in turn is markedly more immunogenic than the recombinant polypeptide lacking a GPI anchor. When specific anti Pbs21 antibody titres exceeded 1 mg/ml all three antigens were capable of inducing transmission blockade > or = 90%, below 1 mg/ml blockade did not correlate with antibody concentration.

Characterisation and expression of pbs25, a sexual and sporogonic stage specific protein of Plasmodium berghei

Following gametogenesis and fertilisation in the bloodmeal within the mosquito midgut, the newly formed zygotes of the malaria parasite develop into motile invasive ookinetes. During this development, surface molecules are synthesised de novo including molecules of 21-28 kDa from the zygote-ookinete stages. An antiserum recognising a 26 kDa protein of Plasmodium berghei was used to clone the corresponding gene from a cDNA library, which was shown to be identical to the reported Pbs25 gene sequence. We show here that Pbs25 was detectable in preparations of gametes 30 min post-gametocyte activation, expression continued on zygotes, ookinetes and oocysts indicating there is a significant overlap of expression of the two immunogenic zygote-ookinete proteins belonging to the P25/28 protein family of sexual stage antigens. Biochemical analysis of Pbs25 demonstrates the presence of a malaria-specific glycosylphosphatidylinositol (GPI) anchor. Antibodies recognising Pbs25 impaired parasite development in the mosquito.

Identification of differentially regulated genes of Plasmodium by suppression subtractive hybridization

Plasmodium, the causative agent of malaria, has many morphologically and functionally distinct developmental stages. In the mosquito host alone, there are five transitions during the development of a gametocyte into a sporozoite. Determining which genes are expressed at the different developmental stages is vital to our understanding of the parasite. There are a growing number of techniques designed to study gene expression, including microarray. Here, Johannes Dessens, Gabrielle Margos, Maria del Carmen Rodriguez and Robert Sinden describe a novel method: suppression subtractive hybridization (SSH) and its successful application in obtaining mosquito midgut stage-specific genes of Plasmodium.

Inhibition of the mosquito transmission of Plasmodium berghei by Malarone (atovaquone-proguanil)

Sera from patients treated with atovaquone-proguanil (Malarone) have previously been shown to inhibit the mosquito transmission of Plasmodium falciparum, though the inhibition was not complete and the effect declined 2 weeks after treatment. In marked contrast, the inhibition of transmission of P. berghei by human sera (fed to mosquitoes, with P. berghei gametocytes, via membrane feeders) from volunteers treated with atovaquone-proguanil was total up to day 28 post-treatment and still very significant at day 56. In view of the short half-lives of atovaquone and proguanil, this was unexpected, and further experiments, reported here, were undertaken. In contrast to the incomplete blockade of infectivity of P. falciparum by serum taken 4 days post-treatment, such serum was totally inhibitory against P. berghei at a 1:10,000 dilution, indicating a remarkable sensitivity of P. berghei and demonstrating an unusual difference between the two Plasmodium species in response to a drug. The inhibitory effect on P. berghei after day 4 was caused by atovaquone and mainly through blockade of development from ookinete to oocyst. Despite previous information on the rapid elimination of atovaquone by patients, the present data indicate that low concentrations of this drug may persist in the plasma for some weeks after treatment.

Anopheles arabiensis and An. funestus are equally important vectors of malaria in Matola coastal suburb of Maputo, southern Mozambique

Transmission characteristics of malaria were studied in Matola, a coastal suburb of Maputo, the capital City, in southern Mozambique, from November 1994 to April 1996. The local climate alternates between cool dry season (May-October) and hot rainy season (November-April) with mean annual rainfall 650-850 mm. Saltmarsh and freshwater pools provide mosquito breeding sites in Matola. Malaria prevalence reached approximately 60% among people living nearest to the main breeding sites of the vectors. Plasmodium falciparum caused 97% of malaria cases, others being P. malariae and P. ovale. Potential malaria vector mosquitoes (Diptera: Culicidae) collected at Matola during daytime indoor-resting (n = 1021) and on human bait at night (n = 5893) comprised 12% Anopheles coustani Laveran (93% biting outdoors), 46% An. funestus Giles (68% biting indoors) and 42% An. gambiae Giles sensu lato (60% biting outdoors). All 215 specimens of An. gambiae s.l. identified genetically were An. arabiensis Patton. Anopheles funestus populations remained stable throughout the year, whereas densities of the An. gambiae complex fluctuated considerably, with An. arabiensis peaking during the rainy season. No concomitant rise in malaria incidence was observed. Human landing indices of An. funestus and An. arabiensis averaged 1.8 and 3.8 per man-night, respectively. Overall Plasmodium sporozoite rates were 2.42+/-1.24% in 2181 An. funestus and 1.11+/-1.25% in 1689 An. arabiensis dissected and examined microscopically. Mean daily survival rates were 0.79 for both vector species. Estimated infective bites/person/year were 15 An. funestus and 12 An. arabiensis. Biting rates were greatest at 2100-24.00 hours for An. funestus (68% endophagic) and 21.00-03.00 hours for An. arabiensis (40% endophagic). The entomological inoculation rate (EIR) declined sharply over very short distances (50% per 90m) away from breeding-sites of the vectors. Consequently, P. falciparum prevalence among Matola residents was halved 350 m within the town. Implications for the protective effectiveness of a 'cordon sanitaire' by residual house-spraying and/or the use of insecticide-treated bednets are discussed.

Determination of mosquito bloodmeal pH in situ by ion-selective microelectrode measurement: implications for the regulation of malarial gametogenesis

Malarial gametocytes circulate in the peripheral blood of the vertebrate host as developmentally arrested intra-erythrocytic cells, which only resume development into gametes when ingested into the bloodmeal of the female mosquito vector. The ensuing development encompasses sexual reproduction and mediates parasite transmission to the insect. In vitro the induction of gametogenesis requires a drop in temperature and either a pH increase from physiological blood pH (ca pH 7.4) to about pH 8.0, or the presence of a gametocyte-activating factor recently identified as xanthurenic acid (XA). However, it is unclear whether either the pH increase or XA act as natural triggers in the mosquito bloodmeal. We here use pH-sensitive microelectrodes to determine bloodmeal pH in intact mosquitoes. Measurements taken in the first 30 min after ingestion, when malarial gametogenesis is induced in vivo, revealed small pH increases from 7.40 (mouse blood) to 7.52 in Aedes aegypti and to 7.58 in Anophĕles stephensi. However, bloodmeal pH was clearly suboptimal if compared to values required to induce gametogenesis in vitro. Xanthurenic acid is shown to extend the pH-range of exflagellation in vitro in a dose-dependent manner to values that we have observed in the bloodmeal, suggesting that in vivo malarial gametogenesis could be further regulated by both these factors.

Systematic screening of Anopheles mosquito genomes yields evidence for a major clade of Pao-like retrotransposons

We developed a degenerate PCR procedure to simultaneously amplify products from divergent retrotransposon families within the genomes of Anopheles mosquitoes. The procedure required cloning of multiple PCR products, but more than half of the clones subsequently sequenced were of retrotransposon origin. These included Copia-like and LINE retrotransposons, as well as the first Gypsy-like retrotransposons reported from mosquitoes. Furthermore, some Anopheles retrotransposon sequences showed similarity to the divergent Pao element from the silkmoth Bombyx mori. Phylogenetic analyses provided consistently strong bootstrap support (> 95%) for a major clade of Pao-like retrotransposons, which includes five mosquito sequences and the recently discovered Drosophila retrotransposons BEL and ninja. This appears to represent a new family of Pao-like LTR-retrotransposons distinct from the Copia and Gypsy families.

The mosquito transmission of malaria: the effects of atovaquone-proguanil (Malarone) and chloroquine

Despite its recognized importance, the prevention of patients with malaria from continuing to infect mosquitoes after treatment is not always achieved in practice. An inevitable consequence of the prolonged life-span and relative metabolic stasis of the mature gametocytes of Plasmodium falciparum is that they are not cleared by most antimalarials, and few antimalarials block infection in the mosquito vector. Previous research on the constituents of Malarone, a new 'combined antimalarial', suggested that the active components, atovaquone and proguanil, might inhibit infectivity of gametocytes to mosquitoes. We contrast here the impact of atovaquone-proguanil and chloroquine on the transmission of P. falciparum and P. berghei. While chloroquine enhanced infectivity of P. falciparum, atovaquone-proguanil caused a significant reduction. Surprisingly, sporontocidal activity against the rodent parasite persisted long after the levels of the constituent drugs would have been expected to have fallen below effective plasma concentrations on the basis of the established pharmacokinetics of atovaquone and proguanil. The P. berghei model may thus have provided a sensitive bioassay, detecting drug(s) at levels below that normally found with the usual chemical assays.

A single-chain antibody fragment specific for the Plasmodium berghei ookinete protein Pbs21 confers transmission blockade in the mosquito midgut

Mouse monoclonal antibody 13.1 (mAb 13.1) directed against Pbs21, a 21-kDa sexual-stage surface protein of Plasmodium berghei, is known to inhibit oocyst development from gametocytes and ookinetes in the mosquito midgut. To examine the properties and potential uses of a single-chain antibody fragment (scFv) for blocking transmission of malaria parasites to mosquitoes, we have cloned and sequenced the genes encoding variable regions of the immunoglobulin heavy and light chains (V(H) and V(L)) of mAb 13.1. The V(H) and V(L) genes were assembled as an scFv gene, and expressed in a baculovirus expression system. Following purification of 13.1 scFv, Western blotting and inhibition ELISA assays confirmed that 13.1 scFv retained the binding specificity of the parent mAb 13.1 for Pbs21. Furthermore, 13.1 scFv bound to the surface of P. berghei ookinetes, and blocked oocyst development in the mosquito midgut by at least 93%, as assessed by oocyst counts in mosquitoes. We suggest that the 13.1 scFv gene could be useful not only in studying the mechanism of transmission blockade, but also in generating, by mosquito germline transformation, a model system to evaluate the production of mosquitoes refractory to malaria.

CTRP is essential for mosquito infection by malaria ookinetes

The malaria parasite suffers severe population losses as it passes through its mosquito vector. Contributing factors are the essential but highly constrained developmental transitions that the parasite undergoes in the mosquito midgut, combined with the invasion of the midgut epithelium by the malaria ookinete (recently described as a principal elicitor of the innate immune response in the Plasmodium-infected insect). Little is known about the molecular organization of these midgut-stage parasites and their critical interactions with the blood meal and the mosquito vector. Elucidation of these molecules and interactions will open up new avenues for chemotherapeutic and immunological attack of parasite development. Here, using the rodent malaria parasite Plasmodium berghei, we identify and characterize the first microneme protein of the ookinete: circumsporozoite- and TRAP-related protein (CTRP). We show that transgenic parasites in which the CTRP gene is disrupted form ookinetes that have reduced motility, fail to invade the midgut epithelium, do not trigger the mosquito immune response, and do not develop further into oocysts. Thus, CTRP is the first molecule shown to be essential for ookinete infectivity and, consequently, mosquito transmission of malaria.

Model systems to evaluate the use of transgenic haematophagous insects to deliver protective vaccines

Insect vector control has proved an effective method for reducing the transmission of disease-causing organisms to human populations in many tropical countries. We are interested in employing direct genetic manipulation of insect vector genomes to use them in beneficial ways so as to have a profound and long-lasting effect on disease transmission. Our research focuses on assessing whether haematophagous insects may be used as a means to deliver protective proteins, such as an antimalarial vaccine, when they take a blood meal. The progress which has been made towards assessing this concept using a number of model systems is described.

Plasmodium differentiation in the mosquito

The essential passage of the malarial parasite through a mosquito vector results in major population bottlenecks in parasite numbers. The volume of the bloodmeal ingested by the female mosquito is 1-2 microliters. This may contain from 1 to 10(5) gametocytes. Of these, it is normal for just 12 to become macrogametes; 5-6 become ookinetes, and 2 develop into oocysts 2-7 days later. Of the 16,000 sporozoites produced from these two oocysts just 10-20 are inoculated by the malaria-infected female mosquito each time she probes when taking a subsequent bloodmeal. These significant population bottlenecks suggest that parasite differentiation is severely constrained by the environment in the mosquito, and therefore by the interactions between the parasite and the vector. This review will describe parasite differentiation in the mosquito and try to highlight the more important interactions between the parasite, the bloodmeal and the mosquito, attempting to identify those interactions which are essential to parasite differentiation, and those where the mosquito may be mounting effective strategies against this important pathogen. The potential exploitation of these interactions as possible mechanisms for intervention will be discussed.

The biosynthesis and post-translational modification of Pbs21 an ookinete-surface protein of Plasmodium berghei

Radiolabelled methionine incorporation into synchronised Plasmodium berghei gametocytes or ookinete cultures, showed that Pbs21 is not synthesised in bloodstage parasites; synthesis was detected within three hours of induction of gametogenesis; synthesis was triggered at gametogenesis, not by fertilisation. We show native Pbs21 to be a hydrophobic membrane protein that was insensitive to cleavage by phosphatidylinositol phospholipase C (PI-PLC), but sensitive to alkaline hydroxylamine, and partially sensitive to glycosylphosphatidylinositol-dependent phospholipase D (GPI-PLD) and HNO2. 3H-myristic and palmitic acid, 3H-glucosamine and mannose incorporation indicated Pbs21 was acylated and glycosylated. Linkage of the acyl group was sensitive to HNO2, which released an acyl-phosphatidylinositol more hydrophobic than that released from P3 of Trypanosoma brucei. All these properties are consistent with the presence of a malaria-specific glycosylphosphatidylinositol (GPI) anchor. In contrast recombinant Pbs21 (rPbs21), expressed in Spodoptera frugiperda cells, was sensitive to both PI-PLC and GPI-PLD, consistent with the protein being modified by a different (S. frugiperda) GPI anchor. Brefeldin A blocked secretion of rPbs21 within a cytoplasmic reticular compartment. Following deletion of the putative GPI anchor addition site (amino acids 189 213), the protein was transported to the cell surface and secreted directly into the aqueous phase of the culture medium. Deletion of amino acids 205-213 disrupted Pbs21 processing, transport through the ER and distribution onto the cell surface. Deletion of amino acids 1-28 prevented transport of Pbs21 into the ER. This suggests that correct processing of the GPI anchor in the ER-Golgi network is essential for the successful secretion of the recombinant protein, which is additionally dependent upon an N-terminal secretory signal sequence.

Heterogeneous ribosome populations are present in Plasmodium berghei during development in its vector

The genome of the rodent malaria parasite, Plasmodium berghei, contains two sets of variant ribosomal RNA (rRNA) genes, termed the A and S types, that are expressed predominantly during the vertebrate and mosquito stages of the parasite's development respectively. Using in situ hybridization, we have examined the transcriptional activity of the A- and S-type rRNA genes, and the switch in expression of the ribosome populations that occurs after parasite transmission to the mosquito. By detection of precursor rRNA molecules, we show that A-type rRNA transcription is downregulated throughout development in the mosquito, whereas the initiation of S-type rRNA transcription is linked to the proliferative phase of the oocyst. Mature A-type rRNA persists during development of the zygote into the ookinete/young oocyst. In contrast, mature S-type rRNA is first detectable in young oocysts and is subsequently present at high levels during further development of oocysts and sporozoites. These results demonstrate that the switch in transcription between the A- and S-type rRNA genes is developmentally regulated, taking place only as the parasite begins to proliferate in the mosquito. A-type ribosomes are therefore not only translationally active in the early stages of development in the mosquito, but are also crucial at this phase.

Protection from Plasmodium berghei infection by priming and boosting T cells to a single class I-restricted epitope with recombinant carriers suitable for human use

The desirability of inducing cytotoxic T cell responses to defined epitopes in humans has led to the development of a variety of recombinant delivery systems. Recombinant protein particles derived from a yeast retrotransposon (Ty) and the modified Ankara vaccinia (MVA) virus can deliver large epitope strings or even whole proteins. Both have previously been administered safely in humans. Immunization with recombinant Ty and MVA containing a single Plasmodium berghei class I-binding epitope provided 95% sterile protection against malaria in mice. The sequence of immunization, Ty followed by MVA, was critical to elicit high levels of IFN-gamma-producing cells and protection. The reciprocal sequence (MVA/TY) or homologous boosting was not protective. Both constructs (Ty and MVA) contain the H-2Kd-restricted pb9 CTL epitope from the circumsporozoite protein of P. berghei among a string of 8-15 human P. falciparum-derived CTL epitopes restricted through 7 common HLA alleles as well as widely recognized CD4 T cell epitopes. Thus, the novel recombinant Ty/MVA prime/boost combination with these constructs provides a safe alternative for evaluation for human vaccination against P. falciparum malaria.

Transgenic expression of a mosquito-stage malarial protein, Pbs21, in blood stages of transformed Plasmodium berghei and induction of an immune response upon infection

Pbs21 is a surface protein of the ookinete of Plasmodium berghei, which can induce a potent transmission-blocking immune response. Pbs21 is normally expressed only by parasite stages in the mosquito, i.e., female gametes/zygotes, ookinetes, and oocysts. However, the Pbs21 gene is transcribed in female gametocytes which circulate in the bloodstream of the host, where translation of the resulting mRNA is totally repressed. Episomal transfection has been used to investigate whether expression of Pbs21 protein could be achieved in blood stages of the parasite. By using plasmid pMD221, the complete mRNA-encoding region of Pbs21, flanked only by 218 nucleotides (nt) of its promoter region and 438 nt of its 3' region downstream from the polyadenylation site, was introduced into the blood stages of gametocyte-producing and non-gametocyte-producing clones of P. berghei. In both of these transformed parasite lines, Pbs21 protein was expressed in asexual trophozoites, schizonts, and, when present, in both male and female gametocytes. Hence, the flanking regions present are sufficient to allow transcription but lack the elements that exert natural control of sex- and stage-specific transcription. The mRNA and the protein expressed by transformed blood stages were indistinguishable from the wild-type forms by the criteria tested, and the protein was recognized by both conformation-dependent and conformation-independent monoclonal antibodies raised against native Pbs21. In mice infected with transformed non-gametocyte-producing parasites, a Pbs21-specific immune response was induced and characterized with respect to isotype (IgG2a/IgG2b) and quantity (11. 5 +/- 10 microg/ml) of antibody produced. However, as found in previous studies, these antibody levels were insufficient to inhibit development of the parasites in the mosquito. The ability to express mosquito midgut-stage antigens in blood-stage parasites will facilitate further investigations of molecular and immunological properties of these proteins.

Identification of xanthurenic acid as the putative inducer of malaria development in the mosquito

Malaria is transmitted from vertebrate host to mosquito vector by mature sexual blood-living stages called gametocytes. Within seconds of ingestion into the mosquito bloodmeal, gametocytes undergo gametogenesis. Induction requires the simultaneous exposure to at least two stimuli in vitro: a drop in bloodmeal temperature to 5 degrees C below that of the vertebrate host, and a rise in pH from 7.4 to 8.0-8.2. In vivo the mosquito bloodmeal has a pH of between 7.5 and 7.6. It is thought that in vivo the second inducer is an unknown mosquito-derived gametocyte-activating factor. Here we show that this factor is xanthurenic acid. We also show that low concentrations of xanthurenic acid can act together with pH to induce gametogenesis in vitro. Structurally related compounds are at least ninefold less effective at inducing gametogenesis in vitro. In Drosophila mutants with lesions in the kynurenine pathway of tryptophan metabolism (of which xanthurenic acid is a side product), no alternative active compound was detected in crude insect homogenates. These data could form the basis of the rational development of new methods of interrupting the transmission of malaria using drugs or new refractory mosquito genotypes to block parasite gametogenesis.

The differing impact of chloroquine and pyrimethamine/sulfadoxine upon the infectivity of malaria species to the mosquito vector

Using serum or infected blood from Danish volunteers and Plasmodium falciparum-infected Mozambican patients, respectively, the impact of curative doses of chloroquine and pyrimethamine/sulfadoxine upon infectivity of P. falciparum to Anopheles arabiensis and An. gambiae or of P. berghei to An. stephensi was studied. Both treatments cleared circulating P. falciparum gametocytes within 28 days. Before this clearance, chloroquine enhanced infectivity to An. arabiensis, whereas pyrimethamine/sulfadoxine decreased infectivity. Patients harboring chloroquine-resistant parasites as opposed to -sensitive ones were 4.4 times more likely to have gametocytes following treatment. In contrast, pyrimethamine/sulfadoxine-resistant parasites were 1.9 times less likely to produce gametocytes. In laboratory infections using replicated P. berghei or P. falciparum preparations, serum from chloroquine-treated, uninfected, nonimmune volunteers enhanced gametocyte infectivity with increasing efficiency for 21 days following treatment, whereas pyrimethamine/sulfadoxine significantly suppressed infectivity. The observed enhancement in infectivity induced by the use of chloroquine combined with increased gametocytemias in chloroquine-resistant strains may in part explain the rapid spread of chloroquine resistance in endemic populations.

The Matola malaria project: a temporal and spatial study of malaria transmission and disease in a suburban area of Maputo, Mozambique

A temporal and spatial study of malaria transmission in a suburban area of Maputo, Mozambique with a mean population density of 2,737/km2 was made from December 1992 to June 1995. A steep but continuous gradient was observed in the Plasmodium falciparum prevalence from 59.0% adjacent to the breeding sites to 5.4% only a few hundred meters distant. The entomologic inoculation rate ranged from a number too low to be determined in some districts to 20 infectious bites per person per year in the others. The risk of malaria was 6.2 times higher for individuals living less than 200 meters from the breeding sites than for individuals living 500 meters or more away from the breeding sites. In areas of high human density, mosquito and parasite dispersion is very limited, and therefore malaria control strategies could be more specifically targeted.

The roles of temperature, pH and mosquito factors as triggers of male and female gametogenesis of Plasmodium berghei in vitro

Developmentally arrested malarial gametocytes undergo gamete formation in the mosquito midgut immediately after ingestion of the infected bloodmeal. In the rodent malaria parasite Plasmodium berghei male gametogenesis (exflagellation) can be induced in vitro by a temperature decrease (from 39 degrees C in the vertebrate host to 20 degrees C) and a concomitant pH increase (from 7.3 in mouse blood to 8.0). We report the presence of additional Gametocyte Activating Factor(s) (GAF) present in Anopheles stephensi tissue extracts, which induce both male and female gametogenesis at the otherwise nonpermissive pH of 7.3 in vitro but are unable to overcome the low temperature requirement. All constituent cellular events of microgametogeneis studied here are induced by the same triggers in vitro. A temperature decrease is also required for exflagellation in the mosquito midgut. The possible role of GAF as a second obligatory natural trigger of gametogenesis is discussed.

Cloning and expression of the thrombospondin related adhesive protein gene of Plasmodium berghei

Sporozoite recognition of host cells is a key step in the life-cycle of malaria parasites. Two sporozoite proteins have so far been characterized in some detail, the circumsporozoite protein (CS) and thrombospondin related adhesive protein (TRAP). We report here the cloning and expression of the TRAP gene homologue from Plasmodium berghei, PbTRAP. The PbTRAP gene encodes a protein of 606 amino acids having a deduced molecular mass of 66 kDa. The overall structure is clearly that of the TRAP family having a signal sequence followed by an integrin A domain, a sulphatide binding motif, followed by a proline based repeat before a transmembrane domain and helical cytoplasmic tail. The observed molecular mass is almost 50% larger than expected, this can be explained almost entirely by the abnormal behaviour in SDS-PAGE of the proline based repeat. As would be expected PbTRAP shows greatest similarity with the P. yoelli TRAP homologue sporozoite surface protein 2 (SSP2) than with PfTRAP, the TRAP gene from P. falciparum. The pattern of expression is similar to that of SSP2.

Epitope mapping on the ookinete surface antigen Pbs21 of Plasmodium berghei: identification of the site of binding of transmission-blocking monoclonal antibody 13.1

The ookinete surface protein of Plasmodium berghei Pbs21 belongs to a class of sexual stage antigens able to induce in the vertebrate host a transmission-blocking immune response. The effectors of this transmission-blocking immunity are antibody molecules directed against particular protein epitopes. The anti-Pbs21 monoclonal antibody 13.1 is known to bind a linear stretch of amino acids within the primary sequence of Pbs21 and to efficiently block the development of P. berghei in the mosquito gut. To map the 13.1 epitope along the amino acid sequence of Pbs21 we assayed the ability of 13.1 antibody to recognize, in Western blot, a series of Pbs21 deletion mutants as well as the ability of synthetic peptides to inhibit 13.1 binding to full length Pbs21. The epitope was identified within the second EGF-like domain of the Pbs21 molecule.

Plasmodium berghei: infectivity of mice to Anopheles stephensi mosquitoes

The infectivity of P. berghei-infected TO mice to mosquitoes declines rapidly 2 to 5 days after blood inoculation, in spite of rising numbers of gametocytes in the blood. This pattern is typical of many malaria infections and various factors, particularly specific and nonspecific immune responses, have previously been implicated in the decline. Here we report that (1) simple physiological changes in the mouse blood, namely, falling pH and bicarbonate levels induced by high parasitaemias, are responsible for the sustained inhibition of infectivity; (2) the inhibition is reversible in vivo by the addition of sodium bicarbonate alone; (3) the inhibition occurs at the point of exflagellation; (4) contrary to previous observations (Kawamoto et al. 1992), exflagellation in P. berghei, like that in P. gallinaceum (Bishop and McConnachie 1956; Nijhout and Carter 1978; Nijhout 1979) and P. falciparum (Ogwan'g et al. 1993), is dependent on extracellular bicarbonate; and (5) induction of exflagellation by a mosquito factor is bicarbonate dependent. These new observations are critical to the design and interpretation of experiments on other transmission blocking phenomena.

Localization of ribosomal RNA and Pbs21-mRNA in the sexual stages of Plasmodium berghei using electron microscope in situ hybridization

A reproducible technique for the ultrastructural localization of RNAs in malaria parasites has been developed which combines excellent structural preservation with high hybridization signals. Signals obtained following in situ hybridization with an antisense rRNA probe which recognizes all forms of small subunit (SSU) rRNA correlate with the density of ribosomes in the parasite cytoplasm and show that a) the male gametocyte has only 12 to 25% the ribosomes found in the female cell and asexual parasite and b) the probe did not hybridize with an electron-dense nuclear body previously called a nucleolus. We suggest this structure is either a transcription-, or a replication-factory. Using a probe for the sexual stage-specific protein Pbs21 mRNA, signal was found only in female gametocytes, zygotes and ookinetes and showed a non-random, clumped cytoplasmic distribution. It is not known at present whether the non-random distribution of the Pbs 21 mRNA is critical to the very delayed translation of the Pbs21 message into protein, which occurs only in the zygote and ookinete.

Nitric oxide synthase activity in malaria-infected mice

Nitric oxide (NO) is implicated in a variety of major cellular functions including defence from invasion by microbical pathogens. Evidence has been presented suggesting that it is an important mediator of protection in the early non-specific responses to malaria in mice infected with Plasmodium chabaudi (Taylor-Robinson et al. 1993). Other data from in vitro studies on the asexual stages of human parasite Plasmodium falciparum indicated that while nitric oxide itself may not be inhibitory to parasite development, its downstream products do have some anti-plasmodial activity (Rockett et al. 1991) and these could be generated by macrophages (Gyan et al. 1994). Similarly, the sexual phases of both rodent (Motard et al. 1993) and human malaria (Naotunne et al. 1993) are reportedly susceptible to the toxic effects mediated by nitric oxide generated by blood leucocytes in the course of transmission to the mosquito vector.

Comparison of numerous delivery systems for the induction of cytotoxic T lymphocytes by immunization

A variety of vaccine delivery systems including peptides with various adjuvants, recombinant particles, live recombinant viruses and bacteria and plasmid DNA were tested for their ability to induce CD8+ cytotoxic T lymphocytes (CTL) against a well-defined epitope (amino acids 252-260) from the circumsporozoite (CS) protein of Plasmodium berghei. We compared routes of immunization that would be applicable for the administration of a malaria vaccine in humans. The majority of these vaccines did not induce high CTL responses in the spleens of immunized mice. However, both a yeast-derived Ty virus-like particle expressing the optimal nine-amino acid epitope SYIPSAEKI from the CS protein (CSP-VLP) and a lipid-tailed peptide of this same sequence induced high levels of the major histocompatibility complex (MHC) class I-restricted CTL with one and three subcutaneous immunizations, respectively. Moreover, these CTL were able to recognize naturally processed antigen expressed by a recombinant vaccinia virus. The levels of CTL induced by CSP-VLP could be augmented by co-immunization with certain cytokines. Target cells pulsed with CSP-VLP were recognized and lysed, showing that the particles were effectively processed and presented through MHC class I presentation pathway. The levels of CTL induced using CSP-VLP and lipopeptides are comparable to those observed after immunization with multiple doses of irradiated sporozoites.