Plasmodium evasion of mosquito immunity and global malaria transmission: The lock-and-key theory

Tripartite interactions between viruses, parasites, and mosquitoes

Mosquito-borne diseases have a major impact on global human health. Biological agents that colonize the mosquito vector are increasingly explored as an intervention strategy to prevent vector-borne disease transmission. For instance, the release of mosquitoes carrying the endosymbiotic bacterium Wolbachia effectively reduced dengue virus incidence and disease. Insect-specific viruses are likewise considered as biocontrol agents against vector-borne diseases. While most studies focused on insect-specific viruses as an intervention against arthropod-borne viruses, we here consider whether mosquito-specific viruses may affect the transmission of the malaria-causing Plasmodium parasite by Anopheles mosquitoes. Although there is no direct experimental evidence addressing this question, we found that viral infections in dipteran insects activate some of the immune pathways that are antiparasitic in Anopheles. These findings suggest that indirect virus-parasite interactions could occur and that insect-specific viruses may modulate malaria transmission. Tripartite interactions between viruses, parasites, and Anopheles mosquitoes thus merit further investigation.

Late sporogonic stages of Plasmodium parasites are susceptible to the melanization response in Anopheles gambiae mosquitoes

The malaria-causing parasites have to complete a complex infection cycle in the mosquito vector that also involves attack by the insect's innate immune system, especially at the early stages of midgut infection. However, Anopheles immunity to the late Plasmodium sporogonic stages, such as oocysts, has received little attention as they are considered to be concealed from immune factors due to their location under the midgut basal lamina and for harboring an elaborate cell wall comprising an external layer derived from the basal lamina that confers self-properties to an otherwise foreign structure. Here, we investigated whether Plasmodium berghei oocysts and sporozoites are susceptible to melanization-based immunity in Anopheles gambiae. Silencing of the negative regulator of melanization response, CLIPA14, increased melanization prevalence without significantly increasing the numbers of melanized oocysts, while co-silencing CLIPA14 with CLIPA2, a second negative regulator of melanization, resulted in a significant increase in melanized oocysts and melanization prevalence. Only late-stage oocysts were found to be melanized, suggesting that oocyst rupture was a prerequisite for melanization-based immune attack, presumably due to the loss of the immune-evasive features of their wall. We also found melanized sporozoites inside oocysts and in the hemocoel, suggesting that sporozoites at different maturation stages are susceptible to melanization. Silencing the melanization promoting factors TEP1 and CLIPA28 rescued oocyst melanization in CLIPA2/CLIPA14 co-silenced mosquitoes. Interestingly, silencing of CTL4, that protects early stage ookinetes from melanization, had no effect on oocysts and sporozoites, indicating differential regulation of immunity to early and late sporogonic stages. Similar to previous studies addressing ookinete stage melanization, the melanization of Plasmodium falciparum oocysts was significantly lower than that observed for P. berghei. In summary, our results provide conclusive evidence that late sporogonic malaria parasite stages are susceptible to melanization, and we reveal distinct regulatory mechanisms for ookinete and oocyst melanization.

Revisiting the determinants of malaria transmission

Malaria parasites have coevolved with humans over thousands of years, mirroring their migration out of Africa. They persist to this day, despite continuous elimination efforts worldwide. These parasites can adapt to changing environments during infection of human and mosquito, and when expanding the geographical range by switching vector species. Recent studies in the human malaria parasite, Plasmodium falciparum, identified determinants governing the plasticity of sexual conversion rates, sex ratio, and vector competence. Here we summarize the latest literature revealing environmental, epigenetic, and genetic determinants of malaria transmission.

The heat shock protein Hsc70-3 mediates an anti-apoptotic response critical for Plasmodium evasion of Anopheles gambiae immunity

IMPORTANCE

Malaria transmission by Anopheles gambiae mosquitoes is very effective, in part because the parasite expresses a surface protein called Pfs47 that allows it to evade the mosquito immune system. Here we investigate how this protein changes the response of mosquito midgut epithelial cells to invasion by the parasite. Pfs47 is known to interact with P47Rec, a mosquito midgut receptor. We found that Pf47Rec inhibits caspase-mediated apoptosis by interacting with the Hsc70-3. This disrupts nitration of midgut epithelial cells invaded by the parasite and the release of hemocyte-derived microvesicles, which are critical for effective activation of the mosquito complement system that eliminates the parasite.

The African mosquito-borne diseasosome: geographical patterns, range expansion and future disease emergence

Mosquito-borne diseases (MBDs) threaten public health and food security globally. We provide the first biogeographic description of the African mosquito fauna (677 species) and the 151 mosquito-borne pathogens (MBPs) they transmit. While mosquito species richness agrees with expectations based on Africa's land surface, African arboviruses and mammalian plasmodia are more speciose than expected. Species assemblages of mosquitoes and MBPs similarly separate sub-Saharan Africa from North Africa, and those in West and Central Africa from eastern and southern Africa. Similarities between mosquitoes and MBPs in diversity and range size suggest that mosquitoes are key in delimiting the range of MBPs. With approximately 25% endemicity, approximately 50% occupying one to three countries and less than 5% occupying greater than 25 countries, the ranges of mosquitoes and MBPs are surprisingly small, suggesting that most MBPs are transmitted by a single mosquito species. Exceptionally widespread mosquito species feed on people and livestock, and most are high-altitude-windborne migrants. Likewise, widespread MBPs are transmitted among people or livestock by widespread mosquitoes, suggesting that adapting to people or livestock and to widespread mosquito species promote range expansion in MBPs. Range size may predict range expansion and emergence risk. We highlight key knowledge gaps that impede prediction and mitigation of future emergence of local and global MBDs.

Role of Pfs47 in the dispersal of ancestral Plasmodium falciparum malaria through adaptation to different anopheline vectors

Plasmodium falciparum malaria originated when Plasmodium praefalciparum, a gorilla malaria parasite transmitted by African sylvan anopheline mosquitoes, adapted to humans. Pfs47, a protein on the parasite surface mediates P. falciparum evasion of the mosquito immune system by interacting with a midgut receptor and is critical for Plasmodium adaptation to different anopheline species. Genetic analysis of 4,971 Pfs47 gene sequences from different continents revealed that Asia and Papua New Guinea harbor Pfs47 haplotypes more similar to its ortholog in P. praefalciparum at sites that determine vector compatibility, suggesting that ancestral P. falciparum readily adapted to Asian vectors. Consistent with this observation, Pfs47-receptor gene sequences from African sylvan malaria vectors, such as Anopheles moucheti and An. marshallii, were found to share greater similarity with those of Asian vectors than those of vectors of the African An. gambiae complex. Furthermore, experimental infections provide direct evidence that transformed P. falciparum parasites carrying Pfs47 orthologs of P. praefalciparum or P. reichenowi were more effective at evading the immune system of the Asian malaria vector An. dirus than An. gambiae. We propose that high compatibility of ancestral P. falciparum Pfs47 with the receptors of Asian vectors facilitated the early dispersal of human malaria to the Asian continent, without having to first adapt to sub-Saharan vectors of the An. gambiae complex.

Knowledge, attitude, and adherence to malaria control guidelines and the prevalence of Plasmodium species infection in localities across transmission and ecological zones in Cameroon

Background

Despite a scale up of control interventions over the years, malaria remains a major public health and economic concern in Cameroon, contributing considerably to hospitalization and deaths. The effectiveness of control strategies depends on the extent of adherence by the population to national guidelines. This study assessed the influence of human knowledge, attitudes, and practices related to malaria and its control on the prevalence of malaria parasite infection, with implications for the elimination of the disease.

Methodology

This is a cross-sectional community and hospital-based study, covering the five ecological and three malaria transmission zones in Cameroon. A pre-tested semi-structured questionnaire was used to document socio-demographic and clinical parameters as well as knowledge, attitudes, and practices toward malaria control and management. Consenting participants were screened for malaria parasite with rapid diagnostic test (mRDT) of the peripheral blood. Association between qualitative variables was determined using the chi-square test and logistic regression analysis.

Results

A total of 3,360 participants were enrolled, 45.0% (1,513) of whom were mRDT positive, with 14.0% (451/3,216) and 29.6% (951/3,216) having asymptomatic parasitaemia and malaria, respectively. Although most participants knew the cause, symptoms, and control strategies, with 53.6% (1,000/1,867) expertly knowledgeable about malaria overall, only 0.1% (2/1,763) individuals were fully adherent to malaria control measures.

Conclusion

The risk of malaria in Cameroon remains high, with the population considerably knowledgeable about the disease but poorly adherent to national malaria control guidelines. Concerted and more effective strategies aimed at improving knowledge about malaria and adherences to control interventions are necessary to ultimately eliminate the disease.

Resolving drug selection and migration in an inbred South American Plasmodium falciparum population with identity-by-descent analysis

The human malaria parasite Plasmodium falciparum is globally widespread, but its prevalence varies significantly between and even within countries. Most population genetic studies in P. falciparum focus on regions of high transmission where parasite populations are large and genetically diverse, such as sub-Saharan Africa. Understanding population dynamics in low transmission settings, however, is of particular importance as these are often where drug resistance first evolves. Here, we use the Pacific Coast of Colombia and Ecuador as a model for understanding the population structure and evolution of Plasmodium parasites in small populations harboring less genetic diversity. The combination of low transmission and a high proportion of monoclonal infections means there are few outcrossing events and clonal lineages persist for long periods of time. Yet despite this, the population is evolutionarily labile and has successfully adapted to changes in drug regime. Using newly sequenced whole genomes, we measure relatedness between 166 parasites, calculated as identity by descent (IBD), and find 17 distinct but highly related clonal lineages, six of which have persisted in the region for at least a decade. This inbred population structure is captured in more detail with IBD than with other common population structure analyses like PCA, ADMIXTURE, and distance-based trees. We additionally use patterns of intra-chromosomal IBD and an analysis of haplotypic variation to explore past selection events in the region. Two genes associated with chloroquine resistance, crt and aat1, show evidence of hard selective sweeps, while selection appears soft and/or incomplete at three other key resistance loci (dhps, mdr1, and dhfr). Overall, this work highlights the strength of IBD analyses for studying parasite population structure and resistance evolution in regions of low transmission, and emphasizes that drug resistance can evolve and spread in small populations, as will occur in any region nearing malaria elimination.

Deceiving and escaping complement - the evasive journey of the malaria parasite

During its life cycle, Plasmodium, the malaria parasite, is exposed to the human and mosquito complement systems. Early experiments demonstrated that activation of complement can pose a serious threat to parasites, but recent studies revealed complement-evasion mechanisms important for parasite survival. Blood-stage parasites and gametes recruit regulators to neutralize human complement activation, while ookinetes inhibit mosquito complement by disrupting epithelial nitration in response to midgut invasion. Here we provide an in-depth overview of the evasion mechanisms currently known and speculate on the existence of others not yet identified. Finally, we discuss how these mechanisms could provide novel targets for urgently needed malaria vaccines and therapeutics.

The Defensive Interactions of Prominent Infectious Protozoan Parasites: The Host's Complement System

The complement system exerts crucial functions both in innate immune responses and adaptive humoral immunity. This pivotal system plays a major role dealing with pathogen invasions including protozoan parasites. Different pathogens including parasites have developed sophisticated strategies to defend themselves against complement killing. Some of these strategies include the employment, mimicking or inhibition of host's complement regulatory proteins, leading to complement evasion. Therefore, parasites are proven to use the manipulation of the complement system to assist them during infection and persistence. Herein, we attempt to study the interaction´s mechanisms of some prominent infectious protozoan parasites including Plasmodium, Toxoplasma, Trypanosoma, and Leishmania dealing with the complement system. Moreover, several crucial proteins that are expressed, recruited or hijacked by parasites and are involved in the modulation of the host´s complement system are selected and their role for efficient complement killing or lysis evasion is discussed. In addition, parasite's complement regulatory proteins appear as plausible therapeutic and vaccine targets in protozoan parasitic infections. Accordingly, we also suggest some perspectives and insights useful in guiding future investigations.

Genome-wide SNP analysis of Plasmodium falciparum shows differentiation at drug-resistance-associated loci among malaria transmission settings in southern Mali

Plasmodium falciparum malaria cases in Africa represent over 90% of the global burden with Mali being amongst the 11 highest burden countries that account for 70% of this annual incidence. The persistence of P. falciparum despite massive global interventions is because of its genetic diversity that drives its ability to adapt to environmental changes, develop resistance to drugs, and evade the host immune system. Knowledge on P. falciparum genetic diversity across populations and intervention landscape is thus critical for the implementation of new strategies to eliminate malaria. This study assessed genetic variation with 12,177 high-quality SNPs from 830 Malian P. falciparum isolates collected between 2007 and 2017 from seven locations. The complexity of infections remained high, varied between sites, and showed a trend toward overall decreasing complexity over the decade. Though there was no significant substructure, allele frequencies varied geographically, partly driven by temporal variance in sampling, particularly for drug resistance and antigen loci. Thirty-two mutations in known drug resistance markers (pfcrt, pfdhps, pfdhfr, pfmdr1, pfmdr2, and pfk13) attained a frequency of at least 2% in the populations. SNPs within and around the major markers of resistance to quinolines (pfmdr1 and pfcrt) and antifolates (pfdhfr and pfdhps) varied temporally and geographically, with strong linkage disequilibrium and signatures of directional selection in the genome. These geo-temporal populations also differentiated at alleles in immune-related loci, including, protein E140, pfsurfin8, pfclag8, and pfceltos, as well as pftrap, which showed signatures of haplotype differentiation between populations. Several regions across the genomes, including five known drug resistance loci, showed signatures of differential positive selection. These results suggest that drugs and immune pressure are dominant selective forces against P. falciparum in Mali, but their effect on the parasite genome varies temporally and spatially. Interventions interacting with these genomic variants need to be routinely evaluated as malaria elimination strategies are implemented.

Understanding pathogen survival and transmission by arthropod vectors to prevent human disease

Many endemic poverty-associated diseases, such as malaria and leishmaniasis, are transmitted by arthropod vectors. Pathogens must interact with specific molecules in the vector gut, the microbiota, and the vector immune system to survive and be transmitted. The vertebrate host, in turn, is infected when the pathogen and vector-derived factors, such as salivary proteins, are delivered into the skin by a vector bite. Here, we review recent progress in our understanding of the biology of pathogen transmission from the human to the vector and back, from the vector to the host. We also highlight recent advances in the biology of vector-borne disease transmission, which have translated into additional strategies to prevent human disease by either reducing vector populations or by disrupting their ability to transmit pathogens.

Plasmodium falciparum Genetic Diversity in Coincident Human and Mosquito Hosts

Population genetic diversity of Plasmodium falciparum antigenic loci is high despite large bottlenecks in population size during the parasite life cycle. The prevalence of genetically distinct haplotypes at these loci, while well characterized in humans, has not been thoroughly compared between human and mosquito hosts. We assessed parasite haplotype prevalence, diversity, and evenness using human and mosquito P. falciparum infections collected from the same households during a 14-month longitudinal cohort study using amplicon deep sequencing of two antigenic gene fragments (ama1 and csp). To a prior set of infected humans (n = 1,175/2,813; 86.2% sequencing success) and mosquito abdomens (n = 199/1,448; 95.5% sequencing success), we added sequences from infected mosquito heads (n = 134/1,448; 98.5% sequencing success). The overall and sample-level parasite populations were more diverse in mosquitoes than in humans. Additionally, haplotype prevalences were more even in the P. falciparum human population than in the mosquito population, consistent with balancing selection occurring at these loci in humans. In contrast, we observed that infections in humans were more likely to harbor a dominant haplotype than infections in mosquitoes, potentially due to removal of unfit strains by the human immune system. Finally, within a given mosquito, there was little overlap in genetic composition of abdomen and head infections, suggesting that infections may be cleared from the abdomen during a mosquito’s lifespan. Taken together, our observations provide evidence for the mosquito vector acting as a reservoir of sequence diversity in malaria parasite populations.

Establishment of a colony of Anopheles darlingi from French Guiana for vector competence studies on malaria transmission

Anopheles darlingi is a major vector of both Plasmodium falciparum and Plasmodium vivax in South and Central America including French Guiana. However, the vector competence and physiology of this mosquito species have been scarcely studied due to difficulties in rearing it in the laboratory. Here, we report the successful establishment of a robust colony, from a mosquito collection in French Guiana. We describe our mosquito colonization procedure with relevant information on environmental conditions, mating ability, larval development, and survival, recorded over the first six critical generations. Experimental infection showed that our An. darlingi colony has a moderate permissiveness to in vitro produced gametocytes of the P. falciparum NF54 strain originating from Africa. This colony, which has reached its 21st generation, will allow further characterization of An. darlingi life-history traits and of Plasmodium–mosquito interactions with South American malaria parasites.

Vector control: agents of selection on malaria parasites?

Insect vectors are responsible for spreading many infectious diseases, yet interactions between pathogens/parasites and insect vectors remain poorly understood. Filling this knowledge gap matters because vectors are evolving in response to the deployment of vector control tools (VCTs). Yet, whilst the evolutionary responses of vectors to VCTs are being carefully monitored, the knock-on consequences for parasite evolution have been overlooked. By examining how mosquito responses to VCTs impact upon malaria parasite ecology, we derive a framework for predicting parasite responses. Understanding how VCTs affect the selection pressures imposed on parasites could help to mitigate against parasite evolution that leads to unfavourable epidemiological outcomes. Furthermore, anticipating parasite evolution will inform monitoring strategies for VCT programmes as well as uncovering novel VCT strategies.

Declines in prevalence alter the optimal level of sexual investment for the malaria parasite Plasmodium falciparum

Like most human pathogens, the malaria parasite Plasmodium falciparum experiences strong selection pressure from public health interventions such as drug treatment. While most commonly studied in the context of drug targets and related pathways, parasite adaptation to control measures likely extends to phenotypes beyond drug resistance. Here, we use modeling to explore how control measures can reduce levels of within-host competition between P. falciparum genotypes and favor higher rates of sexual investment. We validate these predictions with longitudinally sampled genomic data from French Guiana during a period of malaria decline and find that the most strongly selected genes are enriched for transcription factors involved in commitment to and development of the parasite’s sexual gametocyte form.

Successful infectious disease interventions can result in large reductions in parasite prevalence. Such demographic change has fitness implications for individual parasites and may shift the parasite’s optimal life history strategy. Here, we explore whether declining infection rates can alter Plasmodium falciparum’s investment in sexual versus asexual growth. Using a multiscale mathematical model, we demonstrate how the proportion of polyclonal infections, which decreases as parasite prevalence declines, affects the optimal sexual development strategy: Within-host competition in multiclone infections favors a greater investment in asexual growth whereas single-clone infections benefit from higher conversion to sexual forms. At the same time, drug treatment also imposes selection pressure on sexual development by shortening infection length and reducing within-host competition. We assess these models using 148 P. falciparum parasite genomes sampled in French Guiana over an 18-y period of intensive intervention (1998 to 2015). During this time frame, multiple public health measures, including the introduction of new drugs and expanded rapid diagnostic testing, were implemented, reducing P. falciparum malaria cases by an order of magnitude. Consistent with this prevalence decline, we see an increase in the relatedness among parasites, but no single clonal background grew to dominate the population. Analyzing individual allele frequency trajectories, we identify genes that likely experienced selective sweeps. Supporting our model predictions, genes showing the strongest signatures of selection include transcription factors involved in the development of P. falciparum’s sexual gametocyte form. These results highlight how public health interventions impose wide-ranging selection pressures that affect basic parasite life history traits.

A PCR-RFLP Technique to Assess the Geographic Origin of Plasmodium falciparum Strains in Central America

The elimination of malaria requires strengthening diagnosis and offering adequate and timely treatment. Imported cases of falciparum malaria represent a major challenge for pre-elimination areas, such as Central America, where chloroquine and primaquine continue to be used as first-line treatment. The pfs47 gene has been previously described as a precise molecular marker to track the geographic origin of the parasite. The aim of this study was to design a simple and low-cost technique using the polymorphic region of pfs47 to assess the geographic origin of P. falciparum strains. A PCR-RFLP technique was developed and evaluated using the MseI enzyme that proved capable of discriminating, with reasonable precision, the geographical origin of the parasites. This method could be used by national surveillance laboratories and malaria elimination programs in countries such as Honduras and Nicaragua in cases of malaria where an origin outside the Central American isthmus is suspected.

Pfs47 as a Malaria Transmission-Blocking Vaccine Target

Transmission-blocking vaccines (TBVs), pioneered by Richard Carter and others, aim to prevent parasite development in the mosquito vector and are a promising new tool for malaria elimination. Pfs47, recently identified as a TBV target, is a three-domain 6-cysteine protein on the surface of Plasmodium falciparum sexual stages. Pfs47 allows the parasite to evade mosquito immunity and is key for P. falciparum infection of the dominant malaria vectors Anopheles gambiae, Anopheles dirus, and Anopheles albimanus. Antibodies against Pfs47 domain 2 (D2) have significant transmission-blocking activity that prevents Plasmodium ookinete development and is independent of human complement. Strong transmission-blocking activity has been mapped to a region of 52 amino acids in Pfs47 D2. Efforts to optimize the immunogenicity of the Pfs47 D2 antigen with a viral-like particle have been successful, and the efficacy of a P47-based TBV was confirmed in vivo with Pbs47, the orthologue of Pfs47 in the mouse malaria parasite Plasmodium berghei. The current evidence warrants further development and clinical testing of a Pfs47-based TBV.

Malaria oocysts require circumsporozoite protein to evade mosquito immunity

Malaria parasites are less vulnerable to mosquito immune responses once ookinetes transform into oocysts, facilitating parasite development in the mosquito. However, the underlying mechanisms of oocyst resistance to mosquito defenses remain unclear. Here, we show that circumsporozoite protein (CSP) is required for rodent malaria oocysts to avoid mosquito defenses. Mosquito infection with CSP_mut parasites (mutation in the CSP pexel I/II domains) induces nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 5 (NOX5)-mediated hemocyte nitration, thus activating Toll pathway and melanization of mature oocysts, upregulating hemocyte TEP1 expression, and causing defects in the release of sporozoites from oocysts. The pre-infection of mosquitoes with the CSP_mut parasites reduces the burden of infection when re-challenged with CSP_wt parasites by inducing hemocyte nitration. Thus, we demonstrate why oocysts are invisible to mosquito immunity and reveal an unknown role of CSP in the immune evasion of oocysts, indicating it as a potential target to block malaria transmission.

Circumsporozoite protein (CSP), the major surface protein of Plasmodium sporozoites, is important for parasite targeting to mosquito salivary glands and the mammalian liver. Here, Zhu et al. show that CSP is required for rodent malaria oocysts to evade mosquito immunity by inducing hemocyte nitration and causing subsequent defects in sporozoite-release from oocysts.

Evaluation of the Pfs25-IMX313/Matrix-M malaria transmission-blocking candidate vaccine in endemic settings

Malaria control relies heavily on the use of anti-malarial drugs and insecticides against malaria parasites and mosquito vectors. Drug and insecticide resistance threatens the effectiveness of conventional malarial interventions; alternative control approaches are, therefore, needed. The development of malaria transmission-blocking vaccines that target the sexual stages in humans or mosquito vectors is among new approaches being pursued. Here, the immunological mechanisms underlying malaria transmission blocking, status of Pfs25-based vaccines are viewed, as well as approaches and capacity for first in-human evaluation of a transmission-blocking candidate vaccine Pfs25-IMX313/Matrix-M administered to semi-immune healthy individuals in endemic settings. It is concluded that institutions in low and middle income settings should be supported to conduct first-in human vaccine trials in order to stimulate innovative research and reduce the overdependence on developed countries for research and local interventions against many diseases of public health importance.

Plasmodium development in Anopheles: a tale of shared resources

Interactions between the Anopheles mosquito vector and Plasmodium parasites shape how malaria is transmitted in endemic regions. The long association of these two organisms has led to evolutionary processes that minimize fitness costs of infection and benefit both players through shared nutrient resources, parasite immune suppression, and mosquito tolerance to infection. In this review we explore recent data describing how Plasmodium falciparum, the deadliest malaria parasite, associates with one of its most important natural mosquito hosts, Anopheles gambiae, and we discuss the implications of these findings for parasite transmission and vector control strategies currently in development.

Plasmodium falciparum and Plasmodium vivax Adjust Investment in Transmission in Response to Change in Transmission Intensity: A Review of the Current State of Research

Malaria parasites can adjust the proportion of parasites that develop into gametocytes, and thus the probability for human-to-vector transmission, through changes in the gametocyte conversion rate. Understanding the factors that impact the commitment of malaria parasites to transmission is required to design better control interventions. Plasmodium spp. persist across countries with vast differences in transmission intensities, and in sites where transmission is highly seasonal. Mounting evidence shows that Plasmodium spp. adjusts the investment in transmission according to seasonality of vector abundance, and transmission intensity. Various techniques to determine the investment in transmission are available, i.e., short-term culture, where the conversion rate can be measured most directly, genome and transcriptome studies, quantification of mature gametocytes, and mosquito feeding assays. In sites with seasonal transmission, the proportion of gametocytes, their densities and infectivity are higher during the wet season, when vectors are plentiful. When countries with pronounced differences in transmission intensity were compared, the investment in transmission was higher when transmission was low, thus maximizing the parasite’s chances to be transmitted to mosquitoes. Increased transmissibility of residual infections after a successful reduction of malaria transmission levels need to be considered when designing intervention measures.

Genetic diversity and population structure of the human malaria parasite Plasmodium falciparum surface protein Pfs47 in isolates from the lowlands in Western Kenya

Plasmodium falciparum parasites have evolved genetic adaptations to overcome immune responses mounted by diverse Anopheles vectors hindering malaria control efforts. Plasmodium falciparum surface protein Pfs47 is critical in the parasite’s survival by manipulating the vector’s immune system hence a promising target for blocking transmission in the mosquito. This study aimed to examine the genetic diversity, haplotype distribution, and population structure of Pfs47 and its implications on malaria infections in endemic lowlands in Western Kenya. Cross-sectional mass blood screening was conducted in malaria endemic regions in the lowlands of Western Kenya: Homa Bay, Kombewa, and Chulaimbo. Dried blood spots and slide smears were simultaneously collected in 2018 and 2019. DNA was extracted using Chelex method from microscopic Plasmodium falciparum positive samples and used to genotype Pfs47 using polymerase chain reaction (PCR) and DNA sequencing. Thirteen observed haplotypes of the Pfs47 gene were circulating in Western Kenya. Population-wise, haplotype diversity ranged from 0.69 to 0.77 and the nucleotide diversity 0.10 to 0.12 across all sites. All the study sites displayed negative Tajima’s D values although not significant. However, the negative and significant Fu’s Fs statistical values were observed across all the study sites, suggesting population expansion or positive selection. Overall genetic differentiation index was not significant (F_ST = -0.00891, P > 0.05) among parasite populations. All Nm values revealed a considerable gene flow in these populations. These results could have important implications for the persistence of high levels of malaria transmission and should be considered when designing potential targeted control interventions.

Molecular interactions between parasite and mosquito during midgut invasion as targets to block malaria transmission

Despite considerable effort, malaria remains a major public health burden. Malaria is caused by five Plasmodium species and is transmitted to humans via the female Anopheles mosquito. The development of malaria vaccines against the liver and blood stages has been challenging. Therefore, malaria elimination strategies advocate integrated measures, including transmission-blocking approaches. Designing an effective transmission-blocking strategy relies on a sophisticated understanding of the molecular mechanisms governing the interactions between the mosquito midgut molecules and the malaria parasite. Here we review recent advances in the biology of malaria transmission, focusing on molecular interactions between Plasmodium and Anopheles mosquito midgut proteins. We provide an overview of parasite and mosquito proteins that are either targets for drugs currently in clinical trials or candidates of promising transmission-blocking vaccines.

Global diversity of the gene encoding the Pfs25 protein-a Plasmodium falciparum transmission-blocking vaccine candidate

Background

Vaccines against the sexual stages of the malarial parasite Plasmodium falciparum are indispensable for controlling malaria and abrogating the spread of drug-resistant parasites. Pfs25, a surface antigen of the sexual stage of P. falciparum, is a leading candidate for transmission-blocking vaccine development. While clinical trials have reported that Pfs25-based vaccines are safe and effective in inducing transmission-blocking antibodies, the extent of the genetic diversity of Pfs25 in malaria endemic populations has rarely been studied. Thus, this study aimed to investigate the global diversity of Pfs25 in P. falciparum populations.

Methods

A database of 307 Pfs25 sequences of P. falciparum was established. Population genetic analyses were performed to evaluate haplotype and nucleotide diversity, analyze haplotypic distribution patterns of Pfs25 in different geographical populations, and construct a haplotype network. Neutrality tests were conducted to determine evidence of natural selection. Homology models of the Pfs25 haplotypes were constructed, subjected to molecular dynamics (MD), and analyzed in terms of flexibility and percentages of secondary structures.

Results

The Pfs25 gene of P. falciparum was found to have 11 unique haplotypes. Of these, haplotype 1 (H1) and H2, the major haplotypes, represented 70% and 22% of the population, respectively, and were dominant in Asia, whereas only H1 was dominant in Africa, Central America, and South America. Other haplotypes were rare and region-specific, resulting in unique distribution patterns in different geographical populations. The diversity in Pfs25 originated from ten single-nucleotide polymorphism (SNP) loci located in the epidermal growth factor (EGF)-like domains and anchor domain. Of these, an SNP at position 392 (GGA/GCA), resulting in amino acid substitution 131 (Gly/Ala), defined the two major haplotypes. The MD results showed that the structures of H1 and H2 variants were relatively similar. Limited polymorphism in Pfs25 could likely be due to negative selection.

Conclusions

The study successfully established a Pfs25 sequence database that can become an essential tool for monitoring vaccine efficacy, designing assays for detecting malaria carriers, and conducting epidemiological studies of P. falciparum. The discovery of the two major haplotypes, H1 and H2, and their conserved structures suggests that the current Pfs25-based vaccines could be used globally for malaria control.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-05078-6.

Population genomics and evidence of clonal replacement of Plasmodium falciparum in the Peruvian Amazon

Previous studies have shown that P. falciparum parasites in South America have undergone population bottlenecks resulting in clonal lineages that are differentially distributed and that have been responsible for several outbreaks different endemic regions. In this study, we explored the genomic profile of 18 P. falciparum samples collected in the Peruvian Amazon Basin (Loreto) and 6 from the Peruvian North Coast (Tumbes). Our results showed the presence of three subpopulations that matched previously typed lineages in Peru: Bv1 (n = 17), Clonet D (n = 4) and Acre-Loreto type (n = 3). Gene coverage analysis showed that none of the Bv1 samples presented coverage for pfhrp2 and pfhrp3. Genotyping of drug resistance markers showed a high prevalence of Chloroquine resistance mutations S1034C/N1042D/D1246Y in pfmdr1 (62.5%) and K45T in pfcrt (87.5%). Mutations associated with sulfadoxine and pyrimethamine treatment failure were found on 88.8% of the Bv1 samples which were triple mutants for pfdhfr (50R/51I/108N) and pfdhps (437G/540E/581G). Analysis of the pfS47 gene that allows P. falciparum to evade mosquito immune responses showed that the Bv1 lineage presented one pfS47 haplotype exclusive to Loreto and another haplotype that was present in both Loreto and Tumbes. Furthermore, a possible expansion of Bv1 was detected since 2011 in Loreto. This replacement could be a result of the high prevalence of CQ resistance polymorphisms in Bv1, which could have provided a selective advantage to the indirect selection pressures driven by the use of CQ for P. vivax treatment.

A genotyping assay to determine geographic origin and transmission potential of Plasmodium falciparum malaria cases

As countries work towards malaria elimination, it is important to monitor imported cases to prevent reestablishment of local transmission. The Plasmodium falciparum Pfs47 gene has strong geographic population structure, because only those parasites with Pfs47 haplotypes compatible with the mosquito vector species in a given continent are efficiently transmitted. Analysis of 4,971 world-wide Pfs47 sequences identified two SNPs (at 707 and 725 bp) as sufficient to establish the likely continent of origin of P. falciparum isolates. Pfs47 sequences from Africa, Asia, and the New World presented more that 99% frequency of distinct combinations of the SNPs 707 and 725 genotypes. Interestingly, Papua New Guinea Pfs47 sequences have the highest diversity in SNPs 707 and 725. Accurate and reproducible High-Resolution Melting (HRM) assays were developed to genotype Pfs47 SNPs 707 and 725 in laboratory and field samples, to assess the geographic origin and risk of local transmission of imported P. falciparum malaria cases.

A population genetic perspective on the origin, spread and adaptation of the human malaria agents Plasmodium falciparum and Plasmodium vivax

Malaria is considered one of the most important scourges that humanity has faced during its history, being responsible every year for numerous deaths worldwide. The disease is caused by protozoan parasites, among which two species are responsible of the majority of the burden, Plasmodium falciparum and Plasmodium vivax. For these two parasite species, the questions of their origin (how and when they appeared in humans), of their spread throughout the world, as well as how they have adapted to humans have long been of interest to the scientific community. Here, we review the current knowledge that has accumulated on these different questions, thanks in particular to the analysis of the genetic and genomic variability of these parasites and comparison with related Plasmodium species infecting other host species (like non-human primates). In this paper we review the existing body of knowledge, including current research dealing with these questions, focusing particularly on genetic analysis and genomic variability of these parasites and comparison with related Plasmodium species infecting other species of host (such as non-human primates).

Additional Feeding Reveals Differences in Immune Recognition and Growth of Plasmodium Parasites in the Mosquito Host

Mosquitoes may feed multiple times during their life span in addition to those times needed to acquire and transmit malaria. To determine the impact of subsequent blood feeding on parasite development in Anopheles gambiae, we examined Plasmodium parasite infection with or without an additional noninfected blood meal. We found that an additional blood meal significantly reduced Plasmodium berghei immature oocyst numbers, yet had no effect on the human parasite Plasmodium falciparum. These observations were reproduced when mosquitoes were fed an artificial protein meal, suggesting that parasite losses are independent of blood ingestion. We found that feeding with either a blood or protein meal compromises midgut basal lamina integrity as a result of the physical distention of the midgut, enabling the recognition and lysis of immature P. berghei oocysts by mosquito complement. Moreover, we demonstrate that additional feeding promotes P. falciparum oocyst growth, suggesting that human malaria parasites exploit host resources provided with blood feeding to accelerate their growth. This is in contrast to experiments with P. berghei, where the size of surviving oocysts is independent of an additional blood meal. Together, these data demonstrate distinct differences in Plasmodium species in evading immune detection and utilizing host resources at the oocyst stage, representing an additional, yet unexplored component of vectorial capacity that has important implications for the transmission of malaria.

IMPORTANCE Mosquitoes must blood feed multiple times to acquire and transmit malaria. However, the impact of an additional mosquito blood meal following malaria parasite infection has not been closely examined. Here, we demonstrate that additional feeding affects mosquito vector competence; namely, additional feeding significantly limits Plasmodium berghei infection, yet has no effect on infection of the human parasite P. falciparum. Our experiments support that these killing responses are mediated by the physical distension of the midgut and by temporary damage to the midgut basal lamina that exposes immature P. berghei oocysts to mosquito complement, while human malaria parasites are able to evade these killing mechanisms. In addition, we provide evidence that additional feeding promotes P. falciparum oocyst growth. This is in contrast to P. berghei, where oocyst size is independent of an additional blood meal. This suggests that human malaria parasites are able to exploit host resources provided by an additional feeding to accelerate their growth. In summary, our data highlight distinct differences in malaria parasite species in evading immune recognition and adapting to mosquito blood feeding. These observations have important, yet previously unexplored, implications for the impact of multiple blood meals on the transmission of malaria.

A mating-induced reproductive gene promotes Anopheles tolerance to Plasmodium falciparum infection

Anopheles mosquitoes have transmitted Plasmodium parasites for millions of years, yet it remains unclear whether they suffer fitness costs to infection. Here we report that the fecundity of virgin and mated females of two important vectors—Anopheles gambiae and Anopheles stephensi—is not affected by infection with Plasmodium falciparum, demonstrating that these human malaria parasites do not inflict this reproductive cost on their natural mosquito hosts. Additionally, parasite development is not impacted by mating status. However, in field studies using different P. falciparum isolates in Anopheles coluzzii, we find that Mating-Induced Stimulator of Oogenesis (MISO), a female reproductive gene strongly induced after mating by the sexual transfer of the steroid hormone 20-hydroxyecdysone (20E), protects females from incurring fecundity costs to infection. MISO-silenced females produce fewer eggs as they become increasingly infected with P. falciparum, while parasite development is not impacted by this gene silencing. Interestingly, previous work had shown that sexual transfer of 20E has specifically evolved in Cellia species of the Anopheles genus, driving the co-adaptation of MISO. Our data therefore suggest that evolution of male-female sexual interactions may have promoted Anopheles tolerance to P. falciparum infection in the Cellia subgenus, which comprises the most important malaria vectors.

Plasmodium falciparum, the deadliest form of human malaria, is transmitted when female Anopheles mosquitoes bite people and take a blood meal in order to develop eggs. To date, it is still poorly understood whether Anopheles mosquitoes that get infected with P. falciparum suffer fitness costs. Here, we find that the number of eggs produced by Anopheles gambiae and Anopheles stephensi females is not affected by P. falciparum infection, and that the mating status of the mosquitoes does not impact the parasite. However, in field experiments infecting a related species, Anopheles coluzzii, with P. falciparum using blood from donors in Burkina Faso, we find that interfering with the expression of a gene normally triggered by the sexual transfer of the steroid hormone 20-hydroxyecdysone induces increasing costs to egg development as females become more infected with P. falciparum, with no impacts on the parasite. The results of our study suggest that pathways triggered by mating may help Anopheles prevent reproductive costs associated with P. falciparum infection, providing new insights into evolutionary strategies adopted by anophelines in the face of a longstanding association with Plasmodium parasites.

Transmission of Artemisinin-Resistant Malaria Parasites to Mosquitoes under Antimalarial Drug Pressure

Resistance to artemisinin-based combination therapy (ACT) in the Plasmodium falciparum parasite is threatening to reverse recent gains in reducing global deaths from malaria. While resistance manifests as delayed parasite clearance in patients, the phenotype can only spread geographically via the sexual stages and mosquito transmission. In addition to their asexual killing properties, artemisinin and its derivatives sterilize sexual male gametocytes. Whether resistant parasites overcome this sterilizing effect has not, however, been fully tested.

Resistance to artemisinin-based combination therapy (ACT) in the Plasmodium falciparum parasite is threatening to reverse recent gains in reducing global deaths from malaria. While resistance manifests as delayed parasite clearance in patients, the phenotype can only spread geographically via the sexual stages and mosquito transmission. In addition to their asexual killing properties, artemisinin and its derivatives sterilize sexual male gametocytes. Whether resistant parasites overcome this sterilizing effect has not, however, been fully tested. Here, we analyzed P. falciparum clinical isolates from the Greater Mekong Subregion, each demonstrating delayed clinical clearance and known resistance-associated polymorphisms in the Kelch13 (PfK13^var) gene. As well as demonstrating reduced asexual sensitivity to drug, certain PfK13^var isolates demonstrated a marked reduction in sensitivity to artemisinin in an in vitro male gamete formation assay. Importantly, this same reduction in sensitivity was observed when the most resistant isolate was tested directly in mosquito feeds. These results indicate that, under artemisinin drug pressure, while sensitive parasites are blocked, resistant parasites continue transmission. This selective advantage for resistance transmission could favor acquisition of additional host-specificity or polymorphisms affecting partner drug sensitivity in mixed infections. Favored resistance transmission under ACT coverage could have profound implications for the spread of multidrug-resistant malaria beyond Southeast Asia.

Host-Malaria Parasite Interactions and Impacts on Mutual Evolution

Malaria is the most deadly parasitic disease, affecting hundreds of millions of people worldwide. Malaria parasites have been associated with their hosts for millions of years. During the long history of host-parasite co-evolution, both parasites and hosts have applied pressure on each other through complex host-parasite molecular interactions. Whereas the hosts activate various immune mechanisms to remove parasites during an infection, the parasites attempt to evade host immunity by diversifying their genome and switching expression of targets of the host immune system. Human intervention to control the disease such as antimalarial drugs and vaccination can greatly alter parasite population dynamics and evolution, particularly the massive applications of antimalarial drugs in recent human history. Vaccination is likely the best method to prevent the disease; however, a partially protective vaccine may have unwanted consequences that require further investigation. Studies of host-parasite interactions and co-evolution will provide important information for designing safe and effective vaccines and for preventing drug resistance. In this essay, we will discuss some interesting molecules involved in host-parasite interactions, including important parasite antigens. We also discuss subjects relevant to drug and vaccine development and some approaches for studying host-parasite interactions.

Genetic Markers of Adaptation of Plasmodium falciparum to Transmission by American Vectors Identified in the Genomes of Parasites from Haiti and South America

The malaria parasite, Plasmodium falciparum, was introduced into Hispaniola and other regions of the Americas through the slave trade spanning the 16th through the 19th centuries. During this period, more than 12 million Africans were brought across the Atlantic to the Caribbean and other regions of the Americas. Since malaria is holoendemic in West Africa, a substantial percentage of these individuals carried the parasite. St. Domingue on Hispaniola, now modern-day Haiti, was a major port of disembarkation, and malaria is still actively transmitted there. We undertook a detailed study of the phylogenetics of the Haitian parasites and those from Colombia and Peru utilizing whole-genome sequencing. Principal-component and phylogenetic analyses, based upon single nucleotide polymorphisms (SNPs) in protein coding regions, indicate that, despite the potential for millions of introductions from Africa, the Haitian parasites share an ancestral relationship within a well-supported monophyletic clade with parasites from South America, while belonging to a distinct lineage. This result, in stark contrast to the historical record of parasite introductions, is best explained by a severe population bottleneck experienced by the parasites introduced into the Americas. Here, evidence is presented for targeted selection of rare African alleles in genes which are expressed in the mosquito stages of the parasite's life cycle. These genetic markers support the hypothesis that the severe population bottleneck was caused by the required adaptation of the parasite to transmission by new definitive hosts among the Anopheles (Nyssorhynchus) spp. found in the Caribbean and South America.IMPORTANCE Historical data suggest that millions of P. falciparum parasite lineages were introduced into the Americas during the trans-Atlantic slave trade, which would suggest a paraphyletic origin of the extant isolates in the Western Hemisphere. Our analyses of whole-genome variants show that the American parasites belong to a well-supported monophyletic clade. We hypothesize that the required adaptation to American vectors created a severe bottleneck, reducing the effective introduction to a few lineages. In support of this hypothesis, we discovered genes expressed in the mosquito stages of the life cycle that have alleles with multiple, high-frequency or fixed, nonsynonymous mutations in the American populations which are rarely found in African isolates. These alleles appear to be in gene products critical for transmission through the anopheline vector. Thus, these results may inform efforts to develop novel transmission-blocking vaccines by identifying parasite proteins functionally interacting with the vector that are important for successful transmission. Further, to the best of our knowledge, these are the first whole-genome data available from Haitian P. falciparum isolates. Defining the genome of these parasites provides genetic markers useful for mapping parasite populations and monitoring parasite movements/introductions.

Experimental Transmission of Plasmodium malariae to Anopheles gambiae

Our current knowledge of the clinical burden, biology, and transmission of Plasmodium malariae is extremely scarce. To start addressing some of those questions, we experimentally infected Anopheles gambiae mosquitoes with fresh P. malariae isolates obtained from asymptomatic individuals in Lambaréné, Gabon. The proportion of mosquitoes infected via direct membrane feeding assay with either P. malariae monoinfections (16% [19 of 121]) or coinfections (28% [31 of 112]) was higher after serum replacement than in parallel groups without serum replacement (4% [4 of 102] and 4% [2 of 45], respectively; P < .01). Our results show that isolates from asymptomatic carriers can be used for experimental studies of P. malariae transmission.

Effect of irradiation on the survival and susceptibility of female Anopheles arabiensis to natural isolates of Plasmodium falciparum

BACKGROUND

The sterile insect technique (SIT) is a vector control strategy relying on the mass release of sterile males into wild vector populations. Current sex separation techniques are not fully efficient and could lead to the release of a small proportion of females. It is therefore important to evaluate the effect of irradiation on the ability of released females to transmit pathogens. This study aimed to assess the effect of irradiation on the survival and competence of Anopheles arabiensis females for Plasmodium falciparum in laboratory conditions.

METHODS

Pupae were irradiated at 95 Gy of gamma-rays, and emerging females were challenged with one of 14 natural isolates of P. falciparum. Seven days post-blood meal (dpbm), irradiated and unirradiated-control females were dissected to assess the presence of oocysts, using 8 parasite isolates. On 14 dpbm, sporozoite dissemination in the head/thorax was also examined, using 10 parasites isolates including 4 in common with the 7 dpbm dissection (oocyst data). The survivorship of irradiated and unirradiated-control mosquitoes was monitored.

RESULTS

Overall, irradiation reduced the proportion of mosquitoes infected with the oocyst stages by 17% but this effect was highly inconsistent among parasite isolates. Secondly, there was no significant effect of irradiation on the number of developing oocysts. Thirdly, there was no significant difference in both the sporozoite infection rate and load between the irradiated and unirradiated-control mosquitoes. Fourthly, irradiation had varying effects on female survival with either a negative effect or no effect.

CONCLUSIONS

The effect of irradiation on mosquito competence strongly varied among parasite isolates. Because of such isolate variability and, the fact that different parasite isolates were used to collect oocyst and sporozoite data, the irradiation-mediated reduction of oocyst prevalence was not confirmed for the sporozoite stages. Our data indicate that irradiated female An. arabiensis could contribute to malaria transmission, and highlight the need for perfect sexing tools, which would prevent the release of females as part of SIT programmes.

The influence of feeding behaviour and temperature on the capacity of mosquitoes to transmit malaria

Insecticide-treated bed nets reduce malaria transmission by limiting contact between mosquito vectors and human hosts when mosquitoes feed during the night. However, malaria vectors can also feed in the early evening and in the morning when people are not protected. Here, we explored how the timing of blood feeding interacts with environmental temperature to influence the capacity of Anopheles mosquitoes to transmit the human malaria parasite Plasmodium falciparum. In laboratory experiments, we found no effect of biting time itself on the proportion of mosquitoes that became infectious (vector competence) at constant temperature. However, when mosquitoes were maintained under more realistic fluctuating temperatures, there was a significant increase in competence for mosquitoes feeding in the evening (18:00), and a significant reduction in competence for those feeding in the morning (06:00), relative to those feeding at midnight (00:00). These effects appear to be due to thermal sensitivity of malaria parasites during the initial stages of parasite development within the mosquito, and the fact that mosquitoes feeding in the evening experience cooling temperatures during the night, whereas mosquitoes feeding in the morning quickly experience warming temperatures that are inhibitory to parasite establishment. A transmission dynamics model illustrates that such differences in competence could have important implications for malaria prevalence, the extent of transmission that persists in the presence of bed nets, and the epidemiological impact of behavioural resistance. These results indicate that the interaction of temperature and feeding behaviour could be a major ecological determinant of the vectorial capacity of malaria mosquitoes.

The second life of Plasmodium in the mosquito host: gene regulation on the move

Malaria parasites face dynamically changing environments and strong selective constraints within human and mosquito hosts. To survive such hostile and shifting conditions, Plasmodium switches transcriptional programs during development and has evolved mechanisms to adjust its phenotype through heterogeneous patterns of gene expression. In vitro studies on culture-adapted isolates have served to set the link between chromatin structure and functional gene expression. Yet, experimental evidence is limited to certain stages of the parasite in the vertebrate, i.e. blood, while the precise mechanisms underlying the dynamic regulatory landscapes during development and in the adaptation to within-host conditions remain poorly understood. In this review, we discuss available data on transcriptional and epigenetic regulation in Plasmodium mosquito stages in the context of sporogonic development and phenotypic variation, including both bet-hedging and environmentally triggered direct transcriptional responses. With this, we advocate the mosquito offers an in vivo biological model to investigate the regulatory networks, transcription factors and chromatin-modifying enzymes and their modes of interaction with regulatory sequences, which might be responsible for the plasticity of the Plasmodium genome that dictates stage- and cell type-specific blueprints of gene expression.

PIMMS43 is required for malaria parasite immune evasion and sporogonic development in the mosquito vector

Malaria is transmitted among humans through mosquito bites. Here, we characterize a protein found on the surface of mosquito stages of malaria parasites and reveal that it serves to evade the mosquito immune system and ensure disease transmission. Neutralization of PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43), either by eliminating it from the parasite genome or by preincubating parasites with antibodies that bind to the PIMMS43 protein, inhibits mosquito infection with malaria parasites. Differences in PIMMS43 detected between African malaria parasite populations suggest that these populations have adapted for transmission by different mosquito vectors that are also differentially distributed across the continent. We conclude that targeting PIMMS43 can block malaria parasites inside mosquitoes before they can infect humans.

After being ingested by a female Anopheles mosquito during a bloodmeal on an infected host, and before they can reach the mosquito salivary glands to be transmitted to a new host, Plasmodium parasites must establish an infection of the mosquito midgut in the form of oocysts. To achieve this, they must first survive a series of robust innate immune responses that take place prior to, during, and immediately after ookinete traversal of the midgut epithelium. Understanding how parasites may evade these responses could highlight new ways to block malaria transmission. We show that an ookinete and sporozoite surface protein designated as PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43) is required for parasite evasion of the Anopheles coluzzii complement-like response. Disruption of PIMMS43 in the rodent malaria parasite Plasmodium berghei triggers robust complement activation and ookinete elimination upon mosquito midgut traversal. Silencing components of the complement-like system through RNAi largely restores ookinete-to-oocyst transition but oocysts remain small in size and produce a very small number of sporozoites that additionally are not infectious, indicating that PIMMS43 is also essential for sporogonic development in the oocyst. Antibodies that bind PIMMS43 interfere with parasite immune evasion when ingested with the infectious blood meal and significantly reduce the prevalence and intensity of infection. PIMMS43 genetic structure across African Plasmodium falciparum populations indicates allelic adaptation to sympatric vector populations. These data add to our understanding of mosquito–parasite interactions and identify PIMMS43 as a target of malaria transmission blocking.

Plasmodium falciparum evades immunity of anopheline mosquitoes by interacting with a Pfs47 midgut receptor

The surface protein Pfs47 allows Plasmodium falciparum parasites to survive and be transmitted by making them "undetectable" to the mosquito immune system. P. falciparum parasites express Pfs47 haplotypes compatible with their sympatric vectors, while those with incompatible haplotypes are eliminated by the mosquito. We proposed that Pfs47 serves as a "key" that mediates immune evasion by interacting with a mosquito receptor "the lock," which differs in evolutionarily divergent anopheline mosquitoes. Recombinant Pfs47 (rPfs47) was used to identify the mosquito Pfs47 receptor protein (P47Rec) using far-Western analysis. rPfs47 bound to a single 31-kDa band and the identity of this protein was determined by mass spectrometry. The mosquito P47Rec has two natterin-like domains and binds to Pfs47 with high affinity (17 to 32 nM). P47Rec is a highly conserved protein with submicrovillar localization in midgut cells. It has structural homology to a cytoskeleton-interacting protein and accumulates at the site of ookinete invasion. Silencing P47Rec expression reduced P. falciparum infection, indicating that the interaction of Pfs47 with the receptor is critical for parasite survival. The binding specificity of P47Rec from distant anophelines (Anopheles gambiae, Anopheles dirus, and Anopheles albimanus) with Pfs47-Africa (GB4) and Pfs47-South America (7G8) haplotypes was evaluated, and it is in agreement with the previously documented compatibility between P. falciparum parasites expressing different Pfs47 haplotypes and these three anopheline species. Our findings give further support to the role of Pfs47 in the adaptation of P. falciparum to different vectors.

Sexual forms obtained in a continuous in vitro cultured Colombian strain of Plasmodium falciparum (FCB2)

Background

The epidemiological control of malaria has been hampered by the appearance of parasite resistance to anti-malarial drugs and by the resistance of mosquito vectors to control measures. This has also been associated with weak transmission control, mostly due to poor control of asymptomatic patients associated with host-vector transmission. This highlights the importance of studying the parasite’s sexual forms (gametocytes) which are involved in this phase of the parasite’s life-cycle. Some African and Asian strains of Plasmodium falciparum have been fully characterized regarding sexual forms’ production; however, few Latin-American strains have been so characterized. This study was aimed at characterizing the Colombian FCB2 strain as a gametocyte producer able to infect mosquitoes.

Methods

Gametocyte production was induced in in vitro cultured P. falciparum FCB2 and 3D7 strains. Pfap2g and Pfs25 gene expression was detected in FCB2 strain gametocyte culture by RT-PCR. Comparative analysis of gametocytes obtained from both strains was made (counts and morphological changes). In vitro zygote formation from FCB2 gametocytes was induced by incubating a gametocyte culture sample at 27 °C for 20 min. A controlled Anopheles albimanus infection was made using an artificial feed system with cultured FCB2 gametocytes (14–15 days old). Mosquito midgut dissection was then carried out for analyzing oocysts.

Results

The FCB2 strain expressed Pfap2g, Pfs16, Pfg27/25 and Pfs25 sexual differentiation-related genes after in vitro sexual differentiation induction, producing gametocytes that conserved the expected morphological features. The amount of FCB2 gametocytes produced was similar to that from the 3D7 strain. FCB2 gametocytes were differentiated into zygotes and ookinetes after an in vitro low-temperature stimulus and infected An. albimanus mosquitoes, developing to oocyst stage.

Conclusions

Even with the history of long-term FCB2 strain in vitro culture maintenance, it has retained its sexual differentiation ability. The gametocytes produced here preserved these parasite forms’ usual characteristics and An. albimanus infection capability, thus enabling its use as a tool for studying sexual form biology, An. albimanus infection comparative analysis and anti-malarial drug and vaccine development.

Field Relevant Variation in Ambient Temperature Modifies Density-Dependent Establishment of Plasmodium falciparum Gametocytes in Mosquitoes

The relationship between Plasmodium falciparum gametocyte density and infections in mosquitoes is central to understanding the rates of transmission with important implications for control. Here, we determined whether field relevant variation in environmental temperature could also modulate this relationship. Anopheles stephensi were challenged with three densities of P. falciparum gametocytes spanning a ~10-fold gradient, and housed under diurnal/daily temperature range ("DTR") of 9°C (+5°C and -4°C) around means of 20, 24, and 28°C. Vector competence was quantified as the proportion of mosquitoes infected with oocysts in the midguts (oocyst rates) or infectious with sporozoites in the salivary glands (sporozoite rates) at peak periods of infection for each temperature to account for the differences in development rates. In addition, oocyst intensities were also recorded from infected midguts and the overall study replicated across three separate parasite cultures and mosquito cohorts. While vector competence was similar at 20 DTR 9°C and 24 DTR 9°C, oocyst and sporozoite rates were also comparable, with evidence, surprisingly, for higher vector competence in mosquitoes challenged with intermediate gametocyte densities. For the same gametocyte densities however, severe reductions in the sporozoite rates was accompanied by a significant decline in overall vector competence at 28 DTR 9°C, with gametocyte density per se showing a positive and linear effect at this temperature. Unlike vector competence, oocyst intensities decreased with increasing temperatures with a predominantly positive and linear association with gametocyte density, especially at 28 DTR 9°C. Oocyst intensities across individual infected midguts suggested temperature-specific differences in mosquito susceptibility/resistance: at 20 DTR 9°C and 24 DTR 9°C, dispersion (aggregation) increased in a density-dependent manner but not at 28 DTR 9°C where the distributions were consistently random. Limitations notwithstanding, our results suggest that variation in temperature could modify seasonal dynamics of infectious reservoirs with implications for the design and deployment of transmission-blocking vaccines/drugs.

Engineering a Virus-Like Particle as an Antigenic Platform for a Pfs47-Targeted Malaria Transmission-Blocking Vaccine

We recently characterized Pfs47, a protein expressed on the surface of sexual stages and ookinetes of Plasmodium falciparum, as a malaria transmission-blocking vaccine (TBV) target. Mice immunization induced antibodies that conferred strong transmission-reducing activity (TRA) at a concentration of 200 μg/mL. Here, we sought to optimize the Pfs47 vaccine to elicit higher titers of high-affinity antibodies, capable of inducing strong TRA at a lower concentration. We report the development and evaluation of a Pfs47-based virus-like particle (VLP) vaccine generated by conjugating our 58 amino acid Pfs47 antigen to Acinetobacter phage AP205-VLP using the SpyCatcher:SpyTag adaptor system. AP205-Pfs47 complexes (VLP-P47) formed particles of ~22 nm diameter that reacted with polyclonal anti-Pfs47 antibodies, indicating that the antigen was accessible on the surface of the particle. Mice immunized with VLP-P47 followed by a boost with Pfs47 monomer induced significantly higher antibody titers, with higher binding affinity to Pfs47, than mice that received two immunizations with either VLP-P47 (VLP-P47/VLP-P47) or the Pfs47 monomer (P47/P47). Purified IgG from VLP-P47/P47 mice had strong TRA (83-98%) at concentrations as low as 5 μg/mL. These results indicate that conjugating the Pfs47 antigen to AP205-VLP significantly enhanced antigenicity and confirm the potential of Pfs47 as a TBV candidate.

Plasmodium Genomics and Genetics: New Insights into Malaria Pathogenesis, Drug Resistance, Epidemiology, and Evolution

Protozoan Plasmodium parasites are the causative agents of malaria, a deadly disease that continues to afflict hundreds of millions of people every year. Infections with malaria parasites can be asymptomatic, with mild or severe symptoms, or fatal, depending on many factors such as parasite virulence and host immune status. Malaria can be treated with various drugs, with artemisinin-based combination therapies (ACTs) being the first-line choice. Recent advances in genetics and genomics of malaria parasites have contributed greatly to our understanding of parasite population dynamics, transmission, drug responses, and pathogenesis. However, knowledge gaps in parasite biology and host-parasite interactions still remain. Parasites resistant to multiple antimalarial drugs have emerged, while advanced clinical trials have shown partial efficacy for one available vaccine. Here we discuss genetic and genomic studies of Plasmodium biology, host-parasite interactions, population structures, mosquito infectivity, antigenic variation, and targets for treatment and immunization. Knowledge from these studies will advance our understanding of malaria pathogenesis, epidemiology, and evolution and will support work to discover and develop new medicines and vaccines.

Genetic diversity and drug resistance surveillance of Plasmodium falciparum for malaria elimination: is there an ideal tool for resource-limited sub-Saharan Africa?

The intensification of malaria control interventions has resulted in its global decline, but it remains a significant public health burden especially in sub-Saharan Africa (sSA). Knowledge on the parasite diversity, its transmission dynamics, mechanisms of adaptation to environmental and interventional pressures could help refine or develop new control and elimination strategies. Critical to this is the accurate assessment of the parasite’s genetic diversity and monitoring of genetic markers of anti-malarial resistance across all susceptible populations. Such wide molecular surveillance will require selected tools and approaches from a variety of ever evolving advancements in technology and the changing epidemiology of malaria. The choice of an effective approach for specific endemic settings remains challenging, particularly for countries in sSA with limited access to advanced technologies. This article examines the current strategies and tools for Plasmodium falciparum genetic diversity typing and resistance monitoring and proposes how the different tools could be employed in resource-poor settings. Advanced approaches enabling targeted deep sequencing is valued as a sensitive method for assessing drug resistance and parasite diversity but remains out of the reach of most laboratories in sSA due to the high cost of development and maintenance. It is, however, feasible to equip a limited number of laboratories as Centres of Excellence in Africa (CEA), which will receive and process samples from a network of peripheral laboratories in the continent. Cheaper, sensitive and portable real-time PCR methods can be used in peripheral laboratories to pre-screen and select samples for targeted deep sequence or genome wide analyses at these CEAs.

Transmission-Blocking Vaccines: Old Friends and New Prospects

In the progression of the life cycle of Plasmodium falciparum, a small proportion of asexual parasites differentiate into male or female sexual forms called gametocytes. Just like their asexual counterparts, gametocytes are contained within the infected host's erythrocytes (RBCs). However, unlike their asexual partners, they do not exit the RBC until they are taken up in a blood meal by a mosquito. In the mosquito midgut, they are stimulated to emerge from the RBC, undergo fertilization, and ultimately produce tens of thousands of sporozoites that are infectious to humans. This transmission cycle can be blocked by antibodies targeting proteins exposed on the parasite surface in the mosquito midgut, a process that has led to the development of candidate transmission-blocking vaccines (TBV), including some that are in clinical trials. Here we review the leading TBV antigens and highlight the ongoing search for additional gametocyte/gamete surface antigens, as well as antigens on the surfaces of gametocyte-infected erythrocytes, which can potentially become a new group of TBV candidates.

Louse flies of Eleonora's falcons that also feed on their prey are evolutionary dead-end hosts for blood parasites

Host shifts are widespread among avian haemosporidians, although the success of transmission depends upon parasite‐host and parasite‐vector compatibility. Insular avifaunas are typically characterized by a low prevalence and diversity of haemosporidians, although the underlying ecological and evolutionary processes remain unclear. We investigated the parasite transmission network in an insular system formed by Eleonora's falcons (the avian host), louse flies that parasitize the falcons (the potential vector), and haemosporidians (the parasites). We found a great diversity of parasites in louse flies (16 Haemoproteus and 6 Plasmodium lineages) that did not match with lineages previously found infecting adult falcons (only one shared lineage). Because Eleonora's falcon feeds on migratory passerines hunted over the ocean, we sampled falcon kills in search of the origin of parasites found in louse flies. Surprisingly, louse flies shared 10 of the 18 different parasite lineages infecting falcon kills. Phylogenetic analyses revealed that all lineages found in louse flies (including five new lineages) corresponded to Haemoproteus and Plasmodium parasites infecting Passeriformes. We found molecular evidence of louse flies feeding on passerines hunted by falcons. The lack of infection in nestlings and the mismatch between the lineages isolated in adult falcons and louse flies suggest that despite louse flies’ contact with a diverse array of parasites, no successful transmission to Eleonora's falcon occurs. This could be due to the falcons’ resistance to infection, the inability of parasites to develop in these phylogenetically distant species, or the inability of haemosporidian lineages to complete their development in louse flies.