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Plasmodium sporozoites are deposited in the skin by Anopheles mosquitoes. They then find their way to the liver, where they specifically invade hepatocytes in which they develop to yield merozoites infective to red blood cells. Relatively little is known of the molecular interactions during these initial obligatory phases of the infection. Recent data suggested that many of the inoculated sporozoites invade hepatocytes an hour or more after the infective bite. We hypothesised that this pre-invasive period in the mammalian host prepares sporozoites for successful hepatocyte infection. Therefore, the genes whose expression becomes modified prior to hepatocyte invasion would be those likely to code for proteins implicated in the subsequent events of invasion and development. We have used P. falciparum sporozoites and their natural host cells, primary human hepatocytes, in in vitro co-culture system as a model for the pre-invasive period. We first established that under co-culture conditions, sporozoites maintain infectivity for an hour or more, in contrast to a drastic loss in infectivity when hepatocytes were not included. Thus, a differential transcriptome of salivary gland sporozoites versus sporozoites co-cultured with hepatocytes was established using a pan-genomic P. falciparum microarray. The expression of 532 genes was found to have been up-regulated following co-culture. A fifth of these genes had no orthologues in the genomes of Plasmodium species used in rodent models of malaria. Quantitative RT-PCR analysis of a selection of 21 genes confirmed the reliability of the microarray data. Time-course analysis further indicated two patterns of up-regulation following sporozoite co-culture, one transient and the other sustained, suggesting roles in hepatocyte invasion and liver stage development, respectively. This was supported by functional studies of four hitherto uncharacterized proteins of which two were shown to be sporozoite surface proteins involved in hepatocyte invasion, while the other two were predominantly expressed during hepatic parasite development. The genome-wide up-regulation of expression observed supports the hypothesis that the shift from the mosquito to the mammalian host contributes to activate quiescent salivary gland sporozoites into a state of readiness for the hepatic stages. Functional studies on four of the up-regulated genes validated our approach as one means to determine the repertoire of proteins implicated during the early events of the Plasmodium infection, and in this case that of P. falciparum, the species responsible for the severest forms of malaria.

Although the clinical efficacy of antimalarial artemisinin-based combination therapies in Africa remains high, the recent emergence of partial resistance to artemisinin in on the continent is troubling, given the lack of alternative treatments. In this study, we used data from drug-efficacy studies conducted between 2016 and 2019 that evaluated 3-day courses of artemisinin-based combination therapy (artesunate-amodiaquine or artemether-lumefantrine) for uncomplicated malaria in Eritrea to estimate the percentage of patients with day-3 positivity (i.e., persistent parasitemia 3 days after the initiation of therapy). We also assayed parasites for mutations in as predictive markers of partial resistance to artemisinin and screened for deletions in and that result in variable performance of histidine rich protein 2 (HRP2)-based rapid diagnostic tests for malaria. We noted an increase in the percentage of patients with day-3 positivity from 0.4% (1 of 273) in 2016 to 1.9% (4 of 209) in 2017 and 4.2% (15 of 359) in 2019. An increase was also noted in the prevalence of the R622I mutation, which was detected in 109 of 818 isolates before treatment, from 8.6% (24 of 278) in 2016 to 21.0% (69 of 329) in 2019. The odds of day-3 positivity increased by a factor of 6.2 (95% confidence interval, 2.5 to 15.5) among the patients with 622I variant parasites. Partial resistance to artemisinin, as defined by the World Health Organization, was observed in Eritrea. More than 5% of the patients younger than 15 years of age with day-3 positivity also had parasites that carried R622I. In vitro, the R622I mutation conferred a low level of resistance to artemisinin when edited into NF54 and Dd2 parasite lines. Deletions in both and were identified in 16.9% of the parasites that carried the R622I mutation, which made them potentially undetectable by HRP2-based rapid diagnostic tests. The emergence and spread of lineages with both -mediated partial resistance to artemisinin and deletions in and in Eritrea threaten to compromise regional malaria control and elimination campaigns. (Funded by the Bill and Melinda Gates Foundation and others; Australian New Zealand Clinical Trials Registry numbers, ACTRN12618001223224, ACTRN12618000353291, and ACTRN12619000859189.).

Plasmodium falciparum encoded histidine rich protein (HRP2) based malaria rapid diagnostic tests (RDTs) are used in India. Deletion of pfhrp2 and pfhrp3 genes contributes to false negative test results, and large numbers of such deletions have been reported from South America, highlighting the importance of surveillance to detect such deletions. This is the first prospective field study carried out at 16 sites located in eight endemic states of India to assess the performance of PfHRP2 based RDT kits used in the national malaria control programme. In this study, microscopically confirmed P. falciparum but RDT negative samples were assessed for presence of pfhrp2, pfhrp3, and their flanking genes using PCR. Among 1521 microscopically positive P. falciparum samples screened, 50 were negative by HRP2 based RDT test. Molecular testing was carried out using these 50 RDT negative samples by assuming that 1471 RDT positive samples carried pfhrp2 gene. It was found that 2.4% (36/1521) and 1.8% (27/1521) of samples were negative for pfhrp2 and pfhrp3 genes, respectively. However, the frequency of pfhrp2 deletions varied between the sites ranging from 0-25% (2.4, 95% CI; 1.6-3.3). The frequency of both pfhrp2 and pfhrp3 gene deletion varied from 0-8% (1.6, 95% CI; 1.0-2.4). This study provides evidence for low level presence of pfhrp2 and pfhrp3 deleted P. falciparum parasites in different endemic regions of India, and periodic surveillance is warranted for reliable use of PfHRP2 based RDTs.

Due to their remarkable parasitocidal activity, artemisinins represent the key components of first-line therapies against Plasmodium falciparum malaria. However, the decline in efficacy of artemisinin-based drugs jeopardizes global efforts to control and ultimately eradicate the disease. To better understand the resistance phenotype, artemisinin-resistant parasite lines were derived from two clones of the 3D7 strain of P. falciparum using a selection regimen that mimics how parasites interact with the drug within patients. This long term in vitro selection induced profound stage-specific resistance to artemisinin and its relative compounds. Chemosensitivity and transcriptional profiling of artemisinin-resistant parasites indicate that enhanced adaptive responses against oxidative stress and protein damage are associated with decreased artemisinin susceptibility. This corroborates our previous findings implicating these cellular functions in artemisinin resistance in natural infections. Genomic characterization of the two derived parasite lines revealed a spectrum of sequence and copy number polymorphisms that could play a role in regulating artemisinin response, but did not include mutations in pfk13, the main marker of artemisinin resistance in Southeast Asia. Taken together, here we present a functional in vitro model of artemisinin resistance that is underlined by a new set of genetic polymorphisms as potential genetic markers.

The circumsporozoite protein of the human malaria parasite Plasmodium falciparum (PfCSP) is the main target of antibodies that prevent the infection and disease, as shown in animal models. However, the limited efficacy of the PfCSP-based vaccine RTS,S calls for a better understanding of the mechanisms driving the development of the most potent human PfCSP antibodies and identification of their target epitopes. By characterizing 200 human monoclonal PfCSP antibodies induced by sporozoite immunization, we establish that the most potent antibodies bind around a conserved (N/D)PNANPN(V/A) core. High antibody affinity to the core correlates with protection from parasitemia in mice and evolves around the recognition of NANP motifs. The data suggest that the rational design of a next-generation PfCSP vaccine that elicits high-affinity antibody responses against the core epitope will promote the induction of protective humoral immune responses. Characterization of 200 antibodies that bind a malaria parasite protein provides insights that could enable better vaccine design.

The dry season is a major challenge for Plasmodium falciparum parasites in many malaria endemic regions, where water availability limits mosquito vectors to only part of the year. How P. falciparum bridges two transmission seasons months apart, without being cleared by the human host or compromising host survival, is poorly understood. Here we show that low levels of P. falciparum parasites persist in the blood of asymptomatic Malian individuals during the 5- to 6-month dry season, rarely causing symptoms and minimally affecting the host immune response. Parasites isolated during the dry season are transcriptionally distinct from those of individuals with febrile malaria in the transmission season, coinciding with longer circulation within each replicative cycle of parasitized erythrocytes without adhering to the vascular endothelium. Low parasite levels during the dry season are not due to impaired replication but rather to increased splenic clearance of longer-circulating infected erythrocytes, which likely maintain parasitemias below clinical and immunological radar. We propose that P. falciparum virulence in areas of seasonal malaria transmission is regulated so that the parasite decreases its endothelial binding capacity, allowing increased splenic clearance and enabling several months of subclinical parasite persistence. Malaria cases are predominant during the rainy seasons in many endemic regions owing to the life cycle of the mosquito vector. How Plasmodium falciparum adapts in humans during the intervening dry season, without causing malaria symptoms or killing the host, offers new insights into its persistence in humans.

The susceptibility of Anopheles mosquitoes to Plasmodium infections relies on complex interactions between the insect vector and the malaria parasite. A number of studies have shown that the mosquito innate immune responses play an important role in controlling the malaria infection and that the strength of parasite clearance is under genetic control, but little is known about the influence of environmental factors on the transmission success. We present here evidence that the composition of the vector gut microbiota is one of the major components that determine the outcome of mosquito infections. A. gambiae mosquitoes collected in natural breeding sites from Cameroon were experimentally challenged with a wild P. falciparum isolate, and their gut bacterial content was submitted for pyrosequencing analysis. The meta-taxogenomic approach revealed a broader richness of the midgut bacterial flora than previously described. Unexpectedly, the majority of bacterial species were found in only a small proportion of mosquitoes, and only 20 genera were shared by 80% of individuals. We show that observed differences in gut bacterial flora of adult mosquitoes is a result of breeding in distinct sites, suggesting that the native aquatic source where larvae were grown determines the composition of the midgut microbiota. Importantly, the abundance of Enterobacteriaceae in the mosquito midgut correlates significantly with the Plasmodium infection status. This striking relationship highlights the role of natural gut environment in parasite transmission. Deciphering microbe-pathogen interactions offers new perspectives to control disease transmission.

A multi-centre genome-wide association study of severe malaria in African children uncovers a new resistance locus close to a cluster of genes encoding glycophorins, which are receptors used by the malaria-causing parasite to invade red blood cells. Gene variants associated with resistance to malaria This multi-centre genome-wide association study of severe malaria in more than 11,000 African children uncovers a new variant associated with resistance to malaria that is close to a cluster of genes encoding glycophorins — membrane receptors used by the Plasmodium falciparum parasite to invade red blood cells. Interestingly, the variant is linked to polymorphisms previously found to have been maintained for millions of years, based on genome analysis of humans and chimpanzees. The high prevalence of sickle haemoglobin in Africa shows that malaria has been a major force for human evolutionary selection, but surprisingly few other polymorphisms have been proven to confer resistance to malaria in large epidemiological studies 1 , 2 , 3 . To address this problem, we conducted a multi-centre genome-wide association study (GWAS) of life-threatening Plasmodium falciparum infection (severe malaria) in over 11,000 African children, with replication data in a further 14,000 individuals. Here we report a novel malaria resistance locus close to a cluster of genes encoding glycophorins that are receptors for erythrocyte invasion by P. falciparum . We identify a haplotype at this locus that provides 33% protection against severe malaria (odds ratio = 0.67, 95% confidence interval = 0.60–0.76, P value = 9.5 × 10 −11 ) and is linked to polymorphisms that have previously been shown to have features of ancient balancing selection, on the basis of haplotype sharing between humans and chimpanzees 4 . Taken together with previous observations on the malaria-protective role of blood group O 1 , 2 , 3 , 5 , these data reveal that two of the strongest GWAS signals for severe malaria lie in or close to genes encoding the glycosylated surface coat of the erythrocyte cell membrane, both within regions of the genome where it appears that evolution has maintained diversity for millions of years. These findings provide new insights into the host–parasite interactions that are critical in determining the outcome of malaria infection.

The rise of antimalarial drug-resistant Plasmodium falciparum threatens malaria elimination efforts. Mutations in the gene kelch13 (k13) confer artemisinin partial resistance (ART-R), compromising the efficacy of frontline artemisinin-based combination therapies (ACTs). The validated mutation k13 R622I has emerged and expanded rapidly in the Horn of Africa. We conducted a year-long genomic surveillance study in Gondar Zuria and Tach Armachiho, two ecologically distinct districts in northwestern Ethiopia where R622I was first identified. A total of 903 P. falciparum infections were sequenced using molecular inversion probe (MIP) panels targeting major drug resistance mutations and genome-wide informative SNPs. The R622I mutation was found in 44.3% of samples, more frequent in Gondar Zuria than Tach Armachiho (52% vs. 35%; p < 0.001), and persisted year-round in nearly all sites, indicating stable transmission with minimal seasonal variation. Histidine-rich protein 2 (HRP2) based rapid diagnostic test (RDT) negativity was also prevalent (39.3%), with significant district-level variation (48.7% vs. 27.6%; p < 0.001). Concerningly, R622I and HRP2-RDT co-occured in 22% of samples, higher in Gondar Zuria than in Tach Armachiho (28.9% vs. 12.9%; p < 0.001). Overall, HRP2-RDT negativity was significantly more common among R622I mutant parasites than wild-type (48.3% vs. 30.7%; p < 0.05). The k13 C580Y mutation was also detected at very low frequency (0.4%) in Gondar Zuria, representing the first report of this mutation in the Horn of Africa. Long-read whole-genome sequencing showed k13 flanking haplotypes of C580Y isolates were distinct from Southeast Asian lineages, suggesting a local, de novo emergence of African origin. These findings highlight the increasing prevalence and types of ART-R mutations, persistence of k13 R622I and its increasing association with HRP2-RDT negativity, representing a double threat to malaria control and elimination efforts.