Mosquito midgut stem cell cellular defense response limits Plasmodium parasite infection

A novel cellular response of midgut progenitors (stem cells and enteroblasts) to Plasmodium berghei infection was investigated in Anopheles stephensi. The presence of developing oocysts triggers proliferation of midgut progenitors that is modulated by the Jak/STAT pathway and is proportional to the number of oocysts on individual midguts. The percentage of parasites in direct contact with enteroblasts increases over time, as progenitors proliferate. Silencing components of key signaling pathways through RNA interference (RNAi) that enhance proliferation of progenitor cells significantly decreased oocyst numbers, while limiting proliferation of progenitors increased oocyst survival. Live imaging revealed that enteroblasts interact directly with oocysts and eliminate them. Midgut progenitors sense the presence of Plasmodium oocysts and mount a cellular defense response that involves extensive proliferation and tissue remodeling, followed by oocysts lysis and phagocytosis of parasite remnants by enteroblasts.

In-depth transcriptomic analysis of Anopheles gambiae hemocytes uncovers novel genes and the oenocytoid developmental lineage

BACKGROUND

Hemocytes are immune cells that patrol the mosquito hemocoel and mediate critical cellular defense responses against pathogens. However, despite their importance, a comprehensive transcriptome of these cells was lacking because they constitute a very small fraction of the total cells in the insect, limiting the study of hemocyte differentiation and immune function.

RESULTS

In this study, an in-depth hemocyte transcriptome was built by extensive bulk RNA sequencing and assembly of hemocyte RNAs from adult A. gambiae female mosquitoes, based on approximately 2.4 billion short Illumina and about 9.4 million long PacBio high-quality reads that mapped to the A. gambiae PEST genome (P4.14 version). A total of 34,939 transcripts were annotated including 4,020 transcripts from novel genes and 20,008 novel isoforms that result from extensive differential splicing of transcripts from previously annotated genes. Most hemocyte transcripts identified (89.8%) are protein-coding while 10.2% are non-coding RNAs. The number of transcripts identified in the novel hemocyte transcriptome is twice the number in the current annotation of the A. gambiae genome (P4.14 version). Furthermore, we were able to refine the analysis of a previously published single-cell transcriptome (scRNAseq) data set by using the novel hemocyte transcriptome as a reference to re-define the hemocyte clusters and determine the path of hemocyte differentiation. Unsupervised pseudo-temporal ordering using the Tools for Single Cell Analysis software uncovered a novel putative prohemocyte precursor cell type that gives rise to prohemocytes. Pseudo-temporal ordering with the Monocle 3 software, which analyses changes in gene expression during dynamic biological processes, determined that oenocytoids derive from prohemocytes, a cell population that also gives rise to the granulocyte lineage.

CONCLUSION

A high number of mRNA splice variants are expressed in hemocytes, and they may account for the plasticity required to mount efficient responses to many different pathogens. This study highlights the importance of a comprehensive set of reference transcripts to perform robust single-cell transcriptomic data analysis of cells present in low abundance. The detailed annotation of the hemocyte transcriptome will uncover new facets of hemocyte development and function in adult dipterans and is a valuable community resource for future studies on mosquito cellular immunity.

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.

Leishmania genetic exchange is mediated by IgM natural antibodies

Host factors that mediate Leishmania genetic exchange are not well defined. Here we demonstrate that natural IgM (IgMn)1-4 antibodies mediate parasite genetic exchange by inducing the transient formation of a spherical parasite clump that promotes parasite fusion and hybrid formation. We establish that IgMn from Leishmania-free animals binds to the surface of Leishmania parasites to induce significant changes in the expression of parasite transcripts and proteins. Leishmania binding to IgMn is partially lost after glycosidase treatment, although parasite surface phosphoglycans, including lipophosphoglycan, are not required for IgMn-induced parasite clumping. Notably, the transient formation of parasite clumps is essential for Leishmania hybridization in vitro. In vivo, we observed a 12-fold increase in hybrid formation in sand flies provided a second blood meal containing IgMn compared with controls. Furthermore, the generation of recombinant progeny from mating hybrids and parental lines were only observed in sand flies provided with IgMn. Both in vitro and in vivo IgM-induced Leishmania crosses resulted in full genome hybrids that show equal patterns of biparental contribution. Leishmania co-option of a host natural antibody to facilitate mating in the insect vector establishes a new paradigm of parasite-host-vector interdependence that contributes to parasite diversity and fitness by promoting genetic exchange.

Mosquito midgut stem cell cellular defense response limits Plasmodium parasite infection

A novel cellular response of midgut progenitors (stem cells and enteroblasts) to Plasmodium berghei infection was investigated in Anopheles stephensi. The presence of developing oocysts triggers proliferation of midgut progenitors that is modulated by the Jak/STAT pathway, and proportional to the number of oocysts on individual midguts. The percentage of parasites in direct contact with enteroblasts increases over time, as progenitors proliferate. Enhancing proliferation of progenitors significantly decreases oocyst numbers, while limiting proliferation increases oocyst survival. Live imaging revealed that enteroblasts interact directly with oocysts and eliminate them. Midgut progenitors sense the presence of Plasmodium oocysts and mount a cellular defense response that involves extensive proliferation and tissue remodeling, followed by oocysts lysis and phagocytosis of parasite remnants by enteroblasts.

Extending the range of Plasmodium falciparum transmission blocking antibodies

Recent work demonstrating that asymptomatic carriers of P. falciparum parasites make up a large part of the infectious reservoir highlights the need for an effective malaria vaccine. Given the historical challenges of vaccine development, multiple parasite stages have been targeted, including the sexual stages required for transmission. Using flow cytometry to efficiently screen for P. falciparum gamete/zygote surface reactivity, we identified 82 antibodies that bound live P. falciparum gametes/zygotes. Ten antibodies had significant transmission-reducing activity (TRA) in a standard membrane feeding assay and were subcloned along with 9 nonTRA antibodies as comparators. After subcloning, only eight of the monoclonals obtained have significant TRA. These eight TRA mAbs do not recognize epitopes present in any of the current recombinant transmission-blocking vaccine candidates, Pfs230D1M, Pfs48/45.6C, Pf47 D2 and rPfs25. One TRA mAb immunoprecipitates two surface antigens, Pfs47 and Pfs230, that are expressed by both gametocytes and gametes/zygotes. These two proteins have not previously been reported to associate and the recognition of both by a single TRA mAb suggests the Pfs47/Pfs230 complex is a new vaccine target. In total, Pfs230 was the dominant target antigen, with five of the eight TRA mAbs and 8 of 11 nonTRA gamete/zygote surface reactive mAbs interacting with Pfs230. Of the three remaining TRA mAbs, two recognized non-reduced, parasite-produced Pfs25 and one bound non-reduced, parasite-produced Pfs48/45. None of the TRA mAbs bound protein on an immunoblot of reduced gamete/zygote extract and two TRA mAbs were immunoblot negative, indicating none of the new TRA epitopes are linear. The identification of eight new TRA mAbs that bind epitopes not included in any of the constructs currently under advancement as transmission-blocking vaccine candidates may provide new targets worthy of further study.

The Aedes aegypti peritrophic matrix controls arbovirus vector competence through HPx1, a heme-induced peroxidase

Aedes aegypti mosquitoes are the main vectors of arboviruses. The peritrophic matrix (PM) is an extracellular layer that surrounds the blood bolus. It acts as an immune barrier that prevents direct contact of bacteria with midgut epithelial cells during blood digestion. Here, we describe a heme-dependent peroxidase, hereafter referred to as heme peroxidase 1 (HPx1). HPx1 promotes PM assembly and antioxidant ability, modulating vector competence. Mechanistically, the heme presence in a blood meal induces HPx1 transcriptional activation mediated by the E75 transcription factor. HPx1 knockdown increases midgut reactive oxygen species (ROS) production by the DUOX NADPH oxidase. Elevated ROS levels reduce microbiota growth while enhancing epithelial mitosis, a response to tissue damage. However, simultaneous HPx1 and DUOX silencing was not able to rescue bacterial population growth, as explained by increased expression of antimicrobial peptides (AMPs), which occurred only after double knockdown. This result revealed hierarchical activation of ROS and AMPs to control microbiota. HPx1 knockdown produced a 100-fold decrease in Zika and dengue 2 midgut infection, demonstrating the essential role of the mosquito PM in the modulation of arbovirus vector competence. Our data show that the PM connects blood digestion to midgut immunological sensing of the microbiota and viral infections.

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.

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.

Hemocyte differentiation to the megacyte lineage enhances mosquito immunity against Plasmodium

Activation of Toll signaling in Anopheles gambiae by silencing Cactus, a suppressor of this pathway, enhances local release of hemocyte-derived microvesicles (HdMv), promoting activation of the mosquito complement-like system, which eliminates Plasmodium ookinetes. We uncovered the mechanism of this immune enhancement. Cactus silencing triggers a Rel1-mediated differentiation of granulocytes to the megacyte lineage, a new subpopulation of giant cells, resulting in a dramatic increase in the proportion of circulating megacytes. Megacytes are very plastic cells that are massively recruited to the basal midgut surface in response to Plasmodium infection. We show that Toll signaling modulates hemocyte differentiation and that megacyte recruitment to the midgut greatly enhances mosquito immunity against Plasmodium.

Malaria causes hundreds of thousands of deaths each year. This devastating disease is caused by Plasmodium parasites, which are transmitted to people through female Anopheles gambiae mosquitos. Mosquitos become infected with Plasmodium when they ingest blood containing these malaria-causing parasites. However, Plasmodium must avoid the mosquito immune system to survive and spread.

The mosquito immune system is made up of several types of immune cells, including cells known as granulocytes. Granulocytes can further develop into additional cell subtypes, such as megacytes and antimicrobial granulocytes, but it is not clear how these types of cells work to protect mosquitos against infections.

In the mosquitos that transmit malaria, a cell signaling pathway called Toll helps control immune responses to disease-causing microbes, such as Plasmodium. When Toll signaling is strongly triggered in mosquitos, Plasmodium infection is eliminated because immune cell responses are enhanced – which results in lower levels of transmission to humans. But what is the underlying mechanism through which high levels of Toll signaling eradicate Plasmodium infection?

To find out, Barletta et al. collected cell samples from A. gambiae mosquitos and analyzed what happened when Toll signaling was strongly activated. They observed a large increase in the proportion of megacytes in these mosquitos (from 2% to 80% of all granulocytes). Toll signaling also caused megacytes to become bigger, cluster together, and have higher plasticity – meaning they could adopt different shapes. Barletta et al. used microscopy to show that these megacytes were releasing large mitochondria-like structures and membrane vesicles , which may be the trigger activating the mosquito’s immune system. In live mosquitos, megacytes move towards the area of the Plasmodium infection and release microvesicles. These microvesicles are known to activate a part of the the mosquito’s immune system called the complement-like system, destroying the parasites and preventing mosquito infection and disease transmission.

These findings show how strong Toll signaling triggers the mosquito immune system to eliminate Plasmodium infections. Understanding how the mosquito immune system tackles Plasmodium infection may help reveal ways to reduce or block transmission.

Malaria parasite evades mosquito immunity by glutaminyl cyclase-mediated posttranslational protein modification

Glutaminyl cyclase (QC) modifies N-terminal glutamine or glutamic acid residues of target proteins into cyclic pyroglutamic acid (pGlu). Here, we report the biochemical and functional analysis of Plasmodium QC. We show that sporozoites of QC-null mutants of rodent and human malaria parasites are recognized by the mosquito immune system and melanized when they reach the hemocoel. Detailed analyses of rodent malaria QC-null mutants showed that sporozoite numbers in salivary glands are reduced in mosquitoes infected with QC-null or QC catalytically dead mutants. This phenotype can be rescued by genetic complementation or by disrupting mosquito melanization or phagocytosis by hemocytes. Mutation of a single QC-target glutamine of the major sporozoite surface protein (circumsporozoite protein; CSP) of the rodent parasite Plasmodium berghei also results in melanization of sporozoites. These findings indicate that QC-mediated posttranslational modification of surface proteins underlies evasion of killing of sporozoites by the mosquito immune system.

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.

Protective effects of combining monoclonal antibodies and vaccines against the Plasmodium falciparum circumsporozoite protein

Combinations of monoclonal antibodies (mAbs) against different epitopes on the same antigen synergistically neutralize many viruses. However, there are limited studies assessing whether combining human mAbs against distinct regions of the Plasmodium falciparum (Pf) circumsporozoite protein (CSP) enhances in vivo protection against malaria compared to each mAb alone or whether passive transfer of PfCSP mAbs would improve protection following vaccination against PfCSP. Here, we isolated a panel of human mAbs against the subdominant C-terminal domain of PfCSP (C-CSP) from a volunteer immunized with radiation-attenuated Pf sporozoites. These C-CSP-specific mAbs had limited binding to sporozoites in vitro that was increased by combination with neutralizing human “repeat” mAbs against the NPDP/NVDP/NANP tetrapeptides in the central repeat region of PfCSP. Nevertheless, passive transfer of repeat- and C-CSP-specific mAb combinations did not provide enhanced protection against in vivo sporozoite challenge compared to repeat mAbs alone. Furthermore, combining potent repeat-specific mAbs (CIS43, L9, and 317) that respectively target the three tetrapeptides (NPDP/NVDP/NANP) did not provide additional protection against in vivo sporozoite challenge. However, administration of either CIS43, L9, or 317 (but not C-CSP-specific mAbs) to mice that had been immunized with R21, a PfCSP-based virus-like particle vaccine that induces polyclonal antibodies against the repeat region and C-CSP, provided enhanced protection against sporozoite challenge when compared to vaccine or mAbs alone. Collectively, this study shows that while combining mAbs against the repeat and C-terminal regions of PfCSP provide no additional protection in vivo, repeat mAbs do provide increased protection when combined with vaccine-induced polyclonal antibodies. These data should inform the implementation of PfCSP human mAbs alone or following vaccination to prevent malaria infection.

The Plasmodium falciparum (Pf) circumsporozoite protein (CSP) is the major surface protein on sporozoites and is required for these malaria parasites to invade the liver. Antibodies can prevent malaria by neutralizing sporozoites prior to liver invasion. The only approved malaria vaccine (RTS,S) is comprised of the repeat region and C-terminus of PfCSP. RTS,S-mediated protection is associated with vaccine-induced antibodies against both regions. While monoclonal antibodies (mAbs) against the three tetrapeptides in the repeat region potently bind and neutralize sporozoites, mAbs against the C-terminus demonstrate limited sporozoite binding and neutralization. Here, we used flow cytometry to show that the sporozoite binding of C-terminal mAbs are potentiated by combining them with repeat mAbs. This in vitro synergy did not translate into enhanced in vivo protection against sporozoite challenge in mice treated with repeat and C-terminal mAb combinations. Furthermore, combining mAbs against the three tetrapeptides in the repeat region did not provide enhanced protection against sporozoite challenge compared to each mAb alone. However, combining passive and active immunization with repeat mAbs and a RTS,S-like vaccine improved protection against sporozoite challenge compared to each intervention alone. These results have important implications for implementing anti-PfCSP mAbs alone or in combination with vaccines to prevent malaria.

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.

Functional human IgA targets a conserved site on malaria sporozoites

Immunoglobulin (Ig)A antibodies play a critical role in protection against mucosal pathogens. However, the role of serum IgA in immunity to non-mucosal pathogens, such as Plasmodium falciparum, is poorly characterized, despite being the second most abundant isotype in blood after IgG. Here, we investigated the circulating IgA response in humans to Plasmodium falciparum sporozoites that are injected into the skin by mosquitoes and migrate to the liver via the bloodstream to initiate malaria infection. We found that circulating IgA was induced in three independent sporozoite-exposed cohorts: individuals living in an endemic region in Mali, malaria-naïve individuals immunized intravenously with three large doses of irradiated sporozoites, and malaria-naïve individuals exposed to a single controlled mosquito bite infection. Mechanistically, we found evidence in an animal model that IgA responses were induced by sporozoites at dermal inoculation sites. From malaria-resistant individuals, we isolated several IgA monoclonal antibodies that reduced liver parasite burden in mice. One antibody, MAD2-6, bound to a conserved epitope in the N-terminus of the P. falciparum circumsporozoite protein, the dominant protein on the sporozoite surface. Crystal structures of this antibody revealed a unique mode of binding whereby two Fabs simultaneously bound either side of the target peptide. This study reveals a role for circulating IgA in malaria and identifies the N-terminus of the circumsporozoite protein as a target of functional antibodies.

Design of Dissolvable Microneedles for Delivery of a Pfs47-Based Malaria Transmission-Blocking Vaccine

The development of effective malaria vaccines remains a global health priority. In addition to an effective vaccine, there is urgent demand for effective delivery technologies that can be easily deployed. The need for effective vaccine delivery tools is particularly pertinent in resource-poor settings where access to healthcare is limited. Microneedles are micron-scale structures that offer distinct advantages for vaccine delivery by efficiently targeting skin-resident immune cells, eliminating injection-associated pain, and improving patient compliance. Here, we developed and characterized a candidate malaria vaccine loaded and deployed using dissolvable microneedle arrays. Of note, a newly indicated human-relevant antigen was employed, Plasmodium falciparum surface protein P47. P47 and a potent toll-like receptor (TLR9) agonist vaccine adjuvant, CpG, were fabricated into microneedles using a gelatin polymer. Protein binding, ELISA, and fluorescence analysis confirmed the molecular structure, and the function of the P47 antigen and CpG was maintained after fabrication, storage, and release from microneedles. In cell culture, the cargo released from the microneedle arrays triggered TLR9 signaling and activated primary dendritic cells at levels similar to native, unincorporated vaccine components. Together, these studies demonstrate the potential of microneedles as an easily deployable strategy for a P47-based malaria vaccine.

Antibody Therapy Goes to Insects: Monoclonal Antibodies Can Block Plasmodium Transmission to Mosquitoes

Malaria eradication is a global priority but requires innovative strategies. Humoral immune responses attack different parasite stages, and antibody-based therapy may prevent malaria infection or transmission. Here, we discuss targets of monoclonal antibodies in mosquito sexual stages of Plasmodium.

Mosquito cellular immunity at single-cell resolution

Hemocytes limit the capacity of mosquitoes to transmit human pathogens. Here we profile the transcriptomes of 8506 hemocytes of Anopheles gambiae and Aedes aegypti mosquito vectors. Our data reveal the functional diversity of hemocytes, with different subtypes of granulocytes expressing distinct and evolutionarily conserved subsets of effector genes. A previously unidentified cell type in An. gambiae, which we term "megacyte," is defined by a specific transmembrane protein marker (TM7318) and high expression of lipopolysaccharide-induced tumor necrosis factor-α transcription factor 3 (LL3). Knockdown experiments indicate that LL3 mediates hemocyte differentiation during immune priming. We identify and validate two main hemocyte lineages and find evidence of proliferating granulocyte populations. This atlas of medically relevant invertebrate immune cells at single-cell resolution identifies cellular events that underpin mosquito immunity to malaria infection.

BCG vaccine protection from severe coronavirus disease 2019 (COVID-19)

A series of epidemiological explorations has suggested a negative association between national bacillus Calmette-Guérin (BCG) vaccination policy and the prevalence and mortality of coronavirus disease 2019 (COVID-19). However, these comparisons are difficult to validate due to broad differences between countries such as socioeconomic status, demographic structure, rural vs. urban settings, time of arrival of the pandemic, number of diagnostic tests and criteria for testing, and national control strategies to limit the spread of COVID-19. We review evidence for a potential biological basis of BCG cross-protection from severe COVID-19, and refine the epidemiological analysis to mitigate effects of potentially confounding factors (e.g., stage of the COVID-19 epidemic, development, rurality, population density, and age structure). A strong correlation between the BCG index, an estimation of the degree of universal BCG vaccination deployment in a country, and COVID-19 mortality in different socially similar European countries was observed (r2 = 0.88; P = 8 × 10-7), indicating that every 10% increase in the BCG index was associated with a 10.4% reduction in COVID-19 mortality. Results fail to confirm the null hypothesis of no association between BCG vaccination and COVID-19 mortality, and suggest that BCG could have a protective effect. Nevertheless, the analyses are restricted to coarse-scale signals and should be considered with caution. BCG vaccination clinical trials are required to corroborate the patterns detected here, and to establish causality between BCG vaccination and protection from severe COVID-19. Public health implications of a plausible BCG cross-protection from severe COVID-19 are discussed.

In vivo Characterization of Plasmodium berghei P47 (Pbs47) as a Malaria Transmission-Blocking Vaccine Target

An effective vaccine to reduce malaria transmission is central to control and ultimately achieve disease eradication. Recently, we demonstrated that antibodies targeting the Plasmodium falciparum surface protein P47 (Pfs47) reduce parasite transmission to Anopheles gambiae mosquitoes. Here, Plasmodium berghei (Pb) was used as a model to assess the in vivo efficacy of a P47-targeted transmission blocking vaccine (Pbs47). Mice were immunized following a prime/boost regimen and infected with P. berghei. The effect of immunization on infectivity to mosquitoes was evaluated by direct feeding on P. berghei-infected mice. The key region in Pbs47 where antibody binding confers protection was mapped, and the immunogenicity of this protective antigen was enhanced by conjugation to a virus-like particle. Passive immunization with 100 and 50 μg/mL of anti-Pbs47 IgG reduced oocyst density by 77 and 67%, respectively. Furthermore, affinity purified Pbs47-specific IgG significantly reduced oocyst density by 88 and 77%, respectively at doses as low as 10 and 1 μg/mL. These studies suggest that P47 is a promising transmission blocking target and show that antibodies to the same specific region in Pfs47 and Pbs47 confer protection.

BCG Vaccine Protection from Severe Coronavirus Disease 2019 (COVID19)

A series of epidemiological explorations has suggested a negative association between national BCG vaccination policy and the prevalence and mortality of COVID-19. However, these comparisons are difficult to validate due to broad differences between countries such as socioeconomic status, demographic structure, rural vs. urban settings, time of arrival of the pandemic, number of diagnostic tests and criteria for testing, and national control strategies to limit the spread of COVID-19. We review evidence for a potential biological basis of BCG cross-protection from severe COVID-19, and refine the epidemiological analysis to mitigate effects of potentially confounding factors (e.g., stage of the COVID-19 epidemic, development, rurality, population density, and age structure). A strong correlation between the BCG index, an estimation of the degree of universal BCG vaccination deployment in a country, and COVID-19 mortality in different socially similar European countries was observed (r2 = 0.88; p = 8 X 10-7), indicating that every 10% increase in the BCG index was associated with a 10.4% reduction in COVID-19 mortality. Results fail to confirm the null hypothesis of no-association between BCG vaccination and COVID-19 mortality, and suggest that BCG could have a protective effect. Nevertheless, the analyses are restricted to coarse-scale signals and should be considered with caution. BCG vaccination clinical trials are required to corroborate the patterns detected here, and to establish causality between BCG vaccination and protection from severe COVID-19. Public health implications of a plausible BCG cross-protection from severe COVID-19 are discussed.

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.

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.

Mosquito Midgut Prostaglandin Release Establishes Systemic Immune Priming

Anopheles gambiae mosquitoes that have been infected with Plasmodium mount a more effective immune response to a subsequent infection. Priming is established when Plasmodium invasion of the mosquito midgut allows contact of the gut microbiota with epithelial cells. This event is followed by a systemic release of a hemocyte differentiation factor (HDF) consisting of Lipoxin A4 bound to Evokin, a lipocalin carrier, which increases the proportion of circulating hemocytes. We show that mosquito midgut cells produce and release prostaglandin E2 (PGE2), which attracts hemocytes to the midgut surface and enhances their patrolling activity. Systemic injection of prostaglandins (PGs) recapitulates the priming response and enhances antiplasmodial immunity by triggering HDF production. Although insects lack cyclooxygenases, two heme peroxidases, HPX7 and HPX8, catalyze essential steps in PG biosynthesis in mosquitoes. Mosquito midgut PGE2 release attracts hemocytes and establishes a long-lasting enhanced systemic cellular immune response to Plasmodium infection.

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Plasmodium invasion or bacterial exposure triggers midgut prostaglandin synthesis

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Prostaglandins attract mosquito hemocytes and increase their patrolling activity

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Two midgut peroxidases, HPX7 and HPX8, catalyze midgut prostaglandin synthesis

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Systemic release of midgut prostaglandins is essential to establish immune priming

Antibody targeting of a specific region of Pfs47 blocks Plasmodium falciparum malaria transmission

Transmission-blocking vaccines are based on eliciting antibody responses in the vertebrate host that disrupt parasite development in the mosquito vector and prevent malaria transmission. The surface protein Pfs47 is present in Plasmodium falciparum gametocytes and female gametes. The potential of Pfs47 as a vaccine target was evaluated. Soluble full-length recombinant protein, consisting of three domains, was expressed in E. coli as a thioredoxin fusion (T-Pfs47). The protein was immunogenic, and polyclonal and monoclonal antibodies (mAb) were obtained, but they did not confer transmission blocking activity (TBA). All fourteen mAb targeted either domains 1 or 3, but not domain 2 (D2), and immune reactivity to D2 was also very low in polyclonal mouse IgG after T-Pfs47 immunization. Disruption of the predicted disulfide bond in D2, by replacing cysteines for alanines (C230A and C260A), allowed expression of recombinant D2 protein in E. coli. A combination of mAbs targeting D2, and deletion proteins from this domain, allowed us to map a central 52 amino acid (aa) region where antibody binding confers strong TBA (78-99%). This 52 aa antigen is immunogenic and well conserved, with only seven haplotypes world-wide that share 96-98% identity. Neither human complement nor the mosquito complement-like system are required for the observed TBA. A dramatic reduction in ookinete numbers and ookinete-specific transcripts was observed, suggesting that the antibodies are interacting with female gametocytes and preventing fertilization.

Plasmodium P47: a key gene for malaria transmission by mosquito vectors

Malaria is caused by infection with Plasmodium parasites that have a complex life cycle. The parasite protein P47 is critical for disease transmission. P47 mediates mosquito immune evasion in both Plasmodium berghei (Pbs47) and Plasmodium falciparum (Pfs47), and has been shown to be important for optimal female gamete fertility in P. berghei. Pfs47 presents strong geographic structure in natural P. falciparum populations, consistent with natural selection of Pfs47 haplotypes by the mosquito immune system as the parasite adapted to new vector species worldwide. These key functions make Plasmodium P47 an attractive target to disrupt malaria transmission.

Effect of naturally occurring Wolbachia in Anopheles gambiae s.l. mosquitoes from Mali on Plasmodium falciparum malaria transmission

A naturally occurring Wolbachia strain (wAnga-Mali) was identified in mosquitoes of the Anopheles gambiae complex collected in the Malian villages of Dangassa and Kenieroba. Phylogenetic analysis of the nucleotide sequence of two 16S rRNA regions showed that wAnga-Mali clusters with Wolbachia strains from supergroup A and has the highest homology to a Wolbachia strain isolated from cat fleas (Ctenocephalides). wAnga-Mali is different from two Wolbachia strains previously reported in A. gambiae from Burkina Faso (wAnga_VK5_STP and wAnga_VK5_3.1a). Quantitative analysis of Wolbachia and Plasmodium sporozoite infection in field-collected mosquitoes indicates that the prevalence and intensity of Plasmodium falciparum sporozoite infection is significantly lower in Wolbachia-infected females. The presence of Wolbachia in females from a laboratory Anopheles coluzzii (A. gambiae, M form) colony experimentally infected with P. falciparum (NF54 strain) gametocyte cultures slightly enhanced oocyst infection. However, Wolbachia infection significantly reduced the prevalence and intensity of sporozoite infection, as observed in the field. This indicates that wAnga-Mali infection does not limit early stages of Plasmodium infection in the mosquito, but it has a strong deleterious effect on sporozoites and reduces malaria transmission.

Activation of mosquito complement antiplasmodial response requires cellular immunity

The mosquito complement-like system is a major defense mechanism that limits Plasmodium infection. Ookinete midgut invasion results in irreversible damage to invaded cells and triggers epithelial nitration and complement activation. Several lines of evidence suggest that hemocytes participate in early antiplasmodial responses that target ookinetes, but their role remains unclear. The fate of hemocytes in response to Plasmodium infection was investigated by labeling this cell population in vivo. We found that midgut nitration triggers the local release of hemocyte-derived microvesicles (HdMv) into the basal labyrinth of the midgut. Several different strategies, such as gene silencing, immune priming, or systemic injection of polystyrene beads, were used to either enhance or reduce HdMv release. We provide direct experimental evidence that contact of hemocytes with the nitrated midgut basal surface triggers HdMv release and that this response is necessary for effective activation of mosquito complement. Our studies suggest that hemocyte-derived microvesicles may deliver some critical factor(s) that promote activation of thioester-containing protein 1, a key effector of the mosquito antiplasmodial immunity.

Mosquito Vectors and the Globalization of Plasmodium falciparum Malaria

Plasmodium falciparum malaria remains a devastating public health problem. Recent discoveries have shed light on the origin and evolution of Plasmodium parasites and their interactions with their vertebrate and mosquito hosts. P. falciparum malaria originated in Africa from a single horizontal transfer between an infected gorilla and a human, and became global as the result of human migration. Today, P. falciparum malaria is transmitted worldwide by more than 70 different anopheline mosquito species. Recent studies indicate that the mosquito immune system can be a barrier to malaria transmission and that the P. falciparum Pfs47 gene allows the parasite to evade mosquito immune detection. Here, we review the origin and globalization of P. falciparum and integrate this history with analysis of the biology, evolution, and dispersal of the main mosquito vectors. This new perspective broadens our understanding of P. falciparum population structure and the dispersal of important parasite genetic traits.

Plasmodium Oocysts: Overlooked Targets of Mosquito Immunity

Although the ability of mosquitoes to limit Plasmodium infection is well documented, many questions remain as to how malaria parasites are recognized and killed by the mosquito host. Recent evidence suggests that anti-Plasmodium immunity is multimodal, with different immune mechanisms regulating ookinete and oocyst survival. However, most experiments determine the number of mature oocysts, without considering that different immune mechanisms may target different developmental stages of the parasite. Complement-like proteins have emerged as important determinants of early immunity targeting the ookinete stage, yet the mechanisms by which the mosquito late-phase immune response limits oocyst survival are less understood. Here, we describe the known components of the mosquito immune system that limit oocyst development, and provide insight into their possible mechanisms of action.

Co-expression of Interleukin-15 Enhances the Protective Immune Responses Induced by Immunization with a Murine Malaria MVA-Based Vaccine Encoding the Circumsporozoite Protein

Malaria remains a major global public health problem with an estimated 200 million cases detected in 2012. Although the most advanced candidate malaria vaccine (RTS,S) has shown promise in clinical trials, its modest efficacy and durability have created uncertainty about the impact of RTS,S immunization (when used alone) on global malaria transmission. Here we describe the development and characterization of a novel modified vaccinia virus Ankara (MVA)–based malaria vaccine which co-expresses the Plasmodium yoelii circumsporozoite protein (CSP) and IL-15. Vaccination/challenge studies showed that C57BL/6 mice immunized with the MVA-CSP/IL15 vaccine were protected significantly better against a P. yoelii 17XNL sporozoite challenge than either mice immunized with an MVA vaccine expressing only CSP or naïve controls. Importantly, the levels of total anti-CSP IgG were elevated about 100-fold for the MVA-CSP/IL15 immunized group compared to mice immunized with the MVA-CSP construct that does not express IL-15. Among the IgG subtypes, the IL-15 expressing MVA-CSP vaccine induced levels of IgG1 (8 fold) and IgG2b (80 fold) higher than the MVA-CSP construct. The significantly enhanced humoral responses and protection detected after immunization with the MVA-CSP/IL15 vaccine suggest that this IL-15 expressing MVA construct could be considered in the development of future malaria immunization strategies.

The remarkable journey of adaptation of the Plasmodium falciparum malaria parasite to New World anopheline mosquitoes

Plasmodium falciparum originated in Africa, dispersed around the world as a result of human migration and had to adapt to several different indigenous anopheline mosquitoes. Anophelines from the New World are evolutionary distant form African ones and this probably resulted in a more stringent selection of Plasmodium as it adapted to these vectors. It is thought that Plasmodium has been genetically selected by some anopheline species through unknown mechanisms. The mosquito immune system can greatly limit infection and P. falciparum evolved a strategy to evade these responses, at least in part mediated by Pfs47, a highly polymorphic gene. We propose that adaptation of P. falciparum to new vectors may require evasion of their immune system. Parasites with a Pfs47 haplotype compatible with the indigenous mosquito vector would be able to survive and be transmitted. The mosquito antiplasmodial response could be an important determinant of P. falciparum population structure and could affect malaria transmission in the Americas.

A mosquito lipoxin/lipocalin complex mediates innate immune priming in Anopheles gambiae

Exposure of Anopheles gambiae mosquitoes to Plasmodium infection enhances the ability of their immune system to respond to subsequent infections. However, the molecular mechanism that allows the insect innate immune system to ‘remember' a previous encounter with a pathogen has not been established. Challenged mosquitoes constitutively release a soluble haemocyte differentiation factor into their haemolymph that, when transferred into Naive mosquitoes, also induces priming. Here we show that this factor consists of a Lipoxin/Lipocalin complex. We demonstrate that innate immune priming in mosquitoes involves a persistent increase in expression of Evokin (a lipid carrier of the lipocalin family), and in their ability to convert arachidonic acid to lipoxins, predominantly Lipoxin A₄. Plasmodium ookinete midgut invasion triggers immune priming by inducing the release of a mosquito lipoxin/lipocalin complex.

A soluble factor induced by Plasmodium infection promotes hemocyte differentiation and increases mosquitoe resistance to subsequent infections. Here the authors show that this factor consists of a Lipocalin/Lipoxin A4 complex, and that insects can metabolize arachidonic acid to produce lipoxins.

Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease

Malaria is a mosquito-borne disease caused by parasites of the obligate intracellular Apicomplexa phylum the most deadly of which, Plasmodium falciparum, prevails in Africa. Malaria imposes a huge health burden on the world's most vulnerable populations, claiming the lives of nearly one million children and pregnant women each year. Although there is keen interest in eradicating malaria, we do not yet have the necessary tools to meet this challenge, including an effective malaria vaccine and adequate vector control strategies. Here we review what is known about the mechanisms at play in immune resistance to malaria in both the human and mosquito hosts at each step in the parasite's complex life cycle with a view toward developing the tools that will contribute to the prevention of disease and death and, ultimately, to the goal of malaria eradication. In so doing, we hope to inspire immunologists to participate in defeating this devastating disease.

In depth annotation of the Anopheles gambiae mosquito midgut transcriptome

Background

Genome sequencing of Anopheles gambiae was completed more than ten years ago and has accelerated research on malaria transmission. However, annotation needs to be refined and verified experimentally, as most predicted transcripts have been identified by comparative analysis with genomes from other species. The mosquito midgut—the first organ to interact with Plasmodium parasites—mounts effective antiplasmodial responses that limit parasite survival and disease transmission. High-throughput Illumina sequencing of the midgut transcriptome was used to identify new genes and transcripts, contributing to the refinement of An. gambiae genome annotation.

Results

We sequenced ~223 million reads from An. gambiae midgut cDNA libraries generated from susceptible (G3) and refractory (L35) mosquito strains. Mosquitoes were infected with either Plasmodium berghei or Plasmodium falciparum, and midguts were collected after the first or second Plasmodium infection. In total, 22,889 unique midgut transcript models were generated from both An. gambiae strain sequences combined, and 76% are potentially novel. Of these novel transcripts, 49.5% aligned with annotated genes and appear to be isoforms or pre-mRNAs of reference transcripts, while 50.5% mapped to regions between annotated genes and represent novel intergenic transcripts (NITs). Predicted models were validated for midgut expression using qRT-PCR and microarray analysis, and novel isoforms were confirmed by sequencing predicted intron-exon boundaries. Coding potential analysis revealed that 43% of total midgut transcripts appear to be long non-coding RNA (lncRNA), and functional annotation of NITs showed that 68% had no homology to current databases from other species. Reads were also analyzed using de novo assembly and predicted transcripts compared with genome mapping-based models. Finally, variant analysis of G3 and L35 midgut transcripts detected 160,742 variants with respect to the An. gambiae PEST genome, and 74% were new variants. Intergenic transcripts had a higher frequency of variation compared with non-intergenic transcripts.

Conclusion

This in-depth Illumina sequencing and assembly of the An. gambiae midgut transcriptome doubled the number of known transcripts and tripled the number of variants known in this mosquito species. It also revealed existence of a large number of lncRNA and opens new possibilities for investigating the biological function of many newly discovered transcripts.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-636) contains supplementary material, which is available to authorized users.

The role of hemocytes in Anopheles gambiae antiplasmodial immunity

Hemocytes synthesize key components of the mosquito complement-like system, but their role in the activation of antiplasmodial responses has not been established. The effect of activating Toll signaling in hemocytes on Plasmodium survival was investigated by transferring hemocytes or cell-free hemolymph from donor mosquitoes in which the suppressor cactus was silenced. These transfers greatly enhanced antiplasmodial immunity, indicating that hemocytes are active players in the activation of the complement-like system, through an effector/effectors regulated by the Toll pathway. A comparative analysis of hemocyte populations between susceptible G3 and the refractory L3-5 Anopheles gambiae mosquito strains did not reveal significant differences under basal conditions or in response to Plasmodium berghei infection. The response of susceptible mosquitoes to different Plasmodium species revealed similar kinetics following infection with P. berghei,P. yoelii or P. falciparum, but the strength of the priming response was stronger in less compatible mosquito-parasite pairs. The Toll, Imd,STAT or JNK signaling cascades were not essential for the production of the hemocyte differentiation factor (HDF) in response to P. berghei infection, but disruption of Toll, STAT or JNK abolished hemocyte differentiation in response to HDF. We conclude that hemocytes are key mediators of A. gambiae antiplasmodial responses.

The human malaria parasite Pfs47 gene mediates evasion of the mosquito immune system

Plasmodium falciparum transmission by Anopheles gambiae mosquitoes is remarkably efficient, resulting in a very high prevalence of human malaria infection in sub-Saharan Africa. A combination of genetic mapping, linkage group selection, and functional genomics was used to identify Pfs47 as a P. falciparum gene that allows the parasite to infect A. gambiae without activating the mosquito immune system. Disruption of Pfs47 greatly reduced parasite survival in the mosquito, and this phenotype could be reverted by genetic complementation of the parasite or by disruption of the mosquito complement-like system. Pfs47 suppresses midgut nitration responses that are critical to activate the complement-like system. We provide direct experimental evidence that immune evasion mediated by Pfs47 is critical for efficient human malaria transmission by A. gambiae.

An impossible journey? The development of Plasmodium falciparum NF54 in Culex quinquefasciatus

Although Anopheles mosquitoes are the vectors for human Plasmodium spp., there are also other mosquito species-among them culicines (Culex spp., Aedes spp.)-present in malaria-endemic areas. Culicine mosquitoes transmit arboviruses and filarial worms to humans and are vectors for avian Plasmodium spp., but have never been observed to transmit human Plasmodium spp. When ingested by a culicine mosquito, parasites could either face an environment that does not allow development due to biologic incompatibility or be actively killed by the mosquito's immune system. In the latter case, the molecular mechanism of killing must be sufficiently powerful that Plasmodium is not able to overcome it. To investigate how human malaria parasites develop in culicine mosquitoes, we infected Culex quinquefasciatus with Plasmodium falciparum NF54 and monitored development of parasites in the blood bolus and midgut epithelium at different time points. Our results reveal that ookinetes develop in the midgut lumen of C. quinquefasciatus in slightly lower numbers than in Anopheles gambiae G3. After 30 hours, parasites have invaded the midgut and can be observed on the basal side of the midgut epithelium by confocal and transmission electron microscopy. Very few of the parasites in C. quinquefasciatus are alive, most of them are lysed. Eight days after the mosquito's blood meal, no oocysts can be found in C. quinquefasciatus. Our results suggest that the mosquito immune system could be involved in parasite killing early in development after ookinetes have crossed the midgut epithelium and come in contact with the mosquito hemolymph.

Mitochondrial reactive oxygen species modulate mosquito susceptibility to Plasmodium infection

BACKGROUND

Mitochondria perform multiple roles in cell biology, acting as the site of aerobic energy-transducing pathways and as an important source of reactive oxygen species (ROS) that modulate redox metabolism.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate that a novel member of the mitochondrial transporter protein family, Anopheles gambiae mitochondrial carrier 1 (AgMC1), is required to maintain mitochondrial membrane potential in mosquito midgut cells and modulates epithelial responses to Plasmodium infection. AgMC1 silencing reduces mitochondrial membrane potential, resulting in increased proton-leak and uncoupling of oxidative phosphorylation. These metabolic changes reduce midgut ROS generation and increase A. gambiae susceptibility to Plasmodium infection.

CONCLUSION

We provide direct experimental evidence indicating that ROS derived from mitochondria can modulate mosquito epithelial responses to Plasmodium infection.

Epithelial nitration by a peroxidase/NOX5 system mediates mosquito antiplasmodial immunity

Plasmodium ookinetes traverse midgut epithelial cells before they encounter the complement system in the mosquito hemolymph. We identified a heme peroxidase (HPX2) and NADPH oxidase 5 (NOX5) as critical mediators of midgut epithelial nitration and antiplasmodial immunity that enhance nitric oxide toxicity in Anopheles gambiae. We show that the two immune mechanisms that target ookinetes-epithelial nitration and thioester-containing protein 1 (TEP1)-mediated lysis-work sequentially, and we propose that epithelial nitration works as an opsonization-like system that promotes activation of the mosquito complement cascade.

Directional gene expression and antisense transcripts in sexual and asexual stages of Plasmodium falciparum

BACKGROUND

It has been shown that nearly a quarter of the initial predicted gene models in the Plasmodium falciparum genome contain errors. Although there have been efforts to obtain complete cDNA sequences to correct the errors, the coverage of cDNA sequences on the predicted genes is still incomplete, and many gene models for those expressed in sexual or mosquito stages have not been validated. Antisense transcripts have widely been reported in P. falciparum; however, the extent and pattern of antisense transcripts in different developmental stages remain largely unknown.

RESULTS

We have sequenced seven bidirectional libraries from ring, early and late trophozoite, schizont, gametocyte II, gametocyte V, and ookinete, and four strand-specific libraries from late trophozoite, schizont, gametocyte II, and gametocyte V of the 3D7 parasites. Alignment of the cDNA sequences to the 3D7 reference genome revealed stage-specific antisense transcripts and novel intron-exon splicing junctions. Sequencing of strand-specific cDNA libraries suggested that more genes are expressed in one direction in gametocyte than in schizont. Alternatively spliced genes, antisense transcripts, and stage-specific expressed genes were also characterized.

CONCLUSIONS

It is necessary to continue to sequence cDNA from different developmental stages, particularly those of non-erythrocytic stages. The presence of antisense transcripts in some gametocyte and ookinete genes suggests that these antisense RNA may play an important role in gene expression regulation and parasite development. Future gene expression studies should make use of directional cDNA libraries. Antisense transcripts may partly explain the observed discrepancy between levels of mRNA and protein expression.

Energy metabolism affects susceptibility of Anopheles gambiae mosquitoes to Plasmodium infection

Previous studies showed that Anopheles gambiae L3-5 females, which are refractory (R) to Plasmodium infection, express higher levels of genes involved in redox-metabolism and mitochondrial respiration than susceptible (S) G3 females. Our studies revealed that R females have reduced longevity, faster utilization of lipid reserves, impaired mitochondrial state-3 respiration, increased rate of mitochondrial electron leak and higher expression levels of several glycolytic enzyme genes. Furthermore, when state-3 respiration was reduced in S females by silencing expression of the adenine nucleotide translocator (ANT), hydrogen peroxide generation was higher and the mRNA levels of lactate dehydrogenase increased in the midgut, while the prevalence and intensity of Plasmodium berghei infection were significantly reduced. We conclude that there are broad metabolic differences between R and S An. gambiae mosquitoes that influence their susceptibility to Plasmodium infection.

Apolipophorin-III mediates antiplasmodial epithelial responses in Anopheles gambiae (G3) mosquitoes

Background

Apolipophorin-III (ApoLp-III) is known to play an important role in lipid transport and innate immunity in lepidopteran insects. However, there is no evidence of involvement of ApoLp-IIIs in the immune responses of dipteran insects such as Drosophila and mosquitoes.

Methodology/Principal Findings

We report the molecular and functional characterization of An. gambiae apolipophorin-III (AgApoLp-III). Mosquito ApoLp-IIIs have diverged extensively from those of lepidopteran insects; however, the predicted tertiary structure of AgApoLp-III is similar to that of Manduca sexta (tobacco hornworm). We found that AgApoLp-III mRNA expression is strongly induced in the midgut of An. gambiae (G3 strain) mosquitoes in response to Plasmodium berghei infection. Furthermore, immunofluorescence stainings revealed that high levels of AgApoLp-III protein accumulate in the cytoplasm of Plasmodium-invaded cells and AgApoLp-III silencing increases the intensity of P. berghei infection by five fold.

Conclusion

There are broad differences in the midgut epithelial responses to Plasmodium invasion between An. gambiae strains. In the G3 strain of An. gambiae AgApoLp-III participates in midgut epithelial defense responses that limit Plasmodium infection.

Hemocyte differentiation mediates innate immune memory in Anopheles gambiae mosquitoes

Mosquito midgut invasion by ookinetes of the malaria parasite Plasmodium disrupts the barriers that normally prevent the gut microbiota from coming in direct contact with epithelial cells. This triggers a long-lived response characterized by increased abundance of granulocytes, a subpopulation of hemocytes that circulates in the insect's hemocoel, and enhanced immunity to bacteria that indirectly reduces survival of Plasmodium parasites upon reinfection. In mosquitoes, differentiation of hemocytes was necessary and sufficient to confer innate immune memory.

The Anopheles gambiae oxidation resistance 1 (OXR1) gene regulates expression of enzymes that detoxify reactive oxygen species

BACKGROUND

OXR1 is an ancient gene, present in all eukaryotes examined so far that confers protection from oxidative stress by an unknown mechanism. The most highly conserved region of the gene is the carboxyl-terminal TLDc domain, which has been shown to be sufficient to prevent oxidative damage.

METHODOLOGY/PRINCIPAL FINDINGS

OXR1 has a complex genomic structure in the mosquito A. gambiae, and we confirm that multiple splice forms are expressed in adult females. Our studies revealed that OXR1 regulates the basal levels of catalase (CAT) and glutathione peroxidase (Gpx) expression, two enzymes involved in detoxification of hydrogen peroxide, giving new insight into the mechanism of action of OXR1. Gene silencing experiments indicate that the Jun Kinase (JNK) gene acts upstream of OXR1 and also regulates expression of CAT and GPx. Both OXR1 and JNK genes are required for adult female mosquitoes to survive chronic oxidative stress. OXR1 silencing decreases P. berghei oocyst formation. Unexpectedly, JNK silencing has the opposite effect and enhances Plasmodium infection in the mosquito, suggesting that JNK may also mediate some, yet to be defined, antiparasitic response.

CONCLUSION

The JNK pathway regulates OXR1 expression and OXR1, in turn, regulates expression of enzymes that detoxify reactive oxygen species (ROS) in Anopheles gambiae. OXR1 silencing decreases Plasmodium infection in the mosquito, while JNK silencing has the opposite effect and enhances infection.

A peroxidase/dual oxidase system modulates midgut epithelial immunity in Anopheles gambiae

Extracellular matrices in diverse biological systems are cross-linked by dityrosine covalent bonds catalyzed by the peroxidase/oxidase system. We show that a peroxidase, secreted by the Anopheles gambiae midgut, and dual oxidase form a dityrosine network that decreases gut permeability to immune elicitors. This network protects the microbiota by preventing activation of epithelial immunity. It also provides a suitable environment for malaria parasites to develop within the midgut lumen without inducing nitric oxide synthase expression. Disruption of this barrier results in strong and effective pathogen-specific immune responses.

Mosquito immune responses and compatibility between Plasmodium parasites and anopheline mosquitoes

BACKGROUND

Functional screens based on dsRNA-mediated gene silencing identified several Anopheles gambiae genes that limit Plasmodium berghei infection. However, some of the genes identified in these screens have no effect on the human malaria parasite Plasmodium falciparum; raising the question of whether different mosquito effector genes mediate anti-parasitic responses to different Plasmodium species.

RESULTS

Four new An. gambiae (G3) genes were identified that, when silenced, have a different effect on P. berghei (Anka 2.34) and P. falciparum (3D7) infections. Orthologs of these genes, as well as LRIM1 and CTL4, were also silenced in An. stephensi (Nijmegen Sda500) females infected with P. yoelii (17XNL). For five of the six genes tested, silencing had the same effect on infection in the P. falciparum-An. gambiae and P. yoelii-An. stephensi parasite-vector combinations. Although silencing LRIM1 or CTL4 has no effect in An. stephensi females infected with P. yoelii, when An. gambiae is infected with the same parasite, silencing these genes has a dramatic effect. In An. gambiae (G3), TEP1, LRIM1 or LRIM2 silencing reverts lysis and melanization of P. yoelii, while CTL4 silencing enhances melanization.

CONCLUSION

There is a broad spectrum of compatibility, the extent to which the mosquito immune system limits infection, between different Plasmodium strains and particular mosquito strains that is mediated by TEP1/LRIM1 activation. The interactions between highly compatible animal models of malaria, such as P. yoelii (17XNL)-An. stephensi (Nijmegen Sda500), is more similar to that of P. falciparum (3D7)-An. gambiae (G3).

The STAT pathway mediates late-phase immunity against Plasmodium in the mosquito Anopheles gambiae

The STAT family of transcription factors activates expression of immune system genes in vertebrates. The ancestral STAT gene (AgSTAT-A) appears to have duplicated in the mosquito Anopheles gambiae, giving rise to a second intronless STAT gene (AgSTAT-B), which we show regulates AgSTAT-A expression in adult females. AgSTAT-A participates in the transcriptional activation of nitric oxide synthase (NOS) in response to bacterial and plasmodial infection. Activation of this pathway, however, is not essential for mosquitoes to survive a bacterial challenge. AgSTAT-A silencing reduces the number of early Plasmodium oocysts in the midgut, but nevertheless enhances the overall infection by increasing oocyst survival. Silencing of SOCS, a STAT suppressor, has the opposite effect, reducing Plasmodium infection by increasing NOS expression. Chemical inhibition of mosquito NOS activity after oocyte formation increases oocyte survival. Thus, the AgSTAT-A pathway mediates a late-phase antiplasmodial response that reduces oocyst survival in A. gambiae.