An Evolution-Based Screen for Genetic Differentiation between Anopheles Sister Taxa Enriches for Detection of Functional Immune Factors

Nucleotide variation patterns across species are shaped by the processes of natural selection, including exposure to environmental pathogens. We examined patterns of genetic variation in two sister species, Anopheles gambiae and Anopheles coluzzii, both efficient natural vectors of human malaria in West Africa. We used the differentiation signature displayed by a known coordinate selective sweep of immune genes APL1 and TEP1 in A. coluzzii to design a population genetic screen trained on the sweep, classified a panel of 26 potential immune genes for concordance with the signature, and functionally tested their immune phenotypes. The screen results were strongly predictive for genes with protective immune phenotypes: genes meeting the screen criteria were significantly more likely to display a functional phenotype against malaria infection than genes not meeting the criteria (p = 0.0005). Thus, an evolution-based screen can efficiently prioritize candidate genes for labor-intensive downstream functional testing, and safely allow the elimination of genes not meeting the screen criteria. The suite of immune genes with characteristics similar to the APL1-TEP1 selective sweep appears to be more widespread in the A. coluzzii genome than previously recognized. The immune gene differentiation may be a consequence of adaptation of A. coluzzii to new pathogens encountered in its niche expansion during the separation from A. gambiae, although the role, if any of natural selection by Plasmodium is unknown. Application of the screen allowed identification of new functional immune factors, and assignment of new functions to known factors. We describe biochemical binding interactions between immune proteins that underlie functional activity for malaria infection, which highlights the interplay between pathogen specificity and the structure of immune complexes. We also find that most malaria-protective immune factors display phenotypes for either human or rodent malaria, with broad specificity a rarity.

Anopheles gambiae and Anopheles coluzzii are the primary mosquito vectors of human malaria in West Africa. Both of these closely related species efficiently transmit the disease, although they display ecological differences. Previous work showed that A. coluzzii displays distinct genetic patterns in genes important for mosquito immunity. Here, we use this genetic pattern as a filter to examine a panel of potential immune genes, and show that the genetic pattern is strongly predictive for genes that play a functional role in immunity when tested with malaria parasites.

Mosquitoes Trade Fertility for Immune Defense against Malaria

A study of Anopheles gambiae mosquitoes shows that a molecule involved in defense against the malaria parasite also plays a role in male fertility, identifying a potential evolutionary trade-off between immunity and reproductive fitness. Read the Research Article.

A New Role of the Mosquito Complement-like Cascade in Male Fertility in Anopheles gambiae

Thioester-containing protein 1 (TEP1) is a key immune factor that determines mosquito resistance to a wide range of pathogens, including malaria parasites. Here we report a new allele-specific function of TEP1 in male fertility. We demonstrate that during spermatogenesis TEP1 binds to and removes damaged cells through the same complement-like cascade that kills malaria parasites in the mosquito midgut. Further, higher fertility rates are mediated by an allele that renders the mosquito susceptible to Plasmodium. By elucidating the molecular and genetic mechanisms underlying TEP1 function in spermatogenesis, our study suggests that pleiotropic antagonism between reproduction and immunity may shape resistance of mosquito populations to malaria parasites.

The complement-related protein TEP1, which helps kill malaria parasites, also labels damaged cells for removal during mosquito spermatogenesis and promotes male fertility in the malaria vector Anopheles gambiae.

While Anopheline mosquitoes are the most efficient vectors of human malaria, they do have protective mechanisms directed against the causative parasite, Plasmodium falciparum. Their immune system targets the invading parasites through activation of the mosquito complement-like system. A central component of this system, thioester-containing protein 1 (TEP1), is a highly polymorphic gene with four allelic classes. Although one class, called R1, mediates efficient parasite elimination, the other classes render the mosquitoes susceptible to Plasmodium infections. Until now, it was not clear how or why any of these susceptible TEP1 alleles were maintained in the population. Here we discover a new role of TEP1 in male fertility. We demonstrate that mosquitoes use the same mechanism—nitration of target surfaces—to flag both damaged sperm and Plasmodium cells. Binding of TEP1 to, and removal of, the aberrant sperm is critical to preserve high fertility rates. In the absence of TEP1, accumulation of damaged sperm degrades male fertility. Surprisingly, in spite of the common mechanism of TEP1 activation, distinct alleles of TEP1 mediate efficient removal of defective sperm and killing of malaria parasites. Our results suggest that pleiotropic function in immunity and reproduction is one of the mechanisms that maintain TEP1 polymorphism in mosquito populations.

The microbiome modulates arbovirus transmission in mosquitoes

Mosquito-transmitted arthropod-borne viruses (arboviruses) such as dengue virus, chikungunya virus, and West Nile virus constitute a major public health burden and are increasing in severity and frequency worldwide. The microbiota associated with mosquitoes (comprised of viruses, bacteria, fungi and protozoa) can profoundly influence many host phenotypes including vector competence, which can either be enhanced or suppressed. Thus, the tripartite interactions between the mosquito vector, its microbiota and the pathogens they transmit offer novel possibilities to control arthropod-borne diseases.

Immunogenicity and Serological Cross-Reactivity of Saliva Proteins among Different Tsetse Species

Tsetse are vectors of pathogenic trypanosomes, agents of human and animal trypanosomiasis in Africa. Components of tsetse saliva (sialome) are introduced into the mammalian host bite site during the blood feeding process and are important for tsetse’s ability to feed efficiently, but can also influence disease transmission and serve as biomarkers for host exposure. We compared the sialome components from four tsetse species in two subgenera: subgenus Morsitans: Glossina morsitans morsitans (Gmm) and Glossina pallidipes (Gpd), and subgenus Palpalis: Glossina palpalis gambiensis (Gpg) and Glossina fuscipes fuscipes (Gff), and evaluated their immunogenicity and serological cross reactivity by an immunoblot approach utilizing antibodies from experimental mice challenged with uninfected flies. The protein and immune profiles of sialome components varied with fly species in the same subgenus displaying greater similarity and cross reactivity. Sera obtained from cattle from disease endemic areas of Africa displayed an immunogenicity profile reflective of tsetse species distribution. We analyzed the sialome fractions of Gmm by LC-MS/MS, and identified TAg5, Tsal1/Tsal2, and Sgp3 as major immunogenic proteins, and the 5'-nucleotidase family as well as four members of the Adenosine Deaminase Growth Factor (ADGF) family as the major non-immunogenic proteins. Within the ADGF family, we identified four closely related proteins (TSGF-1, TSGF-2, ADGF-3 and ADGF-4), all of which are expressed in tsetse salivary glands. We describe the tsetse species-specific expression profiles and genomic localization of these proteins. Using a passive-immunity approach, we evaluated the effects of rec-TSGF (TSGF-1 and TSGF-2) polyclonal antibodies on tsetse fitness parameters. Limited exposure of tsetse to mice with circulating anti-TSGF antibodies resulted in a slight detriment to their blood feeding ability as reflected by compromised digestion, lower weight gain and less total lipid reserves although these results were not statistically significant. Long-term exposure studies of tsetse flies to antibodies corresponding to the ADGF family of proteins are warranted to evaluate the role of this conserved family in fly biology.

Insect saliva contains many proteins that are injected into the mammalian host during the blood feeding process. Saliva proteins enhance the blood feeding ability of insects, but they can also induce mammalian immune responses that inhibit successful feeding, or modulate the bite site to benefit pathogen transmission. Here we studied saliva from four different tsetse species that belong to two distant species groups. We show that the saliva protein profiles of different species groups vary. Experimental mice subjected to fly bites display varying immunological responses against the abundant saliva proteins and the antigenicity of the shared saliva proteins in different tsetse species differs. We show that one member of the ADGF family with adenosine deaminase motifs, TSGF-2, is non-immunogenic in Glossina morsitans in mice, while the same protein from Glossina fuscipes is highly immunogenic. Such species-specific immune responses could be exploited as biomarkers of host exposures in the field. We also show that short-term exposure of G. morsitans to mice passively immunized by anti-TSGF antibodies leads to slight but not statistically significant negative fitness effects. Thus, future investigations with non-antigenic saliva proteins are warranted as they can lead to novel mammalian vaccine targets to reduce tsetse populations in the field.

Bacterial Infection and Immune Responses in Lutzomyia longipalpis Sand Fly Larvae Midgut

The midgut microbial community in insect vectors of disease is crucial for an effective immune response against infection with various human and animal pathogens. Depending on the aspects of their development, insects can acquire microbes present in soil, water, and plants. Sand flies are major vectors of leishmaniasis, and shown to harbor a wide variety of Gram-negative and Gram-positive bacteria. Sand fly larval stages acquire microorganisms from the soil, and the abundance and distribution of these microorganisms may vary depending on the sand fly species or the breeding site. Here, we assess the distribution of two bacteria commonly found within the gut of sand flies, Pantoea agglomerans and Bacillus subtilis. We demonstrate that these bacteria are able to differentially infect the larval digestive tract, and regulate the immune response in sand fly larvae. Moreover, bacterial distribution, and likely the ability to colonize the gut, is driven, at least in part, by a gradient of pH present in the gut.

Symbiotic microorganisms influence many aspects of the physiology of their hosts. In insects, symbiotic bacteria are able among other things to modulate the immune response and the development of the insect from larval stages to adult. Many bacteria first gain access to insect tissues, such as the gut, during larval development, and are acquired from the environment. Thus, depending on the insect ecology, aquatic vs. terrestrial, the bacterial gut flora found in insects can vary widely. Little is known about the events that follow bacterial infection in larval guts and the driving forces for colonization of the gut by such bacteria. We investigated the distribution of two bacteria, a Gram-positive (Bacillus subtilis) and a Gram-negative (Pantoea agglomerans) fed to sand fly larvae. Our results indicate that bacteria distribution in the larval gut is driven by their ability to multiply at a given pH, as pH in the gut also varies. Gut distribution by these bacteria lead to an immune response that the sand fly larva is able to modulate according to the bacterial species. Our findings can influence development of paratransgenic approaches that utilize bacterial symbionts to control vector population.

Comparison of Bloodmeal Digestion and the Peritrophic Matrix in Four Sand Fly Species Differing in Susceptibility to Leishmania donovani

The early stage of Leishmania development in sand flies is closely connected with bloodmeal digestion. Here we compared various parameters of bloodmeal digestion in sand flies that are either susceptible (Phlebotomus argentipes and P. orientalis) or refractory (P. papatasi and Sergentomyia schwetzi) to Leishmania donovani, to study the effects on vector competence. The volume of the bloodmeal ingested, time of defecation of bloodmeal remnants, timing of formation and degradation of the peritrophic matrix (PM) and dynamics of proteolytic activities were compared in four sand fly species. Both proven vectors of L. donovani showed lower trypsin activity and slower PM formation than refractory species. Interestingly, the two natural L. donovani vectors strikingly differed from each other in secretion of the PM and midgut proteases, with P. argentipes possessing fast bloodmeal digestion with a very high peak of chymotrypsin activity and rapid degradation of the PM. Experimental infections of P. argentipes did not reveal any differences in vector competence in comparison with previously studied P. orientalis; even the very low initial dose (2×103 promastigotes/ml) led to fully developed late-stage infections with colonization of the stomodeal valve in about 40% of females. We hypothesise that the period between the breakdown of the PM and defecation of the bloodmeal remnants, i.e. the time frame when Leishmania attach to the midgut in order to prevent defecation, could be one of crucial parameters responsible for the establishment of Leishmania in the sand fly midgut. In both natural L. donovani vectors this period was significantly longer than in S. schwetzi. Both vectors are equally susceptible to L. donovani; as average bloodmeal volumes taken by females of P. argentipes and P. orientalis were 0.63 μl and 0.59 μl, respectively, an infective dose corresponding to 1-2 parasites was enough to initiate mature infections.

MicroRNA-regulation of Anopheles gambiae immunity to Plasmodium falciparum infection and midgut microbiota

Invasion of the malaria vector Anopheles gambiae midgut by Plasmodium parasites triggers transcriptional changes of immune genes that mediate the antiparasitic defense. This response is largely regulated by the Toll and Immune deficiency (IMD) pathways. To determine whether A. gambiae microRNAs (miRNAs) are involved in regulating the anti-Plasmodium defense, we showed that suppression of miRNA biogenesis results in increased resistance to Plasmodium falciparum infection. In silico analysis of A. gambiae immune effector genes identified multiple transcripts with miRNA binding sites. A comparative miRNA microarray abundance analysis of P. falciparum infected and naïve mosquito midgut tissues showed elevated abundance of miRNAs aga-miR-989 and aga-miR-305 in infected midguts. Antagomir inhibition of aga-miR-305 increased resistance to P. falciparum infection and suppressed the midgut microbiota. Conversely, treatment of mosquitoes with an artificial aga-miR-305 mimic increased susceptibility to P. falciparum infection and resulted in expansion of midgut microbiota, suggesting that aga-miR-305 acts as a P. falciparum and gut microbiota agonist by negatively regulating the mosquito immune response. In silico prediction of aga-miR-305 target genes identified several anti-Plasmodium effectors. Our study shows that A. gambiae aga-miR-305 regulates the anti-Plasmodium response and midgut microbiota, likely through post-transcriptional modification of immune effector genes.

Anopheles midgut epithelium evades human complement activity by capturing factor H from the blood meal

Hematophagous vectors strictly require ingesting blood from their hosts to complete their life cycles. Exposure of the alimentary canal of these vectors to the host immune effectors necessitates efficient counteractive measures by hematophagous vectors. The Anopheles mosquito transmitting the malaria parasite is an example of hematophagous vectors that within seconds can ingest human blood double its weight. The innate immune defense mechanisms, like the complement system, in the human blood should thereby immediately react against foreign cells in the mosquito midgut. A prerequisite for complement activation is that the target cells lack complement regulators on their surfaces. In this work, we analyzed whether human complement is active in the mosquito midgut, and how the mosquito midgut cells protect themselves against complement attack. We found that complement remained active for a considerable time and was able to kill microbes within the mosquito midgut. However, the Anopheles mosquito midgut cells were not injured. These cells were found to protect themselves by capturing factor H, the main soluble inhibitor of the alternative complement pathway. Factor H inhibited complement on the midgut cells by promoting inactivation of C3b to iC3b and preventing the activity of the alternative pathway amplification C3 convertase enzyme. An interference of the FH regulatory activity by monoclonal antibodies, carried to the midgut via blood, resulted in increased mosquito mortality and reduced fecundity. By using a ligand blotting assay, a putative mosquito midgut FH receptor could be detected. Thereby, we have identified a novel mechanism whereby mosquitoes can tolerate human blood.

Mosquitoes are important vectors in the transmission of many human diseases. Their life cycle requires a blood meal to be completed. Ingested blood contains bioactive molecules belonging to the innate immune defense mechanisms against microbes, like the complement system, that can damage foreign cells. We have identified in this study a mechanism whereby mosquitoes can escape the damaging activity of the complement system in the ingested human blood. The mosquito midgut epithelial cell surface captured factor H, a natural regulator of the alternative pathway of complement activation, from the ingested blood. Consequently, the deposition of C3b, a key complement component, on the epithelial cell surface was impaired and cell death was avoided. Interfering with the complement regulatory activity of factor H by monoclonal antibodies, carried to the midgut via blood feeding, increased mosquito mortality and reduced fecundity. The putative Anopheles mosquito factor H binding proteins could be transmission blocking vaccine candidates targeting the malaria parasite carrying vectors.

An overview of malaria transmission from the perspective of Amazon Anopheles vectors

In the Americas, areas with a high risk of malaria transmission are mainly located in the Amazon Forest, which extends across nine countries. One keystone step to understanding the Plasmodium life cycle in Anopheles species from the Amazon Region is to obtain experimentally infected mosquito vectors. Several attempts to colonise Anopheles species have been conducted, but with only short-lived success or no success at all. In this review, we review the literature on malaria transmission from the perspective of its Amazon vectors. Currently, it is possible to develop experimental Plasmodium vivax infection of the colonised and field-captured vectors in laboratories located close to Amazonian endemic areas. We are also reviewing studies related to the immune response to P. vivax infection of Anopheles aquasalis, a coastal mosquito species. Finally, we discuss the importance of the modulation of Plasmodium infection by the vector microbiota and also consider the anopheline genomes. The establishment of experimental mosquito infections with Plasmodium falciparum, Plasmodium yoelii and Plasmodium berghei parasites that could provide interesting models for studying malaria in the Amazonian scenario is important. Understanding the molecular mechanisms involved in the development of the parasites in New World vectors is crucial in order to better determine the interaction process and vectorial competence.

A Host-Pathogen Interaction Reduced to First Principles: Antigenic Variation in T. brucei

Antigenic variation is a common microbial survival strategy, powered by diversity in expressed surface antigens across the pathogen population over the course of infection. Even so, among pathogens, African trypanosomes have the most comprehensive system of antigenic variation described. African trypanosomes (Trypanosoma brucei spp.) are unicellular parasites native to sub-Saharan Africa, and the causative agents of sleeping sickness in humans and of n'agana in livestock. They cycle between two habitats: a specific species of fly (Glossina spp. or, colloquially, the tsetse) and the bloodstream of their mammalian hosts, by assuming a succession of proliferative and quiescent developmental forms, which vary widely in cell architecture and function. Key to each of the developmental forms that arise during these transitions is the composition of the surface coat that covers the plasma membrane. The trypanosome surface coat is extremely dense, covered by millions of repeats of developmentally specified proteins: procyclin gene products cover the organism while it resides in the tsetse and metacyclic gene products cover it while in the fly salivary glands, ready to make the transition to the mammalian bloodstream. But by far the most interesting coat is the Variant Surface Glycoprotein (VSG) coat that covers the organism in its infectious form (during which it must survive free living in the mammalian bloodstream). This coat is highly antigenic and elicits robust VSG-specific antibodies that mediate efficient opsonization and complement mediated lysis of the parasites carrying the coat against which the response was made. Meanwhile, a small proportion of the parasite population switches coats, which stimulates a new antibody response to the prevalent (new) VSG species and this process repeats until immune system failure. The disease is fatal unless treated, and treatment at the later stages is extremely toxic. Because the organism is free living in the blood, the VSG:antibody surface represents the interface between pathogen and host, and defines the interaction of the parasite with the immune response. This interaction (cycles of VSG switching, antibody generation, and parasite deletion) results in stereotypical peaks and troughs of parasitemia that were first recognized more than 100 years ago. Essentially, the mechanism of antigenic variation in T. brucei results from a need, at the population level, to maintain an extensive repertoire, to evade the antibody response. In this chapter, we will examine what is currently known about the VSG repertoire, its depth, and the mechanisms that diversify it both at the molecular (DNA) and at the phenotypic (surface displayed) level, as well as how it could interact with antibodies raised specifically against it in the host.

Flavivirus-mosquito interactions

The Flavivirus genus is in the family Flaviviridae and is comprised of more than 70 viruses. These viruses have a broad geographic range, circulating on every continent except Antarctica. Mosquito-borne flaviviruses, such as yellow fever virus, dengue virus serotypes 1-4, Japanese encephalitis virus, and West Nile virus are responsible for significant human morbidity and mortality in affected regions. This review focuses on what is known about flavivirus-mosquito interactions and presents key data collected from the field and laboratory-based molecular and ultrastructural evaluations.

Chikungunya virus-vector interactions

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that causes chikungunya fever, a severe, debilitating disease that often produces chronic arthralgia. Since 2004, CHIKV has emerged in Africa, Indian Ocean islands, Asia, Europe, and the Americas, causing millions of human infections. Central to understanding CHIKV emergence is knowledge of the natural ecology of transmission and vector infection dynamics. This review presents current understanding of CHIKV infection dynamics in mosquito vectors and its relationship to human disease emergence. The following topics are reviewed: CHIKV infection and vector life history traits including transmission cycles, genetic origins, distribution, emergence and spread, dispersal, vector competence, vector immunity and microbial interactions, and co-infection by CHIKV and other arboviruses. The genetics of vector susceptibility and host range changes, population heterogeneity and selection for the fittest viral genomes, dual host cycling and its impact on CHIKV adaptation, viral bottlenecks and intrahost diversity, and adaptive constraints on CHIKV evolution are also discussed. The potential for CHIKV re-emergence and expansion into new areas and prospects for prevention via vector control are also briefly reviewed.

Mosquito-borne viruses and suppressors of invertebrate antiviral RNA silencing

The natural maintenance cycles of many mosquito-borne viruses require establishment of persistent non-lethal infections in the invertebrate host. While the mechanisms by which this occurs are not well understood, antiviral responses directed by small RNAs are important in modulating the pathogenesis of viral infections in disease vector mosquitoes. In yet another example of an evolutionary arms race between host and pathogen, some plant and insect viruses have evolved to encode suppressors of RNA silencing (VSRs). Whether or not mosquito-borne viral pathogens encode VSRs has been the subject of debate. While at first there would seem to be little evolutionary benefit to mosquito-borne viruses encoding proteins or sequences that strongly interfere with RNA silencing, we present here a model explaining how the expression of VSRs by these viruses in the vector might be compatible with the establishment of persistence. We also discuss the challenges associated with interrogating these viruses for the presence of suppressor proteins or sequences, as well as the candidates that have been identified in the genomes of mosquito-borne pathogens thus far.

Meta-analysis of the effects of insect vector saliva on host immune responses and infection of vector-transmitted pathogens: a focus on leishmaniasis

A meta-analysis of the effects of vector saliva on the immune response and progression of vector-transmitted disease, specifically with regard to pathology, infection level, and host cytokine levels was conducted. Infection in the absence or presence of saliva in naïve mice was compared. In addition, infection in mice pre-exposed to uninfected vector saliva was compared to infection in unexposed mice. To control for differences in vector and pathogen species, mouse strain, and experimental design, a random effects model was used to compare the ratio of the natural log of the experimental to the control means of the studies. Saliva was demonstrated to enhance pathology, infection level, and the production of Th2 cytokines (IL-4 and IL-10) in naïve mice. This effect was observed across vector/pathogen pairings, whether natural or unnatural, and with single salivary proteins used as a proxy for whole saliva. Saliva pre-exposure was determined to result in less severe leishmaniasis pathology when compared with unexposed mice infected either in the presence or absence of sand fly saliva. The results of further analyses were not significant, but demonstrated trends toward protection and IFN-γ elevation for pre-exposed mice.

Arthropod vectors transmit a wide variety of diseases resulting in substantial human morbidity and economic costs worldwide. When hematophagous arthropods blood feed, they release saliva into the host. This saliva elicits a strong immune response and has recently been a focus for vaccine research. There is evidence that the saliva enhances infection in naïve hosts, but that prior exposure to saliva results in less severe infection. This analysis endeavored to determine whether there was a statistically significant enhancement or protective effect with regard to saliva exposure and the progression of disease, and to determine the underlying immune mechanism driving these effects. We found that saliva does indeed enhance infection levels of vector-transmitted pathogens and leishmaniasis pathology in naïve mice and elevates Th2 cytokine levels (IL-4 and IL-10). We also determined that pre-exposure to saliva results in less severe pathology of experimental leishmaniasis in mice. These results are important for vaccine trials and vector control programs, though more studies are needed with regard to pre-exposure.

Characterization of the Rel2-regulated transcriptome and proteome of Anopheles stephensi identifies new anti-Plasmodium factors

Mosquitoes possess an innate immune system that is capable of limiting infection by a variety of pathogens, including the Plasmodium spp. parasites responsible for human malaria. The Anopheles immune deficiency (IMD) innate immune signaling pathway confers resistance to Plasmodium falciparum. While some previously identified Anopheles anti-Plasmodium effectors are regulated through signaling by Rel2, the transcription factor of the IMD pathway, many components of this defense system remain uncharacterized. To begin to better understand the regulation of immune effector proteins by the IMD pathway, we used oligonucleotide microarrays and iTRAQ to analyze differences in mRNA and protein expression, respectively, between transgenic Anopheles stephensi mosquitoes exhibiting blood meal-inducible overexpression of an active recombinant Rel2 and their wild-type conspecifics. Numerous genes were differentially regulated at both the mRNA and protein levels following induction of Rel2. While multiple immune genes were up-regulated, a majority of the differentially expressed genes have no known immune function in mosquitoes. Selected up-regulated genes from multiple functional categories were tested for both anti-Plasmodium and anti-bacterial action using RNA interference (RNAi). Based on our experimental findings, we conclude that increased expression of the IMD immune pathway-controlled transcription factor Rel2 affects the expression of numerous genes with diverse functions, suggesting a broader physiological impact of immune activation and possible functional versatility of Rel2. Our study has also identified multiple novel genes implicated in anti-Plasmodium defense.

Fitness impact and stability of a transgene conferring resistance to dengue-2 virus following introgression into a genetically diverse Aedes aegypti strain

In 2006, we reported a mariner (Mos1)-transformed Aedes aegypti line, Carb77, which was highly resistant to dengue-2 virus (DENV2). Carb77 mosquitoes expressed a DENV2-specific inverted-repeat (IR) RNA in midgut epithelial cells after ingesting an infectious bloodmeal. The IR-RNA formed double-stranded DENV2-derived RNA, initiating an intracellular antiviral RNA interference (RNAi) response. However, Carb77 mosquitoes stopped expressing the IR-RNA after 17 generations in culture and lost their DENV2-refractory phenotype. In the current study, we generated new transgenic lines having the identical transgene as Carb77. One of these lines, Carb109M, has been genetically stable and refractory to DENV2 for >33 generations. Southern blot analysis identified two transgene integration sites in Carb109M. Northern blot analysis detected abundant, transient expression of the IR-RNA 24 h after a bloodmeal. Carb109M mosquitoes were refractory to different DENV2 genotypes but not to other DENV serotypes. To further test fitness and stability, we introgressed the Carb109M transgene into a genetically diverse laboratory strain (GDLS) by backcrossing for five generations and selecting individuals expressing the transgene's EGFP marker in each generation. Comparison of transgene stability in replicate backcross 5 (BC₅) lines versus BC₁ control lines demonstrated that backcrossing dramatically increased transgene stability. We subjected six BC₅ lines to five generations of selection based on EGFP marker expression to increase the frequency of the transgene prior to final family selection. Comparison of the observed transgene frequencies in the six replicate lines relative to expectations from Fisher's selection model demonstrated lingering fitness costs associated with either the transgene or linked deleterious genes. Although minimal fitness loss (relative to GDLS) was manifest in the final family selection stage, we were able to select homozygotes for the transgene in one family, Carb109M/GDLS.BC₅.HZ. This family has been genetically stable and DENV2 refractory for multiple generations. Carb109M/GDLS.BC₅.HZ represents an important line for testing proof-of-principle vector population replacement.

Expression of a DENV2 sequence-derived IR RNA in the mosquito midgut initiates an antiviral intracellular RNAi response that efficiently blocks DENV2 infection and profoundly impairs vector competence for that virus in Aedes aegypti. DENV2-specific IR RNA expression in the Carb109M strain has maintained the RNAi-based, refractory phenotype for 33 generations in laboratory culture. The two transgene integration sites were stable after multiple generations and following introgression into a genetically-diverse (GDLS) Ae. aegypti population. Introgression of the transgene into the GDLS genetic background changed GDLS from a DENV2 susceptible phenotype to a DENV2 refractory phenotype. The DENV2 refractory homozygous line, Carb109M/GDLS.BC₅.HZ, exhibits (relative to GDLS) minimal fitness loss associated with the transgene. This line could be a potential candidate for proof-of-principle field studies.

Differential gene expression in laboratory strains of human head and body lice when challenged with Bartonella quintana, a pathogenic bacterium

Human head and body lice are obligatory hematophagous ectoparasites that belong to a single species, Pediculus humanus. Only body lice, however, are vectors of the infectious Gram-negative bacterium Bartonella quintana. Because of their near identical genomes, yet differential vector competence, head and body lice provide a unique model system to study the gain or loss of vector competence. Using our in vitro louse-rearing system, we infected head and body lice with blood containing B. quintana in order to detect both differences in the proliferation of B. quintana and transcriptional differences of immune-related genes in the lice. B. quintana proliferated rapidly in body lice at 6 days post-infection, but plateaued in head lice at 4 days post-infection. RNAseq and quantitative real-time PCR validation analyses determined gene expression differences. Eight immunoresponse genes were observed to be significantly different with many associated with the Toll pathway: Fibrinogen-like protein, Spaetzle, Defensin 1, Serpin, Scavenger receptor A and Apolipoporhrin 2. Our findings support the hypothesis that body lice, unlike head lice, fight infection from B. quintana only at the later stages of its proliferation.

Transmission-blocking antibodies against mosquito C-type lectins for dengue prevention

C-type lectins are a family of proteins with carbohydrate-binding activity. Several C-type lectins in mammals or arthropods are employed as receptors or attachment factors to facilitate flavivirus invasion. We previously identified a C-type lectin in Aedes aegypti, designated as mosquito galactose specific C-type lectin-1 (mosGCTL-1), facilitating the attachment of West Nile virus (WNV) on the cell membrane. Here, we first identified that 9 A. aegypti mosGCTL genes were key susceptibility factors facilitating DENV-2 infection, of which mosGCTL-3 exhibited the most significant effect. We found that mosGCTL-3 was induced in mosquito tissues with DENV-2 infection, and that the protein interacted with DENV-2 surface envelop (E) protein and virions in vitro and in vivo. In addition, the other identified mosGCTLs interacted with the DENV-2 E protein, indicating that DENV may employ multiple mosGCTLs as ligands to promote the infection of vectors. The vectorial susceptibility factors that facilitate pathogen invasion may potentially be explored as a target to disrupt the acquisition of microbes from the vertebrate host. Indeed, membrane blood feeding of antisera against mosGCTLs dramatically reduced mosquito infective ratio. Hence, the immunization against mosGCTLs is a feasible approach for preventing dengue infection. Our study provides a future avenue for developing a transmission-blocking vaccine that interrupts the life cycle of dengue virus and reduces disease burden.

Dengue virus (DENV), a mosquito-borne flavivirus, is currently the most significant arbovirus afflicting tropical and sub-tropical countries worldwide. No vaccine or therapeutics are available, and dengue has rapidly spread over the last decade. Therefore, additional strategies to combat dengue are urgently needed. In this study, we characterized multiple C-type lectins as susceptibility factors for dengue infection in A. aegypti. These mosGCTLs directly interacted with dengue virus in vitro and in vivo. The combination of antisera against multiple mosGCTLs efficiently reduced DENV-2 infection after a blood meal, suggesting that it is feasible to develop a mosGCTL-based transmission-blocking vaccine to interrupt the life cycle of dengue virus and control disease burden in nature. This study substantially extends our understanding of dengue replication in vectors and provides a research avenue by which the development of therapeutics for preventing the dissemination of mosquito-borne viral diseases can be pursued in the future.

Somatic Wolbachia (Rickettsiales: Rickettsiaceae) levels in Culex quinquefasciatus and Culex pipiens (Diptera: Culicidae) and resistance to West Nile virus infection

The endosymbiotic bacteria Wolbachia pipientis Hertig infects a wide variety of insect species and can increase viral resistance in its host. Wolbachia naturally infects Culex quinquefasciatus Say and Culex pipiens L. mosquitoes, both vectors of West Nile virus (WNV). We recently demonstrated that Wolbachia infection of Cx. quinquefasciatus laboratory strain Ben95 increases host resistance to WNV infection, reducing vector competence. This observation raised the possibility that Wolbachia could impact vector competence in other populations of Cx. quinquefasciatus or Cx. pipiens. To investigate this possibility, Wolbachia densities were measured in Ben95 Cx. quinquefasciatus and compared with densities in a newly established colony of Cx. quinquefasciatus, and in field-collected and colonized Cx. pipiens. Wolbachia densities in somatic tissues of Ben95 Cx. quinquefasciatus were significantly higher than densities in the other mosquito populations tested. There was also no significant spatiotemporal variation in Wolbachia density in the field-collected Cx. pipiens, although significant familial differences were observed. Correlating Wolbachia densities and vector competence in individual colonized Cx. pipiens indicated that the densities of somatic Wolbachia observed in the mosquitoes other than Ben95 Cx. quinquefasciatus were too low to inhibit WNV infection and reduce vector competence. These results suggest that the high Wolbachia densities capable of inducing resistance to WNV in Ben95 Cx. quinquefasciatus are not a general characteristic of Cx. quinquefasciatus or Cx. pipiens mosquitoes and that the impact of Wolbachia on vector competence in field populations of Cx. quinquefasciatus and Cx. pipiens, if any, is likely to be limited to specific populations.

Use of anti-Aedes aegypti salivary extract antibody concentration to correlate risk of vector exposure and dengue transmission risk in Colombia

Norte de Santander is a region in Colombia with a high incidence of dengue virus (DENV). In this study, we examined the serum concentration of anti-Aedes salivary gland extract (SGE) antibodies as a biomarker of DENV infection and transmission, and assessed the duration of anti-SGE antibody concentration after exposure to the vector ceased. We also determined whether SGE antibody concentration could differentiate between positive and negative DENV infected individuals and whether there are differences in exposure for each DENV serotype. We observed a significant decrease in the concentration of IgG antibodies at least 40 days after returning to an "Ae. aegypti-free" area. In addition, we found significantly higher anti-SGE IgG concentrations in DENV positive patients with some difference in exposure to mosquito bites among DENV serotypes. We conclude that the concentration of IgG antibodies against SGE is an accurate indicator of risk of dengue virus transmission and disease presence.

Quantitative determination of sporozoites and circumsporozoite antigen in mosquitoes infected withPlasmodium falciparumorP. vivax

It is essential for malariologists and researchers to have simple and accurate means of assessing the threat of Plasmodium parasites. An attempt was therefore made to re-standardize one of the circumsporozoite (CS) ELISA that can be used to detect and quantify the circumsporozoite antigens of P. falciparum and P. vivax. A two-site, 'sandwich' ELISA based on a monoclonal antibody was used to test for the CS antigen and sporozoites of each Plasmodium species simultaneously. Using the resultant optical-density values, standard curves, that permit the number of sporozoites in an infected mosquito to be estimated from the quantification of the CS antigen, were constructed. Using these plots and the CS ELISA, the presence of just 12.5 sporozoites (i.e. 0.8 pg CS antigen) of P. falciparum, four sporozoites (3.2 pg antigen) of P. vivax-210 or 12.5 sporozoites (32.0 pg antigen) of P. vivax-247 could be demonstrated.

Evaluation of the effectiveness of malaria vector control measures in urban settings of Dakar by a specific anopheles salivary biomarker

Standard entomological methods for evaluating the impact of vector control lack sensitivity in low-malaria-risk areas. The detection of human IgG specific to Anopheles gSG6-P1 salivary antigen reflects a direct measure of human–vector contact. This study aimed to assess the effectiveness of a range of vector control measures (VCMs) in urban settings by using this biomarker approach. The study was conducted from October to December 2008 on 2,774 residents of 45 districts of urban Dakar. IgG responses to gSG6-P1 and the use of malaria VCMs highly varied between districts. At the district level, specific IgG levels significantly increased with age and decreased with season and with VCM use. The use of insecticide-treated nets, by drastically reducing specific IgG levels, was by far the most efficient VCM regardless of age, season or exposure level to mosquito bites. The use of spray bombs was also associated with a significant reduction of specific IgG levels, whereas the use of mosquito coils or electric fans/air conditioning did not show a significant effect. Human IgG response to gSG6-P1 as biomarker of vector exposure represents a reliable alternative for accurately assessing the effectiveness of malaria VCM in low-malaria-risk areas. This biomarker tool could be especially relevant for malaria control monitoring and surveillance programmes in low-exposure/low-transmission settings.