Induced immunity against the mosquito Anopheles stephensi (Diptera: Culicidae): effects of cell fraction antigens on survival, fecundity, and plasmodium berghei (Eucoccidiida: Plasmodiidae) transmission

Mapping Plasmodium transitions and interactions in the Anopheles female

The human malaria parasite, Plasmodium falciparum , relies on Anopheles mosquitoes for transmission. Once ingested during blood feeding, most parasites die in the mosquito midgut lumen or during epithelium traversal. How surviving ookinetes interact with midgut cells and form oocysts is unknown, yet these steps are essential to initiate a remarkable, similarly uncharacterized growth process culminating in the production of thousands of infectious sporozoites. Here, using single-cell RNA sequencing of both parasites and mosquito cells across four time points and two metabolic conditions, we unveil key processes shaping developmental transitions and mosquito-parasite interactions occurring in the midgut. In depth functional analyses reveal processes regulating oocyst growth and identify the transcription factor Pf SIP2 as essential for sporozoite infection of human hepatocytes. By combining the analysis of shared mosquito-parasite barcodes with confocal microscopy, we discover that parasites preferentially interact with midgut progenitor cells during epithelial crossing, potentially using their basal location as an exit landmark. Additionally, we unveil tight connections between extracellular late oocysts and surrounding muscle cells that may ensure parasites adhere to the midgut without damaging it. Ultimately, our study provides fundamental insight into the molecular events characterizing previously inaccessible biological transitions and mosquito-parasite interactions, and identifies candidates for transmission-blocking strategies.

The Bm86 Discovery: A Revolution in the Development of Anti-Tick Vaccines

The presence in nature of species with genetic resistance to ticks, or with acquired resistance after repeated tick infestations, has encouraged the scientific community to consider vaccination as an alternative to the unsustainable chemical control of ticks. After numerous attempts to artificially immunize hosts with tick extracts, the purification and characterization of the Bm86 antigen by Willadsen et al. in 1989 constituted a revolutionary step forward in the development of vaccines against ticks. Previously, innovative studies that had used tick gut extracts for the immunization of cattle against Rhipicepahalus microplus (previously named Boophilus microplus) ticks, with amazingly successful results, demonstrated the feasibility of using antigens other than salivary-gland-derived molecules to induce a strong anti-tick immunity. However, the practical application of an anti-tick vaccine required the isolation, identification, and purification of the responsible antigen, which was finally defined as the Bm86 protein. More than thirty years later, the only commercially available anti-tick vaccines are still based on this antigen, and all our current knowledge about the field application of immunological control based on vaccination against ticks has been obtained through the use of these vaccines.

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

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

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

Mosquitocidal properties of IgG targeting the glutamate-gated chloride channel in three mosquito disease vectors (Diptera: Culicidae)

The glutamate-gated chloride channel (GluCl) is a highly sensitive insecticide target of the avermectin class of insecticides. As an alternative to using chemical insecticides to kill mosquitoes, we tested the effects of purified immunoglobulin G (IgG) targeting the extracellular domain of GluCl from Anopheles gambiae (AgGluCl) on the survivorship of three key mosquito disease vectors: Anopheles gambiae s.s., Aedes aegypti and Culex tarsalis. When administered through a single blood meal, anti-AgGluCl IgG reduced the survivorship of A. gambiae in a dose-dependent manner (LC50: 2.82 mg ml(-1), range 2.68-2.96 mg ml(-1)) but not A. aegypti or C. tarsalis. We previously demonstrated that AgGluCl is only located in tissues of the head and thorax of A. gambiae. To verify that AgGluCl IgG is affecting target antigens found outside the midgut, we injected it directly into the hemocoel via intrathoracic injection. A single, physiologically relevant concentration of anti-AgGluCl IgG injected into the hemocoel equally reduced mosquito survivorship of all three species. To test whether anti-AgGluCl IgG was entering the hemocoel of each of these mosquitoes, we fed mosquitoes a blood meal containing anti-AgGluCl IgG and subsequently extracted their hemolymph. We only detected IgG in the hemolymph of A. gambiae, suggesting that resistance of A. aegypti and C. tarsalis to anti-AgGluCl IgG found in blood meals is due to deficient IgG translocation across the midgut. We predicted that anti-AgGluCl IgG's mode of action is by antagonizing GluCl activity. To test this hypothesis, we fed A. gambiae blood meals containing anti-AgGluCl IgG and the GluCl agonist ivermectin (IVM). Anti-AgGluCl IgG attenuated the mosquitocidal effects of IVM, suggesting that anti-AgGluCl IgG antagonizes IVM-induced activation of GluCl. Lastly, we stained adult, female A. aegypti and C. tarsalis for GluCl expression. Neuronal GluCl expression in these mosquitoes was similar to previously reported A. gambiae GluCl expression; however, we also discovered GluCl staining on the basolateral surface of their midgut epithelial cells, suggesting important physiological differences in Culicine and Anopheline mosquitoes.

An antibody against an Anopheles albimanus midgut myosin reduces Plasmodium berghei oocyst development

Background

Malaria parasites are transmitted by Anopheles mosquitoes. Although several studies have identified mosquito midgut surface proteins that are putatively important for Plasmodium ookinete invasion, only a few have characterized these protein targets and demonstrated transmission-blocking activity. Molecular information about these proteins is essential for the development of transmission-blocking vaccines (TBV). The aim of the present study was to test three monoclonal antibodies (mAbs), A-140, A-78 and A-10, for their ability to recognize antigens and block oocyst infection of the midgut of Anopheles albimanus, a major malaria vector in Latin America.

Method

Western-blot of mAbs on antigens from midgut brush border membrane vesicles was used to select antibodies. Three mAbs were tested by membrane feeding assays to evaluate their potential transmission-blocking activity against Plasmodium berghei. The cognate antigens recognized by mAbs with oocyst-reducing activity were determined by immunoprecipitation followed by liquid chromatography tandem mass spectrometry.

Results

Only one mAb, A-140, significantly reduced oocyst infection intensity. Hence, its probable protein target in the Anopheles albimanus midgut was identified and characterized. It recognized three high-molecular mass proteins from a midgut brush border microvilli vesicle preparation. Chemical deglycosylation assays confirmed the peptide nature of the epitope recognized by mAb A-140. Immunoprecipitation followed by proteomic identification with tandem mass spectrometry revealed five proteins, presumably extracted together as a complex. Of these, AALB007909 had the highest mascot score and corresponds to a protein with a myosin head motor domain, indicating that the target of mAb A-140 is probably myosin located on the microvilli of the mosquito midgut.

Conclusion

These results provide support for the participation of myosin in mosquito midgut invasion by Plasmodium ookinetes. The potential inclusion of this protein in the design of new multivalent vaccine strategies for blocking Plasmodium transmission is discussed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1548-8) contains supplementary material, which is available to authorized users.

Development of malaria transmission-blocking vaccines: from concept to product

Despite decades of effort battling against malaria, the disease is still a major cause of morbidity and mortality. Transmission-blocking vaccines (TBVs) that target sexual stage parasite development could be an integral part of measures for malaria elimination. In the 1950s, Huff et al. first demonstrated the induction of transmission-blocking immunity in chickens by repeated immunizations with Plasmodium gallinaceum-infected red blood cells. Since then, significant progress has been made in identification of parasite antigens responsible for transmission-blocking activity. Recombinant technologies accelerated evaluation of these antigens as vaccine candidates, and it is possible to induce effective transmission-blocking immunity in humans both by natural infection and now by immunization with recombinant vaccines. This chapter reviews the efforts to produce TBVs, summarizes the current status and advances and discusses the remaining challenges and approaches.

Differential roles of an Anopheline midgut GPI-anchored protein in mediating Plasmodium falciparum and Plasmodium vivax ookinete invasion

Novel strategies to directly thwart malaria transmission are needed to maintain the gains achieved by current control measures. Transmission-blocking interventions (TBIs), namely vaccines and drugs targeting parasite or mosquito molecules required for vector-stage parasite development, have been recognized as promising approaches for preventing malaria transmission. However, the number of TBI targets is limited and their degree of conservation among the major vector-parasite systems causing human disease is unclear. Therefore, discovery and characterization of novel proteins involved in vector-stage parasite development of Plasmodium falciparum and Plasmodium vivax is paramount. We mined the recent Anopheles gambiae midgut lipid raft proteome for putative mosquito-derived TBI targets and characterized a secreted glycoconjugate of unknown function, AgSGU. We analyzed molecular variation in this protein among a range of anopheline mosquitoes, determined its transcriptomic and proteomic profiles, and conducted both standard and direct membrane feeding assays with P. falciparum (lab/field) and P. vivax (field) in An. gambiae and Anopheles dirus. We observed that α-AgSGU antibodies significantly reduced midgut infection intensity for both lab and field isolates of P. falciparum in An. gambiae and An. dirus. However, no transmission-reducing effects were noted when comparable concentrations of antibodies were included in P. vivax-infected blood meals. Although antibodies against AgSGU exhibit transmission-reducing activity, the high antibody titer required for achieving 80% reduction in oocyst intensity precludes its consideration as a malaria mosquito-based TBI candidate. However, our results suggest that P. falciparum and P. vivax ookinetes use a different repertoire of midgut surface glycoproteins for invasion and that α-AgSGU antibodies, as well as antibodies to other mosquito-midgut microvillar surface proteins, may prove useful as tools for interrogating Plasmodium-mosquito interactions.

Comparative susceptibility of different biological forms of Anopheles stephensi to Plasmodium berghei ANKA strain

Background

There are varying degrees of compatibility between malaria parasite-mosquito species, and understanding this compatibility may be crucial for developing effective transmission-blocking vaccines. This study investigates the compatibility of different biological forms of a malaria vector, Anopheles stephensi, to Plasmodium berghei ANKA strain.

Methods

Several biologically different and allopatric forms of A. stephensi were studied. Three forms were isolated from different regions of southern Iran: the variety mysorensis, the intermediate form and the native type form, and an additional type form originated from India (Beech strain).The mosquitoes were experimentally infected with P. berghei to compare their susceptibility to parasitism. Anti-mosquito midgut antiserum was then raised in BALB/cs mice immunized against gut antigens from the most susceptible form of A. stephensi (Beech strain), and the efficacy of the antiserum was assessed in transmission-blocking assays conducted on the least susceptible mosquito biological form.

Results

The susceptibility of different biological forms of A. stephensi mosquito to P. berghei was specifically inter-type varied. The Beech strain and the intermediate form were both highly susceptible to infection, with higher oocyst and sporozoite infection rates than intermediate and mysorensis forms. The oocyst infection, and particularly sporozite infection, was lowest in the mysorensis strain. Antiserum raised against midgut proteins of the Indian Beech type form blocked infection in this mosquito population, but it was ineffective at blocking both oocyst and sporozoite development in the permissive but geographically distant intermediate form mosquitoes. This suggests that a strong degree of incompatibility exists between the mosquito strains in terms of midgut protein(s) acting as putative ookinete receptors.

Conclusions

The incompatibility in the midgut protein profiles between two biological forms of A. stephensi demonstrates a well-differentiated population structure according to geographical origin. Therefore, the design of potential transmission-blocking strategies should incorporate a more thorough understanding of intra-species variations in host-parasite interactions.

Control of multiple arthropod vector infestations with subolesin/akirin vaccines

Diseases transmitted by arthropod vectors such as mosquitoes, ticks and sand flies greatly impact human and animal health and thus their control is important for the eradication of vector-borne diseases (VBD). Vaccination is an environmentally friendly alternative for vector control that allows control of several VBD by targeting their common vector. Recent results have suggested that subolesin/akirin (SUB/AKR) is good candidate antigens for the control of arthropod vector infestations. Here we describe the comparative effect of vaccination with SUB, AKR and Q38 and Q41 chimeras containing SUB/AKR conserved protective epitopes on tick, mosquitoes and sand flies vector mortality, molting, oviposition and/or fertility. We demonstrated that SUB vaccination had the highest efficacy (E) across all vector species (54-92%), Q41 vaccination had the highest vaccine E in mosquitoes (99%) by reducing female survival and fertility, and Q38 vaccination had the highest effect on reducing mosquito (28%) and sand fly (26%) oviposition. The effect of vaccination on different developmental processes in several important arthropod vectors encourages the development of SUB/AKR universal vaccines for the control of multiple vector infestations and reduction of VBD.

Functional genomics of the horn fly, Haematobia irritans (Linnaeus, 1758)

Background

The horn fly, Haematobia irritans (Linnaeus, 1758) (Diptera: Muscidae) is one of the most important ectoparasites of pastured cattle. Horn flies infestations reduce cattle weight gain and milk production. Additionally, horn flies are mechanical vectors of different pathogens that cause disease in cattle. The aim of this study was to conduct a functional genomics study in female horn flies using Expressed Sequence Tags (EST) analysis and RNA interference (RNAi).

Results

A cDNA library was made from whole abdominal tissues collected from partially fed adult female horn flies. High quality horn fly ESTs (2,160) were sequenced and assembled into 992 unigenes (178 contigs and 814 singlets) representing molecular functions such as serine proteases, cell metabolism, mitochondrial function, transcription and translation, transport, chromatin structure, vitellogenesis, cytoskeleton, DNA replication, cell response to stress and infection, cell proliferation and cell-cell interactions, intracellular trafficking and secretion, and development. Functional analyses were conducted using RNAi for the first time in horn flies. Gene knockdown by RNAi resulted in higher horn fly mortality (protease inhibitor functional group), reduced oviposition (vitellogenin, ferritin and vATPase groups) or both (immune response and 5'-NUC groups) when compared to controls. Silencing of ubiquitination ESTs did not affect horn fly mortality and ovisposition while gene knockdown in the ferritin and vATPse functional groups reduced mortality when compared to controls.

Conclusions

These results advanced the molecular characterization of this important ectoparasite and suggested candidate protective antigens for the development of vaccines for the control of horn fly infestations.

Interventions against West Nile virus, Rift Valley fever virus, and Crimean-Congo hemorrhagic fever virus: where are we?

ARBO-ZOONET is an international network financed by the European Commission's seventh framework program. The major goal of this initiative is capacity building for the control of emerging viral vector-borne zoonotic diseases, with a clear focus on West Nile virus, Rift Valley fever virus, and Crimean-Congo hemorrhagic fever virus. To evaluate the status quo of control measures against these viruses, an ARBO-ZOONET meeting was held in Istanbul, Turkey, from 19 to 20 November 2009. The symposium consisted of three themes: (1) vaccines: new and existing ones; (2) antivirals: existing and new developments; and (3) antivector vaccines. In addition, a satellite workshop was held on epidemiology and diagnosis. The meeting brought together foremost international experts on the subjects from both within and without the ARBO-ZOONET consortium. This report highlights selected results from these presentations and major conclusions that emanated from the discussions held.

Conservation and immunogenicity of the mosquito ortholog of the tick-protective antigen, subolesin

The control of arthropod vectors of pathogens that affect human and animal health is important for the eradication of vector-borne diseases. The ortholog of the tick-protective antigen, subolesin, was identified in Aedes albopictus and found to have conserved epitopes in ticks and mosquitoes. RNA interference with the tick and mosquito double-stranded RNA in three tick species resulted in significant gene knockdown and decreased tick weight and/or survival. Feeding Anopheles atroparvus, Aedes caspius, and Culex pipiens female mosquitoes on an A. albopictus subolesin hyperimmune serum resulted in 11 +/- 5% to 29 +/- 6% survival inhibition when compared to controls fed on preimmune serum. Feeding sand flies, Phlebotomus perniciosus, on antimosquito subolesin ortholog protein antibodies inhibited female survival and the number of larvae and adults obtained after hatching by 28 +/- 22% and 16 +/- 3%, respectively, when compared to controls. Vaccination with tick and mosquito subolesin ortholog proteins significantly reduced Ixodes scapularis tick infestation and weight in a similar way. However, vaccination with the recombinant mosquito subolesin ortholog antigen did not protect against Amblyomma americanum and Rhipicephalus sanguineus tick infestations. Collectively, these preliminary results provided the first evidence that development of vaccines may be possible for control of multiple arthropod vectors using subolesin orthologs but suggested that multiple antigens may be required to produce an effective vaccine.

Mosquitocidal vaccines: a neglected addition to malaria and dengue control strategies

The transmission of vector-borne diseases is dependent upon the ability of the vector to survive for longer than the period of development of the pathogen within the vector. One means of reducing mosquito lifespan, and thereby reducing their capacity to transmit diseases, is to target mosquitoes with vaccines. Here, the principle behind mosquitocidal vaccines is described, their potential impact in malaria and dengue control is modeled and the current research that could make these vaccines a reality is reviewed. Mosquito genome data, combined with modern molecular techniques, can be exploited to overcome the limited advances in this field. Given the large potential benefit to vector-borne disease control, research into the development of mosquitocidal vaccines deserves a high profile.

Antiserum against perimicrovillar membranes and midgut tissue reduces the development of Trypanosoma cruzi in the insect vector, Rhodnius prolixus

Antiserum raised against Rhodnius prolixus perimicrovillar membranes (PMM) and midgut tissue interfered with the midgut structural organization and reduced the development of Trypanosoma cruzi in the R. prolixus insect vector. SDS-PAGE and Western blot analyses confirmed the specific recognition of midgut proteins by the antibody. Feeding, mortality, molt, and oviposition of the insects were unaffected by feeding with the antiserum. However, the eclosion of the eggs were reduced from R. prolixus females treated with antiserum. Additionally, in vivo evaluation showed that after oral treatment with the antiserum, the intensity of infection with the Dm-28c clone of T. cruzi decreased in the digestive tract of fifth-instar nymphs and in the excretions of R. prolixus adults. These results suggest that the changes observed in the PMM organization in the posterior midgut of R. prolixus may not be important for triatomine survival but the antiserum acts as a transmission-reduction vaccine able to induce significant decreases in T. cruzi infection in the vector.

Immune interactions between mosquitoes and their hosts

The intimate contact between mosquitoes and the immune system of their hosts is generally not considered important because of the transient nature of mosquito feeding. However, when hosts are exposed to many feeding mosquitoes, they develop immune responses against a range of salivary antigens. Understanding the importance of these responses will provide new tools for monitoring vector populations and identifying individuals at risk of mosquito-borne diseases, and allow the development of novel methods for monitoring control and mosquito-release programmes. Antibodies targeting the mosquito midgut are also important in the development of mosquito vaccines. The feasibility of this approach has been demonstrated and future research opportunities are considered in this review. The potential impact of mosquito vaccines is also discussed. Our understanding of the interplay between mosquitoes and the immune system of their hosts is still in its infancy, but it is clear that there is great potential for exploiting this interplay in the control of mosquito-borne diseases.

Enrichment of a single clone from a high diversity library of phage-displayed antibodies by panning with Anopheles gambiae (Diptera: Culicidae) midgut homogenate

A high diversity library of recombinant human antibodies was selected on complex antigen mixtures from midguts of female Anopheles gambiae Giles. The library of phage-displayed single chain variable region fragment constructs, derived from beta-lymphocyte mRNA of naïve human donors, was repeatedly selected and reamplified on the insoluble fraction of midgut homogenates. Five rounds of panning yielded only one midgut-specific clone, which predominated the resulting antibody panel. In A. gambiae, the epitope was found throughout the tissues of females but was absent from the midgut of males. The cognate antigen proved to be detergent soluble but very sensitive to denaturation and could not be isolated or identified by Western blot of native electrophoresis gels or by immunoprecipitation. Nevertheless, immunohistology revealed that this sex-specific epitope is associated with the lumenal side of the midgut. Severe bottlenecking may limit the utility of phage display selection from naïve libraries for generating diverse panels of antibodies against complex mixtures of antigens from insect tissues. These results suggest that the selection of sufficiently diverse antibody panels, from which mosquitocidal or malaria transmission-blocking antibodies can be isolated, may require improved selection methods or specifically enriched pre-immunized libraries.