Evolutionary dynamics of immune-related genes and pathways in disease-vector mosquitoes

Mosquitoes are vectors of parasitic and viral diseases of immense importance for public health. The acquisition of the genome sequence of the yellow fever and Dengue vector, Aedes aegypti (Aa), has enabled a comparative phylogenomic analysis of the insect immune repertoire: in Aa, the malaria vector Anopheles gambiae (Ag), and the fruit fly Drosophila melanogaster (Dm). Analysis of immune signaling pathways and response modules reveals both conservative and rapidly evolving features associated with different functional gene categories and particular aspects of immune reactions. These dynamics reflect in part continuous readjustment between accommodation and rejection of pathogens and suggest how innate immunity may have evolved.

[Mosquito complex (Diptera, Culicidae) in a West Nile fever focus in the Volgograd Region. II. Host-feeding patterns of mosquitoes in different habitats]

Host preference of the mosquitoes collected in the urban and rural habitats of Volgograd and its suburbs was studied by the precipitation reaction test. Human and avian blood was detected in Cx. pipiens, Cx. modestus, Ae. vexans, Ae. behningi, Ae. caspius, Ae. sticticus, and females of the Anopheles maculipennis. The proportion of the mosquitoes fed on birds was similar in the urban and rural biotopes whereas that of the mosquitoes feeding on humans was significantly higher in Volgograd than in its environs. The increase in the number of human blood-fed mosquitoes in the city resulted mainly from the females collected in its multi-storied buildings.

Response of Armigeres subalbatus (Diptera: Culicidae) to intraperitoneally isolated Brugia spp. microfilariae

The relationship between mosquito and parasite involves a delicate balance that is influenced not only by the mosquito but also by parasite determinants. Using the biologically and morphologically similar parasites Brugia malayi and Brugia pahangi and the mosquito Armigeres subalbatus (Coquillett) (Diptera: Culicidae), it should be possible to dissect out the key elements involved in initiating or avoiding an immune response, known as melanotic encapsulation, because in this mosquito B. malayi microfilariae (mf) are melanized and destroyed, but B. pahangi mf develop normally into infective-stage larvae. Because of limitations in isolating sufficient mf from the circulation of an infected mammalian host, Brugia spp. mf that can be obtained in large numbers from the peritoneal cavity of an infected host were tested to ascertain the immune response of Ar. subalbatus to this source of mf. Results indicate that the immune response of Ar. subalbatus against intraperitoneal (i.p.) Brugia spp. mf mimics that which is observed when this mosquito is exposed to mf-infected animals, indicating that i.p. mf are similar to those mf that circulate naturally in the blood of the vertebrate host. Therefore, the i.p. mf should serve as an excellent source of material for genomic and proteomic studies designed to analyze the role of the parasite in influencing the immune response of the mosquito.

Human/vector relationships during human African trypanosomiasis: initial screening of immunogenic salivary proteins of Glossina species

The morbidity and mortality of vector-borne diseases is closely linked to exposure of the human host to vectors. Qualitative and quantitative evaluation of individual exposure to arthropod bites by investigation of the specific immune response to vector saliva would make it possible to monitor individuals at risk of vectorial transmission of pathogens. The objective of this study was to evaluate and compare the antibody (IgG) response to saliva from uninfected Glossina species, vectors, or non-vectors of Trypanosoma brucei gambiense by detecting immunogenic proteins in humans residing in an area endemic for human African trypanosomiasis in the Democratic Republic of Congo. Our results suggest that the immunogenic profiles observed seemed specific to the Glossina species (vector or non-vector species) and to the infectious status of exposed individuals (infected or not infected). This preliminary work tends to support the feasibility of development of an epidemiologic tool based on this antibody response to salivary proteins.

Fz2 and cdc42 mediate melanization and actin polymerization but are dispensable for Plasmodium killing in the mosquito midgut

The midgut epithelium of the mosquito malaria vector Anopheles is a hostile environment for Plasmodium, with most parasites succumbing to host defenses. This study addresses morphological and ultrastructural features associated with Plasmodium berghei ookinete invasion in Anopheles gambiae midguts to define the sites and possible mechanisms of parasite killing. We show by transmission electron microscopy and immunofluorescence that the majority of ookinetes are killed in the extracellular space. Dead or dying ookinetes are surrounded by a polymerized actin zone formed within the basal cytoplasm of adjacent host epithelial cells. In refractory strain mosquitoes, we found that formation of this zone is strongly linked to prophenoloxidase activation leading to melanization. Furthermore, we identify two factors controlling both phenomena: the transmembrane receptor frizzled-2 and the guanosine triphosphate–binding protein cell division cycle 42. However, the disruption of actin polymerization and melanization by double-stranded RNA inhibition did not affect ookinete survival. Our results separate the mechanisms of parasite killing from subsequent reactions manifested by actin polymerization and prophenoloxidase activation in the A. gambiae–P. berghei model. These latter processes are reminiscent of wound healing in other organisms, and we propose that they represent a form of wound-healing response directed towards a moribund ookinete, which is perceived as damaged tissue.

A dangerous journey awaits malaria Plasmodium parasites ingested by a mosquito. Most parasites are destroyed by host responses in the midgut, and in parasite-resistant refractory strains of mosquito the mortality can reach 100%. This midgut “bottleneck” represents an appealing target for reducing malaria transmission by the genetic control of wild mosquitoes. However, the killing mechanisms are still unclear. In this study, electron microscopical analyses followed the entire midgut invasion process in mosquitoes to identify the major site(s) and ultrastructural features of Plasmodium killing. The authors found that invasion can be divided into two steps: a swift passage through a midgut cell, followed by establishment of the parasite in the basal extracellular space, where it becomes an important target for destruction by soluble immunity factors. In refractory mosquitoes, dead parasites are associated with the formation of organelle-free zones of actin in adjacent midgut cells, and melanin deposition on the parasite surface. The authors identify two genes, called frizzled-2 and cell division cycle 42, that control these phenomena. Actin zone formation and melanization are generally thought to be killing mechanisms; however, the authors show by gene silencing that neither is lethal to Plasmodium. Instead, these mechanisms may represent a form of mosquito wound-healing response that is triggered by the presence of a moribund parasite.

Comparative susceptibility of introduced forest-dwelling mosquitoes in Hawai'i to avian malaria, Plasmodium relictum

To identify potential vectors of avian malaria in Hawaiian native forests, the innate susceptibility of Aedes albopictus, Wyeomyia mitchellii, and Culex quinquefasciatus from 3 geographical sites along an altitudinal gradient was evaluated using local isolates of Plasmodium relictum. Mosquitoes were dissected 5-8 and 9-13 days postinfective blood meal and microscopically examined for oocysts and salivary-gland sporozoites. Sporogony was completed in all 3 species, but prevalence between species varied significantly. Oocysts were detected in 1-2% and sporozoites in 1-7% of Aedes albopictus that fed on infected ducklings. Wyeomyia mitchellii was slightly more susceptible, with 7-19% and 7% infected with oocysts and sporozoites, respectively. In both species, the median oocyst number was 5 or below. This is only the second Wyeomyia species reported to support development of a malarial parasite. Conversely, Culex quinquefasciatus from all 3 sites proved very susceptible. Prevalence of oocysts and sporozoites consistently exceeded 70%, regardless of gametocytemia or origin of the P. relictum isolate. In trials for which a maximum 200 oocysts were recorded, the median number of oocysts ranged from 144 to 200. It was concluded that Culex quinquefasciatus is the primary vector of avian malaria in Hawai'i.

Increased melanizing activity in Anopheles gambiae does not affect development of Plasmodium falciparum

Serpins are central to the modulation of various innate immune responses in insects and are suspected to influence the outcome of malaria parasite infection in mosquito vectors. Three Anopheles gambiae serpins (SRPN1, -2, and -3) were tested for their ability to inhibit the prophenoloxidase cascade, a key regulatory process in the melanization response. Recombinant SRPN1 and -2 can bind and inhibit a heterologous phenoloxidase-activating protease and inhibit phenoloxidase activation in vitro. Using a reverse genetics approach, we studied the effect of SRPN2 on melanization in An. gambiae adult females in vivo. Depletion of SRPN2 from the mosquito hemolymph increases melanin deposition on foreign surfaces such as negatively charged Sephadex beads. As reported, the knockdown of SRPN2 adversely affects the ability of the rodent malaria parasite Plasmodium berghei to invade the midgut epithelium and develop into oocysts. Importantly, we tested whether the absence of SRPN2 from the hemolymph influences Plasmodium falciparum development. RNAi silencing of SRPN2 in an An. gambiae strain originally established from local populations in Yaoundé, Cameroon, did not influence the development of autochthonous field isolates of P. falciparum. This study suggests immune evasion strategies of the human malaria parasite and emphasizes the need to study mosquito innate immune responses toward the pathogens they transmit in natural vector-parasite combinations.

Apoptosis-like death as a feature of malaria infection in mosquitoes

Malaria parasites of the genusPlasmodiummake a hazardous journey through their mosquito vectors. The majority die in the process, many as a result of the action of mosquito defence mechanisms. The mosquito too is not unscathed by the encounter with these parasites. Tissue damage occurs as a result of mid-gut invasion and reproductive fitness is lost when many developing ovarian follicles are resorbed. Here we discuss some of the mechanisms that are involved in killing the parasite and in the self-defence mechanisms employed by the mosquito to repair the mid-gut epithelium and to manipulate resources altering the trade-off position that balances reproduction and survival. In all cases, cells die by apoptotic-like mechanisms. In the midgut cells, apoptosis-induction pathways are being elucidated, the molecules involved in apoptosis are being recognised andDrosophilahomologues sought. The death of ookinetes in the mosquito mid-gut lumen is associated with caspase-like activity and, although homologues of mammalian caspases are not present in the malaria genome, other cysteine proteases that are potential candidates have been discussed. In the ovary, apoptosis of patches of follicular epithelial cells is followed by resorption of the developing follicle and a subsequent loss of egg production in that follicle.

[Innate immune defense in anopheline mosquitoes against plasmodium infection]

On the basis of the research on interaction between plasmodium and mosquito vector, the mechanism of innate immune defense responses in anopheline mosquitoes against plasmodium infection has been studied. The innate immune defense may be applied to confine and kill malaria parasites under migration and development, to an effective control strategy on malaria vectors.

Tsetse fly saliva accelerates the onset of Trypanosoma brucei infection in a mouse model associated with a reduced host inflammatory response

Tsetse flies (Glossina sp.) are the vectors that transmit African trypanosomes, protozoan parasites that cause human sleeping sickness and veterinary infections in the African continent. These blood-feeding dipteran insects deposit saliva at the feeding site that enables the blood-feeding process. Here we demonstrate that tsetse fly saliva also accelerates the onset of a Trypanosoma brucei infection. This effect was associated with a reduced inflammatory reaction at the site of infection initiation (reflected by a decrease of interleukin-6 [IL-6] and IL-12 mRNA) as well as lower serum concentrations of the trypanocidal cytokine tumor necrosis factor. Variant-specific surface glycoprotein-specific antibody isotypes immunoglobulin M (IgM) and IgG2a, implicated in trypanosome clearance, were not suppressed. We propose that tsetse fly saliva accelerates the onset of trypanosome infection by inhibiting local and systemic inflammatory responses involved in parasite control.

Sand fly saliva: effects on host immune response and Leishmania transmission

The feeding success of sand flies (Diptera: Phlebotominae) is linked to the vast array of pharmacological substances in their saliva, which interferes with the host haemostasis and immune response. Modification of feeding site plays also an important role in Leishmania transmission. In naive hosts, co-inoculation of saliva and Leishmania parasites increases the chance of successful transmission. Disease exacerbation seems to be associated with enhanced production of type 2 cytokines and selective inhibition of some macrophage functions including the production of NO and H202. On the other hand, hosts repeatedly exposed to sand fly bites develop anti-saliva immune response that results in a protection against Leishmania infection. This led to a new interesting approach to anti-Leishmania vaccine--using salivary components to block parasite transmission. The review is therefore focused on the interactions that run between immunomodulatory molecules in sand fly saliva and host immune response, with the impact on Leishmania infection development. Recent studies revealed that saliva-based vaccine for leishmaniasis might be effective and feasible, however, several questions still require to be solved. The knowledge based on experimental mouse model cannot be fully extrapolated to dogs or humans and due to differences in salivary antigens between sand fly species the protective effect is species-specific. On the other hand, the specificity of salivary antigens enables the use of anti-saliva antibodies for monitoring the exposure of hosts to sand fly bites and might be used as a marker of risks for Leishmania transmission in endemic areas.

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.

Tsetse fly saliva biases the immune response to Th2 and induces anti-vector antibodies that are a useful tool for exposure assessment

Tsetse flies (Glossina sp.) are blood-feeding dipteran insects that transmit African trypanosomes, parasites that are responsible for human sleeping sickness and veterinary infections. Increasing attention is being paid to the effects of tsetse fly saliva deposited at the feeding site, which enables the blood-feeding process and putatively promotes parasite transmission. Here we demonstrate that saliva induces strong humoral responses against the major 43-45 kDa protein fraction (tsetse salivary gland proteins 1 and 2 - Tsal1 and Tsal2) in mice and humans and suppresses murine T and B cell responses to heterologous antigen. The saliva-induced immune response is associated with a Th2-biased cytokine profile and the production of mainly IgG1 and IgE antibody isotypes. Functionally, the antibodies raised in mice exposed to tsetse fly bites or induced after experimental saliva immunisation do not affect the fly's blood-feeding efficiency nor its survival. We propose that anti-saliva as well as anti-Tsal1/2 antibody responses can be used in epidemiological studies as a tool to analyze human exposure to tsetse flies.

Antimicrobial peptides in the interactions between insects and flagellate parasites

Innate immunity has a key role in the control of microbial infections in both vertebrates and invertebrates. In insects, including vectors that transmit parasites that cause major human and animal diseases, antimicrobial peptides (AMPs) are important components of innate immunity. AMPs are induced upon parasitic infections and can participate in regulating parasite development in the digestive tract and in the hemolymph. This review presents our current knowledge of a field that is in its infancy: the role of innate immunity in different models of insects infected with flagellate parasites, and in particular the potential role of AMPs in regulating these parasitic infections.

An antivector vaccine protects against a lethal vector-borne pathogen

Vaccines that target blood-feeding disease vectors, such as mosquitoes and ticks, have the potential to protect against the many diseases caused by vector-borne pathogens. We tested the ability of an anti-tick vaccine derived from a tick cement protein (64TRP) of Rhipicephalus appendiculatus to protect mice against tick-borne encephalitis virus (TBEV) transmitted by infected Ixodes ricinus ticks. The vaccine has a “dual action” in immunized animals: when infested with ticks, the inflammatory and immune responses first disrupt the skin feeding site, resulting in impaired blood feeding, and then specific anti-64TRP antibodies cross-react with midgut antigenic epitopes, causing rupture of the tick midgut and death of engorged ticks. Three parameters were measured: “transmission,” number of uninfected nymphal ticks that became infected when cofeeding with an infected adult female tick; “support,” number of mice supporting virus transmission from the infected tick to cofeeding uninfected nymphs; and “survival,” number of mice that survived infection by tick bite and subsequent challenge by intraperitoneal inoculation of a lethal dose of TBEV. We show that one dose of the 64TRP vaccine protects mice against lethal challenge by infected ticks; control animals developed a fatal viral encephalitis. The protective effect of the 64TRP vaccine was comparable to that of a single dose of a commercial TBEV vaccine, while the transmission-blocking effect of 64TRP was better than that of the antiviral vaccine in reducing the number of animals supporting virus transmission. By contrast, the commercial antitick vaccine (TickGARD) that targets only the tick's midgut showed transmission-blocking activity but was not protective. The 64TRP vaccine demonstrates the potential to control vector-borne disease by interfering with pathogen transmission, apparently by mediating a local cutaneous inflammatory immune response at the tick-feeding site.

Blood-sucking vectors such as mosquitoes and ticks transmit hundreds of micro-organisms that cause diseases like malaria and Lyme disease. Controlling so many diseases is an enormous challenge. A new idea is to make vaccines against the vectors rather than against all the individual disease agents they carry. The authors examined this hypothesis using a vaccine prepared from tick cement. This cement is secreted by ticks to help them attach to a human or animal to feed. A mouse model was used in which mice were infested with ticks infected with tick-borne encephalitis virus (TBEV), the most important vector-borne virus in Europe and northern Asia. The control mice developed fatal encephalitis and died about a week after being bitten by the infected tick. By contrast, the tick cement vaccine gave protection similar to the level seen in mice immunized with a single shot of the commercial TBEV vaccine for humans. However, a commercial tick vaccine used to control cattle ticks did not protect the mice. The authors' tick cement vaccine appeared to work by causing a cellular immune response in the skin where ticks were feeding. These results show that it is feasible to produce a vaccine against a tick that protects against the disease agent it transmits.

Mosquito midguts and malaria: cell biology, compartmentalization and immunology

The malaria parasite Plasmodium has an absolute requirement for both a vertebrate and a mosquito host in order to complete its life cycle, and its interactions with the latter provide the focus for this review. The mosquito midgut represents one of the most challenging environments for the survival and development of Plasmodium, and is thus also one of the most attractive sites for novel targeted malaria control strategies. During their attempts to cross the midgut epithelium en route to the salivary glands, motile ookinetes are swiftly detected and labelled by mosquito recognition factors and targeted for destruction by a variety of immune responses that recruit killing factors both from the midgut and from other tissues in the surrounding body cavity. The exact interplay between these factors and the parasite is highly species- and strain-specific, as are the timing and the route of parasite invasion. These features are paramount to determining the success of the infection and the vector competence of the mosquito. Here we discuss recent advances in genomic analyses, coupled with detailed microscopical investigations, which are helping to unravel the identity and roles of the major players of these complex systems.

Ecology, biology and susceptibility of Phlebotomus papatasi to Leishmania experimental infection in Suez Governorate

Zoonotic cutaneous leishmaniasis (ZCL) is endemic in Sinai Peninsula. The sand fly and reservoirs were investigated in Suez G., since new settlements and land reclamation programs are ongoing. The results showed that Phlebotomus papatasi reached its highest density in September. The successfully colonized P. papatasi facilitated its biology and competence study. An autogenous trait was proven within P. papatasi population indicating its ability to survive and breed during adverse conditions. The vector competence was carried out under laboratory condition through feeding on lesion of a L. major experimentally infected hamster and by membrane feeding technique. Both hamsters and BALB-c mice inoculated with L. major developed ZCL lesions.

Comparative salivary gland transcriptomics of sandfly vectors of visceral leishmaniasis

Background

Immune responses to sandfly saliva have been shown to protect animals against Leishmania infection. Yet very little is known about the molecular characteristics of salivary proteins from different sandflies, particularly from vectors transmitting visceral leishmaniasis, the fatal form of the disease. Further knowledge of the repertoire of these salivary proteins will give us insights into the molecular evolution of these proteins and will help us select relevant antigens for the development of a vector based anti-Leishmania vaccine.

Results

Two salivary gland cDNA libraries from female sandflies Phlebotomus argentipes and P. perniciosus were constructed, sequenced and proteomic analysis of the salivary proteins was performed. The majority of the sequenced transcripts from the two cDNA libraries coded for secreted proteins. In this analysis we identified transcripts coding for protein families not previously described in sandflies. A comparative sandfly salivary transcriptome analysis was performed by using these two cDNA libraries and two other sandfly salivary gland cDNA libraries from P. ariasi and Lutzomyia longipalpis, also vectors of visceral leishmaniasis. Full-length secreted proteins from each sandfly library were compared using a stand-alone version of BLAST, creating formatted protein databases of each sandfly library. Related groups of proteins from each sandfly species were combined into defined families of proteins. With this comparison, we identified families of salivary proteins common among all of the sandflies studied, proteins to be genus specific and proteins that appear to be species specific. The common proteins included apyrase, yellow-related protein, antigen-5, PpSP15 and PpSP32-related protein, a 33-kDa protein, D7-related protein, a 39- and a 16.1- kDa protein and an endonuclease-like protein. Some of these families contained multiple members, including PPSP15-like, yellow proteins and D7-related proteins suggesting gene expansion in these proteins.

Conclusion

This comprehensive analysis allows us the identification of genus- specific proteins, species-specific proteins and, more importantly, proteins common among these different sandflies. These results give us insights into the repertoire of salivary proteins that are potential candidates for a vector-based vaccine.

Long-term study of Japanese encephalitis virus infection in Anopheles subpictus in Cuddalore district, Tamil Nadu, South India

OBJECTIVE

To investigate the role of Anopheles subpictus Grassi as a vector of Japanese encephalitis virus (JEV) transmission in Cuddalore, an area of Tamil Nadu endemic for the disease.

METHOD

We collected 98 pools (4,900 specimens) of wild adult male An. subpictus mosquitoes outdoors during dusk hours and screened them for JEV antigen by antigen-capture Enzyme Linked Immunosorbent Assay. Additionally, over a period of 1 year, we tested 166 pools (8,300 specimens) of wild adult female An. subpictus mosquitoes collected indoors for JEV.

RESULTS

Four pools of male An. subpictus tested positive. This indicates possible natural transovarial transmission of the virus through An. subpictus. Nineteen female pools were positive with a minimum infection rate of 2.3. From January through March the maximum infection rate was highest: 5.0 compared with 1.7 between April and September and 2.1 from October to December, although the difference was not statistically significant. From the 19 positive female pools, four isolates were confirmed as JEV by insect bioassay.

CONCLUSION

The role of An. subpictus as a secondary vector in JEV transmission in Cuddalore, Tamil Nadu lends support to the hypothesis of periodic epidemics in the region.

Considerations on the effect of anti-sandfly antibodies on biological parameters of Lutzomyia longipalpis (Lutz & Neiva, 1912) (Diptera: Psychodidae: Phlebotominae)

The immunization of vertebrate hosts with vector components may be an alternative for the control of diseases transmitted by insects. In the present study we evaluated the effects of anti-sandfly antibodies on some of the biological parameters of female Lutzomyia longipalpis, a vector of visceral leishmaniasis. Rabbits were immunized with extracts of gut from blood-fed (GB) or sugar-fed (GS) females, carcass of sugar-fed (CS) or blood-fed (CB) females, and with repeated sandfly bites (BITE). Immune sera showed increased antibody titers compared to pre-immunized animals, and specific bands were detected by Western Blot. An analysis of biological parameters revealed a decline in fecundity in the group of females fed on rabbits immunized with GB and BITE. Longevity and mortality were studied in females with oviposition (parous) and without oviposition (nulliparous). Nulliparous females that fed on rabbits immunized with bites died in the highest percentage. A mortality analysis after egg laying revealed a peak on the fifth day in all the groups, but females fed on rabbit subjected to repeated bites showed a shift towards the third day.

Immunomodulatory properties of sand fly saliva and its role on vertebrate host

The salivary glands of Phlebotomus papatasi and P. langeroni were investigated for their immunomodulatory properties on vertebrate hosts. Laboratory reared sand fly, were used for feeding on hamsters and for extraction of salivary gland. To determine the influence of the sand fly saliva homogenate (SGH) were performed ELISA using sera from injected hamsters by SGH. Frequent feeding of P. papatasi on hamsters were carried out to relate the anti-body titre with the biting rate. The mosquito Culex pipiens was allowed to feed on pre-exposed hamsters to P. papatasi bites. Antibodies correlated with the saliva of both species showed low titre and not related either to the time (after feeding and injection) or to the number of fed flies. Saliva of P. langeroni (non vector) recorded higher anti-body titre than P. papatasi. The preliminary experiment of the mosquitoes' feeding (C. pipiens) on pre-exposed hamsters on its saliva showed cross reactions between biting of mosquitoes and sand flies. More investigations are needed to study the effects of sand fly saliva on human immune responses to evaluate the fly salivary proteins as vaccines for leishmaniasis and to prevent or decrease the sand flies biting or probably mosquitoes.

Malaria parasites in mosquitoes: laboratory models, evolutionary temptation and the real world

A recent study describing the effect of Plasmodium berghei infection on some Anopheles gambiae immune genes demonstrates that P. berghei is responsible for the upregulation of several genes involved in the immune response that affect parasitic development differently during the ookinete-to-oocyst developmental transition. It is important to question the relevance of such results, which are based on a laboratory model system, when discussing host-parasite interactions and, especially, the development of novel control strategies for malaria.

Galectins as immunoregulators during infectious processes: from microbial invasion to the resolution of the disease

Recent evidence has implicated galectins, a family of evolutionarily conserved carbohydrate-binding proteins, as regulators of immune cell homeostasis and host-pathogen interactions. Galectins operate at different levels of innate and adaptive immune responses, by modulating cell survival and cell activation or by influencing the Th1/Th2 cytokine balance. Furthermore, galectins may contribute to host-pathogen recognition and may serve as receptors for specific interactions of pathogens with their insect vectors. Here we will explore the influence of galectins in immunological processes relevant to microbial infection and will summarize exciting recent work related to the specific interactions between galectins and their glycoconjugate ligands as critical determinants of pathogen recognition. Understanding the role of galectin-sugar interactions during the course of microbial infections might contribute to defining novel targets for disease prevention and immune intervention.

Comparison of immunoreactivity to serotonin, FMRFamide and SCPb in the gut and visceral nervous system of larvae, pupae and adults of the yellow fever mosquito Aedes aegypti

In all life stages, the gut of the mosquito is innervated by a small number (typically 4) of central neurons immunoreactive to serotonin (SI). The serotonergic system appears to pass through metamorphosis largely intact, despite extensive remodeling of the gut. Axons immunoreactive to antibodies raised against molluscan FMRFamide (RF-I) constitute peptidergic innervation that anatomically parallels the serotonergic system. In the larva, two clusters of 3 neurons project to the anterior regions of the gut, whereas in the pupa and adult, typically two large RF-I neurons located next to the esophagus send several processes posteriorly. In adults, these neurons branch throughout the diverticula and anterior stomach. In pupae, but not in larvae or adults, the gut RF-l system coexpresses reactivity to antibodies raised against a member of another peptide family, molluscan small cardioactive peptide b (SCP-I). SCP-I immunoreactivity is localized independently of RF-l immunoreactivity in the ganglia of all stages and in neurons that project along the gut of the adult. We did not find any colocalization of S-I and the peptide markers. Distinct populations of enteroendocrine cells populate different regions of the gut at different life stages. Changes in staining pattern suggest that these cells are replaced at metamorphosis along with the other gut cells during the extensive remodeling of the tract. Distributed in the gut epithelium are subpopulations that express either RF-I or SCP-I; a small fraction of these cells bind antibodies to both peptides. The stomachs of adult females are larger than those of males, and the numbers of SCP-I and RF-I enteroendocrine cells are proportionately greater in females. In all the life stages, the junctions between different regions of the gut are the focus of regulatory input. The larval cardiac valve possesses a ring of cells, the necklace cells, which appear to receive extensive synaptic inputs from both the serotonergic system and the peptidergic system. Another focus of control is the pyloric valve, which is encircled by axon-like processes. The immunoreactive pattern of this region differs across life stages, expressing SCP-I in larvae, S-I in pupae, and both SCP-I and RF-I in adults.

Mosquito immunity against Plasmodium

Understanding the molecular mechanisms of the innate immune responses of Anopheles gambiae against Plasmodium parasites is of great importance for current efforts to develop novel strategies for malaria disease control. The parasite undergoes substantial stage-specific losses during its development in the mosquito, which in some cases lead to complete refractoriness of the mosquito against the parasite. The underlying genetics of refractoriness are complex and multifactorial. Completion of the genome sequence of An. gambiae 2 years ago, together with the development of DNA microarrays in this species and the extension of the RNAi technique to adult mosquitoes, has allowed comparative and functional genomic approaches of the mosquito innate immune system. A variety of factors were shown to negatively affect the development of Plasmodium parasites in the mosquito, in some cases leading to complete transmission blockage. In addition, mosquito factors have been identified that play positive roles and are required for successful transmission of the parasite. These findings indicate a highly complex interplay between parasite and vector. Research is continuing to identify new factors involved in this interaction and to decipher the interplay of these molecules and their regulation.