Selection of Anopheles dirus for refractoriness and susceptibility to Plasmodium yoelii nigeriensis

Ability of TEP1 in intestinal flora to modulate natural resistance of Anopheles dirus

Blocking transmission of malaria is a reliable way to control and eliminate infection. However, in-depth knowledge of the interaction between Plasmodium and mosquito is needed. Studies suggest that innate immunity is the main mechanism inhibiting development of malaria parasites in the mosquito. Recent studies have found that use of antibiotics that inhibit the mosquito gut flora can reduce the immune response of Anopheles gambiae, thereby contributing to the development of malaria parasites. In our study, we used the non susceptible model of Anopheles dirus-Plasmodium yoelii to explore the effect of Anopheles intestinal flora on the natural resistance of A. dirus to P. yoelii. We found that in mosquitoes infected with Plasmodium, the intestinal flora can regulate expression of thioester-containing protein (TEP1) via an RNAi gene-silencing approach. Our results suggest that in the absence of TEP1, the natural microbiota cannot suppress the development of P. yoelii in A. dirus. This suggests that AdTEP1 plays an important role in the resistance of A. dirus to P. yoelii. The intestinal flora may modulate the development of P. yoelii in A. dirus by regulating TEP1 expression.

Continuous exposure to Plasmodium results in decreased susceptibility and transcriptomic divergence of the Anopheles gambiae immune system

Background

Plasmodium infection has been shown to compromise the fitness of the mosquito vector, reducing its fecundity and longevity. However, from an evolutionary perspective, the impact of Plasmodium infection as a selective pressure on the mosquito is largely unknown.

Results

In the present study we have addressed the effect of a continuous Plasmodium berghei infection on the resistance to infection and global gene expression in Anopheles gambiae.

Exposure of A. gambiae to P. berghei-infected blood and infection for 16 generations resulted in a decreased susceptibility to infection, altered constitutive expression levels for approximately 2.4% of the mosquito's total transcriptome and a lower basal level of immune genes expression, including several anti-Plasmodium factors. The infection-responsiveness for several defense genes was elevated in the P. berghei exposed mosquito colonies.

Conclusion

Our study establishes the existence of a selective pressure exerted by the parasite P. berghei on the malaria vector A. gambiae that results in a decreased permissiveness to infection and changes in the mosquito transcriptome regulation that suggest a decreased constitutive immune gene activity but a more potent immune response upon Plasmodium challenge.

Molecular genetics of mosquito resistance to malaria parasites

Malaria parasites are transmitted by the bite of an infected mosquito, but even efficient vector species possess multiple mechanisms that together destroy most of the parasites present in an infection. Variation between individual mosquitoes has allowed genetic analysis and mapping of loci controlling several resistance traits, and the underlying mechanisms of mosquito response to infection are being described using genomic tools such as transcriptional and proteomic analysis. Malaria infection imposes fitness costs on the vector, but various forms of resistance inflict their own costs, likely leading to an evolutionary tradeoff between infection and resistance. Plasmodium development can be successfully completed onlyin compatible mosquito-parasite species combinations, and resistance also appears to have parasite specificity. Studies of Drosophila, where genetic variation in immunocompetence is pervasive in wild populations, offer a comparative context for understanding coevolution of the mosquito-malaria relationship. More broadly, plants also possess systems of pathogen resistance with features that are structurally conserved in animal innate immunity, including insects, and genomic datasets now permit useful comparisons of resistance models even between such diverse organisms.

Transgenic mosquitoes and malaria transmission

As the malaria burden persists in most parts of the developing world, the concept of implementation of new strategies such as the use of genetically modified mosquitoes to control the disease continues to gain support. In Africa, which suffers most from malaria, mosquito vector populations are spread almost throughout the entire continent, and the parasite reservoir is big and continuously increasing. Moreover, malaria is transmitted by many species of anophelines with specific seasonal and geographical patterns. Therefore, a well designed, evolutionarily robust and publicly accepted plan aiming at population reduction or replacement is required. The task is twofold: to engineer mosquitoes with a genetic trait that confers resistance to malaria or causes population suppression; and, to drive the new trait through field populations. This review examines these two issues, and describes the groundwork that has been done towards understanding of the complex relation between the parasite and its vector.

Comparative and functional genomics of the innate immune system in the malaria vector Anopheles gambiae

In much of Africa, the mosquito Anopheles gambiae is the major vector of human malaria, a devastating infectious disease caused by Plasmodium parasites. Vector and parasite interact at multiple stages and locations, and the nature and effectiveness of this reciprocal interaction determines the success of transmission. Many of the interactions engage the mosquito's innate immunity, a primitive but very effective defense system. In some cases, the mosquito kills the parasite, thus blocking the transmission cycle. However, not all interactions are antagonistic; some represent immune evasion. The sequence of the A. gambiae genome revealed numerous potential components of the innate immune system, and it established that they evolve rapidly, as summarized in the present review. Their rapid evolution by gene family expansion diversification as well as the prevalence of haplotype alleles in the best-studied families may reflect selective adaptation of the immune system to the exigencies of multiple immune challenges in a variety of ecologic niches. As a follow-up to the comparative genomic analysis, the development of functional genomic methodologies has provided novel opportunities for understanding the immune system and the nature of its interactions with the parasite. In this context, identification of both Plasmodium antagonists and protectors in the mosquito represents a significant conceptual advance. In addition to providing fundamental understanding of primitive immune systems, studies of mosquito interactions with the parasite open unprecedented opportunities for novel interventions against malaria transmission. The generation of transgenic mosquitoes that resist malaria infection in the wild and the development of antimalarial 'smart sprays' capable of disrupting interactions that are protective of the parasite, or reinforcing others that are antagonistic, represent technical challenges but also immense opportunities for improvement of global health.

Innate immunity in the malaria vector Anopheles gambiae:comparative and functional genomics

The resurgence of malaria is at least partly attributed to the absence of an effective vaccine, parasite resistance to antimalarial drugs and resistance to insecticides of the anopheline mosquito vectors. Novel strategies are needed to combat the disease on three fronts: protection (vaccines),prophylaxis/treatment (antimalarial drugs) and transmission blocking. The latter entails either killing the mosquitoes (insecticides), preventing mosquito biting (bednets and repellents), blocking parasite development in the vector (transmission blocking vaccines), genetic manipulation or chemical incapacitation of the vector. During the past decade, mosquito research has been energized by several breakthroughs, including the successful transformation of anopheline vectors, analysis of gene function by RNAi,genome-wide expression profiling using DNA microarrays and, most importantly,sequencing of the Anopheles gambiae genome. These breakthroughs helped unravel some of the mechanisms underlying the dynamic interactions between the parasite and the vector and shed light on the mosquito innate immune system as a set of potential targets to block parasite development. In this context, putative pattern recognition receptors of the mosquito that act as positive and negative regulators of parasite development have been identified recently. Characterizing these molecules and others of similar function, and identifying their ligands on the parasite surface, will provide clues on the nature of the interactions that define an efficient parasite–vector system and open up unprecedented opportunities to control the vectorial capacity of anopheline mosquitoes.

Insect immunity and its implication in mosquito-malaria interactions

Insects' resistance to infectious agents is essential for their own survival and also for the health of the plant, animal and human populations with which they closely interact. Several of the major human diseases are spread by insects and are rapidly expanding as a result of the development of insecticide resistance in vectors and drug resistance in parasites. A vector insects' permissiveness to a pathogen, and hence the spread of the disease, will largely depend on the compatibility of the molecular interactions between the two species and the capability of the insect immune system to recognize and kill the pathogen. The innate immune system comprises a variety of components and mechanisms that can discriminate between different microorganisms and mount specific responses to control pathogenic infections. An impressive body of knowledge on the insects' innate immunity has been generated from studies in the model organism Drosophila. These studies are now guiding the exploration of the immune system in the vector mosquito of human malaria, Anopheles, and its implication in the elimination of parasites. Anopheles immune responses have been linked to parasite losses and some refractory mosquitoes can kill all parasites through specific defence mechanisms. The recently sequenced Drosophila and Anopheles genomes provide a detailed and comparative view on their immune gene repertoires that in combination with post-genomic analyses is used to further dissect the complex mechanisms of Plasmodium killing in the mosquito.