Mosquito Immune Responses and Malaria Transmission

National Institute Of Allergy And Infectious Diseases

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Abstract

Megacytes enhance antiplasmodial immunity: Activation of Toll signaling in Anopheles gambiae by silencing Cactus, a suppressor of this pathway, enhances local release of hemocyte-derived microvesicles (HdMv), promoting activation of the mosquito complement-like system, which eliminates Plasmodium ookinetes. We uncovered the mechanism of this immune enhancement. Cactus silencing triggers a Rel1-mediated differentiation of granulocytes to the megacyte lineage, a new subpopulation of giant cells, resulting in a dramatic increase in the proportion of circulating megacytes. Megacytes are very plastic cells that are massively recruited to the basal midgut surface in response to Plasmodium infection. We show that Toll signaling modulates hemocyte differentiation and that megacyte recruitment to the midgut greatly enhances mosquito immunity against Plasmodium. Molecular mechanism of P. falciparum evasion of mosquito Immunity: The mosquito immune system limits Plasmodium infection and malaria transmission. Plasmodium falciparum evades the mosquito defense response by expressing Pfs47, a cell surface protein that inhibits midgut epithelial nitration by interacting with P47Rec, a mosquito midgut receptor. However, the mechanism by which P47Rec suppresses caspase-mediated nitration is unknown. Here, we show that epithelial invasion by Pfs47 knockout parasites is followed by an extrusion of cells undergoing caspase-mediated apoptosis, which triggers the release of hemocytes-derived microvesicles (HdMvs), known to promote complement activation and ookinete lysis. In contrast, invasion by P. falciparum parasites that express Pfs47 accelerates the extrusion of damaged cells and disrupts caspase activation and HdMv release. The Anopheles gambiae Heat shock protein 70 cognate 3 (Hsc70-3), the ortholog of Drosophila BiP, was identified as a molecular partner of P47Rec. Silencing of Hsc70-3 promotes lysis of Pfs47 WT parasites through a caspase S2-dependent mechanism. The interaction of Plasmodium Pfs47 with its mosquito receptor (P47Rec) activates a Hsc70-3-mediated anti-apoptotic response that prevents caspase activation. The interaction of P47Rec with Hsc70-3 is necessary for P. falciparum to evade the mosquito early immune responses that target the ookinete stage. Globalization of Plasmodium falciparum malaria: P. falciparum malaria originated when Plasmodium praefalciparum, a gorilla malaria parasite that adapted to humans. We previously showed that Pfs47, a protein on the parasite surface, mediates P. falciparum evasion of the mosquito immune system by interacting with a midgut receptor. Genetic analysis of 4,971 Pfs47 gene sequences from different continents revealed that parasites from Asia and Papua New Guinea harbor Pfs47 haplotypes more similar to its ortholog in P. praefalciparum at sites that determine vector compatibility, suggesting that ancestral P. falciparum readily adapted to Asian vectors. Consistent with this observation, Pfs47-receptor gene sequences from African sylvan malaria vectors, such as Anopheles moucheti and An. marshallii that transmit P. praefalciparum, shared greater similarity with the receptors of Asian vectors than those of vectors of the African An. gambiae complex. Furthermore, experimental infections provide direct evidence that transformed P. falciparum parasites carrying Pfs47 orthologs of P. praefalciparum or P. reichenowi are more effective at evading the immune system of the Asian malaria vector An. dirus than An. gambiae. We propose that high compatibility of ancestral P. falciparum Pfs47 with the receptors of Asian vectors facilitated the early dispersal of human malaria to the Asian continent, without having to first adapt to sub-Saharan vectors of the An. gambiae complex. Post-translational modification of Sporozoite mediates mosquito immune evasion: Glutaminyl cyclase (QC) modifies N-terminal glutamine or glutamic acid residues of target proteins into cyclic pyroglutamic acid (pGlu). Here, we report the biochemical and functional analysis of Plasmodium QC. We show that sporozoites of QC-null mutants of rodent and human malaria parasites are recognized by the mosquito immune system and melanized when they reach the hemocoel. Detailed analyses of rodent malaria QC-null mutants showed that sporozoite numbers in salivary glands are reduced in mosquitoes infected with QC-null or QC catalytically dead mutants. This phenotype can be rescued by genetic complementation or by disrupting mosquito melanization or phagocytosis by hemocytes. Mutation of a single QC-target glutamine of the major sporozoite surface protein (circumsporozoite protein; CSP) of the rodent parasite Plasmodium berghei also results in melanization of sporozoites. These findings indicate that QC-mediated posttranslational modification of surface proteins underlies evasion of killing of sporozoites by the mosquito immune system.