The Effects of A Mosquito Salivary Protein on Sporozoite Traversal of Host Cells

Anopheles gambiae lacking AgTRIO probe inefficiently on a mammalian host

Malaria is initiated as Plasmodium sporozoites are injected into the dermis when an infected mosquito probes on a vertebrate host for a blood meal. Factors in the mosquito saliva, such as AgTRIO, can alter the ability of Anopheles gambiae to transmit Plasmodium. We therefore used CRISPR-Cas9-mediated genome editing to generate AgTRIO knockout (KO) A. gambiae and examined the ability of these mosquitoes to probe on a vertebrate host. AgTRIO KO mosquitoes showed a diminished host probing capacity and required repetitive probing to locate a blood resource to complete a blood meal. This increased probing resulted in enhanced Plasmodium transmission to the vertebrate host. Our data demonstrate the importance of the A. gambiae saliva protein AgTRIO in probing and its influence on the ability of mosquitoes to transmit malaria.

Circadian rhythms mediate malaria transmission potential

Malaria transmission begins when infected female Anopheles mosquitos deposit Plasmodium parasites into the mammalian host's skin during a bloodmeal. The salivary gland-resident sporozoite parasites migrate to the bloodstream, subsequently invading and replicating within hepatocytes. As Anopheles mosquitos are more active at night, with a 24-hour rhythm, we investigated whether their salivary glands are under circadian control, anticipating bloodmeals and modulating sporozoite biology for host encounters. Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian rhythmic expression. Furthermore, we demonstrate that mosquitoes prefer to feed during nighttime, with the amount of blood ingested varying cyclically throughout the day. Notably, we show a substantial subset of the sporozoite transcriptome cycling throughout the day. These include genes involved in parasite motility, potentially modulating the ability to initiate infection at different times of day. Thus, although sporozoites are typically considered quiescent, our results demonstrate their transcriptional activity, revealing robust daily rhythms of gene expression. Our findings suggest a circadian evolutionary relationship between the vector, parasite and mammalian host that together modulate malaria transmission.

Two mosquito salivary antigens demonstrate promise as biomarkers of recent exposure to P. falciparum infected mosquito bites

Background

Measuring malaria transmission intensity using the traditional entomological inoculation rate is difficult. Antibody responses to mosquito salivary proteins such as SG6 have previously been used as biomarkers of exposure to Anopheles mosquito bites. Here, we investigate four mosquito salivary proteins as potential biomarkers of human exposure to mosquitoes infected with P. falciparum: mosGILT, SAMSP1, AgSAP, and AgTRIO.

Methods

We tested population-level human immune responses in longitudinal and cross-sectional plasma samples from individuals with known P. falciparum infection from low and moderate transmission areas in Senegal using a multiplexed magnetic bead-based assay.

Results

AgSAP and AgTRIO were the best indicators of recent exposure to infected mosquitoes. Antibody responses to AgSAP, in a moderate endemic area, and to AgTRIO in both low and moderate endemic areas, were significantly higher than responses in a healthy non-endemic control cohort (p-values = 0.0245, 0.0064, and <0.0001 respectively). No antibody responses significantly differed between the low and moderate transmission area, or between equivalent groups during and outside the malaria transmission seasons. For AgSAP and AgTRIO, reactivity peaked 2-4 weeks after clinical P. falciparum infection and declined 3 months after infection.

Discussion

Reactivity to both AgSAP and AgTRIO peaked after infection and did not differ seasonally nor between areas of low and moderate transmission, suggesting reactivity is likely reflective of exposure to infectious mosquitos or recent biting rather than general mosquito exposure. Kinetics suggest reactivity is relatively short-lived. AgSAP and AgTRIO are promising candidates to incorporate into multiplexed assays for serosurveillance of population-level changes in P. falciparum-infected mosquito exposure.

Aedes aegypti D7 long salivary proteins modulate blood feeding and parasite infection

IMPORTANCE

During blood feeding, mosquitoes inject saliva into the host skin, preventing hemostasis and inflammatory responses. D7 proteins are among the most abundant components of the saliva of blood-feeding arthropods. Aedes aegypti, the vector of yellow fever and dengue, expresses two D7 long-form salivary proteins: D7L1 and D7L2. These proteins bind and counteract hemostatic agonists such as biogenic amines and leukotrienes. D7L1 and D7L2 knockout mosquitoes showed prolonged probing times and carried significantly less Plasmodium gallinaceum oocysts per midgut than wild-type mosquitoes. We hypothesize that reingested D7s play a vital role in the midgut microenvironment with important consequences for pathogen infection and transmission.

A mosquito AgTRIO mRNA vaccine contributes to immunity against malaria

Malaria begins when an infected mosquito injects saliva containing Plasmodium sporozoites into the skin of a vertebrate host. To prevent malaria, vaccination is the most effective strategy and there is an urgent need for new strategies to enhance current pathogen-based vaccines. Active or passive immunization against a mosquito saliva protein, AgTRIO, contributes to protection against Plasmodium infection of mice. In this study, we generated an AgTRIO mRNA-lipid nanoparticle (LNP) and assessed its potential usefulness as a vaccine against malaria. Immunization of mice with an AgTRIO mRNA-LNP generated a robust humoral response, including AgTRIO IgG2a isotype antibodies that have been associated with protection. AgTRIO mRNA-LNP immunized mice exposed to Plasmodium berghei-infected mosquitoes had markedly reduced initial Plasmodium hepatic infection levels and increased survival compared to control mice. In addition, as the humoral response to AgTRIO waned over 6 months, additional mosquito bites boosted the AgTRIO IgG titers, including IgG1 and IgG2a isotypes, which offers a unique advantage compared to pathogen-based vaccines. These data will aid in the generation of future malaria vaccines that may include both pathogen and vector antigens.

Malaria: influence of Anopheles mosquito saliva on Plasmodium infection

Malaria is caused by Plasmodium protozoa that are transmitted by anopheline mosquitoes. Plasmodium sporozoites are released with saliva when an infected female mosquito takes a blood meal on a vertebrate host. Sporozoites deposited into the skin must enter a blood vessel to start their journey towards the liver. After migration out of the mosquito, sporozoites are associated with, or in proximity to, many components of vector saliva in the skin. Recent work has elucidated how Anopheles saliva, and components of saliva, can influence host-pathogen interactions during the early stage of Plasmodium infection in the skin. Here, we discuss how components of Anopheles saliva can modulate local host responses and affect Plasmodium infectivity. We hypothesize that therapeutic strategies targeting mosquito salivary proteins can play a role in controlling malaria and other vector-borne diseases.

A salivary factor binds a cuticular protein and modulates biting by inducing morphological changes in the mosquito labrum

The mosquito proboscis is an efficient microelectromechanical system, which allows the insect to feed on vertebrate blood quickly and painlessly. Its efficiency is further enhanced by the insect saliva, although through unclear mechanisms. Here, we describe the initial trigger of an unprecedented feedback signaling pathway in Aedes mosquitoes affecting feeding behavior. We identified LIPS proteins in the saliva of Aedes mosquitoes that promote feeding in the vertebrate skin. LIPS show a new all-helical protein fold constituted by two domains. The N-terminal domain interacts with a cuticular protein (Cp19) located at the tip of the mosquito labrum. Upon interaction, the morphology of the labral cuticle changes, and this modification is most likely sensed by proprioceptive neurons. Our study identifies an additional role of mosquito saliva and underlines that the external cuticle is a possible site of key molecular interactions affecting the insect biology and its vector competence.

Monoclonal antibodies for malaria prevention

Monoclonal antibodies are highly specific proteins that are cloned from a single B cell and bind to a single epitope on a pathogen. These laboratory-made molecules can serve as prophylactics or therapeutics for infectious diseases and have an impressive capacity to modulate the progression of disease, as demonstrated for the first time on a large scale during the COVID-19 pandemic. The high specificity and natural starting point of monoclonal antibodies afford an encouraging safety profile, yet the high cost of production remains a major limitation to their widespread use. While a monoclonal antibody approach to abrogating malaria infection is not yet available, the unique life cycle of the malaria parasite affords many opportunities for such proteins to act, and preliminary research into the efficacy of monoclonal antibodies in preventing malaria infection, disease, and transmission is encouraging. This review examines the current status and future outlook for monoclonal antibodies against malaria in the context of the complex life cycle and varied antigenic targets expressed in the human and mosquito hosts, and provides insight into the strengths and limitations of this approach to curtailing one of humanity’s oldest and deadliest diseases.

Immunomodulation by Mosquito Salivary Protein AgSAP Contributes to Early Host Infection by Plasmodium

Malaria is caused when Plasmodium sporozoites are injected along with saliva by an anopheline mosquito into the dermis of a vertebrate host. Arthropod saliva has pleiotropic effects that can influence local host responses, pathogen transmission, and exacerbation of the disease. A mass spectrometry screen identified mosquito salivary proteins that are associated with Plasmodium sporozoites during saliva secretions. In this study, we demonstrate that one of these salivary antigens, Anopheles gambiae sporozoite-associated protein (AgSAP), interacts directly with Plasmodium falciparum and Plasmodium berghei sporozoites. AgSAP binds to heparan sulfate and inhibits local inflammatory responses in the skin. The silencing of AgSAP in mosquitoes reduces their ability to effectively transmit sporozoites to mice. Moreover, immunization with AgSAP decreases the Plasmodium burden in mice that are bitten by Plasmodium-infected mosquitoes. These data suggest that AgSAP facilitates early Plasmodium infection in the vertebrate host and serves as a target for the prevention of malaria. IMPORTANCE Malaria is a vector-borne disease caused by Plasmodium sporozoites. When an anopheline mosquito bites its host, it releases Plasmodium sporozoites as well as saliva components. Mosquito proteins have the potential to serve as antigens to prevent or influence malaria without directly targeting the pathogen. This may help set a new paradigm for vaccine development. In this study, we have elucidated the role of a novel salivary antigen, named Anopheles gambiae sporozoite-associated protein (AgSAP). The results presented here show that AgSAP interacts with Plasmodium falciparum and Plasmodium berghei sporozoites and modulates local inflammatory responses in the skin. Furthermore, our results show that AgSAP is a novel mosquito salivary antigen that influences the early stages of Plasmodium infection in the vertebrate host. Individuals living in countries where malaria is endemic generate antibodies against AgSAP, which indicates that AgSAP can serve as a biomarker for disease prevalence and epidemiological analysis.

A mosquito AgTRIO monoclonal antibody reduces earlyPlasmodium infection of mice

Malaria begins when an infected mosquito injects saliva containing Plasmodium sporozoites into the skin of a vertebrate host. Passive immunization of mice with mosquito AgTRIO antisera offers significant protection against Plasmodium infection of mice. Furthermore, passive transfer of both AgTRIO antisera and an anti-circumsporozoite protein monoclonal antibody provides synergistic protection. In this study, we generated monoclonal antibodies against AgTRIO to delineate the regions of AgTRIO associated with protective immunity. Monoclonal antibody 13F-1 markedly reduced Plasmodium infection in mice and recognized a region, VDDLMAKFN, in the carboxyl terminus of AgTRIO. 13F-1 is an IgG2a isotype monoclonal antibody and the Fc region is required for protection. These data will aid in the generation of future malaria vaccines that may include both pathogen and vector antigens.