Malaria: The influence of Anopheles mosquito saliva on Plasmodium infection

Abstract

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.

Anopheles mosquitoes actively participate in the transmission of malaria

Malaria caused over 240 million clinical infections and more than 620,000 deaths worldwide in 2021, 90% of which occurred in sub-Saharan Africa [1]. Plasmodium sp., the causative agent of malaria, is transmitted by the bite of an infected female Anopheline mosquito, arguably the most dangerous animal that humans encounter [2]. Mosquito-borne diseases are expanding their range, and the incidence of malaria is rising [3]. Plasmodium requires both a mosquito and a human host to complete its lifecycle. The completion of parasite development in the mosquito’s midgut results in the production of sporozoites, which migrate to, and invade, the Anopheles salivary glands. When a female mosquito probes for human blood, it deposits saliva and sporozoites into the skin. Understanding the importance of the salivary glands for harboring and transmitting Plasmodium remains relatively unexplored. Of relevance, putative therapeutics targeting Anopheles salivary gland antigens might block parasite transmission and thus control malaria, in part because sporozoites spend several hours at the inoculation site [4, 5]. The success of Plasmodium infection depends on how well these few hundred sporozoites evade local immune responses before reaching blood vessels and migrating to the liver (Key Figure, Figure 1) [6] . While most research efforts have focused on how Plasmodium antigens induce host responses, it is important to consider the tripartite molecular interactions between mosquito salivary proteins, Plasmodium, and the vertebrate host, following deposition of the sporozoites into the skin [7–10]. Recent work on salivary proteins such as AgTRIO, mosGILT, SAMSP1 and AGSAP have revealed a newfound role for salivary proteins in regulating the immune cell function and modulating the infectivity of malaria parasites in the skin. In this opinion, we describe selected host responses to Anopheles bites and the role of individual salivary proteins in Plasmodium transmission. We hypothesize that targeting mosquito salivary proteins might help mitigate or control malaria disease outcomes, and therefore, should be further investigated.

Key Figure, Figure 1:

Role of Anopheles saliva antigens in blood-feeding. This figure was created with Biorender.com

Host responses to Anopheles bites

Mosquito bites lead to a local cutaneous reaction with swelling and erythema, sometimes followed by itching and the development of an indurated papule. Components of mosquito saliva remain in the human skin for up to eighteen hours, and the host response to these antigens contributes to inflammation. Anopheles bites lead to vasodilation of blood vessels, capillary extravasation, and hemorrhage, accompanied by infiltration of polymorphonuclear leukocytes and monocytes [11, 12] (Figure 2). The cutaneous inflammatory response is partially dependent on tissue-resident mast cells, which are activated following mosquito bites [13], and saliva deposits that colocalize with degranulated mast cells [11]. In response to Anopheles bites, mast cells produce the chemokine CXCL-2 and enhance neutrophil recruitment in mice [13, 14]. Moreover, using mosquito bitten mast cell-mouse models (WBB6F1-W/W^v) [mast-cell deficient] vs WBB6F1^+/+ [mast-cell sufficient]), mast cell activation led to the production of anti-inflammatory cytokine IL-10 and downregulation of antigen specific T-cell responses (as evidenced from cytokine production) [13, 14]. These studies suggested that Anopheles bites could have a differential impact on the activation of innate immunity and the inhibition of antigen-specific immune responses.

Figure 2:

Mosquito bite impact on host immunity. Saliva antigens influence host immunity and Plasmodium sp. infection. This figure was created with Biorender.com

Plasmodium sporozoites and associated salivary gland proteins influence the host response:

Upon reaching the dermis, Plasmodium sporozoites navigate through the skin for many hours [9, 15, 16]. During this period, a fraction of the sporozoites invade the blood or lymphatic vessels while the sporozoites that do not successfully enter the blood stream remain in the dermis where they die and are cleared by host defenses [8, 17]. Interactions between sporozoites and immune cells in the skin can influence the protective response to malaria [12, 18–20]. This is attributed to the sporozoite’s ability to migrate in the dermis, and the immunosuppressive responses of Plasmodium or salivary proteins [20–22]. Specifically, saliva components from P. yoelli inhibited CD8⁺ T lymphocyte responses (in terms of T cell expansion frequencies) in sporozoite-primed mice, while purified sporozoites augmented T-cell responses [23]. The inhibition of CD8⁺ T lymphocyte responses might be related to the mobilization of regulatory T-cells (Treg) in the dermis following the bite of Plasmodium-infected mosquitoes [22]. Indeed, relative to controls, mosquito bite-induced Tregs showed an increase in the expression of the homing molecule, L-selectin (CD62L), which is known to facilitate migration and homing of T cells to lymph nodes. The suppressive action on Th1 responses was also evident from an increase in IL-4 and IL-10 after inoculation of sporozoites in the skin in vitro [24]. While a majority of studies suggest that Anopheles salivary components suppress T-helper 1 Th1 responses, another study reported that the exposure to an uninfected mosquito bite polarized the immune response towards a Th1 cell phenotype, as evidenced from the increased concentrations of IL-12, IFN-γ, and inducible nitric oxide synthase in the mouse liver [25]. These non-concordant results are likely due to differences in the numbers of sporozoites inoculated and/or their route of inoculation, i.e., via mosquito bite or intradermal inoculation of homogenized glands; however, further studies should clarify these discrepancies. We speculate that the mosquito bite suppresses Th1 responses in the skin and should be considered an important factor in the design of newer putative malaria vaccines.

Impact of Anopheles salivary gland proteins on Plasmodium transmission and infection:

To develop a successful sporozoite-based vaccine, it is essential to understand the role of Anopheles saliva on the outcome of Plasmodium infection. Early studies with mice immunized with mosquito salivary gland homogenates showed modest protection against P. berghei, as evidenced from the changes in infectivity of mice injected with salivary gland extracts compared to control groups [26, 27]. This was further substantiated by experiments in which the addition of salivary gland homogenates of Aedes mosquitoes increased P. gallinaceum infection in chickens when subcutaneously challenged; this showed that proteins present in salivary glands could influence the outcomes of avian malaria parasite infection [28]. Furthermore, repeated pre-exposure to uninfected Anopheles stephensi has been proposed to evoke a Th1 response that contributes to some protection against P. yoelii infection [25]. The effect of mosquito bites on Plasmodium infection was also shown when mice were infected with P. berghei while concomitantly fed upon by Anopheles. Specifically, the animals exposed to these mosquitoes exhibited increased progression to cerebral malaria with elevated IL-10 concentrations in the skin and draining lymph nodes compared to controls, suggesting that mosquito bites modulated the local immune environment in the skin to favor parasite survival [24]. Moreover, while these studies showed protective effects of mosquito salivary components, as evidenced by a lower Plasmodium burden following exposure to a mosquito bite or salivary components, one study showed that Anopheles stephensi saliva did not influence infectivity of either P. berghei or P. yoelii [29] based on data from Giemsa-stained blood smears. However, in the latter Kebaier study [29], the readout for infection occurred in the parasite blood stage and was detected at 14 days post infection unlike the liver parasite burden measurements which were taken at 40h post infection in the Donovan study [25]; thus, the later study was unlikely able to detect the immediate impact of mosquito saliva. In a recent study, our group observed that antisera against Anopheles gambiae salivary glands, but not repeated mosquito bites, partially protected mice from mosquito-transmitted Plasmodium infection [30], as evidenced from the Plasmodium burden in mouse livers. This finding, together with individual studies on salivary gland antigens AgTRIO, SAMSP1, mosGILT, and AgSAP, suggest that Anopheles saliva has pleiotropic effects on host immunity, as well as on sporozoite mobility and infectivity [30–34].

Overall, these reports suggest that some saliva proteins, and the mosquito bite itself, have an influence on cutaneous host responses. As individuals living in Sub-Saharan Africa may get thousands of bites each season, we hypothesize that the effect of these salivary proteins on skin immunity could be chronic and might play role in suppressing the immune response [35–38] in individuals living in malaria-endemic areas.

Components of Anopheles saliva

Proteomics:

On finding the vertebrate host, an Anopheles mosquito searches for blood by piercing its proboscis into the host skin, which provides pathogens access to the subepithelial microenvironment. Insect saliva helps to optimize blood feeding and can influence capillary extravasation, hemostasis, pain and itch responses, and immune effector mechanisms [13, 25, 39–42]. To identify salivary proteins that are important in these processes and that also potentially alter Plasmodium transmission to the host, it is crucial to define the Anopheles salivary gland proteome. Different studies have characterized up to 150 proteins present in Anopheles saliva. One study identified 69 unique proteins in the salivary gland of A. gambiae which include secretory molecules, housekeeping genes, and proteins of unknown function [43] . The study uncovered the D7r family proteins, apyrases, and the salivary gland-like (SG-like) family, as well as mosGILT, AgTRIO, SAMSP1 homologs in the salivary glands A. gambiae [43]. Another proteomics study characterized 5 proteins in saliva and 122 proteins in the salivary glands of A. gambiae [40]. Using quantitative proteomics, the study showed that the expression of a defense-related protein gVAG was increased two-fold in the infected salivary glands, whereas expression of four proteins was decreased -- gSG6, apyrase, D7 related-1 protein precursor, and D7 precursor allergen AED A2, relative to uninfected Anopheles mosquitoes [44]. Overall, these and other proteomics studies suggest that Anopheles saliva is dynamic and complex, and that Plasmodium infection can modulate the expression of components of saliva. From these studies it is clear that Anopheles not only provide malaria parasites physical access to invade the host but also express individual saliva proteins that help Plasmodium to reach the next stage of its lifecycle [45–48].

Sporozoite-associated salivary proteins:

Upon reaching the skin, sporozoites may be coated with secretory or basement membrane proteins from Anopheles salivary glands via specific interactions or charge-based electrostatic interactions, shown via molecular and cellular studies [33, 34]. To identify sporozoite-associated mosquito proteins, the saliva of infected A. gambiae were subjected to mass spectrometry [33]. The screening led to the identification of mosGILT as a major sporozoite-associated protein [33]. mosGILT binds to both P. berghei and P. falciparum, and homologs are present in other species of Anopheles [33]. The screen also led to the identification and characterization of SAMSP1 and AgSAP as additional proteins that associate with P. sporozoites, as well as other potential targets [33]. This type of analysis provides a focus on proteins that may interact with the sporozoite surface and thereby influence host-pathogen interactions. However, more work is needed to elucidate how Plasmodium interactions with mosquito salivary glands may affect sporozoite infectivity and transmission to the host.

Role of individual salivary gland proteins in blood feeding or Plasmodium infection

Anopheline antiplatelet protein:

Anopheline antiplatelet protein (AAPP) is a protein that was first identified in the saliva of Anopheles stephensi [49]. AAPP binding to collagen can inhibit thrombosis since it interferes with platelet adhesion to collagen [49]. Furthermore, in vitro results show that AAPP binding to collagen I and III limits the ability of immunoglobulin-like receptor glycoprotein VI (GPVI), present on the platelet surface, to bind to collagen [49]. AAPP also inhibits the collagen-mediated increase in intracellular Ca2⁺ concentration in platelets [49]. This is relevant because the platelets trigger the mechanical pathway of the coagulation cascade by binding to collagen and stop blood leakage, and AAPP allows the mosquito to feed on the host. The ability of AAPP to inhibit blood aggregation suggests potential clinical utility as an anticoagulant [50]. Moreover, the neutralization of AAPP in transgenic Anopheles stephensi mosquitoes also affects factors associated with blood feeding, including probing and prediuresis time, blood meal size, and fecundity [51]. The fitness of mosquitoes has been affected when a transgenesis-based protein inactivation approach was used to express anti-AAPP antibody single-chain fragment (scFv) in the salivary glands of Anopheles stephensi. Interestingly, AAPP neutralization did not affect oocyst numbers in mosquitoes when fed upon P. berghei-infected mice. Overall, these studies demonstrate a role for AAPP in blood feeding and as a platelet-modulating agent, certainly meriting further attention.

AgTRIO:

Since mosquito saliva contains hundreds of proteins, a yeast surface display approach was used to identify proteins that react with serum from rabbits immunized with a salivary gland extract [30]. Twenty-one Anopheles gambiae proteins were uncovered, including AgTRIO, D7r1, and 6 other antigens with putative signal sequences. AgTRIO is part of an Anopheles-specific SG1 gene family [52] and AgTRIO expression is increased by the presence of P. berghei in the mosquito’s salivary glands [53, 54]. Of note, AgTRIO antisera reduced the pathogen titers in mice bitten by mosquitoes infected with P. berghei or P. falciparum, and synergized with Plasmodium circumsporozoite protein (CSP) antibodies for enhanced efficacy [30]. To our knowledge, AgTRIO was the first example in which neutralizing a mosquito protein but not parasite proteins provided some degree of protection, as evidenced from the opsonic phagocytosis by neutrophils and the binding of FcγRIII and FcγRIIa by such antibody, of which the titer was higher among protected than non-protected human subjects, as well as from antibody-dependent cellular cytotoxicity assays of NK cells on P. falciparum-infected red blood cells using immune human plasma [55, 56]; moreover, antibodies targeting the carboxyl terminus of AgTRIO were associated with partial immunity (neutrophil phagocytosis) [51, 52]. While the immune mechanism by which AgTRIO provides protection remains unclear, a study showed that AgTRIO mouse antiserum decreased sporozoite movement in the dermis, and that AgTRIO inhibited the expression of TNF-α in mouse skin [32]. Overall, to our knowledge, AgTRIO might be the first example of an Anopheles salivary protein facilitating Plasmodium infection by influencing the local environment in the vertebrate host [30, 32].

mosGILT:

It is also possible that some mosquito salivary gland proteins may negatively impact Plasmodium during the movement of sporozoite to the vertebrate host. One such protein is a mosquito IFNγ-inducible lysosomal thiol reductase (mosGILT). Specifically, mosGILT was identified by a mass spectrometry screen of saliva from A. gambiae infected with P. berghei [33]. mosGILT shares homology with human GILT, an immune-related protein involved in antigen processing and presentation [57]. mosGILT binds to Plasmodium and impacts sporozoite cell traversal activity, an important process used by sporozoites as they migrate through the dermis and towards the blood vessels and liver of the host [33]. Indeed, intravital imaging in mice showed that mosGILT reduced the speed of Plasmodium sporozoites in the skin but did not affect Plasmodium viability [33]. In addition, mosGILT binding to sporozoites reduced Plasmodium infection and, conversely, mice immunized with mosGILT showed higher parasite burdens in the liver than ovalbumin immunized mice, suggesting that mosGILT could modulate the level of initial Plasmodium infection in mice [33]. Overall, the interaction between mosGILT and Plasmodium sporozoites may suggest an unknown mechanism by which sporozoite motility is regulated in salivary glands; this may be interesting to evaluate because such motility might influence the ability of Plasmodium to be released from mosquito salivary glands.

In addition to influencing Plasmodium transmission, mosGILT, is essential for mosquito reproduction and protects Plasmodium oocysts from destruction within the mosquito [58]. This shows that some Anopheles proteins may have multiple functions, including Anopheles development, innate immunity, and control of Plasmodium infection. Therefore, it is important to study the collective role of such proteins both in Anopheles biology and malaria immune responses.

SAMSP1:

Sporozoite-associated mosquito saliva protein 1 (SAMSP1) is another mosquito protein than interacts with Plasmodium during migration from the vector to the vertebrate host. In contrast to mosGILT, cellular assays showed that SAMSP1 binding to P. berghei enhanced sporozoite gliding and traversal [31]. SAMSP1 also affects host immunity at the bite site. Specifically, mouse intradermal inoculation of Anopheles salivary gland extracts (SGE) combined with SAMSP1 decreased the number of neutrophils at the inoculation site compared with controls [31]. The influence of SAMSP1 on skin immunity was associated with neutrophil recruitment since there were no effects on macrophage, Langerhans cell, or dendritic cell numbers compared with controls. This was also evident in an in vitro assay in which SAMSP1 altered neutrophil chemotaxis. Moreover, neutralization of SAMSP1 by active immunization or by passive transfer of antibodies diminished the Plasmodium burden in the liver of mice relative to mice that received ovalbumin antisera [31]. Overall, these findings suggest that SAMSP1 helps facilitate malaria infection. Furthermore, when mice were intravenously given antibodies against both SAMSP1 and CSP, the Plasmodium burden decreased more than when CSP antibodies were given alone [31]. Consistent with this, monoclonal antibodies against CSP are currently being tested in clinical trials to prevent malaria, and we argue that the addition of AgTRIO, SAMSP1, or other Anopheles salivary gland protein antibodies to CSP, might potentially enhance the efficacy of these antibodies.

AgSAP:

Combining proteomic analysis of saliva with sporozoite-associated proteins has provided additional information on targets that were previously not described in sialomes. For example, mass spectrometry of sporozoites isolated from Anopheles saliva led to the identification of a protein named A. gambiae sporozoite-associated protein (AgSAP) that is not a secretory protein. However, it associates with P. falciparum and P. berghei sporozoites, as evidenced from proteomics studies and visualization using specific antibodies [33, 34]. AgSAP has some homology to the Drosophila protein Papilin, an extracellular matrix protein that inhibits metalloproteinases[55]. AgSAP binds to heparan sulfate (HS), a glycosaminoglycan present on the surface of mammalian cells and in the extracellular matrix [34, 60–62]. HS moieties interact with various proteins including selectins (shown by in vitro studies with monocyte–endothelial interaction models and by in vivo mouse studies with endothelial cells); indeed, they play an important role in inflammation [59–63]. For instance, AgSAP-HS interactions can interfere with multiple steps of inflammation: AgSAP modulates local inflammatory response in mouse skin and inhibits expression of TNF-α and various other pro-inflammatory cytokines such as IL-1β, IFN-γ, IL-4, MMP-9, TGF-β and ligands such as ICAM-1 [34]. Further, AgSAP inhibits activation of human Jurkat T-cells in vitro, and AgSAP silencing via RNA interference in Anopheles mosquitoes also reduces effective transmission of sporozoites to mice [34]. Collectively, these studies suggest that AgSAP might help sporozoites suppress immune responses in the skin and facilitate Plasmodium infection in the vertebrate host, although further investigations are warranted.

Apyrase:

Plasmodium infection influences mosquito behavior, including the ability of the mosquito to bite more frequently [68, 69]. Sporozoites inhibit the activity of apyrase, a salivary protein that hydrolyzes ADP and inhibits ADP-induced platelet aggregation. Studies have shown that salivary apyrase concentrations are inversely correlated with probing time, suggesting that Plasmodium infection alters the composition of Anopheles saliva to make mosquitoes more likely to feed again, and more persistent in their host-seeking behavior. [68, 69]. Although intriguing, the influence of apyrase on Plasmodium transmission remains unknown.

D7 protein family:

D7 proteins are among the most abundant in Anopheles saliva and were initially characterized as odorant binding proteins [70]. Anopheles gambiae, the primary vector of malaria, expresses multiple long- and short-form D7 proteins [71]. A recent study showed that A. gambiae D7 long-forms bind to hemostatic agonists such as leukotriene C4 and serotonin (potent activators of vasoconstriction, edema, and postcapillary venule leakage) [71]. AngaD7L1 also binds and inhibits platelet aggregation and the vasoconstrictive activity of thromboxane A2 analog U-46619 [71]. In addition, AngaD7L3 inhibits serotonin-induced platelet aggregation and vasoconstriction [71]. AngaD7L proteins counteract host hemostasis by interacting with factors XII, XIIa, and XIa in blood, and show a dose-dependent anticoagulant effect in vitro [71]. Our group previously showed that neutralizing D7r1 with specific rabbit antiserum did not alter the Plasmodium load in the liver, as evidenced by quantitative PCR; however, the role of other D7 proteins remains to be investigated [30]. Overall, these studies suggest that D7 might play a role in blood feeding.

Concluding remarks

Human populations in sub-Saharan Africa and other malaria endemic areas are continuously exposed to mosquitoes during the transmission season. In these regions, mosquito saliva components and their influence on adaptive immune responses may have an impact on shaping natural immunity to malaria [72–75]. In addition to interfering with host-Plasmodium interactions, mosquito salivary gland proteins may alter sporozoite infectivity. Indeed, as reviewed here, recent work on a few Anopheles proteins suggests that specific saliva factors can have a negative, or positive, effect on Plasmodium transmission and infection. The role of most saliva proteins remains unknown and based on limited studies performed by different groups, we hypothesize that Anopheles saliva may not only help in blood feeding processes, but also actively participate in Plasmodium transmission to the host (see outstanding questions). The examples described in this opinion indicate that detailed studies are needed to decipher the effect of Anopheles proteins on Plasmodium in the arthropod and vertebrate hosts [33, 58] (Key Figure, Key Figure, Figure 1). These issues are relevant as humans living in malaria endemic areas are exposed to both uninfected and infected mosquitoes and therefore, it is difficult to infer from longitudinal studies in malaria endemic areas, the nature of the antibody responses to Anopheles salivary proteins that are related to Plasmodium infection.

Outstanding question.

• How does a mosquito bite regulate the inflammatory response in the skin? What are the unique immunological features of the Anopheles bite compared to other mosquitoes and other arthropods such as ticks?

• How does Plasmodium sporozoite trafficking to the mosquito salivary glands impact the secretion of certain salivary proteins, which in turn influence sporozoite infectivity?

• What host receptor interacts with salivary proteins such as Apyrase, AgTRIO, SAMSP1, and AgSAP?

• Which salivary proteins might serve as putative biomarkers for epidemiological analysis of malarial transmission?

• What role does the skin microbiota plays in mosquito bites and Plasmodium infection?

• Could a cocktail of Anopheles salivary antigens – stand-alone or in combination with Plasmodium antigens -- be used as an effective candidate anti-malarial vaccine?

A vaccine that might block pathogen transmission and restrict mosquitoes from harboring or transmitting malaria parasites is one of the challenging goals of research on arthropod-borne diseases. The field of vector-targeted vaccines is in its infancy but recently an mRNA-based vaccine targeting 19 tick proteins (19ISP) provided protection from infection with the Lyme disease agent, Borrelia burgdorferi [76]. The tick’s ability to feed on guinea pigs immunized with 19ISP mRNA-LNP cocktail was substantially reduced when compared with guinea pigs immunized with a nonspecific mRNA-LNP. Furthermore, quantitative PCR assays showed that the majority of the 19ISP-vaccinated guinea pigs remained uninfected compared to the control group in which most animals were infected with Borrelia burgdorferi [76]. Another advance is the development of a mosquito cocktail containing four A. gambiae saliva antigen peptides, but more work is needed to determine whether this target provides any protection against a vector-borne disease [77]. Unlike Ixodes ticks for which the 19ISP mRNA vaccine induces erythema and restricts the arthropod’s ability to feed on the host [76], a mosquito saliva-targeted vaccine has to work in a different manner because mosquitoes feed rapidly and individuals living in endemic areas are exposed to hundreds of mosquitos bites every day. More immunological and functional studies are needed to understand the function of specific saliva proteins and their role in Plasmodium infectivity during the initial stages of infection in the vertebrate host. Nevertheless. it is essential to study Anopheles salivary proteins since some of these proteins coat Plasmodium sporozoites, and may create an immunoprivileged niche at the bite site that hinders immune cells from eliminating sporozoites in the skin. Therefore, we posit that adding an Anopheles-based component to current Plasmodium-based vaccines might represent a paradigm shift in malaria control and a means to enhance protection against different species of Plasmodium.

Highlights.

• Mosquito saliva, which contains proteins with anti-hemostatic, anti-inflammatory, and immunomodulatory activities, is inoculated into the host during malaria parasite transmission.

• Some saliva proteins associate with the Plasmodium sporozoite surface and may interfere in host-pathogen interactions.

• Early work on a few Anopheles proteins indicates that specific saliva factors can negatively or positively affect Plasmodium transmission and infection.

• Following a mosquito bite, components of its saliva remain in the skin for many hours and may alter the innate and adaptive immune response in the skin.

• Some salivary proteins such as AgSAP, SAMSP1, and AgTRIO can inhibit T cell responses and overall inflammatory reactions that may directly impact the survival of Plasmodium sporozoites.

• Next-generation vaccines and monoclonal antibodies targeting salivary proteins may lead to the development of broad-spectrum therapeutics that can block the transmission of Plasmodium and other mosquito-borne pathogens.

Significance.

Malaria begins when Anopheline mosquitoes inject Plasmodium sporozoites, along with saliva when taking a blood meal, into the dermis of a vertebrate host. Mosquito saliva has pleiotropic effects that can influence local host immune responses. Although the role of mosquito saliva in malaria transmission has been speculated for more than two decades, recent studies identify salivary protein candidates in host-pathogen interactions. Elucidating the role of these proteins in modulating immune responses in the skin may help approaches to curb how Plasmodium establishes infection.

Glossary:

Malaria

serious and sometimes fatal disease caused by a Plasmodium parasite that is transmitted by the bite of an infected female Anopheles mosquito

indurated papule

small, solid, raised, and hardened bump on the skin

Regulatory T-cells

subpopulation of T cells that maintains homeostasis and self-tolerance in the body. Tregs suppress the action of other immune events to keep the immune system from becoming over-active

Th1 responses

Type 1 T helper (Th1) cell response maintains cell-mediated immune responses and produces proinflammatory cytokines such as IFN-γ

Cerebral malaria

one of the most serious malaria complications; characterized by coma and neurological manifestations

Prediuresis

process by which blood-feeding mosquitoes excrete drops of fluid to concentrate ingested blood protein within a small volume that fits in their midgut

yeast surface display

protein engineering technique that expresses recombinant proteins on the surface of yeast and is used to find novel host-vector-pathogen interactions

GILT

Gamma-interferon-inducible lysosomal thiol reductase is a thioredoxin-related oxidoreductase; functions by reducing disulfide bonds of endocytosed proteins in antigen-presenting cells

CSP

The circumsporozoite protein is the major surface protein of the Plasmodium sporozoite; major vaccine target. CSP helps Plasmodium sporozoite adhesion to target cells

Sialomes

Proteins expressed in the salivary glands of mosquitoes and other blood-feeding arthropods