Mosquito Saliva Reshapes Alphavirus Infection and Immunopathogenesis

A novel interaction between the 5' untranslated region of the Chikungunya virus genome and Musashi RNA binding protein is essential for efficient virus genome replication

Chikungunya virus (CHIKV) is a re-emerging, pathogenic alphavirus that is transmitted to humans by Aedesspp. mosquitoes-causing fever and debilitating joint pain, with frequent long-term health implications and high morbidity. The CHIKV replication cycle is poorly understood and specific antiviral therapeutics are lacking. In the current study, we identify host cell Musashi RNA binding protein-2 (MSI-2) as a proviral factor. MSI-2 depletion and small molecule inhibition assays demonstrated that MSI-2 is required for efficient CHIKV genome replication. Depletion of both MSI-2 and MSI-1 homologues was found to synergistically inhibit CHIKV replication, suggesting redundancy in their proviral function. Electromobility shift assay (EMSA) competition studies demonstrated that MSI-2 interacts specifically with an RNA binding motif within the 5' untranslated region (5'UTR) of CHIKV and reverse genetic analysis showed that mutation of the binding motif inhibited genome replication and blocked rescue of mutant virus. For the first time, this study identifies the proviral role of MSI RNA binding proteins in the replication of the CHIKV genome, providing important new insight into mechanisms controlling replication of this significant human pathogen and the potential of a novel therapeutic target.

Research progress toward the influence of mosquito salivary proteins on the transmission of mosquito-borne viruses

Mosquito-borne viruses (MBVs) are a large class of viruses transmitted mainly through mosquito bites, including dengue virus, Zika virus, Japanese encephalitis virus, West Nile virus, and chikungunya virus, which pose a major threat to the health of people around the world. With global warming and extended human activities, the incidence of many MBVs has increased significantly. Mosquito saliva contains a variety of bioactive protein components. These not only enable blood feeding but also play a crucial role in regulating local infection at the bite site and the remote dissemination of MBVs as well as in remodeling the innate and adaptive immune responses of host vertebrates. Here, we review the physiological functions of mosquito salivary proteins (MSPs) in detail, the influence and the underlying mechanism of MSPs on the transmission of MBVs, and the current progress and issues that urgently need to be addressed in the research and development of MSP-based MBV transmission blocking vaccines.

In-vitro assessment of cutaneous immune responses to aedes mosquito salivary gland extract and dengue virus in Cambodian individuals

Dengue virus (DENV) poses a global health threat, affecting millions individuals annually with no specific therapy and limited vaccines. Mosquitoes, mainly Aedes aegypti and Aedes albopictus worldwide, transmit DENV through their saliva during blood meals. In this study, we aimed to understand how Aedes mosquito saliva modulate skin immune responses during DENV infection in individuals living in mosquito-endemic regions. To accomplish this, we dissociated skin cells from Cambodian volunteers and incubated them with salivary gland extract (SGE) from three different mosquito strains: Ae. aegypti USDA strain, Ae. aegypti and Ae. albopictus wild type (WT) in the presence/absence of DENV. We observed notable alterations in skin immune cell phenotypes subsequent to exposure to Aedes salivary gland extract (SGE). Specifically, exposure lead to an increase in the frequency of macrophages expressing chemokine receptor CCR2, and neutrophils expressing CD69. Additionally, we noted a substantial increase in the percentage of macrophages that became infected with DENV in the presence of Aedes SGE. Differences in cellular responses were observed when Aedes SGE of three distinct mosquito strains were compared. Our findings deepen the understanding of mosquito saliva's role in DENV infection and skin immune responses in individuals regularly exposed to mosquito bites. This study provides insights into skin immune cell dynamics that could guide strategies to mitigate DENV transmission and other arbovirus diseases.

Aedes aegypti saliva modulates inflammasome activation and facilitates flavivirus infection in vitro

Mosquito borne flaviviruses such as dengue and Zika represent a major public health problem due to globalization and propagation of susceptible vectors worldwide. Vertebrate host responses to dengue and Zika infections include the processing and release of pro-inflammatory cytokines through the activation of inflammasomes, resulting in disease severity and fatality. Mosquito saliva can facilitate pathogen infection by downregulating the host's immune response. However, the role of mosquito saliva in modulating host innate immune responses remains largely unknown. Here, we show that mosquito salivary gland extract (SGE) inhibits dengue and Zika virus-induced inflammasome activation by reducing NLRP3 expression, Caspase-1 activation, and 1L-1β secretion in cultured human and mice macrophages. As a result, we observe that SGE inhibits virus detection in the early phase of infection. This study provides important insights into how mosquito saliva modulates host innate immunity during viral infection.

Immune Response to Chikungunya Virus: Sex as a Biological Variable and Implications for Natural Delivery via the Mosquito

Chikungunya virus (CHIKV) is a mosquito-borne virus with significant public health implications around the world. Climate change, as well as rapid urbanization, threatens to expand the population range of Aedes vector mosquitoes globally, increasing CHIKV cases worldwide in return. Epidemiological data suggests a sex-dependent response to CHIKV infection. In this review, we draw attention to the importance of studying sex as a biological variable by introducing epidemiological studies from previous CHIKV outbreaks. While the female sex appears to be a risk factor for chronic CHIKV disease, the male sex has recently been suggested as a risk factor for CHIKV-associated death; however, the underlying mechanisms for this phenotype are unknown. Additionally, we emphasize the importance of including mosquito salivary components when studying the immune response to CHIKV. As with other vector-transmitted pathogens, CHIKV has evolved to use these salivary components to replicate more extensively in mammalian hosts; however, the response to natural transmission of CHIKV has not been fully elucidated.

Host Derivation of Sindbis Virus Influences Mammalian Type I Interferon Response to Infection

Arboviruses are defined by their ability to replicate in both mosquito vectors and mammalian hosts. There is good evidence that arboviruses "prime" their progeny for infection of the next host, such as via differential glycosylation of their outer glycoproteins or packaging of host ribosomal subunits. We and others have previously shown that mosquito-derived viruses more efficiently infect mammalian cells than mammalian-derived viruses. These observations are consistent with arboviruses acquiring host-specific adaptations, and we hypothesized that a virus derived from either the mammalian host or mosquito vector elicits different responses when infecting the mammalian host. Here, we perform an RNA-sequencing analysis of the transcriptional response of Human Embryonic Kidney 293 (HEK-293) cells to infection with either mosquito (Aedes albopictus, C7/10)- or mammalian (Baby Hamster Kidney, BHK-21)-derived Sindbis virus (SINV). We show that the C7/10-derived virus infection leads to a more robust transcriptional response in HEK-293s compared to infection with the BHK-derived virus. Surprisingly, despite more efficient infection, we found an increase in interferon-β (IFN-β) and interferon-stimulated gene (ISG) transcripts in response to the C7/10-derived virus infection versus the BHK-derived virus infection. However, translation of interferon-stimulated genes was lower in HEK-293s infected with the C7/10-derived virus, starkly contrasting with the transcriptional response. This inhibition of ISG translation is reflective of a more rapid overall shut-off of host cell translation following infection with the C7/10-derived virus. Finally, we show that the C7/10-derived virus infection of HEK-293 cells leads to elevated levels of phosphorylated eukaryotic translation elongation factor-2 (eEF2), identifying a potential mechanism leading to the more rapid shut-off of host translation. We postulate that the rapid shut-off of host translation in mammalian cells infected with the mosquito-derived virus acts to counter the IFN-β-stimulated transcriptional response.

Mosquito Salivary Proteins and Arbovirus Infection: From Viral Enhancers to Potential Targets for Vaccines

Arthropod-borne viruses present important public health challenges worldwide. Viruses such as DENV, ZIKV, and WNV are of current concern due to an increasing incidence and an expanding geographic range, generating explosive outbreaks even in non-endemic areas. The clinical signs associated with infection from these arboviruses are often inapparent, mild, or nonspecific, but occasionally develop into serious complications marked by rapid onset, tremors, paralysis, hemorrhagic fever, neurological alterations, or death. They are predominately transmitted to humans through mosquito bite, during which saliva is inoculated into the skin to facilitate blood feeding. A new approach to prevent arboviral diseases has been proposed by the observation that arthropod saliva facilitates transmission of pathogens. Viruses released within mosquito saliva may more easily initiate host invasion by taking advantage of the host's innate and adaptive immune responses to saliva. This provides a rationale for creating vaccines against mosquito salivary proteins, especially because of the lack of licensed vaccines against most of these viruses. This review aims to provide an overview of the effects on the host immune response by the mosquito salivary proteins and how these phenomena alter the infection outcome for different arboviruses, recent attempts to generate mosquito salivary-based vaccines against flavivirus including DENV, ZIKV, and WNV, and the potential benefits and pitfalls that this strategy involves.

Evaluation of monophosphoryl lipid A as an adjuvanted for inactivated chikungunya virus

Currently there is no clinically approved chikungunya virus (CHIKV) vaccine for immunization. Though definite need is felt, long disappearance of CHIKV has been a concern. Inactivated CHIKV (I-CHIKV) is an attractive antigen to develop effective vaccines within a short period of time. However, highly purified inactivated CHIKV do not contain necessary triggers for induction of robust antibody response. Monophosphoryl lipid A (MPLA) is a TLR4 ligand which is expressed on immune cells and is known to enhance immune response. Additionally, route of delivery also plays a critical role in modulating the immune response. Thus, antigen, adjuvant and route of delivery might modulate immune response if combined. Therefore in this study, we explored the immunogenicity of inactivated CHIKV-MPLA combination in mice after administration by intradermal or intramuscular route. Long term immune response study was also conducted by varying the antigen concentration and keeping the adjuvant concentration constant. Our study showed that the CHIKV-MPLA combination induced higher binding antibodies as well as neutralizing antibody titers as compared to unadjuvanted CHIKV. No difference in antibody titers was observed after delivery by either of the routes. However, difference in IFNγ and IL4 profiles was observed when a supernatant from stimulated splenocytes was analyzed. Taken together, these data show that both routes could be used for administration of the I-CHIKV-MPLA combination.

Mosquito saliva enhances virus infection through sialokinin-dependent vascular leakage

Viruses transmitted by Aedes mosquitoes are an increasingly important global cause of disease. Defining common determinants of host susceptibility to this large group of heterogenous pathogens is key for informing the rational design of panviral medicines. Infection of the vertebrate host with these viruses is enhanced by mosquito saliva, a complex mixture of salivary-gland-derived factors and microbiota. We show that the enhancement of infection by saliva was dependent on vascular function and was independent of most antisaliva immune responses, including salivary microbiota. Instead, the Aedes gene product sialokinin mediated the enhancement of virus infection through a rapid reduction in endothelial barrier integrity. Sialokinin is unique within the insect world as having a vertebrate-like tachykinin sequence and is absent from Anopheles mosquitoes, which are incompetent for most arthropod-borne viruses, whose saliva was not proviral and did not induce similar vascular permeability. Therapeutic strategies targeting sialokinin have the potential to limit disease severity following infection with Aedes-mosquito-borne viruses.

Mosquito Saliva Modulates Japanese Encephalitis Virus Infection in Domestic Pigs

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that is the leading cause of pediatric viral encephalitis in Asia. Japanese encephalitis virus is transmitted by Culex species mosquitoes that also vector several zoonotic flaviviruses. Despite the knowledge that mosquito saliva contains molecules that may alter flavivirus pathogenesis, whether or not the deposition of viruses by infected mosquitoes has an impact on the kinetics and severity of JEV infection has not been thoroughly examined, especially in mammalian species involved in the enzootic transmission. Most JEV pathogenesis models were established using needle inoculation. Mouse models for West Nile (WNV) and dengue (DENV) viruses have shown that mosquito saliva can potentiate flavivirus infections and exacerbate disease symptoms. In this study, we determined the impact of mosquito salivary components on the pathogenesis of JEV in pigs, a species directly involved in its transmission cycle as an amplifying host. Interestingly, co-injection of JEV and salivary gland extract (SGE) collected from Culex quinquefasciatus produced milder febrile illness and shortened duration of nasal shedding but had no demonstrable impact on viremia and neuroinvasion. Our findings highlight that mosquito salivary components can differentially modulate the outcomes of flavivirus infections in amplifying hosts and in mouse models.

Ixodes ricinus Salivary Serpin Iripin-8 Inhibits the Intrinsic Pathway of Coagulation and Complement

Tick saliva is a rich source of antihemostatic, anti-inflammatory, and immunomodulatory molecules that actively help the tick to finish its blood meal. Moreover, these molecules facilitate the transmission of tick-borne pathogens. Here we present the functional and structural characterization of Iripin-8, a salivary serpin from the tick Ixodes ricinus, a European vector of tick-borne encephalitis and Lyme disease. Iripin-8 displayed blood-meal-induced mRNA expression that peaked in nymphs and the salivary glands of adult females. Iripin-8 inhibited multiple proteases involved in blood coagulation and blocked the intrinsic and common pathways of the coagulation cascade in vitro. Moreover, Iripin-8 inhibited erythrocyte lysis by complement, and Iripin-8 knockdown by RNA interference in tick nymphs delayed the feeding time. Finally, we resolved the crystal structure of Iripin-8 at 1.89 Å resolution to reveal an unusually long and rigid reactive center loop that is conserved in several tick species. The P1 Arg residue is held in place distant from the serpin body by a conserved poly-Pro element on the P′ side. Several PEG molecules bind to Iripin-8, including one in a deep cavity, perhaps indicating the presence of a small-molecule binding site. This is the first crystal structure of a tick serpin in the native state, and Iripin-8 is a tick serpin with a conserved reactive center loop that possesses antihemostatic activity that may mediate interference with host innate immunity.

Alphaviruses: Host pathogenesis, immune response, and vaccine & treatment updates

Human pathogens belonging to the Alphavirus genus, in the Togaviridae family, are transmitted primarily by mosquitoes. The signs and symptoms associated with these viruses include fever and polyarthralgia, defined as joint pain and inflammation, as well as encephalitis. In the last decade, our understanding of the interactions between members of the alphavirus genus and the human host has increased due to the re-appearance of the chikungunya virus (CHIKV) in Asia and Europe, as well as its emergence in the Americas. Alphaviruses affect host immunity through cytokines and the interferon response. Understanding alphavirus interactions with both the innate immune system as well as the various cells in the adaptive immune systems is critical to developing effective therapeutics. In this review, we summarize the latest research on alphavirus-host cell interactions, underlying infection mechanisms, and possible treatments.

Mosquito-bite infection of humanized mice with chikungunya virus produces systemic disease with long-term effects

Chikungunya virus (CHIKV) is an emerging, mosquito-borne alphavirus responsible for acute to chronic arthralgias and neuropathies. Although it originated in central Africa, recent reports of disease have come from many parts of the world, including the Americas. While limiting human CHIKV cases through mosquito control has been used, it has not been entirely successful. There are currently no licensed vaccines or treatments specific for CHIKV disease, thus more work is needed to develop effective countermeasures. Current animal research on CHIKV is often not representative of human disease. Most models use CHIKV needle inoculation via unnatural routes to create immediate viremia and localized clinical signs; these methods neglect the natural route of transmission (the mosquito vector bite) and the associated human immune response. Since mosquito saliva has been shown to have a profound effect on viral pathogenesis, we evaluated a novel model of infection that included the natural vector, Aedes species mosquitoes, transmitting CHIKV to mice containing components of the human immune system. Humanized mice infected by 3-6 mosquito bites showed signs of systemic infection, with demonstrable viremia (by qRT-PCR and immunofluorescent antibody assay), mild to moderate clinical signs (by observation, histology, and immunohistochemistry), and immune responses consistent with human infection (by flow cytometry and IgM ELISA). This model should give a better understanding of human CHIKV disease and allow for more realistic evaluations of mechanisms of pathogenesis, prophylaxis, and treatments.

Vertebrate Responses against Arthropod Salivary Proteins and Their Therapeutic Potential

The saliva of hematophagous arthropods contains a group of active proteins to counteract host responses against injury and to facilitate the success of a bloodmeal. These salivary proteins have significant impacts on modulating pathogen transmission, immunogenicity expression, the establishment of infection, and even disease severity. Recent studies have shown that several salivary proteins are immunogenic and antibodies against them may block infection, thereby suggesting potential vaccine candidates. Here, we discuss the most relevant salivary proteins currently studied for their therapeutic potential as vaccine candidates or to control the transmission of human vector-borne pathogens and immune responses against different arthropod salivary proteins.

A mosquito salivary protein promotes flavivirus transmission by activation of autophagy

Transmission from an infected mosquito to a host is an essential process in the life cycle of mosquito-borne flaviviruses. Numerous studies have demonstrated that mosquito saliva facilitates viral transmission. Here we find that a saliva-specific protein, named Aedes aegypti venom allergen-1 (AaVA-1), promotes dengue and Zika virus transmission by activating autophagy in host immune cells of the monocyte lineage. The AG6 mice (ifnar1^–/–ifngr1^–/–) bitten by the virus-infected AaVA-1-deficient mosquitoes present a lower viremia and prolonged survival. AaVA-1 intracellularly interacts with a dominant negative binder of Beclin-1, known as leucine-rich pentatricopeptide repeat-containing protein (LRPPRC), and releases Beclin-1 from LRPPRC-mediated sequestration, thereby enabling the initialization of downstream autophagic signaling. A deficiency in Beclin-1 reduces viral infection in mice and abolishes AaVA-1-mediated enhancement of ZIKV transmission by mosquitoes. Our study provides a mechanistic insight into saliva-aided viral transmission and could offer a potential prophylactic target for reducing flavivirus transmission.

Mosquito saliva affects transmission of flaviviruses, but underlying mechanisms are incompletely understood. Here, the authors show that Aedes aegypti venom allergen-1 (AaVA-1) promotes dengue and Zika virus transmission by activating autophagy in host immune cells of the monocyte lineage.

Aedes aegypti AgBR1 antibodies modulate early Zika virus infection of mice

A recent epidemic of Zika virus in the Americas, affecting well over a million people, caused substantial mortality and morbidity, including Guillain-Barre syndrome, microcephaly and other fetal developmental defects1,2. Preventive and therapeutic measures that specifically target the virus are not readily available. The transmission of Zika virus is predominantly mosquito-borne, and Aedes aegypti mosquitoes serve as a key vector for Zika virus3. Here, to identify salivary factors that modulate mosquito-borne Zika virus infection, we focused on antigenic proteins in mice that were repeatedly bitten by mosquitoes and developed antibodies against salivary proteins. Using a yeast surface display screen, we identified five antigenic A. aegypti salivary proteins in mice. Antiserum against one of these five proteins-A. aegypti bacteria-responsive protein 1 (AgBR1)-suppressed early inflammatory responses in the skin of mice bitten by Zika-virus-infected mosquitoes. AgBR1 antiserum also partially protected mice from lethal mosquito-borne-but not needle-injected-Zika virus infection. These data suggest that AgBR1 is a target for the prevention of mosquito-transmitted Zika virus infection.