Tick salivary gland extracts promote virus growth in vitro

Discovery of the Role of Tick Salivary Glands in Enhancement of Virus Transmission-Beginning of an Exciting Story

There is increasing evidence that arthropod-borne pathogens exploit saliva of their vectors during the transmission process to vertebrate hosts. Extensive research of the composition of tick saliva and its role in blood-feeding and transmission of pathogens started in the late 1980s and led to a number of discoveries on the composition and function of salivary molecules, some of which are associated with pathogen transmission. The study by Jones et al. published in 1989 can be ranked among the pioneer works in this field as it demonstrated for the first time the role of tick salivary glands in enhancement of transmission of a tick-borne virus. Thogoto virus was used in the model and subsequently similar results were obtained for tick-borne encephalitis virus. After a relatively silent period of almost 20 years, interest in tick-arbovirus-host interactions emerged again in the 2010s. However, no particular salivary molecule(s) enhancing virus transmission has (have) been identified to date. Intensive research in this field will certainly lead to new discoveries with future implications in the control of transmission of dangerous tick-borne viruses.

Vector-Borne Viral Diseases as a Current Threat for Human and Animal Health—One Health Perspective

Over the last decades, an increase in the emergence or re-emergence of arthropod-borne viruses has been observed in many regions. Viruses such as dengue, yellow fever, or zika are a threat for millions of people on different continents. On the other hand, some arboviruses are still described as endemic, however, they could become more important in the near future. Additionally, there is a group of arboviruses that, although important for animal breeding, are not a direct threat for human health. Those include, e.g., Schmallenberg, bluetongue, or African swine fever viruses. This review focuses on arboviruses and their major vectors: mosquitoes, ticks, biting midges, and sandflies. We discuss the current knowledge on arbovirus transmission, ecology, and methods of prevention. As arboviruses are a challenge to both human and animal health, successful prevention and control are therefore only possible through a One Health perspective.

Arboviruses: How Saliva Impacts the Journey from Vector to Host

Arthropod-borne viruses, referred to collectively as arboviruses, infect millions of people worldwide each year and have the potential to cause severe disease. They are predominately transmitted to humans through blood-feeding behavior of three main groups of biting arthropods: ticks, mosquitoes, and sandflies. The pathogens harbored by these blood-feeding arthropods (BFA) are transferred to animal hosts through deposition of virus-rich saliva into the skin. Sometimes these infections become systemic and can lead to neuro-invasion and life-threatening viral encephalitis. Factors intrinsic to the arboviral vectors can greatly influence the pathogenicity and virulence of infections, with mounting evidence that BFA saliva and salivary proteins can shift the trajectory of viral infection in the host. This review provides an overview of arbovirus infection and ways in which vectors influence viral pathogenesis. In particular, we focus on how saliva and salivary gland extracts from the three dominant arbovirus vectors impact the trajectory of the cellular immune response to arbovirus infection in the skin.

Arbovirus-Mosquito Vector-Host Interactions and the Impact on Transmission and Disease Pathogenesis of Arboviruses

Hundreds of viruses, designated as arboviruses, are transmitted by arthropod vectors in complex transmission cycles between the virus, vertebrate host, and the vector. With millions of human and animal infections per year, it is critical to improve our understanding of the interactions between the biological and environmental factors that play a critical role in pathogenesis, disease outcomes, and transmission of arboviruses. This review focuses on mosquito-borne arboviruses and discusses current knowledge of the factors and underlying mechanisms that influence infection and transmission of arboviruses and discusses critical factors and pathways that can potentially become targets for intervention and therapeutics.

Tick-Borne Viruses and Biological Processes at the Tick-Host-Virus Interface

Ticks are efficient vectors of arboviruses, although less than 10% of tick species are known to be virus vectors. Most tick-borne viruses (TBV) are RNA viruses some of which cause serious diseases in humans and animals world-wide. Several TBV impacting human or domesticated animal health have been found to emerge or re-emerge recently. In order to survive in nature, TBV must infect and replicate in both vertebrate and tick cells, representing very different physiological environments. Information on molecular mechanisms that allow TBV to switch between infecting and replicating in tick and vertebrate cells is scarce. In general, ticks succeed in completing their blood meal thanks to a plethora of biologically active molecules in their saliva that counteract and modulate different arms of the host defense responses (haemostasis, inflammation, innate and acquired immunity, and wound healing). The transmission of TBV occurs primarily during tick feeding and is a complex process, known to be promoted by tick saliva constituents. However, the underlying molecular mechanisms of TBV transmission are poorly understood. Immunomodulatory properties of tick saliva helping overcome the first line of defense to injury and early interactions at the tick-host skin interface appear to be essential in successful TBV transmission and infection of susceptible vertebrate hosts. The local host skin site of tick attachment, modulated by tick saliva, is an important focus of virus replication. Immunomodulation of the tick attachment site also promotes co-feeding transmission of viruses from infected to non-infected ticks in the absence of host viraemia (non-viraemic transmission). Future research should be aimed at identification of the key tick salivary molecules promoting virus transmission, and a molecular description of tick-host-virus interactions and of tick-mediated skin immunomodulation. Such insights will enable the rationale design of anti-tick vaccines that protect against disease caused by tick-borne viruses.

Meta-analysis of the effects of insect vector saliva on host immune responses and infection of vector-transmitted pathogens: a focus on leishmaniasis

A meta-analysis of the effects of vector saliva on the immune response and progression of vector-transmitted disease, specifically with regard to pathology, infection level, and host cytokine levels was conducted. Infection in the absence or presence of saliva in naïve mice was compared. In addition, infection in mice pre-exposed to uninfected vector saliva was compared to infection in unexposed mice. To control for differences in vector and pathogen species, mouse strain, and experimental design, a random effects model was used to compare the ratio of the natural log of the experimental to the control means of the studies. Saliva was demonstrated to enhance pathology, infection level, and the production of Th2 cytokines (IL-4 and IL-10) in naïve mice. This effect was observed across vector/pathogen pairings, whether natural or unnatural, and with single salivary proteins used as a proxy for whole saliva. Saliva pre-exposure was determined to result in less severe leishmaniasis pathology when compared with unexposed mice infected either in the presence or absence of sand fly saliva. The results of further analyses were not significant, but demonstrated trends toward protection and IFN-γ elevation for pre-exposed mice.

Arthropod vectors transmit a wide variety of diseases resulting in substantial human morbidity and economic costs worldwide. When hematophagous arthropods blood feed, they release saliva into the host. This saliva elicits a strong immune response and has recently been a focus for vaccine research. There is evidence that the saliva enhances infection in naïve hosts, but that prior exposure to saliva results in less severe infection. This analysis endeavored to determine whether there was a statistically significant enhancement or protective effect with regard to saliva exposure and the progression of disease, and to determine the underlying immune mechanism driving these effects. We found that saliva does indeed enhance infection levels of vector-transmitted pathogens and leishmaniasis pathology in naïve mice and elevates Th2 cytokine levels (IL-4 and IL-10). We also determined that pre-exposure to saliva results in less severe pathology of experimental leishmaniasis in mice. These results are important for vaccine trials and vector control programs, though more studies are needed with regard to pre-exposure.

Antiviral effect of the egg wax of Amblyomma cajennense (Acari: Ixodidae)

The control of viral infections, especially those caused by influenza viruses, is of great interest in Public Health. Bio prospection has shown the presence of active principles in the hemolymph of arthropods, and in the salivary gland of ticks, and some of these are of interest for the development of new pharmacological drugs. Ticks lay their eggs in the environment, and to protect them from desiccation and microbial attack they involve the eggs in a waxy layer produced by an organ known as Gené's Organ. In this study, the eggs wax from tick Amblyomma cajennense (Fabricius) was extracted using ice cold phosphate buffer. The antiviral activity was evaluated with picornavirus and influenza virus. In both cases egg wax was able to inhibit virus replication. For influenza virus, an amount as small as 12 μg/mL of crude egg wax suspension neutralized 128 UHA (hemaglutinant unit) of H(1)N(1) influenza virus. With picornavirus, egg wax led to a 256-fold reduction in virus production by L929 cells. Egg wax was not cytotoxic to VERO, MDCK and L929 cell, being observed that the cell morphology was preserved with concentration as high as 2 mg/mL. In addition no genotoxic effect was observed for Vero cells, suggesting a very interesting potential antiviral activity.

Update on Powassan virus: emergence of a North American tick-borne flavivirus

Powassan virus (POW) (Flaviviridae: Flavivirus) is the cause of rare but severe neuroinvasive disease in North America and Russia. The virus is transmitted among small- and medium-sized mammals by ixodid ticks. Human infections occur via spillover from the main transmission cycle(s). Since the late 1990s, the incidence of human disease seems to be increasing. In addition, POW constitutes a genetically diverse group of virus genotypes, including Deer tick virus, that are maintained in distinct enzootic transmission cycles. This review highlights recent research into POW, focusing on virus genetics and ecology and human disease. Important directions for future research are also discussed.

Mapping of quantitative trait loci controlling tick [Riphicephalus (Boophilus) microplus] resistance on bovine chromosomes 5, 7 and 14

Differences in domestication and selection processes have contributed to considerable phenotypic and genotypic differences between Bos taurus and Bos indicus cattle breeds. Of particular interest in tropical and subtropical production environments are those genetic differences between subspecies that underlie the phenotypic extremes in tolerance and susceptibility to parasite infection. In general, B. taurus cattle are more susceptible to ectoparasites than B. indicus cattle in tropical environments, and much of this difference is under genetic control. To identify genomic regions involved in tick resistance, we developed a B. taurus x B. indicus F(2) experimental population to map quantitative trait loci (QTL) for resistance to the Riphicephalus (Boophilus) microplus tick. About 300 individuals were measured for parasite load in two seasons (rainy and dry) and genotyped for 23 microsatellite markers covering chromosomes 5, 7 and 14. We mapped a suggestive chromosome-wide QTL for tick load in the rainy season (P < 0.05) on chromosome 5. For the dry season, suggestive (P < 0.10) chromosome-wide QTL were mapped on chromosomes 7 and 14. The additive effect of the QTL on chromosome 14 corresponds to 3.18% of the total observed phenotypic variance. Our QTL-mapping study has identified different genomic regions controlling tick resistance; these QTL were dependent upon the season in which the ticks were counted, suggesting that the QTL in question may depend on environmental factors.

Tick immunobiology

Ticks are of vast medical and veterinary public health importance due to direct damage caused by feeding and their roles in transmitting well known and emerging infectious agents. Ticks and tick-borne pathogens stimulate the immune system of the host. Those immune interactions are of importance in tick biology, pathogen transmission and control of ticks and tick-borne diseases. Both innate and specific acquired immune defenses are involved in the responses of vertebrate hosts to infestation. Ticks have evolved countermeasures to circumvent host immune defenses. This review addresses the immunobiology of the tick-host interface from the perspectives of the pharmacology of tick saliva; relationship of tick saliva to pathogen transmission; host immune responses to infestation; tick modulation of host immune defences; and genomic/proteomic strategies for studying tick salivary gland molecules.

The immunomodulatory factors of arthropod saliva and the potential for these factors to serve as vaccine targets to prevent pathogen transmission

In general, attempts to develop vaccines for pathogens transmitted by arthropods have met with little or no success. It has been widely observed that the saliva of arthropods that transmit disease enhances the infectivity of pathogens the arthropod transmits to the vertebrate host. Indeed, it has been observed that vaccinating against components of the saliva of arthropods or against antigens expressed in the gut of arthropods can protect the host from infection and decrease the viability of the arthropod. These results suggest that multi-subunit vaccines that target the pathogen itself as well as arthropod salivary gland components and arthropod gut antigens may be the most effective at controlling arthropod-borne pathogens as these vaccines would target several facets of the lifecycle of the pathogen. This review covers known immunomodulators in arthropod salivary glands, instances when arthropod saliva has been shown to enhance infection and a limited number of examples of antiarthropod vaccines, with emphasis on three arthropods: sandflies, mosquitoes and hard ticks.

A tick B-cell inhibitory protein from salivary glands of the hard tick, Hyalomma asiaticum asiaticum

Some studies done to date suggest that B-cell inhibitory factor occurred in tick saliva. In this study, a novel protein having B-cell inhibitory activity was purified and characterized from the salivary glands of the hard tick, Hyalomma asiaticum asiaticum. This protein was named B-cell inhibitory factor (BIF). The cDNA encoding BIF was cloned by cDNA library screening. The predicted protein from the cDNA sequence is composed of 138 amino acids including the mature BIF. No similarity was found by Blast search. The lipopolysaccharide-induced B-cell proliferation was inhibited by BIF. This is the first report of the identification and characterization of B-cell inhibitory protein from tick. The current study facilitates the study of identifying the interaction among tick, Borrelia burgdorferi, the causative agent of Lyme disease, and host.

Biological transmission of arboviruses: reexamination of and new insights into components, mechanisms, and unique traits as well as their evolutionary trends

Among animal viruses, arboviruses are unique in that they depend on arthropod vectors for transmission. Field research and laboratory investigations related to the three components of this unique mode of transmission, virus, vector, and vertebrate host, have produced an enormous amount of valuable information that may be found in numerous publications. However, despite many reviews on specific viruses, diseases, or interests, a systematic approach to organizing the available information on all facets of biological transmission and then to interpret it in the context of the evolutionary process has not been attempted before. Such an attempt in this review clearly demonstrates tremendous progress made worldwide to characterize the viruses, to comprehend disease transmission and pathogenesis, and to understand the biology of vectors and their role in transmission. The rapid progress in molecular biologic techniques also helped resolve many virologic puzzles and yielded highly valuable data hitherto unavailable, such as characterization of virus receptors, the genetic basis of vertebrate resistance to viral infection, and phylogenetic evidence of the history of host range shifts in arboviruses. However, glaring gaps in knowledge of many critical subjects, such as the mechanism of viral persistence and the existence of vertebrate reservoirs, are still evident. Furthermore, with the accumulated data, new questions were raised, such as evolutionary directions of virus virulence and of host range. Although many fundamental questions on the evolution of this unique mode of transmission remained unresolved in the absence of a fossil record, available observations for arboviruses and the information derived from studies in other fields of the biological sciences suggested convergent evolution as a plausible process. Overall, discussion of the diverse range of theories proposed and observations made by many investigators was found to be highly valuable for sorting out the possible mechanism(s) of the emergence of arboviral diseases.

Haematophagous arthropod saliva and host defense system: a tale of tear and blood

The saliva from blood-feeding arthropod vectors is enriched with molecules that display diverse functions that mediate a successful blood meal. They function not only as weapons against host's haemostatic, inflammatory and immune responses but also as important tools to pathogen establishment. Parasites, virus and bacteria taking advantage of vectors' armament have adapted to facilitate their entry in the host. Today, many salivary molecules have been identified and characterized as new targets to the development of future vaccines. Here we focus on current information on vector's saliva and the molecules responsible to modify host's hemostasis and immune response, also regarding their role in disease transmission.

Exploring tick saliva: from biochemistry to ‘sialomes’ and functional genomics

Tick saliva, a fluid once believed to be only relevant for lubrication of mouthparts and water balance, is now well known to be a cocktail of potent anti-haemostatic, anti-inflammatory and immunomodulatory molecules that helps these arthropods obtain a blood meal from their vertebrate hosts. The repertoire of pharmacologically active components in this cocktail is impressive as well as the number of targets they specifically affect. These salivary components change the physiology of the host at the bite site and, consequently, some pathogens transmitted by ticks take advantage of this change and become more infective. Tick salivary proteins have therefore become an attractive target to control tick-borne diseases. Recent advances in molecular biology, protein chemistry and computational biology are accelerating the isolation, sequencing and analysis of a large number of transcripts and proteins from the saliva of different ticks. Many of these newly isolated genes code for proteins with homologies to known proteins allowing identification or prediction of their function. However, most of these genes code for proteins with unknown functions therefore opening the road to functional genomic approaches to identify their biological activities and roles in blood feeding and hence, vaccine development to control tick-borne diseases.

Tick–host interactions: saliva-activated transmission

The skin site at which ticks attach to their hosts to feed is the critical interface between the tick and its host, and tick-borne pathogens. This site is highly modified by the pharmacologically active molecules secreted in tick saliva. For pathogens, it is an ecologically privileged niche that many exploit. Such exploitation is referred to as saliva-activated transmission (SAT) – the indirect promotion of tick-borne pathogen transmission via the actions of bioactive tick saliva molecules on the vertebrate host. Here we review evidence for SAT and consider what are the most likely candidates for SAT factors among the tick pharmacopoeia of anti-haemostatic, anti-inflammatory and immunomodulatory molecules identified to date. SAT factors appear to differ for different pathogens and tick vector species, and possibly even depend on the vertebrate host species. Most likely we are searching for a suite of molecules that act together to overcome the redundancy in host response mechanisms. Whatever they turn out to be, the quest to identify the tick molecules that mediate SAT is an exciting one, and offers new insights to controlling ticks and tick-borne diseases.

Effect of mosquito salivary gland treatment on vesicular stomatitis New Jersey virus replication and interferon alpha/beta expression in vitro

The sensitivity of vesicular stomatitis (VS) viruses to interferon (IFN)-mediated antiviral effects has been well documented. Previous studies in our laboratory have shown the ability of mosquito saliva to enhance vesicular stomatitis New Jersey (VSNJ) virus infection in mice. To investigate the effect of mosquito saliva on virus replication and IFN alpha/beta expression, virus titers were analyzed at various time points after infection in cells that were treated with mosquito salivary gland homogenate (SGH). Salivary gland treatment of mouse fibroblast cells (L929) resulted in a significant increase in virus growth kinetics compared with untreated controls. In contrast, Vero cells, which are deficient in the IFN alpha/beta response, did not yield increased viral titers in the time points examined. Treatment of L929 cells with an IFN alpha/beta neutralizing antibody also slightly increased virus yield. Ribonuclease protection assays revealed that induction of IFN alpha2 expression was reduced in L929 cells treated with SGH. Modulation of IFN alpha/beta by mosquito saliva may be a critical determinant of the transmission and pathogenesis of VSNJ virus.

Tick salivary gland extract accelerates proliferation of Francisella tularensis in the host

Accelerated proliferation of the tick-borne bacterial pathogen Francisella tularensis was demonstrated in mice when the bacterium was injected together with salivary gland extract from Ixodes ricinus ticks. A significant increase in the numbers of bacteria was recorded in the dermal site of infection,the draining lymph nodes, and the spleen. Analysis of the expression of cytokine messenger ribonucleic acids showed polarization toward a Th2 profile. Salivary gland extract-mediated suppression of interleukin-12 and interferon-gamma, the cytokines required for the expression of the protective immunity against tularemic infection, apparently contributed to the decreased resistance against this tick-transmitted pathogen.

Coinoculation of Borrelia spp. with tick salivary gland lysate enhances spirochete load in mice and is tick species-specific

C3H/HeN mice were inoculated with 10(6) spirochetes, either Borrelia burgdorferi strain N40 or the Portuguese strain of B. lusitaniae, PotiB2. Mice receiving spirochetes coinoculated with salivary gland lysate (SGL) demonstrated significantly higher spirochete loads in target organs as measured by quantitative real-time polymerase chain reaction. This effect was tick dependent, in that Ixodes ricinus SGL specifically enhanced B. lusitaniae load, whereas I. scapularis SGL specifically increased B. burgdorferi N40 load, but did not significantly affect the dissemination of B. lusitaniae. Protein profile analysis indicated at least 5 major protein differences between I. scapularis and I. ricinus SGL, which can possibly account for this specific tick-spirochete interaction.

High-throughput approaches to study salivary proteins and genes from vectors of disease

Blood-feeding arthropods have in their saliva a variety of molecules that affect the vertebrate host's hemostatic, inflammatory, and immune systems. The saliva of blood feeders also helps to facilitate infection of the pathogens they carry to the host, making vector saliva an attractive target to control pathogen transmission. The isolation and identification of salivary molecules from vectors of disease has been slow and difficult. Emerging technologies and new approaches in the fields of molecular biology and protein chemistry are facilitating this work. Massive sequencing of high quality, full-length cDNA libraries, coupled with proteomics and functional genomic approaches has led to the discovery of novel proteins, transcription products (genes), and biologic activities from the salivary glands of blood-feeding arthropods. This review focuses on the biologic activities identified in the saliva of various vectors of disease using classic biochemical and molecular biology approaches and new types of molecules and activities identified with high-throughput strategies.

Progress toward molecular characterization of ectoparasite modulation of host immunity

Ectoparasitic arthropods and vector-borne infectious agents are global medical and veterinary public health concerns. Economic impact due to direct effects of infestation and disease transmission are significant. These problems are increased by development of arthropod resistance to insecticides/acaricides; drug resistance of vector-borne pathogens; and, lack of effective vaccines to prevent many of these diseases. There is much to be gained from understanding the complex array of immunological interactions occurring at the arthropod-host-pathogen interface. One application of that knowledge is the development of novel vaccines for the control of both ectoparasitic arthropods and the diseases they transmit. We now realize that blood-feeding arthropods are not simply flying or crawling hypodermic needles and syringes. Ectoparasitic arthropods are not passive partners in their relationships with the immune systems of their hosts. These clever invertebrates produce numerous pharmacologically active molecules that help them migrate through tissues of their hosts or to successfully obtain blood meals. Arthropod parasites stimulate a spectrum of host immune responses that could potentially impair development, reduce feeding success, or kill the ectoparasite. Not unexpectedly, arthropods have developed sophisticated arsenals of countermeasures that modulate or deviate host immune responses. Not only does arthropod modulation of host immunity facilitate survival in tissues or increase the likelihood of obtaining a blood meal, but it is increasingly recognized as a critical factor in pathogen transmission. Those countermeasures to host immune defenses are the topics of this review. Emphasis is placed on our current understanding of the molecular bases of those changes; the molecules responsible for host immunomodulation; contemporary approaches for studying these complex relationships; and, the potential for using this information to develop innovative vaccine-based control strategies.

Anti-interleukin-8 activity of tick salivary gland extracts

Interleukin-8 (IL-8) is one of many mammalian chemokines (chemotactic cytokines) that direct mammalian inflammatory and immune cells to sites of injury and infection. Chemokines are produced locally and act on leucocytes through selective receptors. The principal role of IL-8 is to control the movement and activity of neutrophils. To date, several tick species have been shown to modulate the production or activity of certain cytokines but none of these are chemokines. Using an IL-8 specific ELISA, we showed that salivary gland extracts (SGE) from several ixodid tick species (Dermacentor reticulatus, Amblyomma variegatum, Rhipicephalus appendiculatus, Haemaphysalis inermis and Ixodes ricinus) reduced the level of detectable IL-8. Analyses of fractionated SGE revealed one similar peak of activity for D. reticulatus, A. variegatum and R. appendiculatus; a second peak, observed for D. reticulatus and A. variegatum, differed between the two species. Using radiolabelled IL-8, SGE and peak activity fractions of D. reticulatus were shown to bind the chemokine, and to inhibit binding of IL-8 to its receptors on human granuolocytes enriched for neutrophils. The biological significance of these observations was demonstrated by the ability of SGE to inhibit IL-8 induced chemotaxis of human blood granulocytes. Future isolation and characterization of the active molecules will enable determination of their functional roles in bloodfeeding and effect on tick-borne pathogen transmission.

Comparison of the protein profiles of salivary gland extracts derived from three species of unfed and partially fed ixodid ticks analysed by SDS-PAGE

Salivary gland extracts (SGE) from unfed and 5 days fed adult female Ixodes ricinus (Linnaeus, 1758); Haemaphysalis inermis (Birula, 1895) and Dermacentor reticulatus (Fabricius, 1794) ticks were prepared. The protein content after feeding increased by 10.6, 8.7 and 6.8 times, respectively. Extracts were equilibrated to the same protein content and submitted to SDS-polyacrylamide gel electrophoresis followed by computer analysis of the scanned gels. Relative differences in protein profiles of extracts obtained from unfed and partially fed ticks were found in all species and some of them were similar in all three species used in the study. Results demonstrate that the increase of the protein content in salivary glands during the feeding does not occur proportionally. Some proteins are synthesised preferentially (67.1 kDa, 13.5 kDa) but other bands (in range of 15-16 kDa) present in the SGE derived from unfed ticks are less discernible in that of fed ticks.

Potentiation of vesicular stomatitis New Jersey virus infection in mice by mosquito saliva

Saliva of arthropod vectors can modulate vertebrate host immunological functions in many ways. To investigate if vesicular stomatitis New Jersey virus (VSNJ) infection could be potentiated by arthropod saliva, mice in three different age groups (3 days, 3 weeks, or > 8 months) were exposed to VSNJ-infected mosquitoes or were needle injected with an equivalent dose of VSNJ (titre 1.5-3 logs). Previous studies have demonstrated that VS viruses do not replicate in mice older than 3 weeks of age. Infection was monitored by examining serum for the presence of VSNJ at 2 days postinfection (PI) or for neutralizing antibody on days 7 and 14 PI. All 3-day-old mice succumbed to viral infection by mosquito transmission or delivery by injection. Ninety-four percent of the 3-week-old mice bitten by infected mosquitoes developed antibody, whereas antibody was detected in only 13% of inoculated mice. Adult mice developed neutralizing antibody (73%) when fed upon by infected mosquitoes, but only 11% developed antibody when virus was injected. Day 2 serum samples from 3-week and adult age groups were negative by virus isolation. These data indicate that mosquito mediated delivery of VSNJ exacerbates virus infection in mice older than 3 weeks.

Inhibition of the antiviral action of interferon by tick salivary gland extract

The saliva of haematophagous arthropods (e.g. mosquitoes, sandflies and ticks) contains potent immunomodulatory activities that counter their hosts' haemostatic, inflammatory and immune responses to facilitate blood-feeding. Such effects are exploited by arthropod-transmitted pathogens to promote their transmission. We investigated the ability of tick saliva to enhance arthropod-borne virus (arbovirus) transmission by determining its effect on the antiviral action of murine interferon (IFN alpha/beta). Salivary gland extract (SGE) was prepared from partially fed adult female Dermacentor reticulatus ticks that had been feeding on mice for either 3 or 5 days (SGED3 and SGED5, respectively). We demonstrated that SGE inhibits the antiviral effect of IFN as measured by a biological assay using vesicular stomatitis virus (VSV), and by two-dimensional electrophoretic analysis of the appearance of selected VSV proteins. The most pronounced effect was observed when mouse L cells were treated with SGE prior to IFN treatment. Following pretreatment with SGE, virus multiplication (which was fully blocked by IFN treatment alone) achieved yields similar to those obtained from infected cells not treated with IFN. Contemporaneous treatment, or treatment with SGE after IFN, was less effective. In parallel with these findings, formation of early viral proteins, N (nucleocapsid protein) and P (phosphoprotein), which was blocked by IFN, was detectable following pretreatment with SGE. The ability to inhibit the antiviral action of IFN was higher for SGED3 compared to SGED5. Demonstration that tick SGE can promote virus replication by suppressing the action of IFN helps explain why ticks are such efficient vectors of arboviruses.

Pathogen-tick-host interactions: Borrelia burgdorferi and TBE virus

Both tick-borne encephalitis (TBE) virus and Borrelia burgdorferi depend for their survival on a tick-borne transmission cycle involving diverse vertebrate hosts and ixodid (primarily Ixodes) tick species. Increasing evidence strongly suggests that a key factor in successful transmission is the ability of these pathogens to exploit the pharmacological properties of their vector tick's saliva. Proteins and other chemicals, secreted in tick saliva, control the vertebrate hosts' haemostatic, inflammatory and immune responses in order to facilitate blood-feeding. Such bioactive saliva molecules include immunoglobulin-binding proteins, histamine-binding proteins, natural killer cell and interferon regulators, and complement inhibitors. By unravelling the contents of tick saliva and understanding their interactions with host and pathogen, we may identify new strategies for disease control.

Immunology of the tick-host interaction and the control of ticks and tick-borne diseases

The first experimental vaccination against ticks was carried out 60 years ago. Since then, progress has been slow, although the recent commercial release of a recombinant vaccine against Boophilus microplus is significant. The nature of naturally acquired protective immunity against ticks is poorly understood, particularly in the important, domesticated ruminant hosts. Characterization of the antigens of naturally acquired immunity remains limited, although more has been achieved with 'concealed' antigens. Crucial questions remain about the true impact of tick-induced immunosuppression and the effect of immunity on the transmission of tick-borne diseases, despite some fascinating and important recent results, as discussed here by Peter Willadsen and Frans Jongejan.

Mx1-based resistance to thogoto virus in A2G mice is bypassed in tick-mediated virus delivery

The interferon-induced mouse Mx1 protein has intrinsic antiviral activity against orthomyxoviruses, including Thogoto virus. Thus, Mx1(+) A2G mice are apparently resistant to infection following needle- or tick-borne virus challenge. However, tick-borne challenge and, to a lesser degree, injection of virus mixed with tick salivary gland extract resulted in virus transmission to uninfected ticks feeding on the A2G mice. The data indicate that immunomodulatory components in tick saliva can overcome a natural antiviral mechanism.

Displaced tick-parasite interactions at the host interface

Reciprocal interactions of parasites transmitted by blood-sucking arthropod vectors have been studied primarily at the parasite-host and parasite-vector interface. The third component of this parasite triangle, the vector-host interface, has been largely ignored. Now there is growing realization that reciprocal interactions between arthropod vectors and their vertebrate hosts play a pivotal role in the survival of arthropod-borne viruses, bacteria, and protozoa. The vector-host interface is the site where the haematophagous arthropods feeds. To obtain a blood meal, the vector must overcome the host's inflammatory, haemostatic, and immune responses. This problem is greatest for ixodid ticks which may imbibe as much as 15 ml blood whilst continuously attached to their host for 10 days or more. To feed successfully, the interface between tick and host becomes a battle between the host's mechanisms for combating the tick and the tick's armoury of bioactive proteins and other chemicals which it secrets, via saliva, into the feeding lesion formed in the host's skin. Parasites entering this battlefield encounter a privileged site in their vertebrate host that has been profoundly modified by the pharmacological activities of their vector's saliva. For example, ticks suppress natural killer cells and interferons, both of which have potent antiviral activities. Not surprisingly, vector-bone parasites exploit the immunomodulated feeding site to promote their transmission and infection. Certain tick-bone viruses are so successful at this that they are transmitted from one infected tick, through the vertebrate host to a co-feeding uninfected tick, without a detectable viraemia (virus circulating in the host's blood), and with no untoward effect on the host. When such viruses do have an adverse effect on the host, they may impede their vectors' feeding. Thus important interactions between ticks and tick-borne parasites are displaced to the interface with their vertebrate host-the skin site of blood-feeding and infection.