Science | vhit

Challenge of arbovirus

The burden of mosquito-borne viral disease is profound. In recent years there has been a rapid increase in both the incidence and geographic range of such diseases. Globalisation has allowed more opportunities for the spread of infections as inter-continental commerce, travel and migration increases. In addition, climate change is enlarging the geographic range of endemic viruses and their vectors. Medically important viruses spread by arthropods (arboviruses) include the chikungunya, yellow fever and dengue viruses, which together infect millions of people each year. Aedes mosquitoes are the primary vector. Temperate climates once confident in their isolation from substantial arbovirus epidemics are now affected, as witnessed by the recent outbreaks of these viruses. Arboviruses also infect economically important farm animals and thus pose an increasing threat to both our food security and human health. VHIT are investigating how viruses spread my mosquitoes cause disease in humans and agriculturally important animals.

Arboviruses are an exceptionally large (>500) and diverse group of viruses. This heterogeneity, combined with the inability to accurately predict the identity of future arboviral threats, makes developing and stockpiling specific drugs and vaccines very challenging. However, something all mosquito-borne viruses have in common is their site of inoculation in mammals; at mosquito bite sites. We suggest that treatment modalities that target this common, shared aspect of their life cycle may prevent both the development of subsequent disease and the onward transmission to uninfected mosquitoes. Many studies have shown that the early events of infection are critically important for survival of the host, with a close relationship between early peripheral virus burden and mortality. However, there remains a critical need to understand the determinants of early peripheral virus burden.

VHIT are undertaking a programme of work that will study the complex immunological interplay between arboviruses, their arthropod vectors and their mammalian hosts. We have a particular interest in understanding the response of mammalian host to infection/mosquito biting in the skin, and how virus spread around the body to cause disease. The eventual aim of this work is to develop novel therapies and identify putative biomarkers that will aid diagnosis and the risk stratification of infected patients.

This coordinated approach aims to determine:

inflammatory processes that occur at inoculation sites following infection / mosquito biting,
the impact that these events have on the development of subsequent disease,
the route by which virus spreads from the inoculation site to the systemic circulation.

In doing so we hope to:

define new mechanisms of innate immune function during the early events of infection,
develop highly relevant model systems for studying innate immune function and viral pathogenesis,
determine the efficacy of a clinically relevant post-exposure prophylaxis to prevent development of arboviral disease.

Why is it important to study arbovirus infections?

Chikungunya: an example of an emerging infectious disease spread by mosquitoes.

Chikungunya is an emerging cause of inflammatory arthritis affecting millions of people worldwide. In humans, the medical importance of arboviruses lies in their ability to induce haemorrhagic fever, encephalitis or arthritis. The importance of understanding arbovirus-induced arthritis has increased substantially in recent years with an expanding chikungunya epidemic in the Americas, Indian and South East Asia. Chikungunya is caused by infection with the chikungunya virus (CHIKV) and is characterised by an initial acute febrile illness, followed by a severe polyarthralgia in 90% of those affected. Recent outbreaks have included those in the island of La Reunion where over a third of the circa 750,000 inhabitants were rapidly afflicted in 2005, followed by spread of the virus to the Indian sub-continent. Recently CHIKV has also emerged in the Americas to infect over 1 million people and is continuing to expand. The Aedes mosquito, whose range is rapidly increasing in Europe and America, is the major vector. Furthermore, CHIKV has adapted to new mosquito vectors, which has resulted in local transmission in Italy, France and Spain. Risk of infection with CHIKV and ensuing arthritis is thus a rapidly emerging reality for millions of people. There are no effective treatments of vaccines available. Therefore CHIKV is an increasingly important cause of arthritis worldwide. CHIKV is in the Semliki Forest virus serogroup of alphaviruses (Togaviridae) and is one of six alphaviruses that can cause human joint disorders. Others include Sindbis, Ross River (RRV), o’nyong-nyong, and Mayaro viruses.

Arboviruses pose an increasing threat to our food security

The ability of arboviruses to mutate rapidly allows them to adapt to new vectors facilitating epidemics in new geographical locations. Europe, once confident in its isolation from substantial arbovirus epidemics is now at risk, as witnessed by the recent spread of viruses such as Bluetongue across Northern Europe, Tick borne encephalitis virus in Eastern Europe and West Nile, and Crimean-Congo haemorrhagic fever viruses in Southern Europe. The Bluetongue virus epidemics are perhaps the most striking recent example of this phenomenon. Bluetongue is spread by biting midges and infects economically important ruminants. Outbreaks in continental Europe have infected millions of animals and inflicted substantial economic hardship. In developing countries, tick-borne African swine fever virus and Nairobi sheep disease virus continue to be significant agricultural problems. In addition, Rift Valley fever virus, which is transmitted by mosquitoes, can cause severe disease in both farm animals and humans, and is endemic to parts of Africa and the Middle East.

Thus, these diseases are both numerous and damaging; affecting both economically important livestock and human health. There are no effective treatments available for most arbovirus infections and their associated diseases, although vaccines do exist for some, such as some strains of Bluetongue virus. Whilst it is inevitable that future epidemics of infection will occur, it is hard to predict the nature of the particular virus, the form of the outbreak or its timing. We suggest that it is essential we understand the basic principles by which these infections are spread so that we can apply this knowledge as needed. In order to better predict future outbreaks and pre-emptively develop novel therapies, there is an acute need to understand the complex biological interplay between the virus, their arthropod vectors and their mammalian hosts. Understanding these processes better will enable us to identify the most relevant aspects for disease control and prevention.

Model systems

A central aim of our work is to develop new in vivo model systems that better recapitulate the most relevant aspects of natural infection with mosquito-borne viruses. To do this we use both model viruses in addition to key human pathogens. We primarily use Semliki Forest Virus as a model virus as it has a number of unique advantages; (1) it is highly related to several important human pathogens, (2) it can be easily manipulated genetically to express marker proteins (e.g. eGFP) and antigens (e.g. OVA), (3) it can infect Aedes mosquito cell lines, and (4) is a category II pathogen, which means it can be used in most laboratories. Key findings, once elucidated, are further studied using important human pathogens with help of our collaborators that have high containment facilities.

Our work focuses on utilising cutting-edge cellular, molecular, systems biology and in vivo-based approaches to understand innate immune processes occurring early during infection, at the site of infection, and how this affects development of disease at distal sites. Because of this, we have developed a novel “naturalised” in vivo mouse model that mimics the most relevant aspects of human infection by virus-infected mosquitoes, with the aim of recapitulating clinically relevant aspects of human and animal disease. An important aspect of this model is the inclusion of biting Aedes aegypti mosquitoes that creates a bite site into which we introduce defined doses of arbovirus. By including mosquito bite inflammation in our model system, it better recapitulates natural infection that is transmitted by mosquitoes.

Chemokines and leukocyte migration

Our work is inter-disciplinary and combines three related but rarely connected areas of research; chemokine-mediated immunobiology, molecular virology and arthropod vector biology. By merging these expertise from these discplines, we aim to uncover fundamental knowledge on arthropod-mammalian interactions and how these processes affect the early stages of arthropod-borne virus infection of mammals. One area that is not well understood is how arboviruses initially establish infection in their mammalian host and the role of innate immunity, chemokines and inflammation in preventing and perhaps facilitating this. Chemokines are a large family of cytokines that help orchestrate immune responses by controlling the migration leukocytes. They have pivotal functions in the immune system, without which coordinated immune responses would not occur. Their importance in virus infections is underlined by the presence of decoy chemokines and chemokine receptors in the genomes of a number of viruses.

We hope that our work is providing new insights that are defining fundamental processes and functions of innate immunity, virology and arthropod vector biology in the initiating stages of arboviral infections, an important class of emerging infections that constitute an increasing threat to animal and human health. By studying the fundamental mechanisms underpinning these processes we can better understand the complex biological interplay between mammals, viruses and their arthropod vectors.