Crimean-Congo hemorrhagic fever virus : interferon-induced antiviral mechanisms and immune evasion strategies

Abstract

The Bunyaviridae family is the largest virus family consisting of more than 350 viruses. Despite the increasing knowledge regarding the biology of members of this family, little is known about Crimean-Congo hemorrhagic fever virus (CCHFV), a virus classified within the Nairovirus genus and the causative agent of a severe hemorrhagic fever in humans with high mortality. The virus is transmitted through the bites of Ixodid ticks or by direct contact with blood or tissues from infected animals. Nosocomial transmission among caregivers has also been reported. CCHFV is endemic in large parts of the world and is regarded as a public health problem in these regions. Research investigations have been limited by the requirement for specialized BSL-4 containment laboratories with high security and the lack of appropriate animal models. As a consequence, the factors determining the pathogenesis of CCHFV are largely unexplored.

Type I Interferons (IFNs) form the first line of defense against a virus infection and is indispensable for the innate immune response in vertebrates. A number of studies have demonstrated that human IFNs have an antiviral effect against a variety of viruses. In this thesis the antiviral effect of IFNs against CCHFV was investigated. In paper II it was demonstrated that IFNs have an antiviral effect against CCHFV and in paper I and II the human MxA protein was identified as a major contributor to these observed interferon-induced effects. However, MxA was found not to be the sole determinant of the observed decrease in virus replication following IFN treatment. The preliminary findings suggest that PKR also contributes to the antiviral effects and most likely other proteins are involved as well.

The importance of the IFN system is illustrated by the numerous viral encoded gene products with IFN antagonistic properties targeting almost all components of the IFN response. Often these antagonists are multifunctional proteins acting on several different pathways of the immune response. In this thesis it was investigated if CCHFV, in conformity with most other viruses, had the ability to interfere with host immune response mechanisms. In paper III it was demonstrated that CCHFV delays the early immune responses, most likely by interfering with the IRF-3 pathway. This in turn has the consequence that secretion of IFNs following infection is a relatively late event.

Recently it was shown that triphosphates in the 5 terminus of viral RNA are key determinants for detection by RIG-I. In paper IV it was shown that some selected viruses, among them CCHFV, avoids RIG-I recognition by processing of their 5 termini to generate monophosphates. These findings not only show that CCHFV can interfere with host antiviral signalling, but also identifies a new strategy by which viruses avoid activation of the innate immune response. If the observed delay in immune responses following a CCHFV infection is the result of the RIG-I avoidance or the combination of avoidance and the expression of an interferon antagonist remains to be investigated.

Taken together, the findings in this thesis show that CCHFV is sensitive to the actions of IFNs and IFN-induced antiviral proteins. However, the virus avoids detection and interferon induction early in the infection and once the virus is replicating, IFN has little effect on virus replication. A better understanding of interactions between the virus and their hosts will facilitate in the future development of improved measurements in the fight against viral infections.