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Ebola virus interplay with host innate immune system as a target for therapeutics against Ebola Virus
Disease
Enzo Tramontano
Laboratory of Molecular Virology, Biomedical Section, Dept. of Life and Environmental Sciences, University
of Cagliari, Cagliari, Italy.
Ebola Virus (EBOV) is the etiological agent of Ebola Virus Disease (EVD), a severe hemorrhagic fever in
humans with case fatality rates of up to 90%, for which there are currently neither licensed vaccines nor
antivirals available. Since its original identification, EVD has been rare and endemic in the Sub-Saharan
Africa, even though it has re-emerged in the last decade with an increase of its geographical incidence and
human outbreaks episodes. Since early 2014, the greatest EVD outbreak is on-going in 4 western Africa
countries with more than 8500 deaths and more than 22000 cases, confirming that EBOV epidemics are
highly unpredictable.
The high EVD lethality has been related to the virus ability to efficiently suppress the host innate immune
response, which begins with the recognition of viral dsRNA, a replicative intermediate, by the cytoplasmic
pattern recognition receptor RIG-I that induces type I interferon (IFN) response. This recognition is
prevented by the dsRNA binding properties of the EBOV VP35 protein that mimics RIG-I 5’-triphosphate (5’ppp) dsRNA ends recognition. EBOV VP35 also interferes with the activation of IRF-3 and inhibits PACTinduced RIG-I ATPase activity, preventing PACT interaction with and activation of RIG-I, allowing EBOV to
evade innate immunity activation. EBOV VP35 is a crucial determinant of EBOV virulence and pathogenesis
and, in fact, mutations in VP35 dsRNA binding domain that reduce its IFN-antagonist capabilities are
associated to loss of EBOV virulence. Considered the importance of the interplay between EBOV and innate
immune system activation, it is possible to develop therapeutic agents targeting either EBOV components
that hide the virus to the innate immune system, or innate immune system components to trigger an
efficacious immune response overcoming the virus action.
Given the specificity EBOV VP35 recognition of viral 5'-ppp dsRNA end-capping, also showed by
crystallographic studies, we are currently pursuing VP35 dsRNA binding as target for drug design. To this
aim we i) established novel methods measuring VP35 ability to bind dsRNA in vitro and determined its
kinetic parameters; ii) established a new luciferase reporter gene assay assessing EBOV VP35 inhibition of
RIG-I-mediated IFN-β induction by viral dsRNA; iii) performed site directed mutagenesis studies
investigating the role of single VP35 amino acid residues involved in the 5'-ppp dsRNA end-cap binding; iv)
defined critical amino acid residues describing a pocket in the VP35 dsRNA domain that can be exploited as
target for development of antiviral agents.