Among arboviruses, some Flaviviruses, Alphaviruses and Bunyaviruses like dengue (DENV), chikungunya (CHIKV) and Rift Valley fever (RVFV) viruses have a large public health impact. Vaccines and antiviral drugs are so far dramatically lacking. Our efforts focus on the pathogenesis of hemorrhagic fevers associated to RVFV and DENV in mammals and on the biology of the mosquito vectors infected by DENV or CHIKV. Our Research unit hosts also the National Reference Center for Borrelia with the aim to extend such studies to the vector borne Lyme disease.

Role of nonstructural proteins in pathogenesis

The RVFV nonstructural protein NSs is a major factor of virulence acting as a multifunctional protein through interaction with cellular partners. It inhibits cellular transcription using two different mechanisms: it prevents assembly of TFIIH through the sequestration of p44 which leads to a drastic inhibition of Pol I and II dependent transcription and it maintains the IFN-ß promoter in a repressed state through the formation of Sin 3A repression complexes interacting with NSs. The NSs protein antagonizes also the functions of PKR by inducing its degradation. More recently, we showed that NSs interacts with chromosomal pericentromeric gamma satellite sequences and induces nuclei abnormalities (Mansuroglu et al 2010). To better understand the pathogenesis, we collaborated with the Reseach Unit "Génétique fonctionnelle de la souris" (JJ. Panthier) and found that, in contrast with Balb/C mice, the MBT mouse is very sensitive to RVFV infection. This finding correlates with an increased viral replication and a delayed and partial type I IFN response in MBT mice (Do Valle et al 2010).

Dengue nonstructural protein NS1 participates to the intracellular stages of viral replication and is associated to the plasma membrane where it modulates the complement system. In addition, it is secreted by infected cells and circulates in the plasma of dengue virus (DENV)-infected patients. We have shown that the NS1 protein represents a hallmark of acute DENV infections and developed a diagnostic assay based on NS1 antigen capture in plasma. The corresponding Platelia dengue NS1 kit from Bio-Rad is now used worldwide. We have further characterized the biological properties of secreted NS1 and showed that it is a viral high-density lipoprotein, pointing to a major role of this protein in the occurrence of severe vascular dysfunction. Our findings should pave the way to novel antiviral therapeutic approaches.

Arthropod vector infections

Emergence of chikungunya virus (CHIKV) has been regularly associated with DENV co-infection. We showed that experimentally infected Ae. albopictus are able to replicate simultaneously both arboviruses and to deliver infectious particles concomitantly in their saliva (Vazeille et al2010). A differential two-dimensional analysis of the proteome of Aedes aegypti midguts infected by dengue-2 and chikungunya viruses showed that 180 spots were modulated by the two viruses, dengue-2 virus being a better modulator (Tchankuo-Nguetcheu et al 2010). The proteins regulated differentially in response to viral infection include structural, redox, regulatory proteins, and enzymes for several metabolic pathways, some of which are probably involved in cell protection and others like transferrin, hsp60 and alpha glucosidase, may favor virus survival, replication and transmission, suggesting a subversion of the insect cell metabolism by the arboviruses.

Aedes albopictus mosquito is naturally infected by two strains (wAlbA and wAlbB) of an endosymbiotic bacterium Wolbachia which are thought to have spread by manipulating their hosts. CHIKV replication in these mosquitoes coincides with a decrease of Wolbachia densities (Mousson et al 2010). Since both Wolbachia and CHIKV multiply in the cytoplasm, the presence of Wolbachia would reduce the pool of cytoplasmic amino acids and interfere with the viral cycle. These results highlight the subtle equilibrium established between Ae. albopictus, Wolbachia and CHIKV making difficult to generalize the design of Wolbachia based strategies to control the transmission of CHIKV.

Borreliosis, another vector borne disease

In 2010, the NRC for Borrelia had an active surveillance of Lyme Borreliosis and Ixodes ricinus populations in France. The NRC also analyzed the consequences of the introduction of a new host in Ile-de-France forests, the Siberian chipmunk. Concerning relapsing fevers, both O. erraticus and O. normandi were found in Tunisia and the high B. crocidurae infection rate found in O. erraticus highlights the risk of TBRF transmission in the southern part of the country. Borrelia samples from the NRC allowed identifying a new Borrelia species, Borrelia kurtenbachii by multilocus sequence analysis of Borrelia bissettii strains from North America.

Keywords: arbovirus, pathogenesis, virus-host interactions, mosquito, vectorial competence

Bouattour A, Garnier M, Youmna M, Sarih M, Gern L, Ferquel E, Postic D, Cornet M. 2010. Borrelia crocidurae infection of Ornithodoros erraticus (Lucas, 1849) ticks in Tunisia. Vector Borne Zoonot Dis. 10 (9): 825-830.

do Valle T, A Billecocq, L. Guillemot, R. Alberts, R. Geffers, K. Schughart, M. Bouloy, X. Montagutelli and JJ Panthier. 2010. A New Mouse Model Reveals a Critical Role of Host Innate Immunity in Resistance to Rift Valley Fever. J. Immunol 185(10):6146-56.

Mansuroglu Z., T. Josse, J. Gilleron, A. Billecocq, P. Leger³, M. Bouloy and E. Bonnefoy. 2010. Non structural NSs protein of Rift Valley Fever Virus interacts with pericentromeric DNA sequences of the host cell inducing chromosome cohesion and segregation defects. J Virol 84 928-939

Tchankouo-Nguetcheu S., Huot Khun, Laurence Pincet, Pascal Roux, Muriel Bahut, Michel Huerre, Catherine Guette, Valérie Choumet. 2010. Differential protein modulation in midguts of Aedes aegypti infected with chikungunya and dengue-2 viruses. PlosOne October 2010, 5, Issue 10, e13149.

Mousson L., Martin E., Zouache K., Madec Y., Mavingui P. and Failloux A.-B. 2010. Wolbachia modulates Chikungunya replication in Aedes albopictus. Molecular Ecology 19(9): 1953-1964.