Mechanisms involved in anti-viral immune response of European bats against Lyssaviruses

Bats (Chiroptera) form the second largest order of mammals and with over 1,250 species, they represent about 20% of all mammalian species worldwide. They are the only mammals with true and sustained flight and distributed all over the world except the arctic regions. Moreover, bats entered specific ecological niches and with their food spectra, they reduce different arthropod populations as well as disperse seeds and pollen of plant species in various regions and habitats. Bats also have a crucial role in spreading high-pathogenic and zoonotic viruses, harbor in general more viruses (zoonotic and non-zoonotic), and, related to the species, number even more than rodents. However, clinical symptoms of viral diseases are rarely reported in bat communities. Also seroconversions after infection were not reported for a variety of viruses found in bats. Since the incidence of virus-positive bats estimated in passive surveillance studies is usually very low, it is a question how such viruses can use bats as reservoir hosts. There is obviously a special evolutionary relationship between the pathogens and bats as hosts, which are based on possibly physiologic adaptations also in resistance and immunity. In this thesis, the two lyssaviruses, European Bat Lyssavirus 1 and 2 (EBLV-1 and -2) were chosen as a model to investigate the immune response of European bats against viral infection in vitro. Lyssaviruses are the causative agents of rabies, a fatal zoonotic disease with neurotropic characteristics. One main question to investigate was in which way bats act as reservoir host and developed a high disease resistance. The present thesis is based on three hypotheses about innate immune response against lyssavirus infection: A) In bats specific peripheral resistance mechanisms evolved which reduce the risk of systemic viral infection after a hypothesized airborne transmission and infection via nasal epithelium supported by the social structure of and communication within bat communities. B) The co-evolution of EBLV and the innate resistance of bats resulted in a very effective type I interferon response to inhibit a systemic lyssavirus infection. C) The specific physiology of body temperature of bats with daily torpor depresses the viral replication but favours the type I interferon response. To analyze the interferon-based resistance mechanisms, the type I interferon (IFN) genes of two European bats species (Eptesicus serotinus and Myotis myotis) were cloned and sequenced. Using established cell lines from the respiratory nasal epithelium (MmNep), olfactory nasal epithelium (MmNol), and Bulbus olfactorius brain (MmBr), the type I IFN response along a possible airborne infection route was investigated. The anti-viral effects and induction of IFNs/interferon stimulated genes (ISGs) in each cell line were also investigated in detail after infection in vitro. Finally, the influence of different temperatures on lyssavirus replication was analyzed in cell culture experiments. The results indicated that (a) along the hypothesized airborne infection route the susceptibility for lyssavirus infections is decreased, (b) the type I IFN activity in contrast is increased contributing to a limitation of lyssavirus replication and (c) an obvious influences of varying cultivation temperatures on the resistance against lyssavirus infections, which favor the IFN response and repressing lyssavirus replication. The result from these in vitro studies supports the hypothesis of a special co-evolution between lyssaviruses and bats. However, in vivo studies on the relevance in infected animals are missing so far. This model could also explain the generally limited pathogenicity of bat-associated viruses.