Dissecting differences and similarities in the host response to Rabies virus and other lyssaviruses in the Syrian hamster model

Rabies is an acute and almost always fatal encephalomyelitis caused by the infection with viruses belonging to the genus Lyssavirus, which accounts for 60,000 reported human deaths annually. The genus prototype, Lyssavirus rabies (RABV), is responsible for most of the human and animal rabies cases worldwide, having established transmission cycles in different mammal hosts, thus occupying different geographical and ecological niches. In addition to RABV, the International Committee on the Taxonomy of Viruses (ICTV) currently recognize seventeen lyssavirus species. Non RABV-lyssaviruses most likely co-evolved with a specific host, mainly one or two sibling bats per viral species, causing rare spillover events in non-flying mammals, including humans. The current knowledge about the mechanisms underlying the pathogenesis and host interaction of lyssaviruses infection is mostly based on studies on RABV, and little is known about other members of the genus. The aim of this study is to characterize and compare the transcriptomic profile of Syrian hamsters brain after intramuscular challenge with three different lyssaviruses (Lyssavirus rabies, Lyssavirus duvenhage - DUVV and Lyssavirus caucasicus - WCBV), in order to investigate the interplay between lyssaviruses and the infected host. We first infected three groups of five Syrian hamsters intramuscularly with a high dose of each virus and observed the animals daily for the development of clinical signs. All infected hamsters developed neurological signs of rabies and were humanely euthanized. We then collected the central nervous system (CNS) for RNA extraction and immunostainings on tissue slices. We found that all the infected hamsters had detectable viral RNA and antigen in their brains, all these findings confirming productive infection with all the challenged viruses. We then performed RNA sequencing and transcriptomic analysis from CNS-extracted RNA, in order to highlight the global mRNA profile and evaluate the differential gene expression between infected and control animals. Our data showed that the three lyssaviruses promoted the activation of classical processes associated with anti-viral response, with some differences between RABV and non-RABV lyssaviruses. We found that DUVV and WCBV promoted the activation of the adaptive immune response, differently from RABV, and that DUVV specifically boosted lymphocyte activation and the deposition of a higher amount of immunoglobulins in the CNS compared to RABV or WCBV. This study describes for the first time the transcriptomic profiles elicited in response to WCBV infection in the Syrian hamster, as a proof of pathogen-host interaction in occasional hosts. Moreover, findings from our study firstly testify the activation of adaptive immunity mechanisms that might partly justify different degrees of viral neuroinvasiveness and pathogenicity, ultimately leading to the occurrence of abortive infections in the spillover hosts, paving the way to a deeper understanding of the ecopathology of non-RABV lyssaviruses in nature.