Defining the antigenic requirements for pan-lyssavirus neutralisation

Rabies, the first zoonotic disease associated with bats, remains one of the most feared viruses known to man and causes approximately 59,000 deaths a year. With no medical intervention prior to symptom onset, rabies-induced encephalitis is invariably fatal. Alongside rabies virus, numerous related lyssaviruses exist. Also capable of causing fatal disease, these lyssaviruses pose a threat to human and animal health. Since 2000, the genus has grown from eight fully characterised species to seventeen fully characterised species and two tentative species. These viruses are genetically and antigenically categorised into phylogroups according to the predicted level of vaccine protection, with protection from current vaccines and therapeutics afforded against phylogroup I but not II or III. The continued discovery of novel lyssaviruses globally, warrants an in-depth assessment of the cross-protection of current vaccines and the protective titre required, to inform occupationally high-risk groups (e.g., scientists, bat workers and speleologists). Current evidence suggests that for a protective neutralising antibody response against rabies virus, a neutralising antibody titre of 0.5 IU/ml is sufficient. However, studies using cross protection assays have suggested that for protection against viruses in phylogroup I, 10-fold greater titres are required. Indeed, this study suggests that a minimum titre of 5 IU/ml is required for the neutralisation of all phylogroup I lyssaviruses in vitro and a minimum titre of 1 IU/ml is required for the neutralisation of novel phylogroup I viruses, Taiwan bat lyssavirus and Kotalahti bat lyssavirus, in vivo. Additionally, experimental data in this study has shown that the antibody repertoire of lyssavirus-specific sera exhibits a strong level of intra-phylogroup neutralisation and very limited inter-phylogroup neutralisation. The data also suggests that a minimum of five distinct lyssavirus glycoproteins, rabies virus, Lagos bat virus lineage D, Ikoma lyssavirus, Lleida bat lyssavirus, and West Caucasian bat virus, would have to be included in any pan-lyssavirus vaccine. Promising cross-protective chimeric glycoproteins were designed and cloned into a rabies vaccine strain full length backbone. Reverse genetics techniques were used to enable virus rescue of these constructs. Where successful, the degree of vaccine-induced protection was assessed in vivo.