The efficacy of current rabies vaccines and novel Nanobody®-based antivirals against highly pathogenic phylogroup -1 and -2 members of the Lyssavirus genus

Current rabies virus (RABV) vaccines and RABV immunoglobulins (RIG) confer protection against phylogroup 1 lyssaviruses. However, they offer partial or no protection against phylogroup 2 lyssaviruses and the high costs of such treatments, especially RIG, can result in poor availability/coverage. Therefore, alternative intervention strategies are required to address these issues. Using a pseudotype-based neutralisation assay (PNA) we first evaluated the efficacy of commercial RABV vaccines against each lyssavirus species. Existing RABV vaccines stimulated high neutralising serum antibody titres against RABV isolates but did not confer protection against antigenically divergent lyssaviruses. Subsequently, selection of anti-rabies llama (Lama glama) heavy chain antibody fragments (VHH/Nanobody®) was undertaken from immune libraries and tested for neutralising potency against a panel of geographically divergent RABV isolates and a range of lyssaviruses using live virus (RFFIT) and PNA. Strong neutralisation of all RABV isolates and the majority of lyssavirus species tested were observed (in the nanomolar range). Bivalent constructs greatly improved neutralising potency. Improved biologicals are required to counter the public health threat posed by these highly pathogenic viruses and Nanobodies® offer an attractive alternative