S. MOESCHLER¹, B. KRAEMER², G. ZIMMER<sup>1

1</sup>Institute of Virology and Immunology (IVI), University of Bern, CH-3147 Mittelhäusern, Switzerland, ²Bundesinstitut für Impfstoffe und biomedizinische Arzneimittel, Paul-Ehrlich-Institut, 63225 Langen, Germany

Rabies virus (RABV) is a zoonotic virus of the genus Lyssavirus that causes fatality rates of nearly 100% in unvaccinated individuals. Dog-mediated human rabies is responsible for at least 55 000 human deaths annually, mostly affecting rural poor communities in Asia and Africa. Rabies infection is 100% preventable and mass dog-vaccination campaigns are implemented in order to eliminate the disease at its source. The quantification of virus neutralizing antibodies (VNA) against RABV is important to assess the efficacy of anti-rabies vaccines and should be coupled to mass vaccination. The conventional methods for the titration of rabies VNA rely on propagation-competent RABV and require enhanced biosafety measures that can hinder their implementation in poor areas. Moreover, these tests are time- consuming, expensive and limited to the detection of VNA directed against the classical RABV (genotype 1). Here we developed a pseudotype-based neutralization test that makes use of Vesicular Stomatitis Virus (VSV) replicons lacking the VSV glycoprotein. VSV replicons are trans-complemented with RABV glycoproteins on helper cell lines in a process called pseudotyping. The use of propagation- incompetent VSV pseudotypes alleviates the enhanced biosafety measures required for the conventional neutralization setups. Furthermore, the readout is facilitated by the expression of reporter genes (GFP, luciferase) in the VSV vector. The pseudotype-based neutralization test turned out to be at least as sensitive as the conventional tests and results strongly correlated (r=0.9). Finally, this novel method is highly versatile as pseudotyping with various lyssavirus glycoproteins extended its use to serological studies on other lyssavirus genotypes.