Rabies and rabies-related viruses can infect all warm-blooded animals where they cause fatal encephalomyelitis. In Europe, apart from classical rabies mainly present in foxes, the European bat lyssaviruses (EBL) 1 and 2 have been isolated and, sporadically, they have been responsible for fatal cases in humans. The commercial vaccines which protect against classical rabies cover only partially these rabies-related viruses, whose potential importance as a reservoir for terrestrial rabies cannot be ruled out. In this work, two approaches of immunization against rabies have been investigated for their capacity to broaden the protection against the European bat lyssaviruses: immunization with recombinant rabies proteins and immunization with "naked" DNA (DNA-based immunization). Recombinant rabies glycoprotein produced by the baculovirus expression system only induces moderate levels of neutralizing antibodies. The addition of recombinant nucleoprotein has an adjuvant effect on the production of virus neutralizing antibodies concerning classical rabies. In opposition to several publications, neither a cross-help effect or priming nor an adjuvant effect concerning the EBLs is observed. The approach of recombinant proteins has therefore turned out to be dissatisfactory for our purposes. On the contrary, DNA-based immunization with a plasmid coding for a hybrid glycoprotein (pEBL1/PV) induces a potent immune response. This hybrid construction contains: 1) the aminoterminal half of the EBL 1 glycoprotein including the antigenic site II and 2) the carboxyterminal half of the Pasteur virus (PV) glycoprotein which comprises the antigenic site III. pEBL1/PV is able to induce T helper cells and virus neutralizing antibodies and to raise a protection against both parental viruses. According to previous results pEBL1/PV should protect as well against the EBL 2; it can therefore be proposed as a possible vaccine against all European lyssaviruses. A plasmid coding for a truncated glycoprotein (pPVIII) containing only the carboxyterminal half with the site III is able to induce a T helper cell response at a similar level as the "full" glycoprotein. The B cell epitope is only functional in a full glycoprotein (pEBL1/PV) where it induces a similarly high level of protection against PV as the EBL1 site II against EBL1 thus demonstrating the important role of the site III region in rabies immunity. A comparison between the in vitro expression of different glycoprotein constructions and their immunogenicity in vivo shows that the induction of virus neutralizing antibodies depends on the correct folding of the glycoprotein, whereas even incorrectly folded glycoproteins are able to induce a potent Th cell response. Thus this model of double analysis can be used to study the relation between the quality of the in vitro expression and the immunological function in vivo of rabies glycoprotein.
