Replacement of the F and G proteins of respiratory syncytial virus (RSV) subgroup A with those of subgroup B generates chimeric live attenuated RSV subgroup B vaccine candidates

Human respiratory syncytial virus (RSV) exists as two antigenic subgroups, A and B, both of which should be represented in a vaccine. The F and G glyc oproteins are the major neutralization and protective antigens, and the G p rotein in particular is highly divergent between the subgroups. The existin g system for reverse genetics is based on the A2 strain of RSV subgroup A, and most efforts to develop a live attenuated RSV vaccine have focused on s train A2 or other subgroup A viruses. In the present study, the development of a live attenuated subgroup B component was expedited by the replacement of the F and G glycoproteins of recombinant A2 virus with their counterpar ts from the RSV subgroup B strain B1, This gene replacement was initially d one for wild-type (,vt) recombinant A2 virus to create a wt AB chimeric vir us and then for a series of A2 derivatives which contain various combinatio ns of A2-derived attenuating mutations located in genes other than F and G, The wt AB virus replicated in cell culture with an efficiency which was co mparable to that of the,ut A2 and B1 parents. AB viruses containing tempera ture-sensitive mutations in the A2 background exhibited levels of temperatu re sensitivity in vitro which were similar to those of A2 viruses bearing t he same mutations. In chimpanzees, the replication of the wt AB chimera was intermediate between that of the A2 and B1 wt viruses and was accompanied by moderate rhinorrhea, as previously seen in this species, An AB chimeric virus, rABcp248/404/1030, which was constructed to contain a mixture of att enuating mutations derived from two different biologically attenuated A2 vi ruses, was highly attenuated in both the upper and lower respiratory tracts of chimpanzees, This attenuated AB chimeric virus was immunogenic and conf erred a high level of resistance on chimpanzees to challenge with,ct AB vir us. The rABcp248/404/1030 chimeric virus is a promising vaccine candidate f or RSV subgroup B and will be evaluated next in humans, Furthermore, these results suggest that additional attenuating mutations derived from strain A 2 can be inserted into the A2 background of the recombinant chimeric AB vir us as necessary to modify the attenuation phenotype in a reasonably predict able manner to achieve an optimal balance between attenuation and immunogen icity in a virus bearing the subgroup B antigenic determinants.