Chimeric bovine respiratory syncytial virus with glycoprotein gene substitutions from human respiratory syncytial virus (HRSV): Effects on host rangeand evaluation as a live-attenuated HRSV vaccine

We recently developed a system for the generation of infectious bovine resp iratory syncytial virus (BRSV) from cDNA. Here, we report the recovery of f ully viable chimeric recombinant BRSVs (rBRSVs) that carry human respirator y syncytial virus (HRSV) glycoproteins in place of their BRSV counterparts, thus combining the replication machinery of BRSV with the major antigenic determinants of HRSV. A cDNA encoding the BRSV antigenome was modified so t hat the complete G and F genes, including the gene start and gene end signa ls, were replaced by their HRSV A2 counterparts. Alternatively! the BRSV F gene alone was replaced by that of HRSV Long. Each antigenomic cDNA directe d the successful recovery of recombinant virus, yielding rBRSV/A2 and rBRSV /LongF, respectively. The HRSV G and F proteins or the HRSV F in combinatio n with BRSV G were expressed efficiently in cells infected,vith the appropr iate chimeric virus and were efficiently incorporated into recombinant viri ons. Whereas BRSV and HRSV grew more efficiently in bovine and human cells, respectively, the chimeric rBRSV/A2 exhibited intermediate growth characte ristics in a human cell line and grew better than either parent in a bovine line. The cytopathology induced by the chimera more closely resembled that of BRSV. BRSV was confirmed to be highly restricted for replication in the respiratory tract of chimpanzees, a host that is highly permissive for HRS V. Interestingly, the rBRSV/A2 chimeric virus was somewhat more competent t han BRSV for replication in chimpanzees but remained highly restricted comp ared to HRSV. This showed that the substitution of the G and F glycoprotein s alone was not sufficient to induce efficient replication in chimpanzees. Thug the F and G proteins contribute to the host range restriction of BRSV but are not the major determinants of this phenotype, Although rBRSV/A2 exp resses the major neutralization and protective antigens of HRSV, chimpanzee s infected with this chimeric virus were not significantly protected agains t subsequent challenge with wild-type HRSV. This suggests that the growth r estriction of rBRSV/A2 was too great to provide adequate antigen expression and that the capacity of this chimeric vaccine candidate for replication i n primates will need to be increased by the importation of additional HRSV genes.