Recombinant respiratory syncytial virus that does not express the NS1 or M2-2 protein is highly attenuated and immunogenic in chimpanzees

Mutant recombinant respiratory syncytial viruses (RSV) which cannot express the NS1 and M2-2 proteins, designated rA2 Delta NS1 and rA2 Delta M2-2, re spectively, were evaluated as live-attenuated RSV vaccines. The rA2 Delta N S1 virus contains a large deletion that should have the advantageous proper ty of genetic stability during replication in vitro and in vivo. In vitro, rA2 Delta NS1 replicated approximately 10-fold less well than wild-type rec ombinant RSV (rA2), while rA2 Delta M2-2 had delayed growth kinetics but re ached a final titer similar to that of rA2. Each virus was administered to the respiratory tracts of RSV-seronegative chimpanzees to assess replicatio n, immunogenicity, and protective efficacy. The rA2 Delta NS1 and rA2 Delta M2-2 viruses were 2,200- to 55,000-fold restricted in replication in the u pper and lower respiratory tracts but induced a level of RSV-neutralizing a ntibody in serum that was only slightly reduced compared to the level induc ed by wild-type RSV. The replication of wild-type RSV in immunized chimpanz ees after challenge was reduced more than 10,000 fold at each site. Importa ntly, rA2 Delta NS1 and rA2 Delta M2-2 were 10-fold more restricted in repl ication in the upper respiratory tract than was the cpts248/404 virus, a va ccine candidate that retained mild reactogenicity in the upper respiratory tracts of 1-month-old infants. Thus, either rA2 Delta NS1 or rA2 Delta M2-2 might be appropriately attenuated for this age group, which is the major t arget population for an RSV vaccine. In addition, these results show that n either NS1 nor M2-2 is essential for RSV replication in vivo, although each is important for efficient replication.