COLD PASSAGED, TEMPERATURE-SENSITIVE MUTANTS OF HUMAN RESPIRATORY SYNCYTIAL VIRUS (RSV) ARE HIGHLY ATTENUATED, IMMUNOGENIC, AND PROTECTIVE IN SERONEGATIVE CHIMPANZEES, EVEN WHEN RSV ANTIBODIES ARE INFUSED SHORTLY BEFORE IMMUNIZATION

A cold-passaged (cp) temperature-sensitive (ts) RSV mutant, designated RSV cpts-530, which possesses host-range mutations acquired during 52 passages at low temperature in bovine tissue culture and one or more ts mutations induced by chemical mutagenesis (shut-off temperature 39 degrees C) was found previously to be tenfold restricted in its replication in mice as compared to wild-type virus and stable genetically in nude mice. In the current study, we introduced additional attenuating mutations, such as small-plaque (sp) or ts mutations, into cpts-530 by chemical mutagenesis with 5-fluorouracil, with the intent of obtaining derivatives of cpts-530 that were move attenuated in mice or chimpanzees and that were more stable genetically following replication in vivo. Fourteen mutants of RSV cpts-530 which had acquired an additional ts mutation were identified and found to be more restricted in replication in BALB/c mice than the cpts-530 parental strain. One mutant, designated cpts-530/1009 (shut-off temperature 36 degrees C), was 30 times more restricted in replication in the nasal turbinates of mice and threefold more restricted in the nasopharynx of seronegative chimpanzees than its cpts-530 parent. Like its parent, this mutant was highly restricted (30 000-fold) in replication in the lower respiratory tract of chimpanzees even following direct intratracheal inoculation. The cpts-530 and cpts-530/1009 mutants exhibited a high level of stability of the ts phenotype during replication in chimpanzees. The immunogenicity and protective efficacy of the cpts-530/1009 mutant and that of two other previously described candidate live attenuated RSV vaccines were compared in seronegative chimpanzees, some of whom were pretreated with RSV immune globulin by the intravenous route to simulate the condition of the very young infant who possesses passively acquired maternal antibodies. The three candidate vaccine strains were immunogenic and induced significant resistance to RSV challenge in both groups of chimpanzees. Interestingly, the chimpanzees infused with RSV antibodies prior to immunization were primed more effectively for an unusually high serum neutralizing antibody response to infection with challenge virus than chimpanzees which did Mot receive such antibodies. This high level booster response occurred despite marked restriction of replication of the challenge virus. Thus, the cpts-530/1009 virus and related mutants exhibit many desirable characteristics which make them promising vaccine candidates.