Immunological properties of plaque puri®ed strains of live attenuated respiratory syncytial virus (RSV) for human vaccine M.L. Herlocher a , M. Ewasyshyn b , S. Sambhara b , M. Gharaee-Kermani a , D. Cho a , J. Lai a , M. Klein b , H.F. Maassab a, * a Department of Epidemiology, School of Public Health, 109 Observatory, University of Michigan, Ann Arbor, MI 48109-2029, USA b Pasteur Merieux Connaught Canada Laboratories, 1755 Steeles Ave, North York, Ontario, Canada Received 20 November 1997; accepted 8 April 1998 Abstract Respiratory syncytial virus (RSV) causes severe lower respiratory tract disease in infants, young children, and the elderly. Eorts to develop satisfactory live or inactivated vaccines have not yet been proven successful. Our research focuses on the development of four puri®ed live attenuated RSV sub-type A human vaccine clones. Temperature sensitive (ts) and attenuated puri®ed clones of either cold-adapted (ca) RSV or high-passage (hp) RSV were administered intra-nasally (i.n.) to BALB/c mice and tested for immunogenicity. All four clones produced signi®cant anti-RSV F IgG 2a and IgG 1 titres in the sera of mice, RSV- speci®c neutralizing titres higher than those produced by their wild-type progenitor viruses, cytotoxic T-lymphocyte (CTL) activity, and total protection against wild-type (wt) viral challenge. These puri®ed vaccine candidates await testing in humans to determine which contain the required balance between immunogenicity and attenuation. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Respiratory synctyal virus; Live vaccine; Immunogenicity; BALB/c mice 1. Introduction Respiratory syncytial virus (RSV) is a member of the pneumovirus genus of the paramyxoviridae family. RSV is the major cause of viral pneumonia and bronchiolitis in infants and young children [1±4]. It has been estimated that RSV is responsible for about 3 million cases of mild respiratory tract infections, one million cases of lower respiratory tract infections, 95 000 cases of hospitalization due to severe bronchio- litis and pneumonia, and 4500 deaths per year in the United States [4, 5]. Among community-dwelling older persons, RSV infection is the cause of serious disease accounting for 27% of respiratory illnesses [6]. In ad- dition, RSV is prevalent worldwide, causing 5 million deaths annually [7, 8], thus making RSV infection a major public health concern. Many studies have focused on control of RSV by vaccination, and although some promising results have been seen, none of the vaccines tested so far provides complete protection [9]. In fact, administration of for- malin-inactivated RSV vaccine in several ®eld trials was shown to be associated with the development of severe disease during subsequent natural infection with RSV [10±15]. As a result of the disease-enhancing eects associ- ated with the formalin-inactivated vaccine, emphasis was placed on the development of live attenuated RSV vaccines. Temperature sensitive (ts) mutants of RSV derived by chemical mutagenesis were developed [16], but clinical trials have shown them to be over-attenu- ated or under-attenuated or genetically unstable [3, 17± 25]. A cold adapted RSV candidate vaccine (cp-RSV) caused upper respiratory tract disease in seronegative infants, although it was completely attenuated in sero- positive adults and children [26, 27]. Additional attenu- ating mutations providing temperature sensitivity (ts) and small plaque (sp) phenotypes were introduced into cp-RSV [28] by chemical mutagenesis. The mutagen- ized and plaque-puri®ed cp-RSV has been shown to be Vaccine 17 (1999) 172±181 0264-410X/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S0264-410X(98)00155-8 PERGAMON * Corresponding author. Tel.: (313) 763 4243; fax: (313) 764 3192.