Differential response of ferrets to infection with virulent and avirulent influenza viruses: a possible marker of virus attenuation

The role of reverse genetics in the development of vaccines against respiratory viruses

Despite their significance, the only available vaccines against respiratory viruses are those for the prevention of influenza. Attempts have been made to produce vaccines against other respiratory viruses using traditional techniques, but have met with little success. Reverse genetics, although still a r-elatively new tool for the manipulation of negative-strand RNA viruses, has great potential for the preparation of vaccines against many of the common respiratory viruses. In the preparation of live vaccines, reverse genetics s-ystems allow the direct modification of the specific regions in the genomes of negative-stranded RNA viruses concerned with attenuation; the ultimate goal is the introduction of site-specific mutations through a cDNA intermediate in order to develop strains with the requisite attenuation, antigenic and growth properties needed in a vaccine. These techniques can also be used to disarm potentially highly pathogenic viruses, such as emerging H5N1 avian influenza viruses, in order to facilitate large-scale preparation of viruses for use in inactivated vaccines under conditions of manufacturing safety. Before these vaccines become available, residual issues concerned with intellectual property rights to the technology and its application will need to be resolved.

In vivo and in vitro hamster models in the assessment of virulence of recombinant influenza viruses

The virulence of five wild-type influenza A viruses and 14 recombinant viruses, prepared from the cold adapted A/Ann Arbor/6/60 virus and various wild-type viruses, was studied by two methods. Firstly, the viruses were inoculated into hamsters, and the titres present in the lungs and turbinates at 1, 3 and 4 days post-infection were measured. Secondly, the effect of five wild-type and ten recombinant viruses on the ciliated epithelium of in vitro hamster tracheal organ cultures was examined. The results obtained were assessed with reference to the known virulence of the viruses for human volunteers. The results showed that virus strains virulent for man grew to higher titres in hamster lungs and turbinates than attenuated strains; and that virulent strains destroyed the ciliary activity of hamster tracheal organ cultures more quickly and to a greater extent than attenuated strains. Comparison of the results with the known virulence of viruses tested for man suggests that the reduced ability of virus to grow in hamster lung tissue and the relatively little effect on ciliary activity may be used as markers of virus attenuation; however, the growth of virus in hamster turbinates overlaps for virulent and attenuated strains and therefore was not considered a useful marker of virulence.

Influenza virus infection of a newborn rats: virulence of recombinant strains prepared from a cold-adapted, attenuated parent

Infant rats were infected with one of a series of influenza A viruses. The growth of viruses in the turbinates or lungs, and the ability of virus infection to potentiate a subsequent bacterial infection by Haemophilus influenzae (HIb), were measured. The three virus strains known to be virulent for man grew to relatively high titres of 10(5.2)--10(6.8) EBID50/ml in the turbinates of infant rats at 48 hours post-infection, and virus infection enhanced subsequent systemic infection following intranasal inoculation of rats with HIb. In contrast, influenza virus A/Ann Arobr/6/60--P17 and the three recombinant viruses prepared from this strain, all of which are attenuated for man, replicated to significantly lower titres of 10(2.6)--10(4.1) EBID50/ml in infant rats turbinates, and failed to promote systemic infection by HIb to the samest that the behaviour of influenza viruses in infant rats may be an indication for virus virulence for man, and thus provide a test which could facilitate the development of live, attenuated virus vaccines.

Antibody responses to vaccination with WRL 105 strain live influenza vaccine in previously vaccinated and unvaccinated volunteers

Intranasal vaccination with a single 0.5 ml dose of 10(7.0) EID 50 WRL 105 strain live influenza vaccine elicited four-fold or greater increases in circulating homotypic haemagglutinating inhibiting (HAI) antibody in 60 (64.5%) of 93 volunteers, or in 58 (74.4%) of 78 volunteers with HAI antibody titres before vaccination of less than or equal to 1/20. In comparison, in a group of volunteers vaccinated 9 months previously re-vaccination elicited antibody responses in only 4 (6.9%) of 58 volunteers, or in 3 (14.3%) of 21 volunteers with antibody titres before vaccination of less than or equal to 1/20. Titres of vaccine-induced antibody and antibody resulting from earlier natural infection appeared to fall slowly and at equivalent rates over a 9 month period.