| In 1997, highly pathogenic avian influenza A (H5N1) viruses crossed the species barrier to infect humans, resulting in clinical outcomes ranging from mild respiratory illness to death. This was the first time that an avian influenza A virus was found to cause respiratory disease in humans, thereby raising concerns about the spread of these viruses and a potential pandemic. Here, the BALB/c mouse was used to better understand the pathogenesis of avian H5N1 viruses in mammals and to evaluate potential vaccine strategies. H5N1 viruses isolated from humans were of high or low pathogenicity phenotypes in BALB/c mice following intranasal inoculation. The highly pathogenic A/Hong Kong/483 (HK/483) virus replicated in systemic organs, including the brain, and caused lymphocyte depletion and 100% mortality, while A/Hong Kong/486/97 (HK/486), a virus of low pathogenicity, replicated only in the lungs and was not lethal. The use of different inoculation routes enabled us to investigate further the correlation of pulmonary replication, lymphocyte depletion, neurotropism, or inflammatory cytokine responses in the lungs or brain, with the lethal outcome of H5N1 disease in mice. We demonstrated that both viruses were equally neurotropic and neurovirulent following intranasal inoculation but that the enhanced virulence of HK/483 virus was due to the ability to spread beyond the site of inoculation by any route whereas the HK/486 virus failed to do so. Thus, neurotropism and the induction of inflammatory cytokines in the brain were associated with the lethality of H5N1 viruses.; The virulence of the influenza A H5N1 viruses for both humans and chickens imposed limitations on the use of traditional chicken egg-grown vaccines. We investigated an alternative approach that would provide safe and rapid vaccine development were an outbreak with pandemic potential to occur. We investigated the impact of altering potential glycosylation sites on a DNA vaccine-expressed hemagglutinin from the H5N1 viruses. The glycosylation pattern of the influenza virus HA1 domain had little impact on the murine antibody response or protective efficacy of the DNA vaccine encoding the H5 HA, thereby minimizing the concern that the pattern of glycosylation sites encoded by the vaccine match those of closely related H5 viruses. |
