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The severity of disease caused in humans by H5N1 influenza viruses remains unexplained. The NS gene of Hong Kong H5N1/97 viruses was shown to contribute to high pathogenicity of reassortants in a pig model. However, the molecular pathogenesis and host immune response underlying this phenomenon remain unclear. Here, in a mouse model, H1N1 A/Puerto Rico/8/34 (PR/8) reassortants that contained the H5N1/97 NS gene, the H5N1/01 NS gene, or an altered H5N1/97 NS gene encoding a Glu92-->Asp substitution in NS1 was studied. The pathogenicity of reassortant viruses, the induction of cytokines and chemokine CXCL1 (KC) in the lungs and specific B- and T-cell responses was characterized. In mice infected with reassortant virus containing the H5N1/97 NS gene, the mouse lethal dose (50%) and lung virus titres were similar to those of PR/8, which is highly pathogenic to mice. This reassortant virus required two more days than PR/8 to be cleared from the lungs of infected mice. Reassortants containing the altered H5N1/97 NS gene or the H5N1/01 NS gene demonstrated attenuated pathogenicity and lower lung titres in mice. Specific B- and T-cell responses were consistent with viral pathogenicity and did not explain the delayed clearance of the H5N1/97 NS reassortant. The reassortant induced elevated pulmonary concentrations of the inflammatory cytokines IL1alpha, IL1beta, IL6, IFN-gamma and chemokine KC, and decreased concentrations of the anti-inflammatory cytokine IL10. This cytokine imbalance is reminiscent of the clinical findings in two humans who died of H5N1/97 infection and may explain the unusual severity of the disease.

A single genetic locus, flavivirus resistance (Flv), controls virus titres and severity of flavivirus infection in mouse brain. It has been mapped to mouse chromosome 5 and shown to include different allelic forms. While the majority of laboratory mouse strains are susceptible to flaviviruses and carry the Flv(s) allele, wild mice and laboratory mouse strains recently derived from them are resistant and carry flavivirus-resistance alleles including Flv(r)-like and Flv(mr) alleles. Although there is a mouse model of flavivirus resistance conferred by the Flv(r) allele, other resistance alleles have not been adequately studied due to a lack of appropriate animal models. In this paper we describe the development of new flavivirus-resistant mouse strains, C3H.M.domesticus-Flv(r) and C3H.MOLD-Flv(mr), which carry the novel resistance alleles Flv(r)-like and Flv(mr) on the genetic background of flavivirus susceptible C3H/HeJ mice. The new strains were created by 10 to 11 generations of backcrossing followed by brother-sister matings resulting in a generation of homozygous founder stocks. Genome analysis of the newly developed mouse strains has revealed chromosomal regions of approximately 9 and 11 cM, respectively, encompassing Flv on chromosome 5, which are derived from resistant donor mice. These segments are much smaller than the segment of approximately 31 cM described in the congenic resistant mouse strain C3H.PRI-Flv(r) (also known as C3H/RV). The new congenic mouse strains, which were created to carry the Flv(r)-like and Flv(mr) alleles on the standardized genetic background of susceptible mice, represent new animal models of flavivirus resistance conferred by these novel resistance alleles.

The lack of an effective animal model has been a major obstacle in attempts to define the role of humoral and cellular immune responses in protection against flavivirus infection. We have used F1 hybrid mice (BALB/c x C3H/RV) that are heterozygous for the flavivirus resistance allele F1vr and show reduced virus replication in the brain after intracerebral inoculation. F1 hybrid mice challenged by intracerebral inoculation with Murray Valley encephalitis (MVE) virus developed encephalitis 2-3 days later than a genetically susceptible strain (BALB/c) but showed a similar mortality rate. This delay in the onset of disease provided more opportunity for virus clearance by primed immune responses. Using F1 hybrid mice we were able to demonstrate protective immunity induced by structural and non-structural proteins of MVE virus by immunization with pure NS1 protein or recombinant vaccinia viruses that expressed various regions of the MVE genome. These constructs included VV-STR (C-prM-E-NS1-NS2A), VV-delta C (prM-E-Ns1-NS2A) and VV-NS1 (NS1-NS2A). VV-delta C vaccinated mice were completely protected (100% survival)from challenge with 1000 infectious units of MVE virus, while mice inoculated with VV-STR, VV-NS1 or pure NS1 were partially protected (40%, 47% and 85% respectively). Analysis of prechallenge sera and in vivo depletion studies revealed that the solid protection induced by VV-delta C was mediated by neutralizing antibody to the E protein and did not require a CD8+ T cell response. The partial protection provided by VV-STR, VV-NS1 and pure NS1 occurred after induction of antibody to NS1. However, depletion of CD8+ cells prior to virus challenge ablated the protection provided by VV-NS1 indicating some requirement for class I restricted cytotoxic T cells.