Host Genetic Variation Affects Resistance to Infection with a Highly Pathogenic H5N1 Influenza A Virus in Mice

Despite the prevalence of H5N1 influenza viruses in global avian populations, comparatively few cases have been diagnosed in humans. Although viral factors almost certainly play a role in limiting human infection and disease, host genetics most likely contribute substantially. To model host factors in the context of influenza virus infection, we determined the lethal dose of a highly pathogenic H5N1 virus (A/Hong Kong/213/03) in C57BL/6J and DBA/2J mice and identified genetic elements associated with survival after infection. The lethal dose in these hosts varied by 4 logs and was associated with differences in replication kinetics and increased production of proinflammatory cytokines CCL2 and tumor necrosis factor alpha in susceptible DBA/2J mice. Gene mapping with recombinant inbred BXD strains revealed five loci or Qivr (quantitative trait loci for influenza virus resistance) located on chromosomes 2, 7, 11, 15, and 17 associated with resistance to H5N1 virus. In conjunction with gene expression profiling, we identified a number of candidate susceptibility genes. One of the validated genes, the hemolytic complement gene, affected virus titer 7 days after infection. We conclude that H5N1 influenza virus-induced pathology is affected by a complex and multigenic host component.The last 10 years have witnessed a spread of highly pathogenic H5N1 avian influenza A virus from Southeast Asia into Europe and Africa, killing millions of chickens and ducks. Mammalian species including tigers, cats, dogs, and humans have also been infected with H5N1 virus, causing severe and often fatal disease. Excess mortality in humans was associated with high pharyngeal viral loads and increased cytokine and chemokine production (12). Some evidence suggests that genetic variation among infected hosts contributes to H5N1 infection and pathogenesis. Compared to the many millions of chickens and ducks that have been infected by H5N1 virus, relatively few humans have been infected. Were these individuals genetically predisposed, and therefore, did they have a greater risk of getting infected by the currently circulating H5N1 influenza viruses? Also, among the identified clusters of human H5N1 virus infections, more than 90% of the cases have occurred in genetically related family members, suggesting a possible genetic susceptibility to infection or severe disease (33). Recently, genetic relatedness was shown to be a significant risk factor for severe disease resulting from H3N2 influenza virus infection (2). However, other recent studies either have been unable to confirm the effect of genetic variation on the outcome and severity of influenza A virus infection (19) or have challenged the role of host genetics in H5N1 virus clusters (36).Genetic polymorphisms in the infected host affect microbial pathogenesis. In some host-pathogen studies, individual genes strongly regulated disease susceptibility or severity. For example, a 32-amino-acid deletion in the CCR5 product has been associated with increased resistance to human immunodeficiency virus infection (26), and more recently, a single amino acid change in the TLR3 product was associated with herpes simplex virus-induced encephalitis (50). Despite these examples, most host-pathogen interactions are more complex and modified by several genetic determinants. In the mouse model, disease severity after infection with viruses, bacteria, or parasites is frequently caused by multiple genetic differences, each affecting the outcome of the disease (3, 7, 8, 17, 47). Genetic modifiers that are associated with increased susceptibility to influenza virus infection or disease are mostly unknown. In humans, the duration of virus shedding was reduced in HLA-A2⁺ individuals, possibly as a result of a stronger cellular immune response (9). In mice, the resistance to influenza virus infection was mapped to the MX1 protein (39, 44, 46). The human MX1 protein also restricts viral replication, but its efficacy depends on the virus strain (13).Although much work is being done to define the viral factors affecting H5N1 influenza virus pathogenesis, little has been done to elucidate the effect of host genetics on H5N1 disease outcome. This study was initiated to assess the effect of the host''s genetic variation on H5N1 influenza virus pathogenesis and to provide the first clues about which host genes are responsible for the increased pathogenesis of H5N1 virus infection. Genome-wide linkage analysis using BXD recombinant inbred (BXD RI) strains was performed to identify areas on the chromosome that contribute to the difference in susceptibility to H5N1 virus seen between C57BL/6J and DBA/2J mice.