Viral Replication Rate Regulates Clinical Outcome and CD8 T Cell Responses during Highly Pathogenic H5N1 Influenza Virus Infection in Mice

Abstract

Since the first recorded infection of humans with H5N1 viruses of avian origin in 1997, sporadic human infections continue to occur with a staggering mortality rate of >60%. Although sustained human-to-human transmission has not occurred yet, there is a growing concern that these H5N1 viruses might acquire this trait and raise the specter of a pandemic. Despite progress in deciphering viral determinants of pathogenicity, we still lack crucial information on virus/immune system interactions pertaining to severe disease and high mortality associated with human H5N1 influenza virus infections. Using two human isolates of H5N1 viruses that differ in their pathogenicity in mice, we have defined mechanistic links among the rate of viral replication, mortality, CD8 T cell responses, and immunopathology. The extreme pathogenicity of H5N1 viruses was directly linked to the ability of the virus to replicate rapidly, and swiftly attain high steady-state titers in the lungs within 48 hours after infection. The remarkably high replication rate of the highly pathogenic H5N1 virus did not prevent the induction of IFN-β or activation of CD8 T cells, but the CD8 T cell response was ineffective in controlling viral replication in the lungs and CD8 T cell deficiency did not affect viral titers or mortality. Additionally, BIM deficiency ameliorated lung pathology and inhibited T cell apoptosis without affecting survival of mice. Therefore, rapidly replicating, highly lethal H5N1 viruses could simply outpace and overwhelm the adaptive immune responses, and kill the host by direct cytopathic effects. However, therapeutic suppression of early viral replication and the associated enhancement of CD8 T cell responses improved the survival of mice following a lethal H5N1 infection. These findings suggest that suppression of early H5N1 virus replication is key to the programming of an effective host response, which has implications in treatment of this infection in humans. Outbreaks of avian influenza (AI) viruses have continued in chickens in Southeast Asia, coupled with regular instances of direct bird to human transmission, with extremely high case fatality rates. The mechanisms underlying the disease pathogenesis and high mortality rate in humans are not well understood. In particular, we lack information on the development and/or failure of adaptive immune responses during AI infection. Our studies in mice have linked the pathogenicity of AI viruses to the virus' rate of replication in the lungs. Surprisingly, a strong T cell response was triggered by the infection, but virus-specific T cells were ineffective in controlling the rapidly replicating virus. The extremely high rate of AI virus replication likely outpaces and overwhelms the developing immune response. However, administration of anti-viral drugs, only early in the infection slowed viral replication, enhanced the number of effector CD8 T cells in the lung, and promoted survival and recovery from infection. These findings highlight the role of viral replication rate in pathogenesis and underscore the importance of controlling viral replication as an adjunct to immunotherapies in the treatment of this infection in humans.