Immunopathology in influenza virus infection: uncoupling the friend from foe

Influenza epidemics and pandemics cause significant morbidity and mortality worldwide associated with severe immunopathology in the lung, and the mechanisms of such immunopathogenesis still remain poorly understood. While human studies help to understand influenza immunopathology, they provide only limited mechanistic information. On the other hand, recent studies using experimental animal models have significantly enhanced our understanding of the complex mechanisms involved in the immunopathogenesis during primary influenza or influenza-associated bacterial superinfection. This includes the involvement of acute inflammatory responses (macrophages, neutrophils, dendritic cells, toll-like receptors, cytokines, chemokines), CD4 and CD8 T cells, tissue remodeling processes, and contribution of bacterial superinfection. In particular, progress has been made in uncoupling the mechanisms that are involved in both anti-viral host defense and in immunopathogenesis from those that solely contribute to lung immunopathology. Uncoupling such events will facilitate the discovery of new intervention strategies to treat pulmonary immunopathology associated with influenza infection.