Despite vaccination, influenza and secondary pneumonias are the fourth leading cause of death in individuals 65 years and older. The recent emergence of the novel 2009-H1N1 influenza, to which vaccines are limited or unavailable, has the potential to cause 30,000-90,000 deaths in people under 65 years of age. These staggering statistics mandate the need to better understand age-related changes in primary immune responses to acute influenza infection. We have recently identified a critical role for natural killer (NK) cells in controlling early influenza infection ex vivo and in vivo. Aged C57BL/6 mice exhibited increased lung virus titers, weight loss, decreased NK cytotoxicity, and a reduced number of NK cells during early influenza infection. Importantly, in vivo NK cell depletion by anti-NK1.1 IgG antibody (PK136) treatment induced marked weight loss and increased lung virus titers after influenza infection of both young and aged mice. These data suggest that NK cells are essential in early control of influenza infection. Thus, our overarching hypothesis is that the induction of an effective NK cell response is vital for controlling early influenza infection in vulnerable populations, including the elderly. Our studies will characterize age-related changes in NK cell function, elucidate potential intrinsic and extrinsic mechanisms for impaired NK cell function, and establish the critical role of NK cells in controlling early infection to influenza virus in vivo. In Aim 1, age-related differences in susceptibility and the role of NK cell function during the first critical four days of infection will be assessed. We will perform influenza dose responses and measure age-related differences lung virus titers, pathology, histology and cellular infiltrates in lung, weight loss, and recovery from infection. Age-related changes in NK subsets, cytolytic function of NK cells, dendritic cell (DCs) function, and local and systemic cytokine production will be measured during infection. In Aim 2, we will elucidate age-related changes in intrinsic and extrinsic mechanism(s) for impaired NK cell function during influenza infection. Age-related differences in lytic efficiency, activation receptor function/signaling, and cytokine receptor/function will be measured in purified NK cells to identify intrinsic NK cell defects. Adoptive transfer studies will delineate between intrinsic and extrinsic effects on NK cell proliferation, apoptosis, and homing as related to susceptibility to influenza. We hypothesize that DCs are critical extrinsic effectors on NK cell function in early infection. Thus, we will measure DC activation of NK cells, DC cytokine production, and the effects of DC ablation on NK cell function. In Aim 3, we will validate the essential role of NK cells in controlling the in vivo response to influenza. In vivo NK cytolysis in young and aged mice will be assessed along with survival, weight loss, and lung virus titers. The specific role of NK cells in controlling susceptibility to influenza will be addressed in mice after NK depletion in vivo with PK136 or in RAG-1-/- or NKD transgenic mice. These three mouse models will afford a clear delineation of the specific contribution of NK cells in controlling the early response to influenza.