Alveolar macrophages (AMs), localized at the pulmonary air-tissue interface, are one of the first lines of defense that interact with inhaled airborne pathogens such as influenza viruses. By using a new CD169-DTR transgenic mouse strain we demonstrate that specific and highly controlled in vivo ablation of this myeloid cell subset leads to severe impairment of the innate, but not adaptive, immune responses and critically affects the progression of the disease. In fact, AM-ablated mice, infected with a normally sublethal dose of PR8 influenza virus, showed dramatically increased virus load in the lungs, severe airway inflammation, pulmonary edema and vascular leakage, which caused the death of the infected animals. Our data highlight the possibilities for new therapeutic strategies focusing on modulation of AMs, which may efficiently boost innate responses to influenza infections.Keywords: Alveolar macrophages r CD169-DTR r Influenza r In vivo depletion r Transgenic mouse Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionSuccessful immunity against influenza virus is the result of interplay between distinct innate and adaptive immune responses, which are activated and controlled by a complex network of heterogeneous subpopulations of myeloid cells distributed in both the airway mucosa and lung parenchyma. An effective CD8 + T-cell and antibody response, initiated and maintained efficiently by lung dendritic cells (DCs) [1][2][3], is widely recognized as one of the main protective mechanisms in controlling influenza virus Correspondence: Prof. Christiane Ruedl e-mail: ruedl@ntu.edu.sg replication [4,5]. However, the exact contribution and relevance of other myeloid cell subsets, such as pulmonary monocytes, alveolar and interstitial macrophages in innate immune responses after virus infection are still under active investigation. Macrophages are suggested as one of the key cells in controlling viral replication at early stages of infection [6][7][8][9]. In fact, clodronatemediated depletion of accessible, actively phagocytizing cells in the lung causes excessive inflammatory response toward harmless antigens [10][11][12][13] as well as significant mortality upon sublethal infection with influenza virus [6]. * These authors equally contributed to this work. Here, we focus specifically on alveolar macrophages (AMs), which are found in the alveolar spaces and conducting airways [14,15]. At steady-state they act as regulatory macrophages controlling tissue homeostasis and repair [16,17], however, upon viral infection they can switch into highly active phagocytic cells [18,19].Their precise contribution in virus clearance, in respiratory inflammation as well as in induction of virus-specific CD8 + T-cell responses, was analyzed during PR8 influenza virus infection using a new CD169-diphtheria toxin receptor (DTR) transgenic mouse that allows highly specific and controlled ablation of these cells in the lungs.CD169, also known as Siglec1 or Sialo...

for their help in optimizing and performing ES cell, lung staining, influenza, and qPCR experiments. Other lab members, for their various help. Their presence and encouragement in the lab brightens up my days.