Adenovirus is a common respiratory pathogen which causes a broad range of distinct clinical syndromes and has recently received attention for its potential for in vivo gene delivery. Although adenovirus respiratory tract infection (ARTI) results in dose-dependent, local inflammation, the pathogenesis of this remains unclear. We hypothesized that alveolar macrophages (AM) rapidly internalize adenovirus following in vivo pulmonary administration and then initiate inflammatory signaling within the lung. To evaluate the role of AM in the induction of lung inflammation during ARTI in vivo, we directly assessed adenovirus uptake by murine AM and correlated uptake with the initiation of proinflammatory gene expression. Adenovirus, a nonenveloped DNA virus with at least 45 serotypes, is an important respiratory pathogen affecting individuals of all ages and accounting for 7 to 10% of all respiratory illnesses in infants and children, with an incidence of between 5 to 10 million infections annually in the United States alone (13, 21). Adenovirus respiratory tract infection (ARTI) occurs sporadically, epidemically, and nosocomially, presents in a wide spectrum of distinct clinical syndromes ranging from self-limited acute pharyngitis to fatal pneumonia, and has been identified as an etiological factor associated with exacerbations in individuals with chronic obstructive lung diseases (13). Despite the frequency and broad range of clinical presentations, the pathogenesis of inflammation in ARTI is poorly understood. Early information regarding host responses to adenovirus was derived from efforts to develop human adenovirus vaccines (42) or to understand the pathology of fatal adenoviral pneumonia (4). Recently, ARTI has been studied in a variety of animal models, including mice (14, 37, 58), Cotton rats (Sigmodon hispidus) (15, 39, 59, 60), and primates (8, 44, 55, 61) as part of preclinical toxicology studies for human gene therapy clinical trials for cystic fibrosis (reviewed in references 7 and 48). Studies in humans showed that administration of replication-deficient adenovirus vectors to the respiratory tract can cause dose-dependent local inflammation (12,26,33).Inflammatory responses to ARTI have been best studied in rodent models. In the Cotton rat, a permissive host for replication of human adenovirus, pulmonary histopathology consists of early and late phases similar to that seen in human ARTI (15,39). The early phase consists primarily of accumulation of neutrophils, macrophages, and monocytes and develops within the first 24 h (60). The late phase, consisting mainly of lymphocytes, is apparent by day 5 (39, 60). In mice, a nonpermissive host for human adenovirus, adenovirus early gene