Regulation of alveolar macrophage immune function

The alveolar macrophage (AM) has a central role in lung homeostasis and host defense of the lung by virtue of its role in clearance of surfactant and microbial pathogens as well as regulation of the acquired pulmonary immunity. Because Granulocyte Monocyte-Colony Stimulating Factor (GM-CSF), a hematological growth factor modulates a number of AM functions including AM surfactant catabolism and is required for normal lung host defense, this thesis was focused on elucidation of the mechanisms by which GM-CSF regulates AM immune functions. Based on the observation that pulmonary GM-CSF expression regulates expression of the transcription factor PU.1 in AMs, the hypothesis tested was that GM-CSF, via PU.1, is a critical regulator of AM maturation and immune functions in lung host defense. To evaluate this hypothesis, mice were used in which GM-CSF expression was normal (GM+/+), absent due to gene targeting (GM−/−), or expressed only in the lung (SPC-GM +/+/GM−/−). In vitro studies utilized AM cell lines derived from GM+/+ and GM−/− mice (MHS, mAM, respectively). To assess the ability of PU.1 to restore normal functions in GM−/− AMs, mAM cells were transduced with a retroviral vector expressing PU.1 (mAM_PU.1+) or a control vector (mAM_GFP+). Results demonstrated that GM-CSF, via PU.1, regulated a number of AM functions. These included: (1) endocytic internalization (and pulmonary clearance) of adenovirus; (2) phagocytic internalization of small and large inert particles; (3) FcγR-mediated phagocytosis; (4) cell adhesion, morphology and filopodia formation; and (5) the capacity to secrete IL-18/IL-12 in response to pathogen exposure in vitro and in vivo. The lack of PU.1-dependent receptor expression was found to mediate impairment of opsonophagocytosis (e.g., FcγRs) but not impaired receptor-mediated endocytosis of adenovirus, the latter being correlated with abnormalities in cytoskeletal organization. An inability to increase IFNγ levels during pulmonary adenoviral infection in GM−/− mice was explained by the inability of AMs from GM−/− mice to express IL-18/IL-12 leading to impaired augmentation of AM FcγR expression following adenoviral infection. Taken together, these results demonstrate that GM-CSF, via PU.1, plays a critical role in antiviral and other lung host defense by regulating a number of AM-mediated innate and adaptive immune functions.