In vivo characterization of alveolar and interstitial lung macrophages in rhesus macaques: Implications for understanding lung disease in humans

摘要

Alveolar macrophages (AM) obtained by bronchoalveolar lavage (BAL) are commonly used to study lung macrophage-mediated immune responses. Questions remain, however, about whether AM fully represent macrophage function in the lung. This study was performed to determine the contribution of interstitial macrophages (IM) of lung tissue to pulmonary immunity and that are not present in BAL sampling. In vivo BrdU injection was performed to evaluate the kinetics and monocyte/tissue macrophage turnover in Indian rhesus macaques (Macaca mulatta). Lung macrophage phenotype and cell turnover were analyzed by flow cytometry and immunohistochemistry. AM and IM in lungs of rhesus macaques comprised about 70% of immune response cells in the lung. AM represented a larger proportion of macrophages, approximately 75–80%, and exhibited minimal turnover. Conversely, IM exhibited higher turnover rates that were similar to those of blood monocytes during steady state homeostasis. IM also exhibited higher staining for TdT-mediated dUTP nick end labeling (TUNEL), suggesting a continuous transition of blood monocytes replacing IM undergoing apoptosis. Although AM appear static in steady state homeostasis, increased influx of new AM derived from monocytes/IM was observed following BAL procedure. Moreover, ex vivo IFN-γ plus LPS treatment significantly increased intracellular expression of TNF-α in IM but not in AM. These findings indicate that the longer-lived AM obtained from BAL may not represent the entire pulmonary spectrum of macrophage responses, and shorter-lived IM may function as the critical mucosal macrophage subset in the lung that helps to maintain homeostasis and protect against continuous pathogen exposure from the environment.