Alveolar and interstitial macrophage populations in the murine lung

Pulmonary macrophages (PM) exist in two general anatomical compartments in the lower respiratory tract: the alveolar space (alveolar macrophages, AM) and the interstitium (interstitial macrophages, IM). We determined the relative contribution that macrophages in each of these compartments make to the size of the total PM population found in the lungs of C3H/OUJ mice, while also evaluating how efficiently bronchoalveolar lavage (BAL) removes AM from the murine lung. These objectives were accomplished by combining extensive BAL with subsequent mechanical and enzymatic dissociation of the lungs in conjunction with in situ and in vitro phagocytic assays involving opsonized erythrocytes (EA) to identify mononuclear phagocytes. On average, 2.5 x 10(6) cells were recovered by extensive BAL, and approximately 78% of these cells ingested EA in vitro. To determine the efficiency of BAL in removing PM from the alveolar space, EA were instilled intratracheally into intact lungs, which had been removed from the chest cavity, and allowed to incubate for 60 min; this was followed by exhaustive BAL and subsequent lung digestion. After these procedures, approximately 4% of the dissociated lung cells contained EA, indicating that these cells were alveolar in origin but had not been removed despite extensive BAL. Subtraction of these AM from the total EA+ cells in lung cell suspensions following a second in vitro incubation with EA indicated that approximately 37% of all PM were within the interstitium. These results suggest that, while AM comprise the majority of lung macrophages, IM constitute a larger component of the total PM population in murine lungs than previously reported. In addition, this study, like several previous investigations using other species, indicates that a significant proportion of AM remain in the lung despite attempts to remove them with BAL. Accordingly, residual AM significantly contaminate the IM population present in murine lung cell suspensions even after extensive lavage.