Are Macrophages Recovered from Mouse Lung by Bronchoalveolar Lavage Representative of the Alveolar Macrophage Pool?

The purpose of this study was to establish whether macrophages recovered from the mouse lung by bronchoalveolar lavage are representative of the alveolar macrophage pool. To this end, mice were exposed by inhalation to an aerosol of monodisperse fluorescent particles (FP) with an actual diameter of 0.89 μm. Croups of animals were killed at 3 and 27 h following exposure and their lungs were lavaged 30 times in situ with normal saline. After excising the lungs, five additional washes were performed with massage to increase the recovery of free cells. The mean total cell recovery was 0.8 × 106—about half the estimated total number of alveolar macrophages in this strain of mouse. The washes were grouped and analyzed using a fluorescence-activated cell sorter (FACS) to give information on the relative concentrations of (a) free FP, (b) macrophage-associated FP, and (c) unlabeled macrophages. Further, for both free and macrophage-associated FP, the fluorescent events were classified as being due to one, two to three, or greater than or equal to four FP. At both time points a higher proportion of AM in the initial washes contained one or more FP and the numbers of FP per labeled AM were also greater than in subsequent washes. The results of this study suggest that representative samples of the general alveolar macrophage population of the mouse lung can be obtained by bronchopulmonary lavage, provided the initial washes are discarded. It was also apparent from the results that free FP were recovered from the lung much more readily than macrophage-associated FP.