Macrophages in bronchoalveolar lavage fluid are not representative of macrophages in granulomas of the lungs of BCG‐infected mice

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Abstract
To find out whether macrophages obtained by bronchoalveolar lavage (BAL) represent the macrophage population in alveoli, we performed a quantitative study to compare the expression of cell‐surface antigens by macrophages in the lavage fluid as well as in the alveolar spaces of sectioned lung tissue from normal and BCG‐infected mice. The amount of antigen on cells, determined with a quantitative immunocytochemical method, was expressed as the mean specific absorbance per 0–25 μm2 cell‐surface area. In normal mice, BAL and intra‐alveolar macrophages are identical. Macrophages in BAL fluid and intra‐alveolar macrophages, both from BCG‐infected mice, expressed antigen F4/80 and Fc receptor II to about the same extent, but the former showed a significantly higher level of antigen expression as defined by antibodies M1/70, M5/114, 30‐G‐12, and M3/38. In granulomatous lesions, expression of antigens by macrophages, assessed semi‐quantitatively, was considerably less than that by BAL macrophages. Lymphocytes in the BAL fluid resembled those in the granulomas. These findings led to the conclusion that in BCG‐infected mice, the macrophages found in BAL fluid do not resemble those occurring in either the alveoli or the granulomatous lesions of the lungs. On these grounds, the relevance of findings in BAL fluid to lung interstitial lung diseases should be reconsidered.