This investigation attempted to determine whether the primary source of alveolar macrophages is pulmonary or hematopoietic. We have utilized an antigenic marker to identify cells of hematopoietic origin. Mouse chimeras were produced by irradiating C57B6/AF1 mice (900 R) and then injecting them intravenously with B10D2/AF1 bone marrow. The donor animal has an antigenic specificity on the H-2 locus, not shared by the recipient. Alveolar macrophages were obtained by repeated lung washings with physiologic saline at 37°C. Cytotoxic tests were done on bone marrow and alveolar macrophages using anti-31 mouse antibody, absorbed rabbit serum as complement, and trypan blue exclusion as a test for viability. Animals were studied at 7, 14, 21, 28, and 35–50 days and 4, 5, 8, and 11 months after irradiation and bone marrow replacement. By 21 days after irradiation, 90% of the animals had greater than 80% replacement of marrow with donor tissue; and white blood cell and alveolar macrophage counts approached normal. At this time and at later intervals the per cent of donor cells in the lung free cell population was not significantly different from the per cent of donor cells in the bone marrow. Similarly, after aerosol particulate exposure, the percentage of marrow cells and alveolar macrophages of donor origin were not significantly different. This immunologic approach suggests that alveolar macrophages in radiation chimeras are entirely of hematopoietic origin.

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