Functional, morphological, and phenotypical differences between rat alveolar and interstitial macrophages

Alveolar macrophages (AM) and interstitial macrophages (IM) from rat lungs were characterized with respect to morphology, phagocytosis, adhesion properties, and phenotype. AM were recovered by lung lavage and IM by treatment of the lung tissue with DNAse and collagenase. The AM were enzyme treated in the same way as the IM. The IM were smaller than AM and had a higher nuclear to cytoplasm ratio. They had markedly lower phagocytic capacity. The attachment of particles to the cell surface was significantly lower in IM than in AM, but the capacity to ingest the particles was the same. Adherence to vitronectin- as well as fibronectin-coated surfaces was significantly higher in AM. The phagolysosomal pH was similar in IM and AM, around pH 5, indicating that dissolution of inorganic particles can take place effectively also in IM. Five surface receptors were studied, and the expression differed significantly in all five between AM and IM. The expression of OX-1 (CD 45), a common leukocyte antigen, was significantly higher on AM as was the expression of CD 71 (transferrin receptor). The receptor density for OX-42 was higher on a fraction of IM. This might be compatible with a stronger interaction between these cells and, for example, matrix components. IM had more surface antigen expressing MHC class Ia (OX-6) and CD 54. Both receptors are important for the antigen presentation capacity of macrophages. These findings show profound differences in phenotype between AM and IM and indicate that IM is a highly immunocompetent cell and should not be regarded only as a precursor to AM.