Studies of membrane receptors and phagocytosis in subpopulations of rat alveolar macrophages.

Recent data suggest that alveolar macrophages may be a heterogeneous group of cells with several subpopulations. This study was undertaken to determine if there is heterogeneity among rat alveolar macrophages with respect to receptors for zymosan, immunoglobulin, and complement. In addition, avidity for the immunoglobulin IgG was defined by opsonizing sheep red blood cells (SRBC) with different amounts of IgG. Alveolar macrophages were harvested by bronchoalveolar lavage and separated into 18 density-defined fractions by centrifugation through a continuous isosmotic gradient of colloidal silica. All density-defined alveolar macrophages exhibited similar abilities to attach zymosan and SRBC opsonized with a high level of IgG. In contrast, macrophages of a density of 1.046 g/ml to 1.075 g/ml exhibited high receptor activity capable of attaching and phagocytizing SRBC opsonized with small amounts of IgG. Similarly, attachment indices for complement-coated SRBC exhibited a peak activity among fractions exhibiting similar high avidity receptors for IgG. These results demonstrate the functional heterogeneity with respect to IgG and complement receptors among density-defined rat alveolar macrophage subpopulations.