Because the expression of
IgG Fc receptors and
complement receptors on macrophages may vary in a tissue-specific manner, we used
monoclonal antibodies and flow cytometry to define the expression and function of
opsonin receptors on fresh normal and
cystic fibrosis (CF) bronchoalveolar lavage (BAL) macrophages. Using flow cytometry to separately analyze individual types of cells, we then determined the relative contributions of
IgG and
complement to phagocytosis of Pseudomonas aeruginosa by fresh BAL cells, avoiding alterations in receptor expression due to in vitro purification or culturing techniques. Neither normal nor CF BAL macrophages express appreciable amounts of the
complement receptors CR1, CR2, or CR3. These results were confirmed by immunohistochemical staining of fixed lung sections. BAL macrophages express a high-affinity
IgG receptor,
Fc gamma RI, that is not found on neutrophils (PMN). In contrast,
chemoattractant-stimulated blood PMN express large amounts of CR1 and CR3 but do not express
Fc gamma RI. These results correlate with phagocytosis assays, which show that phagocytosis by macrophages is enhanced by relatively low concentrations of
IgG but that the addition of
complement does not further increase their phagocytosis. In contrast, low concentrations of
IgG alone do not promote phagocytosis by PMN, whereas addition of
complement markedly enhances phagocytosis by PMN. These results may explain the previously reported sensitivity of macrophages rather than PMN to the "blocking" effects of anti-Pseudomonas
antibodies from CF patients, and emphasize the pathologic significance of interference with
IgG and
complement mediated opsonization in the lung in CF.