Receptors for human IgG subclasses on human alveolar macrophages

The biology of individual heavy chain subclasses of human IgG (IgG1-4) in lung host defenses has become important now that specific deficiencies of certain subclasses (IgG2 and IgG4) can be associated with chronic sinopulmonary infections and that IgG4 can be increased in forms of hypersensitivity lung disease. Because IgG is an important opsonic antibody that promotes attachment of bacteria or particles to phagocytes, the relative binding of IgG subclasses to membrane receptors on human alveolar macrophages might predict the efficacy of specific opsonin-mediated phagocytosis. With in vitro cultured normal alveolar macrophages, various IgG complexes were assessed for receptor binding with a rosetting assay. For respiratory cells in culture for 24 h, about 25% of the macrophages bound IgG3 and about 10% bound IgG1; binding with IgG2 and IgG4 complexes was minimal. In macrophage cultures maintained for as long as 6 days, this pattern of binding persisted. However, in very short-term cultures, 30 min and 105 min after cell adherence had occurred, binding was much greater for IgG3 complexes (about 60%); likewise IgG1 and IgG4 bound to about 20% of the cells. The IgM erythrocyte complexes, usually showing no binding at later time points in culture, bound to 20% of the cells, acutely. Therefore, our studies found that IgG3 consistently bound to more alveolar macrophages than the other subclasses, including IgG1. Also, the duration in culture of adherent cells may significantly affect the pattern of binding.