Monocytes incubated with surfactant: a model for human alveolar macrophages?

Monocytes migrate to the lungs and enter the alveoli where they come into contact with surfactant and differentiate into alveolar macrophages. This study focused on the question of the extent to which monocytes and monocyte-derived macrophages (MDM) incubated with surfactant resemble alveolar macrophages. Surfactant-incubated monocytes shared with alveolar macrophages the intracellular presence of surfactant, efficient phagocytosis of opsonized Staphylococcus aureus, and poor intracellular killing of ingested bacteria. The suppressive effect of surfactant on bactericidal activities of monocytes could not be attributed to either the surfactant lipid fraction or surfactant protein A. Monocytes incubated with surfactant differed from alveolar macrophages with respect to expression of various Fc and complement receptors involved in intracellular killing of bacteria. Surfactant-incubated monocytes produced significantly more H2O2 upon stimulation with phorbol ester than alveolar macrophages, but significantly less than control monocytes. Together, monocytes and MDM incubated with surfactant, although similar to alveolar macrophages in some aspects, are not an adequate model for alveolar macrophages. Most likely, factors other than surfactant in the microenvironment of the alveoli, such as oxygen tension, play a role in the differentiation of monocytes to alveolar macrophages as well.