Human alveolar macrophages inhibit immunoglobulin production in response to direct B cell mitogen

B lymphocytes are crucial participants in pulmonary immune defense. However, excess local antibody production is associated with accelerated lung destruction in several types of lung disease. The purpose of the current study was to study the potential role of alveolar macrophages (AM) in the local regulation of immunoglobulin (Ig) production in the lung in response to a direct B cell mitogen, Staphylococcus aureus Cowan strain (SAC). AM, when added to peripheral blood mononuclear cells, caused a dose-dependent inhibition of IgG and IgM, while not affecting IgA production in response to SAC. The mechanism of the AM-induced inhibition included both membrane-bound and soluble signals. The inhibition was not abrogated by indocin and catalase, or reversed by blocking antibodies to transforming growth factor-beta or interferon-gamma. Mononuclear cells isolated from human lung parenchyma displayed a reduced response to SAC compared with blood cells. However, depletion of macrophages from the parenchymal cells was associated with a restoration of IgG production in response to SAC. The results demonstrate that AM inhibit B cell responses to direct B cell mitogen and suggest that the effect of AM is selective for IgM and IgG.