Resident pulmonary alveolar macrophages (PAM) play an important role in the maintenance of immunological homeostasis in the lung via downmodulation of local T cell responses in the steady state. The present study demonstrates that this pathway for T cell suppression is reversible via granulocyte/macrophage colony-stimulating factor (GM-CSF). Thus, freshly isolated PAM strongly inhibit mitogen-induced T cell proliferation, and pretreatment of the PAM with cytokine-rich lung-conditioned medium (LCM) generated by exposure of lung to bacterial lipopolysaccharide (LPS) abrogated this suppressive activity. LCM from lungs of normal and athymic nude mice exhibited identical activity. Moreover, the PAM-modulating activity of LCM was inhibited by blocking antibody specific for GM-CSF, and the activity of LCM could be reproduced by recombinant GM-CSF. This suggests that secretion of GM-CSF by mesenchymal cells and/or macrophages under stimulation from agents such as LPS provides a potential mechanism for upregulation of local T cell responsiveness during acute inflammation. In addition, experiments with a range of cytokines indicated that interleukin 4, transforming growth factor beta 1 and tumor necrosis factor alpha (TNF-alpha) exhibited weaker (but significant) modulatory effects on PAM, and (in the case of TNF-alpha) amplified the effects of GM-CSF.