Regulation of rat pulmonary dendritic cell immunostimulatory activity by alveolar epithelial cell-derived granulocyte macrophage colony-stimulating factor.

The presentation and recognition of foreign antigen is the critical initial event in the development of local immunity. In the lung, antigen-presenting cell activity is largely attributable to pulmonary dendritic cells (DC) that are distributed along the airways and throughout the pulmonary interstitium in close proximity to overlying alveolar epithelial cells. To test the hypothesis that DC immunostimulatory activity might be locally regulated by overlying alveolar epithelial cells, we evaluated the ability of rat type II alveolar epithelial cells to influence the capacity of purified rat pulmonary DC to stimulate T-cell proliferation in an allogeneic, mixed leukocyte reaction. We found that alveolar epithelial cells greatly enhanced the ability of dendritic cells to induce T-cell proliferation. This effect on DC immunostimulatory activity was mediated by a soluble factor preferentially secreted from the basolateral epithelial cell surface. Alveolar epithelial cultures were found to express mRNA for granulocyte macrophage colony-stimulating factor (GM-CSF), and blocking antibodies against GM-CSF partially neutralized the effect of epithelial cell-conditioned media on DC stimulatory activity, indicating that the effect was due at least in part to alveolar epithelial cell-derived GM-CSF. Through the polar secretion of GM-CSF, alveolar epithelial cells may play an important role in creating distinct immunologic environments within the lung.