MHC class II antigen-bearing dendritic cells in pulmonary tissues of the rat. Regulation of antigen presentation activity by endogenous macrophage populations.

Collagenase digestion of tissue slices from perfused, lavaged SPF rat lung released approximately 10(8) viable mononuclear cells per gram tissue, which comprised 35% T lymphocytes and up to 26% macrophages. A subset of these cells that were Ia+, surface Ig-, nonadherent, FcR- and of ultra low density (putative dendritic cells [DC]), presented protein antigen to immune T cells in vitro, and this function was inhibited by the presence of low numbers of endogenous adherent, FcR+ cells (putative macrophages). APCs were also identified in digests from tracheal epithelium, and were shown to bind antigen in immunogenic form as a result of natural (inhalation) exposure in vivo. Immunoperoxidase staining of frozen sections revealed populations of strongly Ia+ cells with prominent DC-like morphology within the alveolar septal walls and the tracheal epithelium; in both areas, they were closely associated with pleiomorphic cells that expressed macrophage surface markers. We accordingly postulate that interactions between Ia+ antigen-presenting DCs and endogenous tissue macrophages play an important role in regulating T cell activity in the respiratory tract.