Dendritic cells are present in the alveolar region of lungs from specific pathogen-free rats

The purpose of the present study was to establish the presence of dendritic cells in the alveolar region of rat lungs and determine whether they express the antigenic markers suggested in previous studies. We used pathogen-free rats to minimize changes in the cell populations due to subclinical infection and disease as inflammatory products can increase the number of dendritic cells in lungs. As an assay for accessory cells, proliferation of periodate-treated T cells was used. This is an accessory cell dependent response, and most importantly, dendritic cells, not macrophages, can act as accessory cells in this assay. Using this approach, lung cell suspensions, derived primarily from parenchymal tissue, were found to contain accessory cells. The accessory cell population was low density, radiation resistant, Fc receptor negative, nonadherent, and contained cells with similar morphology to dendritic cells from other tissues. About 90% of the accessory cell activity was lost upon depletion of MRC OX-6 (Ia) or MRC OX-1 (leukocyte common antigen, LCA) positive cells. The expression of LCA indicates that the accessory cells were bone marrow derived and not endothelial or epithelial cells. In contrast to suggestions from earlier studies, no loss in activity was seen when cell depletion was performed using CD-4 (W3/25) and macrophage specific (ED-2) antibodies. Thus, these cells appear to be functionally and phenotypically similar to dendritic cells isolated from other rat lymphoid tissues. However, they lack antigenic markers (CD-4 and ED-2) suggested to be expressed by alveolar dendritic cells as described in previous immunocytochemistry studies.