DISTRIBUTION AND IMMUNOPHENOTYPE OF MONONUCLEAR PHAGOCYTES AND DENDRITIC CELLS IN THE HUMAN LUNG

Mononuclear phagocytes and dendritic cells (DC) play an important role in the immune response in the lung. DC act in the afferent phase of t he immune response by presenting antigen to T cells, while macrophages play a role in the efferent phase by exerting phagocytic/cytotoxic fu nctions. We investigated the localization and the marker pattern of th ese cells in the human lung. Macrophages, identified as large, rounded , acid phosphatase-positive cells, were mainly detected in the alveola r spaces, in the lumen of the bronch(iol)us, and in the bronchoalveola r lavage (BAL). They were positive for major histocompatibility comple x (MHC) class II antigens (DR, DQ), CD68, RFD7, RFD9, and partly posit ive for RFD1. Irregularly shaped cells with a marker pattern comparabl e to that of blood-derived DC (positive for DR, DQ, L25, RFD1, and CD6 8) were predominantly observed in the epithelium and subepithelial tis sue of the bronch(iol)us and in the bronchus-associated lymphoid tissu e. In the epithelium, approximately 30% of these cells were positive f or CD1a (OKT6). In the subepithelial tissue, these DC formed character istic small clusters with T cells. The BAL, the alveolar spaces, and t he alveolar walls contained only a small number of DC. These immunohis tologic data suggest that the bronch(iol)us is well equipped to initia te immune responses. The high number of macrophages in the alveolar co mpartment, which have been described to suppress T cell proliferation, together with low numbers of DC, makes the alveolar compartment less suited for mounting an immune response.