Sllnllnal~Class II major histocompatibility complex (Ia)-bearing dendritic cells (DC) from airway epithelium and lung parenchyma express low-moderate antigen presenting cell (APC) activity when freshly isolated. However, this function is markedly upregulated during overnight culture in a manner analogous to epidermal Langerhans cells. The in vitro "maturation" process is inhibited by coculture with pulmonary alveolar macrophages (PAM) across a semipermeable membrane, and the degree of inhibition achieved can be markedly increased by the presence of tumor necrosis factor or. In addition, PAM-mediated suppression of DC function is abrogated via inhibition of the nitric oxide synthetase pathway. Functional maturation of the DC is accompanied by increased expression of surface Ia, which is also inhibited in the presence of PAM. Prior elimination of PAM from DC donors via intratracheal administration of the cytotoxic drug dichloromethylene diphosphonate in liposomes, 24-72 h before lung DC preparation, achieves a comparable upregulation of APC activity, suggesting that (consistent with the in vitro data) the resident PAM population actively suppresses the APC function of lung DC in situ. In support of the feasibility of such a regulatory mechanism, electron microscopic examination of normal lung fixed by intravascular perfusion in the inflated state (which optimally preserves PAM in situ), revealed that the majority are preferentially localized in recesses at the alveolar septal junctions. In this position, the PAM are in intimate association with the alveolar epithelial surface, and are effectively separated by as little as 0.2/~m from underlying interstitial spaces which contain the peripheral lung DC population. A similar juxtaposition of airway intraepithelial DC is demonstrated with underlying submucosal tissue macrophages, where the separation between the two cell populations is effectively the width of the basal lamina. p revious studies from this laboratory initially drew attention to the role of dendritic cells (DC) 1 analogous to those described by Steinman and Nussenzweig (1), as the principal resident APC population in parenchymal lung tissue of rat (2). These observations were confirmed and extended by other investigators in a variety of species including human (3-9), and were further extended to the epithelium of the conducting airways where class II MHC antigen (Ia)-bearing DC were shown to form a tightly meshed network comparable to that of epidermal Langerhans cells (10-12).1 Abbreviations used in this paper: DC, dendritic cell; DPDP, dichloromethylene diphosphate; LNC, lymph node T cell; MMA, monomethylarginine; PAM, pulmonary alveolar macrophage; RLN, regional lymph node; VC, veil cell. 397The epithelial surfaces within the respiratory system occupied by these DC are continuously exposed to an array of pathogenic and nonpathogenic airborne antigens from the environment, and the maintenance of local homeostasis requires fine control of immunological processes, particularly those involving T cell act...

Conventional immunohistochemical analysis of airway intraepithelial class II major histocompatibility complex (Ia) expression demonstrates a morphologically heterogeneous pattern of staining, suggestive of the presence of a mixed population of endogenous antigen presenting cells . Employing a novel tissue sectioning technique in conjunction with optimal surface antigen fixation, we now demonstrate that virtually all intraepithelial la staining throughout the respiratory tree in the normal rat, can be accounted for by a network of cells with classical dendritic cell (DC) morphology. The density of DC varies from 600-800 per mm2 epithelial surface in the large airways, to 75 per mm2 in the epithelium of the small airways of the peripheral lung. All the airway DC costain for CD4, with low-moderate expression of a variety of other leukocyte surface markers . Both chronic (eosinophilic) inflammation and acute (neutrophilic) inflammation, caused respectively by inhalation of chemical irritants in dust or aerosolised bacterial lipopolysaccharide (LPS), are shown to be accompanied by increased intraepithelial DC density in the large airways (in the order of 50%) and up to threefold increased expression of activation markers, including the (i chain of CD11/18 . The kinetics of the changes in the DC network in response to LPS mirrored those of the transient neutrophil influx, suggesting that airway intraepithelial DC constitute a dynamic population which is rapidly upregulated in response to local inflammation . These findings have important theoretical implications for research on T cell activation in the context of allergic and infectious diseases in the respiratory tract.T he maintenance of homeostasis in the lung requires precise control oflocal interactions between inhaled antigens impinging on epithelial surfaces in the airways, and the underlying cells of the respiratory mucosal immune system . In particular, an efficient mechanism is required for discrimination between pathogenic antigens such as those present on bacteria and viruses, and inert nonpathogenic antigens (pollens, animal danders, etc.) which are ubiquitous in the natural environment . The failure on the part of the T cell system to maintain this distinction appears to underly a variety of immunoinflammatory diseases in the respiratory tract (1, 2). A key element of local immune regulation in the lung is thus the nature of the cell population(s) responsible for the initial presentation of inhaled antigens to T cells. Recent studies from a number of laboratories have focused attention on the potential role of dendritic cells (DC)t analogous to those ini-1 Abbreviations used in this paper: SALT, bronchus-associated lymphoid tissue ; DC, dentritic cell ; HRP, horseradish peroxidase; RT, room temperature; SHAM, sheep anti-mouse. tially described by Steinman et al. (3), in antigen recognition in the lung. DC which exhibit strong antigen-presenting activity in vitro have been extracted from enzymatic digests ofparenchymal lung tissue (4-8) and from the trach...

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.

This study utilises a simple technique to section airway epithelium in a plane parallel to the basement membrane, thus providing a unique plan view of the intra-epithelial cell populations. Immunoperoxidase staining of these tissue sections for class II major histocompatibility complex la antigen reveals a virtually contiguous network of Ia-positive dendritic cells (DC) within the epithelium. These DC are shown to be capable of binding inhaled antigens in vivo in a form suitable for presentation to T cells. The strategic location of these cells and the fact that they account for virtually all staining in the airway epithelium during the steady state is convincing evidence that the DC network functions as the ‘first line of defence’ in surveillance for inhaled antigens and further suggests a major role for the intra-epithelial DC in allergic and infectious disease(s) in the respiratory tract.