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...

Summal'yResident pulmonary alveolar macrophages (PAM) play an important role in the maintenance of immunological homeostasis in the lung via downmodulation of local T cell responses in the steady state. The present study demonstrates that this pathway for T cell suppression is reversible via granulocyte/macrophage colony-stimulating factor (GM-CSF). Thus, freshly isolated PAM strongly inhibit mitogen-induced T cell proliferation, and pretreatment of the PAM with cytokinerich lung-conditioned medium (LCM) generated by exposure of lung to bacterial lipopolysaccharide (LPS) abrogated this suppressive activity. LCM from lungs of normal and athymic nude mice exhibited identical activity. Moreover, the PAM-modulating activity of LCM was inhibited by blocking antibody specific for GM-CSF, and the activity of LCM could be reproduced by recombinant GM-CSF. This suggests that secretion of GM-CSF by mesenchymal cells and/or macrophages under stimulation from agents such as LPS provides a potential mechanism for upregulation of local T cell responsiveness during acute inflammation. In addition, experiments with a range of cytokines indicated that interleukin 4, transforming growth factor 31 and tumor necrosis factor o~ (TNF-oe) exhibited weaker (but significant) modulatory effects on PAM, and (in the case of TNF-c~) amplified the effects of GM-CSF.

Repeated exposure of healthy mice to an aerosol of ovalbumen (OA) leads to the development of IgE-isotype-specific immunological tolerance. However, when initial OA exposure occurs during the acute phase of influenza infection, tolerance does not occur, and the mice instead develop high titres of OA-specific IgE in response to subsequent challenge with the allergen.

Intracutaneous injection of purified peritoneal macrophages harvested from ovalbumin (OVA)-hypersensitive high-IgE-responder BN rats into naive animals sensitised the injection sites for subsequent OVA-specific passive cutaneous anaphylaxis (PCA) reactions. The underlying mechanism(s) were investigated using a macrophage cell line (WEHI 265.1), which exhibited comparable sensitising activity in rat or mouse skin, after initial pulsing in vitro with antiserum rich in OVA-specific IgE. Transfer of OVA-hypersensitivity by the cell line (1) was IgE-dependent and did not occur when the cells were pre-exposed to antiserum containing OVA-specific IgG alone, (2) was blockable by saturation of cell surface receptors in the recipient with myeloma IgE (but not myeloma IgG), and (3) did not occur in mast cell-deficient mice carrying the W/W^v mutation, in contrast to their normal heterozygous littermates which developed marked OVA-hypersensitivity at the injection site. These results are consistent with arming of IgE-receptors on cutaneous mast cells by IgE antibody released from macrophages, and hint at a possible role for phagocytes in amplifying IgE-mediated reactions in tissues.