Secretory and accessory cell functions of the alveolar macrophage.

We have attempted to address the requirements necessary for alveolar macrophage accessory cell function. We have also examined the in vitro and in vivo factors that must be taken into account when interpreting results from experimental studies. Differences in phenotypic expression by rat alveolar pleural and peritoneal macrophages are noted, as well as the differing expression of major histocompatibility complex (MHC) class II molecules. Furthermore, alveolar macrophages, harvested from rat lung, do not express the interleukin (IL)-1 cytokines, and lipopolysaccharide (LPS) treatment of quiescent cells (after 24-hr in vitro culture) induces low levels of expression of IL-1 alpha and IL-1 beta. Short-term inhalation of refractory ceramic fibers, however, results in markedly increased IL-1 beta expression after stimulation with LPS. We suggest that, in vivo, IL-1 beta may be involved in the initial recruitment and activation of inflammatory cells rather than in induction of immune responses. We also postulate, based on recent published evidence, that alveolar macrophages activate the dendritic cells within the respiratory epithelium. Thus alveolar macrophages would release cytokines critical for the activation of dendritic cells during the afferent limb of the immune response, and they would respond to products of sensitized T-cells such as interferon-gamma and IL-4 to interact with T-helper cells in an antigen-specific MHC-restricted manner during the efferent limb of the response.