Secretory function of mononuclear phagocytes: a review

The mononuclear phagocytes comprise the bone marrow promonocyte, the blood monocyte, and the tissue macrophages: the histiocyte, Kupffer cells, the macrophages of lungs, lymph nodes, bone marrow, serous cavities, nervous system (microglia) and possibly bone (osteoclasts?). Histiocytes are the same cells as macrophages. Excluded from the mononuclear phagocyte line are the reticular cells, the endothelial cells, and the fibroblasts. Phagocytes are the main cells in chronic inflammation and essential in wound healing and tissue repair. They also represent a key cell in immunity, especially as regards the lymphocyte macrophage interactions. Lymphocytes call forth the phagocytes by a chemotactic substance which can be found in cultures of activated lymphocytes. The uptake of antigens is carried out in part by structures on their surfaces which interact witn the Fc piece of some immunoglobulins or with C3. A third property of the phagocyte is their phagocytosis (activated macrophages). It is the T lymphocyte which activates these macrophages. The main point made in this review, is that the phagocytes may participate in inflammation and immunity as a result of the release of chemical mediators. The products secreted by phagocytes are: enzymes (lysosomal enzymes, lysozyme, plasminogen activators, collagenase and elastase); products affecting cells in culture (lymphostimulatory factor, colony stimulating factor, inhibitors of DNA synthesis and toxins); and, finally, the complement proteins (interferons and pyrogen). There appear to be several patterns of modulation of the secretory process. For many products, a stimulus is the phagocytosis of a particle: plasminogen activator, collagenase, elastase and for the C protein. In contrast, secretion of lysozyme is not triggered by a phagocytic challenge, but the nature of the phagocytized particles appears important. The secretory function gives the phagocytes a regulatory influence in inflammation and tissue repair, in infection, perhaps in the control of hemopoiesis, and in critical steps in immunity; they are also prominent cells in various processes of collagen resorption: the tadpole tail during metamorphosis, the postpartum uterus at the time of collagen resorption and in granulomas. In many of these examples, there was some phagocytosis of collagen fibers. When there is intense macrophage infiltration produced by stimulants which are phagocytized and which activate the macrophage, one can predict that there will be an intense release of the products mentioned above. Injection of streptococcal cell wall preparations produces, granulomatous persistent inflammation, with distortion and alteration of connective tissues. Speculation has arisen with regard to secretion of elastase by inflammatory cells and the destruction of the alveolar wall in emphysema: the activated alveolar macrophage is known to secrete elastase. The importance and implications of the secretory process and phagocytes is obvious. (112 references are cited).