Neutral proteinase expression by human mononuclear phagocytes: a prominent role of cellular differentiation

Human mononuclear phagocytes have the capacity to participate directly in extracellular matrix turnover via the secretion of neutral proteinases. These neutral proteinases include the serine proteinases, elastase and cathepsin G and the metalloproteinases, interstitial collagenase, 92 kD type IV collagenase, 72 kD type IV collagenase and stromelysin. Mononuclear phagocytes also produce the counter-regulatory metalloproteinase inhibitor, TIMP (tissue inhibitor of metalloproteinases). We have studied the capacity of normal human mononuclear phagocytes and of the human monocytic tumor line U937 to elaborate proteinases and inhibitors. The serine proteinases, elastase and cathepsin G, are present only at the earliest stages of mononuclear phagocyte differentiation (U937 cells in the basal state, freshly isolated peripheral blood monocytes) and are stored within intracellular granules. As human mononuclear phagocytes differentiate (U937 cells exposed to phorbol esters, human monocytes cultured in vitro), the cellular content of these serine proteinases declines rapidly. Accompanying the acquisition of a more differentiated state, the ability for regulated secretion of the neutral metalloproteinases is attained. This capacity is acquired in a sequential manner, with secretion of the 92 kD type IV collagenase observed at earlier states of differentiation while release of stromelysin requires a fully differentiated and LPS (lipopolysaccharide)-stimulated alveolar macrophage. Interstitial collagenase and 72 kD type IV collagenase are synthesized at intermediate stages of differentiation. In comparison to human fibroblasts, human mononuclear phagocytes produce approximately 10-30% of the interstitial collagenase, 10% of the stromelysin and 1-2% of the 72 kD type IV collagenase on a per cell basis. Synthesis of the 92 kD type IV collagenase is restricted to the inflammatory cell (but also occurs in neutrophils and keratinocytes).