SummaryFor over 25 years, the cytokine known as macrophage migration inhibitory factor (MIF) has been considered to be a product of activated T lymphocytes. We recently identified the murine homolog of human MIF as a protein secreted by the pituitary in response to endotoxin administration. In the course of these studies, we also detected MIF in acute sera obtained from endotoxin-treated, T cdl-defident (nude), and hypophysectomized mice, suggesting that still more cell types produce MIE Here, we report that ceils of the monocyte/macrophage lineage are an important source of MIF in vitro and in vivo. We observed high levels of both preformed MIF protein and MIF mRNA in resting, nonstimulated cells. In the murine macrophage cell line RAW 264.7, MIF secretion was induced by as little as 10 pg/ml of lipopolysaccharide (LPS), peaked at 1 ng/ml, and was undetectable at LPS concentrations >1/~g/ml. A similar stimulation profile was observed in LPS-treated peritoneal macrophages; however, higher LPS concentrations were necessary to induce peak MIF production unless cells had been preincubated with interferon 3, (IFN-'y). In RAW 264.7 macrophages, MIF secretion also was induced by tumor necrosis factor oe (TNF-c 0 and IFN-% but not by interleukins 1 B or 6. Of note, MIF-stimulated macrophages were observed to secrete bioactive TNF-cz. Although previously overlooked, the macrophage is both an important source and an important target of MIF in vivo. The activation of both central (pituitary) and peripheral (macrophage) sources of MIF production by inflammatory stimuli provides further evidence for the critical role of this cytokine in the systemic response to tissue invasion. N 'umerous observations over the years indicate that the hypothalamic-pituitary-adrenal axis is essential in the normal host response to infection and tissue invasion (1-4). In the course of investigating the role of the pituitary in systemic inflammatory responses, we identified the murine homolog of human macrophage migration inhibitory factor (MIF) 1 as a protein secreted by the anterior pituitary in response to LPS stimulation (5). Pituitary MIF was found to contribute to the MIF present in plasma in the postacute phase (>2 h) of endotoxemia. Recombinant murine MIF markedly increased lethality when coinjected with LPS, and anti-MIF antibody conferred full protection against lethal endotoxemia, suggesting that MIF is an important and possibly critical mediator of endotoxic shock.Historically, MIF has been considered a product of activated T lymphocytes and appears to exhibit a number of 1 Abbreviations used in this ~per: FBS, fetal bovine serum; MIF, macrophage migration inhibitory factor. macrophage-activating properties (6-12). While studying MIF production in experimental endotoxemia, however, we detected MIF in acute sera obtained from LPS-injected, T celldeficient (nude), and hypophysectomized mice, suggesting that yet additional cell types may produce MIF in vivo. Since the macrophage is a major source of the cytokines that appear in respons...
