Macrophage migration inhibitory factor is a critical mediator of the activation of immune cells by exotoxins of Gram-positive bacteria (T lymphocyteycytokineytoxic shock syndrome toxin 1ysuperantigensyseptic shock)

Discovered in the early 1960s as a T cell cytokine, the protein mediator known as macrophage migration inhibitory factor (MIF) has been found recently to be a pituitary peptide released during the physiological stress response, a proinf lammatory macrophage cytokine secreted after LPS stimulation, and a T cell product expressed as part of the antigen-dependent activation response. We report herein that MIF also plays a critical role in the innate host response to staphylococcal and streptococcal exotoxins. In RAW 264.7 or elicited mouse peritoneal macrophages, peak MIF secretion was induced by concentrations of the staphylococcal toxic shock syndrome (TSS) toxin 1 (TSST-1) and the streptococcal pyrogenic exotoxin A as low as 10 pgyml. Moreover, dose-response studies of splenocyte cytokine production showed that lower concentrations of TSST-1 (10 pgyml) were needed to release MIF than to induce interleukin 2 or interferon-g secretion (1 ngyml). We also studied the effect of neutralizing anti-MIF antibodies on TSST-1-induced lymphocyte proliferation and lethal toxic shock. Pretreatment of C57BLy6 mice with anti-MIF antibody 2 hr before TSST-1 injection prevented spleen enlargement and reduced by 50% the proliferation of splenocytes measured ex vivo. In a lethal mouse model of TSST-1-induced shock, anti-MIF antibody increased survival from 8% to 54% (P < 0.0001). These studies indicate that Gram-positive exotoxins are extremely potent inducers of MIF secretion and establish a critical role for MIF and the macrophage in the pathogenesis of the TSSs and in the innate immune response. The biochemical nature and biological function of the protein known as macrophage migration inhibitory factor (MIF), which was described originally to be a T lymphocyte product that inhibited macrophage migration, remained enigmatic for almost three decades (1–4). Recent findings, however, have ascribed a pivotal role to MIF within the immune and endocrine systems. First, the sequence structure of a newly identified protein that was released by pituitary cells in a hormonelike fashion was found to be that of MIF (5–7). Second, monocytes and macrophages that had previously been considered to be the target of MIF action, were observed to be a significant source of MIF after stimulation with endotoxin (lipopolysaccharide, or LPS) or with the cytokines tumor necrosis factor a (TNFa) and interferon-g (IFNg) (8). MIF also was described to mediate certain pro-inflammatory effects, stimulating macrophages to produce TNFa and nitric oxide when given in combination with IFNg (8, 9). Like TNFa and IL-1b, MIF plays a central role in the host response to endotoxemia. Coinjection of recombinant MIF and LPS exacerbates LPS lethality, whereas neutralizing anti-MIF antibodies fully protect mice from endotoxic shock. Similar observations have been made recently in a mouse model of Escherichia coli peritonitis (T.C., unpublished observations). Studies of MIF expression by mouse and human T lymphocytes also established that MIF is a proinflammatory T cell cytokine that is required for T cell activation and antibody production by B cells (10). Finally, the critical regulatory role played by MIF was underscored by the finding that glucocorticoids at low dose stimulated the production of MIF by macrophages and T cells, the first such response ascribed to glucocorticoids to date (6, 10). Importantly, MIF has been shown to function to control or ‘‘counter-balance’’ the anti-inflammatory and immunosuppressive effects of glucocorticoids on macrophages and T cells (6, 10, 11). The proportion of severe infections and septic shock caused by Gram-positive bacteria has increased markedly in recent years, such that these pathogens now account for 40–50 percent of all cases of septic shock occurring in the intensive care setting (12). Staphylococcal and streptococcal toxic-shock syndromes (TSS) and viridans streptococcal infections accompanied by shock or the adult respiratory distress syndrome are examples of the fulminant and often fatal complications of Gram-positive sepsis. In contrast to Gram-negative septic shock, very little is known about the pathophysiology of Gram-positive infections leading to septic shock. In the case of TSS for instance, staphylococcal and streptococcal exotoxins appear to cause a massive activation of macrophages and T lymphocytes, which leads to the production of high levels of proinflammatory cytokines (13–18). Many Gram-positive bacteria do not produce exotoxins however, and they cause shock by mechanisms that remain to be fully unraveled. Given the central regulatory role of MIF in both the macrophage and the T cell limbs of the acute inflammatory and immune responses, we have investigated the extent as well as the role of MIF expression in the host response to Grampositive exotoxins. In this study, we report that the TSS toxin-1 (TSST-1) and the streptococcal pyrogenic exotoxin A (SPEA) are very potent inducers of MIF production by immune cells and that MIF is an important mediator of lymphocyte activation and toxic shock triggered by these toxins. MATERIALS AND METHODS Reagents. TSST-1 and streptococcal pyrogenic exotoxin A (SPEA) were obtained from Toxin Technology (Sarasota, FL). The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. © 1998 by The National Academy of Sciences 0027-8424y98y9511383-6$2.00y0 PNAS is available online at www.pnas.org. Abbreviations: MIF, macrophage migration inhibitory factor; LPS, lipopolysaccharide; SPEA, streptococcal pyrogenic exotoxin A; TSS, toxic-shock syndrome; TSST-1, TSS toxin 1; IL, interleukin; IFN, interferon; TNF, tumor necrosis factor. *Present address: Division of Infectious Diseases, Department of Internal Medicine, Centre Hospitalier Universitaire Vaudois, CH1011 Lausanne, Switzerland. †To whom reprint requests should be addressed. e-mail: rbucala@ picower.edu.