Role of Macrophage Migration Inhibition Factor in Kidney Disease Free

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine and plays an important role in kidney disease. MIF is constitutively produced under normal circumstances but is significantly upregulated in the diseased kidney by T cells,macrophages and various non-immune cells including tubular and glomerular epithelial cells, mesangial cells, endothelial cells, fibroblasts and vascular smooth muscle cells [1, 2, 3]. Renal MIF is released and exerts its biological activities in many pathological conditions such as septic shock,renal inflammation, immune injury and diabetes. Studies using neutralizinganti-MIF antibodies, MIF knockout mice and MIF transgenic animals demonstrate that MIFis a crucial mediator in the pathogenesis of kidney disease [4, 5, 6, 7].

This article will review the current understanding of the general immunopathologic mechanisms of MIF in immune and inflammatory kidney diseases. Also, the signaling mechanisms whereby MIF activates T cells and macrophages, induces the expression of pro-inflammatory cytokines, and interacts with glucocorticoids will be emphasized.

Major progress in understanding the functional role of MIF has been made on the basis of the identification of the MIF receptor CD74 [8], which is the cell-surface form of the MHC class II invariant chain. Although CD74 functions as a cell-surface binding site for MIF, it does not have a typical signal-transducing cytosolic domain and further work indicates that the signal transduction of MIF requires the recruitment and activation of CD44, a genetically polymorphic molecule with an important role in cell-extracellular matrix interaction [9]. As shown in figure 1, it is now clear that CD44 acts as an integral member of the CD74 receptor complex leading to MIF signal transduction. Furthermore, MIF-induced ERK1/2 MAP kinase phosphorylation through CD44 also requires the Src tyrosine kinase [9]. Several of the biologic activities of MIF have been shown to proceed via activation of ERK1/2 MAP kinase. These include modulation of arachidonic acid metabolism and prostaglandin production, regulation of p53 activity and the intracytoplasmic action of c-Jun activation domain binding protein-1, and the activation of additional ERK effectors, such as the Ets family of transcription factors that regulate toll-like receptor expression [reviewed in [10]. Like MIF, upregulation of both MHC class II and CD44 is found in the diseased kidney including glomerular cells, tubulointerstitial cells and infiltrating T cells and macrophages that contribute significantly to the severity of renal inflammation and functional impairment [11]. Thus, coexpression of MIF and its receptor complex CD74 and CD44 in the diseased kidney suggests that MIF produced by inflammatory T cells and macrophages as well as intrinsic kidney cells during renal inflammation may mediate progressive renal injury via the CD74/CD44 receptor complex in a paracrine and autocrine manner.

MIF mRNA and protein is constitutively expressed in many cells in the normal kidney, including glomerular and tubular epithelial cells, endothelial cells, fibroblasts and vascular smooth muscle cells [1, 2]. MIF is markedly upregulated in disease conditions such as experimental and human glomerulonephritis and renal allograft rejection [1, 2, 3]. Indeed, strong MIF expression by glomerular and tubulointerstitial cells is tightly associated with profound macrophage and T-cell accumulation and contributes to focal glomerular and tubulointerstitial damage and particularly glomerular crescent formation. Correlation analysis shows that the level of MIF in both human and experimental kidney diseases including renal allograft rejection is correlated closely with the number of infiltrating macrophages and T cells, resulting in progressive renal dysfunction such as proteinuria, an increased serum creatinine and a fall of glomerular filtration rate. The pathogenic role of upregulation of MIF in the development of kidney injury is evident in MIF transgenic mice in which conditional expression of MIF in podocytes leads to the development of severe proteinuria and glomerulosclerosis, resulting in end-stage kidney disease [6].

MIF is produced by activated T cells. In turn, MIF also causes T cell activation via a paracrine and autocrine manner in kidney diseases such as glomerulonephritis and renal allograft rejection. Evidence for a key role of MIF in T-cell-mediated kidney disease includes the observation that MIF-producing T cells are exclusively localized to the area of severe tissue injury, including glomerular crescentic formation [1, 2], focal glomerular and tubulointerstitial lesions [2], and necrotic vascular inflammation [3]. Furthermore, T-cell-mediated renal injury is prevented in lupus-prone mice targeted for the deletion of MIF whilst administration of anti-MIF antibody is protective in a rat model of anti-GBM crescentic glomerulonephritis, thereby demonstrating a critical role for MIF in T-cell-mediated kidney disease [4, 7].

There are several possible mechanisms in the regulation of T-cell-mediated immune injury by MIF. First, MIF may have a direct or indirect role in recruiting T cells to sites of immune and inflammatory injury. For example, upregulation of chemokines and adhesion molecules is induced by MIF. Second, MIF may act by stimulating T-cell proliferation and activation to mediate renal injury via the delayed-type hypersensitivity (DTH) mechanism. MIF is important in the T-cell-mediated DTH response that is a key mechanism in the pathogenesis of many immune diseases including immunologically-mediated glomerulonephritis and allograft kidney rejection. Indeed, MIF is the first T-cell cytokine identified in association with DTH response. Direct evidence for a role of MIF in the DTH response associated with kidney disease comes from the findings that neutralizing anti-MIF antibody treatment of rats with primed rabbit anti-GBM glomerulonephritis inhibits an antigen-challenge skin DTH reaction induced by the intradermal injection of rabbit IgG [4].

Macrophages are the major cell type that produces MIF and are found to contain significant amountsof preformed MIF protein within intracellular pools. We find that macrophages produce MIF in both experimental and human kidney disease including renal allograft rejection [1, 2, 3, 4, 5]. Indeed, upregulation of MIF is tightly associated with macrophage accumulation and the severity of tissue injury including glomerular crescent formation, suggesting that local production of MIF by macrophages may, in turn, activate macrophages to produce cytokines (IL-1, TNF-α, IL-2, INF-γ), chemokines (MCP-1) and adhesion molecules (ICAM-1). Evidence to support a critical role for MIF in macrophage-mediated renal injury also comes from the finding that administration of a neutralizing anti-MIF antibody is able to prevent or reverse macrophage accumulation and activation in a rat model of crescentic glomerulonephritis [4, 5].

MIF is an established pro-inflammatory cytokine in kidney disease and, unlike classical inducible pro-inflammatory cytokines such as IL-1 and TNF-α, MIF is preformed and stored within the cytoplasm of macrophages and other intrinsic cells within the kidney (fig. 2). MIF is released upon stimulation by cellular stress, endotoxin, exotoxin, infection, inflammation and immune responses. Once released, MIF acts as a pro-inflammatory cytokine to induce expression of other inflammatory cytokines/mediators including IL-1, TNF-α, IL-2, IL-6, IL-8, INF-γ, and iNOS, with these mediators further promoting inflammatory and immune responses. Thus, MIF is a critical mediator and sits upstream in the inflammatory cascade as demonstrated by studies using neutralizing anti-MIF antibody. Treatment of immunologically-induced anti-GBM glomerulonephritis with an anti-MIF antibody prevents and reverses disease in association with a marked reduction of pro-inflammatory cytokines including IL-1 and TNF-α [4, 5], reinforcing the key, upstream role of MIF in pro-inflammatory cytokine expression during the pathogenesis of glomerulonephritis. Furthermore, TNF-α blockade by administration of soluble TNF-α receptors in anti-GBM glomerulonephritis inhibits renal MIF expression and progressive renal injury, demonstrating a pathogenic role of MIF in the inflammatory cascade [12]. Thus, MIF acts not only as a key upstream cytokine in the pro-inflammatory cytokine cascade with MIF-mediated induction of IL-1 and TNF-α production, but also acts as a pro-inflammatory cytokine in response to other pro-inflammatory stimuli.

Unlike other pro-inflammatory cytokines that are generally suppressed by the anti-inflammatory actions of glucocorticoids, MIF is induced by glucocorticoids and acts as an antagonist of glucocorticoid actions within the immune system (fig. 2). Glucocorticoid-induced secretion of MIF is tightly regulated and follows a bell-shaped dose-response curve. Low, physiologic amounts of glucocorticoids stimulate MIF release from murine monocytes and macrophages, with MIF release inhibited at high, anti-inflammatory concentrations of glucocorticoids (>10–⁸M) [13]. Thus, MIF-induced renal injury may be associated with an antagonistic action upon the anti-inflammatory and immunosuppressive effects of glucocorticoids. For example, the reversal of established anti-GBM crescentic glomerulonephritis after treatment with a MIF neutralizing antibody is associated with an increase in serum levels of glucocorticoids, and this may be the result of the counter-regulating interaction between MIF and glucocorticoids in kidney disease.

Although the mechanisms by which MIF antagonizes glucocorticoid actions remain largely unclear, the relationships between hypothalamic-pituitary-adrenal (HPA) hormonesand immunity have received special attention due to the importance of these hormones in the stress response and autoimmune diseases. MIF is rapidly released following activation of the HPA system along with glucocorticoids in response to stimuli. MIF may antagonize the action of glucocorticoids by several mechanisms [reviewed in [14]. First, MIF upregulates cytoplasmic phospholipase A₂, which is an important target for the anti-inflammatory actions of glucocorticoids. Second, MIF may antagonize the anti-inflammatory inhibition of NF-ĸB activation by glucocorticoids. Glucocorticoids prevent the activation of NF-ĸB, in part, by increasing the expression of IĸB which maintains NF-ĸB in an inactive form in the cytosol. MIF may antagonize this effect by overriding the glucocorticoid-mediated increase in the cytosolic concentration of IĸB. Thus, by blocking glucocorticoid-induced IĸB synthesis, MIF promotes the translocation of NF-ĸB into the nucleus, thereby activating key pro-inflammatory pathway involved in mediating renal injury. Finally, MIF is capable of antagonizing glucocorticoid induction of MAP kinase phosphatase, which is important in downregulating the MAP-kinase-associated inflammatory response. Taken together, the antagonistic effect on the anti-inflammatory activities of glucocorticoids may be a key mechanism by which MIF mediates renal injury.

Increasing evidence from both experimental and human kidney disease has shown that MIF is a critical mediator of renal injury. It is clear that MIF acts through many mechanisms to mediate kidney disease including innate and adaptive immunity, induction of cytokines, chemokines, adhesion molecules, and interactions with glucocorticoids and the HAP axis. Despite this, future studies are needed to dissect the signaling pathways of MIF in kidney diseases. The recent discovery of the MIF receptor complex (CD74/CD44) has provided an important tool for such studies. Furthermore, although we have demonstrated the efficacy of anti-MIF treatment in kidney disease using a monoclonal neutralizing antibody, it is necessary to develop new therapeutic strategies to specifically block MIF with small molecules or chemical MIF inhibitors. In addition, it is also important to investigate the counter-regulatory mechanisms of MIF in patients with steroid resistance because MIF can antagonize the beneficial actions of steroids. Indeed, there is a MIF allele polymorphism associated with steroid resistance in patients with nephrotic syndrome [15]. Thus, inhibition of MIF may offer an attractive therapeutic strategy because it would act uniquely to inhibit the antagonistic effects of MIF upon endogenous or prescribed glucocorticoids and thereby provide a new approach for the treatment of kidney diseases in patients who do not respond to steroid therapy.

This work was supported by a grant from the Research Grants Committee of Hong Kong (RGC CERG 759206).