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mechanism of action beyond TNF-α neutralization in inflammatory bowel disease

Kirman, Irenaa; Whelan, Richard La; Nielsen, Ole Hb

European Journal of Gastroenterology & Hepatology: July 2004 - Volume 16 - Issue 7 - p 639-641
doi: 10.1097/01.meg.0000108345.41221.c2
Leading Articles

Infliximab, a chimeric antibody to tumour necrosis factor-alpha (TNF-α), holds much promise for the treatment of patients with Crohn's disease.

• On the cellular level, infliximab affects survival and, as presented by Agnholt et al. in this issue of the journal, inhibits GM-CSF (granulocyte–macrophage colony-stimulating factor) production by intestinal T lymphocytes.

• Future studies will reveal whether the pro-apoptotic effect of infliximab is linked to its inhibition of endogenous GM-CSF expression in T cells.

Treatment of Crohn's disease, a severe chronic intestinal disorder, may at times be challenging as it can be refractory to routine therapy. Among novel therapeutic strategies, agents that neutralize tumour necrosis factor-alpha (TNF-α) are of particular interest because of the crucial role of TNF-α in sustaining chronic mucosal inflammation. The exact mechanism of the anti-TNF action, apart from direct activity that neutralizes cytokines, is not fully understood. Cellular effects of TNF-α neutralizing treatment include an increased susceptibility to apoptosis of intestinal mucosal T cells. A novel pathway of anti-TNF-α interaction with T cells has been presented in the current issue of this journal. Agnholt et al. have found that in-vivo or in-vitro administration of infliximab, a chimeric antibody to TNF-α, resulted in a decreased production of GM-CSF (granulocyte–macrophage colony-stimulating factor) by T cells. Infliximab related down-regulation of TNF-α induced GM-CSF expression may be one of the mechanisms by which this drug increases the rate of apoptosis in T cells.

aDepartment of Surgery, Columbia University, New York, USA and bDepartment of Gastroenterology C, Herlev Hospital, University of Copenhagen, Denmark.

Correspondence to Dr Irena Kirman, Department of Surgery, Columbia University, BB 1702, New York, NY 10032, USA. Tel: +1 212 305 7960; fax: +1 212 305 1690; e-mail:

Received 14 January 2004

Crohn's disease is a chronic inflammatory gastrointestinal disorder of unknown aetiology. Although the precise triggering factors of the intestinal inflammation (the hallmark of this condition) are unclear, an altered response to bacterial antigens is thought to play a role. Thus, mutations in CARD15/NOD2, a gene involved in the host response to bacterial lipopolysaccharide may predispose humans to the development of Crohn's disease [1]. Bacterial superantigens, such as the newly discovered I2, can directly activate CD4+ T cells [2]. The importance of bacterial products in the clinical activation of inflammatory bowel disease (IBD) is suggested from several microorganism dependent experimental models including the recently developed model of spontaneous experimental ileitis in SAMP1/YitFc mice [3,4].

Activated Th1 cells, a central component of the immunoinflammatory process in Crohn's disease, produce the stimulatory cytokines interferon-gamma (IFN-γ), interleukin-2 (IL-2), IL-12 and tumour necrosis factor-alpha (TNF-α) and recruit other cells in the pathological reaction. The role of Th1 cells in the pathogenesis of Crohn's disease is suggested from clinical and experimental evidence. Firstly, the granulomas, which are a characteristic pathological finding in Crohn's disease, are dominated by Th1 cells [5]. Secondly, the production of Th1-type cytokines increases during relapses of Crohn's disease and decreases during remissions [6]. Thirdly, not only do Th1 cells mediate experimental colitis in several models [4,7], but they also have been shown to induce intestinal inflammation when transferred from one animal to another [8,9].

A pivotal component of Th1 mediated colitis is TNF-α; this cytokine stimulates mucosal inflammation regardless of its origin [10]. The importance of TNF-α in sustaining clinical signs of mucosal inflammation is suggested from studies that have demonstrated the efficacy of TNF-α neutralizing therapies both in clinical trials and in animal models. Typically, TNF-α neutralizing agents are administered to patients with chronic inflammatory disorders that are resistant to routine therapy. Currently, two types of TNF-α deactivating strategies are available: (1) administration of a soluble recombinant TNF-α receptor, p75 subunit (etanercept) or a p55 subunit (onercept), and (2) the infusion of TNF-α neutralizing antibodies, chimeric (infliximab), humanized (CDP 571, humicade) and fully human (adalimumab). The results of both etanercept and onercept have been disappointing in randomized, controlled clinical studies on Crohn's disease patients. Infliximab, shown to be effective in 2/3 of Crohn's disease patients, is a neutralizing chimeric monoclonal antibody consisting of the murine variable and human constant immunoglobulin regions. Administration of a chimeric antibody may be accompanied by the development of human anti-chimeric antibodies, which can result in adverse effects such as acute and delayed hypersensitivity reactions. Approximately 10% of patients will experience these problems. A fully human TNF-α neutralizing antibody, adalimumab, recently became available, but because it has been shown to result in the generation of human anti-human antibodies (HAHAs), it may also prove to be associated with side effects.

The anti-inflammatory mechanism of anti-TNF-α antibodies, apart from their direct TNF neutralizing activity, is not completely understood. Infliximab has been shown to exert a pro-apoptotic effect on T cells and to inhibit the production of Th1 type cytokines. Thus, administration of infliximab to patients with Crohn's disease results in a decreased in-vitro production of TNF-α and IFN-γ by intestinal and peripheral blood T cells [11,12]. Further, infliximab therapy is followed by an increase in the number of apoptotic lamina propria T cells [13]. The study by Agnholt et al. in this issue of the journal [14] suggests a new and important effect of infliximab: inhibition of the production of GM-CSF (granulocyte–macrophage colony-stimulating factor) by mucosal T cells. In this work, the authors isolated mucosal lymphocytes from patients. These cells, the majority of which were TCR α/ß+CD4+CD45RO+ cells, were found to secrete GM-CSF. Administration of infliximab to the patients resulted in a significant decrease of the in-vitro GM-CSF production by harvested mucosal lymphocytes. Other in-vitro experiments with stimulated T-cell cultures from this study confirmed the GM-CSF inhibitory potential of infliximab.

GM-CSF, a cytokine originally defined by its ability to stimulate colony formation by myeloid cells, was later found to participate in inflammatory processes. Thus, GM-CSF transgenic mice develop focal accumulation of activated macrophages in the pleural and abdominal cavity, eyes and striated muscles [15], while targeted over-expression of GM-CSF leads to organ-specific autoimmunity [16]. Increased production of GM-CSF has also been reported in IBD. In active IBD, total mucosal cells or lamina propria mononuclear cells, in vitro, secrete increased amounts of GM-CSF [17,18]. In the current study, Agnholt et al. demonstrated that T cells from Crohn's disease patients produced increased amounts of GM-CSF and that this increase significantly correlated with disease activity. Thus, the network of pro-inflammatory cytokines involved in the pathogenesis of IBD includes GM-CSF; its precise role in sustaining mucosal inflammation remains to be elucidated. Surprisingly, several investigators have outlined the rationale for therapeutic usage of GM-CSF in Crohn's disease. Clinical features of Crohn's disease can be observed in human disorders associated with defective function of phagocytes, such as chronic granulomatous disease or glycogen storage disease Ib. Administration of recombinant human GM-CSF (rhGM-CSF) to the patients with active Crohn's disease has been reported to result in a significant decrease of disease activity [19]. It is difficult to compare the range of amounts of rhGM-CSF received by these patients, 4–8 μg/kg/day (a routine therapeutic rhGM-CSF dose) to that produced under pathological or physiological conditions. The study by Agnholt et al. suggests that T cells from Crohn's disease colonic mucosal biopsy specimens produce concentrations of GM-CSF in the picogram per millilitre range; the systemic impact of such GM-CSF production is probably different from that after exogenous administration of GM-CSF in the dose ranges currently utilized. Investigators who have assessed serum GM-CSF concentrations have reported values in the picogram per millilitre concentration range. High therapeutic doses of GM-CSF have been shown to have a broad bioactivity spectrum, including an inhibitory effect on the generation of certain subsets of cells, such as NK cells [20]. Thus, rhGM-CSF therapy may favourably alter bone marrow production of a variety of immune cells and this benefits patients with chronic immunoinflammatory disorders.

Expression of endogenous GM-CSF affects cell survival. Thus, stabilization of GM-CSF mRNA has been shown to significantly prolong the survival of eosinophils [21]. TNF-α induces stabilization of GM-CSF mRNA in cells probably through the activation of MAPK (mitogen activated protein kinase) [22,23]. Future studies will show if the opposite, i.e. neutralization of TNF-α, leads to down-regulation of endogenous GM-CSF with a subsequent apoptosis of T cells. This might be a supplementary mechanism of TNF-α neutralizing therapy in the setting of chronic immunoinflammatory conditions.

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    Crohn's disease; infliximab; granulocyte–macrophage colony-stimulating factor; T cells

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