Article Text

PDF

T cell apoptosis and inflammatory bowel disease
  1. M P Peppelenbosch,
  2. S J H van Deventer
  1. Department of Gastroenterology and Hepatology, Academic Medical Centre, Amsterdam, the Netherlands
  1. Correspondence to:
    Dr S J H van Deventer
    Department of Gastroenterology and Hepatology, C2-330, Academic Medical Centre, Meibergdreef 9, NL-1105 AZ Amsterdam, the Netherlands; s.j.vandeventeramc.uva.nl

Statistics from Altmetric.com

Intrinsic defects in the control of programmed cell death in the mucosal T cell compartment are strongly implicated in the pathogenesis of inflammatory bowel diseases, and there are important differences in the regulation of apoptosis between ulcerative colitis and Crohn’s disease

Approximately 2×1014 bacteria reside in the gastrointestinal tract, and this astonishing microbiological pressure represents an extraordinary challenge to the mucosal immune system, which has to perform a balancing act between appropriate responsiveness to pathogenic organisms and tolerance for harmless organisms. When this balance in mucosal immune system responsiveness is tilted towards an exaggerated or uncontrolled reaction against the commensal flora, inflammatory bowel disease results. Despite many important findings in recent years, including the association of inactivating mutations in the CARD15 (NOD2) receptor with Crohn’s disease, it is not fully understood why the mucosal immune response is over reactive in patients with inflammatory bowel disease, or how current medical treatment precisely affects the mucosal immune system. Two papers in the present issue of Gut1,2 suggest that intrinsic defects in the control of programmed cell death in the mucosal T cell compartment are strongly implicated in the pathogenesis of inflammatory bowel diseases, and important differences in the regulation of apoptosis between ulcerative colitis and Crohn’s disease are identified (see pages 1624 and 1632).

The experiments performed in both papers compare properties of apoptosis induction in lamina propria T cells of patients of Crohn’s disease, ulcerative colitis, and non-inflamed controls. The paper by Doering and colleagues1 reports that induction of apoptosis in mucosal T cell compartment is severely impaired in both Crohn’s disease and ulcerative colitis. This is the latest of a series of observations by several laboratories which show that mucosal T cells of patients with inflammatory bowel disease are highly resistant to apoptosis.3–6 Because apoptosis of activated T cells is an important mechanism of peripheral immune tolerance, this finding may explain why the inflammatory reaction in IBD patients is resilient to resolution. Apoptosis is regulated by two major intracellular pathways. Activation of the Fas receptor leads to recruitment and activation of caspase 8, which activates caspase 3, leading to a cascade of downstream events (DNA cleavage, cell membrane changes, etc), finally causing programmed cell death. Caspase 3 can also be activated by a complex of proteins, including caspase 9, APAF-1, and cytochrome C, know as the apoptosome. This pathway is activated by leakage of cytochrome C from the mitochondria, which is regulated by the Bcl-2 family of proteins (see fig 1). The apoptosis defect in mucosal T cells in Crohn’s disease has been attributed to an imbalance of the antiapoptotic mitochondrial protein Bcl-2 and the proapoptotic mitochondrial protein Bax.3,5 In agreement, in the study of Sturm and colleagues2 in the current issue of Gut, no decreased caspase 8 activity (which acts upstream of both the mitochondrial and non-mitochondrial pathway of apoptosis7) or increased expression of the caspase 8 inhibitor FLIP were observed, but caspase 3 activity (which is downstream of the mitochondria in induction of apoptosis7) was reduced in the mucosal T cells of patients with Crohn’s disease (see fig 1). Hence the available data are consistent with the notion that the apoptosis defect of mucosal T cells in Crohn’s disease occurs at the level of the mitochondria in the mitochondrial pathway to programmed cell death.

Figure 1

 Two major intracellular pathways cause apoptosis. (A) Binding of FasL to CD95 (Fas receptor) leads to recruitment and activation of procaspase 8, which subsequently activates caspase 3. Activated caspase 3 is responsible for induction of several events that lead to apoptosis. A second apoptotic pathway is initiated by leakage of cytochrome C from the mitochondria, which together with caspase 9 and APAF1 forms the apoptosome that can activate caspase 3. The integrity of the mitochondrial membrane is regulated by proteins of the Bcl-2 family, Bax and Bak being proapoptotic, Bcl-2 and Bcl-xl antiapoptotic. (B) Apoptosis resistance in ulcerative colitis results from overexpression of FLIP, leading to impairment of the caspase mediated pathway of apoptosis. The mitochondrial pathway is intact. (C) In contrast, in Crohn’s disease, Fas induced apoptosis is normal but the mitochondrial pathway is affected by an imbalance of pro- and antiapoptotic Bcl-2 family members. As a consequence, activation induced apoptosis of T cells is impaired.

This apoptosis defect in the mitochondrial pathway has important implications for the treatment of Crohn’s disease. Van den Brande et al have shown that the difference in effectiveness of tumour necrosis factor neutralising drugs for treating Crohn’s disease is reflected in their capacity to induce apoptosis of activated T cells.8 Infliximab is highly effective in Crohn’s disease9 and is a strong inducer of apoptosis in lamina propria T cells10 whereas etanercept offers no clinical benefit in Crohn’s disease11 and does not have the capacity to induce apoptosis of these cells.8 In the paper of Doering and colleagues1 in the present issue of Gut, these observations are further extended by showing that sulphasalazine is also a potent inducer of apoptosis in T cells. The sulphasalazine effect was accompanied by downregulation of the antiapoptotic mitochondrial proteins Bcl-xl and Bcl-2, again in agreement with an apoptosis defect at the level of the mitochondria in the mucosal T cells of Crohn’s disease patients. It should be kept in mind that within the colon, sulphasalazine is cleaved by bacterial enzymes to yield sulphapyridine and 5-aminosalicylic acid. It is commonly thought that the therapeutic efficacy of sulphasalazine is mediated by 5-aminosalicylic acid, sulphapyridine being responsible for sulphasalazine related side effects. However, the paper by Doering and colleagues1 indicates that the effects of mesalazine and sulphasalazine on activated T cells are very different, and because the clinical studies that lead to the notion that mesalazine is the therapeutic moiety within sulphasalazine were conducted in patients suffering from ulcerative colitis, and not in Crohn’s disease patients, it is possible that an important therapeutic effect of sulphasalazine has been overlooked. It will be important to know how much uncleaved sulphasalazine remains in the colon, and what fraction of uncleaved sulphasalazine enters the intestinal mucosa.

Two other drugs, the antimetabolites azathioprine/6-mercaptopurine and methotrexate, have been reported to also induce apoptosis of activated T cells. For azathioprine/6-mercaptopurine the mechanism of apoptosis induction has been recently linked to inhibition of CD28 signalling by the 6-mercaptopurine metabolite 6-thioguanine-triphosphate.12 Because activation of the costimulatory molecule CD28 protects T cells against apoptosis, this finding suggests that the therapeutic effect of azathioprine/6-mercaptopurine may also be related to induction of apoptosis of activated T cells. Indeed, CD3/CD28 stimulated T cells undergo apoptosis when exposed to azathioprine or 6-mercaptopurine. Because the molecular apoptotic pathways that are affected by infliximab, azathioprine, methotrexate, and sulphasalazine are likely to be different, these drugs may have additional or even synergistic therapeutic effects. It is of the utmost importance that this possibility be studied in future clinical trials.

In accordance with earlier observations,3,4 Doering et al report that lamina propria lymphocytes isolated from patients with ulcerative colitis were also more resistant to Fas ligand induced apoptosis.1 This apoptosis resistance in ulcerative colitis is clearly different from that observed in Crohn’s disease because lamina propria T cells from ulcerative colitis patients display increased apoptosis or caspase activation on CD2 or CD3 ligation, suggesting that the resistance to apoptosis through the Fas pathway is not reflected in a defect in activation induced apoptosis in the mucosal T cell compartment of these patients. Importantly, Fas ligand mediated apoptosis is mediated through activation of caspase 8 and Sturm and colleagues2 show that ulcerative colitis patients have significantly higher expression of the caspase 8 inhibitor FLIP, suggesting that resistance to apoptosis in this disease is mediated at this level. Thus regulation of programmed cell death seems fundamentally different in mucosal T cells from patients with Crohn’s disease compared with patients with colitis ulcerosa, and probably requires a different therapeutic strategy.

The latter notion is substantiated by the observations of Sturm and colleagues2 in their comparison of the cell cycle properties of lamina propria lymphocytes from patients with Crohn’s disease and ulcerative colitis. Using various parameters it is shown that T lymphocytes of patients with Crohn’s disease cycle faster and have an impressive capacity for cellular expansion when compared with mucosal T cells of inflamed controls, probably as a result of a defect in activation dependent apoptosis. The abnormally rapid cycling of lamina propria T cells is also very different from several other immune mediated diseases that are characterized by a G0/G1 cell cycle arrest of activated T cells.13,14 Such G0/G1 arrested cells will not undergo apoptosis, and in experimental SLE, induction of T cell cycling leads to progression of T cells into apoptosis and has remarkable therapeutic effects.15 Hence although it has become popular to speculate about common mechanisms that lead to immune mediated inflammatory diseases, these observations clearly demonstrate fundamentally different defects that all may lead to impaired T cell apoptosis. These differences are not only important for our understanding of the pathogenesis of these diseases but also for the development of new therapies. For example, it would not be a good idea to try to boost apoptosis of lamina propria T cells in Crohn’s disease by induction of cell cycling.

T cells from ulcerative colitis patients have a limited capacity to expand and this correlates with a strong activation induced apoptosis in these cells. This observation dramatically shows that the defect in the mitochondrial pathway at the mitochondrial level mediates the reduced activation induced cell death in Crohn’s disease whereas inhibition of Fas dependent apoptosis by increased expression of the caspase 8 inhibitor FLIP (that is, upstream of the mitochondria in the apoptotic cascade) in ulcerative colitis does not result in inhibition of activation induced cell death. These molecular differences in apoptosis inhibition between Crohn’s disease and ulcerative colitis strongly indicate that the underlying immune mechanisms are distinct. Furthermore, these observations correspond well with emerging insight that peripheral deletion of T lymphocytes by activation induced cell death is not mediated by Fas activation but by regulation of expression of the mitochondrial proapoptotic bcl-2 family member bim.16,17 As resistance to Fas ligand induced apoptosis in mucosal T lymphocytes seems to be mediated at the level of FLICE inhibition (which acts upstream of the mitochondria), peripheral activation induced cell death in colitis ulcerosa is not impaired.

Hence the papers of Strum et al and Doering et al in the present issue of Gut give substantial novel molecular insights into regulation of apoptosis in the mucosal T cell compartment and provide further support for the concepts that (1) the defect in apoptosis in mucosal T cells is an important contributing factor in the pathogenesis of Crohn’s disease but not colitis ulcerosa, and (2) that restoration of apoptosis in the T mucosal T cell compartment may be an important contributing factor for successful treatment of Crohn’s disease.

Intrinsic defects in the control of programmed cell death in the mucosal T cell compartment are strongly implicated in the pathogenesis of inflammatory bowel diseases, and there are important differences in the regulation of apoptosis between ulcerative colitis and Crohn’s disease

REFERENCES

View Abstract

Request permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.

Linked Articles