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A series of excellent articles published recently in Gut have evaluated the role of T regulatory (Treg) cells in inflammatory bowel disease (IBD) and surrogate animal models.1 2 In principle, self-tolerance in autoimmune disorders may be restored either by specific interference with effector cells and molecular pathways involved in immune activation, or selective enhancement of suppressive cells and cytokines. Based on the long-standing recognition that interleukin 2 (IL2) is a significant stimulatory cytokine involved in immune activation, interference with IL2 signalling has been considered as a potential approach to arrest autoimmunity. Stimulated lymphocytes (B and cytotoxic T cells) secrete IL2 and express the IL2 receptor α chain (CD25) within a positive feedback loop of activation. More physiological approaches to reinstitution of self-tolerance surround the activity of suppressive immune cells, in particular, Treg cells, which prevent and even reverse ongoing autoimmune reactions.3 4 5 6 One particular potent subset of naturally occurring Treg cells is characterised by CD25 expression at high levels in conjunction with transcription factor X-linked forkhead/winged helix (FoxP3). It is not only a contextual relationship between CD25 and suppressive mechanisms, but ample supply of IL2 is essential to the development and function of these Treg cells, and inhibition of IL2 production by cytotoxic cells is one of the primary mechanisms of suppression. Therefore, disruption of IL2 signalling that tilts the equilibrium against suppressive elements evolves as a deleterious mechanism in the context of autoimmunity, rather than an immunosuppressive strategy. Consistently, mice deficient in IL2 develop inflammatory enteritis, and, most convincingly, a similar condition can be induced by IL2 neutralisation in adult wild-type mice.7
Although some studies reported a 3-fold decrease in CD4+CD25high putative Treg cells in peripheral blood of patients with active ulcerative colitis as compared to patients in remission and healthy subjects;8 others present evidence of increased activity of Treg cells in IBD. Murine models showed redistribution of Treg cells in the lamina propria and mesenteric lymph nodes of sick mice9 and also in the enteric wall of patients with ulcerative colitis,9 10 with an important role attributed to IL10 and transforming growth factor β (TGFβ) secretion by these cells.11 Accordingly, CD4+CD25highFoxP3+ Treg cells colonise the lamina propria and draining lymphoid organs in patients with ulcerative colitis12 and their numbers increase during exacerbation (their suppressive activity was demonstrated in vitro).13 It is yet uncertain whether decreased levels of Treg cells in the periphery reflects their recruitment to the site of inflammation or increased levels reflect their reactive proliferation in peripheral lymphoid organs.6
Disruption of IL2 signalling is particularly suited for treatment of IBD, considering that mucosal T cells from active Crohn’s disease lesions express increased levels of mRNA encoding IL2 and its cognate receptor,14 which mediate robust proliferation of the cells.15 A reversible approach consists of blockage of the IL2 receptors using humanised monoclonal antibodies that inhibit lymphocyte activation through blockade of CD25. Although various monoclonal antibodies were shown to exert distinct effects the immune system in the transplant setting,16 one such antibody (daclizumab) failed to provide evident benefit in patients with ulcerative colitis.1 Despite sustained blockage of the IL2 receptor for weeks, this reversible intervention does not affect the unbalanced homeostatic equilibrium that underlies the autoimmune reaction (fig 1). In addition, receptor blockade using non-depleting antibodies exposes to the risk of cytokine storm following initial administration, a physiological mechanism by which the cells attempt to reverse the blockade.17 A more definitive approach involves elimination of the pathogenic cells. Under optimal conditions, therapy would require high selectivity to Teff cells in bowel mucosa while sparing Treg cells, without affecting peripheral immune homeostasis.
In patients with IBD it is particularly appropriate to attenuate the course of disease using agents that induce direct apoptosis in colitogenic cells, as the pathogenic lymphocytes display multiple immunological abnormalities including altered sensitivity of to apoptosis.18 19 20 T cells located in lamina propria of patients with Crohn’s disease display relative resistance to multiple pathways of apoptosis, which is attributed to disturbed ratio between pro- and anti-apoptotic factors.15 21 Likewise, resistance of enteric T cells to apoptosis in patients with ulcerative colitis has been associated with elevated levels of FLICE inhibitory protein (FLIP) (a caspase-8 inhibitor).22 Considering the beneficial effects of IL2 targeted therapy in an animal model of dextran sodium sulfate (DSS)-induced colitis,2 we evaluated two chimeric proteins consisting of IL2 as a targeting moiety and diphtheria toxin (IL2–DT) or caspase-3 (IL2–cas) as toxic conjugates.23 Targeted cells are eliminated after internalisation of the toxin by the IL2/IL2 receptor complex. In a model of murine toxic colitis, which includes secondary lymphocytic infiltration of the colon, administration of these agents yielded three surprising results (table 1). First, at therapeutic doses both agents improved the clinical, morphological and histological disease score. Second, while the protective effect of IL2–DT was achieved under toxic conditions including severe lymphopenia (>50% decrease in T lymphocytes in the spleen and mesenteric lymph nodes), no such toxicity was apparent with IL2–cas. Third, despite depletion of CD25+ T cells, IL2–cas spared CD4+FoxP3+ in the mesenteric lymph nodes, which likely contributed to colon protection under immune sufficient conditions.
Although it is often difficult to extrapolate animal data to human IBD,9 it is evident that non-specific immunosuppression using IL2–DT is accompanied by unacceptable toxicity in particular in young patients, with immediate (infection) and delayed (malignancy) consequences of repeated episodes of lymphopenia. Nevertheless, despite a pivotal role attributed to Treg cells in IBD, IL2 targeted therapy effectively prevents the detrimental contribution of the lymphocytic component of inflammation. As both subsets of activated and regulatory T cells share common CD25 expression, it is likely that eradication of CD4+CD25+ T cells in mesenteric lymph nodes by both IL2 chimeric proteins prevented lymphocyte mobilisation to the inflamed bowel. Importantly, direct killing of colitogenic lymphocytes circumvents their relative resistance to the physiological apoptotic pathways.15 18 19 20 21 22 Most intriguing, CD4+FoxP3+ T cells were relatively preserved in the mesenteric lymph nodes after administration of IL2–cas, possibly as evidence of homeostatic expansion of this putative tolerogenic subset. Indeed, we observed marked increase in CD4+CD25+FoxP3+ T cells in the spleen soon after discontinuation of IL2–cas therapy. A third mechanism might involve expansion of Treg cells secreting IL10 and TGFβ following depletion of the CD4+CD25+ subset. Finally, it may be stipulated that selective IL2 targeting may protect the Treg cell subsets from inactivation within the inflamed mucosa, preventing the repressive effect of high IL2 concentrations on Treg cell function (demonstrated in vitro).24
Funding Support for this study was received from Target-In and the Frankel Trust.
Competing interests SY and YS are employees of GASR Biotechnology Ltd. SY has significant equity in this company. AK and NA have no competing interests.
Provenance and peer review Not commissioned; externally peer reviewed.
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