Indoleamine 2,3-dioxygenase and tumor-induced tolerance (original) (raw)

Acquired tolerance of tumors is undesirable and harmful to the host, but acquired tolerance of other classes of antigens (such as fetal antigens and harmless foreign antigens at mucosal surfaces) is beneficial and necessary (15). IDO has been implicated as a normal, endogenous mechanism of peripheral tolerance and immunosuppression in a number of settings. It was originally described as contributing to maternal tolerance toward the fetus, as shown by the fact that mice treated early in pregnancy with 1-methyl-tryptophan (1MT), which is an inhibitor of IDO, underwent immune-mediated rejection of allogeneic concepti (16–18). The fetus represents an example of a set of foreign antigens to which the immune system is forced to remain tolerant and therefore is conceptually analogous to tumors in this regard.

More generally, it has now been observed that mice treated with IDO inhibitors become refractory to acquired tolerance induction in a number of settings. For example, blocking IDO with 1MT prevents the induction of tolerance to islet cell allografts by the fusion protein cytotoxic T lymphocyte–associated antigen 4–Ig (CTLA4-Ig) (19, 20), and 1MT blocks the tolerance that normally occurs when foreign antigens are introduced into the anterior chamber of the eye (an immunologically privileged site) (21). In settings where self tolerance has already been disrupted (for example, autoimmune disorders), pharmacologic inhibition of IDO causes marked exacerbation of inflammation and worsened symptoms of disease, as shown in models as diverse as inflammatory bowel disease (22), EAE (23), and experimental allergic asthma (24). Conversely, ectopic overexpression of IDO by gene transfer results in suppression of immune responses. For example, MHC-mismatched lung allografts transfected with IDO are protected from rejection without further immunosuppression (25, 26), and similar results have been reported in corneal transplants (27). Therefore, in vivo, IDO functions as a molecular mechanism contributing to acquired peripheral tolerance.

However, IDO does not seem to be required for the constitutive maintenance of tolerance to self. This is shown by the fact that mice genetically modified to lack IDO (_Ido_–/– mice) do not develop lethal autoimmune or lymphoproliferative disorders (20) and mice treated systemically for up to 28 days with pharmacologic IDO inhibitors have not been observed to develop spontaneous autoimmunity (22, 28). Therefore, in certain settings, IDO can be very important for tolerance, but the effects of IDO are selective and are narrowly focused on specific forms of acquired peripheral tolerance. This specificity is potentially an advantage when contemplating the clinical use of pharmacologic IDO inhibitors since these would not be predicted to have severe spontaneous autoimmunity as a limiting side effect.

Regulation of IDO expression

IDO can be expressed by multiple cell types in response to inflammation. The regulation of IDO expression (gene, protein, and functional activity) is complex and remains a subject of active investigation. However, two important points have emerged from the literature. First, within the immune system, certain types or subsets of APCs seem to be preferentially disposed to express functional IDO when challenged with proinflammatory stimuli or exposed to signals from activated T cells. In mice, these “IDO-competent” APCs include a subset of plasmacytoid DCs (29), CD8α+ splenic DCs (or a subset thereof) (30, 31), and doubtless other subsets of DCs and macrophages as well. Second, even in APCs that are IDO competent, the actual presence or absence of functional IDO enzymatic activity is tightly regulated by specific maturation and activation signals (31–36).

Conceptually, this ability to upregulate or downregulate IDO in response to external stimuli seems logical, given the need for APCs to sometimes present antigens in an activating fashion and sometimes in a tolerizing fashion, depending on the context. However, this plasticity somewhat complicates the study of IDO-competent APCs in tumor-bearing hosts since both the relevant APC subsets and the specific signals that turn IDO expression on or off in these cells must be identified. It is even less well understood how IDO expression might be regulated in the tumor cells themselves (for example, in response to local inflammatory mediators such as IFN-γ; ref. 37) and whether IDO expressed by tumor cells has the same properties as that expressed by DCs and macrophages.

Molecular mechanisms of IDO-mediated immune suppression

IDO initiates the degradation of tryptophan along the kynurenine pathway (12). IDO and the downstream enzymes in this pathway produce a series of immunosuppressive tryptophan metabolites (Figure 1A). Some of these metabolites suppress T cell proliferation in vitro or cause T cell apoptosis (38–41), and some can affect NK cell function (42). Even if IDO itself is not present, enzymes downstream of IDO in the kynurenine tryptophan degradation pathway can create immunosuppressive metabolites if supplied with kynurenine (43). In addition, in vivo, rats treated with a mixture of tryptophan metabolites showed prolonged graft survival (44); and the drug _N_-[3′,4′-dimethoxycinnamoyl] anthranilic acid (Tranilast), a synthetic derivative of the tryptophan metabolite anthranilic acid, has been shown to reduce inflammation and reverse paralysis in mice with EAE (45). Therefore, it seems that a number of tryptophan metabolites are immunosuppressive. The molecular mechanism by which these compounds exert their immunologic effects is not known, but at least one recent report has described a receptor able to bind a specific metabolite of tryptophan (kynurenic acid) (46). The biologic function of this orphan G protein–coupled receptor, GPR35, is unknown, but its expression was highest in cells of the immune system and the gut, sites where IDO is known to be expressed (46). Whether there are other such receptors for other tryptophan metabolites and what the biologic effects of such receptors might be in vivo are important questions that deserve further investigation.

In addition to the immunosuppressive effects of tryptophan metabolites, the cellular stress imposed by local depletion of tryptophan also seems to mediate some of the immunosuppressive effects of IDO (Figure 1B). This was first suggested by the observation that some effects of IDO on T cells are reversed by the addition of excess tryptophan in vitro (25, 29, 33, 47). Recently, the stress-responsive kinase general control nonderepressible 2 (GCN2) has been identified as a signaling molecule that enables T cells to sense and respond to stress conditions created by IDO (48, 49). The kinase activity of GCN2 is triggered by a rise in the amount of uncharged transfer RNA (tRNA) in the cell (50); therefore, insufficiency of any amino acid (such as occurs when IDO depletes tryptophan) will activate GCN2 kinase activity and initiate a downstream signaling pathway (51). This results in repression of most protein translation but causes selective upregulation of a small subset of genes that are responsive to signaling by GCN2 (52). These GCN2-responsive genes are different for each cell type, and exactly how this signal transduction pathway modulates immune responses is still under investigation; however, T cells from mice lacking GCN2 are resistant to IDO-induced inhibition of proliferation, and they do not acquire the state of antigen-specific unresponsiveness (anergy) normally induced by exposure to IDO (48). Furthermore, CD4+ T cells from GCN2-deficient mice fail to undergo IDO-induced differentiation into Tregs (48, 49).

Local tryptophan depletion would obviously be a short-range immunosuppressive phenomenon since the total body pool of tryptophan could not be depleted. However, since IDO-expressing APCs and responding T cells are obliged to be in close physical contact, it might be that tryptophan concentrations can be lowered sufficiently in T cells interacting with the IDO-expressing APCs to activate the GCN2 signaling pathway. However, as with the effects of tryptophan metabolites, much of this model of the mechanisms by which local tryptophan depletion elicits immunosuppression remains speculative. It seems probable that both the GCN2 pathway and the metabolite pathways function synergistically to create the full biologic effects of IDO, as has been recently suggested (49).