Regulatory T cells move in when gliomas say “I-DO”

Summary

Indoleamine 2,3-dioxygenase (IDO) is an enzyme with known immunosuppressive and tolerogenic effects in cancer. Mounting evidence has associated IDO expression with the induction of regulatory T cells and malignant progression. IDO inhibition may therefore provide a promising therapeutic approach for glioblastoma, where the need for novel treatment is great.

Commentary

In this issue of Clinical Cancer Research, Wainwright and colleagues present data characterizing the role of IDO-induced immunosuppression in glioma, and specifically address the impact of IDO on the recruitment of regulatory T cells (T_regs) to tumors in the brain (1).

Glioblastoma (GBM) is the most common and aggressive tumor of the central nervous system (CNS). It carries an exceedingly poor prognosis despite aggressive combinations of surgical resection, radiation, and temozolomide chemotherapy, as well as salvage therapy with bevacizumab. These conventional therapies also cause incapacitating, non-specific toxicity to healthy tissues. By contrast, immunotherapy offers an exquisitely precise approach; substantial evidence suggests that T cells have the ability to eliminate large, well-established tumors, even within the “immunologically-privileged” CNS, and without concomitant autoimmunity (reviewed in (2)).

Despite this promise, GBM immunotherapy has been limited to date, in part due to proclivities that the tumor possesses for instigating profound host immunosuppression. While the source of this immune impairment is clearly multifactorial, T_regs are perhaps most frequently implicated as major contributors to cellular immune defects and are present at increased proportions in both the tumors and peripheral blood of patients with GBM (3, 4). In tumors, T_regs curtail the generation and expansion of cellular immune responses, primarily through inhibited production of IL-2 and IFN-γ and the induction of T-cell anergy. Recent studies have suggested that the accumulation of T_regs in brain tumors may be a result of factors associated with the anatomical niche, since intracerebral gliomas in mice were found to contain significantly elevated numbers of T_regs compared to identical tumors implanted subcutaneously (5). While the CCR4/CCL22 axis has certainly been implicated, the data presented here by Wainwright et al. suggest an additional mechanism for T_reg-to-GBM recruitment, centered about the enzyme indoleamine 2,3-dioxygenase (IDO).

IDO is an intracellular oxidoreductase that catalyzes the first step of tryptophan degradation. In a variety of tumor models, the expression of IDO by dendritic cells (DCs) in tumor-draining lymph nodes has been shown to potently facilitate antitumor T-cell tolerance, partly via the induction and recruitment of new and existing T_regs (Fig. 1). Currently, it is believed that IDO also inhibits effector T-cell responses through the depletion of essential tryptophan stores. In the absence of tryptophan, effector T cells have been shown to activate the general control nonrepressed-2 (GCN2) stress kinase pathway, which leads to cell cycle arrest and abortive T-cell activation. Interestingly, IDO expression can be induced in DCs through counter-regulatory mechanisms triggered by pro-inflammatory mediators (e.g., IFNγ), or alternatively through interactions with CTLA-4 on the surface of activated T_regs, thereby creating an immunosuppressive feedback “loop.”

Figure 1.

T_regs accumulate in brain tumors and are associated with poor prognosis. Peripheral depletion with anti-CD25 antibodies and CTLA-4 blockade are methods by which antitumor immune responses may be enhanced. IDO-mediated catabolism of tryptophan occurs in both tumor cells and peripherally in DCs. Here, IDO enzymatic activity potentiates an immune tolerogenic environment resulting in the reciprocal induction of T_regs amidst a suppressed effector T-cell response.

Wainwright and colleagues demonstrate that IDO expression by glioma leads to the recruitment of T_regs, to tumors resulting in impaired immune-mediated rejection of brain tumors in both orthotopic and transgenic models of GBM (1). The effects of IDO proved most impactful when expressed by the tumor itself, with peripheral levels being seemingly less relevant. This was clinically correlated insomuch as a retrospectively reviewed database of tumor samples from GBM patients revealed an inverse relationship between IDO expression and survival. Given that an IDO inhibitor, 1-mehtyl-tryptophan (1MT), is now in clinical trials, these data may be leveraged to support the application of IDO inhibition as a novel therapeutic strategy for GBM. The caveat though will be that such inhibition will need to be demonstrated at the level of the tumor, and not systemically. This is especially pertinent given that much of what is known about IDO and its inhibition to date has been limited to its peripheral activity in DCs, which the authors advance as less influential in the setting of GBM.

Of further import, the authors’ data also suggest that it is the constitution of tumor-infiltrating lymphocytes (TILs), and not their absolute number per se, that impacts survival. Previously, a number of efforts have been made to evaluate the prognostic significance of TILs in GBMs; however, these studies have not yielded definitive results to date. In contrast, when considered as a subset of glioma-infiltrating lymphocytes, intratumoral T_regs have been shown to correlate with tumor grade and malignant features. This notion is similarly supported by Wainwright and colleagues, since increased levels of tumor IDO, despite being associated with increased numbers of TILs in general, were simultaneously associated with an increased ratio of T_regs-to-CD8+ T cells, and accordingly poorer survival.

Numerous efforts have been made to selectively modulate T_reg activity with hopes of improving immunotherapeutic responses against GBM. Importantly, upon removal of T_regs from the peripheral blood of patients with GBM, normal levels of effector T-cell proliferation and cytokine secretion can be restored in vitro, and in vivo depletion of T_regs in tumor-bearing mice has been shown to significantly prolong survival (3, 6, 7). Given the high levels of CD25 on the surface of T_regs, attempts have been made to selectively eliminate T_regs with denileukin diftitox (a fusion protein of diphtheria toxin and IL-2) and LMB-2 (a fusion protein of an anti-IL-2Rα monoclonal antibody and exotoxin). Both have been utilized to deplete T_regs in humans, but with mixed success. A significant limitation is the collateral depletion of effector T cells that transiently upregulate IL-2Rα upon activation. Moreover, due to its IL-2 targeting moiety, denileukin diftitox has the added drawback of unintentionally eliminating an even broader subset of cells that constitutively express lower affinity IL-2βγ receptors.

Whereas T_regs rely heavily on IL-2 for survival and function, activated effector T cells and tumor-specific memory T cells are not necessarily IL-2 dependent especially when undergoing homeostatic proliferation after lymphodepleting chemotherapy. This has prompted hope that systemic blockade with unarmed anti-IL-2Rα antibodies (e.g., daclizumab, basiliximab) may selectively impair T_regs based on their differential requirement for IL-2. Importantly, in preclinical models and early clinical trials, monoclonal antibodies that block IL-2Rα have been shown to significantly reduce T_reg activity without hampering vaccine-induced immunity (8, 9). Although promising, current strategies to deplete T_regs in vivo have not been shown to efficiently eliminate intratumoral T_reg populations, a limitation that IDO inhibition may avoid.

Going forward, it will be important to assess whether IDO inhibition may prove more effective or even synergistic with like-minded therapies. For instance, as noted, one component of IDO activity may be interaction with CTLA-4 on T_regs. CTLA-4 has an established role in facilitating T_reg activity as well as in limiting activated T-cell responses, and its blockade with ipilimumab has potent therapeutic value in metastatic melanoma, for which it is FDA-approved (10). Likewise, CTLA-4 blockade has proven effective in experimental models of GBM (11) and in compassionate use protocols for patients with brain metastases (12). Given their likely activity along two steps in the same pathway, combining IDO inhibition and CTLA-4 blockade may be one example of a rational therapeutic platform to assess. Clearly, the authors demonstrate IDO inhibition as a promising means of countering T_reg recruitment and activity. Clarifying its mechanisms, appropriate immune context, and interaction with other modalities should be emerging priorities.