T lymphocytes undergo proliferation arrest when exposed to tryptophan shortage, which can be provoked by indoleamine 2,3-dioxygenase (IDO), an enzyme that is expressed in placenta and catalyzes tryptophan degradation. Here we show that most human tumors constitutively express IDO. We also observed that expression of IDO by immunogenic mouse tumor cells prevents their rejection by preimmunized mice. This effect is accompanied by a lack of accumulation of specific T cells at the tumor site and can be partly reverted by systemic treatment of mice with an inhibitor of IDO, in the absence of noticeable toxicity. These results suggest that the efficacy of therapeutic vaccination of cancer patients might be improved by concomitant administration of an IDO inhibitor.

Tryptophan catabolism mediated by indoleamine 2,3-dioxygenase (IDO1) is an important mechanism of peripheral immune tolerance contributing to tumoral immune resistance, and IDO1 inhibition is an active area of drug development. Tryptophan 2,3-dioxygenase (TDO) is an unrelated hepatic enzyme that also degrades tryptophan along the kynurenine pathway. Here, we show that enzymatically active TDO is expressed in a significant proportion of human tumors. In a preclinical model, TDO expression by tumors prevented their rejection by immunized mice. We developed a TDO inhibitor, which, upon systemic treatment, restored the ability of mice to reject TDO-expressing tumors. Our results describe a mechanism of tumoral immune resistance based on TDO expression and establish proof-of-concept for the use of TDO inhibitors in cancer therapy.T ryptophan 2,3-dioxygenase (TDO) is a homotetrameric hemecontaining cytosolic enzyme encoded by gene TDO2 and expressed at high levels in the liver. It catalyses the first and ratelimiting step of tryptophan degradation along the kynurenine pathway and thereby regulates systemic tryptophan levels. The same reaction can be catalyzed by another heme-containing cytosolic enzyme, named indoleamine 2,3-dioxygenase (IDO1), which has no sequence similarity with TDO, is not expressed in the liver, and is monomeric. IDO1 has been the focus of attention in recent years because of its immunosuppressive effects on T lymphocytes, resulting partly from tryptophan depletion and partly from direct effects of tryptophan catabolites (1-3). IDO1 is expressed constitutively in the placenta, where it plays a key role in feto-maternal tolerance (2), and in many tumors, where it contributes to tumoral resistance to immune rejection (4-7). IDO1 expression is also inducible in many cells, including dendritic cells, and appears to play a role in peripheral immune tolerance and the retro-control of immune responses (8). In contrast, little is known about the effect of TDO expression on the immune response. A recent report indicated that human cells transfected with TDO depleted tryptophan and thereby prevented both the growth of pathogens and the proliferation of allogeneic T lymphocytes (9). These results suggested that TDO might mediate immunosuppressive effects similar to those of IDO1. We set out to examine whether tumor cells express TDO and thereby inhibit T-cell-mediated immune responses. ResultsWe first observed that many human tumor samples expressed gene TDO2 as measured by real-time RT-PCR (Table 1). This was the case for 41% of bladder carcinomas, 50% of melanomas and 100% of hepatocarcinomas. To confirm the activity of TDO in tumor cells, we selected a series of human tumor cell lines that also expressed the TDO2 mRNA. We incubated cells for 24 h in medium containing a known concentration of tryptophan, and measured by HPLC in the supernatant the concentration of tryptophan and kynurenine, which is the main tryptophan catabolite (Table 2). HEK-293 cells transfected or not with human TDO2 were used as pos...

Tryptophan catabolism by indoleamine 2,3-dioxygenase 1 (IDO1) plays a key role in tumoral resistance to immune rejection. In humans, constitutive expression of IDO1 has been observed in several tumor types. However, a comprehensive analysis of its expression in normal and tumor tissues is still required to anticipate the risks and potential benefits of IDO1 inhibitors. Using a newly validated monoclonal antibody to human IDO1, we performed an extensive immunohistochemical analysis of IDO1 expression in normal and tumor tissues. In normal tissues, IDO1 was expressed by endothelial cells in the placenta and lung and by epithelial cells in the female genital tract. In lymphoid tissues, IDO1 was expressed in mature dendritic cells with a phenotype (CD83 þ ,