Villella et al: The role of IDO in immune system evasion of malignancy summarized the evidence supporting this role in immune tolerance including the role of regulatory T cells. Lastly, we discuss the clinical implications of IDO inhibition as an adjuvant to potentiate cancer vaccine therapy.
(Yuan et al, 1998; Munn et al, 1999). Human and murine T cells enter the cell cycle, but progression terminates midway through the G1 phase before the commencement of S phase when tryptophan is withheld from the culture media (Munn et al, 1999). Growth arrested T cells fail to acquire cytolytic effector functions. This may imply that T cells possess a tryptophan sensitive checkpoint in the cell cycle that determines whether or not they proliferate (Mellor et al, 2001). Thus, tryptophan is important for T cell proliferation, as in its absence T cells arrest in G1 phase of the cell cycle (Mellor et al, 2003). This enzyme is expressed by human macrophages. When present in the tumor microenvironment, it suppress T cell responses locally by limiting tryptophan availability (Mellor and Munn, 2003). IDO is important in maintaining maternal tolerance toward the fetus during pregnancy, as well as in suppressing T cell response to MHC-mismatched allografts (Mellor and Munn, 2004). Dendritic cells are key regulators of immune outcomes as they are the most efficient antigen presenting cells in the body. They are capable of promoting or suppressing T cell responses depending on the circumstances (Moser, 2003). They function as the foreman of the immune system, whereby they integrate incoming signals and direct and appropriate T cell response: immune activation or tolerance (Mosmann and Livingstone, 2004).
II. What is IDO? Tryptophan is an amino acid required for protein synthesis and other metabolic functions. Tryptophan is synthesized from molecules such as phosphoenolpyruvate in bacteria, fungi and plants, and these organisms fuel the tryptophan flux through the food chain (Moffett and Namboodiri, 2003). Animals are incapable of synthesizing tryptophan, so they ingest it in protein form, which is then hydrolyzed into the constituted amino acids. It is delivered through the hepatic portal system, and that portion which is not used for protein synthesis in the liver can either be distributed through the blood stream for protein synthesis or it can be degraded in the liver by the kynurenine pathway (Moffett and Namboodiri, 2003). There is a clear association between tryptophan catabolism and inflammatory reactions, which may be occurring in the immune system rather than in the liver (Moffett and Namboodiri, 2003). Indoleamine 2, 3-dioxygenase (IDO) is a hemecontaining enzyme that catabolizes the first and ratelimiting step in oxidative degradation of tryptophan. IDO is encoded by a single gene and is transcribed in response to inflammatory mediators such as interferon-! (Taylor and Feng, 1991). In the late 1970s, Hayashi et al discovered that de novo synthesis of IDO was induced by immune stimulating agents such as lipopolysaccharide (Hayaishi and Yoshida, 1978; Yoshida and Hayaishi, 1978); and later interferon was found to be one of the primary inducers of IDO synthesis (Yoshida et al, 1986). Tryptophan depletion by IDO was then thought to be limited to areas of inflammation where leukocytes were actively producing inflammatory mediators. In 1984, it was shown that growth of Toxoplasmosis gondii could be inhibited by IFN-mediated IDO induction, and could be controlled by the amount of tryptophan concentrations in the culture media (Pfefferkorn, 1984). Tryptophan depletion by IDO was the principal mechanism responsible for inhibiting parasitic growth. Numerous human cell lines express IDO when exposed to interferon! while it is scarcely detected in freshly isolated tissues (Burke et al, 1995; Varga et al, 1996; Munn et al, 1999; Hwu et al, 2000). Other cytokines such as LPS, TNF" and IL1 act in a synergistic fashion with interferon-! to further increase IDO expression in human dendritic cells (Babcock and Carlin, 2000; Hwu et al, 2000). Antiinflammatory cytokines such as IL4, IL10 and TGF# inhibit IDO expression (Yuan et al, 1998; MacKenzie et al, 1999).
IV. Regulation of IDO gene expression The IDO enzyme is encoded by a single gene with 10 exons spread over approximately 15 kb of DNA located in a syntenic region of human and mouse chromosome 8 (Mellor and Munn, 2004). Transcription is controlled by responding to inflammatory mediators like IFN-"# and IFN-! interferons (Dai and Gupta, 1990; Taylor and Feng, 1991; Hassanain et al, 1993; Mellor and Munn, 1999). Myeloid-lineage cells such as Dendritic cells and macrophages, fibroblasts, endothelial cells and tumor-cell lines express IDO after exposure to IFN-! (Taylor and Feng, 1991; Burke et al, 1995; Varga et al, 1996; Munn et al, 1999; Hwu et al, 2000). Signal transducer and activator of transcription and IFN-regulatory factor 1 function cooperatively to mediate the induction of IDO expression by IFN-!, and mice that lack either IFN-! or IRF1 are deficient in IDO expression during infection (Silva et al, 2002). Due to the synergistic actions of lipopolysaccharide (LPS) and the inflammatory cytokine IL-1 and tumor necrosis factor (TNF), they enhance IDO expression in vitro (Babcock and Carlin, 2000; Robinson et al, 2003). However, in vivo, responsiveness to LPS depends on TNF, but does not require IFN-! indicating an IFN-! independent pathway for IDO expression (Fujigaki et al, 2001). There may be some cell specific inflammatory mediators of transcription, further modulating IDO expression (Yuan et al, 1998; MacKenzie et al, 1999). IDO is functionally an intracellular enzyme. This enzyme is known to be found constitutively at the maternal-fetal interface by human extra-villous trophoblast cells (Kudo and Boyd, 2000; Kudo et al, 2001, 2004;
III. How does Tryptophan deprivation thwart immune response? We know that IDO is released by stimuli such as interferon-!. Tryptophanyl tRNA synthetase is the only amino-acyl tRNA synthetase whose expression is enhanced by inflammatory mediators such as interferon-! 28