RA and experimental inflammatory joint diseases have a progressive character with involvement of increasing numbers of joints; however, the initial and aggressive acute phase in affected joints slows down over time and the inflammatory processes burn out. Several lines of evidence indicate that the effector mechanism that initially attacks the joints is T cell-driven in response to the effect of proinflammatory cytokines, but the mechanisms responsible for the limitation of acute inflammatory processes are much less understood. The novel finding of our study is that IDO activity is upregulated in the acute phase of CIA reflected by the increased K/T ratio in the serum. Furthermore, we could also demonstrate that inhibition of IDO in this experimental model augments the incidence and severity of the disease and increases the immune responses to the autoantigens and alloantigens. These data suggest that IDO plays a central role in the negative regulatory feedback of immunological mechanisms in inflammatory joint diseases.
CIA, like RA in humans, is characterized by the accumulation of T cells, plasma cells, macrophages, B cells, mast cells, natural killer (NK) cells, and dendritic cells in the synovial sublining [17, 18]. Furthermore, inflammatory cells infiltrating the synovial tissue in RA and in the acute phase of CIA exhibit a predominantly Th1 pattern of cytokine expression [10, 11]. By priming the Th1-type inflammatory cell responses, IFN-γ is one of the most important proinflammatory factors in the induction of T cell-driven autoimmune arthritis, such as CIA. However, IFN-γ plays an ambiguous role in autoimmunity. After the activation of self-reactive lymphocyte clones and of bystander and accessory cells in the acute phase, IFN-γ downregulates the autoimmune processes . Indeed, CIA and CFA developed more readily in IFN-γ receptor-deficient mice than in wild-type littermates . As a possible explanation, it has emerged that CFA elicits strong myelopoiesis and expansion of Mac-1+ cells, which play a crucial role in disease pathogenesis, and this process is downregulated by IFN-γ . However, the exact mechanism responsible for the effect of IFN-γ in CIA has not been fully elucidated.
One of the most likely mechanisms for the downregulation of CIA by IFN-γ is the increased expression of IDO by non-T cells . In fibroblasts , macrophages , and dendritic cells , IFN-γ stimulates the enzyme IDO, which degrades the amino acid tryptophan to form kynurenine, resulting in the inhibition of autoimmune processes. According to this assumption, we demonstrated an elevated K/T ratio, indicating high IDO activity during the acute phase of CIA. Moreover, the K/T ratio was even higher in mice that received CII and CFA but developed no clinical and histological signs of arthritis. These data suggest that IDO acts as a negative feedback in this model, and the onset and severity of experimental arthritis are inversely proportional to IDO activity.
To confirm the regulatory role of IDO in CIA, we used 1-MT, a known competitive inhibitor of IDO. 1-MT did not influence IFN-γ, but it significantly suppressed IL-4 production by spleen cells, resulting in an increased Th1/Th2 response. In 1-MT-treated mice, we could demonstrate a significant decrease of K/T ratio in the immediate pre-arthritic and acute phase of arthritis compared to vehicle-treated animals, suggesting the high activity of IDO only in these stages of inflammatory processes. In other words, the low blocking activity of 1-MT either in the pre-arthritic phase or in the chronic phase of arthritis denotes the anti-inflammatory effect of IDO only in the case of upregulation of inflammatory processes, especially Th1 responses.
As a tryptophan-catabolizing enzyme, IDO can induce the peripheral tolerance and reduce the persistent immune activation. On one hand, IDO decreases the tryptophan concentration in the microenvironment of inflammatory cells. Although tryptophan is an essential amino acid indispensable for the biosynthesis of proteins, the low tryptophan concentration results in the arrest of cell proliferation in the mid-G1 arrest point. T cells are specifically sensitive to tryptophan deprivation [2, 5] and thus IDO activity can block the potential harmful autoimmune response. On the other hand, Zhu and colleagues  proposed that synovial T cells derived from RA synovial fluids are resistant to IDO-mediated tryptophan deprivation. This may be one mechanism by which autoreactive T cells are sustained in vivo in patients with arthritis . In addition, selected metabolites on the tryptophan-kynurenine pathway are able to suppress proliferation of allogeneic T cells and, to a lesser extent, B and NK cells . Moreover, some of the kynurenine derivates can induce in vitro the selective apoptosis of Th1 cells, but not Th2 cells . In accordance with these results, 1-MT treatment in our study resulted in an increased, mainly Th1 cell-mediated immune response to the CII and consequently in the significant worsening of severity and increase of onset of CIA mediated by Th1 response.
In this regard, recent studies with the CIA model of arthritis using an antibody to the costimulatory molecule CD137, a member of the tumor necrosis factor receptor superfamily, are of great interest. These results showed that treatment of mice with the anti-CD137 antibody resulted in induction of IDO in vivo, which was associated with significant amelioration of the severity of CIA . Furthermore, pharmacological inhibition of IDO reversed the effects of the anti-CD137 antibody and aggravated the arthritis in this model.
The tryptophan-IDO pathway may have important relevance for the biological therapy of RA. In a study of Boasso and colleagues , CTLA-4-Fc treatment of human peripheral blood CD4+ T cells resulted in increased IDO expression by these cells. This effect was not observed in CD8+ T cells. Thus, abatacept (CTLA4-Ig) therapy may act, at least in part, by the stimulation of IDO production.