JIA is the most common chronic rheumatic disease in children [1]. Incidence rates vary between 1.6 to 23 and prevalence between 3.8 to 400/100,000. Both proportions were almost twice as high in girls than in boys [2]. Oligoarthritis is the most common subtype (pooled incidence rate 3.7 [3.5 - 3.9] with a prevalence of 16.8 [15.9 – 17.7]/100,000) [2], and the most frequent subset in JIA patients, characterized by early-onset asymmetric arthritis predominantly affecting knee joints or ankles. Rheumatoid factor-positive polyarthritis is considered the childhood equivalent of the RF-positive rheumatoid arthritis (RA) seen in adults [1]. This subset represents only 3% of the JIA cases, concerning particularly female adolescents [3]. The etiology of JIA is still poorly understood, but appears to involve a combination of multiple genes, as well as environmental factors [4,5] Patients with systemic JIA (sJIA) present a distinct inflammatory profile . Mounting evidence indicates that a dysregulated innate immune system induces increased production of autoinflammatory cytokines such as IL-6 and IL-18 [6,7]. The sJIA unique pathogenesis was, therefore, an exclusion ground from this study.

The chronic inflammatory processes of joints in JIA are comparable to those observed in adult RA, accompanied by villous hypertrophy and hyperplasia of the synovial lining layer. The subsynovial layers are hyperaemic, oedematous, and massively infiltrated by inflammatory cells such as mononuclear cells, T cells, B cells, macrophages, dendritic cells, and plasma cells [8,9]. The chemokine receptors CCR5, CXCR3, and CD45RO, are primarily expressed by activated T cells type 1 (Th1), which produce the cytokine profiles interferon-g (IFNγ), tumor necrosis factor (TNF) -α, and IL-2 [10]. The diagnosis of JIA is based on clinical examination, patient’s history, and exclusion of other possible causes. Currently, laboratory examination can only support the clinical diagnosis, as there are no reliable laboratory tests or combinations of studies [1]. L-tryptophan (Trp) is the rarest essential amino acid found in food. It is an intermediate for protein synthesis and a building block of several biologically essential metabolites such as 5-hydroxytryptamine (T5H), kynurenines, and nicotinamide adenine dinucleotide (NAD+). The first step of the kynurenine pathway can be catalyz ed either by tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO) [11].

TDO is predominantly expressed in the liver but also in the brain, the epididymis, and the mucous membranes. TDO preserves blood homeostasis of Trp and may also play a role in immunoregulation after liver transplantation [12]. IDO has only been found in mammals and yeasts and is not expressed in prokaryotes [13]. IDO is the rate-limiting enzyme of the kynurenine pathway and transforms L-Trp into N-formylkynurenine, which is rapidly metabolized into L-kynurenine (Kyn) (Figure 1). Kyn can enter the bloodstream or be further converted to downstream kynurenine metabolites like kynurenic acid and quinolinic acid [14]. The highest levels of IDO expression can be detected in professional antigen-presenting cells (APC), like monocyte-derived macrophages, dendritic cells (DC), and fibroblasts. IDO is mainly responsive to the Th1-type cytokine IFN-γ [15,16] but can also be activated by hormones such as human chorionic gonadotropin [17] and estrogen [18]. Besides these soluble molecules, there is also a direct interaction with membrane-anchored co-receptors such as B7 and CTLA4 found on T cells and DCs [19].

The withdrawal of Trp from the micro-environment through stimulation of IDO inhibits the growth of microbes and has a strong antiproliferative effect on malignant cells [20]. IDO plays a pivotal role in peripheral tolerance [21,22]. Kynurenine is a potent start signal for T-cell apoptosis. Furthermore, withdrawal of Trp causes a stop of T-cell proliferation during mitosis and a loss of activity in the T cell. In summary, the activation of IDO is generally considered to have immunosuppressive and anti-inflammatory effects [23-25]. Consequently, the pharmacologic inhibition of IDO promotes the activation of autoreactive T cells, which can lead to autoimmunity and loss of peripheral immune tolerance [26].

Although there have been ongoing, extensive investigations into the enzyme's role in rheumatoid arthritis (RA), research on the activity of IDO in JIA patients remains scarce., Patients with RA had a significantly lower level of Trp than healthy blood donors. In addition, a relation between the progressive stages of RA and decreased levels of Trp was identified [27]. However, a correlation between the subjective disease activity within these stages and Trp levels could not be detected [27,28]. A similar study by Y. Ozkam et al. did not corroborate these results. The authors concluded that serum levels of Trp and Kyn did not differ between RA patients and controls [29]. L. Zhu et al. discovered that DCs isolated from synovial fluid of RA patients expressed higher levels of functionally active IDO than DCs derived from healthy donors. In contrast, there was no detectable IDO in DCs isolated from peripheral blood, neither in patients with RA nor in healthy blood donors [30].

However, IDO is not able to sufficiently suppress autoreactive T cells in RA patients, which is likely caused by highly upregulated levels of tryptophanyl-tRNA-synthetase (TTS) in T cells (also induced by IFN-γ). This enzyme enables T cells to preserve Trp, which results in lower susceptibility to IDO-mediated tryptophan deprivation [30]. This may cause the pathogenic persistence of autoreactive T cells in patients with autoimmune diseases. A laboratory parameter to ascertain JIA and promote early diagnosis has not yet been established. Therefore, the objective of this study was to evaluate the clinical relevance of Trp and its downstream metabolites as biochemical markers for diagnosis or follow-ups during treatment. These parameters are known to be influenced by chronic inflammatory diseases and could facilitate the diagnostic process. Furthermore, an indicator of disease activity might help the practitioner to intervene promptly in case of exacerbations.