Our main insight in this prospective targeted metabolomics study was that at the time of ARDS onset, plasma kynurenine levels and the plasma kynurenine/tryptophan ratio were both independently associated with the likelihood of hospital mortality. Our findings indicate that activation of the kynurenine pathway may play a role in pathogenesis and may therefore serve as a biomarker by which to predict clinical outcomes in cases of ARDS.
Kynurenine can exert immunosuppressive effects through the aryl hydrocarbon receptor, which suppresses the proliferation of effector T cells and natural killer cells and promote the activation of regulatory T cells. Increased kynurenines levels have been shown to alter cellular metabolism and cause cell death via the ROS pathway [15, 19–23, 27]. Under the effects of an inflammatory stimulus or immune system activation, proinflammatory cytokines and chemokines (particularly tumor necrosis factor-α and interferon-γ) can enhance the activity of indoleamine 2,3-dioxygenase (IDO), which is a negative regulator of inflammation and immunization. IDO, which is highly expressed in antigen-presenting cells (e.g., dendritic cells), catalyzes the first and main rate-limiting step in the kynurenine pathway. In this way, it contributes to the breakdown of tryptophan and the accumulation of kynurenine with corresponding effects on immunosuppression and immune tolerance [18–22, 27, 28]. In the current study, the mean concentration of plasma kynurenine was significantly higher among nonsurvivors than among survivors throughout the study period. In our Cox regression model, plasma kynurenine values at day 1 were independently associated with hospital mortality (HR = 1.017, p = 0.017). Urine metabolic profiling revealed higher tryptophan degradation and higher downstream kynurenine pathway metabolites among nonsurvivors at day 1, indicating activation of the kynurenine pathway.
The kynurenine-to-tryptophan ratio has been widely used to estimate the enzyme activity of IDO, where an elevated kynurenine/tryptophan ratio often indicates that the activation of IDO correlates with elevated neopterin levels (an indicator of cellular immune activation and oxidative stress) [8, 9, 18, 22, 29–31]. Kynurenine is a neurotoxic metabolite, and activation of the kynurenine pathway (assessed in terms of plasma kynurenine levels and the kynurenine/tryptophan ratio) was independently associated with acute brain dysfunction (delirium and coma) in mechanically ventilated patients . Sepsis is the leading cause of ARDS, and one previous study reported that the kynurenine/tryptophan ratio was up to 9-fold and was significantly higher in patients with septic shock than in the two control groups (nonseptic, low blood pressure controls and normotensive healthy subjects). The kynurenine/tryptophan ratio was also strongly correlated with inotrope requirements . The kynurenine pathway has also been implicated in tumor-associated immunosuppression, wherein IDO may promote the evasion of tumor cells from the immune system surveillance. The overexpression of IDO is associated with poor prognosis in a variety of cancers, and clinical trials on IDO inhibitors for cancer immunotherapy are currently underway [19, 21, 28, 33].
Researchers have not previously investigated the potential role of the kynurenine/tryptophan ratio or IDO activity in cases of ARDS. Our findings revealed that between day 1 and day 7, the mean kynurenine/tryptophan ratio (an index for IDO activity) was significantly lower among survivors than among nonsurvivors and that the ratio decreased over time. Cox regression models revealed that estimates of hazard of death obtained using the plasma kynurenine/tryptophan ratio at day 1 were higher than estimates obtained using plasma kynurenine at day 1, despite the fact that both factors were independently associated with 90-day hospital mortality (adjusted HR = 1.761 and 1.017, respectively; both p < 0.05). This is an indication that inflammatory signals during ARDS enhanced IDO activity, which subsequently activated the kynurenine pathway, thereby participating in pathogenesis and disease progression with a corresponding effect on clinical outcomes. It is possible that the kynurenine/tryptophan ratio or IDO activity could be used as a prognostic tool for patient stratification or for the development of drugs aimed at improving clinical outcomes of ARDS .
Recent studies described strong associations between metabolites including the kynurenine pathway of tryptophan metabolism and proinflammatory cytokines/chemokines (e.g., interleukin (IL)-1 and IL-6) in COVID-19 patients [7, 8, 34]. Targeting tryptophan metabolism was shown to modulate the release of proinflammatory cytokines by peripheral blood mononuclear cells isolated from rhesus macaques infected ex vivo with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) . These results indicate that by intervening in metabolic dysregulation, it may be possible to suppress the release of cytokines in COVID-19 patients. The kynurenine/tryptophan ratio was also significantly increased among patients infected with SARS-CoV-2 than among those without SARS-CoV-2 infection and healthy controls [8, 31]. Nonetheless, confirming the impact of the activation of the kynurenine pathway on the pathogenesis, disease progression, and clinical outcomes of COVID-19-related ARDS will require further study.
The most common cause of death among ARDS patients is multiple organ failure [1, 2]. In previous research, metabolites of the kynurenine pathway were dysregulated in animal models and clinical observational studies of acute kidney injury . Elevated plasma kynurenine values and/or the kynurenine/tryptophan ratio are predictive of sepsis and multiple organ failure in patients suffering major trauma . Kynurenine-3-monooxygenase is a key enzyme and drug target involved in the conversion of kynurenine into neuroactive metabolites of the immunoregulatory kynurenine pathway, including 3-hydroxykynurenine, which contribute to increased oxidative stress, cellular damage, and apoptosis through the production of ROS. Efforts to inhibit the release of kynurenine-3-monooxygenase have been shown to reduce 3-hydroxykynurenine levels to alleviate acute kidney injury, prevent multiple organ failure, and ameliorate histological changes in lung tissue consistent with ARDS [19, 21, 35, 37]. The IDO-kynurenine-aryl hydrocarbon receptor signaling pathway has also been shown to play an important role in inflammation and multiple organ injuries in cases of SARS-CoV-2 infection [7, 9].
Nonetheless, few reports have explored the links between the kynurenine pathway and multiple organ failure in ARDS. Our findings revealed that plasma kynurenine concentrations were significantly correlated with SOFA scores at the time of ARDS onset, and a Cox regression model revealed that kynurenine levels and SOFA score were both independently associated with hospital mortality. ARDS is characterized by substantial alveolar and systemic inflammation triggered by proinflammatory mediators and cytokines [1, 2]. Thus, it is reasonable to assume that in ARDS patients, proinflammatory cytokines during ARDS promote IDO activity, thereby contributing to an imbalance between tryptophan and kynurenine leading to multiple organ failure and death.
This study was hindered by a number of limitations. First, all of the patients were from a single tertiary care referral center and therefore lacked external validation. This no doubt limits the generalizability and reliability of our findings. Furthermore, we selected unventilated healthy individuals (rather than ventilated ICU patients without ARDS) as our control group. This no doubt poses a potential confounder. Second, we did not examine the precise mechanism involved in activating the kynurenine pathway in the pathogenesis of ARDS. Third, our measurements of circulating cytokines (e.g., IL-6 or interferon-γ) were not integrated with metabolic profiling to identify the potential metabolite-cytokine relationships. Fourth, despite the fact that the kynurenine/tryptophan ratio tends to fluctuate under certain conditions, we used it as a surrogate for IDO activity (as in recent studies) and did not check the exact IDO values. Finally, ARDS is a heterogeneous syndrome with a complex pathogenesis, and the study groups were prone to diverse variations in comorbidities, nutritional status, as well as pharmaceutical and clinical interventions, all of which may have influenced the metabolic fingerprint (metabotypes) of individuals. We observed an association between the activation of the kynurenine pathway and hospital mortality in ARDS patients; however, the causal relationship has yet to be clearly determined.