Intravenous administration of LPS activates the kynurenine pathway in healthy male human subjects: a prospective placebo-controlled cross-over trial

Abstract

Background: Administration of lipopolysaccharide (LPS) from Gram-negative bacteria, also known as the human endotoxemia model, is a standardized and safe model of human inflammation. Experimental studies have revealed that peripheral administration of LPS leads to induction of the kynurenine pathway followed by depressive-like behavior and cognitive dysfunction in animals. The aim of the present study is to investigate how acute intravenous LPS administration affects the kynurenine pathway in healthy male human subjects.

Methods: The present study is a prospective, single-blinded, randomized, placebo-controlled cross-over study to investigate the effects of intravenously administered LPS (Escherichia coli O113, 2 ng/kg) on tryptophan and kynurenine metabolites over 48 h and their association with interleukin-6 (IL-6) and C-reactive protein (CRP). The study included 10 healthy, non-smoking men (18-40 years) free from medication. Statistical differences in tryptophan and kynurenine metabolites as well as associations with IL-6 and CRP in LPS and placebo treated subjects were assessed with linear mixed-effects models.

Results: Systemic injection of LPS was associated with significantly lower concentrations of plasma tryptophan and kynurenine after 4 h, as well as higher concentrations of quinolinic acid (QUIN) after 48 h compared to the placebo injection. No differences were found in kynurenic acid (KYNA) or picolinic acid plasma concentrations between LPS or placebo treatment. The KYNA/kynurenine ratio peaked at 6 h post LPS injection while QUIN/kynurenine maintained significantly higher from 3 h post LPS injection until 24 h. The kynurenine/tryptophan ratio was higher at 24 h and 48 h post LPS treatment. Finally, we report an association between the kynurenine/tryptophan ratio and CRP.

Conclusions: Our findings strongly support the concept that an inflammatory challenge with LPS induces the kynurenine pathway in humans, activating both the neurotoxic (QUIN) and neuroprotective (KYNA) branch of the kynurenine pathway.

Trial registration: This study is based on a study registered at ClinicalTrials.gov, NCT03392701 . Registered 21 December 2017.

Keywords: Experimental endotoxemia; Inflammation; Kynurenine metabolites; Lipopolysaccharides (LPS).

Fig. 1

Schematic overview of the kynurenine pathway. The first and rate-limiting step is catalyzed by tryptophan 2,3-dioxygenase (TDO2) or by indoleamine 2,3-dioxygenase (IDO)1 and 2. N-formyl kynurenine is then converted by kynurenine formamidase to l-kynurenine before entering different possible branches, depending on cell-type or environmental context, to form various metabolites, which can exhibit immunological, antioxidant, or neurological activities. Pathways represented by two arrows involve several metabolites and enzymatic reactions. Nicotinamide adenine dinucleotide (NAD+)

Fig. 2

Kynurenine metabolite levels after LPS (red) or placebo (blue) injection. The red line indicates the injection, the black lines the change between the three consecutive days. Data is presented as mean and standard error of the mean. *p < 0.005 (statistically significant results). KYNA: kynurenic acid, 3-HK: 3-hydroxykynurenine, QUIN: quinolinic acid

Fig. 3

Ratios between metabolites after LPS (red) or placebo (blue) injection. The red line indicates the injection, the black lines the change between the three consecutive days. Data is presented as mean and standard error of the mean. *p < 0.005. KYNA: kynureninic acid, QUIN: quinolinic acid

Fig. 4

Associations between A metabolite ratios and CRP or IL-6 over all timepoints. The size of the circles indicates the strength of the association (−log₁₀p), the color indicates the direction of the association (red: positive, blue: negative). *p < 0.0083. KYNA: kynureninic acid, QUIN: quinolinic acid. B Correlation between the maximum increase in CRP (mg/dL) and the maximum increase in the kynurenine/tryptophan ratio independent of time. The black line indicates the conditional mean, and the grey area indicates the 95% confidence interval