Association of NK cells with a shift in tryptophan catabolism in patients with heart failure after a single exercise exertion.

BACKGROUND: Tryptophan (TRP) metabolism via the kynurenine (KYN) pathway links immune function, energy metabolism, and redox homeostasis. Dysregulation of this pathway has been implicated in inflammatory conditions and heart failure. Here, we investigated the acute effects of exercise on TRP-KYN metabolism and its relationship with natural killer (NK) cell function in controls and patients with heart failure with reduced ejection fraction (HFrEF). METHODS: Control (n=13) and HFrEF (n=16) groups had comparable composition regarding age and sex. Participants were investigated at baseline, immediately after a maximal symptom-limited cardiopulmonary exercise test (CPET), and after 2 hours of resting. Blood samples were obtained at all time points to assess NK cell counts and phenotypic parameters by flow cytometry, as well as tryptophan metabolites and protein secretome by mass spectrometry and targeted proteomics, respectively. NK cells and non-NK cells from blood of healthy donors were stimulated ex vivo prior to flow cytometry-based measurement, indoleamine 2,3-dioxygenase (IDO) mRNA expression analysis and mass spectrometry-based tryptophan metabolite analysis. RESULTS: Plasma TRP levels decreased post-exercise in both study groups, with increased metabolism down the KYN route, albeit only in HFrEF patients, a significant accumulation of quinolinate (QUIN) was seen. Increases in plasma KYN-to-TRP ratios correlated with more circulating NK cell counts and IL-12p70 levels mainly in the HFrEF group. Ex vivo, IL-12 exposure of human total primary NK cells increased representation of the CD56-bright subset, IDO mRNA expression, and TRP-to-KYN conversion, resulting in net KYN accumulation and elevated QUIN production. In non-NK cells, IFN-γ exposure similarly promoted TRP-to-KYN flux and QUIN formation. CONCLUSION: Collectively, our observations confirm earlier descriptive reports of exercise-induced upregulation of KYN production by NK cells and add mechanistic evidence that IL-12 induces a phenotype shift in NK cells, which is accompanied by accelerated TRP metabolism into KYN. Our data point to a concerted interaction between leukocyte subsets upon acute exercise, via the release of IL-12, with potential implications for differential energy metabolism and immune regulation in HFrEF.