Persistent Symptoms and IFN-γ-Mediated Pathways after COVID-19

Abstract

After COVID-19, patients have reported various complaints such as fatigue, neurological symptoms, and insomnia. Immune-mediated changes in amino acid metabolism might contribute to the development of these symptoms. Patients who had had acute, PCR-confirmed COVID-19 infection about 60 days earlier were recruited within the scope of the prospective CovILD study. We determined the inflammatory parameters and alterations in tryptophan and phenylalanine metabolism in 142 patients cross-sectionally. Symptom persistence (pain, gastrointestinal symptoms, anosmia, sleep disturbance, and neurological symptoms) and patients' physical levels of functioning were recorded. Symptoms improved in many patients after acute COVID-19 (n = 73, 51.4%). Still, a high percentage of patients had complaints, and women were affected more often. In many patients, ongoing immune activation (as indicated by high neopterin and CRP concentrations) and enhanced tryptophan catabolism were found. A higher phenylalanine to tyrosine ratio (Phe/Tyr) was found in women with a lower level of functioning. Patients who reported improvements in pain had lower Phe/Tyr ratios, while patients with improved gastrointestinal symptoms presented with higher tryptophan and kynurenine values. Our results suggest that women have persistent symptoms after COVID-19 more often than men. In addition, the physical level of functioning and the improvements in certain symptoms appear to be associated with immune-mediated changes in amino acid metabolism.

Keywords: COVID-19; biomarkers; interferon-gamma; long COVID; neopterin; phenylalanine; tryptophan.

Figure 1

Inflammation-associated biochemical pathways. Inflammatory signaling, most importantly interferon gamma (IFN-γ), stimulates neopterin synthesis via GTP cyclohydrolase 1 (GTP-CH-I) and tryptophan (Trp) catabolism along the kynurenine (Kyn) axis. Neopterin formation occurs mainly in human macrophages (MΦ) and dendritic cells (DC) at the expense of tetrahydrobiopterin (BH4). BH4 is a cofactor of monoxygenases, e.g., phenylalanine 4-monooxygenase (PAH), tyrosine 3-monooxygenase (TH), tryptophan 5-monooxygenases (TPH), and nitric oxide synthases (NOS). BH4 can be synthetized by other cell types, but it is oxidation labile and may diminish in situations of oxidative stress. Abbreviations: KynA = kynurenic acid, indoleamine 2,3-dioxygenase 1 (IDO-1), NAD = nicotinamide adenine dinucleotide, Phe = phenylalanine, QuinA = quinolinic acid, Tyr = tyrosine. Metabolites analyzed in the study are in bold and highlighted grey. Dashed arrows indicate biosynthetic processes in which more than one step/enzyme is involved. Figure licensed by Talia Piater.

Figure 2

Gender comparison of IFN-γ-mediated parameters. All parameters except nitrite (p = 0.003 for men and p = 0.078 for women, n.s.) are significantly different between healthy blood donors and reconvalescent COVID-19 patients, both in women (light boxplots) and in men (dark grey boxplots), with p < 0.001 in each case. * and ° outlier.

Figure 3

Scatter plots demonstrating the relationship between selected amino acids and/or ratios stratified by gender. Kyn/Trp = kynurenine to tryptophan ratio. (a) Higher neopterin values correlated with enhanced IDO-activity (as reflected by Kyn/Trp). (b) Low tryptophan values correlated significantly with low tyrosine levels. (c) Patients with low tryptophan concentrations also presented with low phenylalanine.

Figure 4

Gender comparison of ECOG score and Phe/Tyr ratio. Women = light, men = dark. Lower physical performance of females correlated positively with higher Phe/Tyr values. No significant correlation was found in men. Phe/Tyr = phenylalanine to tyrosine ratio. * and ° outlier.