The first major findings of this systematic review and meta-analysis are that a) the KYN/TRP ratio is significantly increased in COVID-19 patients compared to non-COVID-19 controls with high effect size; and b) the KYN/TRP ratio is dramatically increased in severe/critical COVID-19 as compared with mild/moderate COVID-19 again with a large effect size. Importantly, the severe/critical COVID-19 patient samples included in this study mainly consist of critical patients who did not survive and, therefore, our results also suggest that an increased KYN/TRP ratio is associated with death due to COVID-19.
These results indicate that IDO activity and the TRYCAT pathway are upregulated in COVID-19 and that it predicts critical disease and non-survival. The most probable cause of IDO enzyme activation in COVID-19 is the increased level of pro-inflammatory cytokines including IFN-γ, IL-1β and IL-6 [53, 54] and activated oxidative stress pathways , which both potently stimulate IDO [25, 56].
Further analyses showed that the changes in the KYN/TRP ratio are attributable to significant increases in KYN and decreases in TRP in COVID-19 again with large effect sizes. These results extend the findings of previous studies which showed associations between severity of COVID-19 and increases in the KYN/TRP ratio and KYN and decreases in TRP [25, 51, 52]. Thus, not only aberrations in innate immune potential but also associated TRYCAT pathway activation contributes to a fatal course of the disease [57–59].
The second major finding of this study is that the KA/KYN ratio did not show a significant difference between COVID-19 patients as compared to non-COVID-19 controls, suggesting that COVID-19 is not accompanied by changes in KAT activity. Our meta-analysis performed on KA values in COVID-19 showed important heterogeneity and subsequent groups analysis revealed that serum KA was significantly increased in COVID-19 with medium effect size (0.649), whereas in plasma a non-significant inverse association was found. There are insufficient data to perform meta-analysis on other ratios reflecting KMO and KYNU activity. In this respect, Lawler et al., reported elevated levels of 3HK and QA in patients with COVID-19 compared to healthy controls . Likewise, Marin-Corral et al. reported a high level of 3HK in severe/critical COVID-19 patients compared to those with mild/moderate infection .
During infection, IDO activation and consequent increased TRYCATs but lowered TRP levels are key components of the innate immune response. First, the TRYCAT pathway has major intrinsic scavenging activities by neutralizing ROS . Moreover, some TRYCATs have antioxidant properties on their own as for example, 3-hydroxyanthranilic acid (3HA) and 3HK, which are more effective as radical scavengers than tocopherol, and XA, which has antioxidant activity comparable to that of butylated hydroxytoluene (BHT) [14, 16]. KA has adequate antioxidant effects by protecting tissues from oxidative damage [60, 61]. Second, reduced TRP exerts anti-inflammatory (reduced T cell proliferation and activation, sensitization of apoptosis of activated T cells, and induction of the regulatory phenotype) and antimicrobial (inhibiting the growth of virus, bacteria and parasites) effects through TRP starvation [62–66]. Third, TRYCATs such as KA, KYN, QA, and XA, may exert negative immune-regulatory effects by lowering the production of IFN-γ and/or increasing that of IL-10 [14, 17]. In addition, KA has potent anti-inflammatory effects while diminished KA levels may aggravate tissue damage and cell proliferation . IFN-γ-induced stimulation of antigen-presenting cells upregulates the TRYCAT pathway and results in a counter-regulatory effect that preserves homeostasis . Due to the fact that TRYCATs trigger apoptosis in Th-1, but not Th-2, cells, TRYCAT pathway activation may suppress Th-1 cells but promotes Th-2 cell survival [69, 70]. As such, TRYCAT pathway activation results in a negative feedback loop to limit ROS production, hyperinflammation, and the Th-1 response [17, 69]. Fourth, some TRYCATs have neuroprotective effects including KA, anthranilic acid (AA) and XA. Thus, KA may inhibit N-methyl-D-aspartate (NMDA), kainate glutamate ionotropic, and amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and reduce glutamate liberation through attenuating alpha 7 nicotinic acetylcholine receptors [15, 71]. XA inhibits vesicular glutamate transport (VGLUT), synaptic transmission via the NMDAR receptor, and excitatory postsynaptic potentials . Furthermore, AA has neuroprotective effects by blocking the synthesis of neurotoxic TRYCATs such picolinic acid (PA) and QA from 3HA . Based on the above, it appears that during infection, the TRYCAT pathway activation has major homeostatic effects.
Nevertheless, overproduction of some TRYCATs may cause detrimental effects in COVID-19. KA is implicated in deteriorating male COVID-19 patients through affecting the AhR, one of the master regulators of the immune-inflammatory response . In addition, activation of AhR by TRYCATs, mainly KYN, affects immune resistance against viral infections and the airway basal cells of the lung epithelium, which are responsible for tissue repair [49, 75]. Most importantly, corona viruses activate the same receptor through an IDO-independent mechanism while the IDO-AhR pathway in employed by viruses, bacteria, and parasites to establish infection . Consequently, a positive feedback loop is established between increased TRYCATs levels due to IDO activation and stimulation of the AhR by TRYCATs and corona virus . Moreover, the AhR may enhance IDO transcription and regulates IDO activity . These processes may result in the SAAS which may result in activated immune-inflammatory pathways (increased M1 cytokines), fibrosis (increased IL-22), thromboembolism (increased tissue factor and plasminogen activator inhibitor 1), consequent multiple organ injuries including brain injuries, and eventually death .
Moreover, some TRYCATs have depressogenic, anxiogenic and neurotoxic effects and TRYCATs like KYN are increased in neuropsychiatric illness including major depression, anxiety, and psychosis [29, 77]. Second, some TRYCATs exhibit pro-oxidant properties as evidenced by increased ROS, hydrogen peroxide, and superoxide production, and increased oxidative damage including lipid peroxidation caused by 3HA, 3HK, and QA [18–24]. Third, TRYCATs such as QA and XA and PA may have direct neurotoxic effects by activating hippocampal NMDAR and causing excitotoxicity with apoptosis and hippocampal shrinkage thereby inducing neurocognitive impairments [78, 79]. Elevated XA levels may cause severe neuronal damage, apoptosis, mitochondrial dysfunctions, disrupt glutamate transmission, and impair presynaptic transmission caused by NMDAR stimulation . Such effects may contribute to the development of neuropsychiatric disorders such as depression, anxiety and chronic fatigue due to COVID-19 . Indeed, TRYCATs are confirmed to be associated with various mental disorders, including depression, and anxiety [14, 17], somatization and chronic fatigue syndrome , cognitive impairments , and psychosis . Moreover, some TRYCATs, namely KYN, KA and 3HK are associated with musculoskeletal injuries due to their agonistic effects on the AhR [83–86]. Thus, increased TRYCAT levels could exacerbate the neuro-immune and neuro-oxidative toxicity caused by increased oxidative stress and M1 and Th-1 activation resulting in comorbid affective disorders . Therefore, it is safe to hypothesize that the accumulation of TRYCATs in SARS-CoV2 infected patients may play a role in the neuropsychiatric and cognitive syndromes of long or post-COVID syndrome .
Finally, it may be hypothesized that COVID-associated TRYCAT pathway activation may aggravate the disorders in the TRYCAT pathway in comorbid disorders (obesity, dementia, T2DM, hypertension and heart disease, stroke, chronic obstructive pulmonary disease (COPD) and chronic kidney disease), which increase risk to critical disease and death due to COVID-19 . Indeed, in all those comorbid diseases, the IDO enzyme is activated as indicated by an increased KYN/TRP ratio [88–94]. By inference, when COVID-19 develops in people with those comorbid illnesses, an amplified TRYCATs response may occur, contributing to aggravated toxicity in addition to the consequences of inflammation and oxidative stress.
Another finding of our group meta-analysis revealed differences in the TRYCATs levels between COVID-19 patients and controls depending on whether plasma and serum was examined. For example, the results of KYN/TRP ratio in serum were highly significant with a large effect size (1.359), whereas in plasma no significant differences were found. Group analysis performed on the KA studies showed a significant difference in effect sizes between serum and plasma with serum KA yielding a positive medium effect size for COVID-19 (0.649), whereas in plasma a negative effect size was established. Similar results were detected in the associations between TRYCATs (e.g. KYN and KA) and schizophrenia with positive results in serum and often inverse results in plasma . The reader is referred to the latter paper for a discussion on the differences in TRYCAT measurements in serum versus plasma. Overall, it was concluded that TRYCATs measurements in serum are more adequate than assays performed in plasma .
Some limitations of the current systematic review and meta-analysis should be discussed. Not all studies clearly described the types of medications, the treatment protocol, the relevant comorbidities, and even the vaccination status of the patients. Moreover, non survivors following COVID-19 were sometimes lumped together with survivors. Due the small sample sizes and paucity of data on some TRYCATs, we were unable to estimate KMO and KYNU activity. Therefore, serum TRP and a more complete panel of serum TRYCATs should be determined in well-powered studies in the different stages of COVID-19 (i.e., mild, moderate, severe, critical, non-survival).