A major focus of research in the current coronavirus disease 2019 (COVID-19) pandemic has been the need to understand the mechanisms operating that cause this potentially lethal disease’s various manifestations and complications.
Coronavirus disease COVID-19 infection 3D medical illustration. Image Credit: Corona Borealis Studio / Shutterstock.
A new study claims that severe COVID-19 is caused by a positive feedback loop triggered by systemic inflammation and mediated by abnormal gut permeability. This finding could potentially help open up new avenues of treatment.
Disruption of gut microbiome-lung interactions
Earlier research on respiratory infections has shown that lung damage interrupts the ordinary cross-talk between the lung cells and the gut microbiome, triggering systemic inflammation and causing severe disease. Such inflammation causes gut barrier breakdown as well, allowing gut microbes to cross the intestinal epithelium and enter the systemic circulation. This produces further inflammation and worsens lung injury, completing the positive feedback loop.
Viral infection of the gut cells can also occur, leading to direct damage and further disrupting both gut structure and function, as well as gut barrier integrity. These mechanisms are more likely in the elderly or sick, who are more commonly already suffering from gut dysbiosis.
The current study, published as a preprint in the medRxiv* server in November 2020, is based on a systems biology model, used to examine the plasma from 60 patients with COVID-19 but with a range of clinical severity. Their ages ranged from 50 to 65 years.
Inflammation promotes excessive gut permeability
The researchers found a steep increase in the permeability of the gut epithelial tight junctions in severe COVID-19, signaling a loss of the intestinal barrier function. This was accompanied by a steep increase in the protein zonulin, “the only known physiological mediator of tight junction permeability in the digestive tract.” Increased zonulin was a marker for higher mortality in severe COVID-19.
They also measured the level of plasma lipopolysaccharide (LPS) binding protein (LBP), a marker of acute infection or inflammation, which binds to bacterial LPS and triggers the immune system. This reflects increased microbial access to the bloodstream through the leaky gut, and its level was increased in severe COVID-19, compared to milder cases.
There were also elevations in the levels of the fungal marker, β-glucan, and of the tight junction protein occludin, in severe COVID-19. However, the concentration of the intestinal fatty acid-binding protein (I-FABP) was unchanged, ruling out the death of intestinal enterocytes.
Microbial translocation associated with systemic inflammation
As expected, the gut microbial migration across the intestinal barrier was accompanied by signs of myeloid inflammation, with higher levels of neutrophil and monocyte inflammation markers in severe disease, along with cytokines like IL-6 and IL-1β, and the acute-phase reactants CRP and D-dimer.
Immunomodulatory molecules such as lectins also showed markedly raised levels in severely ill patients who eventually succumbed to the infection. The researchers additionally observed an imbalance in the levels of C3a and GDF15, indicators of complement activation and oxidative stress, respectively, in fatal cases.
The researchers comment, “These data support our hypothesis that disruption of intestinal barrier integrity, which results in microbial translocation, is linked to higher systemic inflammation and immune activation during severe COVID-19.”
Gut permeability modulates metabolite levels in severe COVID-19
Abnormal gut permeability not only causes systemic inflammation directly but is linked to alterations in the concentrations of an array of plasma metabolites. These are not only well-known markers of gut dysfunction, but bioactive compounds, associated with inflammatory and immune responses.
For example, several important amino acid pathways are disrupted. These include some like citrulline that are produced only within enterocytes, or others like tryptophan that are catabolized by enterocytes. This results in an increase in amino acids such as citrulline, succinic acid, and in the breakdown products of tryptophan. These disruptions are markers of gut dysbiosis and of dysregulation of immune interactions with the gut microbiome.
These changes were observed to occur in patients with severe disease versus controls or mild COVID-19.
Metabolomic changes associate with dysregulated inflammation
The observed metabolite alterations were associated with inflammatory markers and markers of gut barrier disruption. For instance, low citrulline and high tryptophan metabolite levels were linked to higher IL-6 levels.
Several lipid pathways were also perturbed by the disturbances in amino acid metabolism in severe COVID-19. Of the 16 pathways most significantly affected, those involving glycerophospholipid and choline metabolism were markedly disrupted. Both of these are linked closely to the gut microbiome, and dysbiosis has a negative effect on the breakdown and absorption of these lipids. Thus, severe COVID-19 is associated with systemic disturbances resulting from gut barrier breakdown.
Abnormalities in glycation cause dysregulation of inflammation
Glycans decorate many proteins and antibodies to regulate the immune response. The enzymes that degrade them come from several gut microbes, and the translocation of the latter may lead to a change in the glycosylation profile. This, in turn, can precipitate more inflammation via complement activation.
Such a change in the glycosylation profile of plasma glycoproteins is known to occur in inflammatory bowel disease (IBD). Additionally, fecal microbiota transplantation in order to correct the composition of the gut microbiome affects glycosylation of both IgG and serum.
A loss of galactose, for example, prevents the activation of the anti-inflammatory checkpoint that suppresses complement-mediated inflammation, which is formed by the galactose-mediated linkage of Dectin-1 to FcyRIIB on myeloid cells. This is seen to cause inflammation and complement activation in IBD.
Hospitalization risk score
The researchers used the machine learning algorithm Lasso to select those markers that could best discriminate mild COVID-19 from severe. The results included zonulin, LBP and sCD14, with an AUC of over 99%. They then used this to estimate the risk of hospitalization, and found that it had a sensitivity of around 98% with a specificity of around 95%, for an overall accuracy of 96%. The Kyn-Trp ratio is also capable of robust discrimination. This emphasizes the close link between gut disruption and severe COVID-19.
What are the implications?
The researchers sum up, “Our data indicate that severe COVID-19 is associated with a dramatic increase in tight junction permeability and translocation of bacterial and fungal products into the blood. This disrupted intestinal barrier integrity and microbial translocation correlates strongly with increased systemic inflammation, increased immune activation, decreased intestinal function, disrupted plasma metabolome and glycome, and higher mortality rate.”
The researchers also suggest that this may indicate the possibility of long-term sequelae due to the disruption of the gut barrier and perturbed gut function. This may include metabolic dysfunction in COVID-19 ‘long haulers’. Studies to understand this should be a priority, accompanied by research on the most effective ways to ameliorate this. This could appropriate for curbs on the widespread use of antibiotics, which impact the course of the disease, especially in older patients and those with metabolic disease.
Secondly, the study may make it possible eventually to predict severe COVID-19, using the identified rise in multiple biomarkers, including plasma lipids, amino acids and their metabolites, and glycans.
Finally, the study reveals some potential therapeutic targets in severe COVID-19. These include zonulin, which could be successfully inhibited to improve gut barrier integrity, and citrulline. The association between severe COVID-19 and a genetic predisposition to high zonulin production should also be examined further.
Again, lectins and other glycans that modulate inflammation may help prevent the cytokine storm in severe COVID-19. The use of highly glycosylated immune complexes is known to inhibit complement-mediated inflammation, and could be adapted for use in COVID-19.
Overall, therefore, the authors conclude, “Greater understanding of the interaction between the gut, intestinal microbiota, and amino acid metabolism during COVID-19 might inform pharmaceutical and diet approaches to improve COVID-19 outcomes.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Giron, L. B. et al. (2020). Severe COVID-19 Is Fueled by Disrupted Gut Barrier Integrity. medRxiv preprint. doi: https://doi.org/10.1101/2020.11.13.20231209. https://www.medrxiv.org/content/10.1101/2020.11.13.20231209v1