key: cord-311673-z4hkw17g
authors: Uzzan, Mathieu; Corcos, Olivier; Martin, Jerome; Treton, Xavier; Bouhnik, Yoram
title: Why is SARS-CoV-2 infection more severe in obese men? The gut lymphatics - lung axis hypothesis
date: 2020-06-23
journal: Med Hypotheses
DOI: 10.1016/j.mehy.2020.110023
sha: 
doc_id: 311673
cord_uid: z4hkw17g

Consistent observations report increased severity of SARS-CoV-2 infection in overweight men with cardiovascular factors. As the visceral fat possesses an intense immune activity, is involved in metabolic syndrome and is at the crossroad between the intestines, the systemic circulation and the lung, we hypothesized that it plays a major role in severe forms of SARS-CoV-2 infection. SARS-CoV2 presents the ability to infect epithelial cells of the respiratory tract as well as the intestinal tract. Several factors may increase intestinal permeability including, direct enterocyte damage by SARS-CoV2, systemic inflammatory response syndrome (SIRS) and epithelial ischemia secondary to SARS-CoV2- associated endothelial dysfunction. This increase permeability further leads to translocation of microbial components such as MAMPS (microbial-associated molecular pattern), triggering an inflammatory immune response by TLR-expressing cells of the mesentery fat (mostly macrophages and adipocytes). The pro-inflammatory cytokines produced by the mesentery fat mediates systemic inflammation and aggravate acute respiratory distress syndrome (ARDS) through the mesenteric lymph drainage.

The recent outbreak of the SARS-CoV-2 has led to a worldwide pandemic responsible for a major health crisis. Coronavirus-induced disease can evolve to severe forms characterized by hyperinflammation, acute respiratory distress syndrome (ARDS) and death, which are more frequently observed in subsets of patients with comorbidities. Observational studies in Chinese, European and US cohorts consistently highlighted a significant association between intensive care unit (ICU) admission and male gender, as well as hypertension, diabetes and cardiovascular diseases, as compared to non-ICU patients [1, 2] . In a cohort of 138 patients hospitalized in Wuhan, males accounted for 61% of ICU patients (n=36) as compared to 52%

(n=102) in conventional units; 58%, 25% and 22% of ICU patients had hypertension, diabetes and cardiovascular history, respectively, as compared to 22%, 11% and 6% in non-ICU patients (all p values below 0.05) [2] . Higher risk of death was further reported for males with cardiovascular comorbidities within COVID-19 ICU patients [3, 4] . [5] . Similar results were obtained in a large cohort from New York City (NY), in which obesity was an independent risk-factor for hospitalization in patients under 60 years old who were admitted to the emergency department [6] . In an Italian cohort of more than 1,000 ICU patients hospitalized in Lombardia, 83% of patients were males; 50% had high blood pressure and about 20% had type 2 diabetes, hypercholesterolemia and/or a cardiovascular disease [7] . Overall, these studies indicate that overweight males with cardiovascular risk factors are at significantly higher risk of developing severe forms of COVID-19.

As the increased volume of mesentery fat in overweight men play a key role in the occurrence of metabolic syndrome [8] , we hypothesized that the visceral adipose tissue plays a central role in severe forms of COVID-19.

The initial phase of the disease is considered as the viral phase. At that phase, COVID-19

behaves as a classical lymphocytic pneumonia, evidenced by CT-scan and/or lung biopsies [9, 10] ,. At a later phase of the disease, lung tissue lesions are unusual, presenting notably with major intra-alveolar fibrin deposition and intraluminal loose connective tissue in alveolar ducts and bronchioles. Additionally, vascular damage is significant with cytoplasmic vacuolization of the endothelium and cell detachment in small to medium-sized pulmonary arteries [10] .

At this later phase, along with endothelial damage, clinical and biological characteristics are suggestive of a "cytokinic storm". In critically ill patients with COVID-19 an intense inflammatory response during a secondary respiratory and systemic worsening at day 7 to day 15 from the onset of initial symptoms has been widely described [7, 11, 12] .

Hadjadj et al. performed an immunological extensive assessment (immune blood cell phenotyping, whole blood transcriptomic and cytokine quantification) in patients with mild form and severe form of COVID-19 at day 8 to day 12 after onset of symptoms [13] .

Interestingly, they found that type 1 interferon response was profoundly impaired. In addition, innate immunity-associated cytokinic pathways were markedly activated. Specifically, they found a strong increase in expression of IL-6-induced genes, such as IL6R, SOCS3 and STAT3

as well as an up-regulation of TNF pathway-related genes, including TNFSF10. Another key finding is the up-regulation of the CCL2-CCR2 pathway suggesting a key role of monocytes and macrophages in the lung and in the intense systemic inflammation state [13] .

Other studies described increased plasma concentrations of a series of inflammatory cytokines, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-6 [14, 15] , TNF-α, and granulocyte colony-stimulating factor (G-CSF), in severe COVID-19. Further investigations demonstrated that peripheral inflammatory monocytes and T cells through their production of GM-CSF may incite cytokine storm in severe COVID-19 patients [15, 16] .

This excess of inflammatory response is a critical cause of mortality from COVID-19. A study of 548 patients showed that, high cytokine levels (IL-2R, IL-6, IL-10, and TNF-α), and high LDH level were significantly associated with severe forms of COVID-19 [17] .

Accordingly, anti-IL-6R therapy is being investigated to treat severely ill patients [18, 19] with preliminary results reporting efficacy in a French multicenter randomized clinical trial. Other anti-cytokines therapies are also under investigation.

SARS-CoV-2 binds ACE2 which is vastly expressed in cardiopulmonary tissues, including endothelial cells. Some hematopoietic cells also express it, and particularly monocytes and macrophages [20] .

From studies focusing on other coronaviruses, it is known that the infection of cells of the myeloid lineage (monocytes, macrophages and dendritic cells) results in their activation and production of inflammatory cytokines including IL-6 [20] .

IL-6 can signal through 2 main pathways, the trans-and the cis-signaling pathways. The cispathway involves gp130 and mIL-6R (membrane-bound IL-6R) mainly restricted to immune cells and results in a pleiotropic effect on the cells of the adaptative and innate immunity.

Trans signaling only involves the soluble form of IL-6R (sIL-6R) forming a complex with gp130 which is virtually express on all hematopoietic and non-hematopoietic cell types.

Therefore, it activates cells that do not express mIL-6R like endothelial cells, resulting in a cytokinic storm [20] .

Overall, the day 7 to day 12 clinical worsening is a typical feature of the course of COVID-19

and involves a major release of cytokines and particularly IL-6 and TNF-α with a key role of myeloid cells. This cytokine release syndrome highly correlates with severity and mortality.

Additionally, A key feature of COVD-19 course, is lymphopenia -a hallmark of severe forms -, suggesting that most of the cytokines released during the cytokinic storm are produced by myeloid cells or non-hematopoietic cells such as adipocytes.

Visceral adipose tissue is more developed in men, as it represents 10 to 20% of total fat as compared to 5-10% in women [21] . Visceral fat possesses multiple properties that make it a key site for intestinal and systemic immune response. In comparison with subcutaneous adipose tissue, visceral adipose tissue including mesenteric fat has a higher vascularization and contains greater number of immune cells. It also expresses higher level of pro-inflammatory cytokines such as IL-6, TNF-α and IL-8 [21] . Visceral adipose tissue is enriched in macrophages that are skewed toward an M2 phenotype (the pro-inflammatory subtype) [22] . Importantly, in case of visceral obesity, the pro-inflammatory state of the visceral fat is further enhanced [21] .

Macrophages of the mesentery fat from patients with nonalcoholic steatohepatitis (NASH) present elevated levels of cytokines and chemokines compared with healthy subjects suggesting, that mesenteric inflammation may uncouples and mediates inflammation through the portal flow [23] .

In addition to the immune response provided by mesentery fat-associated macrophages, adipocytes and stromal cells may also play their own role as immune enhancers [24] . For example, adipokine produced by adipocytes, can regulate metabolic homeostasis and affect immune functions [25] .

In Crohn's disease, it has been shown that mesenteric fat could be a major source of C-reactive protein as well as IL-6. It is also enriched in microbial particles of the microbiota as an evidence of enhanced intestinal microbial translocation [26] . Remarkably, antiTNF-α and antiIL-6 therapies have been shown to reduce intestinal inflammation during Crohn's disease [27, 28] .

Therefore, the visceral adipose tissue is a site with an intense immune activity that is able to produce high level of innate immunity-associated cytokines such as IL-6 and TNF-α. It haspro-inflammatory properties compared with subcutaneous adipose tissue. A systemic proinflammatory potential is further enhanced in case of android obesity. Whether mesenteric fat can be responsible for an intense cytokine production raises the question of the role of the gut in this mesenteric activation. [29] .

It has been demonstrated that SARS-CoV-2 is capable of binding to the angiotensin-converting enzyme 2 (ACE2) which is express in many cell types including the epithelial cells of the respiratory and the gastrointestinal tract [31, 32] .

An Austrian team reported that patient with COVD-19-associated diarrhea had an elevated fecal calprotectin with a mean of 123 µg/g in patients with active diarrhea. The fecal calprotectin could reach values above 200 µg/g in some patients without any preexisting digestive condition [33] . Interestingly, the level of fecal calprotectin did not correlate with the level of viral RNA in the stool but positively correlated with the level of IL-6 in plasma [33] .

CoV-2. Moreover, they showed that the virus can replicate in intestinal epithelial cell lines as well as in human colon organoid models, therefore participating in the spread and enhanced viremia of SARS-CoV-2 [34, 35] .

Overall, SARS-CoV-2 has an intestinal tropism and may induce acute intestinal inflammation which associates with systemic inflammation rather than with intestinal viral load.

The ultimate consequence is an increase translocation of the intestinal luminal content (including bacteria of the microbiota) to the mesentery fat and the systemic blood circulation.

Therefore, the ability of SARS-CoV2 to directly bind and invade the intestinal epithelium and to induce intestinal mucosa inflammation could lead to enhance mesentery antigenic stimulation.

This inflammation of the intestinal tract and subsequently of the mesentery fat could be aggravated by other COVID-19-associated features such as blood coagulopathy and microthrombotic disorders [36] .

Another key digestive characteristic of COVD-19 is the high prevalence of hepatic cytolysis that is associated with severe forms of the disease [37] . While the underlying mechanism is not well understood, multiple factors are likely involved. Increased intestinal translocation to the portal circulation may be one of the participating factors of liver damage.

Overall, the gastrointestinal tract is a key organ with probable direct viral invasion evidenced by fecal shedding of SARS-CoV-2 RNA and prevalent digestive symptoms during COVID-19

course, which are associated with a pejorative outcome

Patients with systemic inflammatory response syndrome (SIRS) have increase intestinal permeability even if they don't have any gastrointestinal condition [38] . Therefore, there is an increase bacterial translocation of either live bacteria, or MAMPS (microbial-associated molecular patterns) that will trigger the innate immunity through toll-like receptors. Increased intestinal permeability was found in ICU patients by measuring urinary excretion of orally administered lactulose or mannitol [39] . Interestingly, increased intestinal permeability was associated with clinical worsening including multiorgan failure in critically ill patients.

Portal venous and the lymph flows are the two main routes that could explain how this modified intestinal permeability as well as the bacterial translocation impact other organs such as the lungs. In a study published in 1991, Moore and al. performed a venous catheterism in the portal vein of critically ill patients to do sequential blood cultures. While 60% of patients were in shock and 30% developed multiorgan failure, only 2% of portal venous blood cultures were positive suggesting that the portal circulation is not the main way for intestinal bacterial translocation [40] . An alternative route of translocation could be the intestinal lymphatics.

Intestinal lymph is drained from the mesenteric lymphatics to the thoracic duct, subsequently merging with the blood circulation in the left subclavian vein. Therefore, products drained by the intestinal lymph are first in contact with the pulmonary arterial vasculature.

In that regard, it is relevant to note that critically ill patients often develop ARDS even without any primary respiratory condition or infections such as patients with severe pancreatitis or trauma developing ARDS. Interestingly, experiments performed in a hemorrhagic shock model in rats showed that lymph collected in the mesenteric lymphatics but not blood from the portal vein could have negative effect in vitro on the endothelium permeability as well as on neutrophils activation and several other relevant cell types. Reinjection of lymph but not portal blood from shocked rat to other rats could worsen lung failure in vivo. Strikingly, the ligation of the major intestinal lymph duct could prevent early lung damage [41] [42] [43] .

Therefore, multiple experimental in vivo and ex vivo data both in human and animal models support a gut lymph-lung axis. SARS-CoV-2 that has a direct effect on the intestine and that triggers a SIRS may then critically increase intestinal permeability leading to the recirculation of pro-inflammatory mediators to the circulation and primarily to the lungs.

Additionally, extensively reported SARS-CoV-2-associated vascular injury and coagulopathy [10, 36] may lead to ischemia of the intestinal epithelium, further favoring microbial translocation.

Male sex, advanced age and metabolic syndrome are factors strongly associated with the severity of COVID-19. They are associated with a high mortality rate. The daily expanding literature shows overwhelming data on the secondary worsening associated with high systemic inflammation including TNF-α and IL-6 as two of the key cytokines. Moreover, it is more and more evident that the intestine is targeted by the SARS-CoV-2 and that the intestinal phase may be critical in severe forms of COVID-19.

Therefore, we propose a pathophysiogeny model of severe forms (figure 1) where the visceral fat, which is already in a pro-inflammatory state in patients with dysmetabolic syndrome, acts as an enhancer of inflammation, being an uncoupling link between the intestine and systemic and pulmonary inflammation through the mesenteric lymphatics. A better understanding of the 

Clinical Characteristics of Coronavirus Disease 2019 in China

Clinical Characteristics of 138 Hospitalized Patients With 2019 Novel Coronavirus-Infected Pneumonia in Wuhan, China

Clinical characteristics of 113 deceased patients with coronavirus disease 2019: retrospective study

Characteristics of and Important Lessons From the Coronavirus Disease 2019 (COVID-19) Outbreak in China

High prevalence of obesity in severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) requiring invasive mechanical ventilation

Obesity in patients younger than 60 years is a risk factor for Covid-19 hospital admission

Baseline Characteristics and Outcomes of 1591 Patients Infected With SARS-CoV-2 Admitted to ICUs of the Lombardy Region

Mesenteric Fat Thickness Is an Independent Determinant of Metabolic Syndrome and Identifies Subjects With Increased Carotid Intima-Media Thickness

COVID-19 pneumonia: different respiratory treatments for different phenotypes?

Time to consider histologic pattern of lung injury to treat critically ill patients with COVID-19 infection

Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus-Infected Pneumonia

Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a single-centered, retrospective, observational study

Impaired type I interferon activity and exacerbated inflammatory responses in severe Covid-19 patients

Pathological inflammation in patients with COVID-19: a key role for monocytes and macrophages

Pathogenic T cells and inflammatory monocytes incite inflammatory storm in severe COVID-19 patients

Coronavirus infections and immune responses

Risk factors for severity and mortality in adult COVID-19 inpatients in Wuhan

Tocilizumab for the treatment of severe COVID-19 pneumonia with hyperinflammatory syndrome and acute respiratory failure: A single center study of 100 patients in

Tocilizumab therapy reduced intensive care unit admissions and/or mortality in COVID-19 patients

Cytokine release syndrome in severe COVID-19

Adipose-Tissue and Intestinal Inflammation -Visceral Obesity and Creeping Fat

Adipose tissue macrophages

Association of Adipose Tissue Inflammation With Histologic Severity of Nonalcoholic Fatty Liver Disease

Macrophage-secreted factors induce adipocyte inflammation and insulin resistance

Adipokines and Their Role in Intestinal Inflammation

Mesenteric fat as a source of C reactive protein and as a target for bacterial translocation in Crohn's disease

Infliximab for the Treatment of Fistulas in Patients with Crohn's Disease

Randomised trial and open-label extension study of an anti-interleukin-6 antibody in Crohn's disease (ANDANTE I and II)

Gastrointestinal Manifestations of SARS-CoV-2 Infection and Virus Load in Fecal Samples from the Hong Kong Cohort and Systematic Review and Meta-analysis

Real-time tracking of self-reported symptoms to predict potential COVID-19

Diarrhoea may be underestimated: a missing link in 2019 novel coronavirus

Receptor Recognition by the Novel Coronavirus from Wuhan: an Analysis Based on Decade-Long Structural Studies of SARS Coronavirus

Faecal calprotectin indicates intestinal inflammation in COVID-19

Critical role of type III interferon in controlling SARS-CoV-2 infection, replication and spread in primary human intestinal epithelial cells

SARS-CoV-2 productively infects human gut enterocytes

Coagulopathy and Antiphospholipid Antibodies in Patients with Covid-19

Liver injury in COVID-19: management and challenges

Gut-origin sepsis in the critically ill patient: pathophysiology and treatment

Increased intestinal permeability is associated with the development of multiple organ dysfunction syndrome in critically ill ICU patients

Gut bacterial translocation via the portal vein: A clinical perspective with major torso trauma

Gut-derived mesenteric lymph but not portal blood increases endothelial cell permeability and promotes lung injury after hemorrhagic shock

Post-hemorrhagic shock mesenteric lymph is cytotoxic to endothelial cells and activates neutrophils

Role of the gut in the development of injury-and shock induced SIRS and MODS: The gut-lymph hypothesis, a review

FIGURE LEGENDS

way that the mesentery fat aggravates the course of COVID-19 may open new and innovative therapeutic avenues.

The authors have no conflict of interest related to this work to declare.
