COVID-19 affects patients by it's entry way: the airways and the deep lung. 20 to 30% of symptomatic adult patients which are hospitalised in or out of the intensive care unit exhibit respiratory distress with major oxygenation index alterations. That syndrome is specifically designated severe acute respiratory syndrome coronavirus 2 (SARS-coV-2). SARS-coV-2 develops 2 weeks after the symptoms start, with a recurring fever, progressing dyspnea and a mortality rate of 10 to 50%. In SARS-coV-2: 1) radiological pulmonary infiltrates are more peripheral and 2) dominancy of hyperinflammation (cytokine storm, of which the interleukine-6 IL-6) and hyperpermeability of the alveolar-capillary barrier is observed, 3) in comparison to a traditional (non COVID) ARDS, the hypoxia generated is exceptionally severe and associated to a pulmonary compliance sparsely reduce in more than 70% of cases.

The Angiotensin I Converting Enzyme 2 (ACE2) is a transmembrane carboxypeptidase of the renin-angiotensin system (ras) implicated in cardiovascular homeostasis. ACE2 counter-regulates the proinflammatory hypertensive ACE1/Angiotensin II (Ang II). ACE2 converts Ang II in Ang1-7 to promote vasodilation, anti-inflammation and tissular protection. ACE2 is expressed abundantly in the lung and is the entry door for many virus like the influenza A, coV-1 and -2. A downregulation of ACE2 activity leads to Ang II excess, with a stimulation of the AngI receptor (AT1R) and an increase of pulmonary vascular permeability, negatively impacting the prognosis of the influenza virus H7N9. More than 85% of pulmonary ACE2 is expressed in apical membrane of alveolar epithelial cell type II (AECII) which are located in the lung distal air spaces at the alveolar-capillary barrier interface. This is where the coV-2 set up and induces the SARS (COVID-19+). AECII regulate the alveolar-capillary barrier permeability. Those produce specific proteins (SP-D or surfactant protein D, and CC-16 or Clara Cell protein) which are released in bloodstream when the alveolar-capillary barrier becomes hyperpermeable and are diagnostic and prognostic biological markers.

The apelin/APJ system could be a protective way by interacting with the renin-angiotensin system (ras) and Angiotensin I Converting Enzyme 2 (ACE2). The apelin/APJ system is recognized to protect and optimize cardiovascular functions. The apelins and receptor APJ operate independently of the catecholaminergic system and constitute a counter regulatory response to the vasopressinergic way with an inodilator activity. The apelin/APJ is largely expressed in the lung and is involved in the reduction of pulmonary inflammation. Apelin-13 stabilizes the mitochondrial function, reduces membrane permeability, prevents apoptosis and stimulates AECII proliferation. The apelin/APJ is reactive to hypoxia like in SARS or ARDS with an increase of blood apelins levels. Apelins are substrates for the ras system and kallikrein/kinin which are producing degradation enzymes like ACE2. Despite this, apelins can reverse a decreased activity of ACE2, and regulate the overproduction of Ang II and the AT1R stimulation which lead to an increase pulmonary vascular permeability and lung edema. APJ is also able to inhibit AT1R by trans-allosteric combination. APJ is a co-receptor for the human and simian immunodeficiency virus, and the apelins block their entry. The proximity between ACE2 and APJ on AECII membranes and their internalization/degradation management in term of the apelins isoform lead to the query of their interaction and the link with their pulmonary protective activity.

Hypothesis: The apelin/APJ system is involved in the protection of the lung affected by the COVID-19 by interacting with the SARS-coV-2 entry door: the Angiotensin I Converting Enzyme 2 (ACE2) and the renin-angiotensin system (ras). Elevated systemic levels of apelins and ACE2 activity are associated to less critical forms of COVID-19 and characterized by less pulmonary hyperpermeability and inflammation.

Goals: Main: In COVID-19+ patients, to establish the basic knowledge of 1) apelins and related systems (ras and degradation enzymes, of which ACE2) pheno-dynamic profile in bloodstream, 2) pulmonary hyperpermeability profile by biomarker's assessment i) comparison of SARS vs. lesser COVID-19 respiratory injury, and with non COVID-19 ARDS and non ARDS acute respiratory condition. Secondary: To set up links between basic and progressive clinical data (data collection system APEL-COVID).

Methods: Observational pilot study of a prospective cohort recruiting in the 36 hours after admission adult patients hospitalized for a symptomatic acute respiratory illness. Groups: 1) COVID+, SARS, MV+ (mechanical ventilation) or not, with more than 6L/min-40% FiO2 for a SpO2 more than 90% for more than 24hrs (n=30); 2) COVID+, non SARS, MV-, with less than 6L/min-40% FiO2 for a SpO2 more than 90% for more than 24hrs (n=30); 3) COVID-, ARDS, MV+, with more than 6L/min-40% FiO2 for a SpO2 more than 90% for more than 24hrs (n=30); 4) COVID-, non ARDS, MV-, with less than 6L/min-40% FiO2 for a SpO2 more than 90% for more than 24hrs (n=30). Given the effects of the pressure generated by mechanical ventilation on epithelial biological markers, 2 sub-groups of 10 controls patients hospitalized for non-respiratory reasons will be constituted post hoc with sex-age matching to the above groups. The patients COVID+ non SARS will be respiratory symptomatic, with or without lung infiltrates and needs in O2 less than 6L/min-40% FiO2 for a SpO2 more than 90% and hospitalized on floors of the pulmonology or internal medicine or in the intensives cares units. The ARDS patients will be selected in the direct form (e.g. pneumonia, aspiration) and the oxygenation index parameters established by Berlin definition.