Apelin; ACE2 and Biomarkers of Alveolar-capillary Permeability in SARS-cov-2 (COVID-19).

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.