Cell Therapy Using Umbilical Cord-derived Mesenchymal Stromal Cells in SARS-CoV-2-related ARDS

General context:

As of March 13, 2020, more than 145,000 cases of 2019-nCoV infection have been confirmed with
5,500 deaths worldwide. As of March 21, 14,469 cases have been confirmed in France, of which
562 have been fatal while 1,525 patients are currently hospitalized in intensive care units.
Whereas the pandemic continues to spread, the severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2) causes Severe Acute Respiratory Distress Syndrome in 30% of patients (Murthy et
al., 2020) with a 30%-60% mortality rate. The main physio-pathological hallmark is an acute
pulmonary inflammation. Currently, there is no treatment.

The objective of this project is to treat intubated-ventilated patients presenting with a
SARS-CoV2-related Acute Respiratory Distress Syndrome (ARDS) of less than 96 hours by three
intravenous infusions of umbilical cord Wharton's jelly-derived mesenchymal stromal cells
(UC-MSC) one every other day (duration of the treatment: one week). The primary endpoint is
the PaO2/FiO2 variation from baseline at day 7. The evolution of several inflammatory
markers, T regulatory lymphocytes and donor-specific antibodies will also be monitored. The
trial will include 40 patients, of whom 20 will be randomized to cell-treatment administered
via intravenous route while the remaining 20 patients will be randomized to receive a placebo
solution in addition to the standard of care. Patients will be followed up to 12 months after
treatment.

State of the art:

Mesenchymal stem cells feature several attractive characteristics: ease of procurement, high
proliferation potential, capacity to home to inflammatory sites, anti-inflammatory,
anti-fibrotic and immunomodulatory properties. Their therapeutic benefits have been
demonstrated in > 100 animal models, including sheep. Specifically, the therapeutic effects
of MSC have been demonstrated in ARDS models induced by H1N1, H5N1, H9N2 influenza
virus-associated pneumoniae and were also shown to reduce bacterial-induced acute lung injury
in a human model of ex vivo perfused lung (Lee et al., 2013). In the clinics, MSC have
demonstrated an excellent tolerance in over 3,000 patients (Thompson et al., 2020),
regardless of the dosing and delivery route. Three phase I/II trials have included patients
with an ARDS and in one (START II), MSC significantly reduced pulmonary endothelial injury
(Matthay et al., 2019). If all MSC share several characteristics regardless of the tissue
source, the highest productions of bioactive molecules and the strongest immunomodulatory
properties are yielded by those from the Wharton's jelly of the umbilical cord (Romanov et
al., 2019) which can be scaled-up to generate banks of cryofrozen and thus readily available
products.

So far, UC-MSC have been used in a wide variety of diseases (reviewed in Scarfe et al., 2018)
and, in most cases, they have been delivered via the intravenous route which is clinically
attractive because of its non invasive nature and the subsequent possibility of repeated
administrations. Labeling techniques have shown that >80% of intravenously injected MSCs are
rapidly trapped in the lungs, followed by a rapid distribution of some of the injected MSCs
to other tissues including liver, spleen, and inflammatory or injured sites (Brooks et al.,
2018). Over all, these biodistribution patterns have been confirmed by human studies using
magnetic resonance imaging (MRI), positron emission tomography (PET) and/or single-photon
emission computed tomography (SPECT).2 A four-parameter model (injection rate, clearance
rate, rate of extravasation and rate of intravasation) predicts that transplanted MSCs are
only therapeutically active for a short period of time (probably less than 24 h) (Parekkadan
and Milwid, 2010), a timescale consistent with that of the biological responses that they
trigger. These assumptions are an incentive to repeated MSC administrations within a short
period of time to induce a sustained therapeutic effect and has rationalized our protocol of
injecting MSC one every other day over a one week-period, a design consistent with the
earlier report of the benefits of delivering MSC at relatively small doses but in a repeated
fashion in patients with graft-versus-host disease (Zhou et al., 2010).

Once they have homed in the lungs, MSC have been reported to first induce an inflammatory
response which is detectable at the tissue level and systemically (Hoogduijn et al., 2013)
and is likely due to their interaction with resident lung cells once they have accumulated in
the microvasculature. This initial response is then followed by a downstream phase of reduced
immune reactivity (Hoogduijn et al., 2013), the mechanisms of which have been extensively
investigated. Thus, 24 hours after their intravenous infusion, most of the UC-MSC that have
accumulated in the lungs are dead after their phagocytosis by monocytes and neutrophils which
then migrate through the blood stream, particularly in the liver (Leibacher et al., 2017; de
Witte et al., 2018). Co-culture experiments have shown that the internalization of MSC
fragments by monocytes triggers a phenotypic shift which translates into the upregulation of
PD-L1 and CD90 along with an increased expression of mRNA levels for IL-1β, IL-6, IL-8 and
IL-10 and a decreased expression of TNF-α. Of note, monocytes polarized towards an
immune-regulatory phenotype increase the expression of Foxp3+ T regulatory lymphocytes while
decreasing that of activated CD4+ cells (de Witte et al., 2018). That apoptosis of
intravenously infused MSC is a requirement for their immunosuppressive function is further
supported by the observation that the cytotoxic activity against MSC is predictive of
clinical responses in patients treated by MSC for graft-versus-host disease, i.e., the best
responders are those with high cytotoxicity; in this study, the postulated mechanistic link
is that phagocytes that have engulfed apoptotic MSC then produce indoleamine 2,3-dioxygenase
(IDO) and thus ultimately deliver MSC immunosuppressive activity (Galleu et al., 2017).

Given the pathophysiology of SARS-CoV2, it is thus sound to hypothesize that the intravenous
administration of UC-MSC during the initial phase of ARDS could control inflammation,
accelerate its recovery with improved oxygenation, reduced mechanical ventilation and
ventilation weaning time and therefore reduced length of stay in intensive care. This
assumption is indeed supported by the recent results of a preliminary Chinese trial in which
MSC (the source of which has not be specified) have been reported to improve pulmonary
function and symptoms in 7 patients with COVID-19 pneumonia along with a rapid clearance of
overactivated cytokine-secreting immune cells (CXCR3+CD4+ T cells, CXCR3+CD8+ T cells, and
CXCR3+ NK cells), a decrease in TNF-α circulating levels and an increase in
CD14+CD11c+CD11bmid regulatory DC cells and IL-10 levels, compared with a placebo group (Leng
et al., 2020).