An effective treatment for Covid-19 is still lacking, although anecdotal
evidence suggests that IL-6 blockade is beneficial (Mehta et al .,
2020), and blockers of other pro-inflammatory cytokines such as IL-1β
are being considered in Covid-19 patients showing hyperinflammation
(Monteagudo et al ., 2020). In such a dramatic situation, it might
be necessary to explore unconventional, but rationale, therapeutic
options. There is evidence that severely ill Covid-19 patients develop a
cytokine storm syndrome causing acute respiratory distress syndrome
(ARDS), the main cause of morbility and mortality (Mehta et al2020). This is thought to be due to uncontrolled, virus-mediated, direct
activation of lung macrophages in a process reminiscent of the
macrophage activation syndrome (MAS) observed in some rheumatologic
diseases such as systemic juvenile idiopathic arthritis (sJIA), adult
onset Still’s disease and systemic lupus erithematosus (LES), although
it is debated wheter hyperinflammation in Covid-19 should be considered
MAS sensu strictu (Crayne et al , 2019; Henderson et al .,
2020). Covid-19 patients show massive infiltration by inflammatory cells
(neutrophils, monocyte/macrophages) in their lungs, and increased blood
levels of IL-1β, IL-6, IL-2, IL-7, TNF-α and several other
pro-inflammatory cytokines and chemokines (Guo et al ., 2019).
While main target of coronaviruses are lung epithelial cells, they also
infect macrophages and dendritic cells, where they cause abortive
infection and sustained activation, that in turn drives
hyperinflammation (Fehr and Perlman, 2015). Thus, macrophage inhibition
is likely to be a crucial step to prevent the extensive lung injury
caused by Sars Cov-2. A promising target receptor to down-modulate
macrophage responses might be the P2X7 receptor (P2X7R).
The P2X7R is a plasma membrane receptor gated by extracellular ATP, the
earliest and most ubiquitous damage-associated molecular pattern (DAMP)
released at all inflammatory sites, the lungs included (Tolle and
Standiford, 2013; Di Virgilio et al ., 2020). Very interestingly,
increased ATP levels are also found in bronchoalveolar lavage fluid
(BALF) from patients with ARDS, or from mice with lipopolysaccharide
(LPS)-induced acute lung injury (ALI) (Cicko et al , 2018).
Increased extracellular ATP levels have also been direclty demonstratedin vivo in the lungs of mice inhaled with LPS thanks to the
bioluminescent luciferase probe pmeLUC (Cicko et al . 2018). The
P2X7R has a central role in inflammation as it is one of the most potent
stimulator of the NLRP3 inflammasome, and therefore of caspase-1
activation and IL-1β and IL-18 release (Di Virgilio et al ., 2017)
(Figure 1). P2X7R stimulation also promotes release of other cytokines
and chemokines, e.g. IL-6, TNF-α, monocyte chemotactic protein-1
(MCP-1), IL-8, CC-chemokine ligand 3 (CCL3) and CXCL2, of pro-fibrotic
factors such as TGF-β, and extracellular matrix remodelling factors such
as metalloproteinase-9 and tissue inhibitor of metalloproteinase
(TIMP)-1 (Di Virgilio et al ., 2017; Riteau et al , 2010).
The P2X7R is a potent trigger of reactive oxygen species (ROS)
production, therefore its overactivation (as it is the case at sites of
heavy inflammation) may inhibit lymphocyte functions by impairing
mitochondrial metabolism, as it has been shown in monocytes during
sepsis (Martinez-Garcia et al 2019). Dysfunctional mitochondria
are a feature of exhausted T lymphocytes (Desdín-Micó et al .,
2018). Although no specific data are available in Covid-19, reduced T
cell function, as documented by the increased expression of T lymphocyte
exhaustion markers, is a feature of coronavirus-induced lung infection
(Diao et al ., 2019). It is known that inability of cytotoxic T
lymphocyte and NK cells to lyse virus-infected cells occurs in MAS, and
a is main cause of the sustained cytokine release driving the cytokine
storm typical of this syndrome and of Covid-19 as well (Crayne et
al ., 2019). In addition, a key feature of ARDS is the extensive
pulmonary edema which is largely dependent on release of vascular
endothelial growth factor (VEGF). Although the pathogenic role of VEGF
in ARDS is controversial, blood VEGF levels are reported to be increased
in Covid-19 patients, and this has prompted a clinical study aimed at
testing the efficacy of bevacizumab administration (NCT04275414)
(https://clinicaltrials.gov/ct2/results?cond=&term=NCT04275414) .
While mostly known for its pro-inflammatory activity, it is also well
documented that the P2X7R is a potent inducer of VEGF release and
neo-angiogenesis in vivo , and accordingly it has been shown that
its blockade inhibits VEGF-dependent increase in vascular permeability
(Clapp et al ., 2019). Thus P2X7R targeting might prove beneficial
to fight the early exudative phase in ARDS. Furthemore, thromboembolic
complications are common among critically ill Covid-19 patients (Tang et
al. 2020a), which has led to explore the protective effect of heparine
administration Tang et al ., 2020b). Inhibition of the P2X7R might
be beneficial also to treat thromboembolism since its stimulation
promotes massive release of tissue factor (Baroni et al , 2007;
Furlan-Freguia et al ., 2011).
A commonly used experimental model for ARDS, is the intratracheal
application of LPS in mice. This causes ALI, an acute lung inflammation
reminiscent of human ARDS (Cicko et al ., 2018). Early work showed
that pharmacological blockade of the P2X7R, or its genetic ablation,
substantially reduced inflammatory cell infiltration, cytokine levels
and lung damage in acute inflammatory response (Wang et al .,
2015; Cicko et al ., 2018). P2X7R deficiency was also shown to
reduce alveolar macrophage death and pro-IL-1α release in the lungs of
LPS-treated mice (Dagvadori et al ., 2015). Monitoring the P2X7R
as an inflammatory biomarker in Covid-19 by recently developed
radiopharmaceuticals has been recently proposed (Juengling et
al. , 2020)
On the basis of these convergent observations, we suggest that the P2X7R
could be an ideal candidate receptor for pharmacological targeting in
ARDS.
Small molecule drugs targeting the P2X7R have been developed by most
Pharma Industries and have undergone extensive Phase I Clinical Trials
revealing an excellent safety profile (Arulkumaran et al ., 2011).
However, therapeutic efficacy in a number of chronic inflammatory
diseases investigated in Phase II turned out to be limited, which led
major Pharma Industries to drop or outsource P2X7R clinical research. A
relevant exception is Johnson & Johnson that in late 2019 started a new
Phase II clinical trial to test the efficacy of P2X7R blockade in
depression (Cully, 2020). Therapeutic efficacy of P2X7R blockade has
never been tested in a disease condition characterized by uncontrolled
hyperinflammation, as in Covid-19-dependent ARDS. We propose that P2X7R
antagonists should be given a chance at least for the compassionate
treatment of Covid-19 patients with rapid evolutive ARDS.