Introduction
Coronavirus disease 2019 (COVID-19), first reported at the end of 2019 in Wuhan, China,
rapidly spread around the globe and was declared a pandemic by the WHO in March 2020
[1]. As of October 2020, more than 30 000 000 cases and 950 000 deaths were reported
worldwide. COVID-19 is caused by the severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2). SARS-CoV-2 shows close relation to SARS-CoV, the underlying viral
cause of the 2002 SARS epidemic, as well as to MERS-CoV, which causes sporadic outbreaks
centered in the Middle East [2]. COVID-19 patients develop an acute respiratory illness, which may cause mild fevers
and coughs but may also lead to acute respiratory distress syndrome (ARDS), hypoxic
respiratory failure, and ultimately death [3]. Immunologically, critical cases of COVID-19 are signified by an excessive inflammatory
response, manifested by strong leukocyte influx into the lungs and increased plasma
levels of inflammatory markers [4]. Additionally, coagulopathies and increased risk of blood clot formation have been
reported as significant contributors of COVID-19 related mortality [5]. The excessive inflammatory response observed in severe and critical COVID-19 patients
raised the possibility of the use of anti-inflammatory drugs to combat the excessive
and damaging immune response. While medical professionals were apprehensive about
the use of immunosuppressants to treat an infectious disease in the early stages of
the pandemic, over time, evidence accrued that glucocorticoids lead to improved patient
outcomes [6].
Glucocorticoids and the glucocorticoid receptor (GR)
Glucocorticoids are stress hormones synthesized in the adrenal cortex under the control
of the hypothalamic-pituitary-adrenal (HPA) axis [7]
[8]. Under normal conditions glucocorticoids are released in a circadian and ultradian
rhythm [7]
[8]. Stress stimuli activate the HPA axis: in particular pro-inflammatory cytokines,
such as IL-6, TNF, and IL-1β, act on the hypothalamus triggering HPA axis activation
and leading to adrenal glucocorticoid production [7]
[8]
[9]. In turn, circulating glucocorticoids exert strong immunomodulatory effects and
constrain inflammation, while also limiting their own production through a negative
feedback on the HPA axis [7]
[8]
[10]. Following their release into the bloodstream glucocorticoid tissue availability
and function are determined by several regulatory mechanisms. Corticosteroid-binding
globulin (CBG) binds and keeps approximately 90% of cortisol in an inactive state.
Neutrophil elastase cleaves CBG, thereby releasing bioactive glucocorticoids at sites
of inflammation [7]
[8]. Moreover, 11β-hydroxysteroid dehydrogenases 1 and 2 (11β-HSD1, 11β-HSD2) regulate
the interconversion of bioactive cortisol to inactive cortisone in target tissues
[7]. Inflammatory signals can regulate the expression of 11β-HSD1 and 2, which determines
local glucocorticoid activity [8].
Glucocorticoids are lipophilic molecules diffusing through the cell membrane and binding
to the cytosolic glucocorticoid receptor (GR), which is abundantly expressed in most
cell types [7]. The GR contains an N-terminal domain, which interacts with co-factors and transcription
factors, a zinc-finger motif-containing DNA-binding domain and a ligand-binding domain
[7]
[11]. The GR is subjected to post-translational modifications, including phosphorylation,
acetylation, ubiquitylation, and sumoylation at multiple sites, which regulate its
compartmentalization, function and degradation [11]. While GRα is the ligand-binding GR isoform, its splice isoform GRβ does not bind
glucocorticoids and exerts dominant-negative effects on GRα. In its inactive state
GR forms complexes with heat-shock proteins, chaperones, and immunophilins. Upon ligation
the receptor undergoes conformational changes, partially disassembles from its interactors,
translocates to the nucleus, and controls gene transcription as a monomer or dimer
[7]. Upon its translocation to the nucleus, GR can directly bind to glucocorticoid response
elements (GRE) to induce or inhibit gene expression [7]. High glucocorticoid doses favor binding of GR homodimers rather than monomers on
palindromic GRE [12]. In addition, GR can tether transcription factors through protein-protein interactions
without the requirement of DNA binding [13]. Through the latter mechanism GR can block the function of transcription factors
mediating inflammatory responses, such as nuclear factor-κB (NF-κB) and activator
protein 1 (AP-1) [8]. However, transrepression can also be mediated by competition for binding to the
DNA or for co-factors [7]. Moreover, GR directly interacts with NF-κB, hindering its translocation to the
nucleus and induces the expression of the NF-κB inhibitor glucocorticoid-induced leucine
zipper protein (GILZ) [14]
[15].
Additionally to the GR-mediated genomic effects, glucocorticoids may also exert non-genomic
effects, leading to rapid biological events [7]
[16]. This mechanism of action has been shown to contribute to the effects of glucocorticoids
on T-cell receptor (TCR) signaling, neutrophil degranulation, macrophage inflammatory
activation, mast cell-mediated histamine release and ion channel function in bronchial
epithelial cells [16]
[17]
[18].
Anti-inflammatory effects of glucocorticoids
Glucocorticoids regulate inflammation through pleiotropic mechanisms [8]
[19]. Their effects on immune responses depend on the cell type, disease, dose and timing
of application, that is, whether application precedes or follows exposure to the inflammatory
stimulus [20]
[21]
[22]. While exposure to low doses of glucocorticoids prior to a noxious stimulus can
promote the inflammatory response, glucocorticoids in high doses applied after the
inflammatory stimulus act in an anti-inflammatory fashion [8]
[19]
[20]
[21]
[22]. In the absence of inflammation, glucocorticoids promote the expression of pattern
recognition receptors (PRR), such as Toll-like receptor (TLR), thereby maintaining
the cells sensitivity to noxious stimuli, such as pathogen- and damage-associated
molecular patterns (PAMP, DAMP) [8]
[19]. In the context of an acute inflammatory response glucocorticoids taper inflammation
through several mechanisms. Glucocorticoids inhibit PRR signaling by tethering NF-κB
and AP-1 and induce the expression of negative regulators of TLR signaling, such as
dual-specificity protein phosphatase 1 (DUSP1) and IL-1 Receptor-Associated Kinase
M (IRAK-M) [8]
[23]. In macrophages glucocorticoids down-regulate the expression of pro-inflammatory
cytokines, such as IL-1β, IL-6, IL-12, IL-17, TNF, and granulocyte–macrophage colony-stimulating
factor (GMCSF), and inducible nitric oxide synthase (iNOS) and inhibit cyclooxygenase
2 in a GILZ-dependent manner, thereby attenuating release of prostaglandins [8]
[22]
[24]
[25]
[26]
[27]. Moreover, in endothelial cells they inhibit the expression of adhesion molecules,
like E-selectin, intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion
molecule 1 (VCAM1). In immune cells they down-regulate the expression of integrins,
such as lymphocyte function-associated antigen 1 (LFA1) and very late antigen 4 (VLA4),
thereby attenuating leukocyte recruitment [8]
[28]
[29]. Glucocorticoids also shift macrophages towards an anti-inflammatory state, which
secretes more IL-10 and transforming growth factor-β (TGFβ), and promote efferocytosis
and cell debris clearance due to increased expression of scavenger molecules (CD163,
CD206 and tyrosine-protein kinase MER (MERTK)), thereby favoring resolution of inflammation
[8]
[19]
[22]
[30]
[31]
[32]
[33]
[34]
[35]. The reaction to antigens is regulated by glucocorticoids at both, the level of
antigen presentation and T cell activation. Glucocorticoids inhibit maturation, promote
apoptosis and attenuate antigen presentation in dendritic cells [8]
[36]
[37]. Glucocorticoids also regulate thymopoiesis through induction of lymphocyte apoptosis
and attenuate T cell receptor signaling [38]
[39]. Moreover, glucocorticoids increase regulatory T cell (Tregs) frequency through
GILZ and TGFβ-dependent induction of Forkhead box P3 (FoxP3), a transcription factor
determining Treg differentiation [40].
Overall, therapeutic glucocorticoids exert potent anti-inflammatory effects across
both the innate and adaptive immune system. Due to these wide-ranging anti-inflammatory
effects glucocorticoids are a mainstay of immunosuppressive therapies. Synthetic glucocorticoid
compounds, such as dexamethasone and prednisolone, show increased potency compared
to cortisol owing to longer half-life in plasma, improved parenteral absorption and
reduced binding to CBG. However, due to their wide-ranging receptor expression and
the large-scale GR-mediated transcriptional changes – overall approximately 20% of
the genome is responsive to GR – glucocorticoids are not only potent immunosuppressants,
but also exert broad off-target effects [7]
[41]
[42].
Adverse effects of glucocorticoids
Adverse effects related to glucocorticoid treatment are common and numerous, and depend
on the dose and duration of therapy [7]
[42]
[43]. A number of glucocorticoid side effects are a result of glucocorticoid-mediated
inhibition of glucose uptake and other metabolic alterations of basic cellular metabolism,
which typically occur in most cells and tissues [42]
[43]. Common side effects include weight gain and the development of diabetes mellitus,
sarcopenia, and osteoporosis. Also, an increase in the incidence of hypertension,
atherosclerosis, cardiovascular disease, and thromboembolism may be observed. Additionally,
therapeutic doses of glucocorticoids raise susceptibility to infections and can cause
impaired wound healing, psychiatric disturbances, and suppression of the HPA axis
with the risk of secondary adrenal insufficiency [42]
[44].
COVID-19 immunopathogenesis and glucocorticoids
Following priming of the viral spike (S) protein through the serine protease TMPRSS2
[45], SARS-CoV-2 gains access to respiratory epithelial cells through the hosts angiotensin-converting
enzyme 2 (ACE2) protein [4]. Upon cellular entry, the viral RNA genome is released into the host cell and new
virions are produced [46]. The viral capture of the host cell is followed by the initiation of an innate immune
response, which features inflammatory cytokine and chemokine production, coupled to
neutrophil and monocyte infiltration of the respiratory tract and lungs. This inflammatory
response is typically associated with increased capillary leakage and alveolar cell
destruction, damaging respiratory function. Systemic inflammation is reflected by
increased plasma concentrations of inflammatory cytokines, such as IL-1β, IL-6, IL-8,
IL-10, IL-17, IP10, and TNF, in COVID-19 patients, whereas levels of type I and III
interferons remain low [47]
[48]
[49]. Increased circulating neutrophil numbers correlate with poor prognosis in COVID-19
and strong neutrophil infiltration of pulmonary capillaries was reported in postmortem
lung tissues [47]
[50]. The complement system is activated during ARDS and activation of C3 exacerbates
disease in SARS-CoV-associated ARDS [51]
[52]. Thus, the complement system may drive part of the inflammatory response and thrombosis
to COVID-19 [51]
[53]
[54]
[55]. Observational studies suggest that, elevated levels of C-reactive protein (CRP)
and lactate dehydrogenase are predictors of COVID-19 severity [4]
[47]; similar correlations have been established for elevated neutrophil/lymphocyte ratio,
low platelet count and increased numbers of CD14+CD16+ monocytes and Th17 cells [4]
[56]
[57].
Due to the rapidly developing nature of the COVID-19 pandemic studies examining the
molecular effects of glucocorticoids in patients suffering from severe disease are
few and far between. Based on the known and extensive immunosuppressive effects of
glucocorticoids outlined above, corticosteroid treatment likely attenuates the COVID-19
induced inflammatory response – particularly in severely affected patients. Indeed,
administration of glucocorticoids was reported to reduce CRP and IL-6 plasma levels
in COVID-19 patients, without affecting virus clearance [58]
[59]. Future studies identifying glucocorticoid target cells and anti-inflammatory mechanisms
in the context of COVID-19 are critical in order enable the development of targeted
therapies in the future.
The clinical use of glucocorticoids in acute respiratory distress syndrome (ARDS)
and COVID-19
ARDS is primarily signified by hypoxic respiratory failure. In general, ARDS is associated
with increased vascular permeability and pulmonary edema as well as increased systemic
inflammation, frequently leading to multi-organ failure and death. Thus, ARDS-related
mortality is as high as 50% [60].
Treatment of ARDS is primarily focused on a combination of low tidal volume mechanical
ventilation and prone positioning [61]
[62]. Even though inflammation likely plays a key role in the pathogenesis of ARDS, anti-inflammatory
therapeutic strategies using glucocorticoids have yielded mixed results. For example,
two meta-analysis examining the subject of glucocorticoid use in ARDS a decade ago
reached opposite conclusions ranging from advising against their use to recommending
steroid treatment in ARDS [63]
[64]. These inconsistent conclusions arose from original research studies, which included
small numbers of patients, and varied in their definition of ARDS, type of steroid
used, as well as timing and dosing strategies. In order to address this issue efforts
have been made to clarify the effectiveness of glucocorticoids in ARDS through large-scale
multi-centered randomized controlled trials. The recent Dexa-ARDS trial found that
patients receiving high-dose dexamethasone had a lower all-cause mortality than patients
receiving standard care (21% vs. 36% respectively) [64]. A meta-analysis, which included the Dexa-ARDS trial, as well as others, also favored
the use of glucocorticoids in ARDS, although some reservations remained regarding
the reliability of the findings [65]. Additionally, a recent meta-analysis attempted to dissect the issues of timing
and dosing of glucocorticoid use in ARDS and came to the conclusion that early initiation
of glucocorticoid therapy and use of a low to medium dose were associated with lower
mortality [66]. Overall, the issue of glucocorticoid use in ARDS is still not entirely resolved,
however, recent evidence has tipped the balance in favor of glucocorticoid treatment
with a recommendation for early commencement of steroids at a low to medium dose.
Of note, the large-scale LUNG SAFE study of 2377 patients with ARDS found an overall
rate of glucocorticoid use of <20% [60].
With the beginning of the COVID-19 pandemic, medical professionals were at a loss
as to whether glucocorticoids could have a role in treating severely affected patients,
particularly in those suffering from hypoxic respiratory failure and ARDS. In the
absence of studies examining the effects of glucocorticoids in COVID-19 patients specifically,
doctors were apprehensive to utilize a potent immunosuppressive agent to treat an
infectious disease. Evidence originating from the SARS und MERS outbreaks showed a
delayed virus clearance in patients receiving glucocorticoids potentially indicating
an impaired host response to the viral infection [67]. Moreover, the use of steroids in the treatment of influenza has been studied to
some degree, and while the evidence is far from conclusive, the general consensus
does not recommend the use of glucocorticoids in patients suffering from influenza
infections [68]. Adding to the concern was an increased risk of bacterial superinfection, which
typically is associated with the use of medium to high dose glucocorticoids [69]
[70].
Following encouraging case reports and small observational studies using glucocorticoids
in severely affected COVID-19 patients, multiple large-scale RCTs were initiated,
most notably the RECOVERY trial in the United Kingdom [71]. As part of this study 2104 intrahospital COVID-19 patients received 6 mg of dexamethasone
for 10 days (or until discharge from hospital, if sooner), while 4321 patients received
standard care [72]. Overall the mortality rate after 28 days was significantly lower in the Dexamethasone
group (22.9 vs. 25.7%). Interestingly, subgroup analysis revealed that patients requiring
mechanical ventilation benefited the most from the pharmacological intervention with
glucocorticoids. In this group of patients, the mortality after 28 days was reduced
by more than 12.1 percentage points in the dexamethasone group (29.3 vs. 41.4%). Dexamethasone
also conferred a survival benefit in patients requiring oxygen – without the need
for invasive mechanical ventilation – however, the effect remained small (28-day mortality:
23.3% vs. 26.2%). In patients who did not require ventilation support or oxygen Dexamethasone
showed no significant effect. Following the publication of the RECOVERY data as a
preliminary report, treatment guidelines were updated to recommend the use of glucocorticoids
in severely affected COVID-19 patients. This also led to an arrest of patient recruitment
for other ongoing trials examining the effects of glucocorticoids for the treatment
of (severe) COVID-19.
The Rapid Evidence Appraisal for COVID-19 Therapies (REACT) Working Group of the WHO
performed a meta-analysis of all available data regarding the use of glucocorticoids
in COVID-19 [6]. The analysis included the RECOVERY trial as well as 6 other trials – some of which
had to be abrogated before reaching their recruitment goal due to the publication
of the RECOVERY data. The meta-analysis concluded that initiation of systemic glucocorticoids
was associated with lower 28-day all-cause mortality in critically ill patients with
COVID-19. Consequently, based on these and other studies, the WHO issued two recommendations
regarding the treatment of COVID-19: First, systemic glucocorticoid therapy should
be initiated in patients with severe and critical COVID-19, whereby therapy is defined
as 6 mg daily of dexamethasone orally or intravenously or 50 mg of hydrocortisone
intravenously every 8 hours for 7 to 10 days. Secondly, the use of glucocorticoids
in non-severe COVID-19 is not advised (for definition of critical, severe and non-critical
COVID-19, see [Table 1]).
Table 1 Mutually exclusive categories of illness severity.
|
Critical COVID-19
|
Defined by the criteria for acute respiratory distress syndrome (ARDS), sepsis, septic
shock or other conditions that would normally require the provision of life-sustaining
therapies, such as mechanical ventilation (invasive or non-invasive) or vasopressor
therapy.
|
|
Severe COVID-19
|
Defined by any of: oxygen saturation <90% on room air.Respiratory rate >30 breaths
per minute in adults and children > 5 years old; ≥60 in children less than 2 months;
≥50 in children 2–11 months; and ≥40 in children 1–5 years old. Signs of severe respiratory
distress (i. e., accessory muscle use, inability to complete full sentences; and in
children, very severe chest wall indrawing, grunting, central cyanosis, or presence
of any other general danger signs).
|
|
Non-severe COVID-19
|
Defined as absence of any signs of severe or critical COVID-19.
|
Reference: Corticosteroids for COVID-19, WHO, 2. September 2020.
Glucocorticoids, COVID-19 and metabolic comorbidities
Diabetes mellitus is associated with an increased risk of infection and infection-related
death [73]
[74]. Accordingly, diabetic patients were at an increased risk of severe or lethal infection
for both SARS as well as MERS during the respective outbreaks [75]
[76]. When the Sars-Cov-2 spread around the world it quickly emerged that in the country
of origin, China, diabetes was associated with increased mortality due to COVID-19
[77] – a finding later confirmed elsewhere [78]. Amongst patients suffering from diabetes mellitus, COVID-19 related mortality was
not only associated with the presence of cardiovascular complications but also with
glycemic control and BMI [79]
[80]. An elevated BMI as well as obesity are also associated with severe outcomes of
COVID-19 such ICU admission and mechanical ventilation in non-diabetic patients, thus
indicating their role as an independent risk factors for COVID-19 related mortality
[81]
[82]. The association between good glycemic control and survival of COVID-19 amongst
diabetics highlights the importance of adequate control of blood sugar levels during
Sars-CoV-2 infection. As a modifiable risk factor, this association has already led
to expert recommendations for the management of diabetes in patients with COVID-19
[83]. The reasons for increased mortality from COVID-19 in diabetic patients and the
potential contribution of blood sugar levels to infection outcomes remain largely
obscure. Multiple potential molecular mechanisms have been described and include a
diabetes associated (i) increase in Coronavirus load, (ii) dysregulated immune response,
(iii) alveolar dysfunction, (iv) epithelial dysfunction and (v) coagulopathy [84]. However, the relative importance of these molecular phenomena needs to be investigated
further and clinically validated. Interestingly, to our current knowledge, no study
investigated the efficiency of glucocorticoid treatment in diabetic COVID-19 patients
specifically. While the overall benefit of glucocorticoid therapy in severe and critical
COVID-19 infections has been established, the efficiency of glucocorticoids in diabetics
is currently less clear. Thus far – to the best of our knowledge – no subgroup analysis
has examined the effect of glucocorticoids in diabetic patients suffering from COVID-19.
This question is of particular relevance as exposure to supraphysiological levels
of glucocorticoids frequently leads to worse glycemic control [85]
[86]. Additionally, it is well established that medium to high doses of therapeutic glucocorticoids
cause hypertension [87]
[88], weight gain [89] as well as an increase in cardiovascular events [90]
[91], all of which are associated with worse outcomes in COVID-19. Whether the adverse
effects of therapeutic glucocorticoids are of concern during short exposure of 7–10
days (as recommended for COVID-19 treatment) remains to be evaluated.
