As Dr. Thomson eloquently notes in his valuable letter [1],
underlying respiratory diseases appear to be less of a risk factor for
poor outcome in COVID-19 patients than either underlying cardiovascular
disease or diabetes. This intriguing finding emerged from several
studies that examined underlying medical conditions in COVID-19
patients.
In a single-center retrospective analysis of critically ill adults
admitted to the intensive care unit of a hospital from China between
late December 2019 and January 26, 2020, 22% of the non-survivors had
cerebrovascular disease, 22% had diabetes, and 6% had chronic
respiratory disease [2]. The analysis of data from patients with
laboratory-confirmed COVID-19 from hospitals in China through January
29, 2020 found that 16.2% of those with serious disease had diabetes,
23.7% had hypertension, and 3.5% had chronic obstructive pulmonary
disease [3]. A study of electronical medical records of COVID-19
patients admitted between January 16 and February 3, 2020 to a hospital
from Wuhan found that hypertension and diabetes mellitus, the most
common comorbidities, were present in 37.9%, 13.8%, of the patients
with severe disease, respectively, but only in 3.4% of the patients
with chronic obstructive pulmonary disease [4]. Finally, an analysis
of all COVID-19 cases reported through February 11, 2020, extracted from
the Infectious Disease Information System in China, found that case
fatality rates in individuals with cardiovascular disease, chronic
respiratory disease, and diabetes were 10.5%, 6.3%, and 7.3%
respectively, as compared to 0.9% among patients with no comorbidities
[5]. In a case series of COVID-19 patients hospitalized in Wuhan,
China, ICU patients were more likely to have underlying diabetes than
patients that did not receive ICU care (22.2% vs 5.9%) [6].
The studies mentioned above did not stratify patients by therapies they
were receiving. However, one commonality between cardiovascular disease
and diabetes is that they are often treated with angiotensin-converting
enzyme (ACE) inhibitors and angiotensin II type-I receptor blockers
(ARBs), widely used to inhibit the formation and action of angiotensin
II.
ACE shares 42% amino acid identity with ACE2 [7], a membrane-bound
aminopeptidase [8] extensively expressed on type II human alveolar
cells [9]. The genes encoding these two proteins are thought to have
emerged by duplication [10]. ACE2 is distributed on many tissues and
shows highest expression levels in the heart, kidney, lung, small
intestine, and testis [11]. On the apical surface of polarized
respiratory epithelial cells, ACE2 is a crucial and primary receptor for
the cellular entry of SARS-CoV, the virus that caused the 2002-2003 SARS
outbreak [12-16]. SARS-CoV binding to ACE2 mediates entry into human
or animal cells [17]. ACE2 is also the receptor for SARS-CoV-2, the
etiologic agent of COVID-19 [18]. Structural analyses indicate that
SARS-CoV-2 binds the ACE2 receptor with a 10-20-fold higher affinity
than SARS-CoV [19, 20].
The entry of SARS-CoV and SARS-CoV-2 into their target cells is mediated
by the viral spike (S) glycoprotein, which is located on the outer
envelope of the virion [21]. The S glycoprotein has two functional
subunits, S1, which binds the cellular receptor, and S2, which contains
domains required for the fusion between viral and cellular membranes
[22, 23]. Viral binding and membrane fusion represent the initial
and critical steps during the infection cycle of the coronavirus
[24] and the first step in establishing the infection [25, 26].
Binding is followed by internalization of ACE2 and down‐regulation of
its activity on the cell surface [27-29].
SARS-CoV binds ACE2 through a region of the viral S1 subunit called the
minimal receptor-binding domain (RBD) [17]. RBD is the most
important determinant of the SARS-CoV host range, and studies about the
“species jump” during the 2002-2003 SARS outbreak revealed that
changes of only one or two amino acids in this region were sufficient to
make the virus “jump” to a new host [26, 30, 31].
ACE and ACE2 are two members of the renin angiotensin system that
negatively regulate each other [32, 33] and are distinct in their
substrate specificity and function [34]. ACE converts angiotensin I
to angiotensin II and mediates aldosterone release, vasoconstriction,
sodium retention, cell proliferation, and organ hypertrophy [35].
ACE2 cleaves a single residue from angiotensin I to form
angiotensin-(1-9), and a single residue from angiotensin II to form
angiotensin-(1-7). In humans, ACE2 has a 400-fold higher catalytic
efficiency when it uses angiotensin II as a substrate as compared to
when it uses angiotensin I [36]. ACE2 and angiotensin-(1-7), through
the Mas receptors, oppose ACE and mediate vasodilation and
anti-proliferative, anti-hypertrophic, cardioprotective, and
reno-protective effects [8, 35, 37]. ACE2 has physiological and
pathological importance [25] and its dysregulation was implicated in
heart disease, hypertension, and diabetes [36, 38-40]. ACE2 is not
inhibited by ACE inhibitors [32] and several studies indicate that
the ACE2/Angiotensin-(1-7)/Mas axis has anti-inflammatory effects [41,
42].
It was recently hypothesized that treatment with ACE inhibitors and/or
ARBs may lead to ACE2 overexpression and this could increase the risk of
severe COVID-19 [43], possibly by increasing the internalization of
SARS-CoV-2. Several lines of evidence indicate that pharmacological
manipulation of the renin-angiotensin-aldosterone pathway could affect
ACE2 receptor levels. In animal studies, the selective blockade of
angiotensin II synthesis or activity increased cardiac Ace2 gene
expression and activity [44, 45], and treatment with ARBs increased
the levels of cardiovascular ACE2 receptors [46-49]. While this link
is thought-provoking as a possibility, there isn’t currently sufficient
evidence to contemplate changing patients’ existing therapeutic regimens
in order to minimize their risk of COVID-19 complications. The first
clinical evidence exploring this link indicated that the use of ACEI and
ARBs appear to improve the clinical outcome of COVID-19 patients with
hypertension [50]. We will only learn about any possible
associations, along with their magnitude and direction, from carefully
conducted and adequately powered clinical trials.
It is also important to consider that an increase in ACE2 levels does
not necessarily entail a negative impact for the course of COVID-19.
ACE2, by forming angiotensin-(1-7) from angiotensin II, could diminish
the deleterious effects of angiotensin II and, consequently, it is also
possible that ACE inhibitors or ARBs could, in fact, lower the risk of
complications [51]. However, increased ACE2 and the formation of
angiotensin-(1-7), by inhibiting COX-2, could exert anti-inflammatory
effects [52, 53], underscoring the multitude of possible effects and
the need to conduct studies to interrogate these connections. Finally,
it is not known whether an increase in the expression of ACE2 would also
lead to an increased shedding and increased levels of soluble ACE2,
which could act as a decoy receptor and lower viral entry into cells
[54]. In support of this, recombinant human ACE2 ameliorated the
lung injury induced by the avian influenza H5N1 virus in mice [55].
It is also important to consider that from the relatively limited amount
of human data, plasma ACE2 activity does not appear to be statistically
different between individuals taking ACE inhibitors or ARBs and those
not taking these medications, but these results do not reflect the
levels of cellular receptors [56]. Structural analyses indicate that
the binding of the SARS-CoV spike protein to ACE2 does not occlude the
catalytically active site of the receptor [26, 57], and it was
hypothesized that angiotensin II binding to ACE2 could induce a
conformational change in the receptor, which will no longer be favorable
for SARS-CoV-2 binding [54]. The mining of existing datasets,
preclinical studies, and clinical trials will help shed light on these
complex and sometimes conflicting scenarios.
A decrease in the number of ACE2 receptors appears to be involved in
acute lung injury and cardiovascular pathology [58, 59], and may be
detrimental during coronavirus infection. A mouse Ace2 knockout
developed severe cardiac contractility defects and increased angiotensin
II levels, and the additional deletion of Ace rescued this
phenotype [60]. In acute lung injury models, the loss of Ace2precipitated severe acute lung failure, and this was attenuated by the
exogenous recombinant human ACE2 in both Ace2 knock-out and in
wild-type mice [59]. Attenuation of the Ace2 catalytic function
perturbed the pulmonary renin-angiotensin-aldosterone system and
increased inflammation and vascular permeability [61], and Ace2
overexpression decreased lung inflammation in an animal model of acute
lung injury [62]. In vitro and in experimental animals,
SARS-CoV and the SARS-CoV spike protein downregulated ACE2 expression
[12, 28]. In mice with lung injury, injection of the SARS-CoV spike
protein worsened the acute lung failure and caused lung edema, increased
vascular permeability, and decreased lung function, and this pathology
was attenuated by blocking the renin-angiotensin-aldosterone pathway
[12]. Thus, animals infected with SARS-CoV or treated with the spike
protein resemble Ace2 knockout animals [12]. It is relevant
that a pilot study of patients with acute respiratory distress syndrome
reported the accumulation of angiotensin I and the decrease of
angiotensin-(1-9), indicating decreased ACE2 activity, among
non-survivors [63]. Thus, SARS-CoV and SARS-CoV-2 might contribute
to severe respiratory symptomatology partly because the viruses, by
binding the ACE2 receptors, also deregulate protective pathways in the
lungs.
Thus, either increased or decreased numbers of pulmonary ACE2 receptors
may be detrimental during SARS-CoV or SARS-CoV-2 infection, most likely
for distinct reasons. An increased number of ACE2 receptors may lead to
a higher viral load and more severe clinical disease. Diabetes increases
ACE2 expression, as shown in several experimental models, and the
resulting increased viral load might explain the more severe course of
COVID-19 in diabetic patients [64, 65]. Interestingly, in a rodent
model of diabetes, ibuprofen inhibited the ACE/angiotensin
II/angiotensin type 1 receptor axis and enhanced the
ACE2/angiotensin-(1-7)/Mas receptor axis [66]. Too few functional
ACE2 receptors, which decrease even more as a result of high viral loads
and enhanced receptor internalization [67], might exacerbate acute
lung injury, increase angiotensin II levels, and alter the balance
between pro- and anti-inflammatory responses. It is relevant that in a
study on twelve COVID-19 patients from China, plasma angiotensin II
levels were markedly elevated as compared to healthy control
individuals, and linearly associated with the viral load and with the
lung injury [68]. The animal studies that documented an
age-dependent decrease in ACE2 expression in the lung and the aortic
might also explain, at least in part, the age-dependent increase in the
risk of serious COVID-19 complications [69, 70].
SARS-CoV can also bind cells through alternative receptors that include
the C-type lectins DC-SIGN (dendritic cell-specific intercellular
adhesion molecule-3-grabbing non-integrin) and/or L-SIGN (liver/lymph
node-SIGN) [14, 71-73]. It will be critical to understand the
potential involvement of the same, or alternative receptors in the
pathogenesis of COVID-19.
It has been less clear why SARS-CoV and SARS-CoV-2 lead to severe lung
disease [57], in contrast to other, previously known coronaviruses,
which usually result in mild upper respiratory infections and cause
pneumonia only rarely, mostly in newborn, the elderly, and
immunocompromised individuals [74-77]. One of the possibilities
advanced for SARS is that the burden of viral replication and the immune
status of the host may both shape the severity of the infection [57,
78, 79]. The same might be true for COVID-19, and further exploring
the link between viral burden, chronic medical conditions, long-term
medication usage, and the severity of the infection will be critical.
An important lesson from SARS and MERS is the association between the
incubation period and disease severity. For any infectious disease, the
incubation period varies among individuals, even for the same outbreak,
and depends on the initial infective dose, the speed of pathogen
replication within a host, and host defense mechanisms [80]. During
the 2002-2003 SARS outbreak, a study in Hong Kong revealed that patients
with shorter incubation times developed more severe disease [81].
The same was found in MERS patients from South Korea, where longer
incubation times were associated with a lower risk of death [82].
Interestingly, during the SARS outbreak in Hong Kong, healthcare
workers, who have a higher infecting dose, had 34% shorter median
incubation times than non-healthcare workers [83]. It will be
interesting to examine whether the same is true for SARS-CoV-2, and
whether the incubation period is different in COVID-19 patients when
they are stratified by age, coexisting morbidities, and therapies they
receive for chronic diseases. While the association between the
incubation period and mortality might simply indicate that the disease
was confirmed earlier in patients with longer incubations, and reflect
earlier treatment opportunities [82], it is also plausible that high
viral loads might mediate the link between the two.
Two factors decisive for the successful control of outbreaks are the
ability to isolate asymptomatic individuals and the ability to trace and
quarantine their contacts [84, 85]. Several studies reported
asymptomatic shedding of SARS-CoV-2, indicating that asymptomatic
carriers, or individuals with very mild symptoms, may sustain
transmission [86-89]. For example, nearly 18% of the passengers who
tested positive for SARS-CoV-2 on the Diamond Princess cruise ship were
asymptomatic [88]. Another valuable finding that emerged from the
COVID-19 outbreak analysis in Singapore, and has a strong impact on
infection control, is that after becoming asymptomatic, some patients
continued to shed the virus for up to several days. In one instance, a
patient continued to have detectable respiratory shedding, as shown by
PCR, for eight consecutive days after becoming asymptomatic [90].
Another study revealed that several children with COVID-19 persistently
tested positive for viral RNA on fecal swabs after their nasopharyngeal
cultures became negative. Even though replication-competent virus was
not detected in the fecal swabs, this finding leaves open the
possibility of SARS-CoV-2 fecal-oral transmission [91]. These
findings illustrate the challenges in understanding SARS-CoV-2
transmission and in identifying infected individuals, tracing their
contacts, and implementing preparedness plans. One of the absolute
requirements, to clarify these questions and overcome these obstacles,
is ensuring the prompt and large-scale testing of symptomatic
individuals and of their asymptomatic contacts. This, together with the
social distancing measures, are currently our only available assets in
facing a pandemic that, even though it was preceded by multiple warnings
in recent years, is unlike any other infectious disease that we
experienced in modern history.