“…As researchers at the University of South
Carolina, we can say one thing is clear: With no effective treatment options,
survival against the coronavirus infection depends completely on the patient’s
immune response. We have been working on how the immune response is
a double-edged sword – on one hand helping the host to fight infections, while
on the other hand causing significant damage in the form of autoimmune diseases.”
The two phases of the immune response:
“The immune response is like a car. To reach a destination
safely, you need both an accelerator (phase 1) and a brake (phase 2) that are
functioning well. Failure in either can have significant consequences.
“An effective immune response against an infectious agent rests
in the delicate balance of two phases of action. When an infectious agent
attacks, the body begins phase 1, which promotes inflammation – a state in
which a variety of immune cells gather at the site of infection to destroy the
pathogen.
“This is followed by phase 2, during which immune cells called
regulatory T cells suppress inflammation so that the infected tissues can
completely heal. A deficiency in the first phase can allow uncontrolled growth
of the infectious agent, such as a virus or bacteria. A defect in the second
phase can trigger massive inflammation, tissue damage and death.
“The
coronavirus infects cells by attaching to a receptor called the
angiotensin-converting enzyme 2 (ACE2), which is present in many tissues
throughout the body, including the respiratory tract and cardiovascular system.
This infection triggers a phase 1 immune response, in which the
antibody-producing B-cells pump out neutralizing antibodies that can bind to
the virus and prevent it from attaching to ACE2. This inhibits the virus from
infecting more cells.
“During phase 1, the immune cells also produce cytokines, a group of proteins that recruit
other immune cells as well as fight infection. Also joining the fight are
killer T cells that destroy the virus-infected cells, preventing the virus from
replicating.
“If the immune system is compromised and works poorly during
phase 1, the virus can replicate rapidly. People with compromised immune
systems include the elderly, organ transplant recipients, patients with autoimmune
diseases, cancer patients undergoing chemotherapy and individuals who are born
with immunodeficiency diseases. Many of these individuals may not produce
enough antibodies or killer T cells to counter the virus, which allows the
virus to multiply unchecked and cause a severe infection.
Lung injury resulting from inflammation:
“Increased replication of SARS-CoV-2 triggers additional
complications in the lungs and other organs. Normally, there is a wide range of
microorganisms, both harmful and benign, that live in harmony in the lungs.
However, as the coronavirus spreads, it is likely that the infection and the
inflammation that ensues will disrupt this balance, allowing harmful bacteria
present in the lungs to dominate. This leads to development of pneumonia, in
which the lungs’ air sacs, called alveoli, get filled with fluid or pus, making
it difficult to breathe.
“This
triggers additional inflammation in the lungs, leading to Acute Respiratory
Distress Syndrome (ARDS), which is seen
in a third of COVID-19 patients. The immune system, unable to
control viral infection and other emerging pathogens in the lungs, mounts an
even stronger inflammatory response by releasing more cytokines, a condition
known as ‘cytokine storm.’
“At this stage, it is also likely that the phase 2 immune
response aimed at suppressing inflammation fails and can’t control the cytokine
storm. Such cytokine storms can trigger friendly fire – destructive, corrosive
chemicals meant to destroy infected cells that are released by the body’s
immune cells which can lead to severe damage to the lungs and other organs.
“Also,
because ACE2 is present throughout the body, the killer T cells from phase 1
can destroy virus-infected cells across multiple organs, causing more
widespread destruction. Thus, patients that produce excessive cytokines and T
cells can die from injury not only to the lungs but also to other organs such
as the heart and kidneys.
The immune system’s balancing act:
“The above scenario raises several questions regarding
prevention and treatment of COVID-19. Because the majority of people recover from coronavirus infection,
it is likely that a vaccine that triggers neutralizing antibodies and T cells
to block the virus from getting into the cells and replicate is likely to be
successful. The key to an effective vaccine is that it doesn’t trigger
excessive inflammation.
“Additionally, in patients who transition to a more severe form
such as ARDS and cytokine storm, which is often lethal, there is an urgent need
for novel anti-inflammatory drugs. These drugs can
broadly suppress the cytokine storm without causing excessive suppression of
immune response, thereby enabling the patients to clear the coronavirus without
damage to the lung and other tissues.
“There may be only a narrow window of opportunity during which
these immunosuppressive agents can be effectively used. Such agents should not
be started at an early stage of infection when the patient needs the immune
system to fight the infection, but it cannot be delayed too long after ARDS
development, when the massive inflammation is uncontrollable. This window of
anti-inflammatory treatment can be determined by monitoring the antibody and
cytokine levels in patients.
“With COVID-19, then, the ‘fittest’ are individuals who mount a
normal phase 1 and phase 2 immune response. This means a strong immune response
in phase 1 to clear the primary coronavirus infection and inhibit its spread in
the lungs. Then this should be followed by an optimum phase 2 response to
prevent excessive inflammation in the form of ‘cytokine storm.’ Vaccines and
anti-inflammatory treatments need to carefully manage this delicate balancing
act to be successful.
“With
this coronavirus, it isn’t easy to know who are the fittest individuals. It
isn’t necessarily the youngest, strongest or most athletic individuals who are
guaranteed to survive this coronavirus. The fittest are those with the ‘right’
immune response who can clear the infection rapidly without mounting excessive
inflammation, which can be deadly” (The Conversation).
Prakash Nagarkatti is Vice
President for Research and Carolina Distinguished Professor, University of
South Carolina. Mitzi
Nagarkatti is Smart State Endowed
Chair of Center for Cancer Drug Discovery, Carolina Distinguished Professor and
Chair, Dept. of Pathology, Microbiology and Immunology, University of South
Carolina
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