The immune ravages of COVID-19
The virus sabotages the body’s chemical defense system.
The design of treatments against SARS-CoV-2 depends on a thorough understanding of its interaction with the cells of the body and the immune response displayed against it.
The virus unleashes a hyper-inflammatory response by the immune system that causes severe damage throughout the body. The ins and outs of this process continue to be elucidated.
In addition to the rapid cell spread facilitated by its route of entry, and its rapid multiplication, the virus causes changes in the ranks of white blood cells to its advantage.
All these aspects are being investigated at a forced pace and with an unprecedented investment of resources that has changed the rules of basic research.
We will surely remember the 21st century divided in two: before and after SARS-CoV-2. Despite decades of warnings about the risk of a deadly global pandemic, health systems have been completely overwhelmed. The first COVID-19 patients were admitted to a hospital from Wuhan on December 16, 2019. Many citizens around the world felt safe because of the great distances, even though China was not able to contain the virus within its borders. This complacent viewpoint ignored that previous coronavirus outbreaks, such as SARS-CoV (causing severe acute respiratory syndrome) and MERS-CoV (causing Middle East respiratory syndrome), spread across multiple continents and that this The latter has not yet been eradicated. The fact is that SARS-CoV-2 spread across the globe in early 2020 and the health response was chaotic and motley. Some governments issued home confinement orders and the requirement to wear a mask, while others simply hoped that everything went well. At the time of writing this article,
Despite the disorganization, health professionals and researchers prepared to combat the new threat in a coordinated manner before its arrival. Less than a year has passed, but thanks to global collaboration we already know quite a bit about the new coronavirus and its impact on the human body. We are beginning to understand why SARS-CoV-2 causes very different diseases: some people show no symptoms, while others cough or have a fever. Most seriously, some people suffer from life-threatening pneumonia and a condition called acute respiratory distress syndrome (ARDS). Today we already know that this virus, like SARS-CoV and MERS-CoV, causes the immune system to become disoriented and causes inflammation that will lead to ARDS and a range of dangerous symptoms. The most handy clinical tests clearly show a very high concentration of immune proteins (IL-6, TNF-a and CRP) in the blood of critically ill patients. A few months after the start of the pandemic, the promising Broad spectrum immunosuppressants , such as the corticosteroids prednisone and dexamethasone , which would end up giving unsatisfactory results. At least this served to confirm the suspicions that, in the sickest patients, the immune system was running rampant and causing hyperinflammation. They were the same anti-inflammatory treatments adopted against severe infections during previous coronavirus outbreaks.
Today we know that in a fraction of patients with COVID-19 an excessive immune response is unleashed that causes damage throughout the body, with blood clots, heart damage and even organ failure. The most serious cases require admission to intensive care units (ICU). The usual cocktail of steroids is not enough to treat them: they need more targeted treatments. We also urgently need rapid tests to examine tissue samples for biological indicators (biomarkers) that predict the course of the disease, such as the likelihood that a mild case will worsen.
The development of biomarkers and pharmacological treatments is based on a thorough understanding of the interaction between SARS-CoV-2 and the cells of the body, as well as the immune response to the arrival of the virus. Last spring, in collaboration with many laboratories, we began to explore the immune dysregulation that occurs in severe cases of COVID-19. From the beginning we knew that the immune system orchestrates an intricate chain of mechanisms to repel invading pathogens. Also that if any of the stages of this response begins at the wrong time, it can trigger an exaggerated inflammation that damages the tissues. There is a quick emergency response and a slower but more durable response to viruses, bacteria, fungi, and more. The first is the “innate” response, in which some receptors on the surface and inside the immune cells detect intruders and activate an elaborate signaling cascade involving proteins called cytokines. These alert neighboring cells to prepare their defenses, initiate the death of infected cells or amplify the alarm with the synthesis of other types of cytokines. The cells responsible for the innate response also attract certain leukocytes from the blood so that they mount a more lasting immunity against the pathogen. Within a week or two, these members of the “adaptive” immune system are activated, reinforcing the amount of antibodies and specific T lymphocytes that will eventually neutralize or kill the invader.
In most COVID-19 patients, the innate immune system does its job as it learns to neutralize and destroy SARS-CoV-2. But in almost 5 percent of cases, the counterattack does not go as planned: When the carefully programmed cascade of signals is twisted, the cells of the innate response react by making too many cytokines, an overproduction reminiscent of the “cytokine storm.” »That appears in other clinical pictures and was thought to contribute to COVID-19 aggravation. The most recent research suggests that, in most cases, inflammation is not the typical storm, although it also poses a threat to the patient’s health. If you cause ARDS, the lung and other tissues will suffer lasting damage. It also involves the synthesis of fibrin, a protein that encourages clot formation. And if all this was not enough, the liquid fraction of the blood begins to leak out of the blood vessels (extravasation), which triggers respiratory failure.
All viruses manipulate cellular machinery to their advantage to reproduce. One strategy of the innate immune system is to sabotage that ability, but it appears to fail against SARS-CoV-2. In recent months, interferons, a type of cytokine that act as the first line of defense, blocking the replication of the virus within the cell, have received a great deal of attention. In theory, the rapid production of type I interferon (IFN-I) would make it possible to contain the virus and not exceed the limits of a mild infection, but some studies indicate that it would continue to reproduce due to the delay in the immune response in the elderly or in patients exposed to large amounts of the virus. What’s more, the entry into the scene of interferons would end up unleashing a hyperreaction that would stimulate the massive manufacture of other cytokines, which would lead to severe inflammation and symptoms. Measuring the response to interferons would provide vital insight into the progression of COVID-19 into a life-threatening disease, as well as some keys to treating the infection.
However, there are so many ways to put sticks in the wheels of the immune response that we learn as we go. For example, the virus could make it difficult for the patient to synthesize interferon. Another possibility is that certain patients produce less IFN-I for genetic reasons. It is even possible that the individual’s immune response is so erratic that it ends up making antibodies against IFN-I. There are several scientists who are investigating whether the presence of these “autoantibodies” would eventually cause COVID-19 symptoms, in which case it would serve as a biomarker to predict worsening. Some patients would also benefit from infusion of laboratory-made interferon. Clinical trials of such treatments have already begun, but the results are still unclear.
Many laboratories around the world have reoriented their line of research to focus on the fight against COVID-19. In Siena (Italy) a biosafety laboratory develops an antibody test. [PAOLO LAZZERONI, GETTY IMAGES]
An inflammatory rash
The cytokine storm was news in the severe cases of the preceding coronaviruses (SARS-CoV and MERS-CoV), so when SARS-CoV-2 appeared, the intervention of a similar mechanism was naturally seen. From the beginning of the pandemic, doctors detected a high concentration of cytokines in patients, but their quantity and the subsequent inflammatory state they caused differed from the typical storm.
These patients were shaken from within by the high concentration of cytokines that, depending on the cell that received them, had disparate consequences, some very harmful. Cytokines such as IL-6, TNF-a, IL-1b, and IL-12 amplify inflammation and tissue damage. Diane Marie Del Valle of the Icahn Mount Sinai School of Medicine in New York and her colleagues described a remarkably high concentration of some in the blood of about 1,500 New York patients. This indicated that an abnormal elevation of IL-6 and TNF-a could serve as a reliable predictor of severity and risk of death.
We observed the same changes in the patients whose evolution we followed closely. Furthermore, we weren’t the only ones to begin to recognize some unusual extreme values in patients’ cytokine profiles relative to a typical cytokine storm. We observed an increase in IL-5 and IL-17 , which are not usually associated with antiviral activity, since they usually appear as a response against parasites and fungi. We still do not know if this causes tissue damage or if it only diverts resources that would otherwise be used to fight the virus.
In some patients we also find a high concentration of chemokines, a subclass of cytokines that guide immune cells to where they are needed. The accumulation of the chemokines CCL2, CCL7, CXCL9 and IL-8 generated in the infection foci would serve as a kick-off. Not only was there local damage caused by cytokines and other immune messengers, chemokines also recruited cells from throughout the body to join the battle.
Numerous research groups have decided to look at blood and lung cells in order to discover the origin of tissue lesions. In the field of immunology, flow cytometry is widely used, as this technique allows the various types of blood cells to be labeled with fluorescent antibodies. Thanks to these markers, our group has been able to detect a large change in the composition of the circulating immune cells of patients, in contrast to that of healthy donors. Two types of innate immunity cells stand out for their abundance: monocytes and neutrophils. Let’s look at an example: in healthy donors, monocytes make up 10 to 20 percent of peripheral blood mononuclear cells, a well-studied set of white blood cells. Instead,
As an integral component of the innate immune system, monocytes patrol the blood and are the first to initiate the elimination or isolation of pathogens. When they perceive a microbial threat, they respond by transforming into two types of white blood cells: macrophages, which engulf pathogens and cell debris, and dendritic cells, which recognize and mark pathogens for target recognition by other defensive cells. The number of monocytes is strictly regulated so that the immune system does not overreact, but this control is lost in severe cases of COVID-19. In the worst cases, monocytes and macrophages infiltrate the lungs. When the team led by Mingfeng Liao of the Chinese Center for Clinical Research of Infectious Diseases in Shenzen, analyzed the interior of these organs in critically ill patients using cell samples obtained from the fluid of the lower respiratory tract with the technique of bronchoalveolar lavage (BAL). ), found monocytes and macrophages in abundance. In line with other findings , both cells expressed an amount of cytokines comparable to that of severe inflammation. As cytokines, synthesized primarily by monocytes and macrophages, are supposed to worsen all these damages, anything that blocks their inflammatory activity would prevent the infection from worsening.
If cytokines turn out to be the main drivers of severe COVID-19, it would make sense for us to try to reduce them in patients. This is achieved with certain drugs, such as tocilizumab, which blocks the receptor to which an important cytokine, IL-6, binds. Unfortunately, this drug does not appear to improve disease outcome in clinical trials. More and more scientists and clinicians are looking beyond the cytokine storm for a more satisfying explanation for the damage caused by the hyperinflammatory response to COVID-19.
A peptide or small protein called bradykinin also appears to contribute to the immune disorder of COVID-19. When they re-analyzed the lung fluid data from the patients, Michael R. Garvin of Oak Ridge National Laboratory in Tennessee and his colleagues hypothesized that bradykinin would induce an inflammatory response, like cytokines. And the latter would make those “bradykinin storms” worse. Too much of it would cause massive dilation of blood vessels and many of the surprising symptoms that COVID-19 patients display, such as arrhythmias and sudden cardiac arrests. A notable increase in hyaluronic acid synthesis has also been observed in severely ill patients. , whose aggregates retain a large amount of water. The flooding of the lungs that is discovered in the autopsies makes clear the dire consequences that for some patients the confluence of this affectation with extravasation from the blood vessels has.
The implication of bradykinin in COVID-19 has not yet been confirmed, but although its direct measurement remains difficult, the incipient successes of an exploratory study with icatibant, a bradykinin receptor inhibitor, support the hypothesis. that lowering the concentration of this peptide would alleviate severe cases.
Defective microbial traps
Bradykinin also appears in another inflammatory pathway in the blood of patients, since its synthesis would be activated by neutrophils , responsible for phagocytosis of pathogens. Different laboratories, including ours, have found a huge accumulation of neutrophils in the blood of some patients. The large amount of IL-8 cytokine that circulates in the blood with COVID-19 attracts these cells to the sources of infection, such as the lungs, and contributes to their abundance. The detection of this elevation of neutrophils on the first day of hospitalization is key, since it reliably predicts transfer to the ICU. Some articles Recent ones point to them as one of the possible culprits of the COVID-19 picture, for emitting the so-called extracellular traps set by neutrophils (TEN). These are meshes of DNA, antimicrobial proteins, and enzymes that isolate and destroy pathogens, but would unfortunately also damage tissue.
In lung autopsy specimens, Moritz Leppkes and his colleagues at the Friedrich-Alexander University in Germany have discovered striking blockages in blood capillaries from TEN aggregation. They have also observed TEN in the vessels of liver and kidney samples. In addition to obstruction, TEN degrade anticoagulant proteins, contributing to clot multiplication in severe cases. Recognizing the possible implication of these aggregates, McGill University has announced a pilot study of a cystic fibrosis drug that sheds DNA from TENs.
All of these studies have shown that SARS-CoV-2 directs the immune system against itself. But it is not only innate defenses that circulate uncontrollably; the adaptive immune system is also impaired. One of the most obvious differences between the blood of some COVID-19 patients and healthy people is the marked lymphocytopenia, specifically the shortage of T lymphocytes, key components of long-term adaptive immunity. It has been seen that the behavior of T lymphocytes in moderate patients differs from that in severe patients: lymphocytes that are specific for an invader, or antigen, normally accumulate as a protective measure, but this is not the case in the sickest.
There are two types of T lymphocytes: some kill virus-infected cells without delay and others only act against any invader once they receive the signal from cytokines. As in other respiratory infections , it has been observed that those hospitalized with COVID-19 lose both, although a small remnant persists for a long time, up to weeks in some cases. Thanks to the study of other respiratory viruses, we know that T lymphocytes leave the blood to settle in infected tissues. Those infected with these viruses raise the levels of chemokines that guide T lymphocytes to infectious foci, such as CXCL9 and CXCL10. On the other hand, although the blood of patients with COVID-19 contains a multitude of chemokines, we did not observe a similar abundance of T lymphocytes. Several studies have focused on the lungs of patients with severe COVID-19 symptoms because that is where they are hosts the virus. A nucleotide sequencing method called scRNA-seq (single cell RNA sequencing) has identified several subsets of immune cells, including a sizable conglomerate of T lymphocytes. But this finding did not provide a complete explanation. . Neither these experiments with lungs nor Autopsy studies of numerous organs explained the shortage of T lymphocytes in the blood. The absence is likely due simply to his death, and in fact data from many research groups support this conclusion.
How then do T lymphocytes disappear? Many of them have a receptor that indicates their propensity for early disappearance in COVID-19 patients. Another possibility is that the bone marrow cannot make enough precursor cells to differentiate into T lymphocytes, so they would be in short supply among mature cells. Studies on aging and other diseases have clearly shown that cytokines modulate the production of T lymphocytes in the bone marrow. But even though the same inflammatory cytokines appear in COVID-19, a similar connection remains to be confirmed. Finally, it could happen that it is the virus itself that kills them. Contrasting these hypotheses would indicate us with what treatments would increase their effectiveness.
Many of the serious immune manifestations of COVID-19 appear in other viral respiratory infections (drastic elevation of cytokines, infiltration of inflammatory cells in the lungs, TEN, and a decrease in white blood cells). SARS-CoV-2 brings with it its own challenges, especially a spread never seen before during the presymptomatic phase and among people who do not have any symptoms.
SARS-CoV, responsible for the 2003 epidemic, reaches its maximum multiplication relatively late, 10 days after the onset of symptoms. MERS-CoV reaches that point between 7 or and 10 or day. SARS-CoV-2 only takes 3-5 days . This precocity of the peak means that before symptoms appear, which in most people happens four or five days after infection, the infected person already releases a large amount of viral particles. That is, you spread the virus everywhere before you feel the slightest itchy throat.
The involvement of so many organs in the symptoms of COVID-19 also seems a unique trait among respiratory viruses. The SARS-CoV-2 anosmia causes, obtundation, digestive problems, blood clots, cardiovascular disease and even covídicos chilblains. It also infects neurons in the brain. In the recovered, the tissue lesions persist for months . All this is not so surprising if we realize that the three types of cells that make up blood vessels (endothelial, pericytes and vascular smooth myocytes) surround all tissues and are dotted with ACE2 receptors. And since this receptor is the gateway of SARS-CoV-2 to the cell, these tissues would constitute a kind of red carpet. To make matters worse, cytokine and bradykinin storms damage the tissues these cells form.
Despite the fact that its predecessor, SARS-CoV, uses the same receptor and causes cytokine storms and ARDS, severe extrapulmonary lesions, similar to those of COVID-19, appear to be rare. 80 percent of the genome of both viruses is almost identical , so it is possible to suspect that the remaining 20 percent is responsible for the differences. Another simpler explanation would be that SARS-CoV-2 has infected many more people than its predecessor (6,700 times more, so far), and it has done so in the eyes of the scientific community.
The past nine months of discoveries and innovation testify to the commitment and dedication of scientists and healthcare professionals. They have never been more united by a common purpose, and never before has the translation from the laboratory to the patient been so rapid as today. This legacy will endure despite the successes or failures of the hundreds of trials seeking treatment for COVID-19. Innovations will remain to fight future pandemics.