Coronaviruses have caused several global challenges for health-care providers all over the world. The notorious SARS-CoV-2 could attack the lower respiratory tract and trigger the immune systems to release massive number of immune cells and pro-inflammatory cytokines and cause immunopathology consequences called cytokine release syndrome. These pro-inflammatory cytokines and other immune cells caused lung injury and severe acute respiratory distress syndrome in COVID-19 (CARDS) and multiple organ failure. There are still many intertwined immune responses that not yet been discovered in SARS-CoV-2 infections. Targeted and specific cell therapy would be reasonable and considered safer to be employed in patients who present with comorbidities and at risk of complications.
Keywords: COVID-19, cytokine storm syndrome, immunology, SARS-COV-2
| Introduction|| |
In the last decade, coronaviruses have caused several global challenges for health-care providers all over the world. There are four types of coronaviruses (α, β, γ, and δ). Typical coronavirus infection usually caused mild upper respiratory symptoms. However, the latest known three strains of β-coronaviruses such as SARS (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and COVID-19 (SARS-CoV-2) are considered to be highly pathogenic and fatal. The notorious SARS-CoV-2 could attack and replicate in the lower respiratory tract and trigger the immune systems to release a massive number of immune cells and pro-inflammatory cytokines and cause immunopathology consequences called cytokine release syndrome (CRS). These pro-inflammatory cytokines and other immune cells caused lung injury and severe acute respiratory distress syndrome in COVID-19 (CARDS) and in the later stage, multiple organ failure (MOF).,
SARS-CoV-2 shares genomic similarity with SARS-CoV or MERS-CoV, approximately 79% and the later about 50%. The similar genomic sequence between these three viruses could help to predict the pathogenesis of SARS-CoV-2 based on the previous ones. By using advance gene mapping, the host innate immune response mechanism as the main contributor of the disease severity could be revealed. The phylogenetic tree of SARS-CoV and SARS-CoV-2 taken from human, bat, civet, and pangolin samples shows the evolution of these β-coronaviruses.,
| Angiotensin-Converting Enzyme-2 Receptor as the Specific Entry Receptor|| |
The SARS-CoV-2 targets the lungs from airway epithelial cells, alveolar epithelial cells, vascular endothelial cells, and macrophages in the lung by binding to the angiotensin-converting enzyme 2 (ACE2) receptor.
Coronaviruses were given its name from a crown-like protein called S-protein (Spike Protein) in its cell surface which given the appearance like a crown or in Latin, Corona. The coronaviruses enter the targeted host cell through the binding of Spike protein/S-protein with the ACE2 receptor. After entering the cell, the viral RNA genome is released into the cytoplasm and begin its replication process.
The nucleic acid sequence analysis shows the spike protein (S-protein) in the protein receptor-binding domain which later binds to ACE2 receptors and helps the virus to enter the host cells. While MERS-CoV uses dipeptidyl peptidase-4 as a specific entry receptor.,,
The theory of its cytokine storm came from this S-protein which is also shown in many mild/benign coronavirus types. The previous exposure to this S-protein in mild coronaviruses causes the memory T cell to respond to the S-protein in COVID-19 in later stage. However, the entirely new S-protein (and another possible mechanism explained below) causing the immune system failed to contain this viremia.,
ACE2 as the entry port for the viruses explains why hypertension and diabetes mellitus become the leading comorbidities and also worse outcome. Since most patients with hypertension and diabetes mellitus consume ACE inhibitors, this causes upregulation of ACE2 receptor which, in turn, increases more infected cell. However, this theory needs much more evidence.,,
The ACE2 is important in the renin–angiotensin system to convert angiotensin 1 to angiotensin 2. The SARS-CoV-2 infection caused downregulation and loss in pulmonary ACE2 which may reduce the blood pressure, causing electrolyte imbalance, and increasing the permeability of airway vascular.,
This arises the idea that blocking the host ACE2 receptors could be one of the treatment strategies. Janus kinase (JAK) inhibitor that used to treat rheumatoid arthritis could inhibit ACE2-mediated endocytosis, along with another JAK inhibitor drugs that are currently tested for COVID-19 disease.
| Normal Immune Response Against Pathogen|| |
In normal individual, the innate and adaptive immune systems work in tandem to identify and kill infectious agents. When an infection occurs, the innate immune system serves as a rapid reaction force that deploys a range of relatively nonspecific weapons to eradicate or at least to keep the infection contained, then the adaptive immune response to an infectious agent reinforces and adds new weapons to the attack mounted by the innate immune system.
The innate immune system uses hard-wired receptors (pattern recognition receptors [PRR]) that are highly reliable in terms of discriminating self from nonself. The precise nature of the PRRs that are engaged on cells of the innate immune system in the initial stages of an infection dictates the type of adaptive immune response that is required.
The beginnings of an immune response are started when upon entry of a microorganism that engages one or more of PRR of macrophages, a transition occurs. The initiation of effective immune response requires engagement of several categories of PRR simultaneously. Various PRR families are involved when a virus attacks the host such as toll-like receptors (TLRs), especially TLR7 and TLR8 in endosome, cytosolic DNA sensors, and RIG-I-like helicase receptors (RLRs). Engagement of TLRs with their respective ligands drives activation of nuclear factor kB (NFkB) and several members of the interferon (IFN)-regulated factor family of transcription factors. These will, in turn, take charges in inducing expression of protein series that could interfere with mRNA translation and viral replication and also degrading viral RNA genomes. The activated macrophage will produce numerous cytokines and chemokines such as tumor necrosis factor (TNF), interleukin-6 (IL-6), IL-8, and IL-12. Each of them has several distinctive roles in the inflammatory process. TNF will activate local endothelium, initiate cytokine production, and upregulate adhesion molecules. IL-6 will trigger acute-phase proteins' production, enhance antibody production from B-cells, and induce T-cell polarization. IL-8 could trigger neutrophil chemotaxis, act as chemotactic for basophils and T-cells, activate neutrophils, and promote angiogenesis. While IL-12 will activate NK cells, polarize T-cells to T helper cells. The capillaries will be more permeable than normal, and the increased vascular permeability permits invading plasma protein to the tissue and incoming neutrophils to the site of infection. Uncontrolled and chaotic production of these inflammatory cytokines will cause a syndrome named CRS which is the main pathology in all three coronaviruses.,
Special considerations of RLRs and cytosolic DNA sensors are brought when viral infection is carried on the process. RLRs are found in the cytoplasm and are activated in response to double-stranded RNA and are capable of directing the activation of NFkB and IFN-related factors that cooperatively induce antiviral type I IFN α and β. IFN α and IFN β, those type 1 IFNs which are induced by the activation of PRRs through cytoplasmic DNA sensors, STING, and RLRs, are also important activators of NK cells. These processes marked examples of the innate immune system cooperation, where cytokines produced by macrophages or other cells upon detection of a pathogen results in the activation of other cells.
Besides macrophages, there are dendritic cells which were among the first immune cells to be recognized. They act as a major conduit between the innate and adaptive arms of the immune system. Their primary role is not the destruction of microbes, but rather than sampling of the tissue environment through continuous micropinocytosis and phagocytosis of extracellular material. On detection and internalization of Pathogen Associated Molecular Patterns (PAMP) through phagocytosis, they will undergo a transition from highly phagocytic but inefficient antigen-presenting cell into a lowly phagocytic but highly migratory cells that is now equipped to present antigen efficiently to T-cells within local lymph nodes.
| Delayed IFN 1 Pathway on Coronaviruses|| |
IFN 1 has a very important role in virus eradication. These polypeptides secreted by viral-infected cells that have potent antiviral properties. IFN α and IFN β, those type 1 IFNs which are induced by the activation of PRRs through cytoplasmic DNA sensors, STING, and RLRs, are also important activators of NK cells. These processes marked examples of innate immune system cooperation, where cytokines produced by macrophages or other cells upon detection of a pathogen results in the activation of other cells.
Coronaviruses will delay the initial innate immune response which includes RNA sensing and interfering with the antiviral IFN pathway which has been described above. Several coronavirus structural and nonstructural proteins act as IFN response antagonize; this includes preventing PRRs to detect the virus RNA and preventing downstream IFN signaling through STAT1. STAT1 mediates IFN signaling. In animal studies, STAT1 deficient shows a complete lack of responsiveness to either IFN-α or IFN-γ and is highly sensitive to infection by microbial pathogens and viruses; it also displays greater weight loss, worsened lung pathology when exposed with SARS-CoV viruses. STAT1 signaling plays an important role in the control of SARS-CoV pathogenesis. We hypothesized that STAT1 is also playing a role in SARS-CoV-2.
IFN type I and its cascade is the main innate immune response against viral infection. All three lethal coronaviruses interfere with this effective viral elimination by delaying the IFN I production results in uncontrolled viral replication in the early phase. This viremia caused an influx in neutrophils and monocytes/macrophages and hyperproduction of pro-inflammatory cytokines. The next phase will show the late excessive IFN1 production and myeloid cell that infiltrated the lung and caused acute pneumonia, followed by distress and respiratory failure. Numerous cytokines and chemokines will continue superfluously and later named as CRS which could be found in SARS-CoV-2 infection as in linearity as previous SARS and MERS. Virus SARS-CoV-2 uses this strategy but possibly with a novel mechanism that yet to understand. This dysregulated immune response worsens in susceptible patients with underlying diseases such as diabetes, cardiovascular, and respiratory diseases. In most young patients with normal healthy immune response, this early innate immune process can effectively eliminate the viruses before it replicates to uncontrolled numbers.
| Adaptive Immune System Against SARS-CoV-2|| |
For the adaptive immune response, T helper type 1 plays the main roles. Patients with COVID-19 presented elevated T helper 2 cytokines (IL-4) in addition to T helper 1 cytokine compared to those in patients with SARS or MERS. CD8+ or T cytotoxic is killing the cell that is already infected by the virus. This response even though important to control the virulence, it can cause lung injury if not controlled well. This proptosis mechanism of the host cell releases damage-associated molecular patterns, including ATP, nucleic acids, and ASC oligomers. These cause a subsequent inflammatory response by triggering the neighboring cell and structure like, endothelial cells, epithelial cells, and circulating immune cells to release the pro-inflammatory cytokines chemokines. These include proteins that would attract monocytes, macrophages, and T cells to the site of infection, promoting further inflammation and increase the IFN produced by T cells. It is proven by increased IL-1 β, an important cytokine released during proptosis. Plasma cell produces antibodies to neutralize and protect the body in late stage and memory cells to prevent reinfection in the future.
Lymphocyte will be recruited to the lungs and target organ, which explains lymphopenia and increased neutrophil-to-lymphocyte ratio that we observed in COVID-19 patients.
Subsequent inflammation in the lung with ongoing process of neutrophils, monocytes, and T lymphocytes infiltration, desquamation of alveolar cells, increased pulmonary vascular permeability will lead to impaired lung oxygen transfer and pulmonary shunting. These clinical findings show that acute respiratory distress syndrome is linked with CRS as the main culprit, and the dysregulation of immune response modulation is the vital key to comprehend and rearrange targeted treatment and strategies.
| Cytokine Release Syndrome|| |
After elimination failure of the virus, the CRS plays a major role in the severity of the diseases. Inflammatory cytokines and chemokines, such as IL-2, IL-7, IL-10, G-CSF, IP-10, MCP-1, MIP-1A, and TNFα, were distinctively higher in ICU patient than non-ICU patient. These pro-inflammatory cytokines lead to lung injury, ARDS, and respiratory failure as the main cause of death. If these cytokines leak into the systemic circulation and find its way to another organ, it causes extrapulmonary manifestations and eventually MOFs such as myocardial damage and hemodynamic instability observed in some patients.
This CRS mimics the exuberant immunopathology expressed in conditions such as macrophage activation syndrome (MAS) induced by rheumatic diseases and primary and secondary hemophagocytic lymphohistiocytosis (HLH). COVID-19 patient has notably similar cytokines profile with MAS/sHLH such as elevated IL-1 β, IL-2, IL-6, IL-17, IL-8, TNF, and CCL2. The laboratory result also shows elevated CRP, hyperferritinemia, deranged liver function tests, and coagulopathy with elevated D-dimer level which commonly found in MAS/sHLH condition. However, in usual MAS/sHLH, hyperactivation of T-cells is not associated with pulmonary dysfunction, but more to the lymphoid organ hyperplasia result in organomegaly which is not found in COVID-19 patients.
From all the pro-inflammatory cytokines, IL-6 attracts the interest when the study found that its level was consistently higher in nonsurvivor patients. IL-6 is essential for the generation of T helper 17 (Th17) cells in the dendritic cell–T cell interactions. The excessive IL-6 may explain the overly activated Th17 cells observed in COVID-19 patients. Preliminary trials demonstrate evidence for efficacy for anti-IL-6R blockade with tocilizumab. However, the full blockade of IL-6 caused opposite outcome instead. It shows that IL-6 also has protective benefit. IL-6 also plays an important role in lung repair following viral or chemical insults. Further, although corticosteroid inhibits IL-6, the study shows the inferior outcome for an unclear reason.
| The Clinical Course of COVID-19|| |
COVID-19 has an incubation period between 2 and 14 days during which the patient is asymptomatic but already infectious. This longer incubation period from usual viral infection which usually lasts 1–4 days may be caused by late immune response and delayed IFN 1 response as explained above.
The clinical course of SARS-CoV-2 infection begins with the viral phase and then responded with the host inflammatory phase. Perturbation in the inflammatory phase would induce a series of vicious circle of inflammation cascade, at first aimed to attack the infected cells, but then inflict self-injury.
| Conclusion|| |
There are still many intertwined immune responses that not yet been discovered in SARS-CoV-2 infections. The bottom line is to control the dysregulated immune response related with IFNs induction, NK cell activation, IL-6 circulation, and also the angiotensin receptor pathways. In the other hand, immunotherapy-specific agent for this infection is still limited. Targeted and specific cell therapy would be reasonable and considered safer to be employed in patients who present with comorbidities and at risk of complications. Future research with larger samples is needed to focus on specific pathways in the immune response to be blocked in order to reduce morbidities and mortalities.
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Conflicts of interest
There are no conflicts of interest.
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Department of Anesthesiology and Intensive Care, Faculty of Medicine, Udayana University, Bali
Source of Support: None, Conflict of Interest: None