Welcome to our e-Learning talk. Today, we will be discussing the role chemokines play in the immune response to SARS-CoV-2 and the subsequent role they play in the onset and progression of COVID-19.

Chemokines are a subset of cytokines that provide migratory signals between cells. They are aptly named for their ability to promote chemotaxis, or movement in response to a chemical stimulus. Chemokines have a fairly consistent structure with approximately 20 – 50% homology to one another. They contain a sequence of four cysteines, which interact to form their characteristic “Greek key” shape.
 
In the figure shown here, C represents the Cysteine and X represents any other amino acid residue. The four families of chemokines are named for the amino acid spacing present between the two N-terminal cysteines of the chemokine. These include CC chemokines with no space, CXC chemokines containing a single amino acid residue separation of the cysteines, CX3C chemokines with a 3 amino acid separation of the cysteines, and C chemokines with a single N-terminal cysteine and a secondary cysteine further downstream.

Functionally, chemokines are divided into two groups: homeostatic and inflammatory. While homeostatic chemokines help recruit cells to maintain normal levels of cells in tissue turnover in healthy organisms, inflammatory chemokines recruit immune cells in response to a pathogen. These immune cells are recruited to sites of inflammation to help remove harmful pathogens and repair damaged tissue.

Inflammatory chemokines recruit immune cells by binding to surface receptors. Different immune cell types express different surface receptors leading to their recruitment by specific chemokines. In this way, various immune cells including monocytes, macrophages, T-lymphocytes, mast cells, eosinophils, and neutrophils can all be recruited as needed to damaged tissue.

Early observations of patients suffering from COVID-19 showed an increased expression of several key chemokines including CCL2, CCL3, and CXCL10. Additionally, patients requiring ICU care presented with higher levels of each of these chemokines. These chemokines attract different classes of immune cells to the damaged lungs. In this way, the body attempts to eliminate the virus and begin repairing the damaged tissue.
 
CCL2 is capable of recruiting monocytes, memory T cells and dendritic cells. CCL3 is capable of recruiting monocytes, macrophages, and neutrophils. CXCL10 is capable of recruiting monocytes, macrophages, T cells, Natural Killer cells, and dendritic cells. In the following slides, we will go into more detail on each of these chemokines and what roles they play in the inflammatory response to SARS-CoV-2.

In response to SARS-CoV-2, CCL2 expression increased in patient populations and correlated with COVID-19 disease severity. From studies on SARS-associated Coronavirus, or SARS-CoV, it is possible to draw some inferences into how the patient responds to an increase in CCL2.
 
In studying SARS-CoV, researchers identified binding of the viral Spike protein to ACE2 on the cell surface initiated a signaling cascade, resulting in CCL2 expression and secretion from a lung carcinoma cell line. Specifically, they identified a role of the Ras-ERK-AP-1 pathway for upregulation of CCL2 expression. Further work found that CCL2 led to the migration of monocytes and macrophages into patients’ lung tissue. In these patients, reduction of CCL2 levels through treatment with corticosteroids had beneficial outcomes. Conversely, high levels of CCL2 in serum correlated with more advanced SARS.
 
It is easy to imagine a similar response to SARS-CoV-2 whereby virus binding initiates a cascade resulting in CCL2 upregulation. In turn, increased CCL2 expression is responsible for recruitment of monocytes and macrophages, resulting in the high levels of inflammation and cytokine storm characteristic of COVID-19.

In response to SARS-Cov-2, CCL3 expression is increased like CCL2 and correlates with COVID-19 disease severity. CCL3 plays a significant role in recruiting neutrophils to sites of injury. In COVID-19, patients with severe cases showed elevated levels of neutrophils in their blood. This correlation between disease severity and neutrophil levels likely corresponds to the increased CCL3 levels observed as well.
 
While neutrophils clear invading pathogens like viruses and bacteria, elevated levels of activated neutrophils can induce further injury to the tissue if uncontrolled. Specifically, TNF-α is capable of priming neutrophils enabling them to have a robust response. In acute respiratory disease syndrome (or ARDS), high levels of TNF-α-primed, hyper-responsive, and reactive oxygen species producing neutrophils are observed. These reactive oxygen species can further damage the surrounding lung tissue leading to poor outcomes. Severe COVID-19 patients often suffer from ARDS.

Similar to both CCL2 and CCL3, CXCL10 expression is increased in response to SARS-Cov-2 and correlates with COVID-19 disease severity. CXCL10 is a potent chemoattractant of monocytes, macrophages, and T lymphocytes.
 
Similar to SARS-CoV, an early increase in CXCL10 occurs in response to SARS-CoV-2. CXCL10 expression is seen in cells of SARS-CoV patient lungs including lung epithelial cells, macrophages, and T lymphocytes at areas of injury. This expression suggests the recruitment of additional immune cells to the area of injury, which can further perpetuate the cytokine storm.
 
In a highly detailed study on CXCL10 in SARS-CoV, the authors suggest that the high levels of CXCL10 may lead directly to damaged lung epithelium and, critically, apoptosis of lymphocytes. They suggest that apoptosis of lymphocytes is the reason for the observed lymphopenia in SARS-CoV patients and the inability to properly respond to secondary complications like pneumonia. Already, scientists have noted elevated CXCL10 and lymphopenia as characteristics of critical COVID-19 patients. It is likely that CXCL10 imparts similar complications on patients infected with both SARS-CoV and SARS-CoV-2.
Chemokines play a critical role in the recruitment of immune cells to the site of SARS-CoV-2 infiltration. Early characterization of COVID-19 patients show elevated levels of key chemokines CCL2, CCL3, and CXCL10, all correlating with disease severity. Their increase leads to local tissue inflammation and perpetuation of the cytokine storm.
 
While the explicit role each of these chemokines play in the immune response to SARS-CoV-2 and COVID-19 is being defined, it may take some time before it is fully understood. Initial hypotheses are being developed based on the understanding of their role in similar viral responses including SARS-CoV.

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