A cytokine ‘hurricane’ centered in the lungs drives respiratory symptoms in patients with severe COVID-19, a new study by immunologists at Columbia University Vagelos College of Physicians and Surgeons suggests.
Two cytokines, CCL2 and CCL3, appear critical in luring immune cells, called monocytes, from the bloodstream into the lungs, where the cells launch an overaggressive attempt to clear the virus.
Targeting these specific cytokines with inhibitors may calm the immune reaction and prevent lung tissue damage. Currently, one drug that blocks immune responses to CCL2 is being studied in clinical trials of patients with severe COVID-19.
Survivors of severe COVID-19, the study also found, had a greater abundance of antiviral T cells in their lungs than patients who died, suggesting these T cells may be critical in helping patients control the virus and preventing a runaway immune response.
The study, published online March 12 in the journal Immunity, is one of the first to examine the immune response as it unfolds in real time inside the lungs and the bloodstream in patients who are hospitalized with severe COVID-19.
“We wanted to look at the immune response in the lung in severe disease, because it’s those responses that are either protecting the organ or causing the damage,” says Donna Farber, Ph.D., professor of microbiology & immunology and the George H. Humphreys II Professor of Surgical Sciences in the Department of Surgery, who led the study.
Numerous COVID studies have focused on identifying immune responses in blood; a few have looked at airway samples from a single time point or from autopsies. Few studies have examined the immune response to SARS-CoV-2 in the respiratory tract as the response unfolds, because obtaining such samples from patients is challenging.
But the Columbia researchers learned several years ago that they can retrieve respiratory immune cells from the routine daily saline washes of the endotracheal tubes that connect intubated patients to a ventilator.
Paired Airway and Blood Samples Show Complete Immune Response in Real Time
In this new study, the researchers collected respiratory immune cells from 15 COVID-19 patients who had been intubated. Each patient spent four to seven days on a ventilator, and airway and blood samples were obtained daily.
All the samples were examined for the presence of cytokines and different types of immune cells. For four of the patients, the researchers measured gene expression in every immune cell to get a detailed picture of the cells’ activities.
“It seems obvious that the immune response in the respiratory tract would drive a disease caused by a respiratory virus, but we didn’t know what the processes were and how they worked together with systematic responses,” Farber says. “What’s new here is that we’ve been able to simultaneously sample both the respiratory tract and the blood over time and put together a more complete picture of the responses involved and how local and systemic responses work together.”
Two Cytokines Appear to Drive Lung Damage
Though the researchers found elevated levels of many cytokines in the blood, many more types of cytokines were present in the lungs and at highly elevated levels.
“People refer to patients experiencing a cytokine storm in the blood, but what we’re seeing in the lungs is on another level,” Farber says. “The immune cells in the lung went into overdrive releasing these cytokines.”
No cytokines were found in the blood that weren’t also found in the lung, suggesting that the signals causing the severe inflammation are driven by lung cytokines rather than systemic ones.
“It has been suggested that systemic cytokines are driving severe disease, but our results suggest that inflammatory processes that perpetuate disease are coming from the lungs,” Farber says.
CCL2 and CCL3 cytokines released by the lung appeared to be particularly important in severe disease, because the monocytes drawn into the lung expressed receptors for these molecules. “Normally, these cells never make it to the airway, but in severe COVID patients, they accumulate throughout the lung and clog up the alveolar spaces,” Farber says.
The findings also may explain why trials of other cytokine inhibitors, including tocilizumab, have shown variable efficacy. Tocilizumab inhibits the cytokine IL-6, which is elevated in patients with severe COVID but does not appear to be a major component of inflammation in the lung, Farber says.
Survivors Have High Level of T Cells in Lung
Of the study’s 15 patients, eight died and all survivors were under 60 years of age.
The lungs of those survivors had significantly more T cells, which are mobilized to the lung to clear virus, and a lower proportion of inflammatory macrophages and monocytes.
In general, younger people have a more robust T cell response while older people have a higher baseline level of inflammatory cells; both factors may help explain why older patients with severe COVID fare worse.
The cell differences between patients who lived and those who died could potentially lead to a way to predict which patients are more likely to develop severe disease, although the differences are only apparent in the lung, not the blood. Importantly, the predictive value of airway immune cell frequencies was better than standard clinical measurements of lung and organ damage.
“Our next step is to try to find a more accessible biomarker that predicts severe COVID so we can try to give treatments earlier to patients who are most at risk,” Farber says.
“Understanding the immune response in severe COVID is really critical at this point,” Farber adds, “because we could see this again with the next coronavirus outbreak. This is what coronaviruses do at their worst; this is their M.O.”
The rapid spread of severe respiratory syndrome coronavirus 2 (SARS-CoV-2) and resulting coronavirus disease 2019 (COVID-19) pose an unprecedented global health crisis. While the majority of cases resolve with mild symptoms or no symptoms at all, some patients develop fatal complications, such as acute respiratory distress syndrome (ARDS), for which effective therapeutic strategies are urgently needed (1, 2).
In these severe cases, a hyperinflammatory response or cytokine storm has been observed and is suspected to be a potential driver of pathology (3, 4). Indeed, transcriptomic profiling and histologic examination of the lungs or bronchoalveolar lavage (BAL) fluid of COVID-19 patients have revealed extensive immune cell infiltration and significantly elevated levels of cytokines, chemokines, and other proinflammatory mediators that correlate with disease severity (5–7). Evidence currently points to immune dysfunction as a potential driver of these hallmark characteristics of COVID-19. However, our understanding of the specific immune responses to SARS-CoV-2 remains extremely limited.
Innate immune sensing serves as the first line of antiviral defense and is initiated by the recognition of conserved pathogen-associated molecular patterns by pattern recognition receptors (PRRs). Single-stranded RNA (ssRNA) viruses, such as SARS-CoV-2, replicate via formation of double-stranded RNA (dsRNA) intermediates, which can be detected by Toll-like receptor (TLR) 3 and cytosolic PRRs MDA-5 and RIG-1, while ssRNA can be detected by TLR7 and TLR8 (8). Indeed, activation of immune cells via PRRs has been postulated to drive the release of proinflammatory cytokines seen in severe COVID-19 patients (8–10).
Mast cells (MCs) are tissue resident immune cells that constitute a major sensory arm of the innate immune system. They are crucially located at sites that interface with the external environment, such as the lungs and gastrointestinal tract, allowing them to be among the first cells to respond during pathogen invasion (11).
MCs are equipped with TLRs and receptors for inflammatory mediators, allowing them to act as sentinels for tissue damage and pathogen exposure (12). Upon activation, MCs release preformed granules containing inflammatory mediators, vasoactive autocoids, and catalytically active MC-specific proteases, including β-tryptase, chymase, and carboxypeptidase (CPA)-3 (13).
In humans, MCs are classified according to their protease content and tissue distribution, with the MCT subclass expressing only tryptase and being primarily found in mucosal tissues and the MCTC subclass expressing tryptase, chymase, and CPA-3 and located mainly in the skin (13).
MC activation also leads to de novo production of cytokines and lipid mediators, including TNF, IL-6, CCL2, CCL3, prostaglandin D2 and E2, and leukotriene B4 and C4 (14, 15), many of which are now known to be associated with the cytokine storm observed in COVID-19 (5–7, 16).
MC responses to viral pathogens have not been extensively studied. Viruses can activate MCs directly or indirectly through viral or inflammatory products such as, ssRNA or dsRNA replication intermediates, complement, and cytokines (17). Many viruses have been shown to induce MC degranulation, protease release, and cytokine production, including dengue (DENV), respiratory syncytial virus (RSV), herpes simplex virus (HSV), Japanese encephalitis (JEV), Zika, and influenza (18, 19).
The interactions between MCs and viruses or pathogen-derived products are complex and can result in either beneficial or detrimental outcomes (17, 18). For example, MCs have been shown to play a protective role against HSV and vaccinia virus infection (17, 19). In contrast, tryptase and chymase are elevated in plasma from patients with severe DENV infection and these MC proteases were shown to induce significant vascular leakage in peripheral tissues in response to the infection (20).
Sialic acid-binding immunoglobulin-like lectin (Siglec)-8 is an inhibitory receptor, selectively expressed on MCs and eosinophils, that inhibits MC activation and induces eosinophil death and depletion when engaged with a monoclonal antibody (mAb) (21–23). Anti-Siglec-8 mAbs have been shown to suppress immune cell infiltration, local and systemic inflammation, protease production, fibrosis, and anaphylaxis (24, 25). Clinical evaluation of lirentelimab (AK002), a humanized anti-Siglec-8 mAb, is currently underway in multiple mast cell and eosinophil-driven diseases (26, 27).
Given the pathogenic role of MCs in many inflammatory diseases and their putative role in COVID-19 pathogenesis (28–30), we sought to evaluate MC activation in SARS-CoV-2 patients and the activity of a Siglec-8 mAb in models of viral inflammation using Siglec-8 transgenic mice. Here we show that MC-derived proteases are significantly elevated in COVID-19 patient sera and lung autopsies.
Surprisingly, we also found evidence of eosinophil activation in these COVID-19 patients. Stimulation of viral-sensing toll-like receptors in vitro and administration of synthetic viral RNA in vivo induced local and systemic inflammation, including cytokine elevation, immune cell airway infiltration, MC-protease production, and eosinophil-granule release.
Treatment of Siglec-8 transgenic mice with an anti-Siglec-8 mAb significantly suppressed airway inflammation induced by either administration of synthetic viral RNA or infection with RSV. These data provide evidence that MC activation is a component of COVID-19 inflammation and demonstrate that targeting Siglec-8 with a mAb suppresses TLR- and RSV-mediated inflammation, supporting anti-Siglec-8 antibodies as a potential therapeutic approach for attenuating excessive inflammation during viral infections.
reference link: https://www.frontiersin.org/articles/10.3389/fimmu.2021.650331/full
More information: Peter A. Szabo et al, Longitudinal profiling of respiratory and systemic immune responses reveals myeloid cell-driven lung inflammation in severe COVID-19, Immunity (2021). DOI: 10.1016/j.immuni.2021.03.005