A new study by researchers in the U.S. further supports the hypothesis that the unregulated release of inflammatory cytokines is responsible for the local and systemic inflammation that is associated with multi-organ damage and death in severe coronavirus disease 2019 (COVID-19). This could lead to the identification of possible drug targets in early disease.
The ongoing COVID-19 pandemic is caused by infection with the severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2). The disease is characterized by severe lung injury and systemic inflammatory damage to multiple organs. The presence of very high levels of inflammatory cytokines in these patients has led to it being termed a cytokine release syndrome (CRS) or cytokine storm.
The study, which was released as a preprint on the bioRxiv* server, reports the value of targeting the NLRP3 inflammasome in early infection with this virus to prevent the CRS and its associated adverse outcomes.
Cytokine storm in COVID-19
In earlier research, the use of the IL-1 Receptor antagonist (IL-1Rα) anakinra in mild to moderate COVID-19 has been reported, indicating a reduction in mortality with this approach. The IL-1β inhibitor, canakinumab, a neutralizing monoclonal antibody, has also been reported to reduce adverse outcomes.
However, IL-1β must be processed within the cell before it is activated. This processing occurs mostly within the inflammasomes. Inflammasomes are macromolecular complexes found within the cytosol.
The NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome has been found to be activated by several SARS-CoV-2 proteins. These include the open reading frame (ORF) 3a, ORF8b, and viroporin 3a.
In vitro studies confirm that the virus induces the formation of this inflammasome. The presence of aggregates of NLRP3 inflammasomes in the lungs of patients who died of COVID-19 further supports this.
These inflammasomes are also found in the peripheral blood mononuclear cells (PBMCs) of such patients. Overall, these findings indicate that NLRP3 inflammasome activation occurs in moderate to severe COVID-19.
The case for early NLRP3 inhibition
If early treatment of such cases was possible, it might prevent the need for hospitalization and ease the enormous burden currently being placed on healthcare services. The current study aims to explore the potential for early inhibition of NLRP3 inflammasome activation and the resulting IL-1β release in COVID-19.
NLRP3 activation leads to CRS
There is a twofold elevation of IL-1β following SARS-CoV-2 infection, while the IL-6 levels are still higher, relative to their levels in healthy individuals. The levels of the natural IL-1 Receptor antagonist (IL-1Ra) are 2.5-fold higher in infected people.
This is a common finding in autoinflammatory conditions rather than infection, with both IL-6 and IL-1Ra being induced by IL-1β. In addition, tumor necrosis factor-alpha (TNFα), IL-10, and urokinase plasminogen activator receptor (uPAR) are all at significantly higher levels in these patients, even without overt disease symptoms or signs. Such a rise in the latter is associated with adverse outcomes in COVID-19.
Circulating white blood cells in SARS-CoV-2 infected patients with asymptomatic or mild COVID-19 showed a twofold rise in NLRP3 levels, while the expression of IL-1β in these cells was 5.5-fold higher. These show a strong correlation with each other at both gene and protein levels.
There was a fourfold rise in the expression of caspase-1, again, in significant association with NLRP3 levels.
The conclusion drawn from these findings is that SARS-CoV-2 infection triggers a molecular cascade that ends in higher levels of NLRP3 activation and IL-1β release, with the latter being the pivot on which disease severity turns.
The observable worsening of the clinical disease features, and the fatal outcomes, occur later in the course of the infection, when viral RNA is often undetectable in the tissues. At this point, the exaggerated secretion of cytokines is the most obvious feature.
This temporal pattern indicates that CRS is a downstream event of IL-1β secretion, the latter triggering the release of other inflammatory cytokines that end in organ damage.
Current anti-inflammatory therapies
Unlike anakinra and canakinumab, therefore, inhibitors of active IL-1β processing and release might help to prevent this often fatal cascade of reactions. As such, patients infected with the virus could be treated by a specific inhibitor of NLRP3 early in the course of the infection, thus bypassing the entire IL-1β-CRS cycle.
The only oral inhibitor specific against NLRP3 at present is OLT1177 (rINN dapansutrile), which has been found to be of use in two inflammatory conditions, gout exacerbations and heart failure.
Colchicine has been reported to be of use in reducing hospitalizations and/or deaths in COVID-19 patients. This drug also reduces cardiovascular events. The underlying mechanism in both cases is likely to be its ability to inhibit IL-1b-mediated inflammation.
Colchicine is not a direct inhibitor of NLRP3, however, and it does have other effects, on the cytoskeleton components called microtubules and integrins, as well as cell migration.
What are the implications?
The study demonstrates that “early in SARS-CoV-2 infection, NLRP3 activation takes place and initiates the CRS. Thus, NLRP3 is a target to reduce the organ damage of inflammatory cytokines of the CRS.”
A reduction in NLRP3 activation could also reduce Il-18 processing, a significant gain as the latter is involved in the onset of the Macrophage Activation Syndrome (MAS)-like disease in COVID-19. High levels of circulating IL-18 are associated with increased severity of disease and poor prognosis in COVID-19.
The use of a compound that specifically inhibits NLRP3 activation would thus prevent both IL-1βas well as IL-18 release, thus combating COVID-19 by two different pathways.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.