In a recent study published on Research Square*preprint server, researchers have demonstrated that impaired immunity to type I interferon (IFN-I) is associated with increasing severity of coronavirus disease 2019 (COVID-19).
The response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is highly variable, resulting in a variety of clinical outcomes, ranging from asymptomatic infection to severe illness and even death. A better understanding of this heterogeneity in responses to IFN-based COVID-19 treatment could help identify alternative therapeutic strategies.
Studies have shown that while responses to blood type I (IFN-I) interferon are essential in early acute infections, they are impaired before clinical worsening of COVID-19. Therefore, researchers observed a dichotomous clinical response to IFN-I-based treatments in late-stage COVID-19.
Consistent with these observations, studies have also identified mutations in the genetic loci that govern IFN-I immunity dependent on the toll-like receptor (TLR) 3 and interferon regulatory factor (IRF) 7 . Similarly, autoantibodies against IFNα, IFNω or IFNβ and the TLR7 pathway are essential for conferring early immunity to SARS-CoV-2 infection. Yet, exogenous IFN-I treatment does not improve clinical outcomes of COVID-19.
Here, it is also important to note the importance of deploying sensitive immunoassays when studying type I IFN directly from patient samples. As is known, IFNα protein levels remain below the detection limits of conventional enzyme immunoassays (ELISA) and Luminex assays. More importantly, plasma IFN levels at the time of patient sampling do not describe how patients’ cells respond to viral exposure.
About the study
In the current study, researchers recruited healthy controls and participants infected with SARS-CoV-2 at St. James’s Hospital in Dublin, Ireland, between March and June 2020. The control group included 61 people in good health that matched the study cohort in age, ± 5 years.
At enrollment, the team assigned all study participants disease severity grades based on hospital admission and supplemental oxygen requirements. Patients who did not require hospitalization were classified as moderately ill, while those hospitalized and receiving supplemental oxygen via a nasal cannula were considered severe. Those classified as critical required more than six liters of oxygen per minute, delivered via high-flow nasal oxygen or a venturi mask.
The team characterized the 13 IFN-alpha subtypes, in particular the IFNα2 proteins using highly sensitive assays, IFN function, interferon-stimulated genes (ISGs), and auto- IFN-I neutralizing antibodies. The researchers also selected a subset of patients and stimulated their whole blood samples with relevant viral antagonists to better assess the functionality of their immune system to external stimuli. Similarly, they obtained 342 and 212 plasma samples for cytokine and autoantibody analysis, respectively. Additionally, they used plasma samples from a small subset of 31 patients for cell phenotyping.
The authors observed multiple disruptions in the IFN-I response with increasing COVID-19 severity. Delayed stimulation with exogenous IFN-I did not benefit patients; moreover, it compromised any apparent clinical benefit of viral agonists. Notably, IFNβ secretion was more disrupted than IFNα, which was unexpected given its low levels in patient plasma. This finding further indicated the challenges in accurately detecting and quantifying IFNβ in blood.
The Gyros test identified four male patients as positive IFNα autoantibodies in the hospitalized group, including two moderately, one severely, and one critically ill. Since the majority of the patients remained negative for anti-IFN autoantibodies, this factor could not lead to serious disease in the patients.
The authors also observed that stimulation of whole blood from hospitalized COVID-19 patients with ex-vivo IFN-I induces a non-canonical inflammatory response. In addition, whole blood TLR gene expression data revealed similar TLR7 expression in all groups, lower TLR3 expression, and higher TLR4 and TLR8 expression in hospitalized patients. Additionally, the authors found eight additional cytokines with significant differences in hospitalized patients, primarily after TLR3 stimulation.
The IFN-I response to stimuli including polyinosinic: polycytidylic acid (Poly: IC), lipopolysaccharide (LPS), and R848 (a TLR7/8 agonist) was impaired in critically ill patients. In fact, Poly:IC stimulation significantly reduced the IFNβ response in the moderate and severe disease groups, but decreased after R848 and LPS stimulation in the hospitalized groups.
The blunting of broad IFN responses in patients with severe COVID-19 remains unexplained. Studies have shown that this phenotype was not attributable to SARS-CoV-2 interference with host immunity. Therefore, with increasing severity, pDCs remained able to produce IFNα intracellularly. Future work should investigate the functionality of intracellular pathways in cells isolated from patients with severe COVID-19. This could help to better understand this phenotype and provide targets for new treatment strategies.
The study highlighted the critical role that IFN-I-mediated immunity plays in determining the outcome of COVID-19. Other studies have implicated IFNβ in the long COVID-19, further highlighting the need to understand IFN-I regulation during SARS-CoV-2 infection.
The present study also identified two main reasons for an impaired IFN-I response in severe COVID-19. They showed a decrease in circulating plasmacytoid dendritic cells (pDC) and deregulated monocytes secreting less IFNα. Second, IFN-I stimulation of leukocytes promoted an inflammatory response in patients with severe COVID-19 but not in moderately ill patients.
In addition, physiological concentrations of IFNα proteins are often below pg/mL levels. The study results therefore highlighted the importance of using sensitive assays to measure circulating IFNα levels in patients with severe COVID-19. The authors also observed differences in plasma IFNα2 protein levels between moderately and severely ill COVID-19 patients.
As new experimental tools become available, they will help researchers fully understand the functions of the 13 IFNα subtypes in COVID-19 patients, which could also be relevant for other viral infections. More importantly, clinical studies should test IFN-I intervention strategies early, that is, before the onset of disease symptoms, and target them to patients with known risk factors.
Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.
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