In a recent study posted to the bioRxiv* pre-print server, researchers developed a highly pathogenic mouse-adapted (MA) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) strain to model mild, moderate, or severe coronavirus disease 2019 (COVID-19). They used this model to study age-dependent impairment of interferon (IFN) immunity and COVID-19 aggravation in standard C57 black 6 (C57BL/6) wild-type (wt) mice.
Small animal models mimicking human disease could help provide novel insights into the critical pathomechanism that drives the age-dependent SARS-CoV-2 virulence. Further, they could help design immunomodulatory treatment and prevention strategies for individuals at high risk of developing severe COVID-19.
About the study
In the present study, researchers analyzed the individual and combined role of type I and III IFNs in limiting SARS-CoV-2 replication in mice lacking functional type I (Ifnar1-/-) or type III IFN receptors (Ifnlr1-/-) and C57BL/6 wt mice infected with MA SARS-CoV-2.
The team compared tissue sections from the upper airway and lung samples of infected adult and aged Ifnar1 -/- mice to evaluate the age-dependent disease phenotype of MA SARS-CoV-2. They performed transcriptome analyses using lung samples of infected or mock-treated adult and aged Ifnar1-/- mice. Furthermore, the reseachers performed gene set enrichment analyses (GSEA) comparing mock-treated or infected adult and aged Ifnar1-/- mice.
Study findings
The adult C57BL/6 mice initiated a rapid innate and adaptive immune response associated with high IFN-γ and low interleukin-10 (IL-10) expression. A timely response limited viral titers in the lungs, mediated rapid viral clearance, and efficiently prevented progression to severe disease.
The adult C57BL/6 mice lacking functional type I (Ifnar1-/-) or type III IFN receptors (Ifnlr1-/-) had about 10-fold increased viral loads in the lungs and upper airways. Notably, Ifnlr1-/- mice continued to have high viral titers in their lungs five days post-infection (dpi). Conversely, within five dpi, WT mice had cleared the SARS-CoV-2 infection.
Loss of type I and type III IFN signaling in Ifnar1-/- Ifnlr1-/- mice led to excessive and prolonged viral replication in the upper airways and lungs. Immunohistochemical analyses confirmed prolonged virus persistence in Ifnar1-/- Ifnlr1-/- mice compared with WT mice, with severe damage to lung tissues and bronchial epithelium.
On the other hand, adult Ifnar1-/- Ifngr1-/- mice lacking functional type I and type II IFN signaling showed severe weight loss and poor survival rates as their aged counterparts. Thus, demonstrating a defining role of diminished type I and type II IFN responses driving the age-dependent virulence of SARS-CoV-2. Interestingly, aged wt mice did not show any weight loss compared with adult controls.
Together these findings showed that age-dependent type I and type III IFN responses synergize to limit excessive SARS-CoV-2 replication, expedite virus clearance, and protection against symptomatic disease in aged mice.
In both adult and aged animals, the lung tissue damage was nearly identical, indicating functional impairment of the infected aged lung. Another plausible explanation could be that aged mice suffered from a systemic disease manifestation due to cytokine storm or viral dissemination to other organs. Accordingly, quantitative reverse transcription-polymerase chain reaction (RT-qPCR) analyses revealed elevated viral ribonucleic acid (RNA) levels in the heart and brain of aged mice but no viral RNA-positive cells, indicating that SARS-CoV-2 does not disseminate systemically in aged animals.
GSEA data showed that the age-dependent increase in viral replication was independent of type I and III IFN signaling but correlated with IFN-γ production, natural killer (NK) cell-mediated immunity, and immune cell activation.
While type I and type III IFNs are produced mainly by dendritic (DC) upon activation of pattern recognition receptors (PRRs), NK and T cells synthesize IFN-γ, with the involvement of transcription factors, Eomes and T-bet. The adult mice had significantly increased expression levels of T-bet in their infected lungs compared with aged mice.
Conclusions
The current study highlighted the need for further research to determine pathways and cell types involved in SARS-CoV-2 infection, specifically identifying cells that produce different types of IFNs.
IFN-γ or IFN-λ alone had limited success in preventing severe COVID-19 and related lethality. However, exogenous treatment by a combination of IFN-γ or IFNλ in test animals prevented age-related impairment of cellular immune responses, compensated for the compromised cell-intrinsic antiviral immunity in epithelial cells, and enhanced DC migration into draining lymph nodes facilitating T cell activation.
At present, both IFN-γ and IFN-λ treatments are already under evaluation in various clinical settings. IFN-γ is licensed to treat chronic granulomatous disease and is under evaluation for idiopathic pulmonary fibrosis treatment. Likewise, a recent clinical trial demonstrated the efficacy of IFN-λ treatment in accelerating SARS-CoV-2 clearance in patients.
In the future, broad-spectrum antivirals like type II and type III IFNs should become accessible for individuals at high risk of developing severe COVID-19.
*Important notice
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.
