Interferon Regulatory Factor 3 Exacerbates the Severity of COVID-19 in Mice

Abstract

Context: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) emerged in 2019, causing the COVID-19 pandemic. While most infected people experienced mild illness, others progressed to severe disease, characterized by hyperinflammation and respiratory distress. There is still much to learn about the innate immune response to this virus. Interferon regulatory factor 3 (IRF3) is a transcription factor that is activated when pattern recognition receptors detect viruses. Upon activation, IRF3 induces the expression of interferon beta (IFN-β) and interferon-stimulated genes, which protect the host from viral infection. However, coronaviruses antagonize this pathway, delaying type 1 IFN production. It is, therefore, unclear how IRF3 influences COVID-19 disease. Our prior reports showed that IRF3 promotes harmful inflammation during bacterial sepsis in mice.

Hypothesis: We hypothesized that IRF3 cannot effectively control the SARS-CoV-2 viral load and instead promotes harmful inflammation during severe COVID-19.

Methods and models: We used mice transgenic for the human angiotensin converting-enzyme 2 transgene, driven by the keratin 18 promoter (K18-ACE2 mice) that were IRF3 deficient or IRF3 sufficient to test how IRF3 influences COVID-19 disease.

Results: Upon infection with SARS-CoV-2, K18-ACE2 mice showed a dose-dependent disease, characterized by mortality, lethargy, weight loss, and lung pathology, reminiscent of clinical COVID-19. However, K18-ACE2 mice lacking IRF3 were protected from severe disease with reduced mortality (84.6% vs. 100%) and disease score. We found that IRF3 promoted IFN-β production in the lungs and reprogrammed the cytokine profile, while viral load in the lungs was similar in the presence or absence of IRF3.

Interpretations and conclusions: These data indicated that IRF3 played a detrimental role in murine COVID-19 associated with changes in IFN-β and inflammatory cytokines.

Keywords: COVID-19; K18-ACE2 mice; cytokines; human angiotensin-converting enzyme 2 transgenic mice; interferon regulatory factor 3; interferons.

Figure 1.

Interferon regulatory factor 3 (IRF3)-knockout (KO) mice are protected from severe COVID-19 disease. IRF3-KO and wild-type (WT) mice (both carrying the human angiotensin converting-enzyme 2 transgene, driven by the keratin 18 promoter [K18-ACE2] ) were infected with 2.5 × 10⁴ plaque forming units of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or saline vehicle as a control. Graphs show animal survival (A), animal disease score (B), and percent weight change (C), relative to the initial animal weight. p values show the results of a log-rank test (animal survival), and a two-way analysis of variance on the rank data, reflecting the group difference (for disease score and weight change). n.s. = not significant.

Figure 2.

Interferon regulatory factor 3 (IRF3) alters the inflammatory cytokine profile in the lungs after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. IRF3-knockout (KO) and wild-type (WT) mice (both carrying the human angiotensin converting-enzyme 2 transgene driven by the keratin 18 promoter [K18-ACE2]) were infected with 2.5 × 10⁴ plaque forming units of SARS-CoV-2 or saline vehicle as a control. Cohorts of animals were euthanized and their lung lobes collected at day 2 and day 4. Graphs show the levels of viral load (A), normalized to lung weight, and interferon beta (IFN-β) (B), interleukin (IL)-12p70 (C), and IL-10 (D), normalized to protein content. Each point represents the individual value for a single mouse lung lobe, and the line indicates the median in each group. p values show the result of a Wilcoxon rank-sum test.