Coronaviruses (CoVs) are by far the largest group of known positive-sense RNA viruses having an extensive range of natural hosts. In the past few decades, newly evolved Coronaviruses have posed a global threat to public health. The immune response is essential to control and eliminate CoV infections, however, maladjusted immune responses may result in immunopathology and impaired pulmonary gas exchange. Gaining a deeper understanding of the interaction between Coronaviruses and the innate immune systems of the hosts may shed light on the development and persistence of inflammation in the lungs and hopefully can reduce the risk of lung inflammation caused by CoVs. In this review, we provide an update on CoV infections and relevant diseases, particularly the host defense against CoV-induced inflammation of lung tissue, as well as the role of the innate immune system in the pathogenesis and clinical treatment.

SARS-CoV-2 is highly pathogenic in humans and poses a great threat to public health worldwide. Clinical data shows a disturbed type I interferon (IFN) response during the virus infection. In this study, we discovered that the nucleocapsid (N) protein of SARS-CoV-2 plays an important role in the inhibition of interferon beta (IFN-β) production. N protein repressed IFN-β production induced by poly(I:C) or upon Sendai virus (SeV) infection. We noted that N protein also suppressed IFN-β production, induced by several signaling molecules downstream of the retinoic acid-inducible gene I (RIG-I) pathway, which is the crucial pattern recognition receptor (PRR) responsible for identifying RNA viruses. Moreover, our data demonstrated that N protein interacted with the RIG-I protein through the DExD/H domain, which has ATPase activity and plays an important role in the binding of immunostimulatory RNAs. These results suggested that SARS-CoV-2 N protein suppresses the IFN-β response through targeting the initial step, potentially the cellular PRR–RNA-recognition step in the innate immune pathway. Therefore, we propose that the SARS-CoV-2 N protein represses IFN-β production by interfering with RIG-I.

One of the fundamental reactions of the innate immune responses to pathogen infection is the release of pro-inflammatory cytokines, including IL-1β, processed by the NLRP3 inflammasome. The stimulator of interferon genes (STING) has the essential roles in innate immune response against pathogen infections. Here we reveal a distinct mechanism by which STING regulates the NLRP3 inflammasome activation, IL-1β secretion, and inflammatory responses in human cell lines, mice primary cells, and mice. Interestingly, upon HSV-1 infection and cytosolic DNA stimulation, STING binds to NLRP3 and promotes the inflammasome activation through two approaches. First, STING recruits NLRP3 and facilitates NLRP3 localization in the endoplasmic reticulum, thereby facilitating the inflammasome formation. Second, STING interacts with NLRP3 and attenuates K48-and K63-linked polyubiquitination of NLRP3, thereby promoting the inflammasome activation. Collectively, we demonstrate that the cGAS-STING-NLRP3 signaling is essential for host defense against HSV-1 infection.

COVID‐19 is infected by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) and can cause severe multiple organ injury and death. Kidney is one of major target organs of COVID‐19 and acute kidney injury (AKI) is common in critically ill COVID‐19 patients. However, mechanisms through which COVID‐19 causes AKI remain largely unknown and treatment remains unspecific and ineffective. Here, the authors report that normal kidney‐specifically overexpressing SARS‐CoV‐2 N develops AKI, which worsens in mice under ischemic condition. Mechanistically, it is uncovered that SARS‐CoV‐2 N‐induced AKI is Smad3‐dependent as SARS‐CoV‐2 N protein can interact with Smad3 and enhance TGF‐β/Smad3 signaling to cause tubular epithelial cell death and AKI via the G1 cell cycle arrest mechanism. This is further confirmed in Smad3 knockout mice and cells in which deletion of Smad3 protects against SARS‐CoV‐2 N protein‐induced cell death and AKI in vivo and in vitro. Most significantly, it is also found that targeting Smad3 with a Smad3 pharmacological inhibitor is able to inhibit SARS‐CoV‐2 N‐induced AKI. In conclusion, the authors identify that SARS‐CoV‐2 N protein is a key mediator for AKI and induces AKI via the Smad3‐dependent G1 cell cycle arrest mechanism. Targeting Smad3 may represent as a novel therapy for COVID‐19‐asscoaited AKI.

Sepsis is characterized by systematic inflammation and contributes to cardiac dysfunction. This study was designed to examine the effect of Akt activation on LPS-induced cardiac anomalies and underlying mechanism(s) involved. Mechanical and intracellular Ca2+ properties were examined in myocardium from wild-type and transgenic mice with cardiac-specific chronic Akt overexpression following LPS (4 mg/kg, i.p.) challenge. Akt signaling cascade (Akt, PTEN, GSK3β), stress signal (ERK, JNK, p38), apoptotic markers (BAX, caspase-3/-9), ER stress markers (GRP78, GADD153, eIF2α), inflammatory markers (TNFα, IL-1β, IL-6) and autophagic markers (Beclin-1, LC3B, Atg7 and p62) were evaluated. Our results revealed that LPS induced marked decrease in ejection fraction, fractional shortening, cardiomyocyte contractile capacity with dampened intracellular Ca2+ release and clearance, elevated ROS generation and decreased GSH/GSSG ratio, increased ERK, JNK, p38, GRP78, GADD153, eIF2α, BAX, caspase-3 and - 9, downregulated Bcl-2, the effects of which were significantly attenuated or obliterated by Akt activation. Akt activation itself did not affect cardiac contractile and intracellular Ca2+ properties, ROS production, oxidative stress, apoptosis and ER stress. In addition, LPS upregulated levels of Beclin-1, LC3B and Atg7, while suppressing p62 accumulation. Akt activation did not affect Beclin-1, LC3B, Atg7 and p62 in the presence or absence of LPS. Akt overexpression promoted phosphorylation of Akt and GSK3β. In vitro study using the GSK3β inhibitor SB216763 mimicked the response elicited by chronic Akt activation. Taken together, these data showed that Akt activation ameliorated LPS-induced cardiac contractile and intracellular Ca2+ anomalies through inhibition of apoptosis and ER stress, possibly involving an Akt/GSK3β-dependent mechanism.