A SARS-CoV-2 Infection Model in Mice Demonstrates Protection by Neutralizing Antibodies.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a pandemic with millions of human infections. One limitation to the evaluation of potential therapies and vaccines to inhibit SARS-CoV-2 infection and ameliorate disease is the lack of susceptible small animals in large numbers. Commercially available laboratory strains of mice are not readily infected by SARS-CoV-2 because of species-specific differences in their angiotensin-converting enzyme 2 (ACE2) receptors. Here, we transduced replication-defective adenoviruses encoding human ACE2 via intranasal administration into BALB/c mice and established receptor expression in lung tissues. hACE2-transduced mice were productively infected with SARS-CoV-2, and this resulted in high viral titers in the lung, lung pathology, and weight loss. Passive transfer of a neutralizing monoclonal antibody reduced viral burden in the lung and mitigated inflammation and weight loss. The development of an accessible mouse model of SARS-CoV-2 infection and pathogenesis will expedite the testing and deployment of therapeutics and vaccines.

Generation of a Broadly Useful Model for COVID-19 Pathogenesis, Vaccination, and Treatment.

COVID-19, caused by SARS-CoV-2, is a virulent pneumonia, with >4,000,000 confirmed cases worldwide and >290,000 deaths as of May 15, 2020. It is critical that vaccines and therapeutics be developed very rapidly. Mice, the ideal animal for assessing such interventions, are resistant to SARS-CoV-2. Here, we overcome this difficulty by exogenous delivery of human ACE2 with a replication-deficient adenovirus (Ad5-hACE2). Ad5-hACE2-sensitized mice developed pneumonia characterized by weight loss, severe pulmonary pathology, and high-titer virus replication in lungs. Type I interferon, T cells, and, most importantly, signal transducer and activator of transcription 1 (STAT1) are critical for virus clearance and disease resolution in these mice. Ad5-hACE2-transduced mice enabled rapid assessments of a vaccine candidate, of human convalescent plasma, and of two antiviral therapies (poly I:C and remdesivir). In summary, we describe a murine model of broad and immediate utility to investigate COVID-19 pathogenesis and to evaluate new therapies and vaccines.

Eicosanoid signalling blockade protects middle-aged mice from severe COVID-19.

Coronavirus disease 2019 (COVID-19) is especially severe in aged populations1. Vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are highly effective, but vaccine efficacy is partly compromised by the emergence of SARS-CoV-2 variants with enhanced transmissibility2. The emergence of these variants emphasizes the need for further development of anti-SARS-CoV-2 therapies, especially for aged populations. Here we describe the isolation of highly virulent mouse-adapted viruses and use them to test a new therapeutic drug in infected aged animals. Many of the alterations observed in SARS-CoV-2 during mouse adaptation (positions 417, 484, 493, 498 and 501 of the spike protein) also arise in humans in variants of concern2. Their appearance during mouse adaptation indicates that immune pressure is not required for selection. For murine SARS, for which severity is also age dependent, elevated levels of an eicosanoid (prostaglandin D2 (PGD2)) and a phospholipase (phospholipase A2 group 2D (PLA2G2D)) contributed to poor outcomes in aged mice3,4. mRNA expression of PLA2G2D and prostaglandin D2 receptor (PTGDR), and production of PGD2 also increase with ageing and after SARS-CoV-2 infection in dendritic cells derived from human peripheral blood mononuclear cells. Using our mouse-adapted SARS-CoV-2, we show that middle-aged mice lacking expression of PTGDR or PLA2G2D are protected from severe disease. Furthermore, treatment with a PTGDR antagonist, asapiprant, protected aged mice from lethal infection. PTGDR antagonism is one of the first interventions in SARS-CoV-2-infected animals that specifically protects aged animals, suggesting that the PLA2G2D-PGD2/PTGDR pathway is a useful target for therapeutic interventions.

COVID-19 treatments and pathogenesis including anosmia in K18-hACE2 mice.

The ongoing coronavirus disease 2019 (COVID-19) pandemic is associated with substantial morbidity and mortality. Although much has been learned in the first few months of the pandemic, many features of COVID-19 pathogenesis remain to be determined. For example, anosmia is a common presentation, and many patients with anosmia show no or only minor respiratory symptoms1. Studies in animals infected experimentally with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of COVID-19, provide opportunities to study aspects of the disease that are not easily investigated in human patients. Although the severity of COVID-19 ranges from asymptomatic to lethal2, most experimental infections provide insights into mild disease3. Here, using K18-hACE2 transgenic mice that were originally developed for SARS studies4, we show that infection with SARS-CoV-2 causes severe disease in the lung and, in some mice, the brain. Evidence of thrombosis and vasculitis was detected in mice with severe pneumonia. Furthermore, we show that infusion of convalescent plasma from a recovered patient with COVID-19 protected against lethal disease. Mice developed anosmia at early time points after infection. Notably, although pre-treatment with convalescent plasma prevented most signs of clinical disease, it did not prevent anosmia. Thus, K18-hACE2 mice provide a useful model for studying the pathological basis of both mild and lethal COVID-19 and for assessing therapeutic interventions.

The NIH Somatic Cell Genome Editing program.

The move from reading to writing the human genome offers new opportunities to improve human health. The United States National Institutes of Health (NIH) Somatic Cell Genome Editing (SCGE) Consortium aims to accelerate the development of safer and more-effective methods to edit the genomes of disease-relevant somatic cells in patients, even in tissues that are difficult to reach. Here we discuss the consortium's plans to develop and benchmark approaches to induce and measure genome modifications, and to define downstream functional consequences of genome editing within human cells. Central to this effort is a rigorous and innovative approach that requires validation of the technology through third-party testing in small and large animals. New genome editors, delivery technologies and methods for tracking edited cells in vivo, as well as newly developed animal models and human biological systems, will be assembled-along with validated datasets-into an SCGE Toolkit, which will be disseminated widely to the biomedical research community. We visualize this toolkit-and the knowledge generated by its applications-as a means to accelerate the clinical development of new therapies for a wide range of conditions.

Adaptation of SARS-CoV-2 to ACE2H353K mice reveals new spike residues that drive mouse infection.

The Coronavirus Disease 2019 (COVID-19) pandemic continues to cause extraordinary loss of life and economic damage. Animal models of severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2) infection are needed to better understand disease pathogenesis and evaluate preventive measures and therapies. While mice are widely used to model human disease, mouse angiotensin converting enzyme 2 (ACE2) does not bind the ancestral SARS-CoV-2 spike protein to mediate viral entry. To overcome this limitation, we "humanized" mouse Ace2 using CRISPR gene editing to introduce a single amino acid substitution, H353K, predicted to facilitate S protein binding. While H353K knockin Ace2 (mACE2H353K) mice supported SARS-CoV-2 infection and replication, they exhibited minimal disease manifestations. Following 30 serial passages of ancestral SARS-CoV-2 in mACE2H353K mice, we generated and cloned a more virulent virus. A single isolate (SARS2MA-H353K) was prepared for detailed studies. In 7-11-month-old mACE2H353K mice, a 104 PFU inocula resulted in diffuse alveolar disease manifested as edema, hyaline membrane formation, and interstitial cellular infiltration/thickening. Unexpectedly, the mouse-adapted virus also infected standard BALB/c and C57BL/6 mice and caused severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions.IMPORTANCEWe developed a new mouse model with a humanized angiotensin converting enzyme 2 (ACE2) locus that preserves native regulatory elements. A single point mutation in mouse ACE2 (H353K) was sufficient to confer in vivo infection with ancestral severe acute respiratory syndrome-coronavirus-2 virus. Through in vivo serial passage, a virulent mouse-adapted strain was obtained. In aged mACE2H353K mice, the mouse-adapted strain caused diffuse alveolar disease. The mouse-adapted virus also infected standard BALB/c and C57BL/6 mice, causing severe disease. The mouse-adapted virus acquired five new missense mutations including two in spike (K417E, Q493K), one each in nsp4, nsp9, and M and a single nucleotide change in the 5' untranslated region. The Q493K spike mutation arose early in serial passage and is predicted to provide affinity-enhancing molecular interactions with mACE2 and further increase the stability and affinity to the receptor. This new model and mouse-adapted virus will be useful to evaluate COVID-19 disease and prophylactic and therapeutic interventions.

Avian influenza A viruses exhibit plasticity in sialylglycoconjugate receptor usage in human lung cells.

IMPORTANCE: It is well known that influenza A viruses (IAV) initiate host cell infection by binding to sialic acid, a sugar molecule present at the ends of various sugar chains called glycoconjugates. These sugar chains can vary in chain length, structure, and composition. However, it remains unknown if IAV strains preferentially bind to sialic acid on specific glycoconjugate type(s) for host cell infection. Here, we utilized CRISPR gene editing to abolish sialic acid on different glycoconjugate types in human lung cells, and evaluated human versus avian IAV infections. Our studies show that both human and avian IAV strains can infect human lung cells by utilizing any of the three major sialic acid-containing glycoconjugate types, specifically N-glycans, O-glycans, and glycolipids. Interestingly, simultaneous elimination of sialic acid on all three major glycoconjugate types in human lung cells dramatically decreased human IAV infection, yet had little effect on avian IAV infection. These studies show that avian IAV strains effectively utilize other less prevalent glycoconjugates for infection, whereas human IAV strains rely on a limited repertoire of glycoconjugate types. The remarkable ability of avian IAV strains to utilize diverse glycoconjugate types may allow for easy transmission into new host species.

The first few days of a SARS-CoV-2 infection viewed at single-cell resolution.

What transpires soon after inhaling Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the respiratory virus causing Coronavirus Disease 2019 (COVID-19)? Where does infection begin? What are the features of subsequent virus spread? How might host responses quickly contain infection? Two recently published manuscripts have evaluated infection in primary cultures of well-differentiated cells to address these questions and bring more light on the proviral and antiviral components operating during the initial days after SARS-CoV-2 exposure.

IL-13 induced inflammation increases DPP4 abundance but does not enhance MERS-CoV replication in airway epithelia.

BACKGROUND: Chronic pulmonary conditions such as asthma and COPD increase the risk of morbidity and mortality during infection with the Middle East respiratory syndrome coronavirus (MERS-CoV). We hypothesized that individuals with such comorbidities are more susceptible to MERS-CoV infection due to increased expression of its receptor, dipeptidyl peptidase 4 (DPP4). METHODS: We modeled chronic airway disease by treating primary human airway epithelia with the Th2 cytokine IL-13, examining how this impacted DPP4 protein levels along with MERS-CoV entry and replication. RESULTS: IL-13 exposure for 3 days led to increased DPP4 protein abundance, while a 21-day treatment increased DPP4 levels and caused goblet cell metaplasia. Surprisingly, despite this increase in receptor availability, MERS-CoV entry and replication were not significantly impacted by IL-13 treatment. CONCLUSIONS: Our results suggest that increased DPP4 abundance is likely not the primary mechanism leading to increased MERS severity in the setting of Th2 inflammation. Transcriptional profiling analysis highlighted the complexity of IL-13 induced changes in airway epithelia, including altered expression of genes involved in innate immunity, antiviral responses, and maintenance of the extracellular mucus barrier. These data suggest that additional factors likely interact with DPP4 abundance to determine MERS-CoV infection outcomes.

Functional correction of CFTR mutations in human airway epithelial cells using adenine base editors.

Mutations in the CFTR gene that lead to premature stop codons or splicing defects cause cystic fibrosis (CF) and are not amenable to treatment by small-molecule modulators. Here, we investigate the use of adenine base editor (ABE) ribonucleoproteins (RNPs) that convert A•T to G•C base pairs as a therapeutic strategy for three CF-causing mutations. Using ABE RNPs, we corrected in human airway epithelial cells premature stop codon mutations (R553X and W1282X) and a splice-site mutation (3849 + 10 kb C > T). Following ABE delivery, DNA sequencing revealed correction of these pathogenic mutations at efficiencies that reached 38-82% with minimal bystander edits or indels. This range of editing was sufficient to attain functional correction of CFTR-dependent anion channel activity in primary epithelial cells from CF patients and in a CF patient-derived cell line. These results demonstrate the utility of base editor RNPs to repair CFTR mutations that are not currently treatable with approved therapeutics.

Middle East Respiratory Syndrome Coronavirus Gene 5 Modulates Pathogenesis in Mice.

Middle East respiratory syndrome coronavirus (MERS-CoV) causes a highly lethal pneumonia that emerged in 2012. There is limited information on MERS-CoV pathogenesis, as data from patients are scarce and the generation of animal models reproducing MERS clinical manifestations has been challenging. Human dipeptidyl peptidase 4 knock-in (hDPP4-KI) mice and a mouse-adapted MERS-CoV strain (MERSMA-6-1-2) were recently described. hDPP4-KI mice infected with MERSMA-6-1-2 show pathological signs of respiratory disease, high viral titers in the lung, and death. In this work, a mouse-adapted MERS-CoV infectious cDNA was engineered by introducing nonsynonymous mutations contained in the MERSMA-6-1-2 genome into a MERS-CoV infectious cDNA, leading to a recombinant mouse-adapted virus (rMERS-MA) that was virulent in hDDP4-KI mice. MERS-CoV adaptation to cell culture or mouse lungs led to mutations and deletions in genus-specific gene 5 that prevented full-length protein expression. In contrast, analysis of 476 MERS-CoV field isolates showed that gene 5 is highly stable in vivo in both humans and camels. To study the role of protein 5, two additional viruses were engineered expressing a full-length gene 5 (rMERS-MA-5FL) or containing a complete gene 5 deletion (rMERS-MA-Δ5). rMERS-MA-5FL virus was unstable, as deletions appeared during passage in different tissue culture cells, highlighting MERS-CoV instability. The virulence of rMERS-MA-Δ5 was analyzed in a sublethal hDPP4-KI mouse model. Unexpectedly, all mice died after infection with rMERS-MA-Δ5, in contrast to those infected with the parental virus, which contains a 17-nucleotide (nt) deletion and a stop codon in protein 5 at position 108. Expression of interferon and proinflammatory cytokines was delayed and dysregulated in the lungs of rMERS-MA-Δ5-infected mice. Overall, these data indicated that the rMERS-MA-Δ5 virus was more virulent than the parental one and suggest that the residual gene 5 sequence present in the mouse-adapted parental virus had a function in ameliorating severe MERS-CoV pathogenesis.IMPORTANCE Middle East respiratory syndrome coronavirus (MERS-CoV) is a zoonotic virus causing human infections with high mortality rate (∼35%). Animal models together with reverse-genetics systems are essential to understand MERS-CoV pathogenesis. We developed a reverse-genetics system for a mouse-adapted MERS-CoV that reproduces the virus behavior observed in humans. This system is highly useful to investigate the role of specific viral genes in pathogenesis. In addition, we described a virus lacking gene 5 expression that is more virulent than the parental one. The data provide novel functions in IFN modulation for gene 5 in the context of viral infection and will help to develop novel antiviral strategies.

Intersubject Variation in ACE2 Protein Expression in Human Airway Epithelia and Its Relationship to Severe Acute Respiratory Syndrome Coronavirus 2.

BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 ) initiates entry into airway epithelia by binding its receptor, angiotensin-converting enzyme 2 (ACE2). METHODS: To explore whether interindividual variation in ACE2 abundance contributes to variability in coronavirus disease 2019 (COVID-19) outcomes, we measured ACE2 protein abundance in primary airway epithelial cultures derived from 58 human donor lungs. RESULTS: We found no evidence for sex- or age-dependent differences in ACE2 protein expression. Furthermore, we found that variations in ACE2 abundance had minimal effects on viral replication and induction of the interferon response in airway epithelia infected with SARS-CoV-2. CONCLUSIONS: Our results highlight the relative importance of additional host factors, beyond viral receptor expression, in determining COVID-19 lung disease outcomes.

Advances in gene therapy for cystic fibrosis lung disease.

Cystic fibrosis (CF) is a multiorgan recessive genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Gene therapy efforts have focused on treating the lung, since it manifests the most significant life-threatening disease. Over two decades have past since the first CF lung gene therapy trials and significant advances in the therapeutic implementation of pharmacologic CFTR modulators have renewed the field's focus on developing gene therapies for the 10% of CF patients these modulators cannot help. This review summarizes recent progress made in developing vectors for airway transduction and CF animal models required for understanding the relevant cellular targets in the lung and testing the efficacy of gene therapy approaches. We also highlight future opportunities in emerging gene editing strategies that may offer advantages for treating diseases like CF where the gene target is highly regulated at the cellular level. The outcomes of CF lung gene therapy trials will likely inform productive paths toward gene therapy for other complex genetic disorders, while also advancing treatments for all CF patients.

Airway Surface Liquid Has Innate Antiviral Activity That Is Reduced in Cystic Fibrosis.

Although chronic bacterial infections and inflammation are associated with progressive lung disease in patients with cystic fibrosis (CF), much less is known regarding the contributions of respiratory viral infections to this process. Clinical studies suggest that antiviral host defenses may be compromised in individuals with CF, and CF airway epithelia exhibit impaired antiviral responses in vitro. Here, we used the CF pig model to test the hypothesis that the antiviral activity of respiratory secretions is reduced in CF. We developed an in vitro assay to measure the innate antiviral activity present in airway surface liquid (ASL) from CF and non-CF pigs. We found that tracheal and nasal ASL from newborn non-CF pigs exhibited dose-dependent inhibitory activity against several enveloped and encapsidated viruses, including Sendai virus, respiratory syncytial virus, influenza A, and adenovirus. Importantly, we found that the anti-Sendai virus activity of nasal ASL from newborn CF pigs was significantly diminished relative to non-CF littermate controls. This diminution of extracellular antiviral defenses appears to be driven, at least in part, by the differences in pH between CF and non-CF ASL. These data highlight the novel antiviral properties of native airway secretions and suggest the possibility that defects in extracellular antiviral defenses contribute to CF pathogenesis.

Mouse-adapted MERS coronavirus causes lethal lung disease in human DPP4 knockin mice.

The Middle East respiratory syndrome (MERS) emerged in Saudi Arabia in 2012, caused by a zoonotically transmitted coronavirus (CoV). Over 1,900 cases have been reported to date, with ∼36% fatality rate. Lack of autopsies from MERS cases has hindered understanding of MERS-CoV pathogenesis. A small animal model that develops progressive pulmonary manifestations when infected with MERS-CoV would advance the field. As mice are restricted to infection at the level of DPP4, the MERS-CoV receptor, we generated mice with humanized exons 10-12 of the mouse Dpp4 locus. Upon inoculation with MERS-CoV, human DPP4 knockin (KI) mice supported virus replication in the lungs, but developed no illness. After 30 serial passages through the lungs of KI mice, a mouse-adapted virus emerged (MERSMA) that grew in lungs to over 100 times higher titers than the starting virus. A plaque-purified MERSMA clone caused weight loss and fatal infection. Virus antigen was observed in airway epithelia, pneumocytes, and macrophages. Pathologic findings included diffuse alveolar damage with pulmonary edema and hyaline membrane formation associated with accumulation of activated inflammatory monocyte-macrophages and neutrophils in the lungs. Relative to the parental MERS-CoV, MERSMA viruses contained 13-22 mutations, including several within the spike (S) glycoprotein gene. S-protein mutations sensitized viruses to entry-activating serine proteases and conferred more rapid entry kinetics. Recombinant MERSMA bearing mutant S proteins were more virulent than the parental virus in hDPP4 KI mice. The hDPP4 KI mouse and the MERSMA provide tools to investigate disease causes and develop new therapies.

A Novel AAV-mediated Gene Delivery System Corrects CFTR Function in Pigs.

Cystic fibrosis is an autosomal-recessive disease that is caused by a mutant CFTR (cystic fibrosis transmembrane conductance regulator) gene and is characterized by chronic bacterial lung infections and inflammation. Complementation with functional CFTR normalizes anion transport across the airway surface. Adeno-associated virus (AAV) is a useful vector for gene therapy because of its low immunogenicity and ability to persist for months to years. However, because its episomal expression may decrease after cell division, readministration of the AAV vector may be required. To overcome this, we designed an integrating AAV-based CFTR-expressing vector, termed piggyBac (PB)/AAV, carrying CFTR flanked by the terminal repeats of the piggyBac transposon. With codelivery of the piggyBac transposase, PB/AAV can integrate into the host genome. Because of the packaging constraints of AAV, careful consideration was required to ensure that the vector would package and express its CFTR cDNA cargo. In this short-term study, PB/AAV-CFTR was aerosolized to the airways of CF pigs in the absence of the transposase. Two weeks later, transepithelial Cl- current was restored in freshly excised tracheal and bronchial tissue. Additionally, we observed an increase in tracheal airway surface liquid pH and bacterial killing in comparison with untreated CF pigs. Airway surface liquid from primary airway cells cultured from treated CF pigs exhibited increased pH correlating with decreased viscosity. Together, these results show that complementing CFTR in CF pigs with PB/AAV rescues the anion transport defect in a large-animal CF model. Delivery of this integrating viral vector system to airway progenitor cells could lead to persistent, life-long expression in vivo.

The tetraspanin CD9 facilitates MERS-coronavirus entry by scaffolding host cell receptors and proteases.

Infection by enveloped coronaviruses (CoVs) initiates with viral spike (S) proteins binding to cellular receptors, and is followed by proteolytic cleavage of receptor-bound S proteins, which prompts S protein-mediated virus-cell membrane fusion. Infection therefore requires close proximity of receptors and proteases. We considered whether tetraspanins, scaffolding proteins known to facilitate CoV infections, hold receptors and proteases together on cell membranes. Using knockout cell lines, we found that the tetraspanin CD9, but not the tetraspanin CD81, formed cell-surface complexes of dipeptidyl peptidase 4 (DPP4), the MERS-CoV receptor, and the type II transmembrane serine protease (TTSP) member TMPRSS2, a CoV-activating protease. This CD9-facilitated condensation of receptors and proteases allowed MERS-CoV pseudoviruses to enter cells rapidly and efficiently. Without CD9, MERS-CoV viruses were not activated by TTSPs, and they trafficked into endosomes to be cleaved much later and less efficiently by cathepsins. Thus, we identified DPP4:CD9:TTSP as the protein complexes necessary for early, efficient MERS-CoV entry. To evaluate the importance of these complexes in an in vivo CoV infection model, we used recombinant Adenovirus 5 (rAd5) vectors to express human DPP4 in mouse lungs, thereby sensitizing the animals to MERS-CoV infection. When the rAd5-hDPP4 vectors co-expressed small RNAs silencing Cd9 or Tmprss2, the animals were significantly less susceptible, indicating that CD9 and TMPRSS2 facilitated robust in vivo MERS-CoV infection of mouse lungs. Furthermore, the S proteins of virulent mouse-adapted MERS-CoVs acquired a CD9-dependent cell entry character, suggesting that CD9 is a selective agent in the evolution of CoV virulence.

Proteolytic processing of Middle East respiratory syndrome coronavirus spikes expands virus tropism.

Middle East respiratory syndrome coronavirus (MERS-CoV) infects humans from zoonotic sources and causes severe pulmonary disease. Virions require spike (S) glycoproteins for binding to cell receptors and for catalyzing virus-cell membrane fusion. Fusion occurs only after S proteins are cleaved sequentially, first during their secretion through the exocytic organelles of virus-producing cells, and second after virus binding to target-cell receptors. To more precisely determine how sequential proteolysis contributes to CoV infection, we introduced S mutations obstructing the first cleavages. These mutations severely compromised MERS-CoV infection into human lung-derived cells, but had little effect on infection into several other cell types. These cell type-specific requirements for proteolysis correlated with S conformations during cell entry. Without the first cleavages, S proteins resisted cell receptor-induced conformational changes, which restricted the second, fusion-activating cleavages. Consistent with these findings, precleaved MERS viruses used receptor-proximal, cell-surface proteases to effect the second fusion-activating cleavages during cell entry, whereas the more rigid uncleaved MERS viruses trafficked past these cell-surface proteases and into endosomes. Uncleaved viruses were less infectious to human airway epithelial and Calu3 cell cultures because they lacked sufficient endosomal fusion-activating proteases. Thus, by sensitizing viruses to receptor-induced conformational changes, the first S cleavages expand virus tropism to cell types that are relevant to lung infection, and therefore may be significant determinants of MERS-CoV virulence.

Attenuation of pulmonary ACE2 activity impairs inactivation of des-Arg9 bradykinin/BKB1R axis and facilitates LPS-induced neutrophil infiltration.

Angiotensin-converting enzyme 2 (ACE2) is a terminal carboxypeptidase with important functions in the renin-angiotensin system and plays a critical role in inflammatory lung diseases. ACE2 cleaves single-terminal residues from several bioactive peptides such as angiotensin II. However, few of its substrates in the respiratory tract have been identified, and the mechanism underlying the role of ACE2 in inflammatory lung disease has not been fully characterized. In an effort to identify biological targets of ACE2 in the lung, we tested its effects on des-Arg9 bradykinin (DABK) in airway epithelial cells on the basis of the hypothesis that DABK is a biological substrate of ACE2 in the lung and ACE2 plays an important role in the pathogenesis of acute lung inflammation partly through modulating DABK/bradykinin receptor B1 (BKB1R) axis signaling. We found that loss of ACE2 function in mouse lung in the setting of endotoxin inhalation led to activation of the DABK/BKB1R axis, release of proinflammatory chemokines such as C-X-C motif chemokine 5 (CXCL5), macrophage inflammatory protein-2 (MIP2), C-X-C motif chemokine 1 (KC), and TNF-α from airway epithelia, increased neutrophil infiltration, and exaggerated lung inflammation and injury. These results indicate that a reduction in pulmonary ACE2 activity contributes to the pathogenesis of lung inflammation, in part because of an impaired ability to inhibit DABK/BKB1R axis-mediated signaling, resulting in more prompt onset of neutrophil infiltration and more severe inflammation in the lung. Our study identifies a biological substrate of ACE2 within the airways, as well as a potential new therapeutic target for inflammatory diseases.

Lack of cystic fibrosis transmembrane conductance regulator disrupts fetal airway development in pigs.

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function causes cystic fibrosis (CF), predisposing the lungs to chronic infection and inflammation. In young infants with CF, structural airway defects are increasingly recognized before the onset of significant lung disease, which suggests a developmental origin and a possible role in lung disease pathogenesis. The role(s) of CFTR in lung development is unclear and developmental studies in humans with CF are not feasible. Young CF pigs have structural airway changes and develop spontaneous postnatal lung disease similar to humans; therefore, we studied lung development in the pig model (non-CF and CF). CF trachea and proximal airways had structural lesions detectable as early as pseudoglandular development. At this early developmental stage, budding CF airways had smaller, hypo-distended lumens compared to non-CF airways. Non-CF lung explants exhibited airway lumen distension in response to forskolin/IBMX as well as to fibroblast growth factor (FGF)-10, consistent with CFTR-dependent anion transport/secretion, but this was lacking in CF airways. We studied primary pig airway epithelial cell cultures and found that FGF10 increased cellular proliferation (non-CF and CF) and CFTR expression/function (in non-CF only). In pseudoglandular stage lung tissue, CFTR protein was exclusively localized to the leading edges of budding airways in non-CF (but not CF) lungs. This discreet microanatomic localization of CFTR is consistent with the site, during branching morphogenesis, where airway epithelia are responsive to FGF10 regulation. In summary, our results suggest that the CF proximal airway defects originate during branching morphogenesis and that the lack of CFTR-dependent anion transport/liquid secretion likely contributes to these hypo-distended airways.