Attenuated replication and damaging effects of SARS-CoV-2 Omicron in an intestinal epithelial barrier model (preprint)

Many COVID-19 patients suffer from gastrointestinal symptoms and impaired intestinal barrier function may play a key role in Long COVID. Despite its importance, the impact of SARS-CoV-2 on intestinal epithelia is poorly understood. To address this, we established an intestinal barrier model integrating epithelial Caco-2 cells, mucus-secreting HT29 cells and human Raji cells. This gut epithelial model allows efficient differentiation of Caco-2 cells into microfold-like cells, faithfully mimics intestinal barrier function, and is highly permissive to SARS-CoV-2 infection. Early strains of SARS-CoV-2 and the Delta variant replicated with high efficiency, severely disrupted barrier function, and depleted tight junction proteins, such as claudin-1, occludin and ZO-1. In comparison, Omicron subvariants also depleted ZO-1 from tight junctions but had fewer damaging effects on mucosal integrity and barrier function. Remdesivir and the TMPRSS2 inhibitor Camostat prevented SARS-CoV-2 replication and thus epithelial barrier damage, while the Cathepsin inhibitor E64d was ineffective. Our results support that SARS-CoV-2 disrupts intestinal barrier function but further suggest that circulating Omicron variants are less damaging than earlier viral strains.

Effect of mRNA vaccination on pulmonary sequelae after mild COVID-19 (preprint)

Background: Previous studies indicate a protective role for SARS-CoV-2 vaccination against development of pulmonary post-acute sequelae of COVID (PASC). We compared clinical, imaging, histopathology and ultrastructural features of pulmonary PASC with and without prior vaccination in a consecutive cohort of 54 unvaccinated, 17 partially vaccinated and 28 fully vaccinated patients who presented with dyspnea on exertion after mild COVID-19 (without hospitalization). Methods: Patients underwent full clinical evaluation including autoantibody (ANA/ENA) serology, high-resolution computed tomography (HRCT), bronchioloalveolar lavage fluid (BAL) analysis and transbronchial biopsy followed by histopathological and ultrastructural analysis and SARS-CoV-2 immunohistochemistry. Results: While vaccinated patients were younger (p=0.0056), included more active smokers (p=0.0135) and a longer interval since infection (35 vs. 17 weeks, p=0.0002), dyspnea on exertion and impaired lung function were not different between vaccinated and unvaccinated patients. Ground glass opacities in HRCT and centrilobular fibrosis were more frequent in unvaccinated patients (p=0.0154 and p=0.0353), but presence of autoantibodies, BAL lymphocytosis and bronchiolitis were common findings in all groups. While vaccination against SARS-CoV-2 is associated with a longer time span between infection and consultation along with a reduced frequency of ground glass opacities and centrilobular fibrosis, impaired lung function, bronchiolitis and presence of autoantibodies are comparable between vaccinated and unvaccinated patients. Residual virus was not detected in lung tissue in all but 1 patient. Conclusion: While differences between the investigated groups with regard to age, smoking status and SARS-CoV-2 variants have to be taken into account, a proposed protective role of SARS-CoV-2 vaccination against pulmonary PASC is so far not fully explained by clinical and histopathology findings.

Determinants of species-specific utilization of ACE2 by human and animal coronaviruses (preprint)

Utilization of human ACE2 allowed several bat coronaviruses (CoVs), including the causative agent of COVID-19, to infect humans either directly or via intermediate hosts. Here, we analyzed the ability of Spike proteins from 24 human or animal CoVs to use ACE2 receptors across nine reservoir, potential intermediate and human hosts. We show that overall SARS-CoV-2 Omicron variants evolved more efficient ACE2 usage but mutation of R493Q in BA.5 Spike disrupts utilization of ACE2 from Greater horseshoe bats. Spikes from most CoVs showed species-specific differences in ACE2 usage, partly due to variations in ACE2 residues 31, 41 or 354. Mutation of T403R allowed the RaTG13 bat CoV Spike to use all ACE2 orthologs analysed for viral entry. Sera from COVID-19 vaccinated individuals neutralized the Spike proteins of a range of bat Sarbecoviruses. Our results define determinants of ACE2 receptor usage of diverse CoVs and suggest that COVID-19 vaccination may protect against future zoonoses of SARS-CoV-related bat viruses.

Clinical, imaging, serological, and histopathological features of pulmonary post-acute sequelae after mild COVID-19 (PASC) (preprint)

Background: A significant proportion of patients experience prolonged pulmonary, cardiocirculatory or neuropsychiatric symptoms after Coronavirus disease 2019 (COVID-19), termed post-acute sequelae of COVID (PASC). Lung manifestations of PASC include cough, dyspnea on exertion and persistent radiologic abnormalities and have been linked to viral persistence, ongoing inflammation and immune dysregulation. So far, there is limited data on lung histopathology and tissue-based immune cell subtyping in PASC. Methods: 51 unvaccinated patients (median age, 40 years; 43% female) with a median of 17 weeks (range, 2-55 weeks) after mild SARS-CoV-2 infection (without hospitalization) underwent full clinical evaluation including high-resolution computed tomography (HR-CT) and transbronchial biopsy. We used RT-PCR/FISH and immunohistochemistry (nucleocapsid/spike/CD3/CD4/CD8) for residual SARS-CoV-2 detection and T lymphocyte subtyping, respectively. We assessed interstitial fibrosis and macrophage profiles by transmission electron microscopy (TEM) and immunofluorescence multiplex staining, while cytokine profiling in broncho-alveolar lavage (BAL) fluid was performed by legendplex immunoassay. Results: Dyspnea on exertion was the leading symptom of pulmonary PASC in our cohort. In 16% and 42.9% of patients, FEV1 and MEF50 were [≤]80% and 35.3% showed low attenuation volume (LAV) in >5% of lung area, in line with airflow obstruction. There was a significant correlation between oxygen pulse and time since COVID (p=0.009). Histopathologically, PASC manifested as organizing pneumonia (OP), fibrinous alveolitis and increased CD4+ T cell infiltrate predominantly around airways (bronchiolitis), while the residual virus components were detectable in only a single PASC patient (2%). T cell infiltrates around small airways were inversely correlated with time since COVID, however, this trend failed to reach statistical significance. We identified discrete interstitial fibrosis and a pro-fibrotic macrophage subtype (CD68/CD163/S100A9) as well as significantly elevated interleukin 1{beta} in BAL fluid from PASC patients (p=0.01), but H-scores for fibrotic macrophage population did not correlate with severity of clinical symptoms or T cell infiltration. Interpretation: We show decreased FEV1/MEF50 and increased LAV in line with obstructive lung disease due to CD4+ T cell-predominant bronchiolitis as well as evidence of pro-fibrotic signaling in a subset of unvaccinated PASC patients. Since our results point towards self-limiting inflammation of small airways without detectable viral reservoirs, it remains unclear whether pulmonary symptoms in PASC are SARS-CoV-2-specific or represent a general response to viral infection. Still, evidence of pro-fibrotic signaling should warrant clincal follow-up and further research into possible long-time fibrotic remodeling in PASC patients.

Determinants of Spike Infectivity, Processing and Neutralization in SARS-CoV-2 Omicron subvariants BA.1 and BA.2 (preprint)

The SARS-CoV-2 Omicron variant rapidly outcompeted other variants and currently dominates the COVID-19 pandemic. Its enhanced transmission, immune evasion and pathogenicity is thought to be driven by numerous mutations in the Omicron Spike protein. Here, we examined the impact of amino acid changes that are characteristic for the BA.1 and/or BA.2 Omicron lineages on Spike function, processing and susceptibility to neutralization. Individual mutations of S371F/L, S375F and T376A in the ACE2 receptor-binding domain as well as Q954H and N969K in the hinge region 1 impaired infectivity, while changes of G339D, D614G, N764K and L981F moderately enhanced it. Most mutations in the N-terminal region and the receptor binding domain reduced sensitivity of the Spike protein to neutralization by sera from individuals vaccinated with the BNT162b2 vaccine or therapeutic antibodies. Our results represent a systematic functional analysis of Omicron Spike adaptations that allowed this SARS-CoV-2 variant to overtake the current pandemic.

SARS-CoV-2 variants of concern remain dependent on IFITM2 for efficient replication in human lung cells (preprint)

The authors have withdrawn this manuscript due to a duplicate posting of manuscript number BIORXIV/2021/468942. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

The Delta variant of SARS-CoV-2 maintains high sensitivity to interferons in human lung cells (preprint)

Interferons are a major part of the anti-viral innate defense system. Successful pathogens, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), need to overcome these defenses to establish an infection. Early induction of interferons (IFNs) protects against severe coronavirus disease 2019 (COVID-19). In line with this, SARS-CoV-2 is inhibited by IFNs in vitro, and IFN-based therapies against COVID-19 are investigated in clinical trials. However, SARS-CoV-2 continues to adapt to the human population resulting in the emergence of variants characterized by increased transmission fitness and/or decreased sensitivity to preventive or therapeutic measures. It has been suggested that the efficient spread of these so-called "Variants of Concern" (VOCs) may also involve reduced sensitivity to IFNs. Here, we examined whether the four current VOCs (Alpha, Beta, Gamma and Delta) differ in replication efficiency or IFN sensitivity from an early isolate of SARS-CoV-2. All viruses replicated in a human lung cell line and in iPSC-derived alveolar type II cells (iAT2). The Delta variant showed accelerated replication kinetics and higher infectious virus production compared to the early 2020 isolate. Replication of all SARS-CoV-2 VOCs was reduced in the presence of exogenous type I, II and III IFNs. On average, the Alpha variant was the least susceptible to IFNs and the Alpha, Beta and Gamma variants show increased resistance against type III IFN. Although the Delta variant has outcompeted all other variants in humans it remained as sensitive to IFNs as an early 2020 SARS-CoV-2 isolate. This suggests that increased replication fitness rather than IFN resistance may be a reason for its dominance. Our results may help to understand changes in innate immune susceptibility of VOCs, and inform clinical trials exploring IFN-based COVID-19 therapies.

SARS-CoV-2 variants of concern remain dependent on IFITM2 for efficient replication in human lung cells (preprint)

ABSTRACT It has recently been shown that an early SARS-CoV-2 isolate (NL-02-2020) hijacks interferon-induced transmembrane proteins (IFITMs) for efficient replication in human cells. To date, several “Variants of Concern” (VOCs) showing increased infectivity and resistance to neutralization have emerged and globally replaced the early viral strains. Here, we determined whether the four SARS-CoV-2 VOCs (Alpha, Beta, Gamma and Delta) maintained the dependency on IFITM proteins for efficient replication. We found that depletion of IFITM2 strongly reduces viral RNA production by all four VOCs in the human epithelial lung cancer cell line Calu-3. Silencing of IFITM1 had little effect, while knock-down of IFITM3 resulted in an intermediate phenotype. Strikingly, depletion of IFITM2 generally reduced infectious virus production by more than four orders of magnitude. In addition, an antibody directed against the N-terminus of IFITM2 inhibited SARS-CoV-2 VOC replication in iPSC-derived alveolar epithelial type II cells thought to represent major viral target cells in the lung. In conclusion, endogenously expressed IFITM proteins (especially IFITM2) are critical cofactors for efficient replication of genuine SARS-CoV-2 VOCs, including the currently dominating Delta variant. IMPORTANCE Recent results showed that an early SARS-CoV-2 isolate requires endogenously expressed IFITM proteins for efficient infection. However, whether IFITMs are also important cofactors for infection of emerging SARS-CoV-2 VOCs that out-competed the original strains and currently dominate the pandemic remained to be determined. Here, we demonstrate that depletion of endogenous IFITM2 expression almost entirely prevents the production of infectious Alpha, Beta, Gamma and Delta VOC SARS-CoV-2 virions in a human lung cell line. In comparison, silencing of IFITM1 had little impact, while knock-down of IFITM3 had intermediate effects on viral replication. Finally, an antibody targeting the N-terminus of IFITM2 inhibited SARS-CoV-2 VOC replication in iPSC-derived alveolar epithelial type II cells. Our results show that SARS-CoV-2 VOCs including the currently dominant Delta variant are dependent on IFITM2 for efficient replication suggesting that IFITM proteins play a key role in viral transmission and pathogenicity.

Spike mutation T403R allows bat coronavirus RaTG13 to use human ACE2 (preprint)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 pandemic, most likely emerged from bats. A prerequisite for this devastating zoonosis was the ability of the SARS-CoV-2 Spike (S) glycoprotein to use human angiotensin-converting enzyme 2 (ACE2) for viral entry. Although the S protein of the closest related bat virus, RaTG13, shows high similarity to the SARS-CoV-2 S protein it does not efficiently interact with the human ACE2 receptor. Here, we show that a single T403R mutation allows the RaTG13 S to utilize the human ACE2 receptor for infection of human cells and intestinal organoids. Conversely, mutation of R403T in the SARS-CoV-2 S significantly reduced ACE2-mediated virus infection. The S protein of SARS-CoV-1 that also uses human ACE2 also contains a positive residue (K) at this position, while the S proteins of CoVs utilizing other receptors vary at this location. Our results indicate that the presence of a positively charged amino acid at position 403 in the S protein is critical for efficient utilization of human ACE2. This finding could help to predict the zoonotic potential of animal coronaviruses.

IFITM proteins promote SARS-CoV-2 infection and are targets for virus inhibition (preprint)

Interferon-induced transmembrane proteins (IFITMs 1, 2 and 3) are thought to restrict numerous viral pathogens including severe acute respiratory syndrome coronaviruses (SARS-CoVs). However, most evidence comes from single-round pseudovirus infection studies of cells that overexpress IFITMs. Here, we verified that artificial overexpression of IFITMs blocks SARS-CoV-2 infection. Strikingly, however, endogenous IFITM expression was essential for efficient infection of genuine SARS-CoV-2 in human lung cells. Our results indicate that the SARS-CoV-2 Spike protein interacts with IFITMs and hijacks them for efficient viral entry. IFITM proteins were expressed and further induced by interferons in human lung, gut, heart and brain cells. Intriguingly, IFITM-derived peptides and targeting antibodies inhibited SARS-CoV-2 entry and replication in human lung cells, cardiomyocytes and gut organoids. Our results show that IFITM proteins are important cofactors for SARS-CoV-2 infection of human cell types representing in vivo targets for viral transmission, dissemination and pathogenesis and suitable targets for therapeutic approaches.

Systematic analysis of innate immune antagonism reveals vulnerabilities of SARS-CoV-2 (preprint)

The innate immune system constitutes a powerful barrier against viral infections. However, it may fail because successful emerging pathogens, like SARS-CoV-2, evolved strategies to counteract it. Here, we systematically assessed the impact of 29 SARS-CoV-2 proteins on viral sensing, type I, II and III interferon (IFN) signaling, autophagy and inflammasome formation. Mechanistic analyses show that autophagy and type I IFN responses are effectively counteracted at different levels. For example, Nsp14 induces loss of the IFN receptor, whereas ORF3a disturbs autophagy at the Golgi/endosome interface. Comparative analyses revealed that antagonism of type I IFN and autophagy is largely conserved, except that SARS-CoV-1 Nsp15 is more potent in counteracting type I IFN than its SARS-CoV-2 ortholog. Altogether, however, SARS-CoV-2 counteracts type I IFN responses and autophagy much more efficiently than type II and III IFN signaling. Consequently, the virus is relatively resistant against exogenous IFN-/{beta} and autophagy modulation but remains highly vulnerable towards IFN-{gamma} and -{lambda} treatment. In combination, IFN-{gamma} and -{lambda} act synergistically, and drastically reduce SARS-CoV-2 replication at exceedingly low doses. Our results identify ineffective type I and II antagonism as weakness of SARS-CoV-2 that may allow to devise safe and effective anti-viral therapies based on targeted innate immune activation.

IFITM proteins promote SARS-CoV-2 infection in human lung cells (preprint)

Interferon-induced transmembrane proteins (IFITMs 1, 2 and 3) restrict numerous viral pathogens and are thought to prevent infection by severe acute respiratory syndrome coronaviruses (SARS-CoVs). However, most evidence comes from single-round pseudoparticle infection of cells artificially overexpressing IFITMs. Here, we confirmed that overexpression of IFITMs blocks pseudoparticle infections mediated by the Spike proteins of {beta}-coronaviruses including pandemic SARS-CoV-2. In striking contrast, however, endogenous IFITM expression promoted genuine SARS-CoV-2 infection in human lung cells both in the presence and absence of interferon. IFITM2 was most critical for efficient entry of SARS-CoV-2 and enhanced virus production from Calu-3 cells by several orders of magnitude. IFITMs are expressed and further induced by interferons in the lung representing the primary site of SARS-CoV-2 infection as well as in other relevant tissues. Our finding that IFITMs enhance SARS-CoV-2 infection under conditions approximating the in vivo situation shows that they may promote viral invasion during COVID-19. HIGHLIGHTSO_LIOverexpression of IFITM1, 2 and 3 restricts SARS-CoV-2 infection C_LIO_LIEndogenous IFITM1, 2 and 3 boost SARS-CoV-2 infection of human lung cells C_LIO_LIIFITM2 is critical for efficient entry of SARS-CoV-2 in Calu-3 cells C_LI

Alpha-1 antitrypsin inhibits SARS-CoV-2 infection (preprint)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). To identify factors of the respiratory tract that suppress SARS-CoV-2, we screened a peptide/protein library derived from bronchoalveolar lavage, and identified 1-antitrypsin (1-AT) as specific inhibitor of SARS-CoV-2. 1-AT targets the viral spike protein and blocks SARS-CoV-2 infection of human airway epithelium at physiological concentrations. Our findings show that endogenous 1-AT restricts SARS-CoV-2 and repurposes 1-AT-based drugs for COVID-19 therapy.