Variant of concern (VOC) Omicron-BA.1 has achieved global predominance in early 2022. Therefore, surveillance and comprehensive characterization of Omicron-BA.1 in advanced primary cell culture systems and animal models are urgently needed. Here, we characterize Omicron-BA.1 and recombinant Omicron-BA.1 spike gene mutants in comparison with VOC Delta in well-differentiated primary human nasal and bronchial epithelial cells in vitro, followed by in vivo fitness characterization in hamsters, ferrets and hACE2-expressing mice, and immunized hACE2-mice. We demonstrate a spike-mediated enhancement of early replication of Omicron-BA.1 in nasal epithelial cultures, but limited replication in bronchial epithelial cultures. In hamsters, Delta shows dominance over Omicron-BA.1, and in ferrets Omicron-BA.1 infection is abortive. In hACE2-knock-in mice, Delta and a Delta spike clone also show dominance over Omicron-BA.1 and an Omicron-BA.1 spike clone, respectively. Interestingly, in naïve K18-hACE2 mice, we observe Delta spike-mediated increased replication and pathogenicity and Omicron-BA.1 spike-mediated reduced replication and pathogenicity, suggesting that the spike gene is a major determinant of replication and pathogenicity. Finally, the Omicron-BA.1 spike clone is less well-controlled by mRNA-vaccination in K18-hACE2-mice and becomes more competitive compared to the progenitor and Delta spike clones, suggesting that spike gene-mediated immune evasion is another important factor that led to Omicron-BA.1 dominance.

Highlights d Ex vivo modeling of SARS-CoV-2 infection at the maternalfetal interface in human d SARS-CoV-2 replicates and propagates in human placenta d Magnitude of infectious SARS-CoV-2 release is related to ACE2 expression d SARS-CoV-2 proteins and/or RNA are detected in different placental cells Authors

BackgroundIn vitro systems of primary cystic fibrosis (CF) airway epithelial cells are an important tool to study molecular and functional features of the native respiratory epithelium. However, undifferentiated CF airway cell cultures grown under submerged conditions do not appropriately represent the physiological situation. A more advanced CF cell culture system based on airway epithelial cells grown at the air-liquid interface (ALI) recapitulates most of the in vivo-like properties but requires the use of invasive sampling methods. In this study, we describe a detailed characterization of fully differentiated primary CF airway epithelial cells obtained by non-invasive nasal brushing of pediatric patients.MethodsDifferentiated cell cultures were evaluated with immunolabelling of markers for ciliated, mucus-secreting and basal cells, and tight junction and CFTR proteins. Epithelial morphology and ultrastructure was examined by histology and transmission electron microscopy. Ciliary beat frequency was investigated by a video-microscopy approach and trans-epithelial electrical resistance was assessed with an epithelial Volt-Ohm meter system. Finally, epithelial permeability was analysed by using a cell layer integrity test and baseline cytokine levels where measured by an enzyme-linked immunosorbent assay.ResultsPediatric CF nasal cultures grown at the ALI showed a differentiation into a pseudostratified epithelium with a mucociliary phenotype. Also, immunofluorescence analysis revealed the presence of ciliated, mucus-secreting and basal cells and tight junctions. CFTR protein expression was observed in CF (F508del/F508del) and healthy cultures and baseline interleukin (IL)-8 and IL-6 release were similar in control and CF ALI cultures. The ciliary beat frequency was 9.67 Hz and the differentiated pediatric CF epithelium was found to be functionally tight.ConclusionIn summary, primary pediatric CF nasal epithelial cell cultures grown at the ALI showed full differentiation into ciliated, mucus-producing and basal cells, which adequately reflect the in vivo properties of the human respiratory epithelium.Electronic supplementary materialThe online version of this article (10.1186/s12931-017-0706-7) contains supplementary material, which is available to authorized users.