One for all - Human kidney Caki-1 cells are highly susceptible to infection with corona- and other respiratory viruses (preprint)

Despite the vast increase in research activity in the coronavirus field over the past two years, researchers are still heavily reliant on non-human cells, for example Vero E6, highly heterogeneous or not fully differentiated cells, such as Calu-3, or not naturally susceptible cell lines overexpressing receptor ACE2 and other accessory factors, such as TMPRSS2. Complex cell models, such as primary cell-derived air-liquid interface epithelial models are highly representative of human tissues but are expensive and time-consuming to develop and maintain and have limited suitability for high-throughput analysis. In vitro investigations of host-pathogen interactions of viruses is highly reliant on suitable cell and tissue culture models and results are only as good as the model they have been validated in. Here, we show the use of a highly characterized human kidney cell line, Caki-1, for infection with three human coronaviruses: Betacoronaviruses severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and Middle Eastern respiratory syndrome coronavirus (MERS-CoV) and Alphacoronavirus human coronavirus 229E (hCoV-229E). Caki-1 cells show equal or superior susceptibility to all three coronaviruses when compared to other commonly used cells lines for the cultivation of the respective virus. Furthermore, we used a panel of antibodies generated against 21 SARS-CoV-2-encoded proteins to identify their location in the infected Caki-1 cells using immunocytochemistry. Most importantly, Caki-1 cells are also susceptible to two other respiratory viruses, Influenza A virus and RSV, making them an ideal model for cross-comparison of not only a broad range of coronaviruses but respiratory viruses in general.

Secreted SARS-CoV-2 ORF8 modulates the cytokine expression profile of human macrophages (preprint)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still adapting to its new human host. Attention has focussed on the viral spike protein, but substantial variation has been seen in the ORF8 gene. Here, we show that SARS-CoV-2 ORF8 protein undergoes signal peptide-mediated processing through the endoplasmic reticulum and is secreted as a glycosylated, disulphide-linked dimer. The secreted protein from the prototype SARS-CoV-2 virus had no major effect on viability of a variety of cell types, or on IFN or NF-{kappa}B signalling. However, it modulated cytokine expression from primary CSF1-derived human macrophages, most notably by decreasing IL-6 and IL-8 secretion. Furthermore, a sequence polymorphism L84S that appeared early in the pandemic associated with the Clade S lineage of virus, showed a markedly different effect, of increasing IL-6 production. We conclude that ORF8 sequence polymorphisms can potentially affect SARS-CoV-2 virulence and should therefore be monitored in sequencing-based surveillance.