One for all – Human kidney Caki-1 cells are highly susceptible to infection with corona- and other respiratory viruses
Authors: Daniels A. , Fletcher S. , Kerr H. , Kratzel A. , Pinto R.M. , Kriplani N. , Craig N. , Hastie C.J. , Davies P. , Digard P. , Thiel V. , Tait-Burkard C. 
Published: October 2022

In vitroinvestigations of host-pathogen interactions of viruses are reliant on suitable cell and tissue culture models. Results are only as good as the model they have been generated in. However, choosing cell models forin vitrowork often depends on what is available in labs or what has previously been used. Despite the vast increase in coronavirus research activity over the past few 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 naturally unsusceptible cells requiring overexpression of receptors and other accessory factors, such as ACE2 and 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. They have limited suitability for large-scale and high-throughput analysis.

Using tissue-specific expression pattern as a selection criteria, we identified human kidney cells as an ideal target for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) and broader coronavirus infection. We show the use of the highly characterized human kidney cell line Caki-1 for infection with three human coronaviruses:BetacoronavirusesSARS-CoV-2 and Middle Eastern respiratory syndrome coronavirus (MERS-CoV) andAlphacoronavirushuman coronavirus 229E (hCoV-229E). Caki-1 cells show equal or superior susceptibility to all three coronaviruses when compared to other commonly used cell lines for the cultivation of the respective virus. Antibody staining against SARS-CoV-2 N protein shows comparable replication rates. Using a panel of 21 antibodies in infected Caki-1 cells using immunocytochemistry shows the location of viral proteins during replication. In addition, Caki-1 cells were found to be susceptible to two other human respiratory viruses, influenza A virus and respiratory syncytial virus, making them an ideal model for cross-comparison of not only a broad range of coronaviruses but respiratory viruses in general.

Author Summary

Investigating how viruses interact with their host relies models used for laboratory research. The closer a model matches the host, the more conclusive results are. Complex cell systems based on primary epithelial or stem cells are the gold standard ofin vitroresearch. However, they are expensive, time consuming, and laborious to establish. Therefore, cell lines remain the backbone of virus research.

Despite vastly increased research into human coronaviruses following the COVID-19 pandemic, researchers continue to rely on suboptimal cell line models, for example ells of non-human origin like the VeroE6 African Green Monkey cell line. Using known expression patterns of the entry factors of the COVID-19 causative agent severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) we identified the Caki-1 cell line as a target for SARS-CoV-2. This cell line could be shown to be infectable with a wide range of coronaviruses including common cold virus hCoV-229E, epidemic virus MERS-CoV, and SARS-CoV-2 as well as other important respiratory viruses influenza A virus and respiratory syncytial virus. We could show that SARS-CoV-2 proteins can be stained for and localized in Caki-1 cells and the cells are competent of forming a cellular immune response. Together, this makes Caki-1 cells a unique tool for cross-virus comparison in one cell line.