Partridge et al. determined which human cell line model to use for studying SARS-CoV-2 S protein attachment based off of the mRNA expression of different membrane proteins important to the pathogenesis of COVID-19. They investigated the following cell types:

• RT4 urinary bladder epithelial cell line
• A549 lung adenocarcinoma alveolar basal epithelial cell line
• Caco2 colorectal adenocarcinoma cell line
• HaCaT skin keratinocyte cell line

RT4 was determined to be the best cell line to use because it expresses both ACE2 and TMPRSS2 which directly interact with SARS-CoV-2 S protein. It also expresses the following accessory proteins relevant to S protein attachment:

• ADAM17: a metalloprotease that processes ACE2
• rhomboid-like 2 (RHBDL2): a protease that is involved with the function of ADAM17
• CD9: an adaptor protein that controls ADAM17