TMPRSS2 and infection: The transfection of the plasmids into a coculture of HEK293T and MDCK-SIAT1-TMPRSS2 cells (45) was followed by passaging selection of the rescued virus at a low multiplicity of infection (MOI) in a monoculture of MDCK-SIAT1-TMPRSS2 cells to ensure a stringent genotype-phenotype link in the final virus pool.