Virus titration of LASV (strain Josiah, Gene Bank Accession Number NC_004297 and NC_004296) was performed by defining the 50% tissue culture infectious dose (TCID 50 ). For this, Vero cells were grown in 96-well plates to 30 to 40% confluence. Cells were inoculated with 10-fold serial dilutions of supernatants from LASV-infected CHO cell lines grown in the presence or absence of doxycycline. The assays were evaluated at 7 to 9 days postinfection. TCID 50 values were calculated using the Spearman-Karber method [51] . All experiments involving LASV-infected samples were performed under biological safety level 4 conditions at the Philipps-University Marburg.