For generation of VSV*DG(Luc) replicon particles, BHK-G43 helper cells were grown in 100-mm diameter culture dishes to 90% confluence and infected with recombinant modified vaccinia virus Ankara (5 pfu per cell) expressing T7 RNA polymerase (MVA-T7, a gift of Gerd Sutter, München, Germany). MVA-T7 has been classified by the German Central Committee for Biosafety as a BSL-1 organism (reference number 6790- [10] [11] [12] [13] [14] . Ninety minutes post infection, the medium was replaced with fresh EMEM containing 5% FBS and 10 29 M mifepristone (Sigma-Aldrich, Buchs, Switzerland) to induce VSV G expression [23] . Subsequently, the cells were transfected with 10 mg of pVSV*DG(Luc), 3 mg of pTM1-N, 5 mg of pTM1-P, and 2 mg of pTM1-L [28] using Lipofectamine TM 2000 transfection reagent (Invitrogen, Basel, Switzerland). The cells were trypsinized 24 hours post transfection and seeded into T75 flasks along with an equal number of fresh BHK-G43 cells. The cells were further incubated at 37uC for 24 hours in the presence of 10 29 M mifepristone. The cell culture supernatant was clarified by low-speed centrifugation and passed through a 0.20-mm-pore size filter to deplete vaccinia virus. VSV*DG(Luc) was further propagated on mifepristoneinduced BHK-G43 cells and stored frozen at 270uC. To determine infectious virus titers, confluent BHK-21 grown in 96well microtiter plates were inoculated in duplicate with 40 ml of serial tenfold virus dilutions for 1 h at 37uC. The wells additionally received 60 ml of EMEM and were incubated for 20 h at 37uC. The infectious titers were calculated according to the number of GFP-expressing cells/well and expressed as fluorescence-forming units per milliliter (ffu/ml).