Analysis of viral replication in U937 cells treated with E64 and NE inhibitor. U937 cells were pre-treated with 300 μM E64 in the absence (-; black bars) or presence (NEI; grey bars) of 200 μM N-Ala-Ala-Pro-Val chloromethylketone, an inhibitor of NE. Following inhibitor pre-treatment, cells were infected with Lang virions or ISVPs at an MOI of 3. Viral yield was analyzed at 2 d p.i. as described in the legend to Fig. 1B.