ERVW-1 and infection: During the early stages of infection, characterized by acute viral replication and vigorous cell-mediated immune responses [18,23], the assay detected levels of Env-specific CTL activity in the PBMCs of infected cats similar to those previously reported with 51Cr CTL assays using SV40-immortalized or primary feline fibroblasts infected by recombinant vaccinia viruses [18,24], with no need for the effector cells to be restimulated ex vivo, a procedure frequently necessary when using the standard Cr51 assay [25].