Flow cytometry plot of the CTL assay and comparative efficiency at restimulating CTL activity in vitro of Env transduced and Env peptides loaded immortalized fibroblasts. A: CTL activity following lymphocyte restimulation with Env-transduced immortalized fibroblasts as measured by F-CTL. Right and left panels show the results obtained with cats BR and BS, respectively. The gray line indicates the target population incubated alone, the thick line indicates residual target cells after incubation with restimulated lymphocytes. B: Comparative efficiency of Env transduced and Env peptides-loaded immortalized fibroblasts at restimulating CTL activity. Prior to the assay, the effector cells (PBMCs of chronically FIV infected cats boosted with p-Env-GMCSF 18 weeks earlier) were restimulated with Env+GFP+ fibroblasts (solid columns) or immortalized fibroblasts pulsed for 1 hour with pooled peptides covering the entire Env (empty columns).