EBNA1-specific Th1 cells, but not Th2 cells, lyse EBNA1-expressing DCs. Cell lines were isolated from PBMCs stimulated with vvEBNA1ΔGA-infected DCs for either 7 hours (IFN-γ) or 18 hours (IL-4). The cells were then positively selected based on the secretion of IFN-γ or IL-4. EBNA1-specific IFN-γ– and IL-4–producing cells were isolated from six of six donors and three of six donors, respectively. These cells were expanded with weekly restimulations of irradiated vvEBNA1ΔGA-infected DCs alternating with DCs pulsed with a rEBNA1 protein. (a) Cells were restimulated with vvEBNA1ΔGA-infected or vvTK^–-infected DCs and tested for either IFN-γ or IL-4 secretion after 3 weeks of expansion. The IFN-γ–secreting cell line isolated from donor 3 representative of all Th1 cell lines is shown in the left panel, and a representative EBNA1-specific IL-4–secreting cell line from donor 1 is illustrated in the right panel. (b) Cells were then tested for their ability to lyse vvEBNA1ΔGA-infected DCs by ⁵¹Cr-release assay. The results of the ⁵¹Cr release from cell line from donor 3 are shown with graded effector-to-target ratios and are representative of all six established IFN-γ cell lines (left panel). Results shown are from a cell line established from donor 1 and are representative of all IL-4–secreting cell lines isolated. (c) CTL assay results at an effector-to-target ratio of 10:1 are shown for five IFN-γ cell lines and three IL-4 lines for three sources of EBNA1 antigen (vvEBNA1ΔGA, rEBNA1, or the physiologically expressed protein in B-LCL) as compared with controls (vvTK, rPCNA control protein, or T2 cells, respectively). Different symbols are representative of each cell line, with Th1 cell lines as open symbols and Th2 cell lines as filled symbols. Th1 and Th2 cell lines isolated from the same donor share symbol shapes.