Proliferation analysis. The proliferation of purified naive CD4+ T cells (no additional MHC II depletion) was measured in triplicate by an ELISA-based BrdU incorporation assay after 4 days of stimulation with EpCAM-transfected CHO cells armed with 250 ng/ml bscAb EpCAM x CD3 (+), providing the first TCR-like signal, with or without a second signal mediated by 500 ng/ml EpCAM-specific single-chain B7-1, B7-2 or LFA-3 construct. Negative controls, in which the first and/or second signal are absent, are indicated by (-). The mean OD₄₀₅ values for the uptake of BrdU are shown.