Flow cytometric analysis of CD45 isoform expression. (A) Unseparated CD4+ T cells. (B) APC-depleted purified naive CD4+ T cells. The latter were subsequently stimulated with EpCAM-transfected CHO cells armed either with (C) 250 ng/ml bscAb EpCAM x CD3 alone, thus providing only a TCR-like primary signal, or with (D) 250 ng/ml bscAb EpCAM x CD3 and 500 ng/ml single-chain B7-1 construct providing an additional costimulatory or second signal. C1 and D1 show the results after 4 days and C2 and D2 after 6 days of stimulation respectively.