CD28 activation-induced BLIMP-1 expression is dependent on the CD PYAP motif. Purified BM (A) WT, (B) CD28-Y170FF, (C) CD28-AYAA PC, and purified splenic (D) WT, (E) CD28-Y170F, and (F) CD28-AYAA PC were cultured ± control Ig or αCD28 mAb-coated beads for 16 h. Lysates were made and BLIMP-1 expression was analyzed by Western blot analysis. Blots representative of two independent experiments; densitometry relative to actin are the aggregate mean ± range of two independent experiments. (G) Quantitative PCR. BM PC purified from WT, CD28-Y170F, and CD28-AYAA mice were incubated for 4 h in serum-free media with control Ig or activating CD28 mAb; prdm1 expression was assessed by quantitative PCR and shown as fold induction (relative quantity values) of prdm1 expression in αCD28-treated over control Ig-treated conditions. Data representative of three independent experiments. *p < 0.05, ***p = 0.0001, ****p < 0.0001.