TACI is necessary and sufficient to induce Blimp-1 in B cells. (A-B) TACI deficiency abrogates Blimp-1 expression by B cells 14 days after immunization. Blimp-1 mRNA expression was detected by RT-PCR (A) and quantified by real-time PCR (B). Relative expression of Blimp-1 was calculated by determining the ratio of Blimp-1/β-actin in immunized B cells compared with the Blimp-1/β-actin ratio in B cells from nonimmunized mice. Results represent the mean ± SEM of 3 measurements/mouse of each of 4 mice/genotype. Values were compared by t test and found to be significantly different: ***P < .0001. (C-D) Expression of TACI is sufficient to induce Blimp-1. The 18.81 B cells were transfected with a vector encoding TACI and EGFP from a single bicistronic mRNA or with a control vector encoding only EGFP. RNA was isolated from sorted GFP-positive transfected cells, 24 hours after transfection and converted to cDNA. Blimp-1 mRNA expression was studied by RT-PCR, after 30 PCR cycles (C) or quantitative RT-PCR (D). Blimp-1 expression by 18.81 B cells expressing TACI was calculated relative to the Blimp-1 expression by 18.81 cells transfected with a GFP control vector. Results represent mean ± SEM of 3 separate measurements. Values were compared by t test and found to be significantly different: *P = .0103. (E) Blimp-1 and TACI expression decreased and BCL-6 expression increased with time in 18.81 B cells transfected with a TACI expression vector. The 18.81 B cells transfected with TACI and EGFP or with EGFP control vectors were cultured up to 3 days with or without 100 ng/mL APRIL, as indicated. TACI, BCL-6, and Blimp-1 mRNA expression was studied by RT-PCR, after 30 PCR cycles. Cells were collected at 0, 24, 48, and 72 hours in culture. Results are representative of 3 independent experiments.