APRIL is critical for the in vivo survival of BM plasmablasts. (A) Adult C57BL/6 mice were primed by intraperitoneal injection of TT/AlOH + CpG₁₈₂₆ and boosted 5 weeks later with TT/AlOH. Four days after boosting, splenocytes were harvested and transferred (100 × 10⁶ per recipient mouse) into adult C57BL/6 BAFF^−/− or WT recipients. Eighteen and 48 hours later, mice were killed and TT-specific IgG ASCs quantified by ELISpot in the BM compartment. Results are expressed as ASCs per organ (mean ± SD) obtained in one experiment including 4 mice per group and representing 2 independent experiments. (B) Adult C57BL/6 mice were primed by intraperitoneal injection of TT/AlOH + CpG₁₈₂₆ and boosted 5 weeks later with TT/AlOH. Four days after boosting, splenocytes were harvested and transferred (100 × 10⁶ per recipient mouse) into adult C57BL/6 APRIL^−/− or WT recipients. Eighteen and 48 hours later, mice were killed and TT-specific IgG ASCs quantified by ELISpot in the BM compartment. Results are expressed as ASCs per organ (mean ± SD) obtained in one experiment including 4 mice per group and representing 3 independent experiments. *P < .05 versus adult WT mice. (C) In an independent experiment, sera from adoptive transfer recipients C57BL/6 APRIL^−/−, BAFF^−/−, or WT adult mice were harvested at different time points after transfer and TT-specific antibody titer was determined by ELISA. *P < .05 WT versus APRIL^−/−; **P < .05 BAFF^−/− versus APRIL^−/−. (D) APRIL^−/− and C57BL/6 control mice were immunized as described in “Antigens, adjuvants, and immunizations” and TT-specific IgG ASCs were quantified by ELISpot in the BM 7 days after boost. Results are expressed as ASCs per million (mean ± SD) obtained in 2 experiments including 6 to 10 mice per group *P < .05 versus adult WT mice.