Enhanced IgM secretion by splenocytes and plasmablast survival require TACI and oligomerized TACI ligands. (A) Purified B splenocytes were cultured for 6 days in the presence of 100 ng/mL of the indicated ligands, after which time IgM titers were measured in culture supernatants. The figure shows the fold increase relative to medium only for the same genotype (titers were comparable for the different genotypes). Results are mean plus or minus SD of triplicate cultures. (B) MMTV-induced plasmablasts of the indicated genotypes were purified and cultured for 2 days in the presence of 500 ng/mL of the indicated ligands or 10 ng/mL of IL-6, after which time IgG2a/c-secreting cells were enumerated by ELISPOT. ASC indicates antibody-secreting cell. Data are expressed as fold increase compared with wild-type B cells in medium only, and represent mean plus or minus SD of triplicate (APRIL) or quadruplicate (medium and BAFF) cultures, representative of 2 experiments with similar results. (C) Tetanus toxoid-specific plasmablasts were purified from spleen at the peak of the booster response and cultured for 2 days in the presence of 500 ng/mL of the indicated ligands or 10 ng/mL of IL-6. Tetanus toxoid-specific plasmablasts were enumerated by ELISPOT, and the percentage of survival determined relative to values obtained at day 0. Values are mean plus or minus SD of 2 independent cell preparations (or 3 preparations for wild-type) cultured in triplicates (*P < .05; **P < .01).