APRIL-mediated effect depends on TACI and not BCMA. (A) Splenic CD19⁺CD5⁺ cells were isolated from TCL1-Tg mice (older than age 11 months) with an overt leukemic phenotype; stained with anti-BCMA, anti-TACI, and anti–BR-3 fluorescent-conjugated antibodies and analyzed by fluorescence-activated cell sorter. Spleens contained more than 70% leukemic cells (CD5⁺B220^dull) among the gated B cells (CD3^–, CD19⁺, left panel). Unfilled bold line represents the specific staining; the shaded curve represents the isotype control. Data shown are from representative mice (n = 6). (B) TACI and BCMA expression are represented as mean specific fluorescence intensity (MSFI) (n = 3). (C) 100 000 splenic CLL cells from TCL1-Tg mice age 8 to 12 months that were >70% CD5⁺CD19⁺B220^dull were stimulated by using either APRIL-WT, mock APRIL, or a mutant of APRIL that binds to BCMA and not TACI (APRIL-R206E). At the indicated time points, cell viability was measured. (D) Absolute number of DioC₆⁺ cells following 4 days of stimulation with indicated conditioned media. **P < .01.