Effect of APRIL on apoptosis in primary myeloma cells. (A) CD138⁺ cells (n = 15; 6 cyclin D2⁺ [1 with circulating plasma cells] and 9 cyclin D1⁺ [1 with circulating plasma cells]) were cultured in CM with or without APRIL (200 ng/mL) for 72 hours and stained with FITC-conjugated annexin V and PI. Viability (annexin V– and PI-negative fraction) in the presence or absence of APRIL is shown according to cyclin D class. Data are mean plus or minus SEM. (B) CD138⁺ cells (patient 13, with circulating plasma cells, Table 1) were cultured in CM with or without APRIL for up to 96 hours. Viability (annexin V– and PI-negative fraction) and proliferation (percentage of CD138⁺ cells in S/G₂M, case 13, Figure 3A) were assessed in parallel in the same experiment. (C) CD138⁺ cells (n = 7) were cultured in RPMI, RPMI/10% FCS, or CM with or without APRIL (200 ng/mL) for 72 hours. Cells were harvested and the viable fraction was determined as in panel A.