APRIL produced by myeloid precursor cells sustains PC survival. (A) Pictures show the cytological analysis of purified mouse Gr-1^lo and Gr-1^hi BM cells. Scale bar represents 3 μm. (B) Purified Gr-1^lo and Gr-1^hi myeloid cells were analyzed by Q-PCR for APRIL mRNA. The results are representative of 4 mice. (C) The indicated purified BM myeloid cells from wild-type (WT) and APRIL^−/− (KO) mice were incubated with purified CD138⁺ PCs. Two days later, CD138⁺ PCs were numerated by flow cytometry. (D) Similar myeloid cell and PC cocultures were performed in the presence of Apry-1-1 or control Ig (cIg) used at 3 μg/mL.