APRIL production peaks in myeloid precursor cells. (A) Left: Stalk-1–stained cells, overexposed to distinguish segmented (arrow) and nonsegmented (arrowhead) nuclei. Right: Absence of Stalk-1 staining for a megakaryocyte (arrow). Scale bars represent 5 μm (left panel), 10 μm (right panel). (B) BM cells were immunostained after cell permeabilization with stalk-1 (thick line) or control rabbit Ig (thin line) and analyzed by flow cytometry. Gating was performed on CD14⁺ and CD34⁺ cells for monocytes and HSCs, respectively, and as indicated in the left panel for myeloid cells. Overlaid histogram plots are shown in the right panel. Stainings are representative of 6 BMs. (C) APRIL mRNA was analyzed by Q-PCR in purified immature CD13⁻CD16⁻ myelocytes and mature CD13⁺CD16⁺ granulocytes from human BM. (D) Furin activity was analyzed in purified myelocytes and mature granulocytes from human BM. Means and minimum/maximum value bars from a duplicate assay are shown.