APRIL production by neutrophils is constitutive. (A) APRIL expression in peripheral neutrophils was observed at the gene-expression level by RT-PCR analysis. DC and T cells were used as positive and negative controls, respectively. (B) Total staining after cell permeabilization or surface staining was performed with Stalk-1 (2 μg/mL) and Aprily-8 (1 μg/mL) Abs. Histogram plots after flow cytometry analysis are shown. Thick and thin lines correspond to indicated Abs and isotype-matched control Abs, respectively. Stainings are representative of 4 donors. (C) Bone marrow cellular suspensions were stained with Stalk-1 and anti-CD10 Abs. Histogram plots after flow cytometry analysis are shown on CD10⁺ and CD10⁻ size-gated granulocytes (right). Thick and thin lines correspond to Stalk-1 and isotype-matched control Abs, respectively. Stainings are representative of 3 donors. (D) Supernatants conditioned by PMNs and PBMCs were immunoprecipitated with BCMA-Ig and analyzed by Western blot with the Aprily-1 mAb.