Abundant secretion of APRIL in BM plasma. (A) Serial decalcified BM sections were immunostained with Stalk-1, Ap-2, 10e4, and 3g10. The pictures are representative of 8 BM. Scale bar represents 250 μm. (B) APRIL concentration in plasma from BM aspirates or supernatant from 3-day cultures of total BM cells was determined by ELISA.