Characterization of anti-APRIL antibodies identifying secreted APRIL and cells producing APRIL. (A) Extracts (20 μg) from IHC-positive and -negative DLBCL tissue samples were analyzed by Western blot with Aprily-2 (2 μg/mL) and Stalk-1 (5 μg/mL). An 18-kDa band corresponding to secreted APRIL was revealed with Aprily-2 (left), whereas Stalk-1 identified a 14-kDa band (middle). Similar 14-kDa reactivity was observed with Stalk-1 on DC lysates (20 μg) (right). The reactivity is representative of 3 IHC-positive and 2 IHC-negative tumor lysates. (B) Western blot specificity was ascertained in blocking experiments. Aprily-2 was preincubated with 10 μg/mL acrpAPRIL or acrpCTRL. Stalk-1 was preincubated with 10 μg/mL stalk peptide or an irrelevant peptide prior to Western blot analysis.