APRIL-induced expression of matrix metalloproteinase 9 (MMP-9) and interleukin-8 (IL-8) is independent of the expression TACI or BCMA in THP-1 cells. (a) THP-1 cells were either transfected with control small interfering (si) RNA or siRNAs specific for BCMA and TACI. Cell surface expression levels of APRIL, TACI and BCMA were measured after 5 days. Histograms from specific staining (open area) and background staining (filled area, stained with mouse immunoglobulin G1; IgG1) were compared. (b) Cells transfected with TACI and BCMA specific siRNAs were stimulated with 1 μg/ml lipopolysaccharide, anti-APRIL monoclonal antibody (mAb), mouse IgG, TACI : Fc, or TACI : Fc plus 1 μg/ml of anti-TACI mAb for 24 hr and the culture supernatant was collected for the analysis of MMP-9 activity and IL-8 measurement. Transfection experiments were independently repeated twice with essentially the same results. ***P < 0·001 when compared with the control.