Concentration of secreted APRIL in normal synovium. (a) Normal synovium was immunostained with control immunoglobulin (cIg) (left panel) and Stalk-1 (middle panel). Stalk-1 stained cells are present in blood vessels (lower insert). A serial section of the same synovium was immunostained with Aprily-2 (right panel). Insert: Aprily-2 staining in the absence (upper) or presence of recombinant soluble A proliferation-inducing ligand (APRIL) (lower). (b) Normal synovium (upper panels, n = 4) and non-inflamed tonsils (lower panels, n = 7) were immunostained with cIg (left panel), Aprily-2 (middle panel), and anti-heparan sulfate proteoglycans (HSPG) (right panel). Blood vessels from these two organs are shown. Original magnification, 100×. (c) Binding of recombinant APRIL on the surface of human umbilical vein endothelial cells was assessed by flow cytometry in the presence or absence of heparin (hep). ACRP30 was used as a negative control.