Synovial plasma cells in rheumatoid arthritis lesions accumulate in zone of secreted APRIL retention. (a) A rheumatoid arthritis (RA) lesion was stained with a control immunoglobulin (cIg) or an anti-CD138. Middle panel: zone with a germinal center (GC) and a low CD138+ plasma cell (PC) density; arrows indicate examples of the many blood vessels harboring PC accumulation; Insert, high magnification of PCs accumulating around a blood vessel. Right panel: same RA lesion in an area with high CD138+ PC density; arrow indicates example of PCs accumulating around blood vessels. (b) CD138+ plasma cells (green) at high density are in close contacts with CD68+ macrophages (red). (c) Ig+ PCs (green) at high density accumulate close to secreted A proliferation-inducing ligand (APRIL) (red). Pictures are representative of three RA lesions with ectopic GC formation and PC accumulation.