ACP5 and arthritic joint disease: The evaluation revealed that, in the mBSA-challenged paw, the mice developed a severe arthritis and peri-articular inflammation characterized by influx of inflammatory cells, hyperplasia of the synovial membrane and pannus formation with the presence of fibroblast-like cells, increased (compared with unchallenged control animals) osteoclast activity (assessed by the increase in number of TRAP-positive cells, stained red, Figure 2a) and bone erosion, and cartilage destruction evidenced by loss of Safranin O staining (Figure 2b).