SPP1 and rheumatoid arthritis: In a model of rheumatoid arthritis, an antibody specifically neutralizing only the SLAYGLR domain of mouse OPN (homologous to the SVVYGLR of human OPN) greatly abrogated monocyte migration towards the thrombin-cleaved form of OPN and inhibited the proliferation of synovial cells, bone erosion, and inflammatory cell infiltration in arthritic joints [21].