S100A9 and Arthritis: We investigated the immunolocalization of S100A8 and S100A9 in sections of antigen-induced arthritis (AIA); the effect of IL-1α on S100a8 and S100a9 expression and immunolocalization in mouse cartilage explants in vitro; the in vivo expression and immunolocalization of S100A8 and S100A9 in cartilage during progressive cartilage destruction in an OA compared with an inflammatory arthritis model in mice; and the effect of S100A8 and S100A9 on the expression by primary adult ovine articular chondrocytes of key extracellular matrix molecules, matrix degrading enzymes, and their inhibitors.