CD4 and Arthritis: Antigen-induced arthritis (AIA) in the rat [15], a severe knee monoarticular arthritis induced by intra-articular administration of methylated bovine serum albumin (mBSA) after systemic immunization, is a suitable arthritis model inasmuch as CD4+ T cells and macrophages infiltrate the SM [16] and its course consists of clearly discernible phases.