GM-CSF is required for optimal synovitis and cartilage breakdown associated with collagenase-induced arthritis. Arthritis was initiated in C57BL/6 and GM-CSF-/- mice by intra-articular collagenase injection and histology performed 1 and 2 weeks later. (A) Representative histologic pictures of knee joints at 2 weeks (original magnification ×100). (B) Quantification of synovitis. (C) Representative DIPEN staining in knee joints at 2 weeks (original magnification ×125). (D) Quantification of DIPEN staining. Results are expressed as the mean ± standard error of the mean; n = 5 to 8 mice per strain per time point (from two independent experiments).*P = 0.05, **P = 0.02, ***P = 0.002, GM-CSF-/- versus C57BL/6 mice. C: cartilage; F: femur; GM-CSF: granulocyte-macrophage colony-stimulating factor M: meniscus; O: osteophyte; T: tibia.