Development of an ex vivo cellular model of rheumatoid arthritis: Critical role of cd14‐positive monocyte/macrophages in the development of pannus tissue

Objective To establish an ex vivo cellular model of pannus, the aberrant overgrowth of human synovial tissue (ST). Methods Inflammatory cells that infiltrated pannus tissue from patients with rheumatoid arthritis (RA) were collected without enzyme digestion, and designated as ST‐derived inflammatory cells. Single‐cell suspensions of ST‐derived inflammatory cells were cultured in medium alone. Levels of cytokines produced in culture supernatants were measured using enzyme‐linked immunosorbent assay kits. ST‐derived inflammatory cells were transferred into the joints of immunodeficient mice to explore whether these cells could develop pannus. CD14 and CD2 cells were depleted by negative selection. Results Culture of ST‐derived inflammatory cells from 92 of 111 patients with RA resulted in spontaneous reconstruction of inflammatory tissue in vitro within 4 weeks. Ex vivo tissue contained fibroblasts, macrophages, T cells, and tartrate‐resistant acid phosphatase–positive multinucleated cells. On calcium phosphate–coated slides, ST‐derived inflammatory cell cultures showed numerous resorption pits. ST‐derived inflammatory cell cultures continuously produced matrix metalloproteinase 9 and proinflammatory cytokines associated with osteoclastogenesis, such as tumor necrosis factor α, interleukin‐8, and macrophage colony‐stimulating factor. More importantly, transferring ST‐derived inflammatory cells into the joints of immunodeficient mice resulted in the development of pannus tissue and erosive joint lesions. Both in vitro development and in vivo development of pannus tissue by ST‐derived inflammatory cells were inhibited by depleting CD14‐positive, but not CD2‐positive, cells from ST‐derived inflammatory cells. Conclusion These findings suggest that overgrowth of inflammatory cells from human rheumatoid synovium simulates the development of pannus. This may prove informative in the screening of potential antirheumatic drugs.