Objective: This work aims to clarify the histogenesis of the cells forming RA pannus: the pannocytes.
Methods: 15 patients with seropositive RA; 5 controls with post-traumatic knee effusion and 5 with OA knee effusion were included in the study. Synovial tissues and fluids, collected during diagnostic arthrocentesis, were used as a source of cells to be cultured. Viable staining and cytocentrifugation were performed. Cell phenotype was investigated by immunofluorescence assays after different culture periods. Cells were also studied by immunohistochemistry to determine the presence of CD5, CD68:KP-1, CD68:PG-M1, vimentin, cytokeratin, a-SM-Actin.
Results: Cells derived from RA samples were sub-divided into two population of lymphocyte-like and macrophage-like cells. Phenotypical characteristics of the first one were analysed after 6 days of culture and suggested they were T lymphocytes. The other population could grow in vitro for undefined time resembling the neoplastic-like proliferation previously described for pannocytes. Phenotypic characterization excluded that these cells were lymphocytes, monocyte-macrophages, fibroblasts, myofibroblasts, endothelial cells or keratinocytes. On the contrary, immunohistochemistry demonstrated that 100% of pannocytes were vimentin positive and 75% of these cells expressed also CD68:KP-1. CONCLUSIONS. The results exclude that pannocytes originate from monocyte-macrophages or from fibroblasts, but strongly support the hypothesis that they belong to the family of primitive embryonal connective tissue-forming cells (residue of the primitive mesenchymal tissue).