Gene expression profiles reveal that chondrogenic progenitor cells and synovial cells are closely related

We showed previously that chondrogenic progenitor cells (CPCs) from the superficial zone of articular cartilage respond vigorously to cartilage wounding by responding chemotactically to cell debris, but the physiologic functions of CPCs remain unclear. To help bridge this knowledge gap we undertook a comparative analysis of gene expression in bovine CPCs, chondrocytes, synovial fibroblasts (synoviocytes), and cells isolated from synovial fluid (SFCs). Analysis of microarrays parsed the four cell types into two distinct groups, one composed only of chondrocytes and the other of CPCs, synoviocytes, and SFCs. The groups differed with respect to metalloendopeptidase, collagen, and cytokine gene expression. Quantitative PCR showed that, relative to chondrocytes, all other cells under-expressed cartilage matrix genes. CPCs significantly over-expressed genes encoding the chemokines interleukin 8 (IL8), and C-C motif ligand 2, while synoviocytes over-expressed the chemokine C-X-C motif Ligand 12. Sulfated glycosaminoglycan deposition in pellet cultures by CPCs was intermediate between chondrocytes and synoviocytes/SFCs. These results indicate that the CPC phenotype more closely resembles synoviocytes and SFCs than chondrocytes. CPCs show a tendency to over-express chemokines that promote immune cell chemotaxis, suggesting they mediate inflammation in response to cartilage wounding.