Objective. To determine the presence of mesenchymal progenitor cells (MPCs) in human articular cartilage.Methods. Primary cell cultures established from normal and osteoarthritic (OA) human knee articular cartilage were analyzed for the expression of CD105 and CD166, cell surface markers whose coexpression defines mesenchymal stem cells (MSCs) in bone marrow and perichondrium. The potential of cartilage cells to differentiate to adipogenic, osteogenic, and chondrogenic lineages was analyzed after immunomagnetic selection for CD105؉/CD166؉ cells and was compared with bone marrow-derived MSCs (BM-MSCs).Results. Up to 95% of isolated cartilage cells were CD105؉ and ϳ5% were CD166؉. Conclusion. These findings indicate that multipotential MPCs are present in adult human articular cartilage and that their frequency is increased in OA cartilage. This observation has implications for understanding the intrinsic repair capacity of articular cartilage and raises the possibility that these progenitor cells might be involved in the pathogenesis of arthritis.

IntroductionMesenchymal stem cells (MSC) are highly attractive for use in cartilage regeneration. To date, MSC are usually recruited from subchondral bone marrow using microfracture. Recent data suggest that isolated cells from adult human articular cartilage, which express the combination of the cell-surface markers CD105 and CD166, are multi-potent mesenchymal progenitor cells (MPC) with characteristics similar to MSC. MPC within the cartilage matrix, the target of tissue regeneration, may provide the basis for in situ regeneration of focal cartilage defects. However, there is only limited information concerning the presence/abundance of CD105+/CD166+ MPC in human articular cartilage. The present study therefore assessed the relative percentage and particularly the zonal distribution of cartilage MPC using the markers CD105/CD166.MethodsSpecimens of human osteoarthritic (OA; n = 11) and normal (n = 3) cartilage were used for either cell isolation or immunohistochemistry. Due to low numbers, isolated cells were expanded for 2 weeks and then analyzed by flow cytometry (FACS) or immunofluorescence in chamber slides for the expression of CD105 and CD166. Following immunomagnetic separation of CD166+/- OA cells, multi-lineage differentiation assays were performed. Also, the zonal distribution of CD166+ cells within the matrix of OA and normal cartilage was analyzed by immunohistochemistry.ResultsFACS analysis showed that 16.7 ± 2.1% (mean ± SEM) of OA and 15.3 ± 2.3 of normal chondrocytes (n.s.) were CD105+/CD166+ and thus carried the established MPC marker combination. Similarly, 13.2% ± 0.9% and 11.7 ± 2.1 of CD105+/CD166+cells, respectively, were identified by immunofluorescence in adherent OA and normal chondrocytes. The CD166+ enriched OA cells showed a stronger induction of the chondrogenic phenotype in differentiation assays than the CD166+ depleted cell population, underlining the chondrogenic potential of the MPC. Strikingly, CD166+ cells in OA and normal articular cartilage sections (22.1 ± 1.7% and 23.6% ± 1.4%, respectively; n.s.) were almost exclusively located in the superficial and middle zone.ConclusionsThe present results underline the suitability of CD166 as a biomarker to identify and, in particular, localize and/or enrich resident MPC with a high chondrogenic potential in human articular cartilage. The percentage of MPC in both OA and normal cartilage is substantially higher than previously reported, suggesting a yet unexplored reserve capacity for regeneration.

The 29-kDa amino-terminal fibronectin fragment (FN-f) has a potent chondrolytic effect and is thought to be involved in cartilage degradation in arthritis. However, little is known about signal transduction pathways that are activated by FN-f. Here we demonstrated that

Articular cartilage is one of the major targets in destructive joint diseases in humans. We studied cellular immune reactions against cartilage cell‐surface membranes, because it has recently been suggested that these represent possible antigenic structures, based upon the observation of autoantibodies with this specificity in certain joint diseases. A striking T cell reactivity toward chondrocyte membranes was found both in blood and synovial tissue from patients with rheumatoid arthritis. This reactivity was strongly dependent on the presence of monocytes and had all the characteristics of an antigen‐driven process. Clonal analysis demonstrated high precursor frequencies in peripheral blood T cells that were reactive against chondrocyte membranes. This response to chondrocyte membranes greatly exceeded the T cell stimulation induced by membranes from other sources such as fibroblasts or epithelial cells. In contrast to patients with rheumatoid arthritis, individuals with osteoarthritis showed a strong peripheral blood and synovial fluid T cell response not only to chondrocyte membranes, but also to fibroblast membrane material. However, there was no reactivity to epithelial cell membranes. Normal donors generally did not show significant responses to any membrane preparation. These data indicate that there is a strong T cell reactivity toward chondrocyte membranes in destructive joint disorders, and this may significantly contribute to the pathogenetic processes that occur in these diseases.

SUMMARYCyclo-oxygenase-2-selective inhibitors produce less gastric damage than conventional non-steroidal antiinflammatory drugs. Valdecoxib is a new orally administered cyclo-oxygenase-2-selective inhibitor, recently approved for use in osteoarthritis, rheumatoid arthritis and primary dysmenorrhoea in the USA. The drug has been evaluated in more than 60 clinical studies involving more than 14 000 patients and healthy volunteers. The analgesic efficacy of valdecoxib at a dose of 10 mg once daily in both osteoarthritis and rheumatoid arthritis is superior to that of placebo and similar to that of traditional non-steroidal anti-inflammatory drugs. Valdecoxib is effective in single doses of up to 40 mg for the alleviation of acute menstrual pain and has a rapid onset of action (within 30 min) and a long duration of analgesia (up to 24 h). Valdecoxib is well tolerated and has safety advantages compared with traditional non-steroidal anti-inflammatory drugs in terms of less gastrointestinal toxicity and a lack of an effect on platelet function. The incidence of adverse effects involving the kidney (fluid retention, oedema and hypertension) is similar to that of non-selective, nonsteroidal anti-inflammatory drugs.

Up-regulation of MMP-1/MMP-3 by IL-1beta and/or TNF-alpha in a fraction of chondrocytes in vitro suggests that a subpopulation of catabolic cells may also exist in osteoarthritis. These cells may undergo considerable dedifferentiation, as indicated by a decreased collagen-II/collagen-I ratio.

It is postulated that class II positive chondrocytes may be actively involved in the destruction or rejection of vital transplanted cartilage grafts. To investigate whether human nasal chondrocytes may also function as accessory cells in ongoing immune reactions with cartilage destruction, mixed leukocyte-chondrocyte cultures and antigen presentation assays were performed. Freshly isolated HLA class II antigen negative chondrocytes obtained from nasal septa were not stimulatory to autologous resting T lymphocytes. HLA class II positive chondrocytes treated with gamma-interferon were able to present antigens to autologous activated T cells derived from an antigen (tetanus) specific T cell line. Upon incubation with activated T cells, initially class II negative changed their phenotype resulting in the expression of class II antigens and enabling them to effectively present antigen. These results suggest an active role of chondrocytes in the rejection of cartilage grafts.

The expression of TNF-a and its receptors in the rheumatoid synovial membrane was investigated using immunohistochemistry and immunocyto¯uorescence. TNF-a cells (< 10% of all cells) were found in all regions, predominantly in sublining and diffuse in®ltrates. The highest percentage of TNF-R cells was found in the lining layer (50±90%), with a slight predominance of TNF-R55. In the sublining, fewer cells expressed TNF-R (< 50%), mostly TNF-R75. TNF-R75 cells were also detectable in diffuse in®ltrates and lymphoid aggregates (10±50%). These contained only individual TNF-R55 cells. In diffuse in®ltrates, there were slightly more TNF-R55 cells than in lymphoid aggregates (in both cases < 10%). In sequential sections, TNF-a cells localized mostly in the vicinity of TNF-R cells. Macrophage-like cells appeared to be the predominant TNF-R cell type. CD3 T cells in lymphoid aggregates expressed exclusively TNF-R75. Subsequently, the expression of membrane-bound TNF-a, TNF-R55 and TNF-R75 was tested by FACS analysis in isolated RA synoviocytes (n 7 patients). Only four specimens expressed mTNF-a, and that on a low percentage of cells (2 6 2.4%; mean 6 SD). In contrast, all specimens expressed higher percentages of TNF-R55 and TNF-R75 (21 6 1% and 14 6 7.1%, respectively). These results demonstrate that: (1) the percentage of cells expressing soluble/transmembrane TNF-a is greatly outnumbered by the percentage of cells expressing TNF receptors; and (2) TNF-a-expressing cells are localized in regions expressing substantial levels of TNF receptors. Therefore, the known pro-in¯ammatory and pro-arthritic effects of TNF-a are probably mediated by local interactions between the receptors and their soluble and transmembrane ligands.