Mesenchymal progenitor cells (MPCs) possess high proliferative potential and can differentiate into several mesenchymal lineages, including bone, cartilage, fat, tendon, and stromal tissue (1–3). Initially identified in the bone marrow (BM), these cells have subsequently been found in trabecular bone, adipose tissue, and synovial tissue (4–9). Although there has been much speculation about the role of MPCs in the pathogenesis of inflammatory or degenerative arthritis, their basic biology, topography, and roles in joint physiology remain unknown (9–12). By definition, MPCs have great potential to repair damaged bone and cartilage and are likely to contribute to joint regeneration, which is a prominent feature of osteoarthritis (OA). A recent study described alterations in the activity of MPCs in the BM of OA patients (13), but their presence in OA synovial fluid (SF) has not yet been investigated. Furthermore, there have been several hypotheses concerning the potential role of MPCs in the pathogenesis of rheumatoid arthritis (RA), but these could not be tested in the absence of data on the phenotypic “fingerprint” of joint MPCs (10–12, 14).
If MPCs play a physiologic role in joint homeostasis and repair, then it is conceivable that they are present in SF, thus permitting direct access to superficial articular cartilage. Fibroblastic cells that expressed phenotypic markers of the mesenchymal lineage upon culture expansion (such as CD44, type I collagen, and vascular cell adhesion molecules) have been found in SF of arthritis patients (12). However, their multipotentiality has not been reported, and hence, it is still unclear whether cells initiating these cultures were true SF MPCs. Also, the inability to clearly define SF MPCs has hampered attempts to enumerate them in RA and OA and, hence, to explore their putative roles in the pathogenesis of these disorders.
The aim of this study was to apply knowledge obtained from our previous investigation of BM MPCs (15) to an evaluation of SF for the presence of MPCs and to compare their numbers in inflammatory arthritis and OA. Our results demonstrate that SF from arthritis patients contained clonogenic, highly proliferative MPCs and that these were similar in phenotype to BM MPCs, more numerous in patients with OA, and uncommon in early and established RA. These findings form the basis for an exploration of the role of SF MPCs in joint pathophysiology in RA and OA.
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- PATIENTS AND METHODS
It has long been known that SF from patients with arthritis contains fibroblastic cells capable of in vitro proliferation (26–28), but their multipotentiality was not investigated. This study demonstrates the presence of rare MPCs in the SF of subjects with arthritis and links them with clonogenic SF fibroblasts. This is also the first demonstration that SF fibroblasts are closely related in phenotype to BM MPCs, including the specific in vivo expression of LNGFR. Furthermore, we showed that individual MPCs can survive in the viscous, antiadhesive medium of the SF in vivo and, in spite of their pathologic environment, can maintain their multipotentiality in vitro. Finally, the fact that MPCs were found to be much more numerous in OA SF compared with other arthropathies suggests their possible role in the pathophysiology of arthritis, and OA in particular (10–12).
What are the tissue origins of SF MPCs? It is possible that these may be derived from disrupted cartilage, bone, synovium, periosteum, or BM itself. The fact that superficial cartilage contains chondroprogenitors has recently been demonstrated in the setting of neonatal animals (29). Moreover, a high degree of plasticity (more or less comparable to multipotential MPCs) has been documented for human articular chondrocytes dedifferentiated due to in vitro culture expansion (30, 31), with more recent studies demonstrating that this dedifferentiation was not required for the capacity of some cartilage-derived cells to differentiate to skeletal muscle in vivo (32). Besides cartilage, SF MPCs could originate from synovium, for instance, from type B synovial lining cells shed into the joint lumen or from vascular pericytes of the subsynovium, because the multipotential nature of pericytes was documented before (33, 34). Although it is well known that synovium contains multipotential MPCs (9, 35), their topography is still unknown.
Alternatively, MPCs could be released into the joint space from the infrapatellar fat pad (36) or could migrate directly from BM via small vascular channels (found in some animals, but not in humans) (14). Even in the absence of channels, cartilage and bone damage, particularly in OA, could allow direct access of BM MPCs into the joint cavity. This seems likely and might explain not only the high numbers of MPCs in OA SF, but also the observed phenotype similarity between multipotential cells in both localities. The significance of in vivo LNGFR expression in this context is unknown; its expression in early fetal subepithelial mesenchyme and restriction to pericytes in adults (37) point to some association with immaturity/multipotentiality, but this has not yet been tested and demonstrated experimentally. Interestingly, LNGFR was recently found to be preferentially expressed on keratinocyte stem cells (38).
It remains a possibility that MPCs are normally present in SF. Early studies described the presence of mesenchymal cells (such as synoviocytes and chondrocytes) in normal SF (39). A recent study of BM MPCs demonstrated that they are capable of adhesion-independent survival and expansion as floating single cells (40). Normal SF cells can indeed produce monolayers of fibroblastic cells in culture, albeit not yet tested for multipotentiality (Pascal E: personal communication). If MPCs are really present in normal SF, one can envision their physiologic role in the repair of slight damage to superficial cartilage by filling-in defects from the top down. Some degree of spontaneous repair in this manner was previously documented in a rabbit model (41). This situation would likely require some altered adhesive cell–matrix interactions at the site of damage, considering the antiadhesive properties of SF in general. Another recent study proposed that fibroblasts “floating” in SF (also suggested to originate from BM MPCs or “dropped out” of synovium) are involved in joint destruction in RA (25). These findings, however, do not preclude the possibility of SF MPCs having some role in physiologic joint repair, both in healthy individuals and those with OA, because regeneration may indeed have gone awry in RA (11). Direct proof of these hypotheses requires further testing.
In conclusion, this is the first study to show that SF from patients with arthritis contains a rare population of clonogenic tripotential MPCs that are greater in number in OA compared with RA and other arthropathies. The prevalence of such cells in OA suggests their local origin, more likely from disrupted cartilage, bone, or BM. Together with many recent studies, this strengthens the idea that joint tissues per se are rich in MPCs. What their physiologic role in healthy subjects is and how resident MPCs can be used for inducing self-repair in arthritis are the subjects of future studies. In addition, we presented a new methodology for the phenotypic analysis of rare MPCs using an adherence-based enrichment. Considering the difficulties of rare cell phenotyping, our study provides a basis for further work aimed at defining phenotypic and molecular signatures of human tissue–resident MPCs in vivo.