Mesenchymal stem cells (MSCs) are a mixture of progenitors that are heterogeneous in their regenerative potential. Development of MSC therapies with consistent efficacy is hindered by the absence of an immunophenotype of MSC heterogeneity. This study evaluates decoy TRAIL receptor CD264 as potentially the first surface marker to detect cellular aging in heterogeneous MSC cultures. CD264 surface expression, regenerative potential, and metrics of cellular aging were assessed in vitro for marrow MSCs from 12 donors ages 20–60 years old. Male and female donors were age matched. Expression of CD264 was compared with that of p16, p21, and p53 during serial passage of MSCs. When CD264+ cell content was 20% to 35%, MSC cultures from young (ages 20–40 years) and older (ages 45–60 years) donors proliferated rapidly and differentiated extensively. Older donor MSCs containing < 35% CD264+ cells had a small size and negligible senescence despite the donor’s advanced chronological age. Above the 35% threshold, CD264 expression inversely correlated with proliferation and differentiation potential. When CD264+ cell content was 75%, MSCs were enlarged and mostly senescent with severely compromised regenerative potential. There was no correlation of the older donors’ chronological age to either CD264+ cell content or the regenerative potential of the donor MSCs. CD264 was upregulated after p53 and had a similar expression profile to that of p21 during serial passage of MSCs. No sex-linked differences were detected in this study. These results suggest that CD264 is a surface marker of cellular age for MSCs, not the chronological age of the MSC donor. CD264 is first upregulated in MSCs at an intermediate stage of cellular aging and remains upregulated as aging progresses towards senescence. The strong inverse correlation of CD264+ cell content to the regenerative potential of MSCs has possible application to assess the therapeutic potential of patient MSCs, standardize the composition and efficacy of MSC therapies, and facilitate aging research on MSCs.

Articular chondrocytes maintain cartilage matrix turnover and have the capacity for anabolic and catabolic activities that can be influenced by injury and disease. This study tested the hypothesis that catabolic genes are upregulated with regional osteoarthritis (OA) disease severity within a joint. With IRB approval, specimens of knee cartilage obtained as discarded tissues from subjects undergoing arthroplasty were partitioned for each subject by OA disease severity and evaluated for gene expression by RT‐PCR. There was regional OA grade‐associated upregulation of expected inflammatory mediators TNF‐α, TNF receptors, IFN‐γ, and interleukins as well as genes encoding proteolytic enzymes, including Adamts‐5 and MMPs. Osteoclast‐related genes, cathepsin K, tartrate‐resistant acid phosphatase (TRAP), RANKL, RANK, M‐CSF, and c‐fms, but not osteoprotegerin, were induced in advanced grades. In vitro treatment of normal human chondrocytes with interleukin‐1β upregulated similar genes; this provides evidence that chondrocytes per se can be the source of osteoclast‐related factors. Immunohistochemical staining showed that RANK‐ and RANKL‐positive cells were abundant in advanced grades, especially in chondrocyte clusters. This suggests a possible autocrine mechanism by which an osteoclast phenotype is induced in articular chondrocytes. In sum, these studies identified gene expression signatures in human OA cartilage based upon regional disease severity within a joint. There was an effect of OA Grade on expression of osteoclastic lytic enzymes and regulatory factors in human articular chondrocytes. Induction of an osteoclast‐like phenotype in chondrocytes may be part of OA progression and suggests specific therapeutic approaches. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:454–462, 2016.