Articular cartilage harbors chondrogenic progenitor cells (CPCs), a population that responds chemotactically to cell death. Because this behavior is reminiscent of macrophages, we hypothesized that CPCs have macrophage-like capabilities for scavenging cell and tissue debris through phagocytosis.
CPCs, chondrocytes, synoviocytes, and macrophages were cultured with fluorophore-labeled chondrocyte debris for 3, 6, 12, or 24 h. Debris internalization was quantified by confocal microscopy and flow cytometry. Confocal microscopy was also used to test CPCs and chondrocytes for uptake of fluorophore-labeled fibronectin fragments (Fn-fs), a form of extracellular matrix debris. Lysosome activity and mass in CPCs and chondrocytes were measured using fluorescent probes. The relative expression of phagocytosis-related genes and proteins was evaluated by polymerase chain reaction (PCR) and immunoblotting, respectively. Pulse-chase experiments were performed to determine if the debris internalized by CPCs and chondrocytes was cleared, and if clearance was affected by a cathepsin B inhibitor.
More macrophages, synoviocytes, and CPCs internalized cell debris than chondrocytes at all time points. While uptake remained flat in chondrocytes at ∼10%, in the other cell types it peaked at more than 60% after 12-24 h. Relative to chondrocytes, CPCs showed significantly higher rates of Fn-fs engulfment, greater lysosome activity and mass, and over-expressed phagocytosis-related genes and proteins. Pulse-chase experiments revealed time- and cathepsin B-dependent clearance of cell debris in CPCs, but not in chondrocytes.
CPCs phagocytized cell and matrix debris much more efficiently than chondrocytes, supporting the hypothesis that they play a macrophage-like role in injured cartilage.