Stiffness- and Bioactive Factor-Mediated Protection of Self-Assembled Cartilage against Macrophage Challenge in a Novel Co-Culture System

Objective: Tissue-engineered cartilage implants must withstand the potential inflammatory and joint loading environment for successful long-term repair of defects. The work's objectives were to develop a novel, direct cartilage-macrophage co-culture system and to characterize interactions between self-assembled neocartilage and differentially stimulated macrophages. Design: In study 1, it was hypothesized that the proinflammatory response of macrophages would intensify with increasing construct stiffness; it was expected that the neocartilage would display a decrease in mechanical properties after co-culture. In study 2, it was hypothesized that bioactive factors would protect neocartilage properties during macrophage co-culture. Also, it was hypothesized that interleukin 10 (IL-10)-stimulated macrophages would improve neocartilage mechanical properties compared to lipopolysaccharide (LPS)-stimulated macrophages. Results: As hypothesized, stiffer neocartilage elicited a heightened proinflammatory macrophage response, increasing tumor necrosis factor alpha (TNF-alpha) secretion by 5.47 times when LPS-stimulated compared to construct-only controls. Interestingly, this response did not adversely affect construct properties for the stiffest neocartilage but did correspond to a significant decrease in aggregate modulus for soft and medium stiffness constructs. In addition, bioactive factor-treated constructs were protected from macrophage challenge compared to chondrogenic medium-treated constructs, but IL-10 did not improve neocartilage properties, although stiff constructs appeared to bolster the anti-inflammatory nature of IL-10-stimulated macrophages. However, co-culture of bioactive factor-treated constructs with LPS-treated macrophages reduced TNF-alpha secretion by over 4 times compared to macrophage-only controls. Conclusions: In conclusion, neocartilage stiffness can mediate macrophage behavior, but stiffness and bioactive factors prevent macrophage-induced degradation. Ultimately, this co-culture system could be utilized for additional studies to develop the burgeoning field of cartilage mechano-immunology.