Effects of particulate debris on macrophage-dependent fibroblast stimulation in coculture

The interactions between the different cell types in periprosthetic tissue are still unclear. We used a non-contact coculture model to investigate the effects of polymethylmethacrylate (PMMA) particles and human macrophage-de rived soluble mediators on fibroblast activation. Macrophages were either e xposed or not exposed to phagocytosable PMMA particles, but fibroblasts wer e not. Increasing numbers of macrophages were tested in cocultures in which the fibroblast cell number was held constant and cultures of macrophages a lone were used for comparison of cytokine release. We used the release of i nterleukin-1 beta (IL-1 beta), interleukin 6 (IL-6), tumour necrosis factor alpha (TNF-alpha), lysosomal enzyme and metalloproteinase activity to asse ss the cultivation of macrophages and fibroblasts. In cocultures, IL-6 release was increased 100-fold for both unchallenged an d particle-challenged cultures when compared with macrophage cultures alone . Furthermore, particle-challenged cocultures had threefold higher IL-6 lev els than unchallenged cocultures. Release of TNF-alpha was similar in cocul tures and in macrophage cultures. IL-1 beta release in cocultures was indep endent of the macrophage-fibroblast ratio. Lysosomal enzyme activity and me talloproteinase activity were increased in cocultures. Our data show that macrophages and fibroblasts in coculture significantly i ncrease the release of IL-6 and to a less degree other inflammatory mediato rs; particle exposure accentuates this effect. This suggests that macrophag e accumulation in fibrous tissue may lead to elevated IL-6 levels that are much higher than those caused by particle activation of macrophages alone. This macrophage-fibroblast interaction represents a novel concept for the i nitiation and maintenance of the inflammatory process in periprosthetic mem branes.