Increased Levels of Tumor Necrosis Factor-α and Interleukin-6 Protein and Messenger RNA in Human Peripheral Blood Monocytes due to Titanium Particles*

Cytokines produced by macrophages in the periprosthetic membranes surrounding joint replacements have been implicated as causal agents in osteolysis and prosthetic loosening. The present study characterizes the response of human peripheral blood monocytes to titanium particles. Monocytes were obtained from volunteers and blood that had been donated to the American Red Cross and were cultured in the presence of titanium particles (one to three micrometers in diameter). There were consistent dose-dependent increases in the production of TNF-α (tumor necrosis factor-α) and IL-6 (interleukin-6) protein, with the greatest stimulation generally observed with a concentration of 6 x 105 to 6 x 106 particles of titanium per milliliter. The level of TNF-α was the greatest (fifty to 1000 times greater than the control level) after eight hours of exposure to titanium particles; the level of IL-6 was two to five times greater than the control level after sixteen hours of exposure. These increases were similar to those observed after stimulation with lipopolysaccharide and depended on de novo synthesis rather than on release from intracellular stores. The production of TNF-α was inhibited in a dose-dependent manner by the translational inhibitor cycloheximide and the transcriptional inhibitor actinomycin D, indicating the requirement for both mRNA (messenger RNA) and protein synthesis for the induction of cytokine synthesis by titanium particles. Although the increase in the levels of cytokine mRNA in response to titanium was rapid (thirty to ninety minutes), the increase in the level of TNF-α mRNA preceded that of IL-6 mRNA. The level of TNF-α mRNA was the greatest at ninety minutes and the level of IL-6 mRNA was the greatest at three hours. After stimulation with titanium particles, the level of TNF-α mRNA was increased as much as fivefold and the level of IL-6 mRNA, as much as twelvefold. CLINICAL RELEVANCE: Awareness of the importance of wear debris particles in cytokine-induced bone resorption has resulted in improvements in the designs of implants and in operative techniques to reduce wear of components. The present study further elucidates the biological mechanisms involved in periprosthetic osteolysis. Titanium-stimulated biosynthesis of the cytokines TNF-α and IL-6, which are both potent stimulators of bone resorption, requires increases in the synthesis of both mRNA and protein by monocytes. An understanding of the complex mechanisms of the induction of cytokine synthesis by particles of wear debris will facilitate the design of pharmacological agents to control periprosthetic bone resorption. These agents, in combination with other efforts to reduce the generation of wear debris, may improve the longevity of orthopaedic implants.