Macrophage-material surface interactions

Abstract

Material surface-stimulated activation of the macrophage and the release of mediators has been suggested to be an important factor for inflammation and tissue regeneration around biomedical implants. Macrophage interactions with particles released from implants as well as macophage-bacteria interactions are also involved in implant loosening processes. The development of relevant in vitro models, which would allow a selective manipulation of biologic and implant components in a less complex environment, is therefore important. The aim of this thesis was to develop experimental model systems and to use these to increase the knowledge of macrophage interactions at an implant surface. An in vitro model of human monocyte-surface interactions was established with a single cell population. The influence of polystyrene (PS), thin metal (TiO2) coating, PS particles of 1 and 3 µm Ø, lipopolysaccharide (LPS) and protein pre-coating on macrophage activation was studied. The phenotype of macrophages was analysed by monoclonal antibodies (CD14, 27E10 and RM3/1). Cytokine (IL-1a, TNF-a and IL-10) and hydrogen peroxide (H2O2) secretion was evaluated. Annexin-V, propidium iodide (PI) and lactate dehydrogenase (LDH), was used to differ between apoptosis and cell death. A co-culture model with monocytes and thyrocytes was established and used to analyse material surface-cell-cell interactions. An animal experimental model with s.c. implant in the dorsum of rats was developed for studies on inflammatory cell recruitment, distribution and activity around and adherent to implants in vivo. Light microscopy (LM) and transmission electron microscopy (TEM) were used for the analysis of cell morphology on the implant surfaces in vitro and in vivo. Secretion of IL-1a was dependent on the type and concentration of exogenous stimuli, culture time and protein pre-adsorption. Although, the mere interaction between the surface and cells caused an IL-1a secretion, no major differences were observed between cells on PS and Ti-sputtered PS. Adherent macrophages changed their phenotype depending on culture time and the type of added stimuli. After 24 h, macrophage oxidative metabolism was downregulated and could not be further stimulated by LPS and particles. LPS upregulated CD14 and stimulated an increased release of IL-1a, TNF-a and IL-10. Fibrinogen was found to exert a modulatory effect on several events, including an increased monocyte adhesion and reduced apoptosis. Fibrinogen also caused a reduction of IL-10 secretion, irrespective of stimuli. The epithelial barrier of cultured thyrocytes was impaired in co-culture with monocytes secreting IL-1a stimulated by particles or LPS without cell-cell contact. A neutralizing antibody markedly reduced the effect of IL-1a, suggesting IL-1a as a potent factor in this model system of cell-cell communication. The macrophage was the predominant cell type between day 3-28 in exudate around PS and Ti in vivo. A spontaneous secretion of H2O2 by adherent macrophages was demonstrated. The oxidative metabolism of the implant-adherent macrophages was downregulated after 7 days post-implantation. In summary, the developed in vitro and in vivo models allow detailed studies on the influence of synthetic material and biologic components on macrophage behaviour at implant surfaces.