On the initial inflammatory response to variations in biomaterial surface chemistry

Abstract: Aims. The aim of the present thesis was, firstly, to develop in vivo and ex vivo models for analysis of peri-implant exudate and implant adherent cells and, secondly, to characterize the early inflammatory response to materials with different surface chemical functionalization as well as materials with inert (Ti) and cytotoxic (Cu) properties.<br/><br/>Materials and Methods. C. p. titanium (Ti) disks were evaporated with gold (Au) or copper (Cu). Au surfaces were functionalized with alkane thiols. Pure Au, Au functionalized with -OH (hydrophilic) or -CH3 (hydrophobic) groups represent material surfaces with small chemical alterations, whereas c. p. Ti and Cu surfaces represent material surface with major variations in chemistry. Surfaces were implanted subcutaneously in rats for time periods ranging between 1 h and 48 h. Early inflammatory events (inflammatory cell recruitment, distribution, activation of transcription factor NF-kappaB, cytokine production, and cell death) and fibrous capsule formation (28 d) were evaluated. Further, surfaces with adherent cells were incubated ex vivo and implant adherent cells were characterized with respect to oxidative burst, cytokine secretion and cells damage (lactate dehydrogenase, LDH, content). Mechanisms of Ti and Cu induced cell death were analysed in vitro upon incubation of human peripheral blood mononuclear cells (PBMC) with different material surfaces.<br/><br/>Results. An experimental rat model was developed, enabling the analysis of implant adherent cells ex vivo. The kinetics of cytokine secretion by implant adherent cells ex vivo was similar to that detected in vivo, indicating that cells associated with the material surface contributed substantially to the total inflammatory response.<br/>Distinct chemical functionalization of material surfaces influenced cellular distribution and oxidative burst during in vivo implantation. Methylated, hydrophobic surfaces were associated with a greater proportion of exudate-located cells than hydroxylated and gold surfaces. Cells associated with hydroxylated, hydrophilic surfaces responded with a relatively higher chemiluminescence after PMA stimulation.<br/>Major variations in surface chemistry (Ti and Cu) influenced cell recruitment, the type of recruited cell, cytokine production, kinetics of cell death, intracellular signalling and fibrous capsule organisation: Ti was associated with a high and transient leukotactic reponse, predominance of mononuclear cells, high and transient amounts of active NF-kappaB in exudates cells and an early and transient cytokine production (IL-1alpha, TNF-alpha and IL-6) in vivo. The fibrous capsule around Ti was thinner than around Cu. In vitro, high cellular viability and activity were characteristic features of human monocytes cultured on Ti. <br/>Cu was associated with a delayed but extended leukotactic response, high degree of cellular damage, predominance of PMN in the inflammatory exudates, and delayed and high amounts of transcriptional factor NF-kappaB and pro-inflammatory cytokines IL-1alpha, TNF-alpha and IL-6 in vivo. The fibrous capsule around Cu was thicker than around Ti and was characterized by accumulation of inflammatory cells close to the implant surface. Further, Cu implants induced rapid cellular death, probably through apoptosis, in vitro. <br/><br/>Conclusions. The present experimental results show that material surface chemical functionality influences specific events in the inflammatory response, including cell distribution and respiratory burst, whereas cytokine secretion seemed not to be affected. It is therefore possible, at least partly, to govern in vivo reactions through tailor-made modifications of material surface chemistry.<br/>The present experimental results show that material surface alterations, which induce cell death also initiate early, multiple cellular and inflammatory processes which in turn are related to the subsequent repair phase. Some of these processes may be determined in the present experimental system, thus providing a pre-clinical screening of materials with adverse effects.<br/>The early and high inflammatory response around Ti contradicts the prevailing view of Ti being an inert material. It is suggested that the observed transient biologic response is of importance for the understanding of the mechanisms of tissue integration and biocompatibility.