title: Harnessing the immune response to enhance osseointegration
Creators: name: Pitchai, Manju Sofia
Publisher: name: Griffith University
Year: 2022
Subjects: subject: osseointegration
subject: immune system biology
subject: immune response

description: New understanding of immune system biology has led a paradigm shift in the development of biomaterials away from classically ‘inert’, to ‘immuno-modulatory’ biomaterials that have the potential to stimulate an immune response able to promote constructive and functional tissue remodeling responses as opposed to persistent inflammation and scar tissue formation. This project aims to examine macrophage behavior and in particular integrin expression at the cell–biomaterial interface in vitro, in order to delineate the underlying molecular events occurring during biomaterial-mediated osseous healing. After reviewing the current literature to identify research gaps that this thesis should focus on, the activation of macrophages with controlled timing, and modulation of their interactions with other cell types involved in wound healing, emerged as key strategies to improve biomaterial efficacy. Careful design of biomaterial structure and controlled release of immunomodulators can be employed to manipulate macrophage phenotype for the maximization of the wound healing response with enhanced tissue integration and repair, however, to elicit predictable immune responses there is a need for a thorough understanding of how the biomaterial properties can be tuned to harness a designed immunological outcome. Our systematic review of in vitro studies suggested that the initial immune response of macrophages to titanium may be modulated by its surface characteristics both topographically and physiochemically. We therefore assessed the potential effects of three novel titanium surfaces; Machined, Blasted and Fluoride-treated, on macrophage phenotype. We chose a monocytic cell-line as our macrophage source rather than using primary macrophages, as the inflammatory status of primary cells could not be readily defined nor reproduced between experiments. This was crucial to the overall study if we were to clearly assess any changes in macrophage phenotype as a result of their II interaction with the titanium surfaces. We subsequently analysed in some detail the experimental processes involved in the optimization of macrophage cell culture, macrophage differentiation and phenotype characterization. We used microscopy, profilometry and elemental analysis to understand how the roughness of the titanium surfaces affected macrophage viability and proliferation in both non-activated ‘M0’ and activated inflammatory ‘M1’ macrophages. Subsequent analysis of one of the key mediators of cell attachment to biomaterials showed for the first time that αM, β1, β2 and α2 integrins play an important role in the adhesion of non-polarised M0 macrophages onto both smooth and rough titanium surfaces. Moreover, the distinct temporal expression profile of αM integrin expression correlated broadly with the subsequent secretion profiles of inflammatory cytokines IL-1β and TNF-α in these cells. Specific integrin expression also played an important role in the adhesion of polarised inflammatory M1 macrophages onto surface-modified titanium surfaces. In these cells, early integrin αM and β1 in particular, along with integrin α2 was associated with modulation of the subsequent cytokine response in these cells. Furthermore, the surface induced differential expression of integrins on the different titanium surfaces showed the Fluoride-modified surface in general induced greater cytokine and integrin modulation than either the Machined or Blasted surfaces. While now biologically plausible, further studies to fully determine how integrin-derived modulation of the macrophage-associated inflammatory response following macrophage attachment as suggested by these studies, can enhance wound healing and osteogenesis are required. Despite this, ‘harnessing the immune response’ is clearly a potential goal of engineered titanium implant surfaces in order ‘to enhance osseointegration’.