Metabolic characterisation and modulation of the immune response to biomaterials for bone tissue engineering

Abstract

The limited capacity of many tissues and organs to adequately repair and regenerate lies at the heart of numerous healthcare challenges. Difficulties faced in promotion of tissue healing and regrowth have begun to be tackled by the incorporation of biomaterial approaches into clinical practice, with the ultimate goal being functional tissue regeneration. While traditional biomaterial design centred around ‘immune-inert’ biomaterials that would elicit minimal immune response from a host system, it is now evident that the immune system functions as a critical determinant of implant integration, failure, and success. Given this, there is a pressing need to thoroughly characterise how the immune system not only recognises, but also responds to biomaterials. Materials intended for bone-tissue applications are one such class of biomaterials that stand to benefit from the incorporation of immunology research into bone-based biomaterial design. As our understanding of ‘osteoimmunology’ expands, so too does our appreciation of the immune processes that can contribute to regeneration of functional bone tissue. As such, efforts are underway to further understand the processes that contribute to destruction at the implant site, and leverage those that facilitate repair and regeneration.

The studies presented in this thesis first examine the effect of micron and nano-sized hydroxyapatite (HA) particles on the metabolic profile of human macrophages, given that metabolic reprogramming is now known to impact on macrophage function. Results from this chapter demonstrate that HA particle size can differentially impact on macrophage metabolism, with micron-sized particles preferentially driving a metabolic shift favouring glycolysis, accompanied by pro-inflammatory macrophage polarisation. Furthermore, it is shown that blockade of particle phagocytosis reduces surrogate markers of glycolysis at the gene expression level and attenuates micronHA-driven pro-inflammatory polarisation, furthering not only our insight into the use of HA in orthopaedic biomaterials, but also our understanding of the destructive processes that can occur at the site of biomaterial implantation that can contribute to orthopaedic implant wear and eventual failure.

This thesis then investigates the effect of novel immunomodulatory ketoacids (indole pyruvate [IP] and hydroxyphenyl pyruvate [HPP]), derived from the parasite Trypansoma brucei (T. brucei), on the inflammatory and regenerative profile of human macrophages. Results reveal differential effects on human macrophages compared to other immune cells and demonstrate for the first time that these ketoacids have potential pro-regenerative effects. This work is furthered by interrogation of the effect of IP on the osteoblastic differentiation of human mesenchymal stem cells (MSC), demonstrating that IP can potentially promote early osteogenesis and mineralisation of MSC.

The final chapter explores the effects of a novel hyaluronic acid hydrogel on primary human macrophages. It is demonstrated that this material is biocompatible, and modulation of hydrogel stiffness in the range achieved (1 – 10 kPa) has no significant effect on macrophage polarisation. It is then attempted to incorporate the ketoacid, IP, into the hydrogel and deliver it into a rat bone defect site. Follow-on assessment of ex vivo tissue demonstrates that IP-loaded hydrogel leads to an upregulation in osteogenic gene expression at seven days post-implantation.

Overall, the work in this thesis furthers our understanding of the human macrophage response to HA biomaterial particulates and posits immunometabolism a worthy strategy to further characterise the immune response to biomaterials, as well as reveal potential novel therapeutic targets for tissue repair and regeneration. This thesis also demonstrates the pro-regenerative potential of a novel immunomodulatory ketoacid for bone tissue engineering applications, and presents a novel hyaluronic acid hydrogel as a potential delivery platform for immunomodulators in a bone regenerative context. Ultimately, by furthering our understanding of biomaterial-immune system interactions, and how we can capitalise on such for therapeutic benefit, we stand to create a new class of ‘immune-informed’ biomaterials that hold the potential to tackle existing healthcare challenges around biomaterial implantation strategies.