Mechanotherapies for Type 1 diabetes: Using actuatable medical implants to understand and modulate the foreign body response

Type 1 diabetes is a chronic, lifelong condition for which there is no cure. The disease is characterised by autoimmune destruction of pancreatic beta cells, rendering them unable to produce insulin. This leads to hyperglycaemia, which can have fatal consequences if untreated. Management of the condition currently necessitates stringent glycaemic control via blood glucose monitoring and insulin therapy which can be burdensome for the patient and result in sub-optimal glycaemic control. Continuous subcutaneous insulin infusion sets, or insulin pumps, have greatly improved therapeutic outcomes for type 1 diabetes patients in recent years. However these devices have a short lifespan and must be replaced approximately every three days, in large part due to device occlusion as a result of the foreign body response. The foreign body response is the body’s defence mechanism towards an implanted object and results in the deposition of a dense, hypopermable, fibrotic capsule around the implant. In the case of insulin infusion sets, the presence of a fibrotic capsule can impede the diffusion of insulin out of the device and intro the surrounding tissue. An alternative therapy for the treatment of type 1 diabetes is the transplantation of insulin producing cells – derived either from cadaveric donors or differentiated from allogenic stem cells – to replace the patient’s dysfunctional beta cell population. Whilst this approach has shown promise, current protocols require co-administration of a harsh immunosuppression regimen to prevent allogenic rejection of the transplanted cells. Immunosuppressive medications are associated with a wide range of side effects, the risks of which often exceed the disease associated risks meaning this is a non-viable treatment option for many patients. To eliminate the need for immunosuppressive medication, there is interest in transplanting insulin producing cells within an immune isolating barrier known as a macroencapsulation device. Macroencapsulation devices house transplanted cells within a semi-permeable membrane which permits the exchange of glucose, insulin, and essential nutrients required for cell survival and function, whilst preventing the infiltration of harmful immune cells. However, the presence of this membrane poses a diffusion barrier between the cells and the host environment, limiting molecular exchange and compromising cell viability. This is exacerbated by formation of a fibrotic capsule which further impedes diffusion of essential molecules, ultimately leading to cell hypoxia. A potential means to modulate the deposition of this fibrotic capsule is to mechanically actuate the implant, which perturbs the local tissue environment and thus interferes with the production of fibrotic tissue. The first study of this thesis (Chapter 3) investigated the effect of intermittent actuation of an implanted insulin delivery device on fibrotic capsule formation and drug delivery kinetics. The device was implanted subcutaneously in mice for up to eight weeks, with five minutes of actuation performed twice daily. This study found that after eight weeks of intermittent actuation, insulin transport was non-significantly different from baseline (three days post implantation) levels, in contrast with non-actuated control devices which had significantly reduced performance and had become functionally redundant at eight weeks. A number of cellular and biological factors could be attributed to this outcome, including that actuation of the device resulted in a significantly reduced the number of inflammatory cells at the implant site (reduction in neutrophils at day five, and reduction in myofibroblasts after two weeks), significantly reduced capsule thickness after two weeks, a significant increase in capsular collagen coherency after eight weeks, in addition to reducing tissue infiltrate present in the device after eight weeks. The combination of these factors facilitated improved insulin diffusion out of the device and into the surrounding tissue to facilitate glucose uptake. Leading from this work, the next study of this thesis (Chapter 4) further investigated the mechanism of action underlying actuation mediated modulation of fibrotic capsule formation using a custom in vitro model. In this study, a human myofibroblast cells (WPMY-1 cell line) were seeded onto actuatable reservoirs, which were actuated for five minutes every 12 hours. This study found that intermittent actuation significantly reduced collagen production by these cells (after nine and 14 days), significantly reduced pro-inflammatory cytokine production (transforming growth factor-β1 after four, nine, and 14 days and interleukin-1β after four and nine days), whilst also upregulating the production of the anti-inflammatory cytokine interleukin-10 after 14 days. Using analytical and computational models to predict the strain and fluid flow levels that elicited these anti-fibrotic responses, this chapter begins to establish design parameters for the design of actuatable implantable cell encapsulation devices that can modulate the foreign body response. These design parameters were then used to inform the design of a novel actuatable macroencapsulation device (Chapter 5). Previous iterations of our actuatable implants have never been used for cell encapsulation. This actuatable macroencapsulation device was conceptualised on the premise that intermittent actuation of implantable drug delivery device (described in Chapter 3) can improve the diffusion kinetics in the local implant environment, which may also promote exchange of essential nutrients required for encapsulated cells to survive and function. Chapter 5 describes the design, manufacture and mechanical characterisation of an actuatable macroencapsulation device, in addition to the evaluation of cell encapsulation materials to support the viability and function of encapsulated cells. A murine mesenchymal cell line previously transfected with either Firefly or Gaussia luciferase was used to evaluate the longitudinal viability and function of the cells respectively. The macroporous, gelatin based scaffold material ‘Spongostan™’ (Johnson & Johnson) was found to support optimal cell viability and function. Actuatable macroencapsulation devices containing cells encapsulated in a Spongostan™ scaffold were then actuated for 10 minutes every 24 hours for the study duration of 13 days at an actuation magnitude that has previously demonstrated immunomodulatory effects (Chapters 3 and 4). Longitudinal bioluminescent imaging indicated that actuation did not compromise the viability of the encapsulated cells. Finally, a preclinical model (C57BL/6 mice) was established to evaluate the performance of the new actuatable macroencapsulation device in vivo, and an initial pilot study was conducted to evaluate the survival of encapsulated cells and understand the local and systemic immune response to the device. Bioluminescent imaging was used to non-invasively monitor the viability of cells encapsulated within the device which was implanted subcutaneously. Longitudinal blood sampling and flow cytometry methods were used to assess immune cell populations in the blood, and ex vivo tissue analysis was performed to evaluate formation of the fibrotic capsule around the implant. Collectively, the research presented in this thesis investigates how intermittent actuation of medical implants can be used to modulate the host foreign body response, and uses a combination of in silico, in vitro, and in vivo methods to design, develop, and evaluate the first actuatable macroencapsulation device. Findings from this thesis will inform further development of the device, which may have the potential to overcome critical limitations of current macroencapsulation approaches.

Funder

Science Foundation Ireland

Publisher

University of Galway

Rights

Attribution-NonCommercial-NoDerivatives 4.0 International

cbn

Collections

University of Galway Theses (PhD Theses)