MOTIVATION AND OBJECTIVES
MOTIVATION AND OBJECTIVES OF THE PRESENT INVESTIGATION
Based on the existing literature, as highlighted in Chapter 1, it is quite evident that the silk biomaterial-based islet encapsulation approach provides an attractive proposition for islet delivery in vivo by providing protective three-dimensional (3D) support for islet function. It has thus come to the forefront as potential therapeutic approach for treating type 1 diabetes. Furthermore, the assistive approach of immunoisolation and immunomodulation associated with this encapsulation technique helps in enhancing the utility of the implant by overcoming immune rejection. The origin of the present investigation and the salient motivating factors stem from the following assertions:
1. Current therapeutic approaches for type 1 diabetes include exogenous insulin injection and pancreas transplantation. However, both the techniques are associated with several limitations viz., rapid blood glucose fluctuations and poor blood glucose control in insulin therapy and major surgery, shortage of donor and life-long immunosuppression in case of pancreas transplantation.
Given this backdrop, there is an urgent need to create alternate therapeutic approaches which could adequately address the dual issue of blood glucose level maintenance and islet rejection. Upon encapsulation in 3D biomimetic microenvironment with suitable biochemical cues, the islet functions are preserved as the 3D matrix compensates for the native extracellular matrix (ECM) with enhanced cell viability, proliferation and desired insulin production.
2. Islet transplantation happens to be a promising treatment strategy for type 1 diabetes owing to its minimal invasiveness, and improved efficacy. However, majority of the work regarding islet encapsulation deals with microencapsulation of single or small groups of islets. However, this technique has inherent limitations like lower mechanical strength of the capsules, poor retrieval and short life span in vivo.
3. Silk sutures already have an existing history of safe use in surgeries. The recent exponential growth in tissue engineering and regenerative medicine has considered silk as a potential biomaterial. Silk, being a structural protein, is biocompatible, non-immunogenic, mechanically robust with tunable degradation rates. Silk protein is easy to process under a mild (aqueous) physiological-like conditions, and its degradation products are amino acids which can easily be metabolized in vivo. Silk bioresources are available in abundance and silk-based matrices have demonstrated exceptional advantages over conventional synthetic and other natural biomaterials in tissue engineering applications.
4. Arg-Gly-Asp (RGD) sequence has conspicuous effect on islet adherence, viability and revascularization owing to its interaction with several integrin variants. Natural and synthetic 3D matrices with externally tethered RGD peptides have shown promising outcomes for islet encapsulation and function maintenance. Recently, it has been reported that the non-mulberry silk, Antheraea assama, is enriched with RGD sequences which makes silk as an essential matrix for islet encapsulation.
5. Rapid to gel, injectable hydrogels are promising vehicles that can deliver encapsulated islets in vivo in a minimally invasive manner. The gelation property of such hydrogels under a mild (aqueous) physiological-like condition allows encapsulation of islets by preserving their functions. Moreover, accelerated gelation without any external stimulus or cross-linker provides an excellent platform for islet encapsulation and delivery. Blend of mulberry B.
mori silk with non-mulberry A. assama fabricates a well integrated matrix with inherent cell binding Arg-Gly-Asp (RGD) sequence.
6. Being an autoimmune disease, pancreatic beta cells are selectively destroyed by the patient’s own immune system Furthermore, surgery-associated tissue damage evokes inflammation which alters the immune environment of the implants. Under such adverse conditions, encapsulated islets undergo tremendous stress and secrete their own pro-inflammatory molecules amplifying the local inflammation. To prevent such pitfalls, the recipients
require life-long administration of systemic immunosuppressive drugs which have detrimental side-effects. In contrast to systemic delivery, localized delivery of immunosuppressant at the implantation site is likely to ensure minimal side-effects and eliminate the need for systemic immunosuppression.
7. Recent advances in tissue engineering and regenerative medicine illustrate the significance of ‘immuno-informed’ biomaterials to regulate the microenvironment of biomedical implants. Macrophages, the key players in immunoregulation, are actively involved in tissue remodelling and vascularization. Macrophages are also the prevalent infiltrating immune cells attacking islets post-transplantation which play crucial role in implant rejection.
Mature macrophages (M0) when exposed to appropriate stimuli (cytokines, drugs, pathogens etc.), are polarized towards pro-inflammatory (M1) or anti- inflammatory (M2) phenotype. Silk has been explored extensively for delivery of biomolecules and growth factors for different tissue-specific applications.
Controlled localized release of anti-inflammatory cytokine interleukin-4 (IL-4) and dexamethasone might polarize resting M0 macrophages to M2 phenotype reducing local foreign body response and maintain immunosuppressive function.
In the light of the enormous scope of exploring silk biomaterial and its attributes for islet encapsulation, combating local inflammation, and improvement in islet engraftment, we analyzed our hypothetic approach through the pursuit of the following essential objectives:
Objectives:
1. Development of localized immunomodulatory silk-alginate/agarose-based scaffolds for islet-like spheroid formation and immunoisolation.
2. Macrophage polarization for immunomodulation and tailoring local biological response.
3. Development of minimally invasive injectable silk hydrogel as potential islet encapsulation and delivery matrix.