Development of a microgel-based cell delivery platform for critical limb ischemia
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Peripheral Arterial Diseases (PAD) is a chronic and debilitating disease caused due to narrowing and hardening of arteries, that results in progressive deterioration of blood flow to the limbs. PAD commonly occurs in the presence of multiple comorbidities, including hypercholesterolemia, diabetes, obesity and stroke. On average one in twenty individuals over the age of 60 have some degree of arterial claudication and it worsens with age. Two thirds of the patients suffering from PAD develop a pathological condition called Critical Limb Ischemia (CLI). Limb ischemia induces severe hypoxia and degeneration of blood vessels resulting in irreversible damage manifested by gangrene or necrosis. 30% of those with CLI will require amputation within one year of diagnosis and 25% will die from circulatory complications. Several surgery-based endovascular interventions are only beneficial for patients with fewer occlusions and availability of autologous vein grafts. Delivery of genes and growth factors demonstrate transient efficacy due to low retention, degradation and non-maintenance of a physiologically relevant dose to trigger a regenerative response. Alternatively, delivery of stem cell factories that manufacture therapeutic proteins and modulate acute inflammatory response accelerate tissue regeneration. However, poor retention of the cells at the site of injection results in limited therapeutic effect. To maximize the therapeutic effect, a cell delivery platform was developed for in vivo delivery of human mesenchymal stem cells. A microgel-based cell delivery platform was fabricated to serve as a ‘restrain’ and ‘re-train’ implantable device, embedding human mesenchymal stem cells. In vitro studies showed that altering the macromolecular concentration can tune the paracrine responses of the mesenchymal stem cells in a time-dependent manner. Further, to test for therapeutic angiogenesis of the microgels there was a need for the development of a relevant pre-clinical animal model which manifests the severity of the CLI. Hence the femoral artery double-ligation model xxxv was established in immunocompromised mouse that exhibits poor perfusion, ambulatory impairments and severe signs of tissue necrosis. Finally, the optimised microgels were tested at a low-cell dose (up to 20 times lower than that of preclinical gold standard) in a pre-clinical hindlimb ischemia model developed in-house. The microgels accelerated formation of new blood vessels with reduced inflammatory response and impeded the progression of tissue damage. Furthermore, molecular analysis revealed that several key mediators of angiogenesis were upregulated in low-cell dose microgel group, providing a mechanistic insight of pathways modulated in vivo. The research adds to the current knowledge in cell encapsulation strategies by highlighting the preconditioning or priming capacity of biomaterials through cell- material interactions. In addition, obtaining therapeutic efficacy at a low-cell dose in the microgel platform is a promising clinical route that would aid in faster tissue repair and reperfusion and benefit several ‘no- option’ patients suffering from CLI.