- How do you grow islet-like structures in vitro?
- How does the scaffold support transplantation of the islet-like structures in vivo?
- How can you modify the scaffold to address inflammation, engraftment, and the immune response?
- What research directions are you pursuing now?
- What motivated you to develop therapies for diabetes?
Like a differentiated stem cell, Lonnie Shea’s strong foundation in fundamental cellular biology underwent just the right prompts to evolve into a proliferative biomedical research career. During graduate school, he studied drug interactions with cell surface receptors and downstream signaling pathways. As the field of tissue engineering gained a foothold, he recognized an opportunity to leverage his expertise to advance the translational potential of new classes of biomaterials. Shea studied biomaterials and tissue engineering as a postdoctoral scholar at the University of Michigan School of Dentistry, and as he encountered unmet needs in regenerative medicine, found applications for his research ranging from ovarian follicle maturation to spinal cord regeneration (1). “I've always tried to approach it from the perspective of how we can design materials with the right cues for the cells to actually be able to have the development or function that would be required of them,” he said.
Now a chemical and biological engineer at the University of Michigan, Shea is taking a biomaterials-based approach to cell therapy for type 1 diabetes. In this form of diabetes, an autoimmune reaction destroys β-cells in the pancreas that secrete insulin in response to glucose. As an alternative to lifelong insulin-based management, the transplantation of pancreatic islets (well-defined clusters of β-cells and other endocrine cells) has emerged as a promising therapy for restoring insulin function. This procedure, however, still faces several hurdles including shortages of donor islets, the specific autoimmune attack on β-cells, and general immune rejection of foreign donor-derived cells. Islet transplant recipients may need to take immunosuppression drugs for decades, increasing their risks for infection.
To overcome these limitations, Shea’s team has developed biocompatible microporous scaffolds that support the assembly of stem cell-derived islet-like structures in vitro and their successful transplantation in vivo. By integrating other cells or biomolecules in the scaffold, the researchers aim to combat the many obstacles that the islet-like structures face when introduced into the body. Armed with this unique form of physical and biological control, Shea hopes to provide a cell therapy strategy for type 1 diabetes with the longevity needed for clinical settings.
How do you grow islet-like structures in vitro?
We use pluripotent stem cells to try to overcome the first major challenge of the donor supply problem because they have an almost indefinite capacity to proliferate in vitro. We can expand them down different lineages to potentially provide a more sustainable source of cells. We’re building upon amazing stem cell work using a six-stage culture system to recapitulate the normal cellular developmental processes. We add different growth factors at each stage to initially maintain pluripotency and then drive pluripotent cells to definitive endoderm, pancreatic progenitors, and endocrine progenitors.
We use a scaffold to recreate a 3D environment for the cells (2). On a typical flat tissue culture surface, the cells begin to pile up at the latter stages of culture and form clusters. Using the scaffold, we can make that clustering happen earlier because rather than a single plastic substrate, the cells have multiple surfaces to adhere to that support a 3D architecture. Three-dimensional structures are particularly important for islet organoids because they enable cell-cell interactions between multiple cell types. So far, our team and other researchers have observed that the cellular maturation process is not complete at the end of in vitro culture; it continues upon transplantation.
How does the scaffold support transplantation of the islet-like structures in vivo?
For cultures on a plastic substrate, researchers often lift the clusters off of the plate to transplant them, which changes the microenvironment of the cells in an uncontrolled and potentially chaotic way. When we grow these organoids on a scaffold, we don't have to manipulate the cells, which have created their own niche and formed connections with other cells. We simply bring the whole scaffold from the culture dish to the transplantation site, preserving those interactions.
The scaffold also helps the cells to obtain nutrients through diffusion before blood vessels infiltrate. If the cells formed one large clump, the outer cells would receive nutrients, but the inner ones would starve. The organoids on the scaffold are evenly distributed across the surface rather than clumped together, facilitating nutrient access. We’ve found that by transplanting islet-like structures into mice, we can reduce blood glucose levels and approach a normoglycemia state (3).
We are modifying the scaffold to address various in vivo challenges such as inflammation, engraftment, and the immune response to the transplanted cells. The material provides a tool that we can adapt to immobilize factors or codeliver cells, allowing us to influence what happens to the cells post-transplantation and ultimately support their function for long periods of time.
How can you modify the scaffold to address inflammation, engraftment, and the immune response?
Inflammation is one of the major challenges in the post-transplantation environment. The material can cause a foreign body reaction, and the surgery itself can initiate a wound healing response and upregulation of inflammatory factors that damage the islet structures. We can design the material to release anti-inflammatory factors, such as the cytokines IL-10 and TGF-β, to limit the immediate inflammatory response (4).
We can also codeliver endothelial cells, a major component of blood vessels, to enhance the rate of vascularization (5). Incorporating endothelial cells in this way provides the cell mass needed to build blood vessels and promotes the formation of sprouts that connect with existing endothelial cells to supply the islet cells with nutrients and signals. We have monitored our mice for almost a year post-transplantation and found that the scaffold has biodegraded and the islets have integrated within the host vasculature. So, the scaffold supports the organization of the cells for a short period of time, but disappears as the islets engraft long-term.
The material provides a tool that we can adapt to immobilize factors or codeliver cells, allowing us to influence what happens to the cells post-transplantation and ultimately support their function for long periods of time.
- Lonnie Shea, University of Michigan
We can also immobilize immunomodulatory factors in the material so that the body doesn’t destroy or reject the islet cells after a certain period of time. We’ve tried to recreate this phenomenon of local immune privilege seen in the eye and the testes, where destructive immune responses are suppressed. Our collaborator developed a molecule called Fas ligand, which binds to the Fas receptor. This Fas ligand-receptor interaction can induce cell death in T cells that attack the foreign islet cells and induce regulatory T cells that reduce this destructive T cell activity. We decorated the scaffold with Fas ligand and transplanted it into the epididymal fat pad in mice, which is analogous to the omentum (the fatty tissue that drapes from the stomach over the intestines) in humans. While the current clinical transplant site is the liver, the omentum is a promising alternative that is more compatible with such immunomodulation strategies. We were able to create a local immunoprivileged site where the destructive T cell response decreases locally, but the immune system remains intact elsewhere (6). We’re trying to block only the immune response to these particular cells and avoid suppressing the general immune response to disease.
What research directions are you pursuing now?
We’re exploring nanoparticle-based approaches to reprogram specific immune responses to reduce inflammation and prevent destruction and rejection of the transplanted cells. We're also collaborating with Scott Soleimanpour, a cellular metabolism researcher at the University of Michigan. As he put it, many people think of islets as bags of insulin that is secreted upon exposure to glucose, but in reality, they're bags of insulin and mitochondria. Since they’re constantly sensing blood glucose and secreting the right amount of insulin, islets have very large energy demands; they make up approximately one percent of the cell mass of the pancreas, but receive 10 to 15 percent of the blood supply. Therefore, we are focusing on enhancing the metabolic maturity of the cells both in vitro and in vivo through strategies such as adding specific pharmacological agents. Metabolism and cell function can become compromised in the stressful transplant environment. By making the cells more metabolically robust, we hope that they can sustain their activity for longer periods of time.
What motivated you to develop therapies for diabetes?
In meeting with patients who have diabetes, I’ve learned that they live with a lot of extra burden in order to manage their diseases. One patient said that his diabetes can cause him to fall unconscious, and he was struggling with how he could take his young child out in public with the risk of experiencing an episode. Everything has to be planned out meticulously to make sure that every trip he takes is successful, and that really stuck with me. The ability to provide a cell therapy where we could remove the energy for those concerns and the burden that patients have to carry, that's what I hope to help accomplish.
This interview has been condensed and edited for clarity.
References
- Thomas, A. M. et al. Channel density and porosity of degradable bridging scaffolds on axon growth after spinal injury. Biomaterials 34, 2213–2220 (2013). https://www.sciencedirect.com/science/article/abs/pii/S0142961212013683
- Youngblood, R.L., Sampson, J.P., Lebioda, K.R., & Shea, L.D. Microporous scaffolds support assembly and differentiation of pancreatic progenitors into β-cell clusters. Acta Biomater 96, 111-122 (2019). https://www.sciencedirect.com/science/article/abs/pii/S1742706119304453
- Rios, P.D. et al. Evaluation of encapsulating and microporous nondegradable hydrogel scaffold designs on islet engraftment in rodent models of diabetes. Biotechnol Bioeng 115, 2356-2364 (2018). https://onlinelibrary.wiley.com/doi/abs/10.1002/bit.26741
- Gibly, R.F. et al. Advancing islet transplantation: from engraftment to the immune response. Diabetologia 54, 2494-2505 (2011). https://link.springer.com/article/10.1007/s00125-011-2243-0
- Clough, D.W., King, J.L., Li, F. & Shea, L.D. Integration of islet/beta-bell transplants with host tissue using biomaterial platforms. Endocrinology 161, bqaa156 (2020). https://academic.oup.com/endo/article/161/11/bqaa156/5902435?login=false
- Skoumal, M. et al. Local immune tolerance from FasL-functionalized PLG scaffolds. Biomaterials 192, 271-281 (2019). https://www.sciencedirect.com/science/article/abs/pii/S0142961218307920