Objective
While we now have better understanding that rejection of transplanted cells and devices by the body are both immune-mediated, we need to discern the underlying mechanism (e.g., which cells, cytokines, etc.) of how our immune system is specifically leading to indirect graft killing through an encapsulating device. By modifying (e.g., surface recognition receptor, sugars) or changing transplant grafts (e.g., from different species closer vs. farther away from a recipient, or by incrementing biomass levels) we hope to be able to discern which immune cells or signaling pathways need to be inhibited to prevent indirect rejection. Further, by simultaneously treating patients with not one but two targeted agents, we will prevent both types of host rejection (one to the biomaterial and one to the graft). We will identify a list of targeted agents, which should dramatically improve treatment success, over classical broad-spectrum anti-inflammatories, which often exhibit non-specific side effects. Further, by delivering and releasing these drugs directly from the device, we can achieve local immune suppression, so that patients’ immune systems are not globally suppressed. Using this improved long-term, slow release technology, we can also increase the duration of success of this therapy to be months to years long.
Background Rationale
Direct integration and controlled-release of various anti-inflammatory drugs have reduced device rejection by the body for various medical device and tissue engineering applications. However, work is still required to identify targeted agents that not only prevent biomaterial rejection but also indirect transplanted cellular or tissue graft rejection by the body. Doing so requires us to probe further at both surface-bound vs. secreted proteins (across pig and human-derived grafts), and to do that we will explore host immune responses against a variety of surface-bound MHCI vs II knockouts as well as post-translational sugar knockouts which will target all proteins, secretome-included. We will further test target robustness by utilizing both wildtype mice as well as humanized (human immune system) mice. Of note, immune cell depletion strategies are available for both in vivo systems, enabling us to confirm any preclinical mouse immune findings by directly looking at human immune cells dynamics (albeit in a mouse) as well. Doing so will add extra credence to our findings. Once this is done, we plan on capitalizing on a new drug delivery platform that we previously optimized for preventing rejection of biomaterial devices alone. Of note, this drug delivery system allows for the integration and slow release of more than one very targeted agent to elongate the prevention of host rejection (of both biomaterial and graft) for as long as possible (already shown to be months to years long). Thus, drug-release strategies, ie., slowly over time and only locally from the transplanted device itself, will be adapted to utilize a two-drug system for improved long-term treatment efficacy in preventing rejection of both the biomaterial device and the live cell or tissue transplant within. While we have already identified improved agents capable of inhibiting only the specific immune population(s) responsible for biomaterial rejection, compounds that prevent immune rejection specifically aimed at living transplants need to be identified.
Description of Project
Even with modern advances, implanted biomedical devices, including islet-containing biomaterials for transplantation and delivery of insulin to diabetic patients, are still sensed as foreign by the body and rejected by the immune system. When this happens, since the body cannot clear or eliminate larger material objects, it does the next best thing by sealing them away behind dense walls of fibrotic scar tissue. This then prevents nutrients, such as oxygen and sugar, from reaching the living islets inside, leading to their death due to starvation. At the same time, islet-produced insulin is also lost once the factory islet cells are gone, leading to device failure, and the need for patients to once again commit to regular, daily insulin injections. We now have a much better understanding of what is needed to prevent the biomaterial rejection response; however, we now also know that the immune system response gets worse when islet grafts, more foreign to the recipient and in higher quantity, are placed inside. Our previous grant allowed us to breakdown and characterize this complex extra layer of host immune rejection, which has also been shown to be responsible for sensing protein that the graft secrete through an outer encapsulating biomaterial meant originally to immunoisolate the cells so that they could not be rejected in the body. The immune system, which normally requires cell contact to kill a foreign graft, shockingly demonstrated the ability to still eliminate the graft (albeit more slowly) by secreting toxic, cell stress/death-inducing factors indirectly over a distance back through the outer immunoisolating biomaterial. Despite identifying which immune cells are responding locally at the capsule surface, there were many responding members of both the innate and adaptive immune systems. Therefore, we propose to first determine which cells are required for this added layer of indirect graft rejection response by removing them one at a time, and potentially in combination, using genetic knockout mice as well as cell depletion tools with which we have extensive experience. Further, we plan to do so using both wildtype as well as humanized (human immune system) mice. Doing so will allow us to better understand which cells should be targeted with new therapeutic interventions to address this added layer of allow long-term tissue transplant survival. We will also be utilizing different inputs to see which graft-associated factors play more vs. less of a role in eliciting rejection: i) renewable SC-beta islet-like transplants that have different surface (major histocompatibility complex, MHC) recognition proteins either intact or knocked out, and ii) testing porcine islets modified to lose specific sugars (relevant for all secreted proteins, not just surface-bound recognition receptors) that have been shown to drive rejection in humans. Such modifications are what allowed recent success with pig heart-into-human transplantation. Ultimately, we believe these efforts will allow us the opportunity to simultaneously treat the patient with anti-inflammatories to prevent host rejection of not only the surrounding, protective biomaterial but also the biologic cell or tissue inside. To this point, we have already developed a long-term crystalline drug reservoir delivery system so that by releasing these drugs directly from the transplanted devices themselves, we can achieve local immune inhibition, so that patients’ immune systems are not globally suppressed. We can further tune the duration of success of this therapy by a significant amount of time up to months or even years-long release. Farther down the road, we feel that tackling the added layer of indirect graft rejection will help enable strategies for grafts to be transplanted without the need for encapsulation at all.
Anticipated Outcome
We will identify additional immune response(s), in both rodent as well as human immune system models, responsible for rejection of encapsulated pancreatic beta cells contained within our biomaterial system. We will utilize correlative kinetics data and conjunction with functional knockout/depletion transplantations to identify which immune cells, and as a result next-generation targets they possess, to do so. Only then will we be able to identify next generation drug targets associated with those cells, as well as associated molecules capable of targeting them for downstream testing to eliminate indirect graft killing. We expect to identify two-drug systems that successfully inhibit rejection responses targeting both encapsulating biomaterial as well as the living tissue grafts contained within. By doing so, we can further elongate therapeutic efficacy of cellular transplantation systems for type 1 diabetes.
Relevance to T1D
Transplantation of donor pancreatic islet cells to restore normoglycemia in diabetic patients has been common practice for decades. However, major hurdles still exist that drastically limit the efficacy of this treatment for patients. One major problem is the need for immunosuppressive therapies to prevent rejection of transplanted cells. Early efforts were placed in developing a bioartificial pancreas to eliminate the need for immune suppression, where islets are encapsulated within an immunoprotective surrounding biomaterial. Originally, this strategy has succeeded in some contexts to isolate transplanted islet cells from host immune rejection while allowing exchange of nutrients, as well as release of insulin. However, the encapsulating material alginate would always eventually be attacked by the host immune system, ultimately leading to death of the islets instead and device/treatment failure. Some of our recent efforts, previously supported by BT1D, thankfully succeeded in identifying and preventing immune-mediated rejection to the encapsulating biomaterial. However, we now appreciate that the farther away the donor tissue or cells is species-wise from the recipient host patient, a stronger, worsened rejection response ensues. Increasing the amount of the graft makes the problem worse as well. As such, depending on the cell or tissue donor source, simultaneous immune suppression against both the biomaterial device as well as the biologic graft contained within is required for true clinical translation and success. Therefore, we need to understand this additional layer of rejection to engineer solutions that may be implemented to help ensure long-term clinical success, short-term for encapsulated grafts and perhaps longer-term for transplants without the need for encapsulation at all.