Engineering Thymic Regeneration to Induce Tolerance

Engineering Thymic Regeneration to Induce Tolerance

Funding Type: 
Transplantation Immunology
Grant Number: 
RM1-01707
Approved funds: 
$1,235,445
Disease Focus: 
Immune Disease
Public Abstract: 
A healthy immune system produces T cells that can recognize and react against foreign molecules (antigens) to protect against infection, while leaving normal host cells with “self antigens” undamaged. All T cells are produced in the thymus from blood stem cells that migrate from the bone marrow. “Tolerant” T cells are those that have been “educated” to not react against self antigen on host cells. The key cells in the thymic microenvironment that control T cell production and tolerance are the thymic epithelial cells (TECs). When TECs are lost or become dysfunctional, T cell production is poor and patients are at risk for a wide range of infections. When tolerance is lost, T cells react to host tissues as if they were foreign, producing inflammation and damage and causing autoimmune diseases such as Type I Diabetes, multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus. The ability of a patient to accept cells or an organ transplant from another person also requires tolerance to occur, or the graft will be rejected. The goal of our studies is to develop a method for engineering and transplanting new, healthy thymus tissue into patients, thus creating a way to generate healthy, tolerant T cells. We have developed a method to engineer one component of the thymic microenvironment (thymic mesenchyme aka Tmes) to produce specific growth factors in a regulated fashion to help TECs grow, and are able to combine TECs and Tmes to form a functional thymus which can then be implanted into mice. This method can be applied to either mouse thymus or human thymus, and in each case the thymic implants allow T cells to develop. We now propose to engineer the thymic implants to produce specific growth factors that we have identified as critical for the rapid thymic growth seen in the fetal and neonatal periods. The ability of engineered thymic aggregates to support the production of mouse and human T cells in cell culture and after transplants in mice will be determined. Importantly, the ability of the thymic implants to educate T cells to become immune tolerant will be assessed. The success of this proposal will have wide-ranging applications for stem cell and regenerative medicine. Re-generation of new thymic tissue for transplantation will be useful for patients who have poor thymic function e.g. during aging, after chemotherapy and bone marrow transplantation and in certain conditions such as AIDS. Induction of immune tolerance with regenerated thymus will be a critical component of the treatment of autoimmune diseases and graft versus host disease of bone marrow transplantation and to prevent rejection of organs and stem cells from mis-matched donors.
Statement of Benefit to California: 
The ability to regenerate and control the immune system is of critical importance in the treatment of a wide-range of life-threatening diseases. A healthy immune system is required to control infections and prevent autoimmune diseases. The thymus gland, which is central in the regulation of the immune system, is damaged by infections such as HIV-1, by chemotherapy and by aging. Each year, tens of thousands of new patients are diagnosed with autoimmune diseases such as Type I Diabetes, Multiple Sclerosis, Systemic Lupus Erythematosus and rheumatoid arthritis. Over 21,000 Californians and 100,000 Americans are estimated to be in need of organ donation for life threatening diseases [Donate Life, California, Organ and Tissue Donor Registry]. Heavy suppression of the immune system is required to prevent rejection of transplanted organs resulting in many serious side effects for patients. Each year, thousands of patients undergo bone marrow transplantation for leukemia and genetic diseases. The main problem for patients after bone marrow transplantation is immune-mediated graft versus host disease and infection from poor thymic function. Thus, understanding how to regenerate and control the function of the thymus would have an enormous potential impact on the treatment of many common and debilitating diseases that affect Californians.
Progress Report: 

Year 1

A healthy immune system produces T cells that can recognize and react against foreign molecules (antigens) to protect against infection, while leaving normal host cells with “self antigens” undamaged. All T cells are produced in the thymus from blood stem cells that migrate from the bone marrow. “Tolerant” T cells are those that have been “educated” to not react against self antigen on host cells. The key cells in the thymic microenvironment that control T cell production and tolerance are the thymic epithelial cells (TECs). When TECs are lost or become dysfunctional, T cell production is poor and patients are at risk for a wide range of infections. When tolerance is lost, T cells react to host tissues as if they were foreign, producing inflammation and damage and causing autoimmune diseases such as Type I Diabetes, multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus. The goal of our studies is to develop a method for engineering and transplanting new, healthy thymus tissue into patients, thus creating a way to generate healthy, tolerant T cells. A major problem with regenerating the thymus is that the TECs, which are so important for T cell growth and differentiation, tend to die during culture. Over the first year of grant support, we have developed a method to engineer one component of the thymic microenvironment (the thymic mesenchyme) to produce specific growth factors that we hope will protect TECs. We have developed specific culture conditions that allow us to grow the thymic mesenchyme separately to the TECs. We then take the mesenchyme and TECs out of culture and spin them together to form a cluster of cells called a thymic aggregate. We have shown that when we combine these “thymic aggregates” with cord blood stem cells (also known as “hematopoietic” stem cells) we can produce T cells from the cord blood. We can make T cells in the aggregates either in culture or after implantation of the aggregates into immune deficient mice. Furthermore, by genetically engineering the thymic mesenchyme to secrete the growth factor VEGF (Vascular Endothelial Growth Factor), we find that we can produce significantly more T cells and improve the architecture and function of the thymic aggregates for at least 5 weeks. We are now in the process of analyzing the function of the T cells produced in the implanted aggregates. Most of the studies in the past year have been conducted using fragments of human thymus discarded after cardiac surgery. We are now modifying the technique to use mouse thymus as this is the only way that we can test whether the T cells produced in the implants can induce tolerance. Other planned studies for the next year include tracking the survival and growth of the aggregates after implantation using an imaging technique called bioluminescence. These studies will help us understand how the initial period of culture affects long-term survival of the implants and how to optimize the number of T cells from the implants. The studies proposed during the next year will be essential for the final phase of the proposal-testing whether the implants can produce healthy, tolerant T cells.

Year 2

A healthy immune system produces T cells that can recognize and react against foreign molecules (antigens) to protect against infection, while leaving normal host cells with “self antigens” undamaged. All T cells are produced in the thymus from blood stem cells that migrate from the bone marrow. “Tolerant” T cells are those that have been “educated” to not react against self antigen on host cells. The key cells in the thymic microenvironment that control T cell production and tolerance are the thymic epithelial cells (TECs). When TECs are lost or become dysfunctional, T cell production is poor and patients are at risk for a wide range of infections. When tolerance is lost, T cells react to host tissues as if they were foreign, producing inflammation and damage and causing autoimmune diseases such as Type I Diabetes, multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus. The goal of our studies is to develop a method for engineering and transplanting new, healthy thymus tissue into patients, thus creating a way to generate healthy, tolerant T cells. A major problem with regenerating the thymus ex vivo is that the TECs, which are so important for T cell growth and differentiation, tend to die during culture. We have developed a method to engineer one component of the thymic microenvironment (the thymic mesenchyme) to produce specific growth factors that we propose will protect TECs. We have developed specific culture conditions that allow us to grow the thymic mesenchyme separately to the TECs. We then take the mesenchyme and TECs out of culture and spin them together to form a cluster of cells called a “thymic aggregate”. We have shown that when we combine these thymic aggregates with cord blood stem cells (also known as “hematopoietic” stem cells) we can produce T cells from the cord blood. We can make T cells in the aggregates either in culture or after implantation of the aggregates into immune deficient mice. Most of the studies in the first year of the grant were conducted using fragments of human thymus discarded after cardiac surgery. In this the second year we have tested the function of T cells produced in the implanted human aggregates and our results suggest they will be able to respond to a wide range of foreign antigens and thus protect against infection and cancer. During the past year we have also focused on modifying our methods to improve the efficiency of aggregation using mouse thymus so that we can test whether the T cells produced in the implants can induce tolerance. We have also been optimizing our culture techniques to increase TEC growth from both mouse and human thymus. These studies have been essential for the experiments in final year of the proposal-testing whether the implants can produce healthy, tolerant T cells.

Year 3

A healthy immune system produces T cells that can recognize and react against foreign molecules (antigens) to protect against infection, while leaving normal host cells with “self antigens” undamaged. All T cells are produced in the thymus from blood stem cells that migrate from the bone marrow. “Tolerant” T cells are those that have been “educated” to not react against self antigen on host cells. The key cells in the thymic microenvironment that control T cell production and tolerance are the thymic epithelial cells (TECs). When TECs are lost or become dysfunctional, T cell production is poor and patients are at risk for a wide range of infections. When tolerance is lost, T cells react to host tissues as if they were foreign, producing inflammation and damage and causing autoimmune diseases such as Type I Diabetes, multiple sclerosis, rheumatoid arthritis and systemic lupus erythematosus. The goal of our studies is to develop a method for engineering and transplanting new, healthy thymus tissue into patients, thus creating a way to generate healthy T cells. A major problem with regenerating the thymus ex vivo is that the TECs, which are so important for T cell growth and differentiation, tend to die during culture. Through this grant support we have developed a method to engineer one component of the thymic microenvironment (the thymic mesenchyme) to produce specific growth factors that we propose will protect TECs. We have developed specific culture conditions that allow us to grow the thymic mesenchyme separately to the TECs. We then take the mesenchyme and TECs out of culture and spin them together to form a cluster of cells called a “thymic aggregate”. We have shown that when we combine these thymic aggregates with cord blood stem cells (also known as “hematopoietic” stem cells) we can produce T cells from the cord blood. We can make T cells in the aggregates either in culture or after implantation of the aggregates into immune deficient mice. During the past year we have focused on modifying our methods to improve the efficiency of aggregation using mouse thymus so that we can test how well the T cells produced in the implants work.

© 2013 California Institute for Regenerative Medicine