Our overall project goal is enhance the survival of stem cell based therapies by understanding if they can be rejected by immune response, and if so, how to manipulate the immune response so that rejection can be avoided. Currently, we are using mouse embryonic stem cells (ESC) and the adult mouse as a prototype of a cell-based therapy and a human patient who requires blood stem cells.
In Year 1, we established mouse ESC culture in the laboratory and began to generate putative blood stem cells, or hematopoietic progenitors (HPs), using established culture methods. However, we noted that our yield of the HPs was too low for what is needed for transplantation. In Year 2, we compared different culture methods to generate higher numbers of HPs, and we found an improved culture method that is easier to scale up and requires less manipulation, which has increased our HP yields and simultaneously reduces the risk of possible contamination. In addition, we have found that HPs using this improved method appear to be similar to HPs found in the natural adult bone marrow. These results suggest that ESC-derived HPs might function similarly to those in the adult, and we will this hypothesis. Moreover, in Year 2 we have developed a strategy to predict if ESC-derived HP will stimulate the immune system of patients. This is important to assess because if the immune system rejects ESC-derived cells, the cell-based therapy could fail in diseased patients. Our data suggests that the culture method used to derive HPs from ESCs correlates with their potential immunogenicity, and we plan to experimentally test this idea in the next reporting period.
Another challenge that could affect the survival of stem cell based therapies in patients can be termed “developmental incompatibility”. HPs derived from ESC are embryonic or fetal in nature, and in cell-based therapies, these embryonic-like tissues will be expected to survive in a mature, adult cellular environment. There is very little evidence to date to show that ESC-derived HP can survive long term in an adult, and there is a paucity of information on how ESC-derived tissues interact at the cellular level with adult microenvironments, or “niches”. In Year 2, we have developed in vitro model systems that we can utilize to answer some of these questions. For example, we have developed a system using bone-building cells, or osteoblasts, which are one adult “niche” cell for blood stem cells, and we have established that the stage of osteoblast maturation correlates with their ability to support adult hematopoiesis. Another cell type that is generated from blood stem cells are T lymphocytes, which interact with thymic epithelial cells (TECs) in the adult mouse. TECs could be considered to be a “niche” cell for developing T lymphocytes. We have also devised an improved method to isolate mouse adult TECs, with the goal of designing an in vitro system similar to the osteoblast system described above. Our next goal is to apply these model systems to the study of ES-HP/niche cell interactions.