Year 2

For the promise of stem cell transplantation therapy to treat or cure human disease to be realized, the key problem of stem cell transplant rejection must be solved. Yet, despite over three decades of research in human embryonic stem cells, little is known about the factors involved in immune system tolerance to grafts derived from embryonic stem cells.

The goal of our CIRM Stem Cell Transplantation Immunology Award is to overcome this formidable hurdle by generating pre-clinical mouse models that have human immune systems. This cutting-edge model system will provide a testing platform to evaluate the importance of matching immune system components, known as human leukocyte antigens (HLAs), between the human embryonic stem (hES) cell-derived neural stem cell (NSC) graft and the patient. Because mouse and human immune systems are fundamentally different, these next-generation ‘humanized’ mice are currently the only animal models within which to conduct our stem cell brain transplant experiments. Such models rely on immunocompromised mice as recipients for human umbilical cord blood stem cells (HSCs). These mice go on to develop a human immune system, complete with HLAs, and can subsequently be used to engraft embryonic stem cell-derived brain cells that are either HLA matched or mismatched and to monitor for graft acceptance vs. rejection.

During this second year of CIRM funding, we have accomplished three main goals leading to completion of Specific Aim 2, which is designed to perform HLA haplotype ‘mix and match’ experiments using hES cell-derived NSCs as donors and ‘humanized’ mice as recipients (year 2). Firstly, we have now successfully generated ‘humanized’ mice that have 50% or more engraftment of human immune cells in lymphoid organs, defined as percentage of human immune cells within the mouse. Secondly, we have successfully HLA haplotyped these human donor CD34+ HSCs, and have additionally transplanted hES cell-derived NSCs with known HLA haplotypes. Finally, we have ‘mixed and matched’ HLA haplotypes in adoptive transfer experiments using human HSC reconstituted mice as recipients and human NSCs as donors. This critically important new tool will allow for a predictive model of human stem cell transplant acceptance vs. rejection.