Through this project, we have been successful in utilizing a gene therapy approach to deliver a newly discovered class of “small interfering RNA” (siRNA) to human stem cells. These siRNAs target and down-regulate proteins involved in recognition and rejection of donor cells by the immune system when they are transplanted into a recipient.
Initially it was thought that human embryonic stem cell (hESC)-derived cells and tissues might not be attacked by the immune system because these cells do not have Human Leukocyte Antigens on their surfaces in their primitive state. Human Leukocyte Antigens (HLA) are proteins that are expressed on the surface of almost all cells in the body. Because HLA sequences are highly variable and each person generally has a different set of HLA gene sequences, these cell surface markers serve as the identifiers of “self” vs. “non-self”. If immune cells in the body encounter foreign cells transplanted from a different individual, in most cases these foreign cells are recognized due to their display of a different “non-self” HLA on their cell surfaces, and attacked by the immune system. It is now known that once hESC start to develop into mature adult-type cells, they also start to increase their display of HLA, marking them as foreign “non-self” transplants. Thus, for hESC-derived cell and tissue transplants face the same problem of immune rejection as adult organ transplants.
To genetically reprogram human stem cells so they will not be rejected when transplanted into a different individual, we have used lentiviruses, a type of retrovirus which includes pathogens such as HIV, but as gene delivery vehicles (“vectors”), they have been completely disabled by removal of the viral genes, which have been replaced with the HLA-silencing siRNA sequences that we want them to deliver, thus turning viral foes into friends. Through this project, we have shown that this genetic reprogramming technique can be used to successfully down-regulate HLA expression in established human embryonic kidney cell lines, primary adult human blood-forming stem cells, primary adult human pluripotent stem cells, and human embryonic stem cells. We have also shown that, since the genetically re-programmed stem cells no longer display their own HLA due to siRNA-mediated silencing, they can no longer be recognized by immune cells or anti-HLA antibodies from other individuals, and are thus protected from immune rejection processes. We are proceeding to testing whether these genetically reprogrammed stem cells can evade the immune system when transplanted in vivo, and we have further developed a novel method for using chemotherapy to selectively expand the population of HLA-silenced stem cells after transplantation into the recipient.