Previous clinical studies have shown that grafting of human fetal brain tissue into the CNS of adult recipients can be associated with long-term (more then 10 years) graft survival even after immunosuppression is terminated. These clinical data represent in part the scientific base for the CNS to be designated as an immune privilege site, i.e., immune response toward grafted cells is much less pronounced. With rapidly advancing cell sorting technologies which permit effective isolation and expansion of neuronal precursors from human embryonic stem cells, these cells are becoming an attractive source for cell replacement therapies. Accordingly, there is great need to develop drug therapies or other therapeutic manipulations which would permit an effective engraftment of such derived cells with only transient or no immunosuppression. Accordingly, the primary goal in our studies is to test engraftment of 3 different neuronal precursors cell lines of human origin once grafted into spinal cord in transiently immunosuppressed minipigs. In addition, because the degree of cell engraftment can differ if cells are grafted into injured CNS tissue, the survival of cells once grafted into previously injured spinal cord will also be tested. Second, we will test the engraftment of neuronal cells generated from pig skin cells (fibroblasts) after genetic reprogramming (i.e., inducible pluripotent stem cells, iPS). Because these cells will be transplanted back to the fibroblast donor, we expect stable and effective engraftment in the absence of immunosuppression. Jointly by testing the above technologies (transient immunosuppression and iPS-derived neural precursors), our goal is to define the optimal neuronal precursor cell line(s) as well as immunosuppressive (or no) treatment which will lead to stable and permanent engraftment of spinally transplanted cells.
Brain or spinal cord neurodegenerative disorders, including stroke, amyotrophic lateral sclerosis, multiple sclerosis or spinal trauma, affect many Californians. In the absence of a functionally effective cure, the cost of caring for patients with such diseases is high, in addition to a major personal and family impact. Our major goal is to develop therapeutic manipulations which are well tolerated by patients and which will lead to stable survival of previously spinal cord-grafted cells generated from human embryonic stem cells. If successful, this advance can serve as a guidance tool for CNS cell replacement therapies in general as it will define the optimal immune tolerance-inducing protocols. In addition, the development of this type of therapeutic approach (pharmacological or cell-replacement based) in California will serve as an important proof of principle and stimulate the formation of businesses that seek to develop these types of therapies (providing banks of inducible pluripotent stem cells) in California with consequent economic benefit.
The ultimate goal of this proposal is to define an optimal immune suppression (IS) regimen and cell source that will allow successful engraftment of neural progenitor cells (NPCs) for the treatment of spinal cord injury (SCI). In the first two aims, the applicant will employ transient immunosuppression to compare engraftment and tolerance induction of human NPCs, human embryonic stem cell-derived NPCs (hESC-NPCS), and human induced pluripotent stem cell-derived NPCs (hiPS-NPCs) in preclinical models of naïve (Aim 1) or traumatically injured (Aim 2) spinal cord. For the third Aim, the applicant will systematically characterize the engraftment properties of isogenic, porcine iPS-NPCs in the absence of immunosuppression.
Reviewers agreed that if successful, the proposed research could have profound clinical impact on the treatment of spinal cord injury. While several of the approaches for Aims 1 and 2 lacked originality, the development and use of the chosen model was considered both innovative and advantageous. Furthermore, the isogenic, iPS experiments of Aim 3 were valued for their novelty and broader significance. Overall, reviewers were optimistic that the combined strengths of this proposal would enable an unmet need to be addressed and could ultimately advance the clinical utility of stem cell derived therapies for SCI.
Reviewers described the research plan as well constructed, straightforward, and logical. The solid preliminary data supported both the feasibility of the approach as well as the technical capabilities of the investigators. Furthermore, the applicants had largely considered potential pitfalls and proposed satisfactory alternative plans. While these strengths were considerable, the reviewers discussed a number of minor deficiencies and one major concern. Minor deficiencies included the lack of a functional readout following successful engraftment of NPCs, the lack of alternative plans for the IS regimen should the proposed method prove unsuccessful, and the lack of sensitivity of the proposed assays in detecting small degrees of graft versus host reactions. Reviewers were also uncertain that IS protocols generated in a xenograft model would inform human allogeneic transplantation and were unsure of the utility of comparing experiments from Aim 1 to Aim 2 due to dramatically different host environments and varying IS regimens. While none of these concerns significantly detracted from reviewers’ enthusiasm, several expressed reservations about the details of the IS regimen. Reviewers noted that the proposed drugs (especially in combination) are not compatible with induction of tolerance and debated the familiarity of the applicant with the current state of the art. Reviewers strongly recommended that the applicants fine tune the IS protocol by considering appropriate modifications to the timing and duration of the various treatment(s).
The Principal Investigator (PI) was praised as an established expert in spinal cord research who is well funded and capable of doing the proposed studies. The Co-Investigator’s extensive expertise in transplant immunology was considered essential to the success of the project, although there was some concern that there was little evidence that the PI and Co-Investigator had collaborated previously.
In summary, while strongly advising the investigators to adjust their IS protocol, reviewers enthusiastically supported this effort for its innovative development of an SCI model as well as its high likelihood for clinical impact.