The human immune system is usually considered to act defensively, recognizing and reacting against invading elements from the outside world. Although such a protective response is often required (for instance, to defend against the entry and spread of infectious agents), it can also prevent the engraftment of foreign cells that might otherwise provide help (for instance, stem cells from an unrelated donor that could be provided as part of a therapy). The goal of this proposal is to find a way to permit such transplants to be tolerated and accepted when necessary. Our experimental strategy is instructed by the single occasion during each of our lives when we must tolerate the presence of “foreign” cells: the time when we are in the womb, in close contact with our related but genetically distinct mother. In recent experiments, we have discovered that this is a time when our immune systems include large numbers of so-called “regulatory T cells,” or Tregs. These cells are generated so that they can suppress the immune response of the fetus against the mother, allowing for a full-term pregnancy to take place without problems. Importantly, we have found that such Tregs are derived from a blood-forming stem cell in the fetus that is distinct from the one that is found after birth. Thus, the fetal stem cell gives rise to an immune system that confers tolerance and prevents an immune response against the mother, whereas the adult stem cell gives rise to an immune system that reacts against anything that is foreign (including cells from the mother). We wish to learn how to isolate the fetal stem cell, preferably from discarded umbilical cord blood, and to learn how to use it after birth so that tolerance to foreign stem cell grafts can be conferred. Two investigators and their respective teams with long experience in human immunology and in stem cell biology, as well as in the use of preclinical models to study stem cell transplantation, will collaborate on these studies. Using resources from CIRM as well as those leveraged from their existing programs, this team will carry out a milestone-driven program to test the feasibility of this approach. Should it work, the next step will be to optimize and to bring this novel approach to the clinic so that it might benefit those in need of stem cell grafts from unrelated donors for a myriad of currently untreatable diseases.
Statement of Benefit to California:
The proposed research will benefit the State of California and its citizens in three ways: • It will immediately leverage existing resources to support a multidisciplinary, multi-site, in-State collaboration that is likely to discover new knowledge in an efficient manner. Such collaborations form the basis for bench-to-bedside research and are best facilitated by goal-oriented programs such as that envisioned by the CIRM request for applications. • Pursuit of this project may move a novel strategy for inducing tolerance to stem cell grants to the clinic, thereby expanding the use of such grafts to a much larger population of patients in need. In particular, our proposed approach could open the door to the development of multiple new stem cell therapies that might be carried out in the absence of lifelong immunosuppression. This step, in turn, would likely encourage creative new applications of stem cells for an even broader constellation of human diseases than previously imagined. • Finally, the knowledge that is obtained could form the basis of a preclinical effort that would garner the input of additional support from non-State (e.g., venture capital) sources while also generating new jobs and taxable income for those in the State. If, for instance, preliminary data from this series of studies appear promising, a much larger infusion of resources would be required to realize the full potential of the proposed strategy. Since our approach will be applicable to all forms of allogeneic stem cell transplantation into humans, the potential demand for its use would be large and the chances for successful pursuit of additional resources should be correspondingly high. To the extent that such resources are forthcoming, new jobs and additional taxable revenues for the State will follow.
The goal of this proposal is to re-create in post birth individuals the naturally occurring tolerance the fetus exhibits towards allogeneic maternal antigens in order to allow for transplantation tolerance to allografts. The applicant suggests that fetal multilineage hematopoietic stem/progenitor cells (HSPC) are functionally distinct from adult hematopoietic progenitor cells (HPCs) and give rise to increased numbers of tolerogenic T regulatory cells (Tregs). These Tregs confer to the fetus an immune system that, unlike the immunogenic immune system generated by adult HPCs, is tolerogenic and permissive to the persistent engraftment of allogeneic maternal cells. Ultimately, if this hypothesis proves correct, fetal cells isolated from umbilical cord blood (UCB) could be injected intrathymically to induce tolerance to allogeneic stem cell grafts. The applicant proposes to develop an appropriate preclinical model to test this hypothesis and develop a protocol whereby this cell population could be used to generate tolerance to allogeneic transplants. In the first aim, the applicant proposes to characterize the preclinical model system and demonstrate that fetal T cell lineage cells from this system are functionally and phenotypically distinct from post birth T cells. In the second aim, the investigator will attempt to purify and in vitro expand fetal HSPCs, including those from UCB. In the third aim, the investigator will intrathymically inject the fetal HSPC population into neonates and analyze whether donor cells engraft and confer tolerance to allogeneic cell grafts. Reviewers agreed that the concept of tolerance induction through intrathymic injection is not novel and has been demonstrated to be largely unsuccessful using adult progenitors, but that the applicants propose an innovative and creative hypothesis: that fetal progenitor cells are inherently different from adult progenitors in producing increased numbers of Tregs and, thereby, generate a tolerogenic rather than immunogenic immune response. Reviewers agreed that if successful, this could provide a strategy for therapy for genetic disorders identified prenatally and impact the development of intrathymic tolerance strategies. However, reviewers agreed that, ultimately, UCB and not fetal cells would be used for tolerance induction, and the potential impact of this proposal was lessened by the lack of experiments demonstrating that UCB progenitor cells resemble the fetal HSPCs in tolerogenic potential. The hypothesis for this proposal is not well supported in the literature, but reviewers agreed that it has potential merit and is supported by compelling preliminary data and rationale. While there was convincing rationale presented to support the hypothesis that fetal HSPCs differ in tolerogenic potential from adult cells, the applicant did not address a possible role for the thymic environment in this process; differences in the thymus between the fetus and newborn may impact the ability of the fetal HSPCs to generate tolerogenic responses. Indeed, in Aim 3, the key experiment to address this issue is missing an essential control: the ability of fetal HSPCs to generate tolerance in the neonatal thymic environment should be compared to that of adult HPCs. Instead, non-injected controls were proposed. Reviewers agreed that the proposed aims were logical, but they had several concerns about the experimental design. Regarding Aim 1, reviewers criticized that the cell population that would ultimately be used to generate tolerance, UCB cells, were not included. In Aim 2, reviewers noted that the investigators do not propose new expansion strategies that would markedly improve upon the modest expansion currently shown in the literature and do not use assays that would allow quantitative determination of expansion achieved. Moreover, reviewers commented that the proposed preclinical animal model for the evaluation of HSPC functionality is complicated. Factors involving three species may confound and influence the success of the experiments. In Aim 3, reviewers expressed concern whether the investigator would be able to recover sufficient numbers of donor cells to perform the proposed analyses. Reviewers considered the principal investigator (PI) to be well established and highly regarded with an outstanding track record in publications and funding. Further the assembled team is superb and has the appropriate stem cell and immunology expertise to carry out the proposed research. Reviewers considered the team highly qualified and the strength of the proposal. Overall, although reviewers were intrigued by the creative hypothesis and thought the team to be excellent, numerous concerns regarding the experimental design and the absence of key experiments to support the hypothesis and feasibility substantially reduced reviewers’ enthusiasm for this project. PROGRAMMATIC REVIEW A motion was made to move this application into Tier 1, Recommended for Funding. Reviewers weighed the programmatic advantages of this proposal - the use of the proposed preclinical model and bringing this investigator to the stem cell field – against concerns regarding the overall scientific merit of the proposal. The motion was withdrawn.