HGF Induces Robust Human Thymopoiesis in Mice

描述（由申请人提供）：在过去的十年中，使用免疫缺陷小鼠作为人类造血干细胞移植的受体极大地改善了干细胞研究。研究人类造血干细胞分化为大多数血细胞谱系的模型已经完成，并且系统已经优化。然而，研究人类血细胞的一种特殊谱系——T淋巴细胞的发育的强大系统尚未完成。最可复制的体外系统，允许人类T细胞从纯化的干细胞发育；胎儿胸腺器官的培养，需要精确的定时交配，然后仔细解剖产生的第15天胚胎。一旦成功，这项技术产生的人类T细胞数量非常少。目前研究人类T细胞在体内发育的最佳系统是scid/hu / live小鼠，它需要获取人类胎儿组织并在小鼠身上完成存活手术，因此许多实验室无法获得。不幸的是，T细胞发育不会发生在最常见的小鼠异种移植受体NOD/SCID小鼠中。在这只小鼠中，移植的人类干细胞和祖细胞强烈而迅速地转向B细胞分化，没有观察到人类T细胞。即使是成熟的人T细胞在移植到NOD/SCID小鼠体内后也能迅速清除。

DESCRIPTION (provided by applicant): Over the past decade, the use of immune deficient mice as recipients for human hematopoietic stem cell transplantation has dramatically improved stem cell research. Development of models to study the differentiation of human hematopoietic stem cells into most blood cell lineages has been accomplished and the systems have been optimized. However, robust systems to study development of one particular lineage of human blood cell, the T lymphocyte, have not yet been accomplished. The most reproducible in vitro system that allows human T cell development from purified stem cells; the fetal thymic organ culture, requires precise timed mating of murine breeders and then careful dissection of the resulting day 15 embryos. When successful, the technique produces very low numbers of human T cells. The best system currently available to study human T cell development in vivo, the scid/hu thy/liv mouse, requires procurement of human fetal tissues and accomplishment of survival surgery in mice, so is not available to many laboratories. Unfortunately, T cell development does not occur in the most popular murine xenograft recipient, the NOD/SCID mouse. In this mouse, transplanted human stem and progenitor cells are strongly and rapidly diverted toward B cell differentiation, and no human T cells are observed. Even mature human T cells are rapidly cleared following transplantation into NOD/SCID mice. In spite of the drawbacks to the existing systems, the scientific community constantly strives to improve these models because the ability to study production of large numbers of human T cells in mice would have an enormous impact on human health care. Delayed immune reconstitution is a major problem in stem cell transplantation, resulting in many deaths from post-transplant infection each year. A murine system to study post-stem cell transplant T cell reconstitution from purified human stem cells could allow dissection of the factors that could help transplant patients develop a functional immune system much more rapidly. The lack of a reproducible and effective T cell development assay that does not require surgical skill has been a major stumbling block for the stem cell field to date. We have discovered that co-transplantation of human mesenchymal stem cells engineered to express hepatocyte growth factor (HGF) with purified human stem cells causes massive human T lymphocyte development and expansion in NOD/SCID/B2M null mice. We hypothesize that human "thymopoiesis" is ongoing in all tissues where the HGF-secreting MSC co-localize with the stem cells. In support of this theory, the MSC/HGF/B2M mice have large numbers of double positive CD4+/CD8+ human cells in their spleen and liver, as well as thymus, but not bone marrow. These are the sites of human MSC homing in the mice. From our initial observations, T cell development appears to mimic normal thymopoiesis, with large numbers of double positive (CD4+/CD8+) human T cells produced in the organs, in addition to mature CD4+ and CD8+ cells in the circulation. The MSC/HGF/HSC cotransplanted mice form the most robust in vivo T cell development system that we have observed in the xenograft field to date. The current application proposes studies to refine this system, to understand it, and to develop it further to be a valuable resource for any investigator that wishes to study development of human T cells from hematopoietic stem cells in vivo.