Nonobese Diabetic/Severe Combined Immunodeficiency (NOD/SCID) Mouse as a Model System to Study the Engraftment and Mobilization of Human Peripheral Blood Stem Cells

Fit-For-All iPSC-Derived Cell Therapies and Their Evaluation in Humanized Mice With NK Cell Immunity

Human induced pluripotent stem cells (iPSCs) can be limitlessly expanded and differentiated into almost all cell types. Moreover, they are amenable to gene manipulation and, because they are established from somatic cells, can be established from essentially any person. Based on these characteristics, iPSCs have been extensively studied as cell sources for tissue grafts, blood transfusions and cancer immunotherapies, and related clinical trials have started. From an immune-matching perspective, autologous iPSCs are perfectly compatible in principle, but also require a prolonged time for reaching the final products, have high cost, and person-to-person variation hindering their common use. Therefore, certified iPSCs with reduced immunogenicity are expected to become off-the-shelf sources, such as those made from human leukocyte antigen (HLA)-homozygous individuals or genetically modified for HLA depletion. Preclinical tests using immunodeficient mice reconstituted with a human immune system (HIS) serve as an important tool to assess the human alloresponse against iPSC-derived cells. Especially, HIS mice reconstituted with not only human T cells but also human natural killer (NK) cells are considered crucial. NK cells attack so-called “missing self” cells that do not express self HLA class I, which include HLA-homozygous cells that express only one allele type and HLA-depleted cells. However, conventional HIS mice lack enough reconstituted human NK cells for these tests. Several measures have been developed to overcome this issue including the administration of cytokines that enhance NK cell expansion, such as IL-2 and IL-15, the administration of vectors that express those cytokines, and genetic manipulation to express the cytokines or to enhance the reconstitution of human myeloid cells that express IL15R-alpha. Using such HIS mice with enhanced human NK cell reconstitution, alloresponses against HLA-homozygous and HLA-depleted cells have been studied. However, most studies used HLA-downregulated tumor cells as the target cells and tested in vitro after purifying human cells from HIS mice. In this review, we give an overview of the current state of iPSCs in cell therapies, strategies to lessen their immunogenic potential, and then expound on the development of HIS mice with reconstituted NK cells, followed by their utilization in evaluating future universal HLA-engineered iPSC-derived cells.

Similar repopulating capacity of mitotically active and resting umbilical cord blood CD34+ cells in NOD/SCID mice

It was hypothesized that during mammalian development, the extensive need for hematopoietic cells requires equal contribution to blood cell production from both quiescent and cycling hematopoietic stem cells (HSCs) while maintaining the stem cell pool. To investigate this hypothesis, the engraftment potential of umbilical cord blood (UCB) CD34+ cells residing in either G0(G0CD34+ cells) or G1(G1CD34+ cells) phases of the cell cycle was assessed in nonobese diabetic/severe combined immune-deficient (NOD/SCID) mice. Whereas the level of chimerism in mice transplanted with UCB G0CD34+ cells was 69.9% ± 24.0%, mice receiving equal numbers of G1CD34+ cells harbored 46.7% ± 21.3% human cells 8 weeks posttransplantation. Both groups of cells sustained multilineage differentiation and the production of CD34+cells in recipient animals. The relationship between the number of transplanted G0CD34+ or G1CD34+ cells and the level of chimerism was analyzed by a general linear models procedure. Although the initial level of chimerism following transplantation of G0CD34+ cells was higher than that sustained by G1CD34+ cells, the increment in the degree of chimerism obtained with each additional 103 cells of either phenotype was identical, suggesting that the reconstitution potential of these 2 types of cells was similar. Of interest is that human cells recovered from primary recipients of both G0CD34+ and G1CD34+cells engrafted in secondary NOD/SCID recipients, albeit at a substantially lower level, confirming the primitive nature of UCB CD34+ cells residing in G1.

β2 Microglobulin-deficient (B2mnull) NOD/SCID mice are excellent recipients for studying human stem cell function

Human SCID repopulating cells (SRC) are defined based on their functional ability to repopulate the bone marrow of NOD/SCID mice with both myeloid and lymphoid cell populations. The frequency of SRC in umbilical cord blood cells is 1 in 9.3 × 105mononuclear cells. We report that as few as 8 × 104 human cord blood mononuclear cells transplanted into NOD/SCID/B2mnull mice resulted in mutlilineage differentiation in the murine bone marrow, revealing a more than 11-fold higher SRC frequency than in NOD/SCID mice. Moreover, as few as 2 to 5 × 103 CD34+ cells recovered from the bone marrow of primary transplanted NOD/SCID mice were sufficient for engrafting secondary NOD/SCID/B2mnull mice with SRC, suggesting SRC self-renewal. Thus, by using NOD/SCID/B2mnull mice as recipients, we established a functional assay for human stem cells capable of engrafting the bone marrow of primary and secondary transplanted immune-deficient mice with SRC, providing a model that better resembles autologous stem cell transplantation.