Basic research in transplantation immunology has relied primarily on rodent models. Experimentation with rodents has laid the foundation for our basic understanding of the biological events that precipitate rejection of non-self or allogeneic tissue transplants and supported the development of novel strategies to specifically suppress allogeneic immune responses. However, translation of these studies to the clinic has met with limited success, emphasizing the need for new models that focus on human immune responses to allogeneic tissues. Humanized mouse models are an exciting alternative that permits investigation of the rejection of human tissues mediated by human immune cells without putting patients at risk. However, the use of humanized mice is complicated by a diversity of protocols and approaches, including the large number of immunodeficient mouse strains available, the choice of tissue to transplant and the specific human immune cell populations that can be engrafted. Here, we present a historical perspective on the study of allograft rejection in humanized mice and discuss the use of these novel model systems in transplant biology.
Transplantation of allogeneic or ‘non-self’ tissues stimulates a robust immune response leading to graft rejection.1, 2, 3 The survival of allogeneic organ transplants requires lifelong immune suppression,4, 5, 6 although recent work has suggested that this may not be necessary in select populations of patients.7 Excellent outcomes notwithstanding, contemporary immunosuppressive medications are toxic, are often not taken by patients, and pose long-term risks of infection and malignancy.8, 9, 10 The ultimate goal in transplantation research is to develop new treatments that will supplant the need for general immunosuppression. While novel protocols have been developed using murine transplantation systems, translation of these approaches to the clinic has been limited. This failure is related to the species-specific differences in the rodent and human immune systems, as well as the difficulty of studying the generation of alloreactive immune responses in patients.
Humanized mice offer a novel approach to study human biology by implanting functional human cells and tissues into immunodeficient mice. Humanized mouse models have been utilized for a number of years in an attempt to understand the basic immunological mechanisms underlying allogeneic transplant rejection. However, these early studies suffered from low levels of chimerism and limited functionality of the engrafted human immune cells. Advancements in generating humanized mice have greatly enhanced the functionality of the engrafted human immune systems and have facilitated the study of human immunobiology. Recent studies have demonstrated robust rejection of human allografts, including skin, islet and artery grafts in humanized models. Moreover, these new model systems are proving to be valuable tools for the preclinical evaluation of human-specific therapeutics to induce transplantation tolerance.
Strain development for humanized mice
The successful transplantation of xenogeneic tissues into mice requires elimination or severe suppression of the murine innate and adaptive immune systems.11 Initial efforts to engraft human immune systems into mice focused on eliminating the adaptive host murine immune response by using the CB17-scid mouse, which has impaired development of B and T cells.12 The scid mutation is within the catalytic subunit of DNA-dependent protein kinase, which is required for the repair of double-stranded DNA breaks and carrying out V(D)J recombination.13, 14 The CB17-scid mouse will engraft with human peripheral blood mononuclear cells (PBMC), 15 hematopoietic stem cells (HSC)16 and fetal tissues,17 but the overall levels of engraftment are extremely low and the engrafted cells have minimal functionality. An alternative to eliminate murine adaptive immunity is the use of mice deficient in the expression of either recombination activating gene (Rag1) (Rag1null) or Rag2 (Rag2null).18, 19 Rag1 and Rag2 function synergistically to create double-stranded DNA breaks and are essential for V(D)J recombination and the development of functional T cells and B cells.20, 21 While these early immunodeficient mouse strains allowed for low-level human immune cell engraftment, the usefulness of these models to study human immunobiology and allograft rejection was limited.
The adaptive immune system is effectively eliminated in scid, Rag1null and Rag2null mice, but the murine innate immune system remains intact and prevents high-level engraftment of human HSC and immune cells. In an attempt to diminish the murine innate immune system, new genetic stocks of scid, Rag1null or Rag2null mice were created that also harbored targeted mutations in the IL2 receptor common gamma chain (IL2rγ) gene.22, 23, 24, 25, 26, 27, 28 The IL2rγ chain is required for high-affinity ligand binding and signaling through multiple cytokine receptors, including IL2, IL4, IL7, IL9, IL15 and IL21.29 Importantly, the absence of IL15 prevents the development of murine natural killer (NK) cells, which are extremely efficient in the rejection of non-self hematopoietic cells in vivo. Immunodeficient mice bearing mutations within the IL2rγ gene support significantly higher levels of human hematolymphoid engraftment than all previous immunodeficient stocks and allow for the development of a functional human immune system comprised of multiple lymphoid and myeloid cell lineages.
An additional variable that will significantly influence the engraftment of human cells and tissues into immunodeficient mice is the specific strain background of the recipient mouse. For example, immunodeficiency mutations on the nonobese diabetic (NOD) mouse background support higher engraftment levels of human hematopoietic cells and immune cells as compared to other backgrounds, such as BALB/c, C3H and C57BL/6.30, 31, 32, 33, 34, 35 The NOD mouse background offers a number of genetic advantages that promote the engraftment of human immune systems.34 A direct comparison of immunodeficient IL2rγnull mice on either a NOD background (NOD-scid IL2rγnull (NSG) and NOD-Rag1null IL2rγnull (NRG)) or BALB/c (BALB-Rag2null IL2rγnull) background revealed that the NOD background supported significantly higher levels of human cell engraftment following injection of human HSC.30, 32 NOD mice have a number of defects in innate immune functionality that may facilitate the engraftment of human cells, including reduced NK cell numbers and function, defects in macrophage function, impaired dendritic cell maturation and a lack of hemolytic complement. In addition, the signal regulatory protein-alpha (SIRPα) polymorphism that is expressed by phagocytic cells on the NOD background allows for heightened self-recognition interactions between murine phagocytic cells and human CD47 expressing hematopoietic cells, minimizing the phagocytosis of human cells.36 In contrast, BALB/c mice express a SIRPα polymorphism that is not protective against phagocytosis. Two recent studies using distinct approaches have demonstrated that the SIRPα–CD47 interactions are critically important for the engraftment of human HSC and immune cell development in IL2rγnull mice.37, 38 It was first shown that transgenic expression of human SIRPα by mice on a mixed (129×BALB/c) strain background significantly improved the engraftment of human HSC.38 The second study showed that retrogenic expression of murine CD47 in human HSC prior to transplantation significantly improved human immune system development.37 Overall, these studies highlight the importance of strain selection in the efficient generation of humanized mice.
Humanized mouse models
There are a variety of humanized mouse models that can be used to study immune cell function (Table 1), including the Hu-PBL-SCID, the Hu-SRC (scid-repopulating cell)-SCID and the SCID-Hu or BLT (bone marrow, liver, thymus) models.39 In the Hu-PBL-SCID model, mice are injected with mature human PBMC, which results in robust engraftment of human T cells, and provides a useful tool to examine the functionality of human T cells in vivo.40, 41 This mouse model has been successfully used to examine both allo-immunity and viral immunity and to recapitulate HIV infection.40, 42, 43, 44, 45 One complicating factor with the Hu-PBL-SCID model is the development of a xenogeneic graft-versus-host disease (GVHD) that develops with successful engraftment as human T cells recognize murine major histocompatibility complex.27, 41, 46, 47, 48 This GVHD limits the time frame of experiments that can be done with the HU-PBL-SCID mice, but T cells expanded during the xenogeneic reaction still are able to mediate rejection of human skin allografts.49 In the Hu-SRC-SCID model, preconditioned mice are injected with human HSC derived from a variety of sources, including umbilical cord blood, bone marrow, fetal liver and peripheral blood of granulocyte colony-stimulating factor-treated individuals. Human HSC engraft at high levels in NSG and NRG stocks, and these mice develop functional innate and adaptive human immune systems.30, 50 The Hu-SRC-SCID model has been used to study many aspects of human immunobiology, including infectious disease, transplantation rejection and immune responses.39, 51 In the SCID-Hu or BLT model, mice are implanted under the renal capsule with human fetal thymus and fetal liver.17, 52 In some instances CD34+ cells derived from the autologous fetal liver tissue are also injected intravenously to provide a peripheral source of HSC. BLT mice develop a functional immune system and have primarily been used to study HIV infection, but have also been used to study other aspects of human infectious and immunological disease.17, 52, 53, 54 The advantage of the BLT model is robust engraftment and the presence of autologous human thymic epithelium for T-cell development. The Hu-PBL-SCID and Hu-SRC-SCID models have been predominantly used in studies of human allograft rejection and have proven to be promising tools to investigate mechanisms of rejection.
HUMAN SKIN ALLOGRAFTS
Human skin allografts have been used extensively to study transplantation immunology in humanized mouse models. There are numerous advantages for the use of human skin in these transplantation experiments. Tissue specimens from healthy individuals are readily available. Preparation of tissue and the transplant protocol is straightforward. The healed-in tissues resemble healthy human skin complete with epidermal and dermal layers and vasculature. It is also possible to recover autologous PBMC from the skin donor. Finally, skin allografts are highly immunogenic and stimulate robust immune responses.55
Early attempts to study rejection of human skin allografts in humanized mice utilized the CB17-scid host.56, 57, 58 In 1992, Kawamura and colleagues evaluated rejection of human skin allografts on CB17-scid mice engrafted with either PBMC from a donor that had been previously sensitized to alloantigen or with PBMC from a donor that had not been previously exposed to alloantigens. PBMC from the non-sensitized donor were unable to reject skin allografts in this model. In contrast, 37% of mice injected with PBMC from the presensitized donor rejected skin allografts from an individual that shared at least one human histocompatibility leukocyte antigen (HLA) allele with the sensitizing donor. During the rejection process human T cells were detectable within the human skin by immunohistochemistry. These findings suggested that PBMC injected into CB17-scid mice do not maintain functionality unless pre-existing alloreactive memory cells were present.
Although human PBMC engraft in CB17-scid mice, the levels of detectable human cells are extremely low. The next effort by transplantation biologists focused on improving engraftment and maintaining functionality of human immune cells following injection into CB17-scid mice. One approach to improve the model included the injection of extremely high numbers (3×108) of human PBMC and the depletion of host murine NK cells by treatment with anti-asialo GM1 polyclonal antibody.58 Although complete rejection of human skin allografts was not demonstrated with this approach, perivascular infiltrates of human T cells were consistently observed and damage to the human microvessels was evident in >95% of engrafted mice. A second strategy to improve the model was to irradiate (2 cGy) the CB17-scid mice prior to injection of human splenocytes.56 The injection of human splenocytes into irradiated hosts resulted in significantly higher levels of human cell engraftment as compared to human PBMC. Consistent with earlier studies, injection of human splenocytes resulted in primarily T-cell engraftment that consisted of both CD4 and CD8 T cells with an activated phenotype: CD25+, HLA-DR+, CD45RA+. Within 3 weeks of human skin transplantation, 75% of splenocyte-engrafted CB17-scid mice had completely rejected the skin allografts, and depletion of T cells prevented rejection. Together, these initial studies suggested that under the appropriate conditions the CB17-scid mouse model could be used to study the rejection of human skin allografts. However, this model required the injection of very high numbers of human cells and preconditioning with either irradiation or depletion of NK cells, and the levels of engraftment were still low.
The enhancement of human cell engraftment by NK cell depletion suggested that the innate immune system of CB17-scid mice was impeding human cell survival. In an effort to improve human cell engraftment and function in immunodeficient mice, Pober and colleagues initiated a series of studies using CB17-scid mice co-expressing the beige (Lystbg) mutation (SCID/beige).59, 60, 61 The beige mutation disrupts a gene required for lysosomal trafficiking and results in impaired NK cell function.62, 63 The use of SCID/beige mice allowed engraftment of human PBMC without depletion of NK cells, but still required the injection of high numbers of cells (1×108–3×108 cells). Moreover, SCID/beige mice readily accepted human skin grafts, with minimal injury or infiltration by murine immune cells.64 Injection of human PBMC or T-cell lines into SCID/beige mice bearing human skin allografts produced allograft injury that was characterized by vascular damage, and this injury was mediated by human CD4 and CD8 T cells.59, 60, 61 The SCID/beige model has since proven useful in characterizing allograft injury mediated by human T cells. Inhibition of human T-cell function by treatment of skin allograft bearing SCID/beige mice with a combination of cyclosporine and rapamycin significantly reduced the level of human cell infiltrate and microvascular damage.59 The blockade of CD2–CD58 interactions, which are important for T-cell interactions with antigen-presenting cells, using an antibody to CD58 or CD58-Ig fusion protein blocked T-cell infiltration of human skin allografts and prevented allograft injury.61 Costimulation blockade by treatment with monoclonal antibodies specific for 4-1BBL, ICOSL or OX40L was not effective in preventing T-cell infiltration of human skin allografts after PBMC injection, but all three treatments were able to diminish allograft injury, including endothelial injury and thrombosis. Moreover, blockade of 4-1BBL, ICOSL or OX40L also reduced the expression of Fas ligand and perforin by the injected T cells.65
The SCID/beige model has also been used to define the role of human T-cell subsets in the rejection of human skin allografts. Preliminary experiments first tested the ability of CD45RA+ naive T cells or CD45RO+ effector T cells to mediate allograft injury.65 CD45RA+ and CD45RO+ T cells were purified by flow cytometry-based sorting and injected into SCID/beige mice. Although both T-cell subsets were able to engraft, only the CD45RO+ cells mediated vascular injury, suggesting that memory phenotype T cells were essential for the rejection of human skin. While CD45RA/RO status is not completely definitive to differentiate memory from naive T cells, greater than 90% of CD45RA+ CD4 T cells and between 50% and 60% of CD45RA+ CD8 T cells identified by this approach are functionally naive.66 More detailed analysis of T-cell subsets involved in skin allograft injury revealed that sort-purified effector memory CD4+ T cells (CD45RO+/CCR7−/CD62L−) mediated rejection of human skin allografts in SCID/beige mice, correlating with the ability of these cells to produce interferon (IFN)-γ in response to stimulation with human allogeneic endothelial cells.67 In contrast, central memory CD4 T cells (CD45RO+/CCR7+/CD62L+) were unable to engraft in SCID/beige mice after transfer, and therefore could not be evaluated for the ability to mediate allograft injury. Together, these data demonstrate that the SCID/beige mouse can be used to study mechanisms by which human T cells mediate injury to human skin allografts.
One limitation for the Hu-PBL-SCID model is that engraftment is dominated by T cells (Table 1), with minimal survival of innate immune cells and B cells.46 In an effort to develop a model that would allow the investigation of human macrophages in skin allograft rejection, adult SCID/beige mice were irradiated and injected with human CD34+ HSC derived from peripheral blood of granulocyte colony-stimulating factor-mobilized individuals to allow the development of a complete human immune system (Hu-SRC-SCID model).64 Unfortunately, SCID/beige mice engrafted only at very low levels with human immune cells in the blood, spleen and bone marrow, but human macrophages (CD68+ and CD14+) were detectable. Following transplantation of human skin allografts onto HSC-engrafted SCID/beige mice, human macrophages were detectable within the graft, but there was no evidence of graft injury. However, injection of autologous PBMC into the HSC-engrafted SCID/beige mice strongly activated the macrophages within the skin allograft and significantly enhanced allograft injury, as compared to mice engrafted with HSC or PBMC alone.64 Overall, these studies indicate that the SCID/beige mouse can be used to study the biology of human skin allograft rejection, but this model is limited by relatively low-level engraftment of human immune cells and the inability to develop a complete human immune system after HSC engraftment.
As described above the NOD background supports high-level engraftment with human cells and tissues. Initial studies on skin allograft rejection in immunodeficient mice on the NOD background were done in NOD-scid mice that were deficient in the expression of β2-microglobulin (Β2mnull).68 The β2-microglobulin deficiency impairs cell surface expression of major histocompatibility complex class I, depressing NK cell development and enhancing human immune cell engraftment.69, 70 For these experiments, NOD-scid B2mnull mice were first depleted of NK cells by pre-treatment with a monoclonal antibody specific for murine CD122. NK cell-depleted mice were then transplanted with human skin allografts and injected with human PBMC or splenocytes. All NOD-scid B2mnull mice injected with human cells were engrafted at high levels and demonstrated significant graft injury characterized by erythema, thrombosis and epithelial sloughing. Antibody-mediated in vivo depletion of human CD4 and CD8 T cells demonstrated that both T-cell subsets could mediate skin allograft rejection. This study demonstrated that T cell-mediated allograft rejection could be studied in immunodeficient mice on a NOD background.68
NSG mice engraft at very high levels with human PBMC in the absence of pretreatment with irradiation or NK cell-depleting antibodies,25, 27, 40 and appeared to be an excellent potential model to study skin allograft rejection mediated by human PBMC. However, human skin grafts on unmanipulated NSG mice showed extensive perivascular infiltration of murine immune cells that resulted in graft injury and precluded the study of rejection mediated by human immune cells.44 The depletion of Gr1+ murine innate immune cells from the recipient NSG mice allowed transplanted human skin to heal efficiently and significantly improved the overall graft morphology, as evident by the maintenance of epidermal and dermal structures. Moreover, human ‘passenger’ leukocytes were readily detectable in human skin grafts on Gr1-depleted NSG, an observation that had not been reported previously. Injection of human PBMC into GR1-depleted NSG mice bearing human skin allografts resulted in severe graft injury within 21 days. This early graft injury was characterized by the remodeling of epidermal and dermal layers, extensive destruction of human endothelium, and a human CD45 infiltrate. By 31 days after PBMC injection, all engrafted mice showed extensive destruction of grafted tissues, with near complete loss of human vasculature and destruction of epidermal and dermal layers. Both human CD4 and CD8 T cells were able to mediate the rejection of human skin allografts.44 The high engraftment of human cells and consistent rejection of human skin allografts in NSG mice and the recent description of additional studies done in other IL2rγnull mouse models71, 72 demonstrate the power and utility of this model system to study transplantation rejection.
Human islet allografts
β-cell replacement therapies are the ultimate goal to cure diabetes.73, 74, 75 One example of a β-cell replacement therapy is human islet transplantation, which offers the advantage of a minimally invasive surgical procedure with limited complications.76, 77, 78 However, to date, the long-term success rate of islet transplantation has not reached the same level as that for pancreatic organ transplants for reasons that are not clear.79 Humanized mouse models have been used to study the rejection of allogeneic human islets and are a valuable resource as a preclinical model for investigating mechanisms of rejection and how to impede them.
As with the study of human skin allograft rejection, the initial experiments to study rejection of human islet transplants were done in CB17-scid mice.80, 81 In 1991, London and colleagues demonstrated that the transplantation of human islets to the renal subcapsular space restored normoglycemia in CB17-scid mice that had been rendered diabetic by the injection of streptozotocin (STZ).80 Injection of human splenocytes allogeneic to the implanted islets resulted in rejection within 7 days. The rejection was characterized by strong infiltration of human CD8 T cells at the graft site. In contrast, human islet grafts were not rejected following the injection of autologous splenocytes. Levels of human cell engraftment were not shown in this manuscript, but based on our current knowledge the levels would be predicted to be low. To improve engraftment of human immune cells in CB17-scid mice, a subsequent study showed that an initial injection of human PBMC followed 2 days later by an injection of autologous PBMC that had been stimulated in vitro with anti-CD3 antibody resulted in higher levels of peripheral engraftment as compared to a single injection.81 Human islet allografts transplanted into CB17-scid mice in this two-stage injection model were rejected within 21 days of implant as determined by a loss of detectable human C-peptide. In contrast, human C-peptide was detectable for over 60 days in unmanipulated CB17-scid transplanted with human islets. Moreover, human T cells recovered from the graft site of PBMC-injected mice showed cytotoxic activity against allogeneic targets cells that shared HLA with the implanted islets, confirming the activation of alloreactive T cells.81
A similar two-stage injection protocol of human PBMC was used in NOD-scid mice to demonstrate human islet allograft rejection.82 In these experiments, NOD-scid mice were first transplanted with human islet allografts and then 3 days later injected with PBMC that were activated by in vitro stimulation with anti-CD3 antibody. Between 2 and 4 days later, these mice received a second injection with PBMC that had been stimulated in vitro with irradiated allogeneic splenocytes from the islet donor. Detection of human C-peptide was significantly reduced in the PBMC-injected mice by day 28 and was undetectable by day 42, indicating that the injected cells were able to reject the graft. Rejection of human islet allografts was prevented by the co-injection of T cell-depleted bone marrow derived from the islet donor, suggesting that the establishment of microchimerism was sufficient to induce tolerance in this humanized model.82 The NOD-scid mouse has also been used to study the rejection of human islet allografts after depletion of murine NK cells with a monoclonal antibody specific for CD122.42 In this study, transplantation of human islet allografts into STZ-treated diabetic NOD-scid mice efficiently restored normoglycemia. Injection of human PBMC that were allogeneic to the graft into the islet-implanted mice resulted in a reversion to hyperglycemia, indicating that the allografts were rejected. Histological evaluation of the graft site revealed a loss of insulin-positive cells and an extensive human T-cell infiltrate, confirming rejection. As an alternative to NOD-scid mice to study allograft rejection, perforin-deficient NOD-Rag1null Prf1null mice were shown to support human PBMC engraftment in the absence of NK cell-depletion and to allow the rejection of HLA-A2 expressing murine islets, isolated from NOD-scid HLA-A2 transgenic mice, by A2-negative human PBMC, but not by A2 positive PBMC.42
As described above, NSG mice support high-level engraftment of human PBMC and allow the study of human T-cell function, following injection of small numbers of human PBMC. Transplantation of human islet allografts into STZ-treated diabetic NSG mice was effective in regulating blood glucose levels but co-injection of human PBMC that were allogeneic to the islets resulted in a rapid reversion to hyperglycemia and a loss of detectable human C-peptide.40 The rejection was confirmed histologically by the absence of insulin-positive cells at the graft site and the presence of a marked cellular infiltrate. This study suggests that immunodeficient mice bearing mutations within the IL2rγ chain are the ideal hosts to study islet allograft rejection in the Hu-PBL-SCID model.
As described above, a limitation of the Hu-PBL-SCID model is the engraftment of predominately T cells. Two recent studies have evaluated the rejection of human islet allografts in the Hu-SRC-SCID model with mixed results.83, 84 Balb/c-Rag2null IL2rγnull mice engrafted with human fetal liver-derived HSC failed to reject human islet allografts.83 Human C-peptide levels were not reduced in HSC-engrafted mice by 35 days postislet transplant, insulin-positive cells were detectable at the graft site and there was minimal human cell infiltration. In contrast, recently published data from our laboratory demonstrated human islet allograft rejection in an NRG Ins2Akita mouse strain that spontaneously develops hyperglycemia.84 NRG mice heterozygous for the Ins2Akita mutation develop a spontaneous hyperglycemia due to misfolding of the insulin-2 protein, leading to endoplasmic reticulum stress and β-cell apoptosis.85, 86, 87 Importantly, the β-cell death occurs in the absence of an immune response and does not require the administration of β-cell toxins such as STZ, which have detrimental effects on many other tissues.88 Euglycemia can be restored following subrenal capsule transplantation with mouse or human islets.84 NRG Ins2Akita mice were injected with human HSC as newborns, and mice that engrafted with human immune systems were transplanted with allogeneic human islets. By 30 days post-transplant, 60% of islet transplanted HSC-engrafted NRG-Akita mice that had initially become normoglycemic reverted back to a hyperglycemic state, and histological evaluation of the grafts confirmed a marked human cell infiltration and loss of most insulin-positive cells. These results are consistent with an immune-mediated rejection of the allogeneic human islets. Interestingly in HSC-engrafted NRG-Akita mice that did not revert back to hyperglycemia, histological examination revealed a human mononuclear cell infiltration at the graft site, but these mice still had abundant insulin-positive cells. Thus, the Hu-SRC-SCID mouse model can be used to study human islet rejection, but this system still requires optimization.
Human artery allografts
A critical target during acute and chronic rejection of allogeneic tissues is the allograft blood vessels, specifically the endothelial cell lining.89 Endothelial cells strongly activate human T cells and are susceptible to direct injury mediated by effector T cells. SCID/beige mice have been used extensively in the Hu-PBL-SCID model to study the process of human vascular rejection.90
In 1999, Pober and colleagues demonstrated that human arterial grafts could be interposed into the murine infrarenal artery of SCID/beige mice and the tissues would remain viable.91 By 28 days after injection of human PBMC allogeneic to the arterial graft, the majority of mice showed severe human cell infiltration of the allograft and histological changes consistent with vascular rejection. IFN-γ production was shown to be critical during the rejection of human artery allografts, mediating vascular dysfunction.92 Transforming growth factor-β produced by arterial allografts diminished the ability of human T cells to produce IFN-γ, and blockade of transforming growth factor-β enhanced vascular rejection.93 In contrast, blockade of IL-1 with a human-specific IL-1R antagonist, diminished the infiltration of human immune cells and IL17 production and minimized arterial graft injury, suggesting the IL1α produced by the human arterial allograft promotes vascular rejection.94 The use of arterial grafts with pre-existing modest atherosclerosis did not accelerate T cell-mediated vascular rejection, suggesting that atherosclerotic plaques do not stimulate more robust immune responses.95 Finally, the Hu-SRC-SCID model has been used to study artery graft rejection in the SCID/beige model.64 As described above, injection of human HSC into SCID/beige mice results in low overall engraftment characterized by the lack of T cells or B cells, but human macrophage are detectable. Human artery allografts were rapidly infiltrated with human macrophages following transplant on HSC-engrafted SCID/beige mice and were extensively calcified. Together, these studies demonstrated that humanized models can be used to study the mechanisms of artery graft rejection.
Significant progress has been made in the study of human allograft rejection in humanized mouse models. New mouse strains, such as those bearing mutations in the IL2rγ chain, allow for optimal engraftment of functional human immune systems and the ability to consistently observe allograft rejection. These advanced model systems are currently being used to test novel immune therapies to suppress rejections. For example, four recent studies have shown that in vitro expanding human T regulatory cells will prevent rejection of human skin and artery grafts in the Hu-PBL-SCID model.71, 72, 96, 97 Efforts are continuing to improve humanized mouse models by creating new immunodeficient strains that express HLA and human cytokines and growth factor to enhance human immune system development.11, 98
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This work was supported by National Institutes of Health research grants AI46629, HL077642, CA34196, DK089572, an institutional Diabetes Endocrinology Research Center (DERC) grant DK32520, a grant from the University of Massachusetts Center for AIDS Research, P30 AI042845 and grants from the JDRF, the Helmsley Foundation and USAMRIID. The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health.
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Brehm, M., Shultz, L. Human allograft rejection in humanized mice: a historical perspective. Cell Mol Immunol 9, 225–231 (2012). https://doi.org/10.1038/cmi.2011.64
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