The assessment of human erythroid output in NOD/SCID mice reconstituted with human hematopoietic stem cells

Preclinical evaluation for engraftment of CD34+ cells gene-edited at the sickle cell disease locus in xenograft mouse and non-human primate models

Sickle cell disease (SCD) is caused by a 20A > T mutation in the β-globin gene. Genome-editing technologies have the potential to correct the SCD mutation in hematopoietic stem cells (HSCs), producing adult hemoglobin while simultaneously eliminating sickle hemoglobin. Here, we developed high-efficiency viral vector-free non-footprint gene correction in SCD CD34+ cells with electroporation to deliver SCD mutation-targeting guide RNA, Cas9 endonuclease, and 100-mer single-strand donor DNA encoding intact β-globin sequence, achieving therapeutic-level gene correction at DNA (∼30%) and protein (∼80%) levels. Gene-edited SCD CD34+ cells contributed corrected cells 6 months post-xenograft mouse transplant without off-target δ-globin editing. We then developed a rhesus β-to-βs-globin gene conversion strategy to model HSC-targeted genome editing for SCD and demonstrate the engraftment of gene-edited CD34+ cells 10-12 months post-transplant in rhesus macaques. In summary, gene-corrected CD34+ HSCs are engraftable in xenograft mice and non-human primates. These findings are helpful in designing HSC-targeted gene correction trials.

Modeling Human Cytomegalovirus in Humanized Mice for Vaccine Testing

Human cytomegalovirus (HCMV or HHV-5) is a globally spread pathogen with strictly human tropism that establishes a life-long persistence. After primary infection, high levels of long-term T and B cell responses are elicited, but the virus is not cleared. HCMV persists mainly in hematopoietic reservoirs, whereby occasional viral reactivation and spread are well controlled in immunocompetent hosts. However, when the immune system cannot control viral infections or reactivations, such as with newborns, patients with immune deficiencies, or immune-compromised patients after transplantations, the lytic outbursts can be severely debilitating or lethal. The development of vaccines for immunization of immune-compromised hosts has been challenging. Several vaccine candidates did not reach the potency expected in clinical trials and were not approved. Before anti-HCMV vaccines can be tested pre-clinically in immune-compromised hosts, reliable in vivo models recapitulating HCMV infection might accelerate their clinical translation. Therefore, immune-deficient mouse strains implanted with human cells and tissues and developing a human immune system (HIS) are being explored to test anti-HCMV vaccines. HIS-mice resemble immune-compromised hosts as they are equipped with antiviral human T and B cells, but the immune reactivity is overall low. Several groups have independently shown that HCMV infections and reactivations can be mirrored in HIS mice. However, these models and the analyses employed varied widely. The path forward is to improve human immune reconstitution and standardize the analyses of adaptive responses so that HIS models can be forthrightly used for testing novel generations of anti-HCMV vaccines in the preclinical pipeline.

Low-Dose Busulfan Reduces Human CD34+ Cell Doses Required for Engraftment in c-kit Mutant Immunodeficient Mice

Humanized animal models are central to efforts aimed at improving hematopoietic stem cell (HSC) transplantation with or without genetic modification. Human cell engraftment is feasible in immunodeficient mice; however, high HSC doses and conditioning limit broad use of xenograft models. We assessed human CD45⁺ chimerism after transplanting varying doses of human CD34⁺ HSCs (2 × 10⁵ to 2 × 10⁶ cells/mouse) with or without busulfan (BU) pretransplant conditioning in c-kit mutant mice that do not require conditioning (non-obese diabetic [NOD]/B6/severe combined immunodeficiency [SCID]/ interleukin-2 receptor gamma chain null (IL-2rγ^-/-) Kit^W41/W41 [NBSGW]). We then tested a range of BU (5-37.5 mg/kg) using 2 × 10⁵ human CD34⁺ cells. Glycophorin-A erythrocyte chimerism was assessed after murine macrophage depletion using clodronate liposomes. We demonstrated successful long-term engraftment of human CD34⁺ cells at all cell doses in this model, and equivalent engraftment using 10-fold less CD34⁺ cells with the addition of BU conditioning. Low-dose BU (10 mg/kg) was sufficient to allow human engraftment using 2 × 10⁵ CD34⁺ cells, whereas higher doses (≥37.5 mg/kg) were toxic. NBSGW mice support human erythropoiesis in the bone marrow; however, murine macrophage depletion provided only minimal and transient increases in peripheral blood human erythrocytes. Our xenograft model is therefore useful in HSC gene therapy and genome-editing studies, especially for modeling in disorders, such as sickle cell disease, where access to HSCs is limited.

Development of a forward-oriented therapeutic lentiviral vector for hemoglobin disorders

Hematopoietic stem cell (HSC) gene therapy is being evaluated for hemoglobin disorders including sickle cell disease (SCD). Therapeutic globin vectors have demanding requirements including high-efficiency transduction at the HSC level and high-level, erythroid-specific expression with long-term persistence. The requirement of intron 2 for high-level β-globin expression dictates a reverse-oriented globin-expression cassette to prevent its loss from RNA splicing. Current reverse-oriented globin vectors can drive phenotypic correction, but they are limited by low vector titers and low transduction efficiencies. Here we report a clinically relevant forward-oriented β-globin-expressing vector, which has sixfold higher vector titers and four to tenfold higher transduction efficiency for long-term hematopoietic repopulating cells in humanized mice and rhesus macaques. Insertion of Rev response element (RRE) allows intron 2 to be retained, and β-globin production is observed in transplanted macaques and human SCD CD34+ cells. These findings bring us closer to a widely applicable gene therapy for hemoglobin disorders.

A Hitchhiker's guide to humanized mice: new pathways to studying viral infections

Humanized mice are increasingly appreciated as an incredibly powerful platform for infectious disease research. The often very narrow species tropism of many viral infections, coupled with the sometimes misleading results from preclinical studies in animal models further emphasize the need for more predictive model systems based on human cells rather than surrogates. Humanized mice represent such a model and have been greatly enhanced with regards to their immune system reconstitution as well as immune functionality in the past years, resulting in their recommendation as a preclinical model by the US Food and Drug Administration. This review aims to give a detailed summary of the generation of human peripheral blood lymphocyte-, CD34⁺ haematopoietic stem cell- and bone marrow/liver/thymus-reconstituted mice and available improved models (e.g. myeloid- or T-cell-only mice, MISTRG, NSG-SGM3). Additionally, we summarize human-tropic viral infections, for which humanized mice offer a novel approach for the study of disease pathogenesis as well as future perspectives for their use in biomedical, drug and vaccine research.

Nonirradiated NOD,B6.SCID Il2rγ-/- Kit(W41/W41) (NBSGW) mice support multilineage engraftment of human hematopoietic cells

In this study, we demonstrate a newly derived mouse model that supports engraftment of human hematopoietic stem cells (HSCs) in the absence of irradiation. We cross the NOD.Cg-Prkdc^scidIl2rg^tm1Wjl/SzJ (NSG) strain with the C57BL/6J-Kit^W-41J/J (C57BL/6.Kit^W41) strain and engraft, without irradiation, the resulting NBSGW strain with human cord blood CD34+ cells. At 12-weeks postengraftment in NBSGW mice, we observe human cell chimerism in marrow (97% ± 0.4%), peripheral blood (61% ± 2%), and spleen (94% ± 2%) at levels observed with irradiation in NSG mice. We also detected a significant number of glycophorin-A-positive expressing cells in the developing NBSGW marrow. Further, the observed levels of human hematopoietic chimerism mimic those reported for both irradiated NSG and NSG-transgenic strains. This mouse model permits HSC engraftment while avoiding the complicating hematopoietic, gastrointestinal, and neurological side effects associated with irradiation and allows investigators without access to radiation to pursue engraftment studies with human HSCs.

•

In engraftment experiments, nonirradiated NBSGW mice show enhanced humanization

•

Similar levels of human chimerism are observed between both irNSG and NBSGW mice

•

NBSGW mice are conducive to serial transplantation without irradiation

•

NBSGW mice harbor a mutant Kit^W41 allele, aiding Gly-A+ development in the marrow

Efficient transduction of human hematopoietic repopulating cells with a chimeric HIV1-based vector including SIV capsid

Innate immune factors, such as TRIM5α and cyclophilin A (CypA), act as a major restriction factor of retroviral infection among species. When HIV1 infects human cells, HIV1 capsid binds to human CypA to escape from human TRIM5α restriction. However, in rhesus cells, the mismatch between HIV1 capsid and rhesus CypA is recognized by rhesus TRIM5α to reduce HIV1 infectivity through proteasomal degradation. To circumvent this block, we previously developed a chimeric HIV1 vector (χHIV) that substituted HIV1 capsid with SIV capsid, and it significantly increased transduction efficiency for nonhuman primate cells. In this study, we evaluated whether the χHIV vector efficiently transduces human cells, and the transduction efficiency might increase by a CypA inhibitor (cyclosporine) and a proteasome inhibitor (MG132). The χHIV vector could transduce human CD34⁺ cells, as efficiently as the HIV1 vector, in vitro and in xenograft mice, even in the mismatch between SIV capsid and human CypA. Cyclosporine decreased transduction efficiency with the HIV1 vector, whereas it slightly increased transduction efficiency with the χHIV vector in human CD34⁺ cells. MG132 increased transduction efficiency with both χHIV and HIV1 vectors in the same manner. However, MG132 was toxic to human CD34⁺ cells at high concentrations, and both drugs had a small range of effective dosage. These findings demonstrate that both χHIV and HIV1 vectors have similar transduction efficiency for human hematopoietic repopulating cells, suggesting that the χHIV vector escapes from TRIM5α restriction, which is independent of human CypA.

Cord blood endothelial progenitor cells as therapeutic and imaging probes

Numerous studies demonstrated that neovascularization processes associated with severe tissue ischemia commonly found in conditions such as cardiovascular disorders and tumor growth occur via angiogenic and vasculogenic mechanisms. Over the past decade, it has been demonstrated that endothelial progenitor cells (EPCs) play a significant role in neo-angiogenic and neovasculogenic processes. Due to their ability to self-renew, circulate, home to the ischemic sites and differentiate into mature endothelial cells, EPCs derived from various sources hold enormous potential to be used as therapeutic agents in pro- or anti-angiogenic strategies for the treatment of ischemic and tumor conditions, respectively. However, the development of EPC-based therapies requires accompanying, noninvasive imaging protocol for in vivo tracking of transplanted cells. Hence, this review focuses on cord blood-derived EPCs and their role in neovascularization with emphasis on the potential use of EPCs as a therapeutic and imaging probe.

Human induced pluripotent stem cells can reach complete terminal maturation: in vivo and in vitro evidence in the erythropoietic differentiation model

BACKGROUND

Human induced pluripotent stem cells offer perspectives for cell therapy and research models for diseases. We applied this approach to the normal and pathological erythroid differentiation model by establishing induced pluripotent stem cells from normal and homozygous sickle cell disease donors.

DESIGN AND METHODS

We addressed the question as to whether these cells can reach complete erythroid terminal maturation notably with a complete switch from fetal to adult hemoglobin. Sickle cell disease induced pluripotent stem cells were differentiated in vitro into red blood cells and characterized for their terminal maturation in terms of hemoglobin content, oxygen transport capacity, deformability, sickling and adherence. Nucleated erythroblast populations generated from normal and pathological induced pluripotent stem cells were then injected into non-obese diabetic severe combined immunodeficiency mice to follow the in vivo hemoglobin maturation.

RESULTS

We observed that in vitro erythroid differentiation results in predominance of fetal hemoglobin which rescues the functionality of red blood cells in the pathological model of sickle cell disease. We observed, in vivo, the switch from fetal to adult hemoglobin after infusion of nucleated erythroid precursors derived from either normal or pathological induced pluripotent stem cells into mice.

CONCLUSIONS

These results demonstrate that human induced pluripotent stem cells: i) can achieve complete terminal erythroid maturation, in vitro in terms of nucleus expulsion and in vivo in terms of hemoglobin maturation; and ii) open the way to generation of functionally corrected red blood cells from sickle cell disease induced pluripotent stem cells, without any genetic modification or drug treatment.

Macrophages prevent human red blood cell reconstitution in immunodeficient mice

An animal model supporting human erythropoiesis will be highly valuable for assessing the biologic function of human RBCs under physiologic and disease settings, and for evaluating protocols of in vitro RBC differentiation. Herein, we analyzed human RBC reconstitution in NOD/SCID or NOD/SCID/γc(-/-) mice that were transplanted with human CD34+ fetal liver cells and fetal thymic tissue. Although a large number of human CD45- CD71+ nucleated immature erythroid cells were detected in the bone marrow, human RBCs were undetectable in the blood of these mice. Human RBCs became detectable in blood after macrophage depletion but disappeared again after withdrawal of treatment. Furthermore, treatment with human erythropoietin and IL-3 significantly increased human RBC reconstitution in macrophage-depleted, but not control, humanized mice. Significantly more rapid rejection of human RBCs than CD47-deficient mouse RBCs indicates that mechanisms other than insufficient CD47-SIRPα signaling are involved in human RBC xenorejection in mice. All considered, our data demonstrate that human RBCs are highly susceptible to rejection by macrophages in immunodeficient mice. Thus, strategies for preventing human RBC rejection by macrophages are required for using immunodeficient mice as an in vivo model to study human erythropoiesis and RBC function.

Humanized bone marrow mouse model as a preclinical tool to assess therapy-mediated hematotoxicity

PURPOSE

Preclinical in vivo studies can help guide the selection of agents and regimens for clinical testing. However, one of the challenges in screening anticancer therapies is the assessment of off-target human toxicity. There is a need for in vivo models that can simulate efficacy and toxicities of promising therapeutic regimens. For example, hematopoietic cells of human origin are particularly sensitive to a variety of chemotherapeutic regimens, but in vivo models to assess potential toxicities have not been developed. In this study, a xenograft model containing humanized bone marrow is utilized as an in vivo assay to monitor hematotoxicity.

EXPERIMENTAL DESIGN

A proof-of-concept, temozolomide-based regimen was developed that inhibits tumor xenograft growth. This regimen was selected for testing because it has been previously shown to cause myelosuppression in mice and humans. The dose-intensive regimen was administered to NOD.Cg-Prkdc(scid)IL2rg(tm1Wjl)/Sz (NOD/SCID/γchain(null)), reconstituted with human hematopoietic cells, and the impact of treatment on human hematopoiesis was evaluated.

RESULTS

The dose-intensive regimen resulted in significant decreases in growth of human glioblastoma xenografts. When this regimen was administered to mice containing humanized bone marrow, flow cytometric analyses indicated that the human bone marrow cells were significantly more sensitive to treatment than the murine bone marrow cells and that the regimen was highly toxic to human-derived hematopoietic cells of all lineages (progenitor, lymphoid, and myeloid).

CONCLUSIONS

The humanized bone marrow xenograft model described has the potential to be used as a platform for monitoring the impact of anticancer therapies on human hematopoiesis and could lead to subsequent refinement of therapies prior to clinical evaluation.