Modeling normal and malignant human hematopoiesis in vivo through newborn NSG xenotransplantation

Characterization of Human Engraftment and Hemophagocytic Lymphohistiocytosis in NSG-SGM3 Neonate Mice Engrafted with Purified CD34+ Hematopoietic Stem Cells

Immunodeficient mice bearing human immune systems, or "humanized" chimeric mice, are widely used in basic research, along with the preclinical stages of drug development. Nonobese diabetic-severe combined immunodeficiency (NOD-SCID) IL2Rγnull (NSG) mice expressing human stem cell factor, granulocyte-macrophage colony stimulating factor, and interleukin-3 (NSG-SGM3) support robust development of human myeloid cells and T cells but have reduced longevity due to the development of fatal hemophagocytic lymphohistiocytosis (HLH). Here, we describe an optimized protocol for development of human immune chimerism in NSG-SGM3 mice. We demonstrate that efficient human CD45+ reconstitution can be achieved and HLH delayed by engraftment of neonatal NSG-SGM3 with low numbers of human umbilical cord-derived CD34+ hematopoietic stem cells in the absence of preconditioning irradiation.

Murine Models of Chronic Viral Infections and Associated Cancers

Viruses are now recognized as bona fide etiologic factors of human cancer. Carcinogenic viruses include Epstein– Barr virus (EBV), high-risk human papillomaviruses (HPVs), hepatitis B virus (HBV), hepatitis C virus (HCV), human T-cell leukemia virus type 1 (HTLV-1), human immunodeficiency virus type 1 (HIV-1, indirectly), and several candidate human cancer viruses. It is estimated that 15% of all human tumors worldwide are caused by viruses. Tumor viruses establish long-term persistent infections in humans, and cancer is an accidental side effect of viral replication strategies. Viruses are usually not complete carcinogens, supporting the concept that cancer results from the accumulation of multiple cooperating events, in which human cancer viruses display different, often opposing roles. The laboratory mouse Mus musculus is one of the best in vivo experimental systems for modeling human pathology, including viral infections and cancer. However, mice are unsusceptible to infection with the known carcinogenic viruses. Many murine models were developed to overcome this limitation and to address various aspects of virus-associated carcinogenesis, from tumors resulting from xenografts of human tissues and cells, including cancerous and virus infected, to genetically engineered mice susceptible to viral infections and associated cancer. The review considers the main existing models, analyzes their advantages and drawbacks, describes their applications, outlines the prospects of their further development.

Absence of CD11a Expression Identifies Embryonic Hematopoietic Stem Cell Precursors via Competitive Neonatal Transplantation Assay

Hematopoietic stem cells (HSCs) are defined by their self-renewal, multipotency, and bone marrow (BM) engraftment abilities. How HSCs emerge during embryonic development remains unclear, but are thought to arise from hemogenic endothelium through an intermediate precursor called "pre-HSCs." Pre-HSCs have self-renewal and multipotent activity, but lack BM engraftability. They can be identified functionally by transplantation into neonatal recipients, or by in vitro co-culture with cytokines and stroma followed by transplantation into adult recipients. While pre-HSCs express markers such as Kit and CD144, a precise surface marker identity for pre-HSCs has remained elusive due to the fluctuating expression of common HSC markers during embryonic development. We have previously determined that the lack of CD11a expression distinguishes HSCs in adults as well as multipotent progenitors in the embryo. Here, we use a neonatal transplantation assay to identify pre-HSC populations in the mouse embryo. We establish CD11a as a critical marker for the identification and enrichment of pre-HSCs in day 10.5 and 11.5 mouse embryos. Our proposed pre-HSC population, termed "11a- eKLS" (CD11a- Ter119- CD43+ Kit+ Sca1+ CD144+), contains all in vivo long-term engrafting embryonic progenitors. This population also displays a cell-cycle status expected of embryonic HSC precursors. Furthermore, we identify the neonatal liver as the likely source of signals that can mature pre-HSCs into BM-engraftable HSCs.

A human SIRPA knock-in xenograft mouse model to study human hematopoietic and cancer stem cells

In human-to-mouse xenogeneic transplantation, polymorphisms of signal-regulatory protein α (SIRPA) that decide their binding affinity for human CD47 are critical for engraftment efficiency of human cells. In this study, we generated a new C57BL/6.Rag2nullIl2rgnull (BRG) mouse line with Sirpahuman/human (BRGShuman) mice, in which mouse Sirpa was replaced by human SIRPA encompassing all 8 exons. Macrophages from C57BL/6 mice harboring Sirpahuman/human had a significantly stronger affinity for human CD47 than those harboring SirpaNOD/NOD and did not show detectable phagocytosis against human hematopoietic stem cells. In turn, Sirpahuman/human macrophages had a moderate affinity for mouse CD47, and BRGShuman mice did not exhibit the blood cytopenia that was seen in Sirpa-/- mice. In human to mouse xenograft experiments, BRGShuman mice showed significantly greater engraftment and maintenance of human hematopoiesis with a high level of myeloid reconstitution, as well as improved reconstitution in peripheral tissues, compared with BRG mice harboring SirpaNOD/NOD (BRGSNOD). BRGShuman mice also showed significantly enhanced engraftment and growth of acute myeloid leukemia and subcutaneously transplanted human colon cancer cells compared with BRGSNOD mice. BRGShuman mice should be a useful basic line for establishing a more authentic xenotransplantation model to study normal and malignant human stem cells.

Zebrafish disease models in hematology: Highlights on biological and translational impact

Zebrafish (Danio rerio) has proven to be a versatile and reliable in vivo experimental model to study human hematopoiesis and hematological malignancies. As vertebrates, zebrafish has significant anatomical and biological similarities to humans, including the hematopoietic system. The powerful genome editing and genome-wide forward genetic screening tools have generated models that recapitulate human malignant hematopoietic pathologies in zebrafish and unravel cellular mechanisms involved in these diseases. Moreover, the use of zebrafish models in large-scale chemical screens has allowed the identification of new molecular targets and the design of alternative therapies. In this review we summarize the recent achievements in hematological research that highlight the power of the zebrafish model for discovery of new therapeutic molecules. We believe that the model is ready to give an immediate translational impact into the clinic.

MISTRG mice support engraftment and assessment of nonhuman primate hematopoietic stem and progenitor cells

Preclinical feasibility, safety, and efficacy testing of hematopoietic stem cell (HSC)-mediated gene therapy approaches is commonly performed in large-animal models such as nonhuman primates (NHPs). Here, we wished to determine whether mouse models would allow engraftment of NHP HSPCs, which would enable more facile and less costly evaluation of promising strategies. In this study, we comprehensively tested two mouse strains for the engraftment of NHP CD34⁺ hematopoietic stem and progenitor cells (HSPCs). No engraftment of NHP HSPCs was observed in NSG mice, whereas the gene-humanized MISTRG model did demonstrate dose-dependent multilineage engraftment of NHP cells in the peripheral blood, bone marrow, spleen, and thymus. Most importantly, and closely mimicking the hematopoietic recovery of autologous stem cell transplantations in the NHP, only HSC-enriched CD34⁺CD90⁺CD45RA^- cell fractions engrafted and reconstituted the bone marrow stem cell niche in MISTRG mice. In summary, we here report the first "monkeynized" mouse xenograft model that closely recapitulates the autologous hematopoietic reconstitution in the NHP stem and progenitor cell transplantation and gene therapy model. The availability of this model has the potential to pre-evaluate novel HSC-mediated gene therapy approaches, inform studies in the NHP, and improve the overall outcome of large-animal experiments.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.

Enhanced Reconstitution of Human Erythropoiesis and Thrombopoiesis in an Immunodeficient Mouse Model with Kit(Wv) Mutations

In human-to-mouse xenograft models, reconstitution of human hematopoiesis is usually B-lymphoid dominant. Here we show that the introduction of homozygous Kit(Wv) mutations into C57BL/6.Rag2(null)Il2rg(null) mice with NOD-Sirpa (BRGS) strongly promoted human multi-lineage reconstitution. After xenotransplantation of human CD34(+)CD38(-) cord blood cells, these newly generated C57BL/6.Rag2(null)Il2rg(null)NOD-Sirpa Kit(Wv/Wv) (BRGSK(Wv/Wv)) mice showed significantly higher levels of human cell chimerism and long-term multi-lineage reconstitution compared with BRGS mice. Strikingly, this mouse displayed a robust reconstitution of human erythropoiesis and thrombopoiesis with terminal maturation in the bone marrow. Furthermore, depletion of host macrophages by clodronate administration resulted in the presence of human erythrocytes and platelets in the circulation. Thus, attenuation of mouse KIT signaling greatly enhances the multi-lineage differentiation of human hematopoietic stem and progenitor cells (HSPCs) in mouse bone marrow, presumably by outcompeting mouse HSPCs to occupy suitable microenvironments. The BRGSK(Wv/Wv) mouse model is a useful tool to study human multi-lineage hematopoiesis.

Humanizing NOD/SCID/IL-2Rγnull (NSG) mice using busulfan and retro-orbital injection of umbilical cord blood-derived CD34(+) cells

Background

Humanized mouse models are still under development, and various protocols exist to improve human cell engraftment and function.

Methods

Fourteen NOD/SCID/IL-2Rγnull (NSG) mice (4‒5 wk old) were conditioned with busulfan and injected with human umbilical cord blood (hUCB)-derived CD34⁺ hematopoietic stem cells (HSC) via retro-orbital sinuses. The bone marrow (BM), spleen, and peripheral blood (PB) were analyzed 8 and 12 weeks after HSC transplantation.

Results

Most of the NSG mice tolerated the regimen well. The percentage of hCD45⁺ and CD19⁺ cells rose significantly in a time-dependent manner. The median percentage of hCD45⁺cells in the BM was 55.5% at week 8, and 67.2% at week 12. The median percentage of hCD45⁺ cells in the spleen at weeks 8 and 12 was 42% and 51%, respectively. The median percentage of hCD19⁺ cells in BM at weeks 8 and 12 was 21.5% and 39%, respectively (P=0.04). Similarly, the median percentage of hCD19⁺ cells in the spleen at weeks 8 and 12 was 10% and 24%, respectively (P=0.04). The percentage of hCD19⁺ B cells in PB was 23% at week 12. At week 8, hCD3⁺ T cells were barely detectable, while hCD7⁺ was detected in the BM and spleen. The percentage of hCD3⁺ T cells was 2‒3% at week 12 in the BM, spleen, and PB of humanized NSG mice.

Conclusion

We adopted a simplified protocol for establishing humanized NSG mice. We observed a higher engraftment rate of human CD45⁺ cells than earlier studies without any significant toxicity. And human CD45⁺ cell engraftment at week 8 was comparable to that of week 12.

Prioritizing therapeutic targets using patient-derived xenograft models

Effective systemic treatment of cancer relies on the delivery of agents with optimal therapeutic potential. The molecular age of medicine has provided genomic tools that can identify a large number of potential therapeutic targets in individual patients, heralding the promise of personalized treatment. However, determining which potential targets actually drive tumor growth and should be prioritized for therapy is challenging. Indeed, reliable molecular matches of target and therapeutic agent have been stringently validated in the clinic for only a small number of targets. Patient-derived xenografts (PDXs) are tumor models developed in immunocompromised mice using tumor procured directly from the patient. As patient surrogates, PDX models represent a powerful tool for addressing individualized therapy. Challenges include humanizing the immune system of PDX models and ensuring high quality molecular annotation, in order to maximize insights for the clinic. Importantly, PDX can be sampled repeatedly and in parallel, to reveal clonal evolution, which may predict mechanisms of drug resistance and inform therapeutic strategy design.

Regulation of hematopoiesis in endosteal microenvironments

After birth, the hematopoietic system develops along with bone formation in mammals. Osteolineage cells are derived from mesenchymal progenitor cells, and differentiate into several types of bone-forming cells. Of the various types of cell constituents in bone marrow, osteolineage cells have been shown to play important roles in hematopoiesis. Early studies have identified osteoblasts as a hematopoietic stem cell niche component. Since that time, the role of endosteal microenvironment as a critical regulator of hematopoietic stem/progenitor cell (HSC/HPC) behavior has been appreciated particularly under stress conditions, such as cytokine-induced HSC/HPC mobilization, homing/engraftment after bone marrow transplantation, and disease models of leukemia/myelodysplasia. Recent studies revealed that the most differentiated osteolineage cells, i.e., osteocytes, play important roles in the regulation of hematopoiesis. In this review, we provide an overview of recent advances in knowledge of regulatory hematopoietic mechanisms in the endosteal area.

Small RNA as a regulator of hematopoietic development, immune response in infection and tumorigenesis

Posttranscriptional gene regulation by small RNAs (15-40-nucleotide noncoding RNAs) is now established as an important branch of the gene regulatory system. It has recently been revealed that noncoding RNAs can be categorized into different types and that they work through novel mechanisms. In addition, it has been shown that noncoding RNAs mediate intercellular communication and, importantly, that cross talk between coding and noncoding RNAs occurs. In this review, we discuss the recent findings concerning small RNAs. It was originally proposed that microRNAs (miRNAs) work to "fine tune" the determination of cell fate. However, critical functions beyond fine tuning have been revealed. In addition to miRNAs, next-generation sequencing has revealed the existence of various species of non-canonical small RNAs: mirtrons, piRNAs, 21U-RNA, endo-siRNAs, snoRNAs, usRNAs, and Y-RNA-derived small RNAs. Some of these species are involved in response to viral infection. Finally, we highlight the intracellular functions of small RNAs, which involve the exosomes.