Humanized Mouse Models of Clinical Disease

Abstract

Immunodeficient mice engrafted with functional human cells and tissues, that is, humanized mice, have become increasingly important as small, preclinical animal models for the study of human diseases. Since the description of immunodeficient mice bearing mutations in the IL2 receptor common gamma chain (IL2rg^null) in the early 2000s, investigators have been able to engraft murine recipients with human hematopoietic stem cells that develop into functional human immune systems. These mice can also be engrafted with human tissues such as islets, liver, skin, and most solid and hematologic cancers. Humanized mice are permitting significant progress in studies of human infectious disease, cancer, regenerative medicine, graft-versus-host disease, allergies, and immunity. Ultimately, use of humanized mice may lead to the implementation of truly personalized medicine in the clinic. This review discusses recent progress in the development and use of humanized mice and highlights their utility for the study of human diseases.

Keywords: allergy; autoimmunity; cancer; humanized mice; immunodeficient mice; infectious disease; regenerative medicine.

Figure 1. Model systems for engraftment of human immune systems into immunodeficient mice

Hu-PBL-SCID; IV or IP injection of mature lymphoid populations including peripheral blood mononuclear cells, lymph node cells, and splenocytes. The advantage of this model is the easy access to clinically relevant samples, but engraftment is limited to predominately human CD3+ T cells. The disadvantage of this model is the development of GVHD leading to a short experimental window. Hu-SRC-SCID; IV or intra-femoral injection of human CD34+ HSCs derived from umbilical cord blood, bone marrow, fetal liver or G-CSF-mobilized peripheral blood stem cells. The advantage of this model is the development of a complete human immune system, but the human T cells are educated on the murine thymus and are H2-restricted. Poor human mucosal immune system development is also observed. BLT; engraftment of human fetal thymus and liver fragments under the renal capsule of the kidney and IV injection of human CD34+ HSCs from the autologous fetal liver. This is a more complete human immune system with improved mucosal immunity. The human T cells are educated on their autologous thymus and are HLA-restricted. The major disadvantage is the development of a wasting syndrome following long-term engraftment that is observed in most laboratories.

Figure 2. Murine models of human hepatocyte engraftment

The engraftment of human hepatocytes into immunodeficient mice requires depletion of murine hepatocytes to create space for human hepatocytes. FAH: expression of fumaryl acetoacetate hydrolase leads to an accumulation of toxic tyrosine metabolic intermediates. Provision of NTBC in the drinking water prevents hepatocyte death until human hepatocytes are engrafted. TK-NOG: herpes simplex virus thymidine kinase transgene under the control of the Albumin promoter. Specific mouse hepatocyte depletion is induced by administration of gancyclovir, but male TK-NOG mice are sterile making breeding of this stock difficult. uPA-SCID: urokinase plasminogen activator gene under the control of the major urinary protein promoter. Over-expression of the uPA gene in an uncontrolled manner leads to high lethality of newborn pups that must be rapidly transplanted with human hepatocytes for their survival. NSG-PiZ: Transgenic expression of SERPINA1 leads to misfolding of human alpha-1 anti-trypsin and death of murine hepatocytes, providing a competitive advantage for human hepatocyte engraftment. AFC8: Expression of active caspase 8 fused with FK506 binding domain that when treated with FK506 selectively kills mouse hepatocytes. Alb-TRECK/SCID: Transgenic expression of the diphtheria toxin receptor under the control of the albumin promoter. Injection of diphtheria toxin selectively kills murine hepatocytes expressing the receptor. Anti-mouse Fas activating antibody: Injection of Jo2 Fas-activating antibody selectively kills murine hepatocytes creating space for human hepatocytes.

Figure 3. Humanized mice in cancer biology and therapy

Humanized mice are used as preclinical models for investigation of cancer cell biology, identification of tumor stem cells, use as an in vivo platform for identifying and testing potential drug targets, investigation of mechanisms of tumor metastasis, and for evaluation of potential new therapeutics prior to their entry into the clinic.