Modeling the Tumor Microenvironment and Cancer Immunotherapy in Next-Generation Humanized Mice

Abstract

Cancer immunotherapy has brought significant clinical benefits to numerous patients with malignant disease. However, only a fraction of patients experiences complete and durable responses to currently available immunotherapies. This highlights the need for more effective immunotherapies, combination treatments and predictive biomarkers. The molecular properties of a tumor, intratumor heterogeneity and the tumor immune microenvironment decisively shape tumor evolution, metastasis and therapy resistance and are therefore key targets for precision cancer medicine. Humanized mice that support the engraftment of patient-derived tumors and recapitulate the human tumor immune microenvironment of patients represent a promising preclinical model to address fundamental questions in precision immuno-oncology and cancer immunotherapy. In this review, we provide an overview of next-generation humanized mouse models suitable for the establishment and study of patient-derived tumors. Furthermore, we discuss the opportunities and challenges of modeling the tumor immune microenvironment and testing a variety of immunotherapeutic approaches using human immune system mouse models.

Keywords: PDX; avatar; chimeric antigen receptor (CAR); colorectal cancer; humanized mice; immune checkpoint blockade; immuno-oncology; metastasis; precision oncology.

Figure 1

Humanized mouse models for translational cancer research and immunotherapy. The scheme illustrates the genealogical tree of different humanized mouse strains. Details on the characteristics of the individual strains and their usability for generating PDXs and for testing cancer immunotherapies can be found in the text and are summarized in Table 1 and Table 2. Humanized mouse strains that are not commonly used or that are commercial regenerations of established mouse models (e.g., NCG, B-NDG) have not been included in this figure. NRG-A2/DR4 mice are also called “DRAGA” mice. Abbreviations: B₆RG, C57BL/6 Rag2^-/- Il2rg^-/-; BRG, BALB/c Rag2^-/- Il2rg^-/-; NOD, non-obese diabetic; NOG, NOD SCID Il2rg^null; NRG, NOD Rag1^-/- Il2rg^-/-; NSG, NOD SCID Il2rg^-/-; KI, knock-in; SCID, severe combined immunodeficiency; SRG, SIRPA Rag2^-/- Il2rg^-/-; MISTRG, M-CSF IL3/GM-CSF SIRPA TPO Rag2^-/- Il2rg^-/-; Tg, transgene.

Figure 2

Modeling patient-derived tumors in immunodeficient mice. (A) Patient-derived tumor xenograft (PDX) models are established by implanting patient tumor into immunodeficient mice. The PDXs are expanded in vivo by serial passages to establish a PDX biobank. (B) Factors that impact the successful establishment of PDXs (initial growth and in vivo propagation for at least 5 passages). (C) Immunodeficient mouse models have unique characteristics (e.g., expression of human cytokines) and therefore differently support the engraftment of various cancer types. Abbreviations: AML, acute myeloid leukemia; APL, acute promyelocytic leukemia; CAR, chimeric antigen receptor; CNA, copy number alteration; HR+, hormone receptor-positive; IL-6, interleukin 6; M-CSF, macrophage colony-stimulating factor; MDS, myelodysplastic syndrome; MM, multiple myeloma.

Figure 3

Establishment of human immune system (HIS) mouse models. (A) Immunodeficient mice can be reconstituted with a human immune system. For this purpose, newborn mice (1–5 days old) are preconditioned (e.g., sublethal irradiation, busulfan treatment). Some humanized mouse strains, such as NSG-W41, NRG-W41, B₆RGS^NOD-K, MISTRG and MISTRG-6 mice, do not need to be preconditioned. Human HSPCs are injected into the liver of newborn mice, which leads to the development of a human immune system. (B) The bar graph illustrates the composition of the human immune system in five different humanized mouse models 10–14 weeks post-engraftment with fetal/neonatal HSPCs [14,149,150]. The development, composition and function of the human immune system depends on the humanized mouse model and the source of HSPCs. Fetal and neonatal HSPCs (fetal liver, umbilical cord blood) engraft ≥ 3-fold better than adult HSPCs (bone marrow, G-CSF-mobilized blood) [18,143]. (C) MHC-deficient humanized mice can be preconditioned and intravenously engrafted with human PBMCs. (D) The bar graph illustrates the composition of human immune system in NSG and MHC-deficient NSG mice 4 weeks post-engraftment with human PBMCs [144]. Abbreviations: cGy, centigray; G-CSF, granulocyte colony-stimulating factor; HSPC, hematopoietic stem and progenitor cell; Hu-PBL, human peripheral blood lymphocyte; MHC, major histocompatibility complex; NK cell, natural killer cell; PBMC, peripheral blood mononuclear cells.

Figure 4

Cancer immunotherapy in humanized PDX mouse models. Efficient and functional development of human immune cell lineages depends on the humanized mouse model. This also influences the composition of the TIME and the suitability of investigating different types of immunotherapies [14,18,83]. Abbreviations: ADCC, antibody-dependent cellular cytotoxicity; ADCP, antibody-dependent cellular phagocytosis; CTLA-4, cytotoxic T lymphocyte-associated antigen 4; HIS, human immune system; KIT (also known as c-kit, CD117 or stem cell factor receptor); NK, natural killer; PD-1, programmed cell death protein 1; TAM, tumor-associated macrophage; TIME, tumor immune microenvironment; VEGF, vascular endothelial growth factor.