Humanized hemato-lymphoid system mice

Abstract

Over the last decades, incrementally improved xenograft mouse models, supporting the engraftment and development of a human hemato-lymphoid system, have been developed and now represent an important research tool in the field. The most significant contributions made by means of humanized mice are the identification of normal and leukemic hematopoietic stem cells, the characterization of the human hematopoietic hierarchy, and their use as preclinical therapy models for malignant hematopoietic disorders. Successful xenotransplantation depends on three major factors: tolerance by the mouse host, correct spatial location, and appropriately cross-reactive support and interaction factors such as cytokines and major histocompatibility complex molecules. Each of these can be modified. Experimental approaches include the genetic modification of mice to faithfully express human support factors as non-cross-reactive cytokines, to create free niche space, the co-transplantation of human mesenchymal stem cells, the implantation of humanized ossicles or other stroma, and the implantation of human thymic tissue. Besides the source of hematopoietic cells, the conditioning regimen and the route of transplantation also significantly affect human hematopoietic development in vivo. We review here the achievements, most recent developments, and the remaining challenges in the generation of pre-clinically-predictive systems for human hematology and immunology, closely resembling the human situation in a xenogeneic mouse environment.

Figure 1.

The principle of humanized mouse models. Normal (healthy) or neoplastic hematopoietic cells are transplanted into immuno-compromised mice to develop humanized mouse models. The models help to learn and understand the physiology and pathophysiology of human hematopoiesis. The knowledge gained with these models can then be translated to humans.

Figure 2.

Prerequisites for successful xenotransplantion of human hematopoiesis. The (mouse) host must be tolerant for the transplanted human hematopoietic cells and provide a supportive niche, including cross-reactive growth factors, cytokines and MHC molecules. Novel models carry multiple genetic modifications in the mouse host to meet these three criteria and support successful human hematopoietic engraftment.

Figure 3.

Engineering of heterotopic human niches in a xenograft mouse model. Human adult BM-derived mesenchymal stromal cells (MSC) are ex vivo-differentiated into hypertrophic cartilage, and implanted into immune-compromised mice. CD34⁺ human umbilical cord blood cells are transplanted into sublethally irradiated mice with implanted ossicles. Upon in vivo implantation the hypertrophic cartilage develops into a fully mature bone organ through endochondral ossification.

Figure 4.

Parameters that influence the engraftment of human hematopoietic cells in vivo. Depending on the experimental conditions, different strains of mice, methods of pre-conditioning and routes of injection can be used to transplant human hematopoietic cells obtained from various sources.

Figure 5.

The expression of certain surface markers is significantly increased on LSC and allows these cells to be distinguished from their normal counterparts. LSC express high levels of CD47 and evade innate immune attacks by macrophages. The expression of TIM-3 has only been reported on LSC, but not on HSC. The expression of CD44 and the v6 isoform of CD44 are increased on LSC. These and other markers such as CLL-1, CD96 and CD123 allow LSC to be distinguished and targeted by monoclonal antibodies.

Figure 6.

Possible disease-specific, tailored mouse models in future research. Different humanized mouse models that might serve specific research questions.