Humanized Mice

Introduction

“Humanized mice are increasingly appreciated as an incredibly powerful platform for infectious disease research. The often very narrow species tropism of many viral infections [Meaning how “various viruses/pathogens have evolved to preferentially target particular host organisms, tissues, or cell types inside those species”such as humans.¹], 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.”²

The idea of using humanized mice is to find a more effective way to study viruses in vivo. The term “in vivo” means in a living organism such as a mouse, as opposed to “in vitro” which means in a test tube or petri dish. Typically this is done using animals which may not be the idea host. The idea behind using humanized mice is that it allows pathogens to be studied in the host cells they actually affect.

Varieties of Humanized Mice

These humanized mice come in a number of “flavours”. But first some definitions so the lingo of the science behind these mice can be more clearly understood.

T-cell: is one of the important white blood cells of the immune system and play a central role in the adaptive immune response.³
B cell: is also a type of white blood cell.⁴
hematopoietic stem cells (HSCs): these are stem cells that differentiate into a variety of cells including Red and white blood cells, platelets, T-cells and B-cells. “HSCs give rise to all blood cells of the immune system through a process called hematopoiesis. As hematopoietic stem cells progress through hematopoiesis they generate the myeloid (monocytes, macrophages, granulocytes, megakaryocytes, dendritic cells, erythrocytes) and lymphoid (T cells, B cells, NK cells) lineages.”⁵ (Fig. 1)
Peripheral blood mononuclear cells (PBMCs): are a diverse mixture of highly specialized immune cells that play key roles in keeping our bodies healthy. These actually originate through the HSCs defined above.⁶ (Fig. 1)

First there are two categories of humanized mice. 1. Immunocompetent mouse models. 2. Immunocompromised mouse models. Regarding the latter: “Immunodeficient mice are essential for establishing mouse models with humanized immune systems. Humanized CD34+ mice have the functional human immune system (HIS) and display T-cell dependent inflammatory responses, with no donor cell immune reactivity toward the host, and yield robust multilineage immune systems. In addition to Humanized CD34+ mice, all immunodeficient mice were engrafted with human hematopoietic stem cells (HSCs) [these are stem cells that differentiate into a variety of cells including Red and white blood cells, platelets, T-cells and B-cells] or human peripheral blood mononuclear cells (PBMCs)”⁷

Figure 1 above. Overview of the different cells that arise from hematopoietic stem cells through the process of hematopoiesis.⁸

In addition mice can be humanized via engraftment or genetically.⁹ “A genetically humanized model is one where a human gene, genomic sequence, or regulatory element is knocked into a mouse. Often, this is performed in tandem with a knockout of the endogenous mouse gene, functionally replacing the mouse gene with the human gene. Genetic humanization is useful when a specific gene or protein target is relevant to research outcomes…Genetic humanization should not be confused with cellular humanization, where mice are engrafted with human cells such as hematopoietic stem cells (HSC) or peripheral blood mononuclear cells (PBMC).”¹⁰

These are just a few examples of the many versions of humanized mice.

HuPBL mice (Human Peripheral Blood Lymphocyte)¹¹First developed in 1983. The first step is to weaken the immune system of the mouse by exposing it to a “sub-lethal dose of irradiation”. This impaired immune system in the mouse allows, at least temporarily the presence of human cells, especially “completely functional, educated T-cell populations”. This type of mouse also allows B cells to be added – at low levels. However there are drawbacks to this type of humanize mouse – the T cells added are activated against the murine cells (that is mouse cells). Thus this model is only useful for short term studies. These do not appear to have been developed from aborted tissues.
HIS mice (Human Immune System)¹²Again mice with impaired immune systems are used. They are injected with haematopoietic stem cells. (These are the stem cells that give rise to other blood cells.¹³) These can come from umbilical cord blood or fetal livers. Thus the tissue from aborted babies is used to create at least some of these mice.
BLT mice (Bone marrow, Liver and Thymus)¹⁴“Co-xenotransplantation of autologous human fetal liver and thymic tissue under the murine renal capsule, alongside an intravenous CD34 + HSC injection…”.¹⁵
(To put this in laymen’s terms, from the FDA: “Xenotransplantation is any procedure that involves the transplantation, implantation or infusion into a human recipient of either (a) live cells, tissues, or organs from a nonhuman animal source”¹⁶ or vice versa. The renal capsule is “a thin membranous protective covering over the external surface of the left as well as the right kidney”.¹⁷)

What follows is the development of what is called a “BLT organoid” which supports functional T-cell populations that are needed in the testing process that these mice serve. “Organoids represent cells grown in specific 3D environments [in this case the mice] to create mini, simplified organs that retain some physiological function.”¹⁸

This type of mouse is “widely used tool for dissecting pathogen transmission, dissemination and pathogenesis, alongside analysis of vaccine candidates and microbicides.¹⁹

Figure 2 below. The tissue from aborted babies is used to create some varieties of the BLT mice. It is used to study how pathogens work – that is infectious microorganisms or agents, such as a virus a or bacterium that cause disease.²⁰

Conclusion

Besides using these humanized mice is to study infections diseases, they are also used to study genetic conditions, conduct therapeutic testing and select vaccine candidates.²¹

This area appears to be a rapidly growing area. With a whole array of humanized mice and even humanized mice versions used to create other humanized mice. For example B-NDG mice are genetically humanized and are “suitable for engraftment with human hematopoietic stem cells, peripheral blood mononuclear cells, and human tumor cells or tissues.”These mice can then be used for “engraftment with human hematopoietic stem cells, peripheral blood mononuclear cells, and human tumor cells or tissues.”²² It appears that the Humanized mice developed from them can used aborted fetal cells. But this is not always the case. Not all humanized mice appear to be based on abortion. This is clearly a rapidly growing field that needs to be regulated to make sure abortion is not used. It also presents a problem for those who wish to avoid using pharmaceuticals that have been developed using aborted fetal tissues.

Introduction

Varieties of Humanized Mice

Conclusion

Further Reading