A humanized mouse is a genetically modified mouse that has functioning human genes, cells, tissues and/or organs. Humanized mice are commonly used as small animal models in biological and medical research for human therapeutics. A humanized mouse or a humanized mouse model is one that has been xenotransplanted with human cells and/or engineered to express human gene products, so as to be utilized for gaining relevant insights in the in vivo context for understanding of human-specific physiology and pathologies. Several human biological processes have been explored using animal models like rodents and non-human primates. In particular, small animals such as mice are advantageous in such studies owing to their small size, brief reproductive cycle, easy handling and due to the genomic and physiological similarities with humans; moreover, these animals can also be genetically modified easily. Nevertheless, there are several incongruencies of these animal systems with those of humans, especially with regard to the components of the immune system. To overcome these limitations and to realize the full potential of animal models to enable researchers to get a clear picture of the nature and pathogenesis of immune responses mounted against human-specific pathogens, humanized mouse models have been developed. Such mouse models have also become an integral aspect of preclinical biomedical research.
History
The discovery of the athymic mouse, commonly known as the nude mouse, and that of the SCID mouse were major events that paved the way for humanized mice models. The first such mouse model was derived by backcrossing C57BL/Ka and BALB/c mice, featuring a loss of function mutation in the PRKDC gene. The PRKDC gene product is necessary for resolving breaks in DNA strands during the development of T cells and B cells. A mutation in the Foxn1 gene on chromosome 11 resulted in impaired thymus development, leading to a deficiency in mature T lymphocytes. Dysfunctional PRKDC gene leads to impaired development of T and B lymphocytes, which gives rise to severe combined immunodeficiency (SCID). In spite of the efforts in developing this mouse model, poor engraftment of human hematopoietic stem cells (HSCs) was a major limitation that called for further advancement in the development of humanized mouse models. Nude mice were the earliest immunodeficient mouse model. These mice primarily produced IgM and had minimal or no IgA. As a result, they did not exhibit a rejection response to allogeneic tissue. Commonly utilized strains included BALB/c-nu, Swiss-nu, NC-nu, and NIH-nu, which were extensively employed in the research of immune diseases and tumors. However, due to the retention of B cells and NK cells, they were unable to fully support engraftment of human immune cells, thus making them unsuitable as an ideal humanized mouse model. The next big step in the development of humanized mice models came with transfer of the scid mutation to a non-obese diabetic mouse. This resulted in the creation of the NOD-scid mice which lacked T cells, B cells, and NK cells. This mouse model permitted for a slightly higher level of human cell reconstitution. Nevertheless, a major breakthrough in this field came with the introduction of the mutant IL-2 receptor (IL2rg) gene in the NOD-scid model. This accounted for the creation of the NOD-scid-γcnull mice (NCG, NSG or NOG) models which were found to have defective signaling of interleukins IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. Researchers evolved this NSG model by knocking out the RAG1 and RAG2 genes (recombination activation genes), resulting into the RAGnull version of the NSG model that was devoid of major cells of the immune system including the natural killer cells, B lymphocytes and T lymphocytes, macrophages and dendritic cells, causing the greatest immunodeficiency in mice models so far. The limitation with this model was that it lacked the human leukocyte antigen. In accordance to this limitation, the human T cells when engrafted in the mice, failed to recognize human antigen-presenting cells, which consequated in defective immunoglobulin class switching and improper organization of the secondary lymphoid tissue. To circumvent this limitation, the next development came with the introduction of transgenes encoding for HLA I and HLA II in the NSG RAGnull model that enabled buildout of human T-lymphocyte repertoires as well as the respective immune responses. Mice with such human genes are technically human-animal hybrids.
Types
Engrafting an immunodeficient mouse with functional human cells can be achieved by intravenous injections of human cells and tissue into the mouse, and/or creating a genetically modified mouse from human genes. These models have been instrumental in studying human diseases, immune responses, and therapeutic interventions. This section highlights the various humanized mice models developed using the different methods.
Hu-PBL-scid model The human peripheral blood lymphocyte-severe combined immunodeficiency mouse model has been employed in a diverse array of research, encompassing investigations into Epstein-Barr virus (EBV)-associated lymphoproliferative disease, toxoplasmosis, human immunodeficiency virus (HIV) infection, and autoimmune diseases. These studies have highlighted the effectiveness of the hu-PBL-SCID mouse model in examining various facets of human diseases, including pathogenesis, immune responses, and therapeutic interventions. Furthermore, the model has been utilized to explore genetic and molecular factors linked to neuropsychiatric disorders such as schizophrenia, offering valuable insights into the pathophysiology and potential therapeutic targets for these conditions. This model is developed by intravenously injecting human PBMCs into immunodeficient mice. The peripheral blood mononuclear cells to be engrafted into the model are obtained from consented adult donors. The advantages associated with this method are that it is comparatively an easy technique, the model takes relatively less time to get established and that the model exhibits functional memory T cells. It is particularly very effective for modelling graft vs. host disease. The model lacks engraftment of B lymphocytes and myeloid cells. Other limitations with this model are that it is suitable for use only in short-term experiments (<3 months) and the possibility that the model itself might develop graft vs. host disease.
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