The lack of appropriate animal models often limits the study of human pathologies. Infectious diseases in humans are primarily studied using mice. However, many human-specific infectious agents do not affect mice.
So-called humanized mice have been created by transplanting human cells into mice over the past twenty years. They have reproduced certain aspects of human disease in mice, which gives hope for new therapeutic approaches in the future.
Immune-deficient laboratory mice are genetically engineered to express human genes or transplanted with human tissues or stem cells. Human genes, organs, pathologies, and drugs intended for human use can be studied using these mice.
A few tips on choosing the suitable immunodeficient mouse model are provided below.
Define Your Research Objectives
The same immunodeficient mice model probably won't work for every situation. A model must choose according to the study's research aim and the constraints imposed by it. Therefore, defining the research needs to address the specific question is a precursor to developing an immunodeficient mice model.
After specifying what specific information the animal model is required to provide, the next step is to determine how the immunodeficient mice model will provide that information. This information will guide the remaining mice model development process.
The common strains of immunodeficiency mice models include
Nude Mice
In 1966, Flanagan explored the first BALB/c Nude (nu) mouse, lacking hair and having a thymus. It is named "nude" due to a genetic mutation producing the hairless state. An athymic state - lack of functioning thymus - is also the result of this mutation. A Foxn1 homolog 11 gene disruption is the genetic basis of the nude mouse mutation. In the nude mice, the T-cells are few, antigen-restricted antibodies are low, and natural killer cells are abundant.
Xenografting tumor cell lines onto nude mice, studying malignancy mechanisms in tumors, and testing treatment options are all possible using this model.
NOD Mice
Previously, type 1 diabetes was studied using special mice that weren't obese or diabetic. NOD mice are valuable models for studying Type 1 diabetes, autoimmune disorders, and tolerance mechanisms. In humans, it shares certain characteristics with Type 1 diabetes (T1D), autoimmune diabetes, autoreactive CD4+ and CD8+ T cells, pancreas-specific autoantibodies, and other genetic similarities.
The model is popular; however, critics feel disappointed that while several applications have been reported to prevent diseases in NOD mice, the same outcome has not yet been achieved by humans. NOD mice and human diseases share many striking similarities, and these are the reasons why NOD mice are used in research so frequently.
NOD SCID Mice
Neither T nor B lymphocytes are present in these immunodeficient mice. A unique set of immune system defects characterized by this model allows the reconstitution of HIV-1 models with human hematopoietic cells.
The early development of B- and T-lineage cells is impaired by this mutation and the recombination of antigen receptor genes. There is no evidence that other hematopoietic cell types are abnormal. Young adult mice can show signs of lymphocyte development halting; some mice make functional B and T cells in young adulthood. These mice are studied for their normal and abnormal development and function. Also, they are useful for studying nonlymphoid cells in the presence of lymphocytes.
C-NKG Mice
Mature T, B, and NK immune cells are absent in these mice, which results in reduced complement activity and compromised phagocytosis of human-derived cells by macrophages. This model can be used to perform transplants using peripheral blood mononuclear cells (PBMCs), patient-derived xenografts (PDXs), and hematopoietic stem cells (HSC). C-NKG mice show high immunodeficiency levels and have been used as models to study tumors, immunity, therapeutic vaccines, autoimmunity illnesses, and GVHD/transplantation.
Outline Intrinsic Pathological Factors
As soon as the experimental requirement has been identified, the next step in developing an immunodeficient mice model is to determine how it is intrinsically related to human disease. A biological agent or pathogen interacts with the host through inherent factors. When comparing the mice model to the human condition, it is crucial to identify all intrinsic factors that are relevant to the biological process being investigated.
The early stages of the model development process may seem obvious and fundamental. Still, they can easily be overlooked, resulting in a not totally appropriate model for the research project. Identifying basic steps in the progression of infection should be the first step in defining the disease's pathological process and the best mice strains for the research.
According to a simple linear scenario, a disease-causing agent must first gain exposure to its host, then bind to that host, enter it, distribute within the host to its target tissues, and finally exert its harmful effects. An infection can be transmitted through injection (via fleas or tick bites) or abrasion.
There may also be an interaction between the pathogen and mucosa, such as the digestive tract and lungs. There may be times when a pathogen binds to a host cell's receptors. The pathogen could enter the host through the mucosal cells and enter the systemic circulation by hijacking the host's cellular processes. A pathogen can spread through the body (such as through the circulatory system) after entering the host. The pathogen binds to receptors on specific tissues to target them during distribution.
In the target cells, specific biochemical processes can be affected by the pathogen, causing disease. Pathogens can produce extracellular substances such as toxins and tissue-damaging enzymes, even when they don't invade a host's body or target tissues. Circulation can distribute these factors to target tissues or cells after they are transported into the body.
There exists an interplay between host and pathogen that cannot be seen in this detailed linear view. As the pathogen interacts with the host and the host interacts with the pathogen, both change dramatically. In some host cells, specific receptors are produced only when the pathogen is present. Further, when the pathogen invades, it triggers innate defenses as well as acquired ones.
In this interaction, the invading organism may directly damage the host's cells and tissues as it harnesses the host's cellular processes to promote its replication. Host response to the pathogen determines the severity of the disease the host experiences as a result of the infection.
The interaction between the pathogen and host determines the level of virulence, not just the organism itself. Host-pathogen interactions are replicated in a model of disease. Therefore, both the host and pathogen collectively form intrinsic factors of a disease model and define it.
Outline Extrinsic Pathological Factors
In addition to intrinsic parameters, external factors also affect the process but are not directly involved in the interaction between the host and the pathogen. Extrinsic factors are factors outside of the relationship between the host and the pathogen or agent. However, they do not typically form part of the animal model, but they are crucial. Host-pathogen interactions, in turn, define the specific animal models and intrinsic factors that impact them.
The methods used to prepare, handle, and formulate the agent, for instance, may affect the results. External factors may also influence a host's response. The bedding used, the temperature and the time of administering the agents, and even the light cycles in which the mice are kept can influence the immunological response of the mice or the pharmacokinetics of the agents being studied.
Extrinsic factors extend a study's experimental design. As a result, these must be defined and documented to allow for comparisons and enhance the ability to extrapolate results to human diseases.
While there is a difference of opinion with respect to the extrinsic factors that affect a model, it is widely recognized that these factors collectively influence the design of experiments that use animals. Host animals are exposed to pathogens or biological agents in animal models of disease. To understand and control an animal model, extrinsic and intrinsic factors must be evaluated collectively as part of the experimental design.
Define a Search Framework
The relevance of the intrinsic and extrinsic factors identified may require a preliminary, brief literature review using freely available resources. Studies conducted on immunodeficient mic models or human clinical data should be reviewed as part of the preliminary review. A detailed search strategy can be developed through this review.
Using the considerations identified through the preliminary literature review, a comprehensive literature search strategy can be designed. Often overlooked, a literature review is a necessary step to help inform decisions regarding immunodeficient models during their identification and development. It is important to design a search strategy that enables a comprehensive search of available data and publications.
It can be determined through a systematic literature review which factors are relevant to the biological phenomenon of interest, based on the identification of the relevant intrinsic and extrinsic factors. Often overlooked, such a review is essential for obtaining necessary information on which to base scientifically informed decisions throughout the process of identifying and developing animal models. A search strategy should be developed to provide a complete overview of the relevant data and publications.
A comprehensive search strategy should include researching all relevant information sources because no single database can contain all the information. Several types of search strategies may be more suitable, starting with the most relevant and free informational sources and incorporating proprietary information.
Identify the Right Genetic Background
Mice with varying genetic backgrounds exhibit very different phenotypes and applications. A variety of NKG mouse models is widely used in biomedical research and is highly immunodeficient. Among their most striking features is their NOD genetic background, which contributes to innate immunity defects, including less natural killer cell (NK) activity, decreased complement activity, diminished macrophages, and diminished antigen-presenting cells.
Research Your Model
You should carefully consider the immune components of a model before choosing it. Using each immunodeficient model, determine the mechanism of endogenous immune response by examining the remaining function of B cells, T cells, dendritic cells, macrophages, NK cells, and complement.
The presence of NK cells in nude mice with intact innate immunity can limit primary human cell engraftment by preventing human cells from homing and sustaining. Human tissues, hematopoietic stem cells, and peripheral blood mononuclear cells (PBMCs) seem to engraft more readily in models lacking multiple cytokine signaling, such as NKG variants.
To determine whether an immunodeficient model has healthy immune components, determine where they stand before choosing it. To get an overview of how various strains of immunodeficient mice and xenografts differ, review more on immune system components.
Understand the Characteristics of the Model
Leakiness. Certain strains are leaky because of a mutation called Scid. The number of functional T and B cells can be limited in some SCID mice, as in the B6-scid, between three and nine months of age. A NOD-Scid mouse has a low leakiness rate to none, and an NKG mouse has no leakiness at all.
Lifespan: When studying long-term engraftment with primary tumors that can take months to grow, it would be prudent to avoid models with relatively short lifespans. They would only live for around 5 to 9 months if they did not develop IL-2-dependent lymphomas in their thymus.
Husbandry: A special housing and care are required for immunodeficient strains, as opposed to immune-competent strains.
Radiosensitivity. Radioresistant mice may prove to be the best host for experiments evaluating radiation therapy. The mutation in this protein is involved in DNA repair in Scid mice. They are, however, excessively radiosensitive. Some radiation therapy studies might be more conducive to Rag1 or Rag2 KO mice due to their tolerance to higher radiation levels. Moreover, Rag KO mice have a reduced sensitivity to chemotherapeutics that damage DNA.
The background features: Consider important background strain characteristics, such as the haplotype and behavior of the genotype and disease susceptibility. In the NOD strain, for example, diabetes is a problem, and the natural killer (NK), macrophage, antigen-presenting cell (APC), and complement systems are inadequate.
Breeding performance. Despite their poor breeding abilities, female nude mice begin ovulating when they are 2.5 months old; they stop ovulating by four months.
We hope that this review has presented a thorough examination of the various immunodeficient mouse models and helpful, practical advice on selecting and using them. To make the selection process easier, please call Cyagen at 800-921-8930 /+1 408-969-0306 or email us at animal-service@cyagen.com.
