Q&A: In-vivo models of infectious disease

We speak to Adriano Flora, associate director of the Scientific Program Management Group at Taconic Biosciences

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With the special report on infectious disease planned for our September issue, Taconic Biosciences reached out to talk to us about disease models. As noted in the email to us, a representative of the company noted:
In-vivo models of infectious diseases can be used to:
  1. Study the basic biology of the pathology (e.g. how a virus spreads during the infection, what is the kinetic of infection, etc.)
  2. Study how an organism is affected by a pathogenic agent (e.g. a virus)
  3. Study how the immune system of the affected host reacts to the infection
  4. Test specific drugs for efficacy
The limitation of mouse models is linked to the fact that pathogenic microorganisms (e.g. viruses) use specific membrane receptors as entry point to infect the host cells and, oftentimes, these receptors are different between mice and humans. In this case, humanization of the receptor can help in generating a model that might be relevant to study the pathology (for example, see recent publication using a Taconic humanized model). Also, conditional knockout or knock-in models can be used to dissect the cellular mechanisms essential for pathogens to thrive and for infections to spread, and to test if cellular proteins can be used as targets to develop new drugs.
We spoke to Adriano Flora, associate director of the Scientific Program Management Group at Taconic Biosciences, to dig a little deeper into the topic.
DDNews: Before we get into the nitty-gritty, why don't you review the main uses for in-vivo animal models of infectious disease? (Some of this is answered in the info you provided initially)
Adriano Flora: Animal models are commonly used in almost all of the different steps of the development of drugs for treating and preventing infectious diseases. If appropriately used, animals models are precious tools to understand how an infection can spread within the organism and what are the cellular mechanisms affected by the infecting agent, allowing the identification and development of novel therapeutic strategies. In addition, animal models can be then used to test the efficacy of a proposed therapeutic approach and even to evaluate potential toxic effects of the new therapy.
DDNews: What are the specific advantages and disadvantages of small animal models (like mice) vs. large animal models (if any) in terms of infectious disease research?
Flora: The use in infectious disease research of small animal models vs. large animal models must be driven by the specific scientific question that one needs to address. Large animal models share many physiological similarities with humans and are therefore much more suitable than mice or rats to mimic many aspects of the human disease, such as how the immune system responds to the infection or the development of the pathology in the respiratory system. On the other hand, the possibility to easily manipulate the rodent genome provides researchers with an incredibly powerful tool to test a plethora of important hypothesis. One interesting example of this powerful technology is the humanization of specific viral receptors by replacing the mouse gene with its human counterpart, creating a small animal model showing a human-like response to a virus. Additional genetic alterations of this humanized model can then be used to identify modifiers of the infection and unravel the mechanisms used by the infectious agent to spread over the organism.
DDNews: Looking at mice, since they are such a commonly used tool, what are their pros and cons in infectious disease research when compared to human models of disease (whether in-vitro, ex-vivo or in-vivo)?
Flora: The main advantage of using mouse models over in-vitro or ex-vivo models is the possibility to model the complex interwoven responses of multiple tissues to infective agents. Infections rarely affect a single tissue or cell type, but cause complex diseases by disrupting the normal physiology of multiple organs and systems. A bacterial infection, for example, can cause injury to the affected tissue both by damaging directly the cells of the infected organism and by activating a strong inflammatory response at the site of the infection. This complex interplay between the pathogen and the host is virtually impossible to model using in-vitro or ex-vivo approaches due to the reductive nature of these experimental systems.
DDNews: I know the distinction between transgenic mice generally and “humanized” mice is subtle, but are there significant differences worth noting between that larger transgenic group and the subset of humanized model in infectious disease research?
Flora: Even if subtle, the distinction between transgenic and genetically humanized mouse models is quite useful from a practical point of view. Genetically humanized mice are mostly used to model very specific features of infectious diseases such as viral kinetics or organ distribution of a pathogen, whereas other types of transgenic models are used to address a larger class of questions spanning from the definition of the role of a specific gene in the progression of the disease to the identification of new potential drug targets.
DDNews: Can you discuss some of the key advantages of humanizing the membrane receptors of mice in infectious disease research and drug development? Also, what are the specific advantages of conditional knockout/knock-in mice vs. more general gene knockout/knock-in?
Flora: Expression of a human receptor on the surface of mouse cells can be achieved by two different approaches: transgenesis or gene replacement. The key advantage of the second approach (also known as knockout/knock-in) lies in the fact that the humanized receptor is expressed at a level and with a pattern similar to the endogenous gene. This is a critical feature, since it allows researchers to  mimic the kinetics of infection and develop new therapeutic approaches based on these observations (e.g. if an infectious agent is able to spread to the brain tissues, it becomes crucial to identify a therapeutic molecule able to cross the blood-brain barrier). Expression of a human receptor by transgenesis, on the other hand, might result in over-expression or even ectopic expression of the human protein, therefore leading to a model that does not mimic the human pathology.
An additional twist to the genomic replacement approach is the addition of recombinase sites to the modified allele in order to make it amenable to deletion when the recombinase is expressed (i.e. loxP sites to be recombined upon expression of the Cre gene). This approach permits the tissue-specific deletion of the humanized allele, allowing the dissection of the role of different cell types in the progression of the disease and identifying which organ should be the focus for the development of new drugs.

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