Animal Models of Asthma

Asthma has reached epidemic proportions worldwide and typically starts early in life. It is a complex disease that can be both chronic and heterogeneous, its causes are multifactorial, including genetic, viral, obesity, and a wide arrange of environmental factors (i.e. cigarette smoke, pollution, or inhaled airborne antigen exposure).

Given its complex etiology, there are many ways to approach the development of a clinically relevant animal models. This article provides a high-level summary of the current literature to assist pulmonary researchers with the development and assessment of pre-clinical models of asthma.

Developing an Ideal Asthma Model

Pre-clinical models are designed to replicate the clinical presentation of the disease under study. For asthma, this is a difficult task as multiple phenotypes have been identified. There are many factors to replicate, in terms of clinical symptoms, physiological responses to stimuli, environmental triggers, and genetic biomarkers¹.

The principal characteristics of asthma include reversible airflow obstruction, airway hyperresponsiveness, airway inflammation, mucus hypersecretion and airway remodeling. These phenotypes can vary depending on the inflammatory profile, and if the disease presents in an acute form, as an airway hyperresponsiveness (AHR) reaction, or in a chronic form, leading to permanent changes to the structure and function of the lungs¹.

Mullane and Williams (2003) describe the ideal animal model for asthma as follows:

Genetics: Similar genetic basis to human disease, either naturally or engineered
Anatomy: Similar lung structure or a clear understanding of the anatomical differences
Pathology: Similar pathological response and underlining mechanisms involved with acute and chronic cases of disease
Validation: Similar endpoints are investigated pre-clinically that would be investigated clinically such as AHR, Spirometry (FEV₁), immunological profile (BAL), etc.
Response to intervention: Similar response to drugs or challenges of external stimuli (such as Methacholine) to what is observed clinically.

Current Pre-Clinical Asthma Models

Most animals do not naturally develop asthmatic disease (with the rare exception of cats and horses) and require human intervention to become susceptible to developing the disease⁵. Historically, guinea pigs were the most popular model, given they are easily sensitized and have natural AHR responses⁹. In the early 1990s murine models of asthma were developed and they rapidly became the most widely used pre-clinical model.⁸BALB/c , C57BL/6 and A/J mice are the current preferred strains, with BALB/c mice being the most popular, as they are considered to be immunologically Th-2 biased¹. Mice are ideal given they are convenient, have a lower cost, and a wide variety of transgenic models are available for study. Mice are largely responsible for our current understanding of the pathogenic mechanisms of asthma⁵.

Larger animals have also been used to study asthma such as cats, dogs, non-human primates, and even horses. Non-human primates in particular, perhaps unsurprisingly, demonstrate similar responses to allergen challenges observed in humans, with an early and late phase bronchoconstriction response and a similar increase in airway eosinophilia. However, as commonly seen with larger subjects, using primates is expensive, labour intensive, and it can take over 18 months to develop a sensitized model⁵.

Although no singular pre-clinical model will replicate clinical asthma perfectly, a variety of animal models can be used to assess specific aspects of the disease (Table 1).

Table 1: Comparison of small vs large animal subjects for pre-clinical assessment of asthma

OVA and HDM Models

There have been an array of allergens used to develop asthmatic models, such as ovalbumin (OVA), house dust mite (HDM Dermatophagoides pteronyssinus(Der p) or D. farinae (Der f)), fungi (Aspergillus fumigatus,Alternaria alternata), cockroach extracts, Ascaris antigens, cotton dust, ragweed and latex (Hevea brasiliensis). The allergen of choice depends on the condition to be replicated and can be used separately or in combination⁷. The most popular allergens used experimentally today are OVA and HDM, summarized in this document, based on their ability to produce a Th-2 mediated inflammatory response.

Validation of Pre-Clinical Asthma Models

Clinical asthma has varied etiology, environmental triggers, and endotypes, making it challenging for animal models to mimic human disease. However, animal models play a critical role in our ability to understand the pathogenesis of disease and test therapeutic interventions prior to clinical testing.

Pre-clinical asthma models require validation and understanding of the physiological differences between the animal model and humans. A few notable considerations when developing and validating asthmatic murine models include:

Model development protocol– Clearly identifying the sensitization allergen, challenge antigen and specific murine strain, as each factor can have significant influences on the endpoints of the resulting model⁹.
Acute vs. Chronic models- Acute and chronic models of asthma will have significant differences in their AHR response, inflammatory profile, and lung remodelling responses. It is important to evaluate both models when evaluating mechanisms of disease, pathogenesis, AHR and drug development⁴.
Structural and physiological- Mice lungs differ from humans in their anatomy and physiology. Structurally, the lungs differ in their branching and bronchiole to alveolar ratio, the type and location of cells within the lungs (i.e. basal cells), less smooth muscle, and a lack of bronchial circulation⁸. It is important to keep in mind the differences and limitations when evaluating pre-clinical models.
Structure to function validation- AHR is one of the main functional endpoints used to assess asthma and can be influenced by a variety of factors such as the antigen used, airway remodeling, smooth muscle cell hyperplasia, mucous section and blood flow distribution⁵. In order to validate and assess the structural and functional changes in pre-clinical models a combination of techniques is required to develop a unified hypothesis assessment including FOT, histology, BALF and computational modelling (if possible).

Refrences

Vivolo Aun, M. et al. (2017). Animal models of asthma: utility and limitations. Journal of Asthma and Allergy.
Daubeuf, F. and Frossard, N. (2013). Acute asthma models to ovalbumin in the mouse. Current protocols in mouse biology. 3:31-37
Woo et al. (2018). A 4 week model of house dust mite (HDM) induced allergic airway inflammation with airway remodeling. Scientific Reports.
Piyadasa, H. et al. (2015). Biosignature for airway inflammation in a house dust mite challenged murine model of allergic asthma. Company of Biologists.
Holmes et al. (2011). Animal models of asthma: Value, limitations, and opportunities for alternative approaches. Drug Discovery Today. 16: (15).
Bates et al. (2009). Animal Models of Asthma. Am J Physiol Lung Cell Mol Physiol. 297.
Doras et al. (2017). Lung responses in murine models of experimental asthma house dust mite over ovalbumin sensitization. Respiratory Physiology and Neurobiology. 43-51.
Hapeslagh et al. (2017). Murine models of allergic asthma. Inflammation: Methods and Protocols, Methods in Molecular biology. Chapter 10. Vol.1559
Mullane and Williams (2013). Animal models of asthma: Reprise or reboot?. Biochemical Pharmacology.