Allergic asthma is a condition that causes coughing, wheezing, shortness of breath, rapid breathing and tightness in the chest. These symptoms are the result of the immune system overreacting to harmless antigens (allergens), such as tree pollen. Allergic asthma is characterized by early and late asthmatic responses (EARs and LARs) following the introduction of an allergen. An EAR is an asthma response that occurs immediately and usually resolves after a couple hours; a LAR is a delayed asthma response, which can occur hours after exposure to an allergen. LARs are followed by airway hyperresponsiveness (AHR)—increased airway sensitivity to a bronchoconstrictor stimulus. Among humans, the time point at which the response occurs is highly variable.
Researchers are learning more about asthmatic responses to allergens by using guinea pigs as a model for functional parameters of the condition. Guinea pigs are a valuable animal model because compared to humans they have a similar distribution of mast cells, which play a key role in inflammation. Also, compared to mice models, guinea pig EAR bronchoconstriction is pronounced and responsive to more inflammatory mediators. However, studies in recent years have shown that guinea pigs have become less sensitive to a standard protocol used for eliciting EAR, LAR, AHR and airway inflammation. Thus, to re-establish the conditions necessary to use guinea pigs in asthma research, this study1 assessed lung function and inflammation in relation to several different protocols.
The study used the antigen ovalbumin (Ova)—the main protein found in egg white—to elicit asthmatic responses. (Ova is commonly used in allergen challenges, and an Ova protocol had been previously developed and successfully used in guinea pig studies.) Six male guinea pigs were sensitized with Ova injections. Aluminum hydroxide (Al(OH)3) was administered with the Ova to enhance immune response. After allowing time for antibodies and immune responses to develop, the guinea pigs underwent an allergen challenge: exposure to inhaled Ova for one hour. A control group was sensitized in the same way, but exposed to aerosolized saline. Six different sensitization and challenge conditions were performed, with variable Ova challenge doses, numbers of sensitizations, Ova sensitization doses, Al(OH)3 doses, and challenge days. Following the allergen challenges, specific airway conductance (sGaw) measurements were used to assess EAR and LAR. Airway response to histamine was measured to assess AHR. After lung function tests, the guinea pigs were euthanized. Lung tissue was sampled and eosinophils, macrophages, lymphocytes, and neutrophils were counted to determine the effect of the sensitization and challenges on pulmonary inflammation.
For this study, functional asthmatic responses were assessed using DSI’s Buxco® respiratory solutions. Using the patented Allay™ restraint in a double-chamber plethysmograph, Buxco Non-Invasive Airway Mechanics (NAM) technology measured sGaw in conscious, spontaneously breathing guinea pigs. Airway responses to aerosolized histamine before and after the Ova challenge were also measured using the same technology.
Results confirmed the protocol that had been used in previous studies no longer achieved the full range of desired effects. Ultimately, the authors found that increasing the Ova sensitization and challenge concentrations restored AHR, increased the peak of the EAR, and increased eosinophils. From that point, either increasing the Al(OH)3 concentration during sensitization or extending the duration between Ova sensitization and challenge induced EAR, LAR, AHR and pulmonary inflammation. In addition, allowing more time for the immune response to develop before the challenge prolonged the EAR and LAR. Interestingly, there was also a dissociation between AHR and the influx of inflammatory cells, highlighting the importance of assessing asthmatic responses directly, rather than relying on cell counts alone.
1Lowe, A. P. P., Broadley, K. J., Nials, A. T., Ford, W. R. & Kidd, E. J. (2015). Adjustment of sensitisation and inflammatory responses to ovalbumin in guinea-pigs. Journal of Pharmacological and Toxicological Methods, 72: 85-93. doi: 10.1016/j.vascn.2014.10.007
To read the complete article, visit: http://www.ncbi.nlm.nih.gov/pubmed/25450500