Effect of age on allergen-induced structural airway changes in brown Norway rats

Airway wall remodeling in young and adult rats with experimentally provoked bronchial asthma

BACKGROUND

Airway wall remodeling is a typical finding in patients suffering from bronchial asthma. While morphological changes have been thoroughly described in adults, less is known about such changes in children because of the limited accessibility of relevant material. To overcome this constraint, animal asthma models may be used instead of human specimens. This study examined rats with artificially stimulated chronic asthma-like symptoms.

METHODS

Brown Norway rats of two age categories (young and adult) were sensitized by ovalbumin (OA), and their intrapulmonary airways (IA) were studied using morphometric and histochemical methods.

RESULTS

OA administration induced a significant increase in lung resistance in young animals but not in adults. The total IA wall area was significantly increased in both young and adult OA rats. In young animals, thickening of the adventitia played a more crucial role in this increase than it did in adults, in which the mucosa and the submucosa participated to a higher degree. The IA walls of young OA rats had significantly higher levels of infiltrating eosinophils than those of adult OA animals. The multiplication of goblet cells was more pronounced in adult rats, which was associated with a tendency to produce a higher proportion of acidic glycoconjugates.

CONCLUSIONS

OA stimulation affected the IA of young rats differently than those of adult animals. Changes in the outer IA layer of young rats can be triggered by activated eosinophils; however, stimulated airway epithelium can be a source of factors that influence the inner IA layers in adult rats.

Dose-Response Relationships and Threshold Levels in Skin and Respiratory Allergy

A literature study was performed to evaluate dose-response relationships and no-effect levels for sensitization and elicitation in skin- and respiratory allergy. With respect to the skin, dose-response relationships and no-effect levels were found for both intradermal and topical induction, as well as for intradermal and topical elicitation of allergenic responses in epidemiological, clinical, and animal studies. Skin damage or irritation may result in a significant reduction of the no-effect level for a specific compound. With respect to the respiratory tract, dose-response relationships and no-effect levels for induction were found in several human as well as animal studies. Although dose-response relationships for elicitation were found in some epidemiological studies, concentration-response relationships were present only in a limited number of animal studies. Reported results suggest that especially relatively high peak concentrations can induce sensitization, and that prevention of such concentrations will prevent workers from developing respiratory allergy. Moreover, induction of skin sensitization may result in subsequent heightened respiratory responsiveness following inhalation exposure. The threshold concentration for the elicitation of allergic airway reactions in sensitized subjects is generally lower than the threshold to induce sensitization. Therefore, it is important to consider the low threshold levels for elicitation for recommendation of health-based occupational exposure limits, and to avoid high peak concentrations. Notwithstanding the observation of dose-response relationships and no-effect levels, due to a number of uncertainties, no definite conclusions can be drawn about absolute threshold values for allergens with respect to sensitization of and elicitation reactions in the skin and respiratory tract. Most predictive tests are generally meant to detect the potential of a chemical to induce skin and/or respiratory allergy at relatively high doses. Consequently, these tests do not provide information of dose-response relationships at lower doses such as found in, for example, occupational situations. In addition, the observed dose-response relationships and threshold values have been obtained by a wide variety of test methods using different techniques, such as intradermal exposure versus topical or inhalation exposure at the workplace, or using different endpoints, which all appear important for the outcome of the test. Therefore, especially with regard to respiratory allergy, standardized and validated dose-response test methods are urgently required in order to be able to recommend safe exposure levels for allergens at the workplace.

Brown Norway rat asthma model of diphenylmethane-4,4'-diisocyanate (MDI): analysis of the elicitation dose-response relationship

The known human asthmagen polymeric diphenylmethane-diisocyanate (MDI) was investigated in the Brown Norway rat skin asthma model. Two types of dose-response relationships are addressed with the following focus: (1) does sensitization dose and surface area influence the subsequent elicitation response and (2) is the elicitation response more dependent on previous elicitation doses or more on skin sensitizing dose? These two aims are investigated in two elaborated experiments, using inflammatory (bronchoalveolar lavage, BAL) and physiologic (Penh) endpoints to characterize asthma-like responses in rats. Postchallenge measurements of Penh focused on responses delayed in onset. Inflammatory endpoints in BAL were performed one day after the fourth challenge. Both protocols utilized a dermal sensitization phase with two administrations on days 0 and 7 followed by four inhalation challenges with approximately 38 mg MDI/m(3) in intervals of 2 weeks. In the first protocol three groups of rats were topically dosed with 40, 10, and 2.5 mul of MDI per rat. Each dose group consisted of three subgroups with dosed surface areas of 3.1-12.6 cm(2), 0.8-3.1 cm(2), and 0.4-0.8 cm(2), respectively. In the second protocol groups of rats were topically dosed with 40 microl of MDI per rat followed by three challenges with 37 mg MDI/m(3). At the fourth challenge subgroups of rats were either challenged with 8, 18, or 39 mg MDI/m(3). Independent of the protocol used, response was characterized by increased influx of neutrophilic granulocytes in BAL and delayed respiratory response. All groups from the first study sensitized to and challenged with MDI elicited a distinct response relative to similarly challenged naive rats. A sensitization dose dependence of the elicitation response was not found. The second protocol revealed that the elicitation dose correlates with increased neutrophils in BAL and delayed-onset respiratory responses. In summary, these data suggest that the vigor of asthma-like responses appear to be more dependent on the inhalation elicitation dose of previously challenged rats rather than the dermal induction dose.

Local pulmonary immune stimulation by the Toll-like receptor 2 and 6 ligand MALP-2 in rats is age dependent

Recent studies indicate that the pulmonary immune response in humans and experimental animals is different in newborn, adult and elderly age groups. The aim of this study was to investigate the influence of age on the leukocyte composition in different lung compartments and peripheral blood of weaned and adult rats. A mycoplasma-like inflammatory response was mimicked by intratracheal application of the synthetic macrophage-activating lipopeptide-2 (MALP-2) which activates macrophages and other cells via the Toll-like receptor (TLR) 2 and 6. TLR 2 and 6 mRNA expressions were investigated by semiquantitative RT-PCR in cells of the bronchoalveolar lavage (BAL) and lung interstitium. Weaned Lewis rats (3-4 weeks old) and adults (12-14 months old) were treated with vehicle control or MALP-2. Cytokines and cell infiltration were measured in the BAL and lung interstitium. In control rats, no differences in TLR 2 and 6 mRNA expression level were found between the age groups. After MALP-2 treatment, the maximum of MCP-1 concentration in the BAL fluid was reached in weaned rats after 4h and in adults after 2h. The TNF-alpha maximum was measured after 2h in both age groups. Three days after MALP-2 the numbers of different leukocyte subsets were significantly increased in the BAL of both groups. In contrast, in the lung interstitium MALP-2 induced a leukocyte increase in adult rats but not in weaned rats. In conclusion, data on pulmonary immune responses from one age group and one lung compartment should not be generalized or extrapolated to other groups.

Experimental approaches to evaluate respiratory allergy in animal models

Asthma is defined as a chronic disease of the entire lung and asthma attacks may either be immediate, delayed or dual in onset. Allergic asthma is a complex chronic inflammatory disease of the airways and its etiology is multifactorial. It involves the recruitment and activation of many inflammatory and structural cells, all of which release mediators that result in typical pathological changes of asthma. A wealth of clinical and experimental data suggests that allergic asthma is due to an aberrant lung immune response mediated through T-helper type 2 (Th2) cells and associated cytokine-signaling pathways. The pathology of asthma is associated with reversible narrowing of airways, associated with prominent features that involve structural changes in the airway walls and extracellular matrix remodeling including abnormalities of bronchial smooth muscle, eosinophilic inflammation of the bronchial wall, hyperplasia and hypertrophy of mucous glands. The primary objective of respiratory allergy tests is to determine whether a low-molecular-weight chemical (hapten) or high-molecular-weight compound (antigen) exhibits sensitizing properties to the respiratory tract. This may range from reactions occurring in the nose (allergic rhinitis), in the bronchial airways (i.e., allergic bronchitis, asthma) or alveoli (e.g., hypersensitivity pneumonitis). Current assays utilize several phases, viz. an induction phase, which includes multiple exposures to the test compound (sensitization) via the respiratory tract (e.g., by intranasal or intratracheal instillations), by inhalation exposures or by dermal contact, and a single or multiple challenge or elicitation phase. The challenge can either be with the chemical (hapten), the homologous protein conjugate of the hapten or the antigen. The choice depends both on the irritant potency and the physical form (vapor, aerosol) of the hapten. The appropriate selection of concentrations (dosages) both for the induction and elicitation of respiratory allergy appears to be paramount for the outcome of test. Endpoints to characterize positive response range from the induction of immunoglobulins, cytokine or lymphokine patterns in serum (or the lung) to (patho-)physiological reactions typifying asthma. None of the currently applied animal models duplicate all features of human asthma. Accordingly, the specific pros and cons of the selected animal model, including protocol variables, animal species and strain selected, must be interpreted cautiously in order to arrive at a meaningful extrapolation for humans.

Repeated inhalation challenge with diphenylmethane-4,4'-diisocyanate in brown Norway rats leads to a time-related increase of neutrophils in bronchoalveolar lavage after topical induction

Diphenylmethane-4,4'-diisocyanate (MDI) is a low-molecular-weight chemical known to cause occupational asthma. The objective of this study was to evaluate the topical and inhalation routes of sensitization on the elicitation response of MDI in the Brown Norway (BN) rat model following repeated challenge exposures. BN rats were either induced topically (150 microl MDI on the flanks, booster administration to the skin of the dorsum of both ears using 75 microl/dorsum of each ear) or by inhalation (5x3 h/d, 28.3+/-3.0 mg MDI/m3 [+/-SD]). Inhalation challenge exposures with MDI (15.7+/-1.4 mg/m3, duration 30 min) were made on d 21, 35, 50, and 64. One day after each challenge, rats were rechallenged with methacholine (MCh) aerosol. Respiratory changes were monitored during challenges. One day after the MCh challenge, selected endpoints in bronchoalveolar lavage (BAL), the weights of lungs, and auricular and lung-associated lymph nodes were determined. After the first and last challenge, lymph nodes and lungs were examined by histopathology. Repeated challenge with MDI or MCh did not elicit marked changes in respiratory patterns at any time point. Mild but consistent time-related increased BAL neutrophils and slightly increased lung and lymph-node weights occurred in topically sensitized rats as compared to the remaining groups. In topically sensitized rats, in the lung histopathology revealed activated lymphatic tissue and an increased recruitment of airway eosinophils. Immunoglobulin (Ig) E determinations (serum and BAL) did not show any differences amongst the groups. Thus, high-dose topical induction with MDI was associated with a neutrophilic and eosinophilic inflammatory response in the lung after repeated inhalation challenge with MDI, with magnitude of effect dependent on the specific methodology used.

Induction and maintenance of airway responsiveness to allergen challenge are determined at the age of initial sensitization

Age is an important factor in determining the quantity and quality of immune responses when challenged with allergen. In a model of allergen-induced airway hyperresponsiveness and inflammation, where the sensitization phase and challenge phases can be dissociated in time, we examined the impact of age on these two phases. Sensitization of young mice (1-20 wk), but not older animals (30-40 wk), led to the development of airway hyperresponsiveness, airway eosinophilia, Th2 cytokine responses, and allergen-specific IgE, regardless of the age when the challenge phase was conducted. Thus, age at the time of initial sensitization was shown to be the critical factor dictating the nature of the response to later allergen challenge, as older mice remained responsive to allergen challenge if sensitized at a young age. These effects were shown to be mediated by lung T cells from sensitized young mice. Moreover, the failure of old sensitized mice to mediate these effects was shown not to be the result of active suppression of the responses. These data define the importance of age at initial allergen exposure in dictating subsequent responses in the lung when exposed to allergen and may help to define why asthma, even in adults, is most often initiated in early childhood.