Introduction

Asthma is a disease of major importance in human medicine, and the mechanisms underlying the various forms of human asthma are the subject of much research. By contrast, spontaneously arising allergic respiratory disease in the dog and cat is poorly defined and it remains unclear whether these species actually develop an equivalent to human asthma.

Human Asthma

The prototype allergic respiratory disease is human asthma. Asthma may have a range of non­immunological causes (e.g. exercise-, anxiety- or drug-induced) where it is defined as intrinsic or non-atopic asthma. By contrast, many cases do have an allergic pathogenesis and these patients are considered to have atopic asthma. This involves a classical type I hypersensitivity response to inhaled aeroallergens that are generally derived from innocuous environmental agents. As for most immune-mediated diseases, the causation of asthma is multifactorial and there are strong genetic influences. Although human asthma clearly runs within families, strong genetic linkages have proven difficult to identify. Candidate genes that have been examined include those of the major histocompatibility complex (MHC) and the Th2 cytokine gene cluster. Immunologically, allergic asthma involves sensitization of the affected individual by capture of inhaled allergen by mucosal dendritic cells that process the antigen and present it to antigen specific lymphocytes within the draining lymphoid tissue of the lung. A classical type 2 (Th2) immune response is engendered, which results in production of allergen-specific IgE (and probably IgG of a particular subclass) that subsequently coats mast cells in the respiratory mucosa. On subsequent exposure to allergen, there is cross-linkage of IgE with mast cell degranulation leading to immediate (5-60 min) bronchoconstriction, vasodilation and late phase (4-24 h) recruitment of inflammatory cells (e.g. eosinophils, macrophages) to the affected area. It is now understood that such allergic responses progress such that the nature of the immune regulation changes towards a more cell-mediated (type 1 or Th1) immunity in chronic phases. The immunological control of the allergic response is mediated by regulatory T cells producing cytokines such as IL-10 or TGF-β. Such Treg are induced following microbial activation of the innate immune system, particularly the dendritic cells. The increasing prevalence of allergic diseases such as asthma is thought to relate to decreased exposure to infectious agents and an associated decline in Treg populations (the 'hygiene hypothesis').

Experimental Models of Asthma

The immunology of allergic pulmonary disease has largely been studied in rodent models. It is possible to immunologically sensitize mice or guinea-pigs of particular strains to standard antigens such as ovalbumin (OVA; derived from chicken eggs), such that when they are challenged by the same antigen (via the respiratory tract), they develop clinical signs and pathology compatible with asthma. The Der p1 allergen of house dust mite has also been used to induce murine experimental asthma in such studies. Similar experimental studies have been performed in the dog and cat. Beagle dogs sensitized systemically by repeated injection of Ascaris, OVA or ragweed antigen develop clinical signs of respiratory disease when challenged by the same allergen via the airways. Physiological studies in these models have confirmed increased airway resistance in challenged dogs. Bronchoalveolar lavage fluid (BALF) collected from ragweed-sensitized dogs contains a high proportion of eosinophils, and serum allergen-specific IgE and IgG1/IgG4 antibodies are produced. OVA sensitized dogs have been shown to have BALF allergen-specific IgE in the absence of detectable serum levels of the same antibody. The latest of these studies shows that the offspring of ragweed-sensitized dogs show an allergic response to aerosol allergen challenge, whereas offspring of non-sensitized dogs do not. Experimental sensitization and challenge studies have also been performed in the cat. In one model system, cats were sensitized and challenged with Bermuda grass allergen and shown to mount a serum allergen-specific IgE response that was greatest after aerosol challenge. This was associated with evidence of bronchial hyper-reactivity, together with the presence of eosinophils and allergen-specific IgG and IgA in BALF. Analysis of cytokine mRNA production by BALF cells and blood mononuclear cells was determined in this model by quantitative RT-PCR. A Th2 cytokine profile (IL-4, IL-6, IL-10) was shown. Subcutaneous administration of Th1 enhancing bacterial 'CpG oligonucleotide motifs' prior to sensitization led to reduced production of allergen specific IgE in sensitized cats. The same group has used house dust mite antigen to similarly sensitize cats, and OVA has been utilized by other research groups. Some studies of exercise-induced asthma have also been reported in the dog. Sled dogs working in cold environments have mucus accumulation and neutrophilic inflammation of the lower airways. The extent of these changes is significantly greater than control dogs that were not exercised for a two week period before sampling.

Canine Eosinophilic Bronchopneumopathy

For many years a syndrome has been recognized in the dog that was most often described as 'pulmonary infiltration with eosinophils' or PIE. Recently, we have undertaken research into this condition and proposed the more descriptive name of 'eosinophilic bronchopneumopathy' or EBP. EBP is not a common disease, but young dogs of the Husky or Malamute breed appear predisposed. The clinical presentation is of cough, gagging, retching and dyspnoea. The disease can be complicated in some dogs by the presence of concurrent bacterial bronchopneumonia. Up to 50% of affected dogs may also have concurrent eosinophilic rhinitis, and the pathogenesis of this response is presumptively similar to that of the lower respiratory tract. On radiography, the most common features are a moderate to severe bronchointerstitial pattern. Bronchoscopy generally reveals the presence of abundant yellow-green mucoid material with thickening or polypoid change to the mucosa and occasional partial airway closure during expiration. BALF from dogs with EBP is characterised by a striking eosinophilic component, sometimes with mucus and other chronic inflammatory cells. Bronchial mucosal biopsies similarly show a spectrum of change from early eosinophilic infiltration and exocytosis, through to late-stage chronic mixed mononuclear inflammation with eosinophils with tissue destruction and ulceration. Similarly, pulmonary biopsies demonstrate the classical intense eosinophil infiltration of alveoli. Approximately 60% of cases have blood eosinophilia. Immunological studies have shown that there are high levels of immunoglobulins with the BALF of affected dogs - including IgA, although this immunoglobulin may be reduced in concentration in the serum. Current studies have also shown the presence of allergen-specific IgE within the blood and BALF of dogs with EBP. The ratio of CD4 to CD8 lymphocytes in the BALF is also altered compared with normal dogs. Dogs with EBP have a relatively greater proportion of CD4 cells compared to CD8. The ratio of these cells reverts to normal following glucocorticoid therapy. Histochemical staining of biopsy material suggests elevated numbers of mast cells within the respiratory tissue of EBP dogs. Other studies have shown increased expression of the collagenolytic matrix metalloproteinases MMP-8 and MMP-13 in the BALF of dogs with this disease, suggesting a role for these molecules in airway destruction. Immunohistochemical investigations of bronchial mucosal biopsies have shown an infiltration comprising MHC class II⁺ and CD1⁺ APCs, IgG plasma cells (in greater number than IgA or IgM cells) and numerous CD3⁺ T cells that are predominantly CD4⁺ and express the αβTCR. Assessment of the function of these cells by measurement of expression of genes encoding cytokines and chemokines by real-time RT-PCR yielded inconclusive results. There was no apparent elevation in gene expression (above normal bronchial tissue) for the cytokines IL-4, IL-5, IL-6, IL-8, IL-10, IL-12p40, IL-13, TNF-α TGF-β or IFN-γ or for the chemokines MCP-1, -2, and -4 and the chemokine receptor CCR3. By contrast, there was elevation of gene expression for MCP-3 and eotaxin-2 and -3 consistent with the eosinophil recruitment into tissue, and a reduction in expression of the gene encoding RANTES. The inciting causes of EBP are poorly defined. Some studies have suggested an association with heartworm, pulmonary parasites or chemicals/drugs. Evidence that EBP might have an allergic basis comes from limited intradermal testing performed in these patients and our more recent IgE serological data. It is our hypothesis that this is an allergic disease (akin to asthma) driven by aeroallergens such as those derived from house dust mites.

Feline Asthma

For many years, it has been recognized that cats develop a spontaneously arising idiopathic respiratory disease of allergic phenotype which has been termed 'feline asthma'. Reports of immunological mechanisms underlying this disorder are sparse. Feline asthma should be distinguished from respiratory disease related to bacterial/mycoplasmal infection, pulmonary parasitism or heartworm infection. The disease is characterised by recurrent episodes of coughing, wheezing or respiratory distress. Examination of BALF should demonstrate a prominent eosinophilic component, although normal cats may have numerous eosinophils within BALF. Bronchial biopsies may reveal hyperplasia of glandular tissue with excess mucus secretion, thickening of airway smooth muscle and eosinophilic infiltration of the mucosa. Blood eosinophilia is uncommonly present. Radiographically there is evidence of increased bronchial and/or interstitial pattern in the lungs. Affected cats do test positive on intradermal testing and there are reports of clinical improvement following restricted access to allergen or hyposensitization. Human dander has been incriminated as a causative allergen in some individuals.

Acknowledgements

The author's work on allergic respiratory disease involves a long-standing collaboration with Professor Cecile Clercx and Dr Dominique Peeters of the University of Liège in Belgium.

Further Reading

A list of further reading can be supplied on request.