Regulation of immune responses at mucosal surfaces: allergic respiratory disease as a paradigm

Mucosal immunotherapy with CpG oligodeoxynucleotides reverses a murine model of chronic asthma induced by repeated antigen exposure

Murine models of acute atopic asthma may be inadequate to study the effects of recurrent exposure to inhaled allergens, such as the epithelial changes seen in asthmatic patients. We developed a murine model in which chronic airway inflammation is maintained by repeated allergen [ovalbumin (OVA)] inhalation; using this model, we examined the response to mucosal administration of CpG DNA (oligonucleotides) and specific antigen immunotherapy. Mice repeatedly exposed to OVA developed significantly greater airway hyperresponsiveness and goblet cell hyperplasia, but not airway eosinophilia, compared with those exposed only twice. CpG-based immunotherapy significantly reversed both acute and chronic markers of inflammation as well as airway hyperresponsiveness. We further examined the effect of mucosal immunotherapy on the response to a second, unrelated antigen. Mice sensitized to both OVA and schistosome eggs, challenged with inhaled OVA, and then treated with OVA-directed immunotherapy demonstrated significant reduction of airway hyperresponsiveness and a moderate reduction in eosinophilia, after inhalation challenge with schistosome egg antigens. In this model, immunotherapy treatment reduced bronchoalveolar lavage (BAL) levels of Th2 cytokines (IL-4, IL-5, IL-13, and IL-10) without changing BAL IFN-gamma. Antigen recall responses of splenocytes from these mice demonstrated an antigen-specific (OVA) enhanced release of IL-10 from splenocytes of treated mice. These results suggest that CpG DNA may provide the basis for a novel form of immunotherapy of allergic asthma. Both antigen-specific and, to a lesser extent, antigen-nonspecific responses to mucosal administration of CpG DNA are seen.

Understanding airway wall remodeling in asthma: a basis for improvements in therapy?

Current strategies for the management of asthma focus on suppressing airway inflammation. Other characteristic features of human asthma, such as airway hyperreactivity and the structural changes collectively referred to as airway remodeling, are largely ignored in existing guidelines for monitoring the effectiveness of treatment. Evidence is accumulating that pharmacologic therapy targeting airway wall remodeling may be valuable in treating asthma. However, development of appropriate therapeutic agents will require a better understanding of the pathogenesis of remodeling, which appears to be regulated by a variety of cytokines and growth factors produced by inflammatory, epithelial, and stromal cells. Furthermore, testing the effectiveness of novel agents that specifically target the process of remodeling will require appropriate experimental models, but most currently available animal models of asthma have major limitations. A recently described murine model of chronic human asthma offers considerable potential for dissection of the mechanisms of airway wall remodeling, as well as for investigation of the therapeutic potential of drugs that can modulate chronic inflammation and remodeling.

Allergen immunotherapy: current and new therapeutic strategies

Allergic individuals respond to an environmental allergen encounter by producing T-cell cytokines, predominantly IL-4 and IL-5, which in turn drive the production of allergen-specific IgE antibodies and recruitment of an eosinophil-rich inflammatory infiltrate. Allergen-specific immunotherapy (SIT) involves the repeated injection of the allergen to specifically downregulate this predominantly Th2-type immune response. SIT is a clinically proven effective treatment for allergic diseases, including rhinoconjunctivitis and asthma. However, despite having been in clinical practice since early this century, its use remains empirical. Best practice protocols are based on clinical experience and include recommendations for selecting patients for treatment, SIT regimes and avoidance of adverse events. More rational and safer SIT regimes will result from new insights into the underlying immune mechanisms for allergic disease, in particular the critical role of helper T-cells in orchestrating this response. The development of recombinant techniques for producing purified allergens and allergen derivatives has led to a dramatic improvement in the ability to standardise allergen preparations and to develop novel vaccines for allergy treatment. Potential vaccines include short peptides based on dominant T-cell epitopes of allergens, allergen fragments and mutant allergens. All of these preparations are designed to target T-cells without binding IgE and inducing local and systemic side effects. Additional strategies under consideration include DNA vaccines and fusion protein constructs incorporating immunomodulatory elements such as bacterial cell proteins, cytokines and immunostimulatory sequences of DNA. Different forms of allergens are being evaluated for the more practical mucosal administration of allergy vaccines. The identification of recombinant allergens suitable for diagnostic use and the development of reliable laboratory assays, based on T-cell function to monitor clinical efficacy of SIT, are important practical outcomes from this research.