Role of Toll-like receptors in airway inflammation
Abstract: Toll-like receptors (TLRs) are germline-encoded pathogen-recognition receptors that detect different microbial structures and activate the immune system. The human TLR family presently comprises ten members (TLR1-TLR10), each with distinct properties. Although the TLRs protect the host from infection, they have become increasingly implicated in the pathogenesis of various inflammatory diseases, including asthma and allergy. The aim of this thesis was to characterize the presence and function of TLRs in different cell types involved in upper airway inflammation, with special emphasis on recurrent tonsillar infection and allergic rhinitis. We also sought to map cellular and molecular changes occurring as a result of allergen-specific immunotherapy.
In the first two studies, a distinct expression of TLRs was demonstrated in subsets of B and T lymphocytes isolated from human tonsils. The expression levels seemed to be affected by ongoing tonsillar infection. This was most evident in T cells. By studying the functional activity of the expressed receptors, it was found that the corresponding ligands promoted B cell activation. These data indicate that TLRs have a direct role not only in activation of the innate part of our immune system, but also of the adaptive branch.
In the next two studies, presence of the virus-sensing receptors TLR3, TLR7 and TLR9 was shown in eosinophils. Stimulation with the cognate ligands gave rise to an activation manifested by an increased eosinophil survival, release of damaging mediators and ability to migrate to inflamed areas. It was also found that the response was higher in atopic patients with allergic rhinitis than in healthy subjects, and in the presence of a Th2-like cytokine milieu. Thus, activation of eosinophils via these TLRs might engender a link between viral infection and allergic exacerbations.
In the following work, the effect of nasal administration with the TLR9 agonist CpG was described in healthy subjects and in patients with allergic rhinitis. CpG exposure resulted in an increased nasal resistance, production of nasal nitric oxide, infiltration of inflammatory cells and release of pro-inflammatory Th1-type cytokines. These responses were generally more marked among the healthy subjects, most likely due to the ongoing persistent inflammation seen in the allergic group. Taken together, these results show that CpG induces a local inflammation that skews the immune response towards a Th1-like phenotype.
Lastly, cellular and molecular effects induced by rush immunotherapy were analyzed in patients with birch pollen-induced allergic rhinitis. The clinical improvements seen as a result of three years of treatment were mirrored by several changes in antibody and receptor levels. Of these, a reduction in serum allergen-specific IgE antibodies, correlating with cell-bound IgE and IgG expression, along with a general activation of T cells were most prominent. The novel findings in this report included a shift in monocyte populations and decrease in levels of TLR2 and TLR4.
To summarize, the results presented in this thesis demonstrate a role for TLRs in lymphocyte and eosinophil biology, as well as an involvement in tonsillar and allergic inflammation. Although their exact role in airway inflammation is unclear, the TLR system holds great promise in the development of new therapeutic alternatives for allergic and inflammatory diseases.
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