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Background: Despite constant exposure to micro-organisms and other immunogenic environmental factors, relatively very few immunological responses are initiated in the nasal mucosa. Although several mechanisms could play a role in maintaining this immune suppressive milieu, none of them have been validated. Previous data from our group suggested that locally produced interleukin (IL)-10 could be involved in maintaining local homeostasis.
Methods: To investigate the role of epithelial IL-10 expression in the manifestation of allergic symptoms, we used immunohistochemistry to study the expression of IL-10 in the nasal epithelium of healthy individuals and house dust mite allergic patients. In the allergic patients, we determined potential correlations of epithelial expression with allergic symptoms, both at baseline and after allergen provocation.
Results: IL-10 is expressed in the basal and differentiated epithelial cells of both healthy individuals and allergic rhinitis patients. In the allergic individuals, there is a strong negative correlation at baseline between the epithelial expression level of IL-10 and rhinorrhoea and sneezing, but not between that expression level and nasal blockage or peak nasal inspiratory flow (PNIF). This correlation disappears with steroid treatment or after allergen provocation, although the expression at baseline seems to predict PNIF scores after provocation.
Conclusions: Our data not only reveals IL-10 expression by human nasal epithelial cells, but also suggests that nasal epithelial IL-10 regulates allergic symptoms. Targeting the regulation mechanisms affecting IL-10 or targeting the regulation mechanism affected by IL-10 could constitute new options for the treatment of allergic disease.
The incidence of allergic diseases in the western world is continuously increasing (1–5). Although this has led to extensive research, many aspects of the pathogenesis of allergic diseases are still unclear. In particular, there is no clear understanding of the factors that predict or are linked to the severity of symptoms in patients.
The local mucosal immune system, as present in the airways or the gastro-intestinal tract, guards major entry sites of the body against intrusion by foreign antigens. It, therefore, must be well-equipped to regulate the outcome of the immune response according to the type of antigen encountered. The concept of the inappropriate control of a local response to a harmless antigen as an important contributor to the pathogenesis of allergy is intriguing. As the nasal epithelium is an integral structural part of the local mucosal immune system it should also be seen as a functional part of a local regulation mechanism. The major function of the respiratory epithelium was thought to be primarily that of a physical barrier. However, the importance of the airway epithelial cells in regulating many of the inflammatory responses seen in respiratory diseases is increasingly being recognized (6–8).
Previous investigations by our group suggested that nasal epithelial cells expressed the immunomodulatory cytokine interleukin-10 (IL-10) (9). Given the immune regulatory roles described for IL-10 it is tempting to hypotheses that locally produced IL-10 may contribute to the regulation of the immune response and affect the manifestation of allergic symptoms. IL-10 production by various cells of the immune system has been found earlier to mediate immune suppression in dendritic cells (10, 11) and T lymphocytes (12–15) and to control IgE-isotype switching in B-cells (16, 17). Furthermore, IL-10 promotes IgA production (18, 19), cytolysis, neutrophil activation, and inhibits other aspects of effector responses, like chemokines, cytokines, and PGE2 production (20). The evidence that IL-10 contributes to the tolerogenic phenotype generally observed in the gut is undisputed (21, 22).
This study used the immunohistochemical staining of nasal biopsies from healthy and allergic individuals. The location of epithelial IL-10 expression at the border of the external and internal milieus suggests a role in the maintenance of mucosal homeostasis. To investigate this possibility, we studied the expression of IL-10 in the nasal epithelium of allergic patients at baseline and after allergen provocation, and correlated IL-10 expression levels to the severity of allergic symptoms in the allergic individuals. We also investigated the effect of local steroid treatment on these potential correlations.
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The study shows for the first time that the potent immunosuppressive cytokine IL-10 is expressed by human nasal epithelium and that its expression level is negatively correlated with allergic symptoms.
The link between the signs and symptoms of disease and a cellular or inflammatory marker is a research topic of great interest. The identification of such a marker is highly relevant as such a marker would be an important target for treatment. Although allergen-specific IgE is required for symptoms to develop, the level of IgE has no predictive value for the level of symptoms (27). Indeed, a substantial part of the population has IgE for a given allergen, but shows no symptoms at all (28–30). In these individuals (and also in allergic individuals) there is no clear relationship between the amount of mediators released from mast cells and the level of symptoms (27, 31). Our data show that in a group of patients with persistent rhinitis and proven HDM allergy, individuals with relatively high levels of IL-10 expression in basal epithelium suffer less from allergy, conversely, those with relatively low IL-10 expression levels suffer more from their allergy. Given the statistical correlation at baseline between low symptom scores for rhinorrhoea and sneezing with high IL-10 levels it is tempting to speculate that there is a causal relationship between the two.
Because of the continuous all-year-round exposure to HDM it is impossible to establish a clear, symptom-free baseline in perennial rhinitis patients, as is evidenced by the varying level of symptoms in our patient group. Consequently, the observed pattern of variable IL-10 expression and disease symptoms might be the result of low-grade and variable exposure to allergen. Allergen provocation in our model (and also in real life) leads to an increase in signs and symptoms. In our model, this is accompanied by an increase in the expression of IL-10 in the epithelium. Furthermore, we found nasal epithelial IL-10 expression and symptom VAS scores at baseline to be inversely related. Combining these two observations, we can begin to conclude that allergic symptoms and IL-10 levels are not regulated independently of each other. Moreover, since high allergen exposure evidently does not result in a fall in IL-10 levels, high symptom scores can not be held responsible for low IL-10 levels at baseline. Conversely, low symptom scores cannot be responsible for high IL-10 expression. At baseline, then, high IL-10 expression must be responsible for low allergic symptoms. Our data, therefore, provides a strong argument for the idea that epithelial IL-10 expression protects people from allergic symptoms.
This hypothesis raises questions about a possible mechanism. A hint comes from the observation that not all symptom scores were correlated with epithelial IL-10 expression. Whereas rhinorrhoea and sneezing do correlate with epithelial IL-10 expression, nasal blockage does not. Interestingly, as medication for allergic rhinitis patients, antihistamines can suppress symptoms of rhinorrhoea and sneezing, but are usually less efficient in treating nasal blockage (32, 33). This indicates that there is a mechanism in which IL-10 influences allergic symptoms via histamine. In vitro data supports the concept that IL-10 might reduce symptoms by inhibiting histamine. Royer et al. studied the effects of IL-10 on the release of pro-inflammatory mediators by activated mast cells and found that histamine release was inhibited in the presence of recombinant IL-10 (32, 34, 35).
The relationship between IL-10 and histamine is complex, as histamine has been shown to increase the synthesis and the release of IL-10 from inflammatory cells (36). Not only in macrophages, but also in T cells, IL-10 production was up-regulated after histamine stimulation (37, 38). As nasal epithelial cells have histamine receptors (39–41), it is conceivable that expression of epithelial IL-10 is also up-regulated in response to histamine. In this way, histamine, induced by repeated allergen challenge, appears to induce a complex negative feedback loop by up-regulating IL-10. However, the expression of IL-10 could also be a direct effect of epithelial cells being exposed to the HDM antigen. Epithelial cells have been shown to recognize allergenic mixtures through protease activated receptors (PARs) (42). This could be particularly relevant to HDM where a number of the dominant allergens (DerP1 and DerP2) exhibit protease activity (43, 44). To distinguish between a direct and an indirect effect of allergen exposure upon IL-10 expression it would be relevant to study the IL-10 response to HDM in nonallergic individuals.
After allergen provocation, the inverse linear relationship between symptoms and IL-10 levels is completely lost in the placebo group. Currently we can only speculate that allergen loads in our provocation model are sufficiently high to induce maximum symptom levels and, directly or indirectly, induce similarly maximum IL-10 levels. Furthermore, in the FPANS group, there is no correlation between IL-10 levels and symptoms. In this group, the levels of symptoms are lower than in the placebo group, which renders an argument inplausible for the placebo group after provocation. Our current model is not suitable for addressing the issue of corticosteroid influence on IL-10 expression, histamine release, and allergic symptoms. Steroids may directly affect IL-10 or histamine levels or the levels of their respective receptors. In vitro experiments using primary epithelial cells exposed to allergen, histamine, or steroids are suggested to resolve this rather complex interplay.
Our data not only reveal IL-10 expression by human nasal epithelial cells, but also show that nasal epithelial IL-10 both regulates and responds to allergic symptoms. This effect may well be mediated through histamine. These observations suggest that IL-10 may not only regulate the onset of the immune response, but also influence the effector phase of the allergic response. In this concept, targeting the regulation mechanisms affecting IL-10 or targeting the regulation mechanism affected by IL-10 might constitute new options for the treatment of allergic disease.