This study shows that TLR4 and TLR9 are differently expressed in the nasal mucosa of healthy subjects and of the major rhinitis phenotypes. Our findings allow some speculations about the role of TLRs in physiological conditions and in the different inflammatory mechanisms responsible for the different forms of rhinitis.
Both TLR4 and TLR9 are expressed in the healthy nasal mucosa. TLR4 is mainly present in the epithelial layer and in the glandular cells, while TLR9 is mainly detectable at the stromal level. In the submucosal layer, TLR4 is present in scattered cells while the expression of TLR9 is widely diffuse. In the nasal glands, TLR4 was highly expressed while a lower expression was shown for TLR9. The observation of a high TLR4 and TLR9 in the healthy nasal mucosa is consistent with TLR function as the first line of defence against pathogens [7
Confocal analysis of the inflammatory nasal mucosa in allergic rhinitis revealed a marked downregulation of both TLR4 and TLR9.
TLR4 is mainly expressed by dendritic cells, macrophages and monocytes, as well as by CD3-positive T lymphocytes in the nasal mucosa of young children [9
]. It has been suggested that exposure to microbial products in the early life induces a shift of the immune response toward a Th1-type cytokine profile, which protects against the development of allergies and asthma [11
]. In fact, exposure to bacterial endotoxin—a ligand for TLR4—was shown to be inversely related to the incidence of atopic asthma, hay fever, and sensitization against aeroallergens in school-aged children [12
]. A farming environment is also associated with a lower prevalence of both atopic and nonatopic asthma and an increased exposure to domestic animals implies a decreased atopic risk [13
]. LPS/allergen coexposure resulted in the inhibition of allergic inflammation and bronchial hyper-responsiveness, although exposure to LPS after allergen challenge exacerbated the allergic response [15
Our finding of a lower TLR4 expression in the nasal mucosa allergic rhinitis is in line with the reported low expression of TLR4 in T cells and the lack of response to LPS in atopic adults [10
Endotoxin-rich environments also contain other immunostimulatory microbial products that act as ligands for TLR2 and TLR9 [17
]. While TLR4 is specialized in the recognition of (Gram negative) bacterial products, TLR9 recognizes unmethylated CpG DNA of bacteria and viruses, as well as nucleic acids, which are not unique to the microbial world [2
]. Synthetic immunostimulatory sequence oligodeoxynucleotides (ISS-ODNs) activate TLR9 and are more effective than steroids in attenuating the hypersensitivity response of asthma, allergic conjunctivitis, and allergic rhinitis [19
]. ISS-ODNs rapidly inhibit the activities of effector Th2 cells and other cells that contribute to allergic response [16
]. Interestingly, vaccination with allergen and ISS-ODN and TLR9 ligands has been proposed as a promising approach in immunotherapy of allergic diseases [16
Therefore, our finding of a lower expression of TLR4 and TLR9 in the nasal mucosal of allergic rhinitis patients may be related to the Th2-type allergic inflammation and to the increased susceptibility to upper respiratory infections usually observed in rhinitis patients.
In chronic rhinosinusitis we found that TLR4 is significantly overexpressed. Our data appear in line with previous findings of Dong and coworkers who observed an increased TLR4 expression in chronic rhinosinusitis epithelial cells of inferior turbinate nasal mucosa, as compared to healthy adult volunteers [23
]. Accordingly, You and coworkers also reported a significant TLR4 increase in epithelial/glandular cells of chronic rhinosinusitis, compared to nasal polyps and control tissues [24
]. On the other hand, a low TLR9 expression has been reported by Ramanatan and coworkers in chronic rhinosinusitis with nasal polyps patients, in comparison to healthy specimens [25
In rhinosinusitis specimens, confocal analysis showed a clear TLR4/TLR9 co-localization in the cellular infiltrate of the sub-mucosa associated with marked damage of the epithelium. These findings are consistent with the chronic inflammatory state typical of rhinosinusitis.
Although in vasomotor rhinitis both TLR4 and TLR9 appeared to be significantly down-regulated, their co-localization in the cell infiltrate similar to that observed—but in a definitely higher degree—in rhinosinusitis may indicate that a neurogenic minimal inflammation does also occur in this “noninflammatory” rhinitis phenotype [27
In conclusion, our data indicate that the pattern of expression of TLR4 and TLR is different in healthy subjects and in different forms of rhinitis, possibly in relation to the different pathophysiological mechanisms involved. These findings may contribute to a more accurate phenotyping of rhinitis and may suggest new therapeutic approaches based on the interplay between innate immunity and the adaptive effector response to allergens and microbial agents.