In addition to their involvement in innate immune responses, the airway epithelium is also capable of driving the exacerbation of established allergic airway diseases by the production of pro-Th2 cytokines and chemokines such as IL-4, IL-13, TSLP, and TARC/CCL17 [49
]. DCs, which densely line the airways, are critically involved in the pathogenesis of allergic diseases and are known to be potent inducers of CD4 T cell differentiation, expansion, and polarization [51
]. However, the mechanism by which immature pulmonary DCs undergo maturation and become effector T cell-inducing antigen presenting cells (APCs) is unclear.
Using bone marrow chimeric mice to restrict TLR4 signalling to either the SC compartment (SC+
) or the hematopoietic cell (HPC) compartment (SC-
), we showed that TLR4 expression on lung radioresistant SCs, but not on DCs, is necessary and sufficient for DC activation in the lung and for priming of Th2 responses to HDM [43
]. TLR4 triggering on SCs induced the activation of airway WT DCs as read out by CD86 and CD40 expression [53
]. Moreover, in a WT animal exposed to LPS, DCs that had migrated to the draining lymph nodes were able to induce effector T cell responses characterized by the production of IL-17A and IFN-g. It was however intriguing to see that in chimeric mice lacking TLR4 expression on stromal cells, WT DCs in the airways were no longer able to induce affector T cell differentiation. The same held true when HDM was used instead of LPS. It is therefore very likely that TLR4-expressing stromal cells release factors that instruct airway DCs to induce a particular type of immune response. Such factors might include cytokines such as GM-CSF, known to induce DC activation [54
], or other cytokines such as TSLP or IL-33 which might contribute to set the stage for Th2 response development [55
]. In agreement with this, the absence of TLR4 on structural cells, but not on hematopoietic cells, prevented the development of HDM-driven allergic airway inflammation and the production of Th2 cytokines by mediastinal lymph node T cells. Interestingly, in the same mice, the levels of instructing cytokines were severely impaired. Interestingly, inhalation of a TLR4 antagonist to target ECs suppressed the salient features of asthma, including bronchial hyperreactivity. In a similar way, Th2 sensitization to inhaled ovalbumin (OVA), an antigen often used to induce asthma features in mice but often criticized for its content in LPS, seems to depend on recognition by stromal TLR4. When it comes to LPS, it is generally approved that the concentration of LPS determined the type of immune response induced, with high concentrations (LPShigh
) inducing Th1 responses and low concentrations (LPSlow
) inducing Th2 responses [57
]. A recent study reported that using contaminated OVA contaminated with high levels of LPS, the stromal recognition of LPS by TLR4 led to a robust Th2 response, indicating that in the presence of higher concentrations of LPS, stromal cell expression of TLR4 is sufficient for Th2 sensitization [47
]. In view of these results, one can wonder about the level of contamination of allergen preparation such as HDM extracts. When addressing this issue in our experiments showing a crucial role for stromal TLR4 expression in Th2 responses to HDM [43
], we found that the degree of endotoxin contamination of HDM extract was in the subnanogram range, far below the dose previously reported to promote TH2 responses to OVA [57
]. If HDM extracts contain such a low level of LPS contamination, why are they triggering TLR4? A very elegant study by Trompette et al. showed that Der p 2, one major allergen of the house dust mite Dermatophagoides pteronyssinus, was found to enhance the response of mouse bronchial ECs to endotoxin by acting as an MD2-like chaperone that promotes TLR4 signalling [59
], providing an explanation to the profound proallergic innate response to HDM.
Altogether, these studies demonstrate that stromal cell TLR4 signalling is critically involved in Th2 but not Th1 sensitization to inhaled allergen [47
]. Stromal TLR4 signalling leads to the maturation of Th2-inducing DCs that fail to produce proinflammatory cytokines or to upregulate the Th1-inducing Notch ligand Delta-4. Following intranasal administration of LPS or HDM into the airways, stromal cells upregulate mRNA expression or synthesis of TSLP, suggesting a stromal cell-dependent instruction of DCs in the priming of allergic Th2 responses (Figure ).