Role of the effector and regulatory arms of the adaptative immune response in the pathophysiology of experimental asthma

Zusammenfassung

Classic murine models of experimental asthma based on intraperitoneal sensitization followed by airway challenge do not reflect the way in which humans acquire allergic disease to airborne allergens. The interaction of the airway mucosa with the allergens may be essential for the triggering of the subsequent immune response. In the present work, we developed a murine model of allergic disease based on primary airway exposure to antigen followed by continuous airway challenge. Following sensitization and after only three intranasal challenges, a classical intraperitoneal model developed airway hyperresponsiveness to MCh, increased airway contractile tissue mass, eosinophilic inflammation and mucous hypersecretion in the upper and lower airways. We detected in this model a high number of Foxp3 mRNA+ cells in the lung inflammatory infiltrates. Conversely, murine models based on primary airway allergen exposure and continuing challenge showed attenuated asthma features, including borderline airway hyperresponsiveness, a modest increase in airway contractile tissue mass and mucous production, and an attenuation of the eosinophilic inflammatory infiltrates with a drift towards a lymphocytic profile. In such models based on primary airway exposure, mucous overproduction was found as an early response prior to the development of any adaptive immune response, which suggests the participation of and innate immune response prior to the onset of chronic airway inflammation. Subepithelial collagen deposition behaved differently from the airway contractile tissue mass, and increased progressively over time. Contrary to the intraperitoneal model, moderate numbers of Foxp3 mRNA-expressing cells were found in the pulmonary inflammatory infiltrates upon primary airway antigen exposure. However, such Foxp3 mRNA+ cells increased over time proportionally to the duration of the allergen instillations, and their numbers were proportionally balanced with Il10 and Tgfb mRNA expression. These findings suggest a role of Tregs in the attenuation of the asthma features in the primary airway exposure models, whereas most Foxp3+ cells in the intraperitoneal sensitization model may have been non-functional. In all, these results suggest that classic animal models of experimental asthma based on intraperitoneal sensitization represent the effector arm of the adaptive immune response, but fail to reflect the regulatory arm, which is likely operated by Treg subpopulations. The data are also consistent with the hypothesis that primary contact of the antigen with the respiratory mucosa is crucial to determine the profile of the subsequent immune response in terms of attack versus tolerance, and that the innate immune system plays a central role in such decisions. To demonstrate that FOXP3-expressing T cells may drive immune regulation in an experimental asthma setting, versus the induction of asthma features by effector CD4+ T cells, we performed two series of adoptive transfer experiments employing genetically modified T cells, by means of retroviral gene transduction. Adoptive transfer of 100,000 antigen-specific, effector, GFP-expressing, CD4+ T cells was sufficient, upon antigen airway exposure, to elicit migration of GFP+ cells to the airways and induce airway hyperresponsiveness and inflammation. On the contrary, antigen-specific Treg cells generated by forced FOXP3 expression, of which 75,000 were adoptively transferred to recipients with established experimental asthma, led to an attenuation of asthma features. Finally, to find translational evidence on a role of Treg cells in human asthma, we studied the presence of FOXP3+ cells in bronchial biopsies from severe asthmatics versus control subjects. FOXP3+ cells were virtually absent in the control subjects, whereas the specimens from asthmatics revealed a significant influx of FOXP3+ lymphocytic cells in subepithelial location. Further work is warranted to better define the phenotype and Treg function of such FOXP3+ cells.