Abstract
Asthma is an inflammatory airway disease characterized by airway eosinophilia, airway obstruction due to an increased mucus production by goblet cells, airway remodeling, and airway hyperresponsiveness (AHR) to a variety of stimuli. In allergic airway inflammation, dendritic cells (DC) deliver antigen to T helper cells in the draining lymph nodes and induce a Th2 immune responses. An emerging concept is that DC subsets differentially skew T cell responses towards Th1 or Th2 immunity, or tolerance. Flt3-ligand (Flt3-L) is an indispensible synergistic hematopoietic growth factor for DC development and differentiation in hematopoietic and peripheral lymphoid organs. Administration of Flt3-L specifically affects the generation, phenotype and function of DCs in several organs. It has been observed that Flt3-L treatment not only prevents the development of allergic airway inflammation and airway hyperresponsiveness, but also reverses the established allergic asthma. In this study the lung DC subsets were defined and their functional responses in OVA-sensitizationinduced allergic airway inflammation and Flt3-L-induced immunomodulation were examined in a mouse model of allergic asthma.|Two dendritic cell subsets, CD11chighCD11blow and CD11clowCD11bhigh, with distinct expression of CD8α, B220, CD19, F4/80, MHC II, CCR7, CD40, PDL1, PDL2, CD80, and CD86, were identified in the lungs of PBS-treated and OVA-sensitized and challenged mice with and without treatment with Flt3-L. Functional tests revealed that CD11chighCD11blow and CD11clowCD11bhigh dendritic cells were more prone to induce Th1 and Th2 responses, respectively, thus playing a suppressive and immunogenic role in allergic asthma, respectively. The two lung DC subsets demonstrated dynamic changes in numbers following OVA-sensitization and Flt3-L treatment. Most importantly, Flt3-L administration increased the number of suppressive CD11chighCD11blow DCs in the lungs of antigen-sensitized mice. This DC subset acquired a heightened regulatory capacity after Flt3-L treatment. Thus, Flt3-L exerts its therapeutic effect in allergic asthma by increasing the Th1 regulatory lung DC subset.|The migratory pattern and antigen uptake ability were examined to gain a more in-depth understanding with regard to the role of Flt3-L on DC recruitment and migration to lymph node. Flt3-L did not alter the expression of CCR2, CCR5, and CCR6 in CD11chighCD11blow regulatory DCs. In addition, immunogenic CD11clowCD11bhigh DCs in Flt3-L-treated-OVA-sensitized mice demonstrated a less mature phenotype characterized by a lower expression of CCR7 and the costimulatory molecules CD86 and CD40, inefficient antigen uptake, and impaired migration in vitro to various lymphatic chemokines than those in OVA-sensitized mice. In vivo studies showed that fewer antigen-carrying cells were detected in the lungs and lymph nodes of Flt3-L-treated- OVA-sensitized mice than OVA-sensitized mice with a significant decrease in numbers of CD11clowCD11bhigh DCs. Consistent with previous findings, mediastinal lymph node cells from Flt3-L-treated mice secreted higher levels of Th1 cytokines and IL-10 than OVA-sensitized mice in vitro. These data suggest that Flt3-L increases numbers of CD11chighCD11blow DC merely through enhancing hematopoiesis, and the Flt3-L generated lung immunogenic CD11clowCD11bhigh DCs have a less mature phenotype than those in OVA sensitized mice, impaired antigen uptake and impaired migration to draining lymph nodes.|Finally, the expression and involvement of the calcium-activated potassium channel KCa3.1 in lung DC migration was examined. Both CD11chighCD11blow and CD11clowCD11bhigh dendritic cells expressed KCa3.1 mRNA and cell surface protein. OVA sensitization and challenge upregulated the expression of KCa3.1 in both lung DC subsets with a greater change observed in the immunogenic CD11clowCD11bhigh subset. Lung DCs that expressed highest levels of KCa3.1 also had highest levels of CCR7 expression and more loaded OVA antigen. The Blockade of KCa3.1 channels in vitro by TRAM-34 impaired the chemotactic migration of both lung DC subsets to lymphatic chemokines CCL19 and CCL21. Thus, an involvement of KCa3.1 in lung DC migration is strongly implicated.|In conclusion, the administration of Flt3-L in the mice with allergic asthma causes several changes on lung DC properties leading to suppression of Th2 responses. First, Flt3-L specifically increases the hematopoiesis of a Th1-prone regulatory lung DC subset. Second, Flt3-L contributes to a less mature phenotype of an immunogenic lung DC subset in asthmatic mice, functionally demonstrated by impaired antigen uptake and migration to draining lymph nodes. Third, the calcium-activated potassium channel KCa3.1 participates in lung DC migration, which opens a new direction for lung DC biology research. These findings suggest that Flt3-L could be used in the therapy of allergic asthma.