PU.1 Is Required for Macrophage Polarization and the Development of Allergic Inflammation

Rational: Alternatively activated macrophages (AAM) play a pivotal role in Th2 immune response. Although it was well proven that the transcription factor PU.1 has an important role in regulating differentiation and maturation of macrophages as well as modulating neutrophilic lung inflammation during endotoxemia, the roles of PU.1 in AAM polarization and in the pathogenesis of asthmatic inflammation have not been investigated yet. We hypothesized that PU.1 plays a critical role in polarizing AAM and the development of allergic airway inflammation. Methods and Results: PU/ER(T)+/- mice are heterozygotes of conditional PU.1 knock-out mice carrying a single PU.1 locus fused to the modified estrogen (E) receptor (R) ligand binding domain that is responsive to tamoxifen. IL-4, a cytokine polarizing macrophages towards AAM, significantly increased expression of PU.1 in bone marrow derived macrophages (BMDMs), which was similar with STAT6 phosphorylation, suggesting PU.1 is associated with AAM polarization. Upon treatment of IL-4 for 24 hours, BMDMs from PU/ER(T)+/- mice, compared to that from wide type (WT) mice, displayed attenuated expression of AAM specific markers Ym-1 and Fizz-1 by both western blot and real-time PCR assay. To determine whether PU.1 is involved for asthma development, we established triple allergen (DRA: dust mite, ragweed and aspergillus) combined murine asthma model. After repeated airway challenge with DRA, mice with deficient PU.1 showed attenuated cellular infiltration and goblet cell hyperplasia, which are characteristics of airway inflammation and remodeling, that corresponded to lower expression of arginase-1, Ym-1 and Fizz-1 in lungs than that of WT mice. Conclusion: Our results indicate that functionally intact PU.1 is required for AAM polarization and contributes to the eosinophilic inflammation and airway remodeling, suggesting that PU.1 is important for the allergic Th2 immune response through maintaining an AAM-regulated microenvironment.