Allergic asthma is associated with airway inflammation and airway hyperresponsiveness. Macrophage polarization has been shown to have a profound impact on asthma pathogenesis. On exposure to local microenvironments, recruited macrophages can be polarized into either classically activated (or M1) or alternatively activated (or M2) phenotypes. Macrophage polarization has been heavily associated with development of asthma. The process of regulation of macrophage polarization involves an intricate interplay between various cytokines, chemokines, transcriptional factors, and immune-regulatory cells. Different signals from the microenvironment are controlled by different receptors on the macrophages to initiate various macrophage polarization pathways. Most importantly, there is an increased attention on the epigenetic changes (eg, microRNAs, DNA methylation, and histone modification) that impact macrophage functional responses and M1/M2 polarization through modulating cellular signaling and signature gene expression. Thus, modulation of macrophage phenotypes through molecular intervention by targeting some of those potential macrophage regulators may have therapeutic potential in the treatment of allergic asthma and other allergic diseases. In this review, we will discuss the origin of macrophages, characterization of macrophages, macrophage polarization in asthma, and the underlying mechanisms regarding allergen-induced macrophage polarization with emphasis on the regulation of epigenetics, which will provide new insights into the therapeutic strategy for asthma.

Exposure to cockroach allergen leads to allergic sensitization and increased risk of developing asthma. Aryl hydrocarbon receptor (AhR), a receptor for many common environmental contaminants, can sense not only environmental pollutants but also microbial insults. Mesenchymal stem cells (MSCs) are multipotent progenitor cells with the capacity to modulate immune responses. In this study, we investigated whether AhR can sense cockroach allergens and modulate allergen-induced lung inflammation through MSCs. We found that cockroach allergen treated AhR-deficient (AhR−/−) mice showed exacerbation of lung inflammation when compared to wild-type (WT) mice. In contrast, 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an AhR agonist, significantly suppressed allergen-induced mouse lung inflammation. MSCs were significantly reduced in cockroach allergen challenged AhR−/− mice as compared to WT mice, but increased in cockroach allergen-challenged WT mice when treated with TCDD. Moreover, MSCs express AhR and AhR signaling can be activated by cockroach allergen with increased expression of its downstream genes, cyp1a1 and cyp1b1. Furthermore, we tracked the migration of intravenously injected GFP+ MSCs and found that cockroach allergen-challenged AhR−/− mice displayed less migration of MSCs to the lungs compared to WT. The AhR mediated MSC migration was further verified by an in vitro Transwell migration assay. Epithelial conditioned medium (ECM) prepared from CRE-challenged epithelial cells significantly induced MSC migrations, which was further enhanced by TCDD. The administration of MSCs significantly attenuated cockroach allergen-induced inflammation, which was abolished by TGFβ1 neutralizing antibody. These results suggest that AhR plays an important role in protecting lungs from allergen-induced inflammation by modulating MSC recruitment and their immune-suppressive activity.