Allergic asthma is a chronic Th2 inflammation in the lungs that constricts the airways and presents as coughing and wheezing. Asthma mostly affects boys in childhood and women in adulthood, suggesting that shifts in sex hormones alter the course of the disease. Alveolar macrophages have emerged as major mediators of allergic lung inflammation in animal models as well as humans. Whether sex differences exist in macrophage polarization and the molecular mechanism(s) which drive differential responses are not well understood. We found that IL-4- stimulated bone marrow-derived and alveolar macrophages from female mice exhibited greater expression of M2 genes in vitro and after allergen challenge in vivo. Alveolar macrophages from female mice exhibited greater expression of the IL-4 receptor (IL-4R)-α and estrogen receptor (ER)-α compared to macrophages from male mice following allergen challenge. An ERα-specific agonist enhanced IL-4-induced M2 gene expression in macrophages from both sexes but more so in macrophages from female mice. Further, IL-4-stimulated macrophages from female mice exhibited more transcriptionally active histone modifications at M2 gene promoters than did macrophages from male mice. We found that supplementation of estrogen into ovariectomized female mice enhanced M2-polarization in vivo upon challenge with allergen and that macrophage-specific deletion of ERα impaired this M2-polarization. The effects of estrogen are long-lasting; bone marrow derived macrophages from ovariectomized mice implanted with estrogen exhibited enhanced IL-4-induced M2-gene expression compared to macrophages from placebo-implanted littermates. Taken together, our findings suggest that estrogen enhances IL-4-induced M2-gene expression and thereby contributes to sex differences observed in asthma.

Asthma is a chronic inflammation of the airways that exhibits sex differences, affecting mostly boys in childhood and women in adulthood. Alveolar macrophages have emerged as major mediators of allergic lung inflammation. We hypothesized that alveolar macrophages respond to estrogen by enhancing M2 responses and thereby contribute to sex differences in asthma. We found that macrophages from female mice exhibited increased expression of canonical M2 markers like Ym1 and Arg1 in response to IL-4. Signaling through ERα further enhanced IL-4-induced M2 gene expression in macrophages from female mice. The promoters of these genes contained increased abundance of transcriptionally active modifications like H3Ac and H3K4Me3 in macrophages from female mice. Using an OVA model of allergic lung inflammation, we found that female mice exhibited enhanced M2-polarization in vivo after allergen challenge. M2-polarization was impaired in ovariectomized (OVx) mice and but was restored with estrogen replacement. Further, we established a mixed bone marrow chimera model whereby irradiated mice were reconstituted with a 1:1 mix of bone marrow from CD45.1+ WT and CD45.2+ LysMCREERαflox/flox mice. In these mice, ERα-sufficient alveolar macrophages exhibited enhanced M2-polarization compared to ERα−/− alveolar macrophages. ERα−/−cells were largely retained in the bone marrow, suggesting that E2 regulates leukocyte trafficking during lung inflammation. Together these data suggest that sex and hormonal factors contribute to sex differences in macrophage responses during asthma. Understanding the role of estrogen signaling in M2-polarization is paramount for identifying novel therapeutic targets to better treat women with asthma.

Asthma is a chronic inflammation of the airways that affects over 300 million people worldwide. Asthma exhibits sex differences, affecting mostly boys in childhood and women in adulthood. Alveolar macrophages have emerged as major mediators of allergic lung inflammation. Macrophages express receptors that recognize and respond to estrogen. We hypothesized that estrogen enhances M2 polarization of alveolar macrophages and thereby contributes to asthma. We found increased expression of the canonical mouse M2 marker YM1 by alveolar macrophages from allergic female mice compared to male mice. Consistently, bone marrow derived macrophages (BMMs) from female mice exhibited higher levels of YM1 and ARG1 than did BMMs from male mice following stimulation with IL-4. This sex difference is reflected on the epigenetic level as M2 promoters in BMMs from female mice exhibited enhanced acetylated H3 compared to BMMs from male mice following stimulation with IL-4. Pretreatment of BMMs from female mice with estrogen receptor ligands enhanced IL-4-induced M2-gene expression. Using ovariectomized (OVx) and LysMCREERaflox/floxmice, we show that an absence of either estrogen or the estrogen receptor-a (ERa) impairs M2 polarization following challenge with OVA. Thus, macrophages exhibit sex differences in their ability to polarize to an M2-phenotype in vitro and in vivo. Estrogen and ERa engagement augment IL-4-induced M2-gene expression. Together these data suggest that sex and hormonal factors contribute to sex differences in macrophage responses during asthma. Understanding the role of estrogen signaling in M2-polarization is paramount for identifying novel therapeutic targets to better treat women with asthma.