Introduction
Asthma is a chronic inflammatory disease affecting approximately 300 million people worldwide [1]. Asthma symptoms can range from mild and manageable on low dose inhaled corticosteroids (CS) to severe and uncontrolled despite intense treatment [2]. Attempts to understand this heterogeneity have led to phenotyping asthma according to clinical, physiologic and cellular phenomena [3] and ultimately to efforts to determine the molecular mechanisms, or endotypes, driving disease [4]. Currently, at least two endotypes of severe asthma have been identified; patients exhibiting persistent type 2 inflammation with eosinophilia (type 2-high) and those with type 1/type 17 inflammation with neutrophilia (type 2-low) [3].
Type 2-high asthma is characterized by increased numbers of blood and airway Th2 cells, eosinophils, group 2 innate lymphoid cells (ILC2) and type 2 cytokines IL-4, IL-5 and IL-13 [3, 4]. Woodruff et al. examined bronchial biopsies from mild-moderate asthmatics and healthy controls and showed that patients with high expression of type 2 cytokines improved following inhaled CS (ICS) treatment [5]. In type 2-high severe asthma, however, symptoms and inflammation persist despite high dose inhaled and/or oral CS [6, 7]. Though anti-type 2 therapies are effective in these patients, indicating the pathway mediates their symptoms [2], the mechanism(s) driving persistence of type 2 inflammation remain elusive.
Expression of CRTh2 (chemoattractant-homologous receptor expressed on Th2 cells) by CD4+ T cells is considered a marker of Th2 cells [8-10], but CRTh2 is also expressed by eosinophils, basophils, ILC2 and some CD8+ T cells [10-12]. CRTh2 is a receptor for prostaglandin D2 (PGD2), a lipid mediator released by mast cells within the airways following allergen-induced activation [13-15]. PGD2 activation of CRTh2 regulates Th2 cell function mediating chemotaxis, type 2 cytokine expression and inhibition of apoptosis [10, 13, 16, 17]. In murine models of asthma, CRTh2 deficiency reduced eosinophil infiltration and IL-5 production within the lung [18, 19]. Administration of CRTh2 antagonists or depleting antibodies also showed a reduction in airway hyper-reactivity, serum IgE levels, mucus secretion and leukocyte infiltration into the airways [18, 20, 21]. CD4+CRTh2+ T cells (i.e. Th2 cells) circulating within the blood have a memory phenotype and their frequency is considered to influence one’s susceptibility to respond to subsequent allergen exposures [9, 22-24]. Although severe asthmatics have been shown to have higher levels of Th2 cells, CRTh2 mRNA and PGD2 in the blood and airways [25, 26], whether this pathway influences Th2 cell response to CS and/or plays a role in persistence of type 2 inflammation has not been examined.
Women are more likely to be diagnosed with severe asthma [27], to have severe exacerbations requiring hospitalization [28] and to relapse following treatment for exacerbation [29]. Serum estrogen levels have been associated with amount of IL-5 in sputum of women reporting peri-menstrual asthma symptoms [30]. CD4+ T cells express estrogen receptors (ER) [31] and in a mouse model of asthma estrogen administration induced expression of type 2 cytokines [32] through ER alpha (ERα) activation [33]. Here we examined the relationship between asthma severity and type 2 inflammation in a sex-stratified analysis and assessed the ability of estrogen receptor signaling to influence Th2 cell response to in vitro exposure to CS.