Asthma, which characterized by chronic airway inflammation and bronchial hyperresponsiveness, is identified as a major globally common chronic respiratory disease affecting 1%–18% of child and adult in different countries [1]. Recently, a large comprehensive asthma survey in China indicated that the overall prevalence of asthma is 4.2% in adult, which represents 45.7 million individuals aged 20 years or older were affected [2]. Moreover, the incidence of asthma is still increased rapidly and its control is suboptimal in many countries.

Persistent allergic inflammation in asthma is primarily based on an adaptive immune response. However, innate immunity is reported to crosstalk with adaptive immunity, and they work together to drive the pathogenesis of asthma [3]. Toll-like receptors (TLRs), a receptor family of innate immunity containing 11 members, serve as the first line of defense to recognize and process invading pathogens and aeroallergens. Among these members, we and others have evidenced the significant promoting role of TLR2 in allergic airway inflammation induced by ovalbumin (OVA) [4,5], house dust mite (HDM) [6] or Aspergillus fumigatus [7]. Such promoting role of TLR2 in asthma has been suggested to be relied on its activation of NLRP3 inflammasome [8] and oxidative stress [9]. Recently, metabolic reprogramming is reportedly significantly associated with inflammation [10], and enhanced glycolysis has been evidenced to accompany with airway inflammation [11,12]. TLR2-mediated glycolysis has been detected in rheumatoid arthritis [13], however, no studies have shown a direct link between TLR2 and glycolysis in allergic airway inflammation as well as glycolysis of which cell is directly affected by TLR2.

Macrophages contribute significantly to the development of asthma. Besides an increase in the number of F4/80+ macrophages [14], macrophage polarization changes also clearly contribute to asthma pathology, asthmatic patients have higher numbers of IRF5+ M1 and CD206+ M2 macrophages in bronchial biopsies than do healthy controls [15], and lung sections of murine allergic asthma contain both IRF5+ M1 and YM1+ M2 alveolar macrophages [16]. One important type of macrophages in the lung is tissue-resident alveolar macrophages (r-AMs), they account for 95% of the leukocytes in the lower respiratory tract under homeostatic conditions [17] and reside in the alveolar lumen, where they are at the first line to response to inhaled particulates, including pathogens and allergens, making them being critical in allergen-induced airway inflammation. Although several reports have been documented the regulatory effect of TLR2 on immune responses of bone marrow-derived macrophages (BMDMs) and mouse alveolar macrophage cell line MH-S in response to rhinovirus [18] and HDM [6], respectively, the regulatory roles of TLR2 in resident AMs immune response during allergic inflammation is still limited.

Resident AMs is considered to rely predominantly on oxidative phosphorylation (OXPHOS) under homeostatic state, however, during inflammatory and hypoxic environment, their rapid activation induces profound metabolic shift from OXPHOS to glycolysis [19,20]. However, previous reports have attributed the enhanced glycolysis in asthma to glycolytic reprogramming in airway epithelial cells [21,22], leaving AMs rarely studied. During the development of asthma, non-resident leukocytes flooding, airway remodeling and angiogenesis result in an hypoxic and inflamed lung microenvironment, which is associated with an elevation of hypoxia inducible factor 1α (hif1α) [23], which has been reported to induce the transcriptional activation of many glycolytic and inflammatory genes in macrophages [24]. In a study focusing on r-AMs glycolytic reprogramming in acute lung injury revealed that hypoxia dose-dependently increases hif1α, which is responsible for glycolysis augmentation in r-AMs [19]. Besides, the activation of TLR2 can increase glycolysis in RA-synovial fibroblast cell [13] and CD8+ T cells [25], supporting that both hif1α and TLR2 activation are involved in glycolysis augmentation. Despite these findings, it is unknown whether TLR2 may regulate resident AMs metabolic reprogramming via Hif1α and contributes to allergic airway inflammation.

In the present study, we first examined the necessity of TLR2 for OVA-induced allergic airway inflammation. Then, RNA-sequencing analysis revealed that the OVA-induced hif1 signaling pathway and glycolysis were markedly inhibited in TLR2−/−OVA group. Glycolysis inhibitor 2-DG or hif1α stabilizer EDHB was used to further reveal the involvement of TLR2-hif1α-mediated glycolysis in pyroptosis, oxidative stress and lung macrophage activation in allergic airway inflammation. Next, using in vivo AMs from OVA-challenged mice or ex vivo cultured resident AMs, we investigated whether OVA allergen impacts AMs immune responses and glycolysis reprogramming in a TLR2 dependent manner. Finally, we depleted resident AMs in TLR2−/− mice, or adoptively transferred TLR2−/− resident AMs to OVA-sensitized WT mice before OVA challenge to confirm that loss of TLR2-hif1α-mediated glycolysis in resident AMs protects against allergic airway inflammation.