**1. Introduction**

Neutrophilic asthma is considered to be a poorly controlled disease, showing severe pathological symptoms [1,2]. While mild and moderate asthma can be treated effectively with glucocorticoids, severe neutrophilic asthma is poorly controlled by steroid treatment [3]. Increased numbers of neutrophils are closely correlated with the severity of the disease, and their presence is suggested to be associated with the exacerbation of asthmatic inflammation [4]. Since there is no effective therapy for neutrophilic asthma, identifying a target for developing an effective therapy for severe asthma is urgently needed.

Colony-stimulating factors (CSFs) were previously defined as regulators of the generation of myeloid populations but were later demonstrated to also regulate the survival, proliferation, and function of myeloid cells, which are functions closely related to inflammation [5]. Among CSFs, granulocyte colony-stimulating factor (G-CSF)/CSF3 is a major regulator of neutrophil differentiation, migration, and recruitment to inflammation sites, as well as survival [6,7]. Various types of cells, including epithelial cells, stromal cells, endothelial cells, and macrophages, produce G-CSF [8]. Recently, G-CSF was reported to be associated with severe neutrophilic asthma development and exacerbation, and increased levels of G-CSF were reported in the sputum of patients with asthma [9,10]. A transcriptomic sputum dataset from patients with severe asthma showed that CSF3 and CSF3R expression levels showed a positive correlation with the severity of the neutrophilic asthma [11]. Also, the neutralization of the G-CSF receptor (G-CSFR) attenuated neutrophilic asthma and mucus secretion in a murine model [11]. Together, these results suggest the role of G-CSF as a critical player in neutrophilic asthma development and

**Citation:** Kwak, D.-W.; Park, D.; Kim, J.-H. Leukotriene B4 Receptor 2 Mediates the Production of G-CSF That Plays a Critical Role in Steroid-Resistant Neutrophilic Airway Inflammation. *Biomedicines* **2022**, *10*, 2979. https://doi.org/ 10.3390/biomedicines10112979

Academic Editor: Stanislawa Bazan-Socha

Received: 31 October 2022 Accepted: 17 November 2022 Published: 19 November 2022

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exacerbation [12]. Despite the suggested critical roles of G-CSF in neutrophilic asthmatic pathogenesis, little is known about the regulators of the production of G-CSF in severe neutrophilic asthma.

Eicosanoids such as leukotrienes (LTs) and prostaglandins (PGs) are well-defined lipid metabolites that were reported to be associated with asthma pathology [13–15]. Among them, leukotriene B4 (LTB4) interacts with two distinct receptors, BLT1 and BLT2 [16]. The majority of studies have focused on BLT1, which is the high-affinity receptor for LTB4 and is expressed mainly on immune cells, such as leukocytes [17–19]. On the other hand, BLT2 is a low-affinity receptor for LTB4 and is expressed ubiquitously, including in airway epithelial and mast cells [20,21]. BLT2 is also known to interact with 12(*S*)-hydroxyeicosatetraenoic acid (12(*S*)-HETE), a lipid mediator derived from arachidonic acid by the enzymatic action of 12-lipoxygenase (12-LO) [22]. Recently, we observed that BLT2 and its ligand 12(*S*)-HETE played critical roles in asthmatic airway inflammation [23–28]. Especially, BLT2 was shown to play mediatory roles in the production of IL-17 and IL-1β, eventually contributing to the neutrophilic asthma development in the murine models [29–31]. However, the mediatory role of BLT2 in G-CSF production in neutrophilic asthma has not been elucidated yet.

In the present study, we examined whether BLT2 was involved in G-CSF production in lipopolysaccharide/ovalbumin (LPS/OVA)-induced steroid-resistant severe neutrophilic airway inflammation. The data showed that BLT2 critically mediated the production of G-CSF, thereby contributing to the progression of neutrophilic airway inflammation. We also observed that 12-LO was located upstream of BLT2, mediating G-CSF production. Collectively, our results suggest that a 12-LO-BLT2-linked network mediates the production of G-CSF, thus contributing to neutrophilic airway inflammation. The present study can provide a potential new target for the therapy of severe neutrophilic asthma.
