Placenta growth factor augments airway hyperresponsiveness via leukotrienes and IL-13

摘要

Airway hyperresponsiveness (AHR) affects 55%–77% of children with sickle cell disease (SCD) and occurs even in the absence of asthma. While asthma increases SCD morbidity and mortality, the mechanisms underlying the high AHR prevalence in a hemoglobinopathy remain unknown. We hypothesized that placenta growth factor (PlGF), an erythroblast-secreted factor that is elevated in SCD, mediates AHR. In allergen-exposed mice, loss of Plgf dampened AHR, reduced inflammation and eosinophilia, and decreased expression of the Th2 cytokine IL-13 and the leukotriene-synthesizing enzymes 5-lipoxygenase and leukotriene-C4-synthase. Plgf~(–/–) mice treated with leukotrienes phenocopied the WT response to allergen exposure; conversely, anti-PlGF Ab administration in WT animals blunted the AHR. Notably, Th2-mediated STAT6 activation further increased PlGF expression from lung epithelium, eosinophils, and macrophages, creating a PlGF/leukotriene/Th2-response positive feedback loop. Similarly, we found that the Th2 response in asthma patients is associated with increased expression of PlGF and its downstream genes in respiratory epithelial cells. In an SCD mouse model, we observed increased AHR and higher leukotriene levels that were abrogated by anti-PlGF Ab or the 5-lipoxygenase inhibitor zileuton. Overall, our findings indicate that PlGF exacerbates AHR and uniquely links the leukotriene and Th2 pathways in asthma. These data also suggest that zileuton and anti-PlGF Ab could be promising therapies to reduce pulmonary morbidity in SCD. Keywords: Genetics, Hematology, Immunology, Inflammation, PulmonologyIntroductionSickle cell disease (SCD) is one of the most prevalent genetic disorders worldwide. It is caused by a mutation in the β-globin gene that results in sickle-shaped rbc during deoxygenation (1, 2). Adhesion of rbc and leukocytes to the endothelium, rbc sickling, and adhesion of sickle rbc to wbc result in vascular occlusions and multiple end-organ complications. However, the oxygen tension in lungs is too high for sickle hemoglobin polymerization. Indeed, the lung is the only organ where sickled rbc returning from the venous vasculature can unsickle. Despite this, acute and chronic lung diseases are major complications of SCD, and independent risk factors for morbidity and mortality in these patients (3, 4) and mechanisms behind them are largely unknown. Airway hyperresponsiveness (AHR), a dynamic narrowing of the lower airways, is highly prevalent in patients with SCD, affecting 55%–77% of children (5–7) and 33% of adults with SCD (8, 9). In contrast, the reported prevalence of AHR is 6%–20% in the African-American and black Caribbean pediatric population (6, 10).Clinically, AHR is a key feature of asthma and is assessed using a provocative challenge (e.g., methacholine challenge) to induce a measurable bronchoconstriction. In SCD, the incidence of AHR is much higher than that of clinically symptomatic asthma (5, 6, 10–12) and is present without clinical features of asthma (wheezing, exhaled NO, and positive skin test to aeroallergens). But unlike asthma, which is a known risk factor for morbidity in SCD (13–16), the mechanism behind high AHR and its clinical significance is unknown. In SCD, biomarkers of hemolysis rather than those of asthma show a strong correlation with AHR (7). This led us to hypothesize that the etiology of the high AHR prevalence in SCD may be related to erythroid cells.Following allergen stimulation, AHR is driven by a potent Th2 immune response that primes B cells to release allergen-specific IgE and drives lung inflammation. Th2 cytokines, such as IL-13 and IL-4 (17), stimulate airway smooth muscle contraction, recruit inflammatory cells, and increase mucus production; congestion and constriction of the airway lumen contributes to its remodeling (18, 19). In SCD, AHR has been presumed to occur secondarily to the leukocytosis and inflammation present at steady state. Notably, SCD patients also have elevated levels of LTE_(4) and LTB_(4) at steady state (20, 21). LTB_(4), a potent neutrophil inflammatory molecule (22), and cysteinyl LTs (CysLT) have been implicated in different aspects of allergen-induced asthma (18, 23). CysLT receptor inhibitors of 5-lipoxygenase (5-LO, encoded by Alox5 ) inhibitors are currently licensed for use for the treatment of asthma in humans (24, 25).Our group previously reported that placenta growth factor (PlGF), an erythroid cell–secreted growth factor that belongs to the vascular endothelial growth factor family, is increased in plasma of patients with SCD and contributes to the baseline inflammation and features of pulmonary hypertension (26, 27). Herein, we hypothesized that PLGF mediates the high prevalence of AHR in SCD. We utilized genetic and pharmacological ablation of Plgf in WT mice and Berkeley sickle mice (hereafter termed HbS mice) (28). We have previously reported that crosses between Plgf KO and HbS mice were born extremely rarely, and the few bo