Fibrinogen cleavage products, or cryptokines, generated during airway mycosis may contribute to glucocorticoid-resistant airway hyperresponsiveness through Toll-like receptor 4 signaling.
In preclinical models, researchers used 4- to 8-week-old female mice to investigate how fungal exposure, lipopolysaccharide (LPS), and fibrinogen cryptokines influenced glucocorticoid responsiveness. The mice were challenged intranasally with Aspergillus niger, with or without low-dose LPS, and treated with inhaled fluticasone propionate or systemic dexamethasone. Additional experiments compared wild-type with Toll-like receptor (TLR) 4–deficient mice. The researchers also exposed primary human sinonasal epithelial cells to cryptokines to assess coagulation-factor production.
The researchers assessed airway hyperresponsiveness (AHR), measured as respiratory system resistance following acetylcholine challenge, airway inflammatory cells, cytokine-producing T cells, fungal burden, coagulation-related gene expression, and fibrinogen and prothrombin secretion.
In the A niger model, fluticasone failed to suppress fungus-induced AHR at any tested dose, contrasting with the dose-dependent reduction in AHR observed with fluticasone in an ovalbumin model. Higher fluticasone doses reduced airway inflammatory cells, including eosinophils, interleukin (IL)-13– and IL-17–producing T cells, but AHR persisted. At the 0.5-µg/day dose, fluticasone significantly enhanced AHR compared with vehicle and higher doses.
Low-dose LPS further altered the response to fungal exposure. Adding 10 or 30 ng of LPS to A niger challenge increased AHR, whereas doses of 100 ng or higher abrogated AHR. LPS also reduced fungus-associated eosinophilia while inducing neutrophilia, shifting the inflammatory phenotype toward T2-low, neutrophil-predominant disease.
Glucocorticoid treatment did not reverse AHR in mice exposed to both A niger and low-dose LPS. Fluticasone reduced airway neutrophils and eosinophils and decreased IL-5–, IL-13–, and IL-17–producing T cells, but AHR remained elevated. Acute and chronic systemic dexamethasone similarly failed to reduce AHR. Fluticasone also impaired fungal clearance, whereas LPS enhanced clearance.
Low-dose LPS alone induced modest AHR after 3 consecutive days of intranasal exposure. Wild-type mice challenged with A niger developed AHR that was further enhanced by LPS, whereas A niger–challenged TLR4-deficient mice did not develop significant AHR compared with controls. Low-dose LPS enhanced AHR through TLR4.
Primary human sinonasal epithelial cells secreted fibrinogen and prothrombin at baseline, and cryptokine exposure increased secretion of these coagulation factors, with fibrinogen secretion increasing by up to 10,000-fold in vitro. In mice, cryptokines induced greater AHR compared with controls or intact fibrinogen, whereas intact fibrinogen alone did not produce a significant difference. Neither fluticasone nor dexamethasone reduced cryptokine-induced AHR.
The murine models may not reflect the chronic course of asthma in humans. The in vitro models used primary sinonasal epithelial cells rather than bronchial epithelial cells, although the researchers cited evidence that sinonasal cells may serve as surrogates for bronchial epithelium. Longer-term models involving at least 6 weeks of A niger exposure are needed to determine whether the findings extend to chronic disease.
The research suggested TLR4 ligands generated or encountered during fungal airway disease may contribute to persistent AHR despite the anti-inflammatory effects of glucocorticoids. The researchers emphasized that the potential therapeutic implications of targeting these pathways require further investigation.
“[G]lucocorticoid resistance in asthma may in part result from airway-specific production of the TLR4 ligands cryptokines and LPS,” wrote lead study author Y. Zeng, of the Division of Pulmonary and Critical Care Medicine at The First Affiliated Hospital of Sun Yat-sen University in China, and colleagues.
The study authors reported no conflicts of interest.
Source: Mucosal Immunology