Circadian and Sex-Specific Responses to Alternaria alternata in Allergic Airway Inflammation

Technical Summary: Circadian and Sex-Specific Responses to Alternaria alternata in Allergic Airway Inflammation

Problem Statement
Asthma severity and immune responses are known to be regulated by circadian rhythms, with symptoms and airway obstruction typically peaking at night or in the early morning. However, the specific role of circadian timing in fungal allergen-induced airway inflammation remains poorly defined. Alternaria alternata, a clinically relevant fungal allergen strongly associated with severe asthma exacerbations and persistent airway dysfunction, exhibits diurnal oscillation in environmental spore counts. Despite this, it is unclear whether pulmonary responses to Alternaria exposure are influenced by the time of day or biological sex, and how these factors interact with allergen-driven inflammatory pathways, particularly STAT signaling and epithelial barrier integrity.

Methodology
The study employed a preclinical in vivo mouse model using adult C57BL/6 wild-type mice (both females and males, 2–3 months old). The experimental design involved an acute Alternaria alternata (ALT) asthma model where mice received intranasal doses of ALT extract (10 µg) or PBS control once daily for four consecutive days. Exposure was timed at two distinct Zeitgeber times (ZT): ZT0 (06:00, beginning of the rest phase) and ZT12 (18:00, beginning of the active phase).

Key assessment methods included:

• Airway Hyperresponsiveness (AHR): Measured 24 hours post-final exposure using the flexiVent system with methacholine challenges to assess respiratory system resistance, elastance, compliance, and tissue damping.
• Cellular Analysis: Bronchoalveolar lavage (BAL) fluid was collected to quantify total cellularity and specific immune populations (eosinophils, neutrophils, macrophages, dendritic cells, ILCs, and T cells) via flow cytometry.
• Histopathology: Lung sections were stained with Hematoxylin and Eosin (H&E) for semi-quantitative scoring of peribronchial, perivascular, and alveolar inflammation.
• Molecular Profiling: Cytokine and chemokine levels in BAL fluid were quantified using cytometric bead arrays. Lung tissue was analyzed for gene expression (qRT-PCR) and protein levels (Western blot) of circadian clock components (CLOCK, BMAL1, REV-ERBα, PER2), epithelial junctional proteins (E-cadherin, ZO-1, occludin, claudin-4), and signaling molecules (STAT1, STAT3, STAT6, ERK).
• Statistical Analysis: Data were analyzed using Two-way ANOVA and generalized linear modeling to assess interactions between Treatment, Sex, and Time.

Key Contributions and Results
The study demonstrates that Alternaria-induced allergic airway inflammation is regulated by both circadian timing and sex, revealing distinct temporal and sexual dimorphisms:

1. Sex-Specific Temporal Immune Infiltration:

   • Females: Exhibited enhanced infiltration of eosinophils, neutrophils, interstitial macrophages, and dendritic cells predominantly at ZT0 (rest phase). Lymphoid populations (ILC2, Th2, Th17) also peaked at ZT0.
   • Males: Showed stronger inflammatory responses, particularly in eosinophils and dendritic cells, at ZT12 (active phase).
   • Total Cellularity: While ALT increased total BAL cellularity in both sexes, males showed significantly greater infiltration during the active phase (ZT12) compared to the resting phase (ZT0), whereas females did not show this specific total cellularity trend.

2. Cytokine and Chemokine Profiles:

   • Females: Showed higher levels of Th2 cytokines (IL-4, IL-5, IL-13) and proinflammatory/Th17 mediators (IL-6, TNFα, IL-17A, IL-22) at ZT12.
   • Males: Displayed maximal induction of these same mediators at ZT0.
   • IL-33: The epithelial alarmin IL-33 was significantly elevated at ZT0 in both sexes.
   • Tissue Injury Markers: Lactate dehydrogenase (LDH) and uric acid levels followed the same circadian-dependent trends as the inflammatory mediators (higher at ZT12 in females, ZT0 in males).

3. Airway Hyperresponsiveness (AHR):

   • Functionally, ALT-exposed mice displayed greater AHR (increased resistance, elastance, and tissue damping; decreased compliance) during the resting phase (ZT0).
   • This effect was driven largely by males, who showed significant time-of-day variation in AHR. Females exhibited minimal diurnal variation in airway mechanics despite pronounced inflammatory responses, suggesting a sex-specific uncoupling between inflammation and functional obstruction.

4. Epithelial Barrier and Circadian Clock Disruption:

   • Barrier Integrity: ALT exposure disrupted epithelial junctional proteins (E-cadherin, ZO-1). In females, ZO-1 and E-cadherin were reduced, particularly at ZT0. In males, ZO-1 levels were higher at ZT0 in controls but increased further with ALT exposure at ZT0.
   • Clock Proteins: ALT exposure altered the expression of core circadian proteins. In females, CLOCK and BMAL1 were reduced at both time points, while REV-ERBα was lower at ZT12. In males, CLOCK and BMAL1 were reduced at ZT12, and REV-ERBα showed a marked reduction at ZT12 compared to ZT0.
   • Signaling Pathways: Peak inflammation coincided with enhanced activation (phosphorylation) of STAT1, STAT3, and STAT6 at ZT0 in both sexes. ERK1/2 phosphorylation showed sex-specific patterns, increasing at ZT0 in males but decreasing in females.

Significance and Claims
The authors claim that this study identifies circadian timing and sex as key determinants of fungal allergen-induced airway inflammation. By defining the "circadian gating" of inflammatory and STAT-dependent pathways specifically for Alternaria, the study provides new insight into the temporal regulation of allergen-driven airway responses.

The paper posits that the interaction between circadian clock components and allergen-driven pathways is bidirectional: fungal allergens perturb lung circadian rhythms (altering CLOCK/BMAL1/REV-ERBα), which in turn influences immune and epithelial responses. The findings suggest that chronotherapeutic approaches—timing interventions based on the time of day and biological sex—may improve asthma outcomes, particularly for severe fungal-allergen-driven asthma. The study highlights that responses to fungal allergens differ significantly from those to house dust mite (HDM) allergens, underscoring the need for allergen-specific chronobiological considerations in asthma management.