CX3CL1/CX3CR1 Signaling Pathway in Fungal-Associated Allergic Asthma: Good or Bad? The Devil is in the Method

Asthma is a chronic condition characterized by narrowing and hyperresponsiveness of airways upon exposure to triggers such as allergens, including fungal agents. Clinically, this inflammatory disorder presents with symptoms of shortness of breath, chest tightness, coughing, and wheezing which all lead to airflow limitation¹. Asthma affects more than 300 million individuals worldwide² but most cases can be well-managed by administrating low doses of anti-inflammatory agents¹. Nonetheless, up to 10% of patients present with the severe form of asthma which considerably impacts their quality life³.

A subset of human asthmatics who are sensitized to fungal species such as Alternia, Aspergillus, Cladosporium, and/or Penicillium are described as having the asthma phenotype termed “severe asthma with fungal sensitization” (SAFS)⁴. Fungal sensitization is associated with high rates of hospitalizations and increased risk of life-threatening exacerbations ^{5, 6}. Moreover, the inflammatory response of SAFS is neutrophils-based; its treatment is challenging because neutrophils do not respond to treatment with corticosteroids⁷.

In earlier publications, Dr. Steele and colleagues sought to find potential therapeutic targets by identifying factors that play a part in the immunopathogenesis of SAFS. Previously, they established that 1) cytokine IL-7 is a potential promoter of immunopathogenesis in both human and experimental fungal-associated allergic airway inflammatory response⁸, 2) IL-1 contributes to fungal asthma severity in human and treatment with IL-1 receptor antagonist improves asthmatic disease in an experimental animal model ⁹. The unique chemokine C-X3-C motif ligand 1 CX3CL1/Fractalkine is synthesized as a transmembrane molecule consisting of an extracellular N-terminal domain, a mucin-like stalk, a transmembrane α helix, and a short cytoplasmic tail^10-12.  Previous studies reported that CX3CL1 appears to worsen the severity of allergic asthma^13-15.

Assessment of airway hyperresponsiveness (AHR) is considered the gold standard in experimental asthma models. Though studies investigating CX3CL1 within in-vivo models of allergic asthma are scarce, a previous study has indirectly assessed AHR in-vivo by measuring enhanced pause via whole-body plethysmography; correlative method for AHR assessment^{2, 16, 17}. The report concluded that the loss of CX3CR1 improves disease pathology and suggested that the blockage of CX3CR1/CX3CL1 signalling pathway represents a promising therapeutic alternative in allergic asthma¹³.

 In their recent publication, Dr. Steele and colleagues’ objective was to shed light on the immunopathogenic role of CX3CL1/Fractalkine because of its elevated levels in fungal-sensitized asthmatics. Their methodology comprised the measurement of changes in pulmonary function via a computer-controlled volume ventilator (flexiVent system; SCIREQ) followed by histological analysis to further support their results. Total lung resistance (Rrs) and Newtonian resistance (Rn) were measured via direct cannulation of the trachea in mice following the model of Aspergillus fumigatus-induced allergic airway inflammation.

Surprisingly, their findings are in direct contradiction to what was reported in the literature. Compared to asthmatic wild type, asthmatic mice deficient in CX3CR1 (Cx3cr1-/-) showed a significant increase in central airway resistance (AHR; Rn) as well as total lung resistance twenty-four hours post the last organism challenge. Histological analysis of Cx3cr1-/- asthmatic mice lung sections show a significant increase in inflammatory cells surrounding blood vessels and airways, in addition to higher infiltration of inflammatory cells around the airways and in the alveolar spaces. The pathological alterations revealed by the lung sections histological analysis of Cx3cr1-/- asthmatic mice corroborate the changes in pulmonary function, specifically the enhancement of AHR.

In conclusion, the absence of CX3CR1/CX3CL1 signalling pathway resulted in a dramatic impairment in lung functions. The contradictory findings of Dr Steele and colleagues’ might be attributed to the authors using the gold standard instrument for in-vivo lung function measurement well known for high accuracy, sensitivity, and reproducibility due to precise control of experimental conditions. Further studies investigating the potential CX3CR1/CX3CL1 signalling as a novel therapeutic target to limit disease severity in fungus sensitized asthmatics should be conducted.

References

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