Gut permeability and its correlation with patient's traits and blood inflammatory markers in severe asthma: Real‐world assessment from the Korean severe asthma registry‐2 (KoSAR‐2)

Recently, interest has grown in the role of epithelial barriers in chronic inflammatory disease pathogenesis, particularly concerning environmental interactions.1 Notably, the gut is exposed to external antigens from food and microbiota, significantly impacting our immune system.2 Building on this background, our study aimed to answer three questions: Do patients with severe asthma (SA) exhibit increased gut permeability? Which patient traits are associated with gut permeability? What is the relationship between plasma calprotectin, plasma immunoglobulins and gut permeability in SA? We conducted a cross-sectional study using data from the Korean Severe Asthma Registry-2 (KoSAR-2) and two independent control groups: non-asthma (NA) and non-severe asthma (NSA). We compared gut permeability (measured by plasma zonulin, intestinal fatty acid-binding protein [I-FABP], and LPS) and some blood immune-related markers (calprotectin, IgG, IgA, IgM, and IgE). The study included 30 NA, 25 NSA and 99 SA patients. NSA and SA had statistically similar baseline characteristics, except for asthma severity, while significant differences were observed between the NA and SA groups. Notably, the average age and BMI were higher in the SA group compared to the NA group. The SA group had high proportions of T2 inflammation (86.7%) and atopy (71.3%). Plasma calprotectin levels were higher in the SA group (10.69 ± 5.62 ng/mL) compared to the NA group (6.84 ± 1.95 ng/mL, p < .001) and the NSA group (7.29 ± 1.59 ng/mL, p < .001). Plasma IgE and plasma IgA levels were elevated in the SA group compared to the NA group. (Table S1). To determine if gut permeability is increased in SA, we measured gut permeability markers and found that plasma zonulin (NA: 6.16 ± 2.66, NSA: 7.38 ± 2.44, SA: 10.24 ± 6.8 ng/mL) and LPS (NA: 8.28 ± 1.84, NSA: 9.59 ± 3.73, SA: 12.86 ± 5.20 pg/mL) were significantly elevated in SA (Figure 1A–C). These findings remained significant even after multivariate analysis (Figure 1D,E). We used linear regression, adjusting for age and sex, to analyze which patient traits most significantly impacted gut permeability within the SA group. Plasma zonulin was linked to airway indicators like cough severity and chronic sinusitis, while LPS was strongly correlated with T2 high inflammation (β = .288; p = .007) and atopy (β = .530; p < .001) (Table 1). Notably, LPS levels were significantly elevated in the T2 high allergic SA group, while no increase was observed in the T2 high non-allergic SA group (Figure 1E–G, Figure S1). Additionally, although not statistically significant, gut permeability markers tended to increase with the presence of atopy even in the NSA group (Figure S2). This finding highlights that atopy is the most influential patient trait affecting plasma LPS levels. Although an upward trend in plasma LPS levels was observed in BMI over 30 kg/m2, this increase did not reach statistical significance (Figure S3). To address the third question, significant associations were found between LPS, I-FABP, and calprotectin. In particular, in allergic SA group, calprotectin strongly correlated with I-FABP (β = .516) and LPS (β = .564) (Figure 1H, Table S2). In previous study, we reported increased gut permeability in symptomatic eosinophilic asthma.3 This study reconfirmed increased gut permeability in SA within a multicenter cohort. Furthermore, we further investigated the key factors influencing gut permeability in SA and identified atopy as the most significant. Increased gut permeability in allergic SA patients was also associated with elevated calprotectin levels. Calprotectin, also known as S100A8/A9, is a marker of neutrophil activation and migration, indicating ongoing inflammation. This association has also been observed in asthma, where calprotectin is recognized as a marker of neutrophilic inflammation.4 Given these findings, increased gut permeability in allergic SA patients may contribute to heightened neutrophilic inflammation, potentially playing a role in the progression to treatment-resistant SA.5 There likely exists a bidirectional interaction between gut permeability and airway inflammation highlighting the importance of mitigating gut permeability to break this vicious cycle. This study found plasma LPS to be more reliable than zonulin for indicating gut permeability in SA. More specifically, zonulin is secreted from the airways6 as well as the gut and showed a higher correlation with airway indicators in our study. Additionally, unlike zonulin, LPS showed strong correlations with I-FABP and calprotectin (Figure S4). This study had several limitations. First, there was no healthy control group matched for age and sex, but since no significant correlation was found between age and gut permeability markers, and adjustments for age and sex preserved statistical significance. Second, the high prevalence of T2 high inflammation and atopy in the cohort may have biased the results regarding gut permeability. Third, the study did not control for asthma medications or dietary factors. Finally, as a cross-sectional study, it could not establish causality, and only a limited set of systemic inflammatory markers was used. Further research with various inflammatory markers with advanced techniques and mechanistic studies is needed. In conclusion, this study demonstrated that SA is associated with increased gut permeability, with atopy identified as the most significant patient trait influencing this condition. HKP: Conception and design; HKP, DYC, SYP, JHK, WJS, SHK: acquisition of data; HKP, DYC: Analysis of data; HKP: Interpretation of data; HKP, DYC: Drafting the manuscript; HKP, DYC, WJS, SHK: Revising the manuscript; All authors: samples and clinical information collection; All authors: Final approval of the version submitted for publication. This research was supported by the National Institute of Health (NIH) research project (project No.2022-ER1205-00 and No. 2024-ER2113-00). The authors thank the investigators, clinical research coordinators, and participants for their time and efforts in the KoSAR. No additional information is needed regarding funding. The authors declare no competing financial interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions. Figure S1. Table S1. Table S2. Appendix S1. 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