Staphylococcus aureus enterotoxin A- and B-specific IgE in chronic obstructive pulmonary disease

Sensitization to Staphylococcus aureus enterotoxins A (SEA) and B (SEB) has been associated with asthma severity, exacerbations, and disease control. Our study aimed to investigate if there are differences in serum SEA-IgE and SEB-IgE levels between patients with chronic obstructive pulmonary disease (COPD), asthma, and controls, and to assess the association between SE sensitization and COPD clinical parameters and Th2 inflammation biomarkers in two well-defined COPD cohorts. Our findings suggest that COPD patients do not exhibit higher SEA and SEB sensitization compared to asthma patients and controls. However, in COPD patients, the presence of atopy and allergy is associated with positivity for SEA-IgE and SEB-IgE. Consequently, these allergens may aid in identifying atopic or allergic subgroups within the COPD population, but they are not directly associated with the diagnosis of COPD, elevated circulating blood eosinophils, or fractional exhaled nitric oxide (FENO) levels.


Introduction
Staphylococcous aureus (S. aureus) is a common opportunistic pathogen that colonizes the airways [1].Among more than 20 staphylococcal enterotoxins, staphylococcal enterotoxin A (SEA) and B (SEB) are the best characterized and defined as superantigens, because they are
Clinical, laboratory, lung function, as well as Th2 inflammation characteristics [total IgE levels (tIgE), skin prick test, blood eosinophils, fractional exhaled nitric oxide (FeNO)] were collected from all patients.The ALEX 2 Allergy Explorer assay (Macro Array Diagnostics, Wien, Austria) was utilized to measure tIgE.Patients were considered atopic when tIgE ≥ 100 kU/L and allergic when their skin prick test was positive, as previously described [11].Additionally, serum specific SEA-IgE and SEB-IgE were measured using the monoplex Immuno-CAP assay (ThermoFisher Diagnostics, Uppsala Sweden), following the manufacturer's instructions.Values above > 0.35 kU A /L were considered positive.
In the confirmation cohort, there was a significant difference with respect to frequency of SEA-IgE positivity between men and women (p = 0.014, Table 1).SEB-IgE positivity was associated with lower incidence of severe exacerbation as compared to SEB-IgE negativity (20.00% vs. 27.15%,p = 0.006, Table 1).Indeed, SEB-IgE positive subjects had a longer time to first AECOPD than SEB-IgE negative subjects (571.769days vs. 383.968days, p = 0.023).Additionally, an increase of 1kUA/L in SEB-IgE prolonged the time to first AECOPD from baseline by 42.995 days (p = 0.009).Allergy, atopy, and positivity to a rhinitis-related skin prick allergen were, as expected, more commonly observed among SEA-IgE and SEB-IgE positive patients (Table 1).
Age, cigarette exposure in pack years, Global Initiative for Chronic Obstructive Lung Disease grade, post bronchodilator body plethysmography values, walking distance in the 6-Minute Walking Test, health related life quality as assessed by the St. George Respiratory Questionnaire, symptoms as assessed by the COPD assessment test and Modified Medical Research Council Dyspnea Scale scores, number and etiology of AECOPD in the previous year and during the PREVENT study, circulating blood eosinophils and sputum microbiology were similar between SEA-IgE-and SEB-IgE-positive and negative patients (Table 1). .

Discussion
This study has demonstrated a similar prevalence of SEA and SEB sensitization among asthma, COPD and controls.SEB-IgE positivity is more commonly encountered than SEA-IgE positivity among COPD patients.A previous study including 18 stable and 54 exacerbated COPD patients reported that SE-IgE positivity is 4 times more frequent in COPD patients compared to healthy controls [10].However, in contrast to the current study, specific IgE was determined for a mix of SE, including SEA, SEC and TSST-1 [10].Additionally, our study reveals that a higher proportion of SEB-IgE-positive patients, compared to SEB-IgE-negative patients, remained free of severe AECOPD.In accordance, we found that SEB-IgE positivity, as well as increase in SEB-IgE concentration were associated with a longer time to first AECOPD from baseline.Thus, these findings refute the hypothesis that SEB sensitization is linked to a higher risk of AECOPD.It is worth noting that, to the best of our knowledge, this is the first study ever examining the association across SEB-IgE positivity and SEB-IgE levels and the risk and time to AECOPD.The cause of this association remains so far, however, a conundrum.A previous study including a selected population of 77 patients with COPD suggested that specific IgE to perennial allergens could be linked to selected negative COPD outcomes such as emergency department visit and/or hospitalization, but not steroid use or pneumonia.Unfortunately, sensitization to SAB or SEB has not been evaluated in that study [12].S. aureus is frequently found colonizing patients with Th2-biased diseases, such as atopic dermatitis and chronic rhinosinusitis with nasal polyps.In these patients, there is evidence that S. aureus colonizes the nasal mucosa, enclosed in a biofilm or hiding inside immune cells, and constantly produces a panel of factors that could initiate and aggravate Th2-biased immune responses as immune escape mechanisms [13,14].Moreover, SE may damage epithelial layers and thus facilitate allergen entry, therefore promoting Th2 airway inflammation [15].Indeed, our results indicate that in COPD, more SEA-IgE and SEB-IgE positive patients were allergic, atopic, or positive to at least one rhinitis-related skin prick test allergen, when compared to SEA-IgE and IgE with specificity for staphylococcal antigens demonstrates exposure of the immune system to staphylococcal products and does not necessarily indicate presence of S. aureus in the airways.S. aureus-derived extracellular vesicles are present in the environment and specific staphylococcal proteins (SEA, SEB, SEC) have been detected in dust samples from houses of asthmatic patients [16,17].
S.aureus may even be part of the microbiome of house dust mites found in dust samples [16,18].However, it would be of great interest to investigate whether there is an association between the presence of S. aureus in the airways of asthmatic and COPD patients and SEA-IgE or SEB-IgE positivity, as relative studies are lacking.
The relatively small sample size of the derivation cohort, which includes populations of unequal size, is a limitation of our study and larger trials with balanced groups are needed to further validate the results.Another limitation of the study can be considered the absence of a  confirmation cohort for patients with asthma that would confirm the results obtained in the derivation cohort.

Concluding remarks
This is, to the best of our knowledge, the first study investigating the association of SEA and SEB sensitization with COPD clinical parameters and Th2 inflammation biomarkers in two well-defined COPD cohorts.This study demonstrates that although COPD patients do not appear to have a higher frequency of SEA and SEB sensitization than asthma patients and controls, atopy and allergy are both associated with SEA-IgE and SEB-IgE positivity in COPD patients.Thus, these allergens may help to identify atopic or allergic COPD subgroups but cannot be directly associated with the diagnosis of COPD.

Table 1
Comparisons of COPD clinical parameters between patients that were SEA-/ SEB-IgE positive and negative in the confirmation cohort of the study (N = 342)

Positive to at least one asthma related allergen, N (%)
Diffusing capacity of lungs for carbon monoxide, DLCOc: hemoglobin adjusted DLCO, SB: single breath, VA: alveolar volume, FeNO: fractional nitric oxide concentration in exhaled breath, MMRC: Modified Medical Research Council Dyspnea Scale, 6ΜWT: 6-Minute Walking Test, GOLD: Global Initiative for Chronic Obstructive Lung Disease, AECOPD: Acute exacerbations of COPD.Comparisons of numeric variables were conducted utilizing the Mann-Whitney U test, while comparisons of categorical variables were conducted utilizing the Pearson's chi-squared test.