Staphylococcus aureus and its IgE-inducing enterotoxins in asthma: current knowledge

S. aureus and its products in the environment

S. aureus is robust and can survive hostile environmental conditions. Of all the microbial components present in indoor dust, including Gram-positive and Gram-negative bacteria, Pseudomonas spp., Acinetobacter spp., Enterobacter spp. and Staphylococcus spp. are the dominant genera [28]. The presence of S. aureus and its allergenic components in house dust has been associated with allergic diseases such as asthma. In a recent study evaluating the prevalence of environmental ses in households of adolescents with asthma, exposure to detectable levels of se proteins (seA–seF) in house dust was frequent, ranging from 36% (seB and seC) to more than 60% (seA) [29]. The presence of S. aureus and S. aureus-derived extracellular vesicles (EVs) in house dust was also reported; these EVs in indoor dust can evoke inflammatory lung disease, including asthma [28]. The house dust also typically contains house dust mites (HDMs) as a potent source of allergens, and their microbiomes contain a wide variety of Gram-negative and Gram-positive bacteria, including S. aureus and Escherichia coli [28]. The mites thus act as potential carriers for IgE-reactive bacterial antigens, and sensitisation to HDM is common in patients suffering from respiratory problems [30]. Dzoro et al. [30] demonstrated the presence of bacterial antigens in purified HDM bodies using specific antibody probes and verified that HDM contain IgE-reactive bacterial antigens by qualitative IgE immunoblot and quantitiative ImmunoCAP inhibition experiments. Additionally, in a small study in 24 homes, 50% of bedrooms were found to be contaminated with S. aureus based on culture for detection, but when real-time PCR methods for detection and quantification of S. aureus (femB) were used, the micro-organism was discovered in 96% of the bedrooms. Prevalence and se gene detection in bedroom dust were 21 to 63% for seA, seB, seC and seD, respectively [31].

S. aureus and its products in the airways

In humans, the anterior nares are the main site of colonisation for S. aureus; however, the bacterium may also colonise other parts of the body, such as the skin of the axilla, groin, chest/abdomen, as well as the pharynx, intestine and perineum [17, 18]. The prevalence of S. aureus nasal carriage varies considerably with geographical location. In Europe and the USA, approximately 20% of the healthy population are permanent carriers of S. aureus (carry a single strain most of the time), 30% are intermittent carriers (strains change with varying frequency) and 50% are non-carriers (no evidence of S. aureus despite repeated sampling) [18, 32]. A meta-analysis of five general population-based studies from different geographical locations, including America, Europe, and the Middle East, reported an overall mean prevalence of nasal S. aureus colonisation of around 30% [33]. Patients with airway diseases, such as asthma and nasal polyps, show significantly higher rates of S. aureus colonisation than healthy adults [25]. The bacterium can be found colonising the airways in up to 90% of patients with nasal polyps, with the highest prevalence in patients with comorbid asthma and aspirin sensitivity. In CRwNP patients, S. aureus grows intramucosally and even intracellularly [34, 35]. More than 600 proteins released by S. aureus were identified in the upper airway mucosa of patients with CSRwNP by high resolution mass spectrometry; among these were SEs and Spls [36].

S. aureus-specific IgE in severe asthma

In accordance with the increased colonisation rates, the formation of serum IgE against staphylococcal enterotoxins (se-IgE) is also increased in patients with chronic airway diseases compared with the general population [27, 34]. In nasal polyps, type 2 inflammation characterised by IL-5 and se-IgE expression is related to the severity of the local mucosal inflammation and also significantly associated with asthma comorbidity [37, 38]. Numerous studies show that se-IgE-positive individuals have an increased risk of asthma. Moreover, in asthmatics, se-IgE is associated with more severe disease [39]. In the general adult European population (>2900 subjects in 19 centres), se-IgE was significantly associated with asthma (OR 2.10, 95% CI 1.60–2.76; p=0.001) in a concentration-dependent manner [26]. Logistic regression analyses in a British asthma cohort demonstrated significantly increased risks for se-IgE positive subjects to have any asthma (OR 7.25, 95% CI 2.7–19.1) or severe asthma (OR 11.09 95% CI 4.1–29.6) versus negative subjects [23]. In contrast, the presence of grass pollen or house dust mite specific IgE antibodies was not associated with either increased risk for asthma or asthma severity. se-IgE in asthmatics was indicative for a significantly increased oral steroid use, more frequent hospitalisations within the last year, and a lower lung function (forced expiratory volume in 1 s (FEV1) % predicted value). se-IgE positivity in serum was significantly more frequent in patients with severe asthma than in healthy control subjects (60 versus 13%; p<.001). Furthermore, 21% of the patients with severe asthma and se-IgE were otherwise non-atopic, i.e. had only se-specific IgE but no IgE response to common inhalant allergens. Another study confirmed that se-IgE could be found in atopic and non-atopic patients; in non-atopic asthma, total serum IgE levels were significantly increased in patients sensitised to ses compared with patients non-sensitised to ses. Out of 224 patients with non-atopic and/or late-onset asthma, 105 (47%) patients were sensitised to ses [40]. The study concluded that se sensitisation may contribute to Th2-mediated inflammation in non-atopic patients.

In adolescents, the presence of se-IgE in serum has been also been associated with polyclonal IgE and multimorbidity in terms of airway disease [41]. Corroborating the European studies, Song et al. [42] reported similar observations from Korea. Serum se-IgE concentrations were significantly higher in Korean patients with asthma than in controls [42]. Elderly asthma patients with high se-IgE levels had more severe asthma, sputum eosinophilia and chronic rhinosinusitis compared with those with lower se-IgE levels. In multivariate logistic regression analyses, the association between serum se-IgE concentrations and severe asthma was significant, independently of co-variables (OR 7.47, 95% CI 1.86–30.03; p=0.005). Multiple correspondence analyses also showed that high serum se-IgE level had close relationships with severe asthma, sinusitis and sputum eosinophilia all together [42]. This was the first report on significant associations of se-IgE sensitisation with late-onset asthma in the elderly, particularly severe eosinophilic asthma with CRS comorbidity.

The IDENTIFY study gathered data on sensitisation towards aeroallergens in patients with severe asthma, in whom no allergen could be detected in previous routine tests and who are therefore considered to have non-atopic asthma [7]. In the non-atopic asthma group (n=188), allergic sensitisation to at least one allergen was additionally detected in 52% of patients using an extended panel of aeroallergens. The most frequent sensitizations found were to seB (∼25%) and seA (∼15%), and 8% of the patients were mono-sensitised to seA and/or seB. This still may be an underestimation of sensitisations to S. aureus ses, as the species harbours 23 superantigen genes, of which only two have been tested. This suggests that in addition to standard perennial allergens, other allergens including ses should be tested, specifically in those patients who have “non-atopic” asthma. The exact consequences of these sensitizations require additional studies.

Similarly, in a small study comparing COPD patients with controls, se-IgE positivity was four times higher in both stable and exacerbated COPD subjects compared to healthy controls, either non-smokers or smokers, and se-IgE serum concentrations were significantly higher in both COPD groups versus controls. se-IgE decreased significantly during hospitalisation in the exacerbated patients going along with a significant increase in FEV₁. Thus, similar to severe asthma, se-IgE may be relevant in a subgroup of COPD patients [43].

Serum se-IgE predicts severe asthma

In a nested case–control study from the 20-year Epidemiological study on the Genetics and Environment of Asthma (EGEA) cohort, including 225 adults (75 without asthma, 76 with mild and 75 with severe asthma) in EGEA2 (2003–2007), se-sensitisation was measured on samples collected 11 years earlier (EGEA1) [44]. Cross-sectional associations were conducted for both, EGEA1 and EGEA2, and longitudinal analyses estimated the association between se-sensitisation in EGEA1 and the risk of severe asthma and asthma exacerbations assessed in the follow-up. Models were adjusted on sex, age, smoking, parental asthma/allergy and skin prick test to house dust mite. se-sensitisation varied between 39% in controls to 58% and 76% in mild and severe asthma in EGEA1, respectively. se-sensitisation was associated with an increased risk for severe, but not for mild asthma, clearly supporting former findings of an independent cohort [23]. Furthermore, se-sensitisation in EGEA1 was associated with severe asthma (adjusted-OR 2.69, 95% CI 1.18–6.15) and asthma exacerbations (adjusted-OR 4.59, 95% CI 1.40–15.07) assessed 10 to 20 years later. Thus, for the first time, this study showed that being sensitised to ses is associated with an increased subsequent risk of severe asthma and asthma exacerbations. Inhalant allergens did not show such association, making se-IgE a possibly unique predictor of natural course of disease development.

How could S. aureus trigger or exacerbate type 2 airway inflammation?

S. aureus has numerous means of promoting type 2 airway inflammation. These are assumed to represent immune escape mechanisms whose effectiveness is underlined by the increased prevalence of S. aureus colonisation in allergic and type 2 inflammatory patients [25]. The bacteria release α-toxin, highly damaging to epithelial layers and presumably facilitating allergen entry [45]. Nakamura et al. [46] reported that δ-toxin can trigger mast cell degranulation independent of IgE. ses and other superantigens (see below) can induce an excessive cytokine release from tissue T cells, including Th2 cytokines such as IL-5, IL-4 and IL-13, but they can also induce the rapid release of typical mast cell mediators when specific se-IgE is present [47, 48]. In cultures of fresh nasal polyp tissue with intramucosal S. aureus, staphylococcal proteins can be measured in increasing concentrations over 24–72 h paralleled by the spontaneous release of IL-5 [49]. Killing the intramucosal micro-organisms in the tissue by antibiotics or specific bacteriophages significantly reduces the release of its proteins and suppresses IL-5 production. Recent findings on the migration of eosinophils through the human airway epithelium in response to S. aureus exposure, releasing eosinophil extracellular traps (EETs) to immobilise the bacteria, add another aspect to the multifaceted role of S. aureus in type 2 inflammation [50]. The IL-5-activated eosinophils may be instrumentalised by S. aureus to further break down the epithelium by inducing the release of major basic protein, an epithelium-toxic constituent of eosinophil extracellular traps. With the release of EETs, galectin 10 is released into the epithelial layer, forming Charcot–Leyden crystals (CLCs), which further contribute to maintain the persistent inflammatory response [51].

Promoting functional IgE formation

A prominent group of S. aureus exotoxins, including the ses, act as superantigens (see below). In Th2 cells these will induce the release of IL-4 and IL-13, which are known to promote class switching to IgE and IgE formation. Both cytokines can be found increased in nasal polyps together with increased levels of B cell activating factor (BAFF) supporting B cell activation [52], and in the inflamed airway tissues IgE-positive B cells, soluble IgE as well as follicle-like structures have been demonstrated [14]. seB facilitates sensitisation to ovalbumin and consecutive bronchial inflammation with features of allergic asthma in mice, likely due to augmentation of dendritic cell migration and maturation as well as allergen-specific T cell proliferation [53]. Besides the numerous T cell superantigens, S. aureus also produces a B cell superantigen, the staphylococcal protein A (SpA). SpA can activate all B cells that use a V_H3 element in the B cell receptor [54, 55]. It is plausible that T cell superantigens can also act as “super-allergens” providing help to pre-existing B cells in an antigen non-specific manner, thereby stimulating IgE responses directed against unrelated allergens [53]. The pleiotropic effects of T cell- and B cell superantigens is discussed below.

It needs to be clarified whether S. aureus, besides boosting type 2 inflammation via non-antigen-specific mechanisms, can also produce allergens, i.e., antigens which typically induce specific allergic responses in susceptible individuals [56]. Many humans have S. aureus-reactive type 2 T cells as well as IgE that binds to S. aureus factors, indicating that the micro-organism is capable of producing allergens [25, 57].

Role of Spls

The majority of clinical S. aureus isolates can release Spls, a group of up to six proteins, Spls A–F, which are encoded in one operon [58, 59]. Remarkably, the human T cell response to Spls is characterised by IL-4, IL-5 and IL-10, while IFN-γ- and IL-17 production is very low. This is in strong contrast to the IFN-γ- and IL-17-dominated immune reactions typically elicited by other S. aureus antigens [20, 21, 26]. Moreover, the majority of asthma patients and a minority of healthy adults have Spl-specific serum IgE. In a mouse model, repeated intra-tracheal administration of SplD triggered allergic lung inflammation characterised by bronchial hyper-responsiveness, strong eosinophilic infiltration in the lung tissues and bronchoalveolar lavage fluid (BALF), and T cell accumulation in the BALF [26]. This allergic inflammation is dependent on IL-33 as well as on T cells. It is accompanied by the generation of SplD-specific IgE and IgE antibodies to other antigens simultaneously administered, and the development of Th2 cells, which are found in the lung draining lymph nodes. Interfering with IL-33 receptor binding with soluble ST2 will decrease most of the type 2 immune reactions, such as the numbers of eosinophils and IL-5/IL-13 production by lymph nodes [24].

S. aureus may also directly contribute to the type 2 immune reaction via the release of IL-33, triggering ST2 positive ILC2s and Th2-cells [60, 61]. Interestingly, significantly increased numbers of ILC2s can be found in nasal polyps in patients with comorbid asthma, supporting the pathogenic role of IL-33 [62]. Human ILC2s, driven by GATA3 [63], can produce at least small amounts of IL-4 [64–67], and the stimulation of adult human ILC2s with IL-33 in combination with IL-2 or IL-7 induces release of IL-4 [68]. This may result in increased IgE production without antigen specificity, comprising hundreds of functional IgE specificities and contributing to often very high total tissue IgE levels [48], along with pronounced eosinophilic inflammation (type 2 inflammation) [50, 69] The functionality of se-IgE antibodies from nasal polyp tissue has been repeatedly demonstrated, using mast cell and basophil activation or the promotion of IgE-facilitated allergen binding to B cells (IgE-FAB) [48, 70]. Interestingly, in nasal polyps, high concentrations of both IgE and IgG antibodies can be found. IgG antibodies are able to limit IgE-mediated inflammation by competition, because IgG repertoires widely share the antigen targets with the IgE repertoires in both allergic and non-allergic subjects. This resembles the working principle of allergen-specific immunotherapy in allergic subjects, which elicits blocking IgG antibodies that bind the allergens before cell-bound IgE can do so [71].

ses and other T cell superantigens

T cell superantigens (SAgs) are well studied virulence factors of S. aureus, which have been implicated in allergy. The species harbours 23 SAg genes encoding the staphylococcal enterotoxins or enterotoxin-like proteins A to U and toxic shock syndrome toxin 1 (TSST-1) [72–74]. The molecules are structurally related but of limited sequence similarity. The equipment with SAg genes as well as their regulation is highly variable as many of them are encoded on mobile genetic elements. Around 80% of clinical S. aureus isolates harbour SAg genes, on average 5–6 [75]. SAgs are exceptionally stable and have remarkable effects on the immune system. They can circumvent the T cells’ requirement for antigen recognition by directly crosslinking T cell receptors using specific Vβ elements with major histocompatibility complex (MHC) class II molecules, e.g. on dendritic cells or B cells, thereby activating large proportions of T cells irrespective of their antigen specificity (figure 1a) [72, 73]. Some SAgs also are known to cause food poisoning by inducing vomiting and diarrhoea. These are called ses, the others se-like [76]. Apart from their function as SAgs, the molecules are also conventional antigen targets for the immune system. In fact, they are strongly immunogenic, and adults have a SAg-neutralising IgG response, which in S. aureus carriers is specific for the colonising strain (figure 1b) [77]. As described above, patients with allergic diseases often develop SAg-specific IgE antibodies (se-IgE; figure 1b), suggesting that ses may also function as allergens [78]. se-IgG can be measured in serum using commercially available tests for IgE binding to SEs A-E and TSST-1 (se-IgE). Whether IgE responses are also directed against the other 17 SAgs is not known. As indicated, se-IgEs are functional, degranulate mast cells and basophils, and are, like inhalant allergens, partially inhibited by IgG antibodies due to overlapping repertoires [70]. This recent publication based on findings in nasal polyps demonstrated: the local specific IgE antibodies in nasal polyp homogenates from patients with CRSwNP are functional and promote FcεRI- and FcεRII-mediated allergic responses; IgG-associated inhibitory responses are based on the fact that antibody repertoires are widely shared by IgG- and IgE-producing B cells; and local se-specific IgG antibodies are also induced and can suppress se-IgE and FcεRII-mediated facilitated allergen presentation by B cells to T cells.

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FIGURE 1

Possible functions of staphylococcal T cell and B cell superantigens in allergy. Staphylococcal enterotoxins (ses) and other T cell SAgs can act as a) T cell superantigen: SAgs have superantigen activity and can circumvent conventional antigen processing and peptide presentation by directly cross-linking conserved areas of the T cell receptor (TCR) with areas on major histocompatibility complex (MHC) class II molecules outside the peptide binding cleft. This leads to the polyclonal activation of T cells, including T helper (Th)2 cells, independent of their antigen specificity. b) Conventional antigen: antigen-presenting cells (in this case a SAg-specific B cell) can handle SAgs as conventional antigens: They internalise and process them into peptides, which are then presented to specific T cells on MHC class II molecules. Th cells binding these peptide–MHC complexes with their specific TCRs will provide help for the B cells. The B cells differentiate and produce SAg-specific antibodies. Staphlococcal protein A (SpA) can act as c) B cell superantigen: SpA can bind to V_H3-positive immunoglobulins. If V_H3-positive immunoglobulin (Ig)E is bound to a mast cell via its Fcε receptor, SpA will mimic allergen binding and induce degranulation. APC: antigen presenting cell; B: B cell; BCR: B cell receptor; FcεR: Fcε receptor; MC: mast cell; MHC: major histocompatibility complex; SAg: T cell superantigen; SpA: staphylococcal protein A; T: T cell; Th: T helper cell; TCR: T cell receptor, V_H: variable element of the immunoglobulin heavy chain.

Findings from this study suggest that IgE antibodies including se-IgEs are functional in patients with CRSwNP and comorbid asthma, and that the IgG repertoire is regulating IgE-mediated inflammatory responses, representing a natural blocking mechanism as observed in allergen specific immunotherapy. Besides total IgG, specifically IgG4 is known to block IgE function and cannot bind to FcRs that are present on mast cells [79]; especially S. aureus derived Spls are strong IgG4-inducers [80] and thus could exert a strong IgE blocking activity.

SpA: a B cell superantigen

Almost all S. aureus strains express SpA, a 45 kDa multi-domain protein with pleiotropic functions in pathogen–host interaction [81]. Protein A is produced by clinical S. aureus isolates in varying amounts and anchored in the bacterial cell wall, from where it can be released by proteolytic cleavage. The spa gene contains repetitive sequence motifs that are highly variable and can be used for S. aureus molecular typing [82]. The protein SpA is best known for its ability to bind to the Fc-domain of human IgG, interfering with IgG Fcγ receptor binding and downstream functions, importantly opsonophagocytosis [81]. Furthermore, SpA can function as a B cell superantigen by crosslinking B cell receptors that use V_H3 elements. 83 SpA can bind to many different V_H3-positive immunoglobulins but not to immunoglobulins of the other six V_H families [83]. In humans, S. aureus infection SpA skews the antibody response: V_H3+ plasmablasts are over-represented in the peripheral blood, and these predominantly bind to SpA but not to other S. aureus antigens [84]. In mice, SpA-neutralising antibodies, induced by active vaccination or passively transferred, increase the antibody response to a broad spectrum of S. aureus virulence factors. This improves protection from S. aureus infection as well as clearance of colonising S. aureus [85, 86]. Both S. aureus cells and soluble SpA can also bind to V_H3-positive IgE antibodies. Mast cells which are decorated with such antibodies via their Fcε receptors are then triggered to degranulate (figure 1c) [83, 87]. To explain the high local IgE concentrations in CRSwNP, it has been proposed that SpA can induce IgE production in V_H3-positive B cells [88–91]. Experiments aiming to induce IgE secretion in human B cells with soluble SpA or S. aureus cells in cell culture were partially successful so far [92–94].

Clinical implications in asthma and it comorbidities

S. aureus is a frequent opportunistic pathogen, playing a prominent role as infectious agent in human disease. In this situation, the pathogen elicits a neutrophilic response driven by type 1 and 3 immune reactions, interferons and IL-17. In addition, evidence is accumulating that S. aureus may have another so far underestimated role in type 2 immune reactions, specifically in atopic dermatitis, nasal polyps (CRSwNP) and asthma [95]. There, the germ may aggravate the inflammation contributing to IgE formation and eosinophil activation (figure 2). In those conditions, S. aureus is found accumulating at sites of disease, as the type 2 immune response module counteracts the protective type 1 and type 3 response modules, giving the microorganism a survival advantage [25, 96]. In CRSwNP, staphylococcal proteins such as SEs, se-like SAgs and Spls have been identified within the inflamed tissue, which, when added to the human mucosal tissues ex vivo, evoke strong pro-inflammatory and type 2 deviating immune reactions [36, 97]. Furthermore, the bacteria themselves colonising the airway mucosa, as well as Spls released from S. aureus, induce epithelial alarmins, specifically IL-33, which consecutively activates ILC2s and Th 2 cells [24, 60]. S. aureus also produces a B-cell superantigen, SpA, which can activate all B cells that use a V_H3 element in their B cell receptor [81]. We propose that in an IL-4 and IL-13 rich mucosal environment, also expressing BAFF and follicle-like structures, the B-cell superantigen may induce polyclonal IgE and high total IgE production, as it is typically seen in severe CRSwNP, asthma and atopic dermatitis. Innovative ways of interference with these triggers, such as immunisation against S. aureus proteins or IL-33 receptor inhibition, may be offering relief in severe disease atopic dermatitis, asthma and CRSwNP in the future, besides conferring protection from S. aureus colonisation by restoration of the epithelial barrier.

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FIGURE 2

Summary of impacts of Staphylococcus aureus and its proteins on the human airway immune response. S. aureus colonises the airway epithelium, and induces the release of interleukin (IL)-33 and thymic stromal lymphopoietin (TSLP) [60]. These recruit and activate innate lymphocyte cells (ILC)2s to release IL-5 and IL-13 [24], activating T helper (Th) 2 cells and macrophages; those host S. aureus intracellularly [49], releasing proteins such as serine protease like proteins (spls) and T-cell superantigens (ses) [24], and further trigger T-cells [87, 89]. IL-33 also activates dendritic cells (DCs), which may via superantigens prime naive T-cells to develop into Th2 cells. IL-4 and IL-13 then activate B-cells and initiate polyclonal immunoglobulin (Ig)E production, further supported by mast cells and basophils. Also mast cells have been shown to host intracellular S. aureus. IL-5 released by ILC2s or TH2 cells activate eosinophils, which upon exposure to S. aureus migrate to the epithelium and release extracellular eosinophilic traps (EETs) [50], consisting of major basic protein, DNA and galectin 10 (gal10). Gal10 crystalises to Charcot–Leyden–Crystals (CLCs), leading to persistent stimulation of the respiratory epithelium [51].

Furthermore, IgE with specificity for staphylococcal SAgs (ses) is formed within the diseased airways, demonstrating productive exposure of the immune system to staphylococcal products. se-IgE has been shown to be a biomarker for severe disease, and also to predict disease progression [44]. se-IgE in the airways is functional, as it has been demonstrated to rapidly degranulate mast cells and basophils upon contact with the specific antigen, functioning as allergen in this situation, and to facilitate antigen binding (FAB assay) to B cells [48, 70] in CRSwNP. se-IgE measured within CRSwNP tissue is associated with asthma comorbidity and recurrence of disease, as well as severe type 2 immune inflammation [98]. Europe-wide se-IgE is associated with asthma [26], and independent studies in patients cohorts with severe asthma, the marker was correlated with asthma severity and its clinical expressions, such as hospitalisation, systemic corticosteroid usage and impaired lung function [23]. Recently, in the EGEA cohort, se-IgE was again demonstrated to be associated with disease severity, but for the first time it was also shown to be a significant risk factor to have moderate–severe asthma about a decade later [44]; moreover, being sensitised to ses was a risk factor for having asthma exacerbations years later, whereas a positive SPT to house dust mite did not contribute to that risk. To date se-IgE thus was the only biomarker to predict the development of severe disease. In atopic dermatitis, skin colonisation with S. aureus is significantly increased versus healthy subjects, reaching up to 90% of atopic dermatitis patients, and se-IgE levels are again associated with severe disease [99, 100]. A possible approach to reduce se-IgE may be based on anti-IgE strategies, such as omalizumab, which has been demonstrated to be effective in se-IgE positive, allergic and non-allergic CRSwNP and “non-allergic” asthma; [101] however, its efficacy in “se-IgE only” positive subjects with severe airway disease without further sensitizations remains to be shown.

S. aureus, belonging to the microbiome of house dust mites, also was found in house dust samples, similar to other bacteria, and could be cultured from it [29, 30]. Furthermore, S. aureus-derived extracellular vesicles are present in the environment [28], and specific staphylococcal proteins (seA, seB and seC) have been found in about 50% of dust samples from houses of asthmatic subjects and have been associated with asthma [29]. Sensitisations to house dust mites and bacterial antigens often occur together in atopic dermatitis subjects, indicating that, like HDM allergens, also staphylococcal allergens induce specific IgE antibodies; again, IgE sensitisation to bacterial antigens but not to HDM allergens was associated with severe atopic dermatitis [30].

Future research needs

However, we need to further dissect the pathophysiological relevance of the different functions of S. aureus proteins, from superantigens, superallergens to sole allergens, specifically to estimate the impact on humans when exposed to environmental S. aureus proteins. Does se-IgE in the serum of an asthmatic patient predict allergic reactions upon contact with the allergen? The main obstacles for skin testing are the toxic effects of ses, which are potent T-cell mitogens, precluding their use in humans. As ses are very stable, recalcitrant to heat inactivation, their superantigenicity has to be eliminated by gene engineering. The structure function relationships of several ses were already clarified in the 1990s. On that basis inactivating mutations have been predicted and experimentally confirmed for a number of staphylococcal SAgs [73, 102]. Detoxified seB has been generated by introducing point mutations for use as an anti-seB vaccine. Such reagents have already successfully passed a clinical phase I study, demonstrating safety and immunogenicity [103]. Detoxified seB could first be used in human tissue models to show the allergenic properties, followed by skin prick and nasal/bronchial provocation tests just like those performed with other inhalant allergens. Correlating the results of the in vivo tests with measurements of serum se-IgE by immunoassay systems would reveal the latter's diagnostic capacity such that skin testing might later be avoided in severe asthmatics. A similar research programme should be focussed on S. aureus Spls and extended in perspective to the entire allergome of the species. The SEs and the other S. aureus allergens could then be added to the allergologists’ diagnostic and therapeutic armamentarium to improve the fate especially of “non-allergic” patients with severe asthma, presenting with high total serum IgE without evidence of sensitisation to HDM or other common inhalant allergens.

Antibiotics and oral corticosteroids unfortunately only have temporary effects on this disease, unless given long term, which increases the risk for adverse events; the impact on se-IgE therefore is unknown. For therapy, detoxified ses are in development for vaccination in the future, which might impact on the allergenic, but also on the superantigenic properties of ses. Another approach in se-IgE positive subjects may be anti-IgE treatment, as discussed above; as studies in severe CRSwNP and asthma with Omalizumab have already been performed, the further analysis of such patient's IgE pattern may easily provide a perspective, comparing the efficacy of the drug in se-IgE only positive subjects to placebo treated and the whole group of verum treated subjects. If effective, se-IgE should be added to the allergens regularly tested in severe asthma patients, offering a new treatment option to asthmatics formerly considered “non-atopic”.

Summary

In summary, several groups of staphylococcal proteins are able to interact with the human airway mucosa and its immune system, including the epithelium, ILCs, dendritic cells, T and B cells, mast cells, basophils and eosinophils. Some of these interactions are shared by other bacteria or infectious agents, such as the release of IL-33 from the epithelium; others are specific to the superantigenic properties of the S. aureus proteome. Using the human nasal mucosa as a surrogate for bronchial mucosa, the pathomechanisms used by S. aureus to induce asthma have been partially deciphered in the human, not only in mice. In large epidemiological studies as well as in smaller cohorts, the association of se-IgE with upper and lower airway disease, and severity of CRSwNP and asthma, has been demonstrated repeatedly in independent populations and studies. These studies clearly show that se-IgE is associated with disease severity in asthma, atopic dermatitis and CRSwNP, and predicts severe disease prospectively.

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