Cytokines in atopic diseases: revisiting the Th2 dogma

ARTICLE

Auteur(s) : Emilie Mamessier, Antoine Magnan

UPRES 3287, Pathologie respiratoire liée à l’environnement, Faculté de médecine, 27, Boulevard Jean Moulin, 13285 Marseille, France

accepté le 6 Novembre 2005

Immunological mechanisms (( figure 1 ))

Initial T_H2 deviation: a role for dendritic cells

In allergic experimental models, both naturally occurring CD4+CD25+^high regulatory T cells and inducible populations of antigen-specific interleukin-10-secreting regulatory T cells suppress inappropriate T_H2 responses [5]. In atopic subjects, there is an alteration of the T_H1 /T_H2 balance towards a T_H2 bias, because of a missing T_H1 deviation [6]. Immune cells from atopic patients intrinsically display a deficient IFN-γ production compared to healthy volunteers, favoring a T_H2 differentiation of T cells in response to antigens, i.e. the preferential production of IL-4, IL-5, IL-9 and IL-13. In this pro-T_H2 context, DCs present allergen peptides loaded on MHC_II, to naïve T_H0 CD4+ T cells and drive their T_H2 deviation. In this case, T_H2 cytokines secreted from APC, and especially IL-4, initiate the subsequent cascade of T_H2 transcriptional factors in the naïve CD4+ T cells in absence of IFN-γ and IL-12 (( figure 2 )). The transcription factors STAT-6, and then c-maf and GATA-3 allow the T_H2 commitment of T_H0 cells by activating T_H2 cytokine genes promoters [7]. In parallel, GATA-3 inhibits expression of other T_H1 transcription factors [8].

Maintenance of the sustained T_H2 inflammation (table 1)

IgE-dependent cascade: early phase reaction

Eosinophil-dependent cascade: late and chronic phase reaction

Other Th2 cytokines

IL-16 is localized in bronchial epithelial cells from allergic patients, even in the absence of allergen, whereas no immunoreactivity was found in non allergics [23]. IL-16 is chemotactic, induces activation and proliferation of CD4+T_H2 cells, eosinophils and monocytes [24]. However, several recent studies have demonstrated that both in vitro and in vivo, IL-16 down regulates rather than exacerbates antigen-driven T cell activation, Th2 cytokine production and allergic airway inflammation.

IL-25 is secreted by mast cells after FcεRI-mediated activation, and induces the production of IL-4, IL-13 and IL-5, thus helping mast cells to enhance and sustain the Th2 activation [25].

However, even if T_H2 cytokines remain the corner stone of allergic reaction, several works tend to demonstrate that T_H1 cytokines, notably IFN-γ, act concurrently with T_H2 cytokines during the chronic inflammation. This was first described in atopic dermatitis and now appears in other allergic diseases, especially in uncontrolled asthmatic reactions. The best approach to get the right picture of the respective role of T_H2/T_H1 activation in these diseases is to describe current knowledge of their immunopathology.

Cytokines in human atopic diseases

Atopic dermatitis (( figure 3 ))

Indeed, in atopic dermatitis, the allergen is rapidly up-taken by Langerhans cells in the epidermis and DCs in the dermis [26]. These APCs secrete classical T_H2 cytokines and also IL-16, MDC and TARC. TARC induces CTACK (literally a cutaneous T cell-attracting chemokine for T_H2 cells) production from keratinocytes [27]. Then CTACK, and the other chemokines (IL-16, RANTES, MDC), allow CD8+T_H2 cells to infiltrate the epidermis. Once activated, recruited cells, through the secretion of MCP-4, RANTES and eotaxin, activate eosinophils and maintain the T_H2 phenotype [28]. The toxic mediator release due to T_H2 cell activation leads to the early lesions, which give rise to pruritus and subsequent scratching of the epidermis. In a mouse model of atopic dermatitis, a recent report demonstrated that T_H2 cells also produce IL-31, which could be responsible for itch sensation, promoting scratching behavior and severe skin lesions [29]. Thereafter, tissue damage induces an inflammatory mediator synthesis, such as IL-6, TNF-α and IL-1β, or IFN-γ, which recruits and stimulates new T_H2 cells and allows T_H1 inflammatory response development in the dermis [30]. TNF-α induces keratinocytes to produce more RANTES and CTACK, maintaining the T_H2 phenotype [31]. However in parallel, a very recent study has demonstrated that antigen binding on FcεRI at the surface of IDEC (Inflammatory Dendritic Epidermal Cell)-DCs induces the production by these cells of IL12, IFN-γ and GM-CSF [32]. IDEC cells are then programmed to prime naïve T cells toward a T_H1 phenotype. Rapidly, i.e. 48 hours after a novel contact with allergen, T_H1 CD4+ and CD8+ cells infiltrate the epidermis and produce IFN-γ, leading to chronic T_H0 skin lesions [33, 34].

Asthma (( figure 4 ))

IL-5 regulates eosinophil accumulation and activation in the respiratory tract of allergic asthmatics, as elegantly shown in occupational asthma [38]. It is noteworthy that in other allergic reactions, such as allergic rhinitis, chronic sinusitis, allergic eye diseases, a tight positive correlation between IL-5 and eosinophil activation has also been reported compared to control subjects, after specific allergen challenge. Subsequent activation of eosinophils by IL-5 provokes local release of major basic protein, eosinophil cationic protein and eosinophil-derived neurotoxin, leading to the destruction of the asthmatic bronchial epithelium [39].

Currently, IL-5 is considered as an important biological predictive marker for asthma exacerbation. Indeed, it has been demonstrated that more the disease was uncontrolled, the more IL-5 and ECP were produced. In asthmatic patients, induced sputum eosinophils, directly or indirectly recruited by IL-5, are considered as predictive markers of exacerbations [40].

Recent evidence suggests that whereas this exclusive T_H2 face of allergic asthmatic inflammation could be relevant to the initial phase of the disease, a T_H1 face could arise during uncontrolled episodes and chronic phases. Indeed, in humans, we and others have shown that uncontrolled chronic asthma was associated to IFN-γ increase [41], both in blood [42] and in induced sputum [43]. More recently, in induced sputum from asthmatics, Cho et al. also demonstrated an increase in both CD4+ and CD8+ type 1 and type 2 cytokines, absent in healthy controls [44]. Moreover, this production was close related to the severity of the disease. Accordingly, in asthmatic children, BHR was associated with eosinophilia and IFN-γ production the more atopic they were [45].

Thus, as in atopic dermatitis, it appears that IFN-γ, absent from the initiation of the allergic reaction, is induced after tissue damage during the chronic inflammatory phase.

IFN-γ could actively maintain and increase the reaction severity. Indeed, IFN-γ induces expression of HLA-II, facilitating antigen presentation and T cell activation by increasing the Tcell-APC contact. IFN-γ enhances expression of FcεRI both on polynuclear cells and mononuclear phagocytes. IFN-γ also increases adhesion molecule expression on various cell types, resulting in enhanced adhesiveness and diapedesis for leukocytes, maintaining an inflammatory state. Importantly, IFN-γ is related to BHR [41]. In a mouse model of asthma, T_H1 cells induced strong AHR independent of IL-4 and IL-13. Moreover, this AHR was associated with the presence of lymphocytes and macrophages, but not neutrophils [46].

The implication of T_H1 cytokines in uncontrolled asthma could be an explanation for the incomplete success of specific anti-T_H2 treatment compared to anti-inflammatory molecules that inhibit both T_H1 and T_H2 cells.

Therapeutic implications

The second approach was to enhance T_H1 activation in order to inhibit the T_H2. In mice, administration of recombinant IL-12 suppressed allergen-induced eosinophilia, AHR and inhibited allergen sensitization. However, complete inhibition of allergen-specific IgE synthesis was obtained only when IL-12 was administered during sensitization, but not during subsequent allergen challenges [50]. IL-12-induced suppression of IgE synthesis by IL-4-stimulated lymphocytes was dose-dependent. However in humans, IL-12 administration was inefficient. IL-18 gene transfer prevented the development and reversed established allergen-induced AHR in mice [51]. Recently IL-18 injected simultaneously with antigen, induced severe airway inflammation and AHR in a mouse model previously injected with T_H1 cells [52]. IFN-γ nebulized in mice induced a decrease in IgE production and airway inflammation, and normalized airways functions, but parenteral administration had no effect. IFN-γ gene transfer prevented and suppressed allergen-induced AHR [53]. The use of plasmids encoding either IFN-γ or IL-12 genes was also promising in animal models. However administration of IFN-γ was inefficient in human asthma. In AD, some IFN-γ trials were encouraging. In a double-blind, placebo-controlled trial, recombinant IFN-γ improved clinical parameters. However, failure of IFN-γ therapy in children with severe refractory AD was also reported. Long-term IFN-γ therapy for severe atopic dermatitis is currently considered as a safe procedure [54].

To induce a T_H1 profile, in vivo administration of infectious agents was also tested and yielded various results according to the type of agent used. Suppression of allergen-induced airways eosinophilia via IL-12 and IFN-γ production caused by BCG administration has been reported in some experimental models. These findings are consistent with human reports, showing reduced prevalence of allergy in BCG vaccinated children [55].

The more promising treatment to induce a T_H1 profile was provided by the use of bacterial CpG motifs. CpG motifs introduced with allergen, during or after the sensitization phase, could reduce or eliminate allergic asthma in mice in increasing the IFN-γ/IL-4 ratio, decreasing the IgE allergen specific amount and IgG1, increasing IgG2a and influx of inflammatory cells in the lung [56].

A recent publication showed that IL-27 plays an important role in the down-regulation of airway hyper-reactivity and lung inflammation during the development of allergic asthma through its suppressive effect on cytokine production [57]. This pro-T_H1/ regulatory cytokine is therefore a new potential therapeutic agent for allergy.

The last strategy tested consisted in tolerance induction through allergen specific immunotherapy and/or regulatory populations and related cytokine induction. IL10 favors immunoglobulin isotype switching towards IgG4 production instead of IgE. IL-10 can turn off immune reactions with basophil, mast cell, eosinophil and T cell deactivation. IL-10 can prevent inflammation by inhibiting APC maturation, down regulating MHC_II and costimulatory molecules. In a mouse model of asthma, IL-10 was effective in decreasing serum IgE and suppressing eosinophilia and AHR [58]. However IL-10 induces hematological adverse events. The current strategy is thus to induce naturally occurring T cell and spontaneous IL10 producing T cells, notably T regulatory cells (Treg). Several recent reports demonstrated that the absence of deleterious immune responses against allergens in healthy volunteers was due to impaired activation of Treg cells. Regulatory CD4+CD25+^high cells have been found to be deficient in allergic subjects [59]. Accordingly, spontaneous recovery from milk allergy in children was related to an increase in CD4+CD25+^high Treg cells, which did not occur in non-recovering children [60]. AD patients have normal CD4+CD25+Treg cells, with normal activity, but when exposed to super antigens, these cells loose their suppressive functions [61]. We have recently demonstrated that in ultra-rush wasp venom immunotherapy, CD4+CD25+ and CD4+IL10+ Treg cell increases were one of the first immune modifications observed during tolerance induction. In addition, both CD4+CD25+^high and IL10+ Treg cells increased earlier and in higher proportion in the less severe patients, suggesting a facilitated tolerance induction in these subjects [62]. During grass pollen immunotherapy, Francis et al. have shown that CD4+CD25+^high Treg cells increase in treated patients and not in controls [5]. Jutel et al. also confirmed these results [63] in house dust-mite allergy. In addition specific immunotherapy induced IFN-γ producing T cells correlated to tolerance induction and improvement of symptoms [64].

Conclusions

A complete picture of the cytokine network is still lacking in allergic diseases. Such a picture will come from longitudinal studies, evaluating not only patients to controls but also symptomatic and controlled periods in the same patients.

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