Decreased percentage of CD4⁺Foxp3⁺TGF-β⁺ and increased percentage of CD4⁺IL-17⁺ cells in bronchoalveolar lavage of asthmatics

Asthma is a chronic inflammatory disorder of the airways characterized by variable airflow obstruction and presence of airway hyperresponsiveness that leads to typical symptoms like recurrent episodes of wheezing, breathlessness, chest tightness and coughing [1]. CD4⁺ T cells are one of the most important cells in the pathogenesis of airway inflammation in asthma. CD4⁺ T cells are traditionally divided into Th1 and Th2 subsets based on profile of their cytokines production [2, 3]. Th2 cells, which are characterized by IL-4, IL-5, IL-9 and IL-13 cytokine production, predominate in asthma, whereas Th1 cells producing IFN-γ, IL-2 and TNF have rarely been associated with asthma [2, 3]. More recently new subsets of CD4⁺ T cells have been described: anti-inflammatory (suppressive) T regulatory (Treg) cells and proinflammatory Th17 cells [2, 3].

CD4⁺ Treg cells can be divided into natural regulatory T cells (nTreg cells) and inducible or adaptive regulatory T cells (iTreg cells) [4]. nTreg got their name because they are always present in the body and constantly perform their function during normal surveillance of self-antigens [4]. They arise in thymus and leave it fully active, ready to inhibit immunological responses. They constitute the population of long-living cells, resident in peripheral organs and one of their main activities is protection against autoimmunological disorders [4]. The first described marker of these cells was CD25 with a more recent marker of nTreg cells being the constitutively expressed transcription factor Foxp3 [4]. Therefore CD4⁺Foxp3⁺ or CD4⁺CD25⁺Foxp3⁺ cells are often regarded as nTreg cells, in spite of the fact that newer markers have also been proposed [5]. However it should be noted that we still lack precise and specific markers of nTreg cells [6].

iTreg, in contrast to nTreg, obtain their regulatory ability in peripheral tissues. Naïve or mature CD4⁺ cells are activated by dendritic cells in the secondary lymphoid organs [7]. The main function of iTreg is therefore to counteract excessive or the pathological inflammatory response [7, 8]. There are no good surface markers of iTreg and according to some authors they lack CD25 expression (CD4⁺CD25^- cells) [8] whereas others have shown that CD4⁺CD25⁺ iTreg can be generated from naïve CD4⁺ cells after antigen stimulation in the presence of IL-2 and TGF-β [9]. Furthermore it was shown that TGF-β-induced iTreg cells may express Foxp3 [10] and that Foxp3⁺ Treg cells are a rich source of TGF-β [11‐13]. These latter cells are also known as Th3 cells. Phenotypic characterization of Th3 cells is very difficult as not all TGF-β-secreting T cells are Th3 cells and not all Th3 cells express Foxp3 [11‐13].

Tr1 cells are another subset of iTreg cells, which is difficult to study in great details as we still lack precise markers, which could help us to reliably distinguish IL-10-secreting Treg cells from other T cells [14]. Tr1 cells produce IL-10, TGF-β and little or no IL-2, IL-4 and can be sometimes defined as: IL-10⁺IL-5⁺IL-4^-IL-2^+/-IFN-γ⁺. Many studies have demonstrated that IL-10-secreting T cells did not express Foxp3 [15‐17] but there are also reports of subsets of Foxp3-expressing Treg cells with the ability to secrete IL-10 [18, 19]. Phenotypic characterization of Tr1 cells could be therefore defined as CD4⁺IL-10⁺IL-4^-.

The percentage of CD4⁺CD25^high cells was decreased in bronchoalveolar lavage (BAL) fluid of pediatric asthma patients [20]. Also Foxp3 protein expression was decreased within blood CD4⁺CD25^high cells [21]. However, Smith et al. have recently shown an increased percentage of CD4⁺Foxp3⁺ and CD4⁺CD25⁺CD127^- cells in BAL fluid of moderate to severe asthma [22]. In another study the percentage of CD4⁺Foxp3⁺ cells in BAL increased after allergen provocative test in asthmatic patients [23].

Th17 cells are a new population of proinflammatory CD4+ T cells which are important during activation of the inflammatory response against bacterial infection for example [24]. Th17 cells have an opposite function to Treg cells (especially iTreg cells) in relation to Th1 and Th2 cells. Interestingly, TGF-β in the presence of IL-6 induced the development of Th17 cells from naïve T cells whereas TGF-β alone induced the development of CD4⁺CD25⁺Foxp3⁺ Treg cells [25, 26]. Mouse Th17 cells produce large amounts of IL-17, but no IL-4, IFN-γ or Foxp3 and contribute to airway hyper-responsiveness [25, 27]. Several studies showed a possible involvement of Th17 cells in the pathogenesis of asthma. The number of IL-17 positive cells in sputum and BAL as well as the expression of mRNA for IL-17 in sputum cells is increased in asthma [28, 29]. The levels of IL-17 in sputum correlated with hyperresponsiveness to methacholine in asthmatic subjects [30]. Increased levels of IL-17 in sputum were also seen in patients with allergic rhinitis after a nasal allergen challenge [31]. Interestingly, two recent reports showed the opposite results [32, 33]. In the first study an increased expression of IL-17 in the bronchial epithelium was observed in severe asthma but not in mild or moderate asthma [32] whereas increased expression of IL-17 was reported in mild-moderate, but not severe asthma, in the second study [33].

In this study we isolated CD4⁺ cells from BAL fluid of asthmatics and age-matched control subjects and after short in vitro stimulation we measured simultaneously the expression of up to six parameters in these cells:

set 1: TGF-β, FoxP3, CD25, IFN-γ, IL-4, TNF-α;

set 2: IL-10, FoxP3, CD25, IFN-γ, IL-4, MIP-1β;

set 3: IL-17A, IL-8, IFN-γ, IL-4, MIP-1β.

The aim of our study was to analyze the incidence of different subsets of CD4⁺ T cells in BAL fluid of asthmatics and healthy subjects, in particular CD4⁺CD25⁺Foxp3⁺ cells, CD4⁺Foxp3⁺ cells, CD4⁺IL-10⁺IL-4^- cells, CD4⁺TGF-β⁺ cells, CD4⁺TGF-β⁺Foxp3⁺ cells and CD4⁺IL-17⁺ cells together with the co-expression of various cytokines: IFN-γ - a marker of Th1 cells, IL-4 – a marker of Th2 cells, IL-8 (CXCL8) – a chemokine for neutrophils, MIP-1β (CCL4) – a chemokine for monocytes and macrophages and TNF-α - a proinflammatory cytokine.

In this study we analyzed the incidence of different subsets of CD4⁺ T cells together with the co-expression of various cytokines in BAL fluid of asthma patients and healthy subjects. We found that in BAL fluid of patients with chronic asthma have a decreased population of CD4⁺Foxp3⁺TGF-β⁺ cells and at the same time an increased population of CD4⁺IL-17⁺ cells, CD4⁺IL-17⁺ IFN-γ⁺ cells and CD4⁺IL-4⁺ cells. We also found that decreasing FEV₁ correlated with the decreasing percentage of CD4⁺Foxp3⁺TGF-β⁺ cells and simultaneously with the increasing percentage numbers of CD4⁺IL-17⁺ cells. Additionally, CD4⁺Foxp3⁺ cells derived from the BAL fluid of asthmatics were characterized by an increased expression of IFN-γ, IL-4 and CD25, whereas CD4⁺IL-10⁺ cells had a decreased expression of Foxp3.

Our study is the first to show the decreased population of CD4⁺Foxp3⁺TGF-β⁺ cells in BAL fluid of asthma patients. We did not observe a significant difference between asthma and controls with respect to the incidence of other Foxp3 or IL-10 positive cells: CD4⁺Foxp3⁺, CD4⁺Foxp3⁺CD25⁺, CD4⁺IL-10⁺IL-4^- or CD4⁺Foxp3⁺IL-10⁺. Therefore the results of our study do not confirm either the recent report of Smith et al., who showed increased percentage of CD4⁺Foxp3⁺ and CD4⁺CD25⁺CD127^- cells in BAL fluid of moderate to severe asthma [22] or the results of Hartl et al. who found a decreased percentage of CD4 + CD25^high cells in BAL fluid of pediatric asthma patients [20]. Hartl and colleagues also found that the suppressive capacity of CD4 + CD25^high cells in BAL fluid was depended on TGF-β, so they speculated that these cells may represent iTreg cells. The results of our study are therefore much closer to those of Hartl et al. as we showed a decreased percentage of CD4⁺Foxp3⁺TGF-β⁺ cells in BAL fluid of asthma patients. Similarly, we speculate that this subset of T cells may represent Th3 iTreg cells as these cells express TGF-β. Additionally, the percentage of CD4⁺Foxp3⁺CD25⁺ did not differ between the two groups studied.

Asthma is an inflammatory disease of a peripheral organ, the lung, so the involvement of locally-induced iTreg cells instead of nTreg cells in the pathogenesis of this disease seems to be more probable. The decreased population of Th3 iTreg cells may, therefore, contribute to the presence of airway inflammation and to asthma progression since there is a correlation between the percentage of CD4⁺Foxp3⁺TGF-β⁺ cells and FEV₁.

Although several studies have previously shown the importance of IL-17 and IL-17 producing cells in asthma pathogenesis our study is, to our knowledge, the first to show using flow cytometry and intracellular cytokine staining that the percentage of CD4⁺IL-17⁺ cells in BAL fluid of asthma patients is increased. Th17 cells are known inducers of neutrophilic inflammation as they stimulate other cells (e.g. epithelial cells) to produce IL-8/CXCL8, which is one of the most potent neutrophil chemoatractants [2]. In this study we showed that CD4⁺IL-17⁺ cells in humans are able to produce IL-8/CXCL8, but we observed no differences in the percentage of CD4⁺IL-17⁺IL-8⁺ cells between asthmatic and control groups. Th17 cells may also cooperate with Th2 cells to induce eosinophilic airway inflammation in mice [35]. In our study we confirmed an increased population of Th2 cells, which may indicate that Th17 and Th2 may work together to play an important role in the pathogenesis of human asthma.

The role of CD4⁺IL-17⁺ cells in pathogenesis of asthma is further supported by the presence of a negative correlation with FEV₁. Interestingly, approximately half of the CD4⁺IL-17⁺ cells in our study expressed INF-γ⁺ and the percentage of these CD4⁺IL-17⁺INF-γ⁺ cells was significantly increased in asthma (P = 0.047). Th17 cells show great degree of plasticity and during the inflammatory response Th17/Th1 cells can be readily detected [36] indicating a close relationship between Th1 and Th17 cell differentiation programs [37, 38]. Our results suggest therefore that part of increased activity of Th17 cells in asthma could be attributed to Th17/Th1 cells.

Although we observed no difference in the percentage of CD4⁺IL-10⁺, CD4⁺IL-10⁺IL-4^- or CD4⁺IL-10⁺Foxp3⁺ cells between the asthma and control groups, we found that CD4⁺IL-10⁺ cells of asthmatics expressed less Foxp3. Many studies have shown that IL-10-secreting Treg cells (Tr1) generally do not express Foxp3, but IL-10-secreting Foxp3⁺CD4⁺ cells have also been reported by others. Our findings suggest therefore that Foxp3^- IL-10-secreting CD4⁺ cells predominate in asthmatic patients.

Increased expression of IFN-γ and IL-4 by CD4⁺Foxp3⁺ cells will suppress the immune response in asthmatic patients. nTreg are poor producers of proinflammatory cytokines such as IL-4, IFN-γ or IL-17 [17]. However, recent studies indicate that a subset of CD4⁺Foxp3⁺ cells in humans are not regulatory but effector CD4⁺ cells [39‐41]. It may be possible, therefore, that CD4⁺Foxp3⁺ cells expressing IFN-γ or IL-4 represent activated effector CD4⁺ cells. Alternatively, since iTreg cells show much higher level of plasticity than nTreg cells [37, 38] and are able to transform into CD4⁺Foxp3⁺IL-17⁺ cells in the presence of an inflammatory cytokine milieu [42]. A similar conversion of Foxp3⁺ Treg cells into effector cells secreting IL-4 or IFN-γ has been observed and these cells were able to express low levels of Foxp3 [43, 44]. It may therefore possible that a subset of CD4⁺Foxp3⁺ cells expressing IL-4 or IFN-γ cells are iTreg cells. Additionally, the increased population of CD4⁺Foxp3⁺IFN-γ⁺ and CD4⁺Foxp3⁺IL-4⁺ cells in asthmatics suggest that CD4⁺Foxp3⁺ cells in those patients are very plastic and depending on different circumstances may show different phenotypes.

In conclusion, our results suggest that in the airways of chronic asthma patients there is an imbalance between the increased population of CD4⁺IL-17⁺ cells, CD4⁺IL-17⁺IFN-γ⁺ cells and CD4⁺IL-4⁺ cells and the decreased population of CD4⁺Foxp3⁺TGF-β⁺ cells. This imbalance may be important for development of airway obstruction in asthma as we found that decreased FEV₁ correlated with the decreasing percentage of CD4⁺Foxp3⁺TGF-β⁺ cells and simultaneously with the increasing percentage numbers of CD4⁺IL-17⁺ cells.