Cytokine production by Th17 cells in CF

Cytokine production by Th17 cells in cystic fibrosis

Cystic Fibrosis (CF) is a potentially lethal genetic disease that typically results in the development of bronchial inflammation, bronchiectasis, the progressive loss of lung function and, ultimately, death [1].

CF is caused by genetic defects in Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene which encodes a chloride channel regulating chloride transport in the lung. CFTR mutations are associated with severe lung disease are generally found along with little to no CFTR protein expression in the membranes of airway epithelial cells [2]. The most common CF-causing mutation is the homozygous deletion of a phenylalanine at amino acid position 508 (deltaF508), which accounts for nearly 70% of defective alleles and causes 90% of disease cases [1], [2].

In the absence of normal CFTR activity, CF airways get colonized by several bacterial species, which results in chronic stimulation of the proinflammatory signaling. Pseudomonas aeruginosa is the predominant pathogen of CF chronic lung infection [3].

The deltaF508 mutation results in misfolding and ubiquitynation which target the protein for degradation. This leads to decreased amounts and poor functioning of CFTR in the cell membrane resulting in inadequate chloride transport. Reduced secretion of chloride and fluid hydration, as well as excessive secretion of mucins, produce a biological matrix that facilitates growth of P. aeruginosa in biofilm. In the absence of functional CFTR, the height of the airway surface liquid is significantly reduced, resulting in defective volume of ciliary movement and reduced mucociliary clearance. This defect in mucociliary clearance results in mucus stasis and impaired antigen clearance and potentates bacterial infection [1].

Epithelial response to bacterial ligands is mediated by Toll-like receptors (TLRs) resulting in the Nuclear Factor kappa-B (NF-kB) activation that ultimately induces transcription of proinflammatory cytokines, including Interleukins IL-6 and IL-8 [3]. These interleukins are also produced by antigen presenting cells (presumably by lung macrophages and dendritic cells) in response to bacteria. Bacterial lipopolysaccharides (LPS) in the presence of the LPS binding protein (LBP) are recognized by TLR4/ MD-2/ CD14 complex followed by NF-kB activation. Production of Interleukin IL-23 (which is composed of two subunits, alpha (IL23A) and beta (IL-12 beta chain)) by lung macrophages and dendritic cells in response to mucoid P. aeruginosa is critical for the induction of Interleukin IL-17 and the subsequent T-cell differentiation and neutrophilic inflammation [1], [4], [5].

IL-17 is expressed by a distinct subset of CD4+ T helper cells called Th17cells [6]. IL-6 induces naive T-cells to promote Th17 differentiation, whereas IL-23 maintains and expands the population Th17cells [1], [5], [7].

In mice models, in addition to IL-6, the Transforming growth factor (TGF-beta 1) has been shown to be critical for promoting Th17 differentiation, but due to limitations of studying pure naive T-cells in humans, the role of TGF-beta signaling in the differentiation of human naive T-cells into Th17 cells remains unclear [7], [8], [9], [10], [11]. Several studies indicate that TGF-beta 1 is not needed for IL-17 production in human T cells; in fact, TGF-beta 1 inhibits IL-17 production [7], [12], [13].

IL-6 induces the T cell Interleukin 6 Signal Transducer (gp130) / Janus Kinase 1 and 2 (Jak1 and Jak 2)/ Signal Transducer and Activator of Transcription 3 (STAT3) pathway [14]. Jak2/ STAT3 signaling, activated both by IL-6 and IL-23, plays a critical role in the subsequent Th17 development [15], [16], [17]. STAT3 upregulates the expression of Retinoic Acid Receptor-Related Orphan Receptor Gamma-T (ROR-gamma) [7], [18], a Th17 specific transcriptional regulator that is critical for the expression of two members of Interleukin-17 family, IL-17A (IL-17) and IL-17F [19], [20].

Th17cells in CF lung can signal to fibroblasts, airway epithelial cells and vascular or/and microvascular endothelial cells [1].

IL-17 is a key cytokine in CF lung that regulates granulopoiesis and neutrophil migration. IL-17 acts through the Interleukin 17 Receptor A (IL-17 receptor) that is associated with Interleukin 17 Receptor C (IL-17RC) to form a multimeric receptor complex [21]. IL-17RC binds both IL-17F and IL-17 [22].

Little is known about the mechanisms of IL-17 receptor signaling. After stimulation with IL-17, TRAF3 Interacting Protein 2 (CIKS) is supposed to be recruited to IL-17 receptor, followed by activation of E3 ubiquitin ligase TNF Receptor-Associated Factor 6 (TRAF6) and Mitogen-Activated Protein Kinase Kinase Kinase 7 (TAK1), which mediates downstream activation of transcription factor NF-kB [23], [24].

The majority of IL-17 target genes are NF-kB-dependent. IL-17 signaling results in the induction of IL-6, granulopoietic growth factors, such as Granulocyte colony-stimulating factor (G-CSF) and Granulocyte-macrophage colony-stimulating factor (GM-CSF), chemokines, particularly Chemokine (CXC) Ligand 1 (GRO-1), Chemokine (CXC) Ligand 6 (GCP2) and IL-8, and Intercellular Adhesion Molecule 1 (ICAM1) [1], [5]. The subsequent signaling of these cytokines results in neutrophil recruitment followed by the development of persistent bronchial inflammatory process in CF disease [1].

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