Dysregulated signalling pathways in innate immune cells with cystic fibrosis mutations

Abstract

Cystic fibrosis (CF) is one of the most common life-limiting recessive genetic disorders in Caucasians, caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). CF is a multi-organ disease that involves the lungs, pancreas, sweat glands, digestive and reproductive systems and several other tissues. This debilitating condition is associated with recurrent lower respiratory tract bacterial and viral infections, as well as inflammatory complications that may eventually lead to pulmonary failure. Immune cells play a crucial role in protecting the organs against opportunistic infections and also in the regulation of tissue homeostasis. Innate immune cells are generally affected by CFTR mutations in patients with CF, leading to dysregulation of several cellular signalling pathways that are in continuous use by these cells to elicit a proper immune response. There is substantial evidence to show that airway epithelial cells, neutrophils, monocytes and macrophages all contribute to the pathogenesis of CF, underlying the importance of the CFTR in innate immune responses. The goal of this review is to put into context the important role of the CFTR in different innate immune cells and how CFTR dysfunction contributes to the pathogenesis of CF, highlighting several signalling pathways that may be dysregulated in cells with CFTR mutations.

Keywords: CFTR and autoinflammation; Cystic fibrosis; Inflammation; Macrophages; Monocytes; Neutrophils.

Fig. 1

CF airway and altered AECs mechanisms. a In this panel, a cross-section of the CF airways is represented, which showing the airway lumen on top and different epithelial cells on the bottom. In CF, the lack of CFTR function leads to increased Na⁺ influx by ENaC, followed by water absorption leading to dehydration of the periciliary layer (PCL), with accumulation of a thick, dense mucus in the apical surface and persistent colonisation by opportunistic pathogens. The chronic inflammatory microenvironment in the lung facilitates neutrophilic infiltration, with subsequent release of excessive amounts of neutrophil extracellular traps (NETs) upon activation. b In CF AECs the CFTR malfunction decompensates the intracellular ionic balance, leading to overactivity of ENaC and increased Na⁺ influx and K⁺ efflux, as a consequence. This exaggerated K⁺ efflux, combined with increased ER stress and reactive oxygen species (ROS) production, activates the NLRP3 inflammasome and further increases IL-1β and IL-18 secretion. The ionic imbalance is also associated with increased ER stress, ROS and metabolic turnover. The misfolded CFTR, combined with the ionic imbalance, causes IRE1α activation with the generation of the spliced form of XBP1 (XBP1s), which, in turn, activates a number of UPR-related genes inducing inflammation. The overstimulation of both surface and intracellular receptors, through DMAPs and PAMPs, combined with all the other dysfunctional signalling pathways, causes an exacerbated inflammatory response with increased production of TNF, IL-6 and IL-8

Fig. 2

Altered signalling pathways in CF macrophages. Macrophages with CFTR mutations show alterations in multiple cellular pathways. The mutated CFTR causes ionic imbalance, with accumulation of misfolded protein in the case of the ∆F508 mutations and primes these myeloid cells towards an altered immune response or chronically activating other signalling pathways. CFTR malfunction primes the overactivation of ENaC, leading to increased Na⁺ influx, which is then compensated by K⁺ efflux. The increased K⁺ efflux, combined with increased ROS and ATP production, activates the NLRP3 inflammasome with further increased IL-1β and IL-18 secretion. CF macrophages have raised levels of TLR4 expression, and the resultant overactivation of NF-κB leads to increased TNF and IL-6 production. Induction TNF and IL-8 may also occur through NETs by an unknown mechanism. Similarly, chronic TLR4 activation, possibly due to the persistent bacterial colonisation in the lungs, leads to the overactivation of IRE1α; thereby, triggering XBP1s. This production of XBP1s induces transcriptional activation of several UPR responsive genes involving metabolism, inflammation and protein folding. XBP1s overexpression induces a low-grade chronic induction of IL-6 and TNF, which exacerbates the inflammatory response when combined with other signalling pathways. XBP1s also regulate metabolic pathways and, in CF macrophages, the increased metabolic state can be reduced by IRE1α inhibition. Macrophages with CFTR mutations also show increased glycolytic flux and mitochondrial respiration. It is known that in M1 macrophages the Krebs cycle favours the accumulation of succinate and citrate. Succinate accumulation leads to stabilisation of HIF-1α, which can induce IL-1β production and activation of glycolytic genes. It may be possible that in CF macrophages, this axis is favouring a proinflammatory response and increased glycolytic function. Alternatively, citrate is converted into aconitate, facilitating the synthesis of itaconate, which is a potent anti-inflammatory metabolite; however, the role of itaconate in CF is unknown. CF macrophages also display deficient bacterial killing with intracellular accumulation of phagocytic vesicles. Altogether, these mechanisms influence the altered innate response elicited by macrophages. LDHA lactate dehydrogenase A, GLUT glucose transporter, SDH succinate dehydrogenase, PDH pyruvate dehydrogenase, ER endoplasmic reticulum, HIF-1α hypoxia inducible factor 1 subunit alpha, NRF2 nuclear factor erythroid-2-related factor 2, ATF3 activating transcription factor 3, ROS reactive oxygen species