Di et al.
[20] were the first to report a specific alteration in CF macrophages, showing that CFTR is expressed in the phagolysosomes of macrophages and has a critical role in phagosome acidification, a fundamental function for efficient bactericidal activity. These data were challenged in J774 and murine models where the authors failed to detect a difference in pH
[21]. Another study confirmed no differences in lysosomal acidification between either uninfected (CFTR)-defective macrophages or normal macrophages treated with a CFTR-specific drug inhibitor. However, after ingestion of
B. cenocepacia, acidification and phagolysosomal fusion of the bacteria-containing vacuoles occurred in a lower percentage of CFTR-negative macrophages than CFTR-positive cells
[22]. Interestingly, Painter et al. reported defective intraphagolysosomal HOCl production in granulocytes from CF patients
[23] and reported three mechanisms mediating the neutrophil-mediated killing of Pseudomonas Aeruginosa: (1) CFTR-dependent and oxidant-dependent, (2) chloride-dependent but not CFTR- and oxidant-dependent, and (3) independent of any of the tested factors
[24]. Thus, they suggested that the alteration described in mono-macrophages might be subtle; under specific circumstances, alternative mechanisms might contribute to the intracellular killing defect identified in several of the studies mentioned below. Recently CRISPR/Cas9 deletion of the CFTR gene in human monocyte-derived macrophages has indicated that CF macrophages are unable to effectively phagocyte bacteria, and they are subsequently unable to mount effective intracellular killing responses. The authors suggested the cause was a mechanism linked to a reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase assembly. At variance with most studies performed in primary human macrophages, it was notable that CFTR KO was associated with decreased production of several inflammatory cytokines in response to infection, suggesting the need to deepen our understanding of the role of CFTR and the influence of the microenvironment in macrophage reprogramming
[25]. Another alteration that might influence innate response was reported by Wright et al., studying the role of scavenger receptors present in macrophages in CF. These authors reported a failure to express MARCO and mannose receptors in small sputum macrophages in CF. As MARCO mediates the phagocytosis of unopsonized particles, this finding suggests an impaired clearance of inhaled particles with increased inflammation and damage to the CF lung
[26][27]. Another step forward was provided by the work of Bruscia et al., who, using a mice model of CF disease, demonstrated an increased inflammatory response in CF animals in comparison to WT mice, with an increased number of neutrophils and proinflammatory cytokines detected in their bronchoalveolar lavage fluid after LPS exposure
[28]. Interestingly, the same authors show, by means of hemopoietic cell transplant, that CFTR
+/+ hematopoietic cells, including macrophages, significantly dampen the proinflammatory response to LPS in CFTR
−/− animals. Along this line, Bonfield et al., showed that the absence of Cftr in myeloid-derived cells slows the resolution of inflammation and infection
[29]. Indeed, in young children with CF and no active infection, inflammation can be detected in bronchoalveolar lavage (BAL) fluid, a process that was shown to be mainly driven via NLRP3-inflammasome activation
[30][31]. Studies in primary macrophages indicate the presence, in the serum and monocytes of patients with CF, of an enhanced NLRP3-inflammasome signature with increased IL-18, IL-1b, caspase-1 activity and ASC speck release
[32].
Another study performed RNA-Seq in 20 patients with CF pre- and 6 months post-lumacaftor/ivacaftor initiation, and 20 non-CF healthy controls. The transcriptomic profile revealed marked overexpression of inflammation-related and apoptosis genes and significant under-expression of T and NK cell-related genes compared to the non-CF controls, which were decreased in CF cells following treatment with lumacaftor/ivacaftor, alongside the normalization of intracellular calcium levels [33]. Overall, these data indicate that CFTR modulators have potent innate anti-inflammatory properties, indicating IL-18 and IL-1b are potential reliable biomarkers for drug effectiveness at downregulating inflammation in CF[34]. Another class of inflammatory mediators is represented by the endogenous lipid mediator resolvin (Rv) D1. This is a potent regulator of resolution, a member of the family of pro-resolving lipid mediators (SPMs) produced mainly by macrophages and neutrophils via separate pathways from omega-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The role of RvD1 in CF was recently investigated and found to reduce chronic P. aeruginosa lung infection, inflammation, and damage-promoting resolution in vivo in CF mice. The mechanism was related to enhanced P. aeruginosa phagocytosis and reduced genes and proteins associated to NF-κB activation and leukocyte infiltration. Concentration of RvD1 in sputum from patients with CF was also inversely correlated to those of the cytokines and chemokines involved in CF lung pathology, suggesting an important role of this mediator in the pathogenesis of lung disease in CF, as well as a potential therapeutic application[35][36]. Recently, a specific defect altering agonist-dependent integrin activation (corrected in vitro by lumacaftor) was described in CF monocytes, leading to the proposal of the classification of CF as a new type of leukocyte adhesion deficiency (LAD) disease named LAD IV[37][38], adding further strength to the notion that an overall alteration in monocyte function, recruitment and activation is dysregulated in CF. Altogether, these data suggest that part of the CF phenotype expressed in humans might depend on the function of myeloid cells and can therefore represent a relevant target for current therapeutic strategies where the anti-inflammatory properties of CFTR modulator combinations, in addition to their ability to stimulate CFTR function, might contribute to improved clinical outcomes. The notion is further supported by studies in model animals where specific defects were recorded at birth, suggesting an intrinsic alteration to cell biology when CFTR function is altered. Indeed, studies on CFTR deficient pigs have shown altered responses of monocytes to pathogens tested on Pseudomonas aeruginosa infections[39], are in agreement with other modeling systems demonstrating a difference between the CFTR deficient environment and the healthy, non-CF environment[40][41][42][43]. These data not only underline the clinical relevance of these cells, but also suggest that probing CFTR expression/activity and associated biomarkers such as metalloproteinase 9 (MMP9) in peripheral blood leukocytes might have a possible clinical application such as the monitoring the efficacy of CFTR modulators[44][45][46].