Clinical Manifestations

Individuals with CF have exocrine gland dysfunction, which results in progressive suppurative obstructive lung disease, pancreatic insufficiency (85–90%), elevated sweat electrolytes, male infertility (>95%), and female infertility. Less common manifestations include hepatobiliary disease, osteoarthropathy, diabetes mellitus, nasal polyposis, and meconium ileus ( Box 25.1 ). With the institution of newborn screening and improved treatment, CF has become a disease of both children and adults, with roughly 50% of affected individuals living beyond the age of 18 years. Onset of disease in the respiratory and digestive systems begins within 6 months of age.

A strong association between pancreatic function and genotype has been reported for individuals homozygous for p.Phe508del. Most patients homozygous for p.Phe508del have pancreatic insufficiency. Obstruction of the pancreatic duct begins in utero, resulting in fibrosis and loss of exocrine pancreatic function. Pancreatic fluid from patients with CF is low in enzyme and bicarbonate concentrations, resulting in maldigestion of fat and protein. Clinically, children can present with steatorrhea, protein-calorie malnutrition, muscle wasting, and progressive failure to thrive. A voracious appetite is characteristic, and stools are described as bulky, greasy, and foul smelling. Approximately 10% to 15% of patients have enough preservation of pancreatic function to allow for normal digestion of food (pancreatic sufficiency). At least five mutations from classes 3 to 5 described earlier are associated with pancreatic sufficiency, whereas almost all patients homozygous for p.Phe508del have pancreatic insufficiency.

Failure to thrive was a common complication observed at the time of diagnosis before newborn screening was implemented; however, this complication is now rare. If malnutrition is present and severe, hypoproteinemia and edema are observed. In addition, malabsorption can lead to vitamin deficiency, especially of fat-soluble vitamins A, D, E, and K. These problems can be reversed with pancreatic enzyme replacement therapy, oral nutritional supplements, and routine vitamin supplements. Infants diagnosed by newborn screening may have normal absorption of food for several months after birth; however, with time, pancreatic function is lost and symptoms of malabsorption develop.

Glucose metabolism often becomes impaired with age, because fibrosis of the pancreas occurs in patients with exocrine pancreatic insufficiency. In the 2014 CF registry, 35% of adults were reported to have impaired glucose tolerance or CF-related diabetes with and without fasting hyperglycemia. Decreased secretion of insulin and reductions in peripheral glucose use and hepatic insulin sensitivity are observed in patients with impaired glucose tolerance. CF-related diabetes has features of type 1 and type 2 diabetes. The prevalence increases with age and is associated with increased morbidity and mortality disproportionately in women. Progressive fibrosis of the pancreas causes destruction of insulin-producing β cells within the islets of Langerhans. In addition, destruction of islet α cells impairs glucagon secretion; therefore, ketoacidosis is rare. Microvascular complications have been reported in up to 20% of patients with CF-related diabetes. Diabetes in CF is common and strongly affected by genetic modifiers. Affected-twin studies have reported that genetic modifiers were primarily responsible for the age at onset of diabetes.

Liver disease in CF is associated with pancreatic insufficiency. Approximately 25% of patients with CF develop focal biliary cirrhosis, but fewer than 5% progress to multilobar biliary cirrhosis and portal hypertension. In the absence of CFTR, bile becomes inspissated and associated with periductal inflammation and fibrosis. Liver function tests frequently are abnormal, as is a small, poorly functioning gallbladder. Cholelithiasis has been reported in 12% of patients and may be related to loss of bile acids in the stool. Meconium ileus, the thick inspissated meconium that mechanically obstructs the distal ileum, occurs in 8% to 20% of newborns with CF. It also is associated with pancreatic insufficiency. Both CFTR and modifier genes are thought to play an important role in the development of meconium ileus. A similar syndrome (distal intestinal obstructive syndrome) mimicking meconium ileus can occur in older children and young adults with CF. Complete or incomplete obstruction can occur in the terminal ileum or proximal colon. Higher rates are observed in patients with pancreatic insufficiency, positive history of meconium ileus, and previous episodes of distal intestinal syndrome.

Absence of the vas deferens with secondary aspermia renders 98% of men with CF infertile. Sexual potency is normal, and with microsurgical techniques for sperm aspiration, affected men can become biologic fathers. Men with congenital absence of the vas deferens can have abnormal CF alleles with little clinical expression of disease other than the reproductive system. Fertility in women with CF may be decreased secondary to amenorrhea (malnutrition) and dehydrated cervical mucus. Pregnancy in women with mild to moderate disease appears to be well tolerated and not associated with an increased risk for death. The live birth rate is 1.9/100.

Pulmonary disease is the primary cause of morbidity and mortality in patients with CF. Expression of CFTR has been localized to the airways and submucosal glands of the lung. Clinical studies in young children with CF have found significant inflammatory changes in the airways in bacterial-positive and bacterial-negative patients. Large-animal models of CF suggest that infection occurs first, followed by inflammation. Imaging studies using high-resolution computed tomography (CT) in infants with CF describe the presence of thickened airway walls and nonhomogeneous air trapping. Smaller airways and tracheal ring abnormalities are noted at birth in the CF porcine model. Human lungs are reported to be morphologically normal at birth; within weeks, they begin showing evidence of small-airway abnormalities and inflammation. Small and medium-sized airways become obstructed, and neutrophils are the inflammatory cells primarily recovered from bronchoalveolar lavage (BAL) fluid. An intense neutrophilic response leads to the release of proteases that cause chronic injury to the respiratory epithelium and supporting airway structure. The massive numbers of neutrophils subsequently release elastase, which overwhelms the antiproteases in the airway, contributing to enhanced destruction of tissue. Large amounts of neutrophil-derived DNA and cytosol proteins are released into the airway lumen, increasing sputum viscosity and worsening airway obstruction.

Progressive bronchiectasis develops with time, leading to advanced destruction of the airways and parenchyma ( Figs. 25.2 and 25.3 ). Bronchiectatic cysts are prominent, especially in the upper lobes. Death eventually occurs from respiratory failure. Progressive deterioration of pulmonary function occurs despite the routine use of antiinflammatory therapy, mucolytics, airway clearance, and antimicrobial agents. Progressive destruction of the airway and increasing obstruction lead to air trapping, hyperinflation, hemoptysis, and spontaneous pneumothorax.

Early stages of lung disease in cystic fibrosis are visible in this lung specimen. Airway inflammation and bronchiectasis are present. The surrounding lung parenchyma is normal.

Late stages of pulmonary disease. Epithelial ulceration of the airway, loss of smooth muscle from the airway wall, inflammation, and bronchiectasis are present in the large airway at the top of the photomicrograph. Compression of the surrounding lung parenchyma occurs as bronchiectasis increases.

Many children with CF present during infancy with recurrent wheezing or persistent bronchiolitis. Most infants are asymptomatic at birth; however, many develop tachypnea, wheezing, hypoxia, and hyperinflation after having a respiratory viral infection. These findings often resolve with therapy. As mucopurulent secretions increase, chronic cough develops. Digital clubbing occurs gradually and correlates with severity of lung disease. Cough is the earliest symptom in infants and precedes persistent sputum production, crackles, or clubbing. It is present in half the infants in a prospective study of CF infants diagnosed by newborn screening. Predictive risk factors included the presence of pancreatic insufficiency, infection with Pseudomonas aeruginosa, socioeconomic status, and ethnicity. On examination, evidence of crackles and decreased breath sounds secondary to mucopurulent secretions is seen. Acute exacerbations may develop, requiring intravenous antibiotic therapy and frequent hospitalization. More than a third of patients with CF in the 2014 US CF patient registry experienced a pulmonary exacerbation requiring intravenous antibiotics. As lung disease progresses, tolerance for exercise is reduced, dyspnea increases, and respiratory failure develops. Marked heterogeneity occurs in the rate of progression of pulmonary disease. Some patients live to the sixth decade of life, whereas others die as a result of respiratory failure before their 20th birthday. Since newborn screening was instituted, patients are being treated at an earlier age, improving the health in children.

In 2014, the median predicted survival for individuals with CF was 39 years. Survival for children born in 2005 is expected to increase, with the median age approaching the fifth decade of life. In the United States, 50% of people with CF are 18 years old or older, and within 5 years, half are expected to be older than 18 years.

Diagnosis

The diagnosis of CF has changed over the past 15 years. Newborn screening for CF has been adopted by the United States, Europe, Australia, New Zealand, and many Latin American countries. The CF newborn screening program identifies patients at risk for the disease by measuring values of immunoreactive trypsinogen in dried blood spots. Trypsinogen is produced in the pancreas and is released into the serum secondary to pancreatic duct dysfunction. After an abnormal immunoreactive trypsinogen value is identified, many programs perform DNA testing to identify known CFTR mutations. Other programs repeat the immunoreactive trypsinogen testing 2 weeks after the initial test. Both programs report 90% to 95% sensitivity and identify infants with varying disease severity. After a positive screen, infants are referred for diagnostic testing (sweat test or molecular genetic testing). For most patients, the sweat test remains the best diagnostic test to establish the diagnosis of CF. If the sweat test results are in the intermediate range, then DNA analysis may help in the diagnosis. The clinical significance of all 2000 CF mutations is unknown. Many are sequence variants without known clinical disease. Others are known to cause loss of CFTR function and are associated with clinical abnormalities in one or more organ systems. Sequence analysis of the exons, introns, and promoter regions and detection of deletions and duplications identifies 98% of CFTR mutations. Infants are generally asymptomatic at the time of diagnosis, although some may be underweight. Data from the 2014 US CF registry reported that newborn screening accounted for more than 60% of newly diagnosed patients. In the absence of newborn screening, an in utero diagnosis, or a family history of CF, a strong clinical suspicion is required for early recognition. Most children present with a history of recurrent lower respiratory tract disease and symptoms secondary to malabsorption. Approximately 15% to 20% of children have meconium ileus at birth, a family history of CF, or both. A consensus panel convened by the US Cystic Fibrosis panel recommended a combination of phenotypic features, family history of CF, or positive newborn screen and one or more laboratory tests to diagnose CF. A new CF consensus panel was convened in 2015, but results have not been published. Laboratory tests include identification of known CF mutations, abnormal bioelectric transepithelial membrane properties, and elevated concentrations of sweat chloride. The World Health Organization adopted similar recommendations.

The quantitative pilocarpine iontophoresis sweat test is the primary test to establish a diagnosis of CF. A sweat chloride concentration greater than 60 mEq/L is consistent with a diagnosis of CF. Values between 40 and 60 mEq/L are considered borderline, and values less than 40 mEq/L are normal. Infants with a positive newborn screen and sweat chloride values between 30 and 59 mEq/L under 6 months of age are considered to have values in the intermediate range. Molecular genetic testing should be pursued in those infants with indeterminate sweat chloride values. In the presence of two CF-causing mutations, a diagnosis of CF can be made. Infants with intermediate sweat test values and one CF-causing mutation cannot be diagnosed definitively with CF and will need to be followed longitudinally. Infants with an elevated immunoreactive trypsinogen level and inconclusive CFTR functional and genetic testing should be designated CFTR-related metabolic syndrome (United States) or CF Screen Positive, Inconclusive Diagnosis (Europe). If the sweat test is positive, it should be repeated on two separate occasions. Identification of two CF mutations by genotype is highly specific but less sensitive and should be confirmed with a sweat test. Mutational analysis can be done by several different techniques, with commercial laboratories testing for the most common 30 to 87 mutations. These laboratories identify approximately 90% of CF mutations but leave more than 1000 mutations unidentified. Extensive screening for the remaining 10% of mutations is expensive. For rare mutations, gene sequencing is available. The mutational classifications in the CFTR2 project ( http://www.cftr2.org/index.php ) should be used to help with diagnosis. The p.Phe508del mutation is found in 89% of patients with CF in the United States and a similar number of CF patients in the United Kingdom. Because each patient has two chromosomes, however, only 50% of patients are homozygous for p.Phe508del.

Nasal potential difference measurements assess the transepithelial electric potential difference that exists across nasal epithelium. Different patterns of potential difference are found in patients with CF. Abnormalities in chloride transport and sodium absorption alter the transepithelial electric potential difference in CF in contrast to normal epithelia. The test can be useful in individuals with mild or atypical phenotypic features of CF, but it is not readily available and requires experienced personnel to perform.

Newborn screening programs are associated with long-term benefits in children with CF diagnosed shortly after birth. Improved nutritional status as a result of newborn screening has been reported in a controlled, randomized trial in Wisconsin as well as in countries other than the United States. A total of 90% of screened infants in the Wisconsin study diagnosed with CF at birth maintained their weight greater than the 10th percentile in contrast to only 60% of unscreened controls. Children in the screened group were less likely to fall below the 10th percentile for weight and height from early childhood through 16 years of age. In addition, cognitive function improved significantly in the screened group. No long-term improvements in pulmonary status were observed in the Wisconsin study; however, several observational studies have reported improved pulmonary outcomes, less colonization with P. aeruginosa, decreased hospitalization for complications, and improved nutrition in children diagnosed by newborn screening.

In addition to improving nutritional outcomes, newborn screening may result in improved survival rates for children. A systematic literature review of mortality in children with CF reported a survival benefit for patients diagnosed by newborn screening. A survival effect also was shown in a study from Wales. Without screening, approximately 60% of patients are diagnosed by the time they reach 1 year of age and almost 90% by the time they are 5 years old. Early diagnosis through neonatal screening improves nutritional outcome, with increasing evidence that these programs are associated with improved pulmonary outcomes and improved long-term survival.

The diagnosis of CF should be based on the presence of one or more clinical features ( Box 25.2 ), a positive newborn screening test, and laboratory evidence of abnormal CFTR function. Laboratory tests include elevated sweat chloride concentration, two identifiable CF mutations, or abnormal in vivo nasal potential difference measurements made across the nasal epithelium.

Pathogenesis

The CFTR protein is a cyclic adenosine monophosphate–regulated chloride channel that resides primarily in the apical membranes of epithelial cells. CFTR is highly expressed in airways; its loss of function leads to defective secretion of chloride and bicarbonate and subsequent dehydrated and acidic airway secretions. Sweat glands, pancreas, liver, intestinal tract, and reproductive organs are among the other organ systems affected by dysfunctional CFTR. In affected organs, the abnormal chloride and fluid secretion impairs fluid movement and leads to ductular obstruction and organ damage. Mucociliary clearance is impeded, and a destructive cycle of inflammation and chronic infection in the airways results.

Chronic inflammation and resultant lung damage in CF results from failure of the innate immune system to protect the airways from invading pathogens. The complex process of CFTR dysfunction leading to inflammation in the airways is still not fully understood. In the CF lung, decreased volume of airway surface liquid (ASL) impairs mucociliary clearance (MCC), which is the body’s primary defense against inhaled particulate matter and various microbial pathogens. The ASL is composed of a gel and sol layer that function together to help trap microbial pathogens to be propelled up and out of the lungs by beating cilia. The sol or periciliary liquid layer consists of low-viscosity fluid to hydrate mucins and allows ciliary movement to occur. The periciliary liquid layer is thin, 7 µm in height, bathes the cilia, and allows them to beat freely. The gel or mucous layer floats on top of the sol and is composed of high-molecular-weight mucins with carbohydrate side chains that bind inhaled particulate matter and pathogens.

The ASL and mucus layer is a dynamic structure that changes in response to the environment and host. ASL volume is regulated by the respiratory epithelium through ion transport processes by CFTR. Salt concentrations can be changed to regulate the hydration of the airway lining fluid to maintain optimal mucociliary function. The absence of CFTR in the apical cellular membrane decreases the ability of cells to secrete chloride into the periciliary fluid. CFTR inhibits the epithelial sodium channel; in its absence, excessive absorption of sodium occurs. Other channels are available for secretion of chloride (i.e., calcium-activated chloride channel) in the respiratory epithelium; however, they cannot compensate for the loss of CFTR. The net effect is increased absorption of sodium, chloride, and water, which reduces the periciliary volume, alters the composition of mucins, and decreases mucociliary clearance. The reduction in the periciliary fluid impairs ciliary movement, as they are weighed down by the heavy gel layer and mucus cannot be transported and cleared. This initiates the cycle of airway obstruction, chronic infection, inflammation, and progressive lung destruction.

Altered chloride channel function and fluid secretion also help to explain the presence of disease in the sweat gland, intestine, pancreas, and male genital tract. The epithelial cells affected by CFTR mutations in various organs represent different channel and regulatory activities of CFTR but result in deficient secretion of fluids. This deficiency causes accumulation of mucus, obstruction, and various degrees of organ damage. Plugging of pancreatic ducts leads to chronic fibrosis, pancreatic atrophy, and loss of digestive enzymes and islet cells. Similar obstruction in the biliary tract can cause inflammation and focal biliary cirrhosis. Glandular obstruction of the vas deferens causes involution of the Wolffian duct and infertility in more than 95% of men with CF. Women with CF produce abnormally tenacious cervical mucus, with reported rates of infertility of 20%.

Human cell culture models of normal and CF epithelia have shown that the airway surface liquid is decreased in CF ; this remains the primary hypothesis of the impaired MCC in the disease.

Work with large-animal models of CF has revealed that there is likely a more complex pathology for early defects in MCC. CF piglets showed abnormal MCC not due to periciliary liquid depletion but rather due to abnormal CF submucosal gland secretion. Even when periciliary fluid volume was replaced, abnormal mucins remained tethered to glandular ducts, thus preventing normal upward movement on the mucociliary ladder.

Submucosal glands have high expression of CFTR. Loss of CFTR function alters the composition of mucins produced by the submucosal glands, leading to ductular dilation with mucus and obstruction. Mucus is tightly adhered to the respiratory epithelium and prevents normal flow of the mucociliary elevator. Failure to clear mucous plugs, continued mucin secretion, and abnormal adherent mucus to the airway surface provides the focus for infection. Pili extending from the surface of the bacterium are able to bind to mucin. In CF, dysfunctional bicarbonate secretion also results in ASL that is 8-fold more acidic than that of individuals without CF. This low pH can inactivate ASL and mucus antimicrobial peptides.

Obstruction of small terminal airways and submucosal glands with thickened mucopurulent secretions are often the first signs of early disease in infants with CF. Ductular dilation, neutrophil infiltration, glandular hyperplasia, and peribronchiolar inflammation are classic findings of the disease. Thick mucus adherent to epithelial cell surface is seen in the lungs of even newborn CF pigs. A recent study showed that CFTR activity of bicarbonate transfer is greater in small-airway epithelial cells compared with larger airways and the effect of nonfunctioning CFTR in small airways leads to more acidic, viscous ASL with impaired bacterial killing. This may help explain the vulnerability of the small airways to bacterial infection and disease. Air trapping is found in infants with CF as young as a few months old even when clinically asymptomatic. Also, piglets with CF have been shown to have air trapping prior to the onset of infection and inflammation in the airways. These piglets were found to have a smaller trachea and proximal airways compared with non-CF piglets. These data suggest that working CFTR is necessary for normal development of early airways.

Early and exaggerated inflammation occurs in the CF airways that begins in infancy and may precede bacterial infection. In CF lung disease, various immune cells migrate to the airways, contributing to the chronic unrelenting inflammation. Neutrophils predominate to fight bacterial and fungal pathogens, but the activation of these cells can lead to tissue destruction through oxidant and protease release. The serine proteases released have been shown to predict the development of bronchiectasis in CF. Neutrophil elastase mediates killing of P. aeruginosa by degrading its outer membrane protein but also breaks down structural airway matrix proteins, cleaves proteins important for host defense, and increases mucus secretion. Neutrophil elastase has a clear role in the development of bronchiectasis and tissue destruction in progressive CF lung disease. Other proteases, such as cathepsins (found to be increased in BAL samples of infants and children with CF) and matrix metalloproteinases, may play a role in early CF disease. Calgranulins are proinflammatory proteins that are also increased in CF sputum and have been shown to activate key processes of CF lung inflammation.

The inflammatory response in CF involves different immune pathways. Neutrophils, macrophages, TLR4-dependent responses, and T lymphocytes have all been shown to have defective functioning associated with CFTR deficiency.

Lipid abnormalities, such as an accumulation of epithelial ceramide, have been suggested in CF to increase cell death, increase bacterial binding to extracellular DNA, and release inflammatory chemokines. Other investigators highlight the importance of ceramides in lipid rafts needed to clear infection. CF cell membranes are also reported to have increased arachidonic acid compared to docosahexaenoic acid. Docosahexaenoic acid has important antiinflammatory properties. Chronic infection, with retention of the by-products of inflammation, ultimately leads to the severe bronchiectatic changes and derangements of gas exchange characteristic of end-stage CF.

A hypoxic environment develops within the thickened mucous plugs that form in the airways, which may hinder host defenses and favor bacterial growth and inflammation. Pseudomonas aeruginosa, after binding to mucin, is able to penetrate the thickened mucus and grow in an anaerobic environment. P. aeruginosa is able to grow in anaerobic conditions because of the production of nitrate reductase, which allows it to cleave oxygen from nitrate ( Fig. 25.4 ). When it is in the anaerobic environment, an alginate polysaccharide is formed. Biofilm-containing macrocolonies of P. aeruginosa are then established. The established macrocolonies remain within the airway lumen. These macrocolonies are very resistant to antibiotics and host defense and allow chronic infection, inflammation, and airway destruction to occur.

Schematic model of the pathogenic events hypothesized to lead to chronic Pseudomonas aeruginosa infection in airways of patients with cystic fibrosis (CF). (A) On normal airway epithelia (NL), a thin mucous layer ( light gray ) resides atop the periciliary liquid (PCL; clear ). The presence of the low-viscosity PCL facilitates efficient mucociliary clearance ( vector ). A normal rate of epithelial oxygen (O ₂ ) consumption ( ; left) produces no O ₂ gradients within this thin airway surface liquid (ASL). (B–F) CF airway epithelia. (B) Excessive CF volume depletion ( vertical arrows ) removes the PCL, mucus becomes adherent to epithelial surfaces, and mucus transport slows or stops ( bidirectional vector ). The increased O ₂ consumption ( left ) associated with accelerated CF ion transport does not generate gradients in thin films of ASL. (C) Persistent mucus hypersecretion (denoted as mucus secretory gland/goblet cell units; dark gray ) with time increases the height of luminal mucus masses and plugs. The increased CF epithelial generates steep hypoxic gradients in thickened mucus masses. (D) P. aeruginosa bacteria deposited on mucus surfaces penetrate actively or passively or both (as a result of mucus turbulence) into hypoxic zones within the mucus masses. (E) P. aeruginosa adapts to hypoxic niches within mucus masses with increased alginate formation and the creation of macrocolonies. (F) Macrocolonies resist secondary defenses, including neutrophils, setting the stage for chronic infection. The presence of increased macrocolony density and, to a lesser extent, neutrophils renders the now mucopurulent mass hypoxic.

(From Worlitzsch D, Tarran R, Ulrich M, et al. Effects of reduced mucus oxygen concentration in airway Pseudomonas infections of cystic fibrosis patients. J Clin Invest. 2002;109:317–25.)

Several in vivo studies have assessed the location of bacterial adherence within the lung of CF patients and the attachment of bacteria to CF and non-CF cells in vitro. P. aeruginosa is found within the lumen of airways of patients with CF obtained from autopsy specimens. These organisms are observed within the inflammatory exudates of the airway and not within epithelial cells lining the lung or in alveolar spaces. In vitro studies have shown adherence of P. aeruginosa in areas of epithelial cell destruction. No adherence to the apical membrane of intact epithelial cells has been noted, however. In contrast, evidence in cell culture models indicates that defects in CFTR enhance bacterial binding to immortalized airway epithelial cells. A tetrasaccharide (asialo GM ₁ ) is expressed more on CF than on non-CF cells and promotes P. aeruginosa binding to the epithelial membrane. Pseudomonal exoproducts, such as neuraminidase, increase the amount of asialo GM ₁ available for bacterial binding and facilitate bacterial adherence to airway epithelial cells. CFTR itself can act as a receptor for P. aeruginosa and initiate an effective innate immune response in the initial stages of infection to clear Pseudomonas from the airway. CFTR binding to P. aeruginosa induces release of IL-1β, nuclear translocation of nuclear factor-κB (NF-κB), and neutrophil influx. Endocytosis of Pseudomonas by the epithelial cells and subsequent clearance of infected epithelial cells by desquamation has been reported. In the absence of CFTR, clearance of infection with P. aeruginosa is impaired.

Mucins also are important in binding bacteria within the airway. Sialylated and neutral forms of mucins bind P. aeruginosa. Removal of sialylic acid from mucin by neuraminidase reduces adherence of P. aeruginosa . Mucus dehydration can lead to increased concentration of mucins, decreased pH, and a reduction in glutathione, which decreases mucus viscoelasticity. Decreased bicarbonate secretion results in mucin cross-linking by calcium. Polymeric macromolecules within the gel matrix reduce pore size from 500 to 150 nm, immobilizing bacteria within the mucus and inhibiting neutrophil migration and clearance. The concentrated mucus impairs neutrophil migration, promotes an anaerobic environment, and reduces mucociliary transport. Mucin, a component of mucus, is decreased in CF; its decreased content may promote development of infection.

The balance between oxidants and antioxidants is known as redox balance. Imbalance can lead to acute or long-term oxidative or reductive stress. Chronic redox imbalance favoring an oxidative environment is hypothesized to contribute to the disease state in CF.

Increased oxidant production by neutrophils has been proposed as a mechanism of airway injury in CF. Evidence of the effects of oxidative damage in airway epithelial cells and extracellular fluids can be seen in the peroxidation of lipids and modifications of proteins. Markers of reactive oxygen species increase during acute pulmonary exacerbations and improve with treatment but do not normalize.

An intracellular imbalance in oxidant metabolism has been proposed for airway epithelial cells in CF. ASL in CF is characterized by lower levels of glutathione (GSH) and nitric oxide. Mucins, reduced glutathione, α-tocopherol, and metal-binding proteins function in the airway as antioxidants. The cysteine residues and carbohydrates of mucin account for its antioxidant properties. Secretion of mucins is increased with oxidative stress; however, dehydration of mucins may impair its ability to scavenge reactive oxidative metabolites. Glutathione is important in regulating inflammation related to oxidative stress. GSH is reduced in ASL either from increased consumption or secondary to CFTR dysfunction. GSH is found in large amounts in airway lung fluid from normal patients ; however, it is reduced in patients with CF. CFTR may alter glutathione transport, impairing defense to oxidant injury. High-throughput metabolomic analyses of human primary airway epithelia measured decreases in GSH, glutathione disulfide, and a metabolite of S -nitroglutathione in CF versus non-CF cells. Also, the GSH/glutathione disulfide ratio was diminished in CF cells, suggesting oxidative stress.

Reduced intracellular concentrations of GSH affect hydrogen peroxide content of airway cells and increase NF-κB production, contributing to airway inflammation. GSH acts to scavenge free radicals in epithelial lining. GSH is also important in the immune response with chemotaxis, phagocytosis, and oxidative burst. Impaired absorption of antioxidants from the gut in CF may contribute as well. Thus, increased production of reactive oxygen species from neutrophils and impaired antioxidant biosynthesis associated with mutant CFTR create an environment within the airway for excessive inflammation and cellular injury.

There are repeated descriptions of abnormalities in oxidants and antioxidants in CF plasma, cells, and ASL. However, several studies failed to show significant clinical benefit of antioxidant therapies in CF. Further studies using omics technology may help us to better understand redox in CF and potential therapies.

Production of cell adherence molecules and inflammatory cytokines is intact in patients with CF. Defects in bacterial opsonization, reduction in antiinflammatory cytokines, and proinflammatory effects of bacterial DNA have been reported. Impaired phagolysosomal function and decreased respiratory burst have been found in neutrophils in CF with upregulation of Toll-like receptor 4 (TLR4) and downregulation in TLR2. These abnormalities would be expected to reduce the ability of neutrophils to clear bacteria. Inflammation and infection can increase intracellular Ca ²⁺ and affect the immune response mediated by airway epithelial cells and T-helper cells. Increased intracellular Ca ²⁺ in CF mice helper T-cells affect gene expression and favor a Th2 differentiation rather than the Th1 needed for bacterial clearance. Altered T-helper lymphocytes impair macrophage activation for bacterial killing. Humoral antibody responses are normal; however, increased elastase in the CF airway can alter opsonic receptors, impairing phagocytosis. Large amounts of elastase are present in lavage fluid from airways of patients with CF, overwhelming the normal antiprotease activity. This condition can lead to local impairment in phagocytosis of bacteria within the airway lumen, contributing to bacterial persistence.

Proinflammatory mediators are elevated in BAL fluid from young patients with CF, as are macrophages expressing intracellular cytokines. Increased numbers of neutrophils and interleukin 8 (IL-8) in BAL fluid are seen in bacterial culture–negative and bacterial culture–positive infants with CF in contrast to controls. Proinflammatory mediators IL-1, IL-2, tumor necrosis factor-α, and IL-8 are markedly elevated in children with advanced disease. In contrast, some antiinflammatory cytokines (e.g., IL-10) are found in reduced amounts in BAL from patients with CF. These observations have prompted some investigators to assess production of IL-10 from bronchial epithelial cells in normal subjects and subjects with CF. Bronchial epithelial cells from normal individuals produce significantly greater amounts of IL-10 than cells of subjects with CF. NF-κB is a transcription factor for several proinflammatory mediators and is activated persistently in CF. Inhibitors to NF-κB, including IL-10, are downregulated.

Enhanced intracellular Ca ²⁺ signaling in response to infection and inflammation in both airway epithelial and immunomodular cells may account for the hyperinflammatory response noted in CF. Both phases of Ca ²⁺ release, either by Ca ²⁺ influx or endoplasmic reticulum release, can upregulate NF-κB and production of proinflammatory cytokines. Chronic airway inflammation and infection increase endoplasmic reticulum Ca ²⁺ stores, enhancing the inflammatory response. Therefore, the CF gene defect may result in an inability to control airway inflammation after exposure to repeated infections, altering the innate and immune responses to specific pathogens. Although the understanding of this disease pathogenesis is incomplete, airway damage that occurs from excessive inflammation and chronic infection remains a target for new therapeutic modalities.

The range of severity of disease is related to the production of functioning CFTR. Patients with at least one “mild” mutation in CFTR have some low level of CFTR expression. They retain some degree of pancreatic function, have less severe lung disease, and have sweat chloride concentrations that are borderline normal. Male patients with congenital bilateral absence of the vas deferens are pancreatic sufficient, exhibit little or no lung disease, can have normal sweat chloride concentrations, and are thought to have CFTR expression that is 10% of normal. Carriers for CFTR who are heterozygotes expressing half the expected amount of CFTR are at increased risk for developing pancreatitis, allergic bronchopulmonary aspergillosis (ABPA), and sinusitis. As we begin to see the clinical effects of pharmacologic therapies that correct and potentiate CFTR function, we are learning more about the direct effect of CFTR on the symptoms of CF. The potentiator ivacaftor has been able to show that improvement in CFTR function, evidenced by dramatic improvement in sweat chloride tests, leads to improvement in multiple clinical factors. Lung function assessed by forced expiratory volume in 1 second and MCC improved and rates of P. aeruginosa infection dropped by 18% in 6 months after initiation of the drug in one study.

Variation in phenotype also exists in individuals with the same genotype, indicating that other factors—including modifier genes and epigenetic and environmental factors—influence the disease process.

Specific Cystic Fibrosis Pathogens

Anaerobic Bacteria

Anaerobes are organisms that do not require oxygen for growth. Steep oxygen gradients exist within CF mucus such that even at relatively shallow depths within mucus, the environment is considered to be hypoxic or even frankly anaerobic. Conventional culture-dependent approaches are not optimized for identifying anaerobes. Specific anaerobic culture methods, or culture-independent techniques, may be more appropriate. Several studies on tracheal aspirates, sputum, or BAL fluid have confirmed the presence of anaerobes in the lower airways in CF in up to 80% of samples and at bacterial densities of between 10 ⁷ and 10 ⁹ cfu/mL in sputum. The most common genera identified were Prevotella , Veillonella , Propionibacterium , Actinomyces , Staphylococcus saccharolyticus , Peptostreptococcus , and Clostridium . No data linking anaerobes to inflammation or clinical outcomes in CF are available. Longitudinal studies are also lacking, although a comprehensive longitudinal study conducted in a single patient suggested that anaerobes of the Streptococcus milleri group contributed to the development of pulmonary exacerbations. Ulrich and colleagues reported that 16 of 17 patients with CF produced antibodies against two immunoreactive antigens of Prevotella intermedia compared with 0 of 30 controls, suggesting that anaerobes are, indeed, immunogenic in CF. In studies in which anaerobes were specifically targeted during treatment for pulmonary exacerbations, the results have been conflicting. Worlitzsch and colleagues did not identify any significant reduction in the density of anaerobes in sputum after treatment with antibiotics despite an increase in pulmonary function during the period of treatment. Similarly, Tunney and colleagues identified only limited reduction in the density of anaerobes at the end of 2 weeks of treatment. An important factor to consider when treating anaerobic infections is that anaerobic organisms are often resistant to the commonly administered antibiotics.

Treatment of Pathogens in Cystic Fibrosis Patients

Airway infections are a key component of CF lung disease. Whereas the approach to common pathogens such as P. aeruginosa is guided by a significant body of evidence, other infections often pose a considerable challenge to the treating clinicians. A cornerstone of CF care is the aggressive use of oral, intravenous, and aerosolized antibiotics. The increasing longevity of patients with CF has paralleled the development of effective antibiotics. Antibiotics may be used during several stages of CF lung disease (1) to prevent acquisition of pathogens, most commonly MSSA; (2) to eradicate initial acquisition of pathogens, most commonly P. aeruginosa and MRSA, in efforts to prevent or delay chronic infection; (3) to treat pulmonary exacerbations caused by classic CF pathogens; (4) to treat patients infected with P. aeruginosa, as long-term suppressive therapy; and (5) to treat emerging multidrug-resistant pathogens. Newer molecular techniques have demonstrated that the airway microbiome consists of a large number of microbes, and the balance between microbes, rather than the mere presence of a single species, may be relevant for disease pathogenesis. A better understanding of this complex environment could help define optimal treatment regimens that target pathogens without affecting others. The recognition that there is a diverse microbiota in sputum samples from people with CF raises questions about how we approach antibiotic therapy. Conventional bacterial culture in aerobic conditions allows isolation of a limited number of organisms. Extended culture methods identify a much wider range of bacteria, which include more difficult to culture bacteria, such as anaerobic bacteria. At present, there is no readily available methodology to identify all of these organisms in a way that makes this information valuable for clinical treatment. Studies are under way to develop technologies to allow molecular identification without prior culture.