What is cystic fibrosis (CF)?

Cystic fibrosis is a genetic disorder which affects various organs, such as the lungs and pancreas. CF causes the production of thick and sticky mucus in these organs, which can block the airways. This could cause severe respiratory issues and block the pancreas, leading to digestive problems. The genetic principles that determine cystic fibrosis involve both parents carrying a recessive defective gene. CF is an autosomal recessive disease, as the gene responsible for CF is located on a non-sex chromosome (chromosome 7), and it is recessive, so a person needs two faulty copies of the gene; one is not enough, as it would be overpowered by the functioning gene.¹

Which gene causes cystic fibrosis?

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene regulates the development of cystic fibrosis. CFTR is found on chromosome 7 and it controls chloride channels that sit on the membranes of epithelial cells (cells lining organs such as the lungs, pancreas and intestines). CFTR controls the movement of chloride ions and bicarbonate in and out of cells. This helps to regulate water transport across membranes, ensuring hydration in our organs to facilitate activities such as gas exchange, absorption and digestion. The control of water transport through CFTR also determines the viscosity (thickness) of mucus.¹

What happens when the CFTR gene is mutated? Gene mutations are frequent and mostly involve small changes in the genetic code, most of which have a harmless effect on humans. However, a mutation in the CFTR gene is crucial as it can cause cystic fibrosis. When the CFTR gene is mutated, the chloride ion channels which it controls become faulty. This means there is impaired water transport within the organs, making mucus very thick and sticky (due to lack of water) rather than thin and slippery. This thick mucus can build up in the airways, causing issues in ventilation and increasing susceptibility to other respiratory infections. Mucus can also block the pancreatic ducts, leading to poor digestion and malnutrition.²

Pathophysiology of airway infections caused by cystic fibrosis

Mucociliary dysfunction

Typically, our airways and lungs are coated with a thin layer of watery mucus, which helps to trap dust, microbes, and other unwanted particles. Microscopic hair-like structures known as cilia sweep this mucus up so that it can be coughed out or swallowed. This is known as the mucociliary escalator system, largely controlled by the CFTR genes and proteins. 

In cystic fibrosis, as the CFTR protein is faulty, chloride ions do not leave the cells properly, which prevents water from moving. This produces thick, sticky, and dehydrated mucus, which lines the airways. This becomes hard to clear, and the cilia are unable to push such thick mucus. The dehydrated mucus can also stick to the walls of the airways, clinging to the cells of the lungs and causing blockage.³ This buildup of mucus within the respiratory system can trap bacterial and fungal pathogens as the mucus is not effectively cleared, leading to severe infections.

Cystic fibrosis vs healthy airways. In normal airways (right), functional CFTR channels control chloride ion and water transport, maintaining a thin, hydrated layer of mucus. In cystic fibrosis, defective CFTR impairs chloride secretion, increases sodium absorption and reduces airway hydration. This forms a thick, sticky mucus which promotes airway obstruction and bacterial infections. 

Bacterial infections

Pseudomonas aeruginosa is a common bacterial pathogen that can become trapped in the lungs in CF patients and initiate infection. P. aeruginosa is commonly found in the environment, inhabiting areas such as soil, water and plants. It can also colonise moist and wet environments such as bathtubs and sinks. P. aeruginosa infections are rare in those with a healthy and working immune system; however, in cases of CF, an infected person is immunocompromised, as they cannot clear P. aeruginosa from the lungs due to thick mucus production. This ensures that P. aeruginosa (and other pathogens) are trapped in the mucus and cannot spread, causing infection.⁴

The methicillin-resistant strain of Staphlococcus aureus (MRSA) is becoming an increasingly common pathogen infecting CF patients. The MRSA strain is incredibly resistant to common antibiotics such as methicillin, penicillin and oxacillin. This allows MRSA to cause infection in CF patients if it enters the lungs. MRSA infections are common in children and adolescent patients, as MRSA transmission in schools is often high. MRSA spreads through the environment by direct contact with infected people, wounds, or through objects that have made contact with infected skin (e.g. clothes and soft toys). MRSA is therefore easy to contract in young CF patients, and harder to clear due to antibiotic resistance.⁵

Burkholderia cepacia is a group of closely related bacteria found in soil and water. Normally, these bacteria are harmless in healthy people; however, in CF patients, they can lead to severe, persistent lung infections. Those with CF are at a high risk of contracting an infection with B. cepacia, as it easily spreads through direct and indirect contact amongst CF patients - CF clinics usually separate patients to prevent this from happening. B. cepacia is also naturally resistant to antibiotics, and so it can trigger rapid and fatal declines in lung function, with fevers, sepsis and necrotising pneumonia.⁶

Once established, these bacteria are difficult to eradicate in those suffering from CF. P. aeruginosa and MRSA both have the ability to produce biofilm, a protective layer of microorganisms which helps the bacteria spread. It provides a niche for them to inhabit. P. aeruginosa switches to a mucoid form, producing alginate, which helps to protect it from antibiotics and immune attacks. This biofilm grows on any surface, making these infections highly transmissible. Immune evasion is another key factor in bacterial persistence. B. cepacia can survive inside immune cells (macrophages), and P. aeruginosa prevents neutrophil action. The behaviour of these bacteria may also change once they have entered the lungs, P. aeruginosa undergoes genetic changes inside the infected lungs, which help it survive in low-oxygen and nutrient-poor mucus, whereas MRSA can switch between active infection and low-level colonisation. ^4,5,6

Immunopathology of cystic fibrosis

Innate immune dysfunction

The innate immune system is the body’s first line of defence against harmful bacteria. CF causes dysfunction in the innate system by impairing mucociliary clearance. This is due to the thick mucus it produces, allowing bacteria to remain in it, making it harder for the cilia to clear them. CF also results in dehydrated acidic airway surface liquid (ASL). This is because mutations in the CFTR gene cause less chloride secretion and more sodium absorption. Dehydrated ASL results in antimicrobial proteins (such as lysosomes) working poorly, as the environment is too salty and acidic for them to operate.⁷

Neutrophils rush into the airways of CF-infected lungs, but they fail to clear the infection effectively. They release neutrophil elastase, which damages the airway tissue and leads to chronic inflammation. This worsens damage and produces even thicker mucus.

Adaptive immune responses

The adaptive immune response is also affected by CF. These responses are skewed toward a T-helper cell 2 and 17 profile, recruiting these T-helper cells to the site of infection. The Th2 response promotes antibody production and allergic-type inflammation, whereas the Th17 response drives neutrophil recruitment and chronic inflammation. Together, these responses lead to chronic inflammation of the airways and worsening airway damage. CF overstimulates T and B cells; however, the clearance of pathogens from the mucus is poor. There is also a lack of control over this overinflammation as the regulatory T cells are impaired, reducing immune control and promoting further infection.⁸

Antimicrobial therapeutics

Antbiotics

Antibiotics are crucial for the clearance of airway infections caused by bacteria. Antibiotics can be used as a prophylactic (preventive measure) in children. Children can be treated with flucloxacillin to reduce the likelihood of contracting an MRSA infection. 

Antibiotics are also used for early eradication to clear bacteria before they become chronic. For example, P. aeruginosa can be treated with inhaled tobramycin or colistin. Antibiotics can be used for chronic suppressive therapies once bacteria have established long-term colonisation. Inhaled antibiotics are favoured as they deliver high doses of drugs directly into the lungs, preventing systemic side effects.⁹

In the hospital, antibiotics can be administered via IV in cases of acute pulmonary infections in CF. P. aeruginosa is often treated with ceftazidime, piperacillin-tazobactam and IV aminoglycosides. MRSA is treated with vancomycin or linezolid. However, B. cepacia is difficult to treat as it has high natural resistance to many drugs, including antibiotics that could be used in meropenem and ceftazidime.⁹

Biofilm targeting and adjunctive therapies

As many bacteria that cause infections in CF produce biofilm, this is a key target in eradicating infection. DNase helps to break down the extracellular DNA from dead neutrophils, which makes mucus and biofilms less sticky. This improves antibiotic access and clears the airways. Other adjunctive and experimental techniques include phage therapy, which uses bacteriophages to target biofilm-embedded bacteria. Antibiotics are also being developed as nanoparticles to specifically target and penetrate biofilms more effectively. ¹⁰

Future directions

Future directions in managing airway infections in CF aim to control infections but also help to prevent them entirely.

Better antibiotic strategies are needed, as current CF does not respond to many of the antibiotics currently being used in treatment. Novel inhaled antibiotics involve new formulations with enhanced lung penetration and less resistance. Nanoparticle drug delivery can help to penetrate bacterial biofilms, target the lungs specifically and release drugs slowly. Combination therapies may also be used to target multiple bacterial resistance pathways.

Immune modulation therapies could help to rebalance T-cell responses, shifting away from the Th2/Th17 heavy response in CF. This reduces the inflammation caused by Th2 and Th17, relieving symptoms. Neutrophil elastase inhibitors work to inhibit the elastase proteins that neutrophils normally secrete to cause inflammation. By inhibiting this, the airways suffer less damage and inflammation.

Gene editing approaches such as CRISPR-Cas9 are programmed to cut DNA at the defective CFTR site. This section can be replaced or repaired with a functioning sequence of DNA. This can permanently correct the CFTR mutation in airway epithelial stem cells. However, this is a difficult task as there are over 2000 different mutations in the CFTR gene, and so covering them widely is much harder.¹¹ On the other hand, mRNA therapy would deliver synthetic CFTR mRNA into the lung cells via nanoparticles. The lung epithelial cells would then use this DNA to make a functional CFTR protein, despite any mutations in other proteins. This could work regardless of the mutation type and is a promising technique.¹²

Summary

Cystic fibrosis is an autosomal recessive disease caused by CFTR gene mutations. This leads to defective chloride ion transport, hindering effective water movement within the airways of the lungs. This lack of water movement results in thick, sticky mucus in the lungs, which can trap bacteria such as P. aureginosa, B. cepacia and MRSA. The mucus thickness in CF cases prevents the lungs from clearing it, allowing bacteria to remain in the lungs and causing infection. 

Patients dealing with CF can suffer from chronic infections, inflammation and malabsorption. This is often due to the innate immune system becoming dysregulated. The cilia in the lungs are unable to push out thick, sticky mucus, macrophages are unable to engulf bacteria, and neutrophils are overstimulated, causing chronic inflammation. 

Therapeutics for treating CF involve CTFR modulators, antibiotics for airway infections, airway clearance and anti-inflammatories. Emerging strategies include gene therapy (e.g. CRISPR-Cas9) and stem cell therapies that aim to correct the underlying defects and reduce the burden of chronic secondary infections in CF.