Human bronchial epithelial cells (HBECs) are the primary cells forming the inner lining of the airways in our lungs. These cells create a continuous protective barrier against the external environment. They are fundamental to maintaining healthy respiratory function, performing various roles to keep our lungs clear and safe from inhaled threats.
What Are They and Where Are They Found?
HBECs are located within the bronchial tubes, the branching air passages extending from the trachea into the lungs. This lining is a pseudostratified columnar epithelium, appearing to have multiple layers due to varying cell heights, though all cells connect to the basement membrane. The epithelial layer thins as airways branch, becoming cuboidal in smaller passages.
The bronchial epithelium comprises several specialized cell types. Ciliated cells are columnar cells covered with hair-like projections called cilia. Goblet cells, named for their wine-goblet shape, are columnar cells that secrete mucus. Basal cells are small, nearly cuboidal cells at the base of the epithelium, serving as progenitor cells for renewing other epithelial cell types.
How Do They Protect Our Lungs?
A primary protective function of HBECs is mucociliary clearance, often called the mucociliary escalator. This process involves the coordinated action of goblet cells and ciliated cells. Goblet cells produce and secrete mucus, a sticky, viscoelastic gel that forms a blanket over the ciliated cells. This mucus layer effectively traps inhaled particles, dust, allergens, and microorganisms, preventing them from reaching the delicate lung tissue.
Once particles are trapped in the mucus, the ciliated cells take over. Each ciliated cell possesses approximately 200 cilia that beat constantly at a rate of 10 to 20 times per second in a coordinated, wave-like motion. This rhythmic beating propels the mucus layer and its trapped contents upwards towards the pharynx. The mucus is then typically swallowed, allowing stomach acid and enzymes to neutralize any pathogens.
Beyond mucociliary clearance, HBECs act as a physical barrier against inhaled irritants, allergens, and pathogens. They form a continuous sheet with tight junctions between adjacent cells, which restricts the passage of large particles and microbes into deeper tissues. This physical barrier helps to maintain the sterility of the lungs.
HBECs also participate in the initial immune responses of the lungs. They are equipped with various receptors that detect foreign organisms and can respond by producing signaling molecules such as chemokines and cytokines. These molecules help to recruit immune cells, like neutrophils and T cells, to the site of infection. Additionally, these cells produce antimicrobial proteins, such as lysozyme, lactoferrin, and mucins, which directly combat pathogens.
Their Role in Lung Diseases
Dysfunction or damage to HBECs can contribute to the development and progression of various respiratory conditions.
Asthma
In asthma, HBECs can exhibit abnormal immune responses and contribute to the chronic inflammation and hyperresponsiveness characteristic of the disease. This includes altered production of inflammatory mediators and a modified response to viral infections.
Chronic Obstructive Pulmonary Disease (COPD)
Chronic obstructive pulmonary disease (COPD) also involves significant HBEC dysfunction. In COPD, there can be impaired mucociliary clearance due to goblet cell hyperplasia, leading to excess mucus accumulation and airway obstruction. The integrity of the epithelial barrier can be compromised, and these cells may release inflammatory mediators like IL-6, IL-8, and IL-1β, perpetuating inflammation and contributing to lung damage.
Cystic Fibrosis (CF)
Cystic fibrosis (CF) is a genetic disorder where mutations in the CFTR gene lead to altered ion transport and hydration of the airway surface liquid. This results in thick, sticky mucus that impairs mucociliary clearance, making individuals with CF highly susceptible to chronic bacterial infections and inflammation. HBECs in CF patients show an inherent pro-inflammatory state and overproduction of neutrophil chemokines like IL-8.
Infections
HBECs also serve as targets and entry points for various viral and bacterial infections. Respiratory viruses, such as influenza and rhinovirus, directly infect these cells, leading to cellular damage and inflammatory responses. Viral infections can increase susceptibility to secondary bacterial infections, as seen with influenza increasing CD47 expression on ciliated cells, providing attachment sites for bacteria like Staphylococcus aureus and compromising the epithelial barrier.
Studying HBECs for Health Solutions
Human bronchial epithelial cells are valuable tools in scientific research and drug development. Scientists can isolate and grow these cells in laboratory settings to create models that mimic the human airway.
A common method involves culturing HBECs at an air-liquid interface (ALI), where the cells are grown on a porous membrane with nutrients supplied from below and air from above. This environment encourages the cells to differentiate and form a pseudostratified epithelium with beating cilia and mucus-producing goblet cells, closely resembling the in vivo human airway.
These laboratory models allow researchers to study fundamental lung biology, including how the epithelium maintains homeostasis and responds to various stimuli. They are also used to investigate the mechanisms of lung diseases, observing how HBEC dysfunction contributes to conditions like asthma, COPD, and cystic fibrosis. HBECs are also utilized to test the efficacy and safety of new medications, including inhaled drugs, providing a physiologically relevant system for drug screening and the development of targeted therapies for respiratory illnesses.