What Is the Nasal Epithelial Barrier?

An epithelial barrier is a lining of cells that separates and protects our body from our environment. When the epithelial barrier doesn’t work properly, it exposes us to disease. Simply speaking, our skin is an epithelial barrier on the outside of our body. But we also have epithelial barriers inside our body. They are a little bit different to skin (yet still linings of sort) and are in our digestive, our reproductive and our nose/respiratory systems. This paper focuses on the nasal (nose) epithelial barrier.

But first, let’s examine the four features of the nasal epithelial barrier, as described by the authors. These four features work together to form the nose’s protection against external risk factors and the body’s immune response.

The Physical Barrier

A physical barrier is created from cells touching cells, dividing the internal and external environment. These are called cell junctions, which the authors describe in detail, alongside some other cell functions. They are important for protection against allergens, disease and other irritants. The junctions are in charge of immune surveillance and prevent the invasion of foreign particles into lower, deeper layers of the barrier lining.

The Chemical Barrier

This is where snot plays a role! Snot, or mucus, is a chemical barrier. Mucus is the main component in the chemical barrier. It protects the nasal epithelium from drying, preserves the local wetness, and humidifies the inhaled air. It is the first place where inhaled allergens and germs will land. The mucus traps them and prevents their invasion. Mucus can exchange molecules, transfer, and remove foreign particles, cleaning as it goes and forming a mucosal protective layer. It works alongside cilia, which are tiny protrusions from the epithelial cells that beat in a coordinated manner, conveying mucus to drainage sites such as the nostril or throat. When there is an infection or allergic response, the normal structure and function of the cilia are altered, thus weakening this clearance function.

The Immune Barrier

An immune barrier is formed with molecules called immunoglobulins (Igs) and antimicrobial proteins and peptides (AMPs). These are given out by cells in the nasal lining. The authors describe the detailed role that Igs play in immune response and in suppressing inflammation and allergic reactions. This is a frontline defense mechanism of the respiratory tract. Defense molecules, including AMPs, don’t just inhibit inflammation but also promote repair of the epithelial lining. They are also a first-line response for the body’s immune system because they have antibacterial effects: They inhibit bacterial multiplication and capture and kill germs.

The Microbiological Barrier

The microbiological barrier is the microbiota that colonize the nasal mucosa. Microbiota are often known as “good” bacteria. The microbiota play a protective and regulatory role in the mucosal immune system. They are usually grown in infancy and continuously remodeled by environmental exposure as an infant grows up. When the microbiota move beneath the epithelium, the immune system is stimulated, and the inflammatory process is promoted, thus helping protect and defend.

What Can Cause Problems with the Epithelial Barrier?

Allergens Containing Proteases

Protease-containing allergens include house dust mites, pollen, pet dander, insects, and fungi. It is known that they can induce immune reactions and break down the epithelial barriers.

Bacteria

Although good bacteria (the microbiota) are needed for a healthy epithelium, some other bacteria can have a damaging effect on the physical and chemical barriers.

Viruses

Viral infections cause impairment to the physical barrier and increase the permeability of the epithelial cells, allowing germs to enter the body. Human rhinovirus (HRV) infection is one of the most common viral infections in the nasal mucosa.

Particulate Matter and Diesel Exhaust Particles

Numerous studies have confirmed that these substances are connected to the prevalence of nasal diseases. These particles can undermine the integrity of the physical barrier and also affect the chemical barrier.

Cigarette Smoke

This compromises the physical, chemical, and immune barriers of the nasal epithelium, stimulating and exacerbating the nasal mucosal immune response.

Inflammatory Cytokines

Inflammatory cytokines are signaling proteins produced during immune reactions. They can further induce and exacerbate damage to the epithelial barrier. For example, by impairing cell junctions.

Approaches for Restoring Epithelial Barrier in Nasal Diseases

The epithelial barrier is the first line of defense against disease. The most common problems caused by issues with the nasal epithelial barrier are allergic rhinitis (AR) and what is commonly known as sinusitis (chronic rhinosinusitis (CRS)).

AR is a chronic, noninfectious inflammatory disease and essentially a hypersensitivity reaction, primarily caused by environmental allergens that disrupt the epithelial barrier. After repeated exposure to the same allergen, many allergic chemical mediators (histamine, prostaglandin, etc.) are produced, which induce a range of nasal allergic symptoms such as sneezing, nasal itching, and a watery nose. Recently, it was found that people with AR tend to develop CRS.

Restoring the epithelial barrier could result in improvement of both AR and CRS symptoms. However, the research is only just beginning. Most studies have limited participants and variable methods. In addition, current studies have not yet sought the ideal dose and timing for treatment, or whether there are adverse effects.

There is still a lot of unclear information on the processes (pathways) that lead to epithelial problems. Some studies have confirmed that drugs with histone deacetylase (HDAC) inhibitors can restore the nasal epithelial physical and chemical barriers. Corticosteroid drugs have also been shown to have a positive effect on the physical barrier. Also, natural plant products have a protective and repairing effect. For the microbiological barrier, there is increasing evidence suggesting that nasal administration of probiotics such as Lactobacilli can have a protective effect.

Take-Home Message

Although there are still deficiencies and challenges to overcome in the future, restoring the epithelial barrier may be a promising strategy for the development of nasal disease therapies.