Introduction:
The main function of the lung is to help during the process of gaseous exchange. During this process, several chemical substances are released by lung epithelial cells. Surfactants are one of the most common chemicals released by lung cells. Surfactant D, one of the surfactants released by lung cells, has great prognostic values. This molecule not only helps in various functions but also can be used as a marker for several systemic conditions.
What Is Surfactant-D?
Gaseous exchange in the lung occurs through many physio-chemical complex procedures. Pulmonary surfactants are phospholipids and proteins in nature. They are secreted by the type II epithelial cells of the pulmonary epithelium. The function of the surfactants are:
-
Surfactants control the expansion and shrinkage of the lung during the respiratory process. Surfactants reduce the surface tension of the lung at the end of the expiration and help in the expansion of the lungs.
-
Reduced surface tension created by surfactants prevents the collapse of the lung and alveoli during the respiratory process.
-
It is also important for the healing process.
There are four types of surfactants. These are known as surfactants B, C, A, and D. Primary function of the surfactants B and C is to decrease the surface tension. At the same time, surfactants A and D are responsible for the host defense function. Surfectant D is also known as a pattern recognition molecule which consists of collectins (a combination of collagen and lectins).
Surfectsnt D consists of four structural domains. Among these domains, there is one carbohydrate recognition domain that acts as an interaction site for pathogens and facilitates phagocytosis. It is secreted from type II alveolar cells and also from the Clara cells of the tracheal and bronchial glands of the lower respiratory glands.
Surfactant D mainly binds to gram negatives bacteria like g Pseudomonas aeruginosa, Klebsiella pneumoniae, and Stenotrophomonas maltophilia. Apart from this, gram-positive bacteria like Streptococcus pneumoniae and Staphylococcus aureus are also recognized by surfactant D. Herpes simplex virus (HSV), human immunodeficiency virus (HIV), and respiratory syncytial virus (RSV) are recognized by this molecule. Surfactant protein D also acts in fungal infections like Pneumocystis carini, Aspergillus fumigatus, and Cryptococcus neoformans.
Surfactant D increases the phagocytosis action of the macrophages. It also helps in the migration of monocytes and neutrophils.
The role of surfactant D is also crucial in allergic inflammation. Surfactant D can prevent histamine release from basophil and allergen-induced lymphocyte proliferation. It also prevents the binding of immunoglobulin E (IgE) to allergens. It is an essential molecule for the protection of the airway from inflammatory damage.
What Are the Significance of Surfactant D in Respiratory Disorder?
1.Allergic Asthma:
In this condition, the lung is inflamed by the ingestion of the allergens. The allergens can be of several types, like dust, pollens, and foreign substances. The level of surfactant D in bronchoalveolar lavage (a procedure in which samples are collected from inside of the lung) and in the serum is elevated. Exposure to these allergens increases the level of surfactant D via inflammatory mediators like interleukin 4 and thirteen. During such periods surfactant d is secreted by club cells (cubical-shaped cells that secret mucus) and hyperplastic goblet cells (modified epithelial cells that secret mucus).
The increased level of surfactant D reduces the level of inflammatory substances through negative feedback mechanisms. Thus, airways are protected from the noxious effects of inflammatory substances. The function of surfactant D in allergic asthma is as follows:
-
Surfactant D is associated with increased aggregation and accelerated binding of the allergens with macrophages.
-
It prevents the nitrous oxide production and polarization (a process through which macrophages adopt different functional approaches) of the macrophages.
-
Prevent the binding of the immunoglobulin E to the allergens and prevent the release of histamine and degranulation of the mast cells.
-
Chemical activity, degranulation, and movement of the eosinophil are suspended by the activity of the surfactant D.
2.Chronic Obstructive Pulmonary Disease
Chronic obstructive pulmonary disease (COPD) is one of the most common non-infectious inflammatory pulmonary disorders.
One of the main causes of chronic obstructive pulmonary disorder is smoking. Smoking is associated with a reduced amount of surfactant secretion. Injury of the epithelial cells due to carcinogenic agents and nicotine is responsible for the reduced amount of surfactant D in the blood. Also, it disrupts the quarternary structure of surfactant D. Though the level of serum surfactant D is higher in chronic obstetric pulmonary disease, the level of surfactant D in bronchoalveolar lavage (a procedure in which samples are collected from inside of the lung) sampling is less. As a result, patients suffering from chronic obstructive pulmonary disease are prone to develop lung infections and lung inflammations.
It is observed that the serum level of elevated surfactant D is associated with progressive pulmonary diseases and emphysema. Also, an increased level of surfactant D is associated with an increased risk of lung cancer.
3.Bronchopulmonary Dysplasia
This is a common complication of premature delivery. In this condition, the development of the pulmonary epithelium and vascularity is impaired in this condition. Several factors like mechanical ventilation, secondary infections, the presence of the reactive oxygen species (generated due to oxygen supply and from the inflammatory procedures), and immaturity of the lung epithelium are responsible for these conditions.
Also, low levels of pulmonary surfactant in such conditions are associated with an increased risk of lung damage.
Full-length recombinant human SP-D (rhSP-D) can be produced artificially. This can be used in the treatment of bronchopulmonary dysplasia.
What Are the Significance of Surfactant D in Non-Respiratory Disorders?
As discussed, surfactant D is also synthesized by the non-pulmonary tissues also. These disorders are:
1.Cardiovascular Diseases:
Assessment of the level of serum surfactant D level is indicative of pulmonary disorders. The level of surfactant D is significantly higher in coronary artery disorders. It is closely associated with atherosclerosis (change in the elasticity of the blood vessels) changes in the blood vessels.
2.Metabolic Disorders:
The exact role of surfactant D in various metabolic disorders is not known. The level of serum surfactant D is higher in patients with obesity and type II diabetes. Whereas, the level is not elevated in non-obese patients with type II diabetes.
Conclusion:
Surfactant D is one of the useful proteins secreted by lung epithelial cells. This can help in the process of respiration. Also, protection of the lung tissues against pathogens and allergens is provided by this substance. Surfactant D can be used as a marker for several conditions like lung cancer, cardiovascular diseases, and metabolic disorders. Artificial surfactant D can be used in infants for the treatment of conditions like bronchopulmonary dysplasia.
