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Surfactant Protein D: Its Role in Various Diseases

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Surfactant d is useful for the protection of lung tissues. It can be used as a marker for several pathological conditions.

Medically reviewed by Dr. Kaushal Bhavsar
Published At September 7, 2023
Reviewed At March 26, 2024

Education:

Master of Dental Surgery

Professional Bio:

Dr. Partha Sarathi Adhya is a Dental Surgeon with three years of clinical experience. He was posted to NRS Medical College and Hospital as a House Surgeon. He has experience working under some of the best dental surgeons in West Bengal. He completed his MDS in Prosthodontics and Crown and Bridge in 2020. He is specialized in placing crowns and bridges, dentures, smile designing, and TMJ disorders. He has skilled hands in routine procedures like RCT, scaling, extraction, etc.

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Education:

MBBS

Professional Bio:

Dr. Kaushal Bhavsar is an Internal Medicine specialist and Pulmonologist. He completed his MBBS at M. P. Shah Medical College, Jamnagar, Gujarat, MD at PDU Medical College, Rajkot. He has 13 years of clinical experience and is currently working at GMERS Medical College and Sola Civil Hospital, Gujarat.

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Table of Contents

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:

  1. 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.

  2. Reduced surface tension created by surfactants prevents the collapse of the lung and alveoli during the respiratory process.

  3. 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.

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Frequently Asked Questions

What Conditions Are Related to the Deficiency of Surfactant Protein D?

Surfactant protein D deficiency is not directly linked to any medical condition but plays a role in lung health. The role of surfactant protein D is to clear infectious pathogens from the lungs. A deficiency of surfactant D compromises the ability of the lungs to fight infection. As a result, the individual is at higher risk of developing respiratory infections and respiratory diseases such as pneumonia, bronchitis, bronchopulmonary dysplasia, and chronic obstructive pulmonary disease.

Can Lung Disorders Be Diagnosed Using Surfactant Protein D as a Marker?

Various lung disorders can be diagnosed using surfactant protein D (SP-D) as a marker.SP-D levels can be measured in the blood or lung fluid. Increased or decreased SP-D levels can be a sign of certain lung diseases.

What Role Does Surfactant Protein D Play in the Function of Lung Surfactants?

Like other surfactants, Surfactant protein D is mainly produced in lung cells. It plays a significant role in the lungs' innate immunity, protecting the body from various potential pathogens such as bacteria, fungi, and viruses. It is also an essential molecule that protects the airways from inflammatory damage.

Are There Genetic Factors Linked to Surfactant Protein D Levels?

Yes, some evidence suggests genetic factors can influence surfactant protein D levels. However, the exact link between genetic factors and surfactant protein D needs further investigation.

What Are the Possible Uses of Surfactant Protein D in Medicine?

Surfactant D is one of the useful proteins produced by lung cells. It may benefit respiration and protect lung tissues against allergies and infections. Numerous ailments, including metabolic problems, lung cancer, and cardiovascular diseases, can be detected with surfactant D as a marker. Conditions such as bronchopulmonary dysplasia can be treated in newborns with artificial surfactant D.

Can COVID-19 Patients Be Treated With Surfactant Protein D?

Currently, there is no evidence of COVID-19 patients being treated with surfactant protein D. The possibility of using surfactant protein D to treat COVID-19 patients is being explored, but there are no definitive results yet.

In What Way Is Pulmonary Fibrosis Connected to Surfactant Protein D?

Research reveals that patients with pulmonary fibrosis may have altered, more elevated levels of SP-D. It is unclear if these changes in SP-D contribute to the development of fibrosis or result from the disease process itself.

Are There Any Drugs That Specifically Target D-Surfactant?

No, there are currently no drugs specifically targeting SP-D; researchers are looking into synthetic forms of SP-D or molecules that can enhance its activity as potential therapeutic interventions in lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), and respiratory infections.

What Function Does Surfactant Protein D Provide in Protecting the Host Against Infections?

Surfactant protein D plays a crucial role in host defense against infections. It helps identify, immobilize, and eliminate invading pathogens, protecting the host from respiratory infections.

Is It Possible to Measure the Amounts of Surfactant Protein D in Regular Medical Tests?

Surfactant protein D levels can be measured in blood or lung fluid samples using sandwich enzyme-linked immunosorbent assays and other laboratory techniques.

Does Surfactant Protein D Play a Role in Controlling Lung Inflammation?

Yes, surfactant protein D plays an important role in controlling lung inflammation. Surfactant protein D (SP-D) controls lung inflammation by inhibiting inflammatory signaling pathways, promoting anti-inflammatory responses, and facilitating the clearance of inflammatory stimuli.

In What Way Is Surfactant Protein D Related to Neonatal Respiratory Distress Syndrome?

Neonatal Respiratory Distress Syndrome (RDS) is characterized by breathing difficulties in premature infants. Sometimes, a deficiency of surfactant protein D in developing lungs leads to developing RDS.

Can Lifestyle Factors Influence Surfactant Protein D Levels?

Certain lifestyle factors, such as smoking and air particulate matter, can affect SP-D levels and contribute to lung diseases.

Can Lung Cancer Be Diagnosed Using Surfactant Protein D as a Marker?

A higher risk of lung cancer is linked to elevated levels of surfactant D, but it cannot be used as a reliable tool for diagnosing lung cancer. Tests like chest X-rays, CT scans, and biopsies are more accurate and reliable for diagnosing lung cancer.

How Stable Is Surfactant Protein D in Different Types of Environments?

The surfactant protein D (SP-D) stability varies according to environmental conditions such as pH, temperature, and denaturing chemicals. SP-D is generally stable at pH levels ranging from neutral to slightly acidic and at physiological temperatures. However, it is susceptible to pH extremes, high temperatures, and certain denaturing chemicals, such as detergents. Interactions with other environmental molecules, such as lipids and carbohydrates, may alter SP-D's stability.

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Dr. Kaushal Bhavsar from iCliniq
Dr. Kaushal Bhavsar

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