Surfactant SP-D Could Be a Good Blood Biomarker for Pulmonary Fibrosis

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Surfactant SP-D

Surfactant SP-DScientists from Japan have found that blood levels of the surfactant protein SP-D could be a good biomarker for idiopathic pulmonary fibrosis, since it leaks into the blood more easily than another similar surfactant protein, called SP-A. The study appeared in the journal BioMedCentral Pulmonary Medicine on December 8, 2014.

Pulmonary fibrosis occurs when the lung tissue becomes thick, stiff, and scarred. The scarring is called fibrosis. An “idiopathic” form of any disease means that doctors are not sure what is causing it. People with pulmonary fibrosis live on the average for 3 years following diagnosis, so studies to help understand and direct treatment for pulmonary fibrosis are greatly needed.

Understanding of surfactant proteins could advance research on pulmonary fibrosis. Surfactant proteins are made in the lungs and aid in maintaining the lung’s immune system. Blood levels of these proteins have been used to show how severe pulmonary fibrosis may be. Two types are thought to be particularly useful as clinical biomarkers (indicators), called surfactant protein-A (SP-A) and surfactant protein-D (SP-D). Although these two proteins are very similar in their structure and function, they may have different properties and could act differently in pulmonary fibrosis, leading to different levels of usefulness as biomarkers for the disease.

The researchers, led by Hirotaka Nishikiori of Sapporo Medical University School of Medicine, studied 36 patients with idiopathic pulmonary fibrosis and compared them to 18 patients with sarcoidosis as well as 20 healthy subjects. Sarcoidosis refers to the growth of immune cells in various body regions, and can occur in the lungs. The purpose of the study was to compare the two different surfactant proteins and to see if they act differently in pulmonary fibrosis.

The scientists compared fluid that had been washed inside the lung (called bronchoalveolar lavage fluid, or BALF for short) to blood serum samples. Serum is the portion of blood that separates out when the blood clots.

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The BALF SP-A levels were the same in all three of the groups; however, the amount of SP-D levels was lower in idiopathic pulmonary fibrosis patients than the other two groups. The comparison of SP-D in the BALF to blood serum levels was more dramatic for the idiopathic pulmonary group than the other two groups, and showed a lower SP-D in the lung fluid and higher SP-D in the blood serum. Serum SP-D was elevated in all of the patients with idiopathic pulmonary fibrosis, but only some of the pulmonary fibrosis patients had elevated serum SP-A — those with more severe thickening of the lung tissue. The researchers also found that the SP-D leaked more easily into the bloodstream than the SP-A, which could explain the consistent blood elevations of SP-D. SP-A was more likely to remain in the air spaces of the lungs.

The study authors concluded “Although SP-A and SP-D are serum biomarkers of IPF having high homology, the current study suggests that the difference in hydrophilicity of the two proteins could be the cause of their difference in migration from the air space into the bloodstream. Because SP-D leaks into the circulation more easily, serum levels of SP-D may reflect pathological changes of the disease more sharply than those of SP-A.”

Based on this study, SP-D might have more use in a clinical blood test for pulmonary fibrosis.

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