Abstract
Clinical research in chronic obstructive pulmonary disease (COPD) has been hampered by the lack of validated blood biomarkers. The ideal COPD biomarker would have the following characteristics: (1) it would be a lung specific protein that could be assayed in blood; (2) it would change with disease severity or during exacerbations; (3) it would be specific for COPD; and would be responsive to change with effective treatments. One such candidate is the lung specific protein surfactant protein D (SP-D). In this review, we discuss the evidence supporting SP-D as a COPD biomarker.
| Abbreviations | ||
| BALF | = | bronchoalveolar lavage fluid |
| COPD | = | chronic obstructive pulmonary disease |
| FEV1% pred.% Predicted Forced Expiratory Volume in 1 Second | = | |
| SP-D | = | surfactant protein D |
| LPS | = | lipopolysaccharide |
Chronic obstructive pulmonary disease (COPD) is the third-leading cause of death in the United States (Citation1) and both worldwide COPD morbidity and mortality are expected to increase dramatically in the next 10 years. Cigarette smoke is the major risk factor for COPD; however, not all smokers are diagnosed with COPD. A COPD-specific biomarker would ideally allow us to identify smokers who are at higher or lower risk for developing clinically important COPD or to assess disease complications such as acute exacerbations and would identify COPD risk in non-smokers. Surfactant protein D (SP-D) is a strong candidate for such a biomarker.
SP-D is a large multimeric, calcium binding glycoprotein that is a member of the collectin family. The protein serves as an innate immune regulatory molecule and is produced predominantly in the lungs. SP-D promotes the elimination of pathogens by its ability to recognize carbohydrate structures on the surface of large numbers of bacteria, viruses and fungi. Both environmental and genetic factors have been shown to contribute to SP-D expression (Citation2).
SP-D is recognized as an important regulator of innate immunity capable of binding pathogens and facilitating phagocytosis (Citation3). The protein also exerts direct microbicidal activity on selected bacteria and fungi (Citation4, 5). Lower levels of SP-D caused by cigarette smoking may thus weaken lung immunity. Mice harboring SP-D null alleles show reduced bacterial clearance and elevated basal levels of inflammation (Citation6, 7). In vitro, SP-D can suppress inflammatory responses elicited by lipopolysaccharide (LPS) and peptidogylcan (Citation8, 9). Thus, chronically reduced levels of SP-D are expected to cause defects in microbial recognition and impaired suppression of inflammation. Cigarette smoking as has other strong effects on innate immunity of the lung independent of surfactants and phospholipids (Citation10). These effects include impairment of T-cell function leading to susceptibility of infection, dysfunctional ciliary epithelium, and reduction of macrophage and neutrophils phagocytic capabilities.
Recent evidence also implicates anionic pulmonary surfactant phospholipids as important negative regulators of TLR4 activation and inflammation as well as respiratory syncytial virus-induced inflammation (Citation11, 12). Thus, reduction of both SP-D and surfactant phospholipids in smokers is likely to increase inflammation and predispose these individuals to certain viral infections. Genetic association studies for the gene encoding for SP-D (SFTPD) as well as studies of blood and bronchoalveolar lavage fluid have suggested that SP-D could serve as a biomarker for COPD.
Genetic association studies have revealed associations between COPD and single nucleotides polymorphisms (SNPs) in SFTPD (). The most promising SNP is rs721917, which results in a methionine being exchanged for threonine at amino acid 11 (Met11Thr). This SNP is associated with lower serum levels of SP-D (Citation13). The polymorphism was first reported to be associated with COPD in a study of 278 Mexicans (Citation14), but has also been associated with emphysema in 1342 (Citation15) and 1131 (Citation13) Japanese subjects. The association between rs721917 and COPD has been replicated in a subjects of European descent (National Emphysema Treatment Trial (NETT) compared to control subjects from the normative aging study (NAS)), but was not replicated in the Boston Early-Onset COPD Study, the Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints (ECLIPSE) Study, and the Bergen Cohort (Citation16). Additional SP-D SNPs were associated with COPD phenotypes in the NETT-NAS and EOCOPD cohorts (). For unclear reasons, the same investigators reported in an earlier publication that Rs721917 was not associated with COPD in a case control study of NETT-NAS subjects (Citation17).
Table 1. SFTPD SNPs associated with COPD
SP-D protein is made predominantly in the lung, associated with phospholipids, and is in highest concentrations in bronchoalveolar lavage fluid (BALF). The first study of the effect of smoking on phospholipids was reported by Finley and Ladman (Citation18), who noted lower BALF return of phospholipids in smokers. Interestingly they noted that three of the smokers who quit had a rapid rise in BAL phospholipids after as little as 1 month. The earliest study to document the effects of smoking on human BAL SP-D was a small study of 8 smokers and 12 nonsmokers by Honda et al (Citation19) who reported that current smokers had lower BAL SP-D and phospholipids compared to never smokers. Similar results in 22 nonsmokers and 82 smokers were reported by Betsuyaku et al (Citation20). In smokers with COPD, BALF SP-D has been associated with forced expiratory volume at one second (FEV1%) (Citation21). There are caveats to interpreting studies of BALF SP-D. First, smoking does not appear to affect gene expression of SP-D in lung (Citation21). Second, Honda et al (Citation19) and More et al (Citation21) reported that correcting surfactant protein levels for phospholipid content made differences in BALF SP-D statistically non-significant. Third, Schmekel et al have postulated that alveolar macrophages in smokers may increase clearance of phospholipids (Citation22). Thus, although BALF SP-D may be a good marker for smoking and COPD, one must take into account both smoking status and put levels into the context of overall phospholipid metabolism in the lung.
Unfortunately the invasiveness of bronchoscopy means that BALF levels of SP-D are not suitable for biomarker studies in large numbers of subjects. Fortunately SP-D is one of the few lung specific proteins that can be measured in peripheral blood. Serum levels of SP-D have been reported to be higher in COPD patients (Citation23) and those with high serum SP-D have more COPD exacerbations (Citation24), but other reports have found that SP-D is not affected by smoking status (Citation25). The use of SP-D as a biomarker for COPD has also been suggested in a report that found regular inhalation of salmeterol and fluticasone lowers serum SP-D levels in COPD patients (Citation26). Winkler et al. (Citation27) reported that COPD patients had higher SP-D serum levels and SP-D was associated with severity of COPD (FEV1%). Foreman et al found that SP-D serum level was associated with severity of COPD (FEV1%) independent of SP-D SNP status (Citation16). Serum SP-D levels also rise during an exacerbation (Citation28).
Use of specific monoclonal antibodies to detect modified SP-D (cleaved or nitrosylated) was not associated with specific COPD phenotypes (Citation29). Some investigators have reported an opposite relationship between BAL and blood SP-D (Citation27, Citation30), but the mechanism for this is unclear. Sin et al has proposed that the lower levels of BAL SP-D may be due to increased transmigration of SP-D from the alveolar space into blood (Citation31). This may be due to disruption of the multimeric aggregates of SP-D in the lung into lower molecular weight forms that can more easily translocate into blood or to an increase permeability of the lung due to inflammation. There are, however, few data to support or refute this theory.
In conclusion, multiple studies have demonstrated associations between SP-D SNPs and COPD. Additionally, COPD and COPD exacerbations are associated with higher levels of serum SP-D, which is inversely correlated with BALF SP-D. Although the mechanisms linking association to increased risk of COPD are unknown, the lower levels of SP-D in the lung lining fluid may result in reduced immune function and increased susceptibility to infections. Whatever the mechanisms involved, the increased levels of SP-D that are observed in blood suggest it is a promising biomarker for COPD. In particular, it appears to track both with exacerbation and with response to therapy.
Declaration of Interest Statement
The author reports no conflicts of interest. The author alone is responsible for the content and writing of the paper.
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