Pulmonary surfactant protein A: a serum marker of pulmonary fibrosis in patients with collagen vascular diseases

Surfactant protein-D predicts prognosis of interstitial lung disease induced by anticancer agents in advanced lung cancer: a case control study

BACKGROUND

Interstitial lung diseases induced by anticancer agents (ILD-AA) are rare adverse effects of anticancer therapy. However, prognostic biomarkers for ILD-AA have not been identified in patients with advanced lung cancer. Our aim was to analyze the association between serum biomarkers sialylated carbohydrate antigen Krebs von den Lungen-6 (KL-6) and surfactant protein D (SP-D), and clinical characteristics in patients diagnosed with ILD-AA.

METHODS

Between April 2011 and March 2016, 1224 advanced lung cancer patients received cytotoxic agents and epidermal growth factor receptor tyrosine kinase inhibitors at Juntendo University Hospital and Juntendo University Urayasu Hospital. Of these patients, those diagnosed with ILD-AA were enrolled in this case control study. ΔKL-6 and ΔSP-D were defined as the difference between the levels at the onset of ILD-AA and their respective levels prior to development of ILD-AA. We evaluated KL-6 and SP-D at the onset of ILD-AA, ΔKL-6 and ΔSP-D, the risk factors for death related to ILD-AA, the chest high resolution computed tomography (HRCT) findings, and survival time in patients diagnosed with ILD-AA.

RESULTS

Thirty-six patients diagnosed with ILD-AA were enrolled in this study. Among them, 14 patients died of ILD-AA. ΔSP-D in the patients who died was significantly higher than that in the patients who survived. However, ΔKL-6 did not differ significantly between the two groups. Moreover, ΔSP-D in patients who exhibited diffuse alveolar damage was significantly higher than that in the other patterns on HRCT. Receiver operating characteristic curve analysis was used to set the optimal cut off value for ΔSP-D at 398 ng/mL. Survival time for patients with high ΔSP-D (≥ 398 ng/mL) was significantly shorter than that for patients with low ΔSP-D. Multivariate analysis revealed that ΔSP-D was a significant prognostic factor of ILD-AA.

CONCLUSIONS

This is the first research to evaluate high ΔSP-D (≥ 398 ng/mL) in patients with ILD-AA and to determine the risk factors for ILD-AA in advanced lung cancer patients. ΔSP-D might be a serum prognostic biomarker of ILD-AA. Clinicians should evaluate serum SP-D during chemotherapy and should carefully monitor the clinical course in patients with high ΔSP-D.

Roflumilast-N-oxide induces surfactant protein expression in human alveolar epithelial cells type II

Surfactant proteins (SPs) are important lipoprotein complex components, expressed in alveolar epithelial cells type II (AEC-II), and playing an essential role in maintenance of alveolar integrity and host defence. Because expressions of SPs are regulated by cyclic adenosine monophosphate (cAMP), we hypothesized that phosphodiesterase (PDE) inhibitors, influence SP expression and release. Analysis of PDE activity of our AEC-II preparations revealed that PDE4 is the major cAMP hydrolysing PDE in human adult AEC-II. Thus, freshly isolated human AEC-II were stimulated with two different concentrations of the PDE4 inhibitor roflumilast-N-oxide (3 nM and 1 µM) to investigate the effect on SP expression. SP mRNA levels disclosed a large inter-individual variation. Therefore, the experiments were grouped by the basal SP expression in low and high expressing donors. AEC-II stimulated with Roflumilast-N-oxide showed a minor increase in SP-A1, SP-C and SP-D mRNA mainly in low expressing preparations. To overcome the effects of different basal levels of intracellular cAMP, cyclooxygenase was blocked by indomethacin and cAMP production was reconstituted by prostaglandin E2 (PGE2). Under these conditions SP-A1, SP-A2, SP-B and SP-D are increased by roflumilast-N-oxide in low expressing preparations. Roflumilast-N-oxide fosters the expression of SPs in human AEC-II via increase of intracellular cAMP levels potentially contributing to improved alveolar host defence and enhanced resolution of inflammation.

Genetic variant associations of human SP-A and SP-D with acute and chronic lung injury

Pulmonary surfactant, a lipoprotein complex, maintains alveolar integrity and plays an important role in lung host defense, and control of inflammation. Altered inflammatory processes and surfactant dysfunction are well described events that occur in patients with acute or chronic lung disease that can develop secondary to a variety of insults. Genetic variants of surfactant proteins, including single nucleotide polymorphisms, haplotypes, and other genetic variations have been associated with acute and chronic lung disease throughout life in several populations and study groups. The hydrophilic surfactant proteins SP-A and SP-D, also known as collectins, in addition to their surfactant-related functions, are important innate immunity molecules as these, among others, exhibit the ability to bind and enhance clearance of a wide range of pathogens and allergens. This review focuses on published association studies of human surfactant proteins A and D genetic polymorphisms with respiratory, and non-respiratory diseases in adults, children, and newborns. The potential role of genetic variations in pulmonary disease or pathogenesis is discussed following an evaluation, and comparison of the available literature.

Genetic complexity of the human innate host defense molecules, surfactant protein A1 (SP-A1) and SP-A2--impact on function

Innate immunity mechanisms play a critical role in the primary response to invading pathogenic microorganisms and other insulting agents. The innate lung immune system includes lung surfactant, a lipoprotein complex that carries out a function essential for life, that is, reduction of the surface tension at the air-liquid interphase of the alveolar space. By means of this function, pulmonary surfactant prevents lung collapse, therefore ensuring normal lung function and lung health. Pulmonary surfactant contains a number of host-defense molecules that are involved in the elimination of pathogens, viruses, particles, allergens, and other insults, as well as in the control of inflammation. This review is concerned with one of the surfactant proteins, the human (h) surfactant protein A (hSP-A), which, in addition to its role in surfactant-related functions, plays an important role in the modulation of lung host defense. The hSP-A locus has been identified with extensive complexity that may have an impact on its function, structure, and regulation. In humans, two genes--SP-A1 (SFTPA1) and SP-A2 (SFTPA2)--encode SP-A, with SP-A2 gene products being more biologically active than SP-A1 in most of the in vitro assays investigated. Although the two hSP-A genes share a high level of sequence similarity, differences in the structure and function between SP-A1 and SP-A2 have been observed in recent studies. In this review, we discuss the human SP-A complexity and how this may affect SP-A function.

Immunohistochemical distribution of SP-D, compared with that of SP-A and KL-6, in interstitial pneumonias

The immunohistochemical distribution of SP-D was compared with that of SP-A and KL-6 using a monoclonal antibody in lung tissues of 15 cases of collagen vascular disease-associated interstitial pneumonia (CVD-IP), 4 cases of hypersensitivity pneumonitis (CHP), and 6 cases of other diseases to determine their differences in distribution. In this study, the main targets were alveolar epithelial cells, especially those in the regenerating stage, as well as lymph vessels and stroma. The cytoplasm of type II alveolar epithelial cells and Clara cells was positive for SP-D, with sharp margins; interestingly, however, during the process of regeneration large positive cells were intermingled with relatively small negative cells, even in the same row of cells. In sharp contrast, staining for SP-A and KL-6 was positive in the cytoplasm of all the regenerating alveolar epithelial cells, as well as Clara cells. Staining for KL-6 was usually positive in the surface of air spaces in linear fashion. Staining for SP-A was also positive in elastic fibers in vascular walls. In areas of destruction of pulmonary structures, loose stroma and the endothelial cells of lymph vessels as well as their contents were distinctly positive for SP-A and/or KL-6 but not SP-D. Judging from these results in pulmonary tissues of CVD-IP and HP, SP-D might be a marker for maturity of regenerating epithelial cells. Both SP-A and KL-6 were detected in intimate relationship to the stage of regeneration of alveolar epithelial cells and were expressed before SP-D. In addition, the lymph vessels play a very important role in transfer of KL-6 into the bloodstream.

Surfactant protein A and surfactant protein D variation in pulmonary disease

Surfactant proteins A (SP-A) and D (SP-D) have been implicated in pulmonary innate immunity. The proteins are host defense lectins, belonging to the collectin family which also includes mannan-binding lectin (MBL). SP-A and SP-D are pattern-recognition molecules with the lectin domains binding preferentially to sugars on a broad spectrum of pathogen surfaces and thereby facilitating immune functions including viral neutralization, clearance of bacteria, fungi and apoptotic and necrotic cells, modulation of allergic reactions, and resolution of inflammation. SP-A and SP-D can interact with receptor molecules present on immune cells leading to enhanced microbial clearance and modulation of inflammation. SP-A and SP-D also modulate the functions of cells of the adaptive immune system including dendritic cells and T cells. Studies on SP-A and SP-D polymorphisms and protein levels in bronchoalveolar lavage and blood have indicated associations with a multitude of pulmonary inflammatory diseases. In addition, accumulating evidence in mouse models of infection and inflammation indicates that recombinant forms of the surfactant proteins are biologically active in vivo and may have therapeutic potential in controlling pulmonary inflammatory disease. The presence of the surfactant collectins, especially SP-D, in non-pulmonary tissues, such as the gastrointestinal tract and genital organs, suggest additional actions located to other mucosal surfaces. The aim of this review is to summarize studies on genetic polymorphisms, structural variants, and serum levels of human SP-A and SP-D and their associations with human pulmonary disease.

Surfactant protein A2 (SP-A2) variants expressed in CHO cells stimulate phagocytosis of Pseudomonas aeruginosa more than do SP-A1 variants

Surfactant protein A (SP-A) enhances phagocytosis of Pseudomonas aeruginosa. Two functional genes, SP-A1 and SP-A2, encode human SP-A. As we showed before, baculovirus-mediated insect cell-expressed SP-A2 enhances the association of P. aeruginosa with rat alveolar macrophages (rAMs) more than does SP-A1. However, true phagocytosis (internalization) was not shown, and insect cell derived proteins lack or are defective in certain mammalian posttranslational modifications that may be important for SP-A1 and SP-A2 activity and specificity. Here we used SP-A1 (6A(2), 6A(4)) and SP-A2 (1A(0), 1A(1)) allele variants expressed by CHO (Chinese hamster ovary) mammalian cells to study their effect on association and/or internalization of P. aeruginosa by rAMs and/or human AMs (hAMs) and to study if phagocytosis can be modulated differentially and/or more effectively by CHO cell-expressed SP-A variants than by insect-cell expressed SP-A variants. For cell association and internalization assessments, light microscopy and fluorescence-activated cell sorter analyses were used, respectively. We found the following for the first time. (i) SP-A2 variants enhanced phagocytosis (cell association and/or internalization) of P. aeruginosa more than SP-A1 variants did, and the cell association correlated with internalization. (ii) Differences in the activities of SP-A variants were observed in the following order: 1A(1)>1A(0)>6A(2)>6A(4). (iii) rAMs, although more active than hAMs, are an appropriate model, as SP-A2 variants exhibited activity higher than that seen for SP-A1 variants with either rAMs or hAMs. (iv) CHO cell-expressed SP-A was considerably more active than insect cell-expressed variants. We conclude that SP-A2 variants stimulate phagocytosis of P. aeruginosa more effectively than SP-A1 variants and that posttranslational modifications positively influence the phagocytic activity of SP-A.

Elevated serum surfactant protein A and D in pulmonary alveolar microlithiasis

Pulmonary alveolar microlithiasis (PAM) is a rare disease characterized by widespread localization of calcispherites in the alveolar spaces. The authors report two cases of PAM, with markedly elevated sera concentrations of surfactant protein-A and surfactant protein-D, which showed a tendency to increase as the disease progressed. Therefore, surfactant protein-A and surfactant protein-D may function as serum markers to monitor the disease activity and progression of PAM.

Serum KL-6 and Surfactant Proteins A and D in Pediatric Interstitial Lung Disease

OBJECTIVE

To determine if serum KL-6, surfactant protein A (SP-A), and surfactant protein D (SP-D) levels are elevated in pediatric interstitial lung disease (ILD) and associated with pulmonary function and disease severity score.

METHODS

Serum KL-6, SP-A, and SP-D levels were measured by enzyme-linked immunosorbent assay in 10 children with ILD and in 10 healthy volunteers. In the ILD group, FEV1 percentage of predicted, FVC percentage of predicted, and ILD disease severity score were measured and correlated with serum KL-6, SP-A, and SP-D levels.

RESULTS

For the ILD and control groups, respectively, mean serum KL-6 was 4,523 U/mL and 206 U/mL (p = 0.007), mean serum SP-A was 133 ng/mL and 21 ng/mL (p = 0.003), and mean serum SP-D was 304 ng/mL and 75 ng/mL (p = 0.004). There was an inverse relationship between SP-A and FVC (p = 0.05), and between SP-D and FEV1 (p = 0.05). There was a direct relationship between SP-D and ILD score (p = 0.05).

CONCLUSIONS

Serum KL-6, SP-D and SP-D levels are elevated in children with ILD. SP-A and SP-D levels appear to correlate with some measures of disease severity.

SP-A1 and SP-A2 variants differentially enhance association ofPseudomonas aeruginosawith rat alveolar macrophages

Chronic airway inflammation caused by Pseudomonas aeruginosa is an important feature of cystic fibrosis (CF). Surfactant protein A (SP-A) enhances phagocytosis of P. aeruginosa. Two genes, SP-A1 and SP-A2, encode human SP-A. We hypothesized that genetically determined differences in the activity of SP-A1 and SP-A2 gene products exist. To test this, we studied association of a nonmucoid P. aeruginosa strain (ATCC 39018) with rat alveolar macrophages in the presence or absence of insect cell-expressed human SP-A variants. We used two trios, each consisting of SP-A1, SP-A2, and their coexpressed SP-A1/SP-A2 variants. We tested the 6A2and 6A4alleles (for SP-A1), the 1A0and 1A alleles (for SP-A2), and their respective coexpressed SP-A1/SP-A2 gene products. After incubation of alveolar macrophages with P. aeruginosa in the presence of the SP-A variants at 37°C for 1 h, the cell association of bacteria was assessed by light microscopy analysis. We found 1) depending on SP-A concentration and variant, SP-A2 variants significantly increased the cell association more than the SP-A1 variants (the phagocytic index for SP-A1 was ∼52–95% of the SP-A2 activity); 2) coexpressed variants at certain concentrations were more active than single gene products; and 3) the phagocytic index for SP-A variants was ∼18–41% of the human SP-A from bronchoalveolar lavage. We conclude that human SP-A variants in vitro enhance association of P. aeruginosa with rat alveolar macrophages differentially and in a concentration-dependent manner, with SP-A2 variants having a higher activity compared with SP-A1 variants.

Elevated serum surfactant protein A and D in a case of acute eosinophilic pneumonia

A 29-year-old man was admitted to our hospital because of fever elevation, dry cough, malaise, skin eruption, and dyspnea with hypoxemia. His serum levels of surfactant protein (SP) -A and SP-D were markedly high, but serum KL-6 was not. He was diagnosed as acute eosinophilic pneumonia (AEP) on the basis of CT imaging, bronchoalveolar lavage findings and the clinical course. He showed good response to steroid therapy and serum levels of SP-A and SP-D returned to almost normal levels. Our experience suggested that serum SP-A and SP-D might be helpful markers for monitoring the clinical course in AEP.

Family-based transmission disequilibrium test (TDT) and case-control association studies reveal surfactant protein A (SP-A) susceptibility alleles for respiratory distress syndrome (RDS) and possible race differences

A key cause of respiratory distress syndrome (RDS) in the prematurely born infant is deficiency of pulmonary surfactant, a lipoprotein complex. Both low levels of surfactant protein A (SP-A) and SP-A alleles have been associated with RDS. Using the candidate gene approach, we performed family-based linkage studies to discern linkage of SP-A to RDS and identify SP-A susceptibility or protective alleles. Moreover, we performed case-control studies of whites and blacks to detect association between RDS and SP-A alleles. Transmission disequilibrium test (TDT) analysis revealed that the frequency of transmission (from parent to the offspring with RDS) of alleles 6A(2) and 1A(0) and of 1A(0)/6A(2) haplotype in RDS was increased, whereas transmission of alleles 1A(5) and 6A(4) and of haplotype 1A(5)/6A(4) was decreased. Extended TDT analysis further strengthened the observations made. The case-control studies showed that in whites or blacks with RDS the frequencies of specific genotypes, 1A(0) and 6A(2) or 1A(0), were increased, respectively, but the frequency of specific 6A(3) genotypes was increased in certain white subgroups and decreased in blacks. Regression analysis revealed gestational age (GA) and 6A(3) genotypes are significant factors in blacks with RDS. In whites with RDS, GA and antenatal steroids are important factors. The data together indicate linkage between SP-A and RDS; certain SP-A alleles/haplotypes are susceptibility (1A(0), 6A(2), 1A(0)/6A(2)) or protective (1A(5), 6A(4), 1A(5)/6A(4)) factors for RDS. Some differences between blacks and whites with regard to SP-A alleles may exist.

Enhanced clearance of surfactant protein D during LPS-induced acute inflammation in rat lung

Pulmonary surfactant participates in the regulation of alveolar compliance and lung host defense. Surfactant homeostasis is regulated through a combination of synthesis, secretion, clearance, recycling, and degradation of surfactant components. The extracellular pool size of surfactant protein (SP) D fluctuates significantly during acute inflammation. We hypothesized that changes in SP-D levels are due, in part, to altered clearance of SP-D. Clearance pathways in rats were assessed with fluorescently labeled SP-D that was instilled into control lungs or lungs that had been treated with lipopolysaccharide (LPS) 16 h earlier. SP-D clearance from lavage into lung tissue was time dependent from 5 min to 1 h and 1.7-fold greater in LPS-treated lungs than in control lungs. Analysis of cells isolated by enzymatic digestion of lung tissue revealed differences in the SP-D-positive cell population between groups. LPS-treated lungs had 28.1-fold more SP-D-positive tissue-associated neutrophils and 193.6-fold greater SP-D association with those neutrophils compared with control lungs. These data suggest that clearance of SP-D into lung tissue is increased during inflammation and that tissue-associated neutrophils significantly contribute to this process.

Diagnostic significance of surfactant proteins A and D in sera from patients with radiation pneumonitis

Radiation pneumonitis (RP) is the most common complication of radiotherapy for thoracic tumours. The aim of this study was to evaluate the significance of pulmonary surfactant proteins (SP)-A and SP-D as new serum markers for RP. Twenty-five patients with lung tumour, who had received radiotherapy, were studied. At the completion of radiotherapy, the presence of RP was judged by chest plain radiography and chest high resolution computed tomography (HRCT). RP findings detected on chest plain radiography were seen in only three of 12 patients in whom RP was detected by HRCT. Nevertheless, both SP-A and SP-D concentrations in sera from the patients with RP were significantly higher than those from the 13 patients without RP (p = 0.0065, p = 0.0011, respectively). As with SP-A, ratios of SP-D at the completion, compared to at the initiation (1 week post/pre ratio), were also significantly higher in patients with RP than in patients without RP. When a post/pre ratio > 1.6 was considered positive, the SP-A and SP-D assays showed an 83% and 85% specificity, respectively. In conclusion, serum assays of surfactant proteins A and D may be of diagnostic value for detection of radiation pneumonitis, even when the radiographic change is faint.

Serum levels of surfactant proteins A and D are useful biomarkers for interstitial lung disease in patients with progressive systemic sclerosis

To find a less-invasive and lung-specific clinical biomarker, we measured serum levels of surfactant proteins A and D (SP-A and SP-D) by sandwich enzyme-linked immunosorbent assays in 42 patients with progressive systemic sclerosis (PSS) to evaluate their significance in relation to the presence of interstitial lung disease (ILD) and to assess their diagnostic merits. The patients were divided into two groups based on findings by chest computed tomography (CT): 30 patients with ILD (CT-positive ILD group), and 12 patients without any lung abnormalities (CT-negative ILD group). The CT-positive ILD group was further divided into two groups: 24 patients with ILD detectable by chest plain radiography (X-ray-positive ILD group) and six patients with ILD showing no abnormality (X-ray-negative ILD group). The levels of SP-A and SP-D in sera were significantly higher in the CT-positive ILD group than in the CT-negative ILD group. They were also significantly higher in the X-ray-positive ILD group than in the CT-negative ILD group. In the X-ray-negative ILD group, their levels were higher than those of the CT-negative ILD group. We next estimated sensitivity and specificity of SP-A, SP-D, and X-ray for detecting ILD on CT. Sensitivity of SP-D was high (77%) as well as that of X-ray (80%), whereas SP-A showed a low sensitivity (33%). Remarkably, five of six patients in the X-ray-negative ILD group showed SP-D concentrations over its cut-off level, thereby demonstrating that an SP-D assay contributes to the detection of ILD overlooked by X-ray. Moreover, a combination of X-ray and SP-D dramatically increases sensitivity to 97%. Specificity of SP-A, SP-D, and X-ray to the CT-negative ILD group was 100%, 83%, and 100%, respectively. In conclusion, this study indicates that elevated levels of serum SP-A and SP-D reflect well the presence of ILD and that the combination of SP-D and X-ray contributes to reduce the risk of clinicians overlooking ILD complicated by PSS, although a repetition in another set of subjects is needed to confirm these indications.

Genetics of the hydrophilic surfactant proteins A and D

The use of candidate genes has increased the ability to identify genetic factors involved in diseases with complex and multifactorial etiology. The surfactant proteins (SP) A and D are involved in host defense and inflammatory processes of the lung, which are often components of pulmonary disease. Therefore, the SP-A and SP-D genes make particularly good candidates to study factors contributing to pulmonary disease etiopathogenesis. Moreover, SP-A also plays a role in the surface tension lowering abilities of pulmonary surfactant, which is essential for normal lung function. Although genetic variability at the SP-D locus may exist among humans, allelic variants have not yet been characterized. On the other hand, the human SP-A genes (SP-A1 and SP-A2) are characterized by genetically dependent splice variants at the 5' untranslated region and allelic variants. The polymorphisms that give rise to SP-A1 and SP-A2 alleles are contained within coding regions, potentially having an effect on protein function. There appears to be a correlation between SP-A genotype and SP-A mRNA content. Furthermore, one SP-A2 allele (1A0) shown to associate with low SP-A mRNA levels is found with higher frequency in a subgroup with respiratory distress syndrome. The evidence gathered thus far indicates that SP-A, possibly by interacting with other surfactant components, may play a role (e.g. be a susceptibility factor) in the development of respiratory disease.

Surfactant proteins A and D: disease markers

The abundant and restricted expression of surfactant proteins SP-A and SP-D within the lung makes these collectins specific markers for lung diseases. The measurement of SP-A and SP-D in amniotic fluids and tracheal aspirates reflects lung maturity and the production level of the lung surfactant in infants with respiratory distress syndrome (RDS). The SP-A concentrations in bronchoalveolar lavage (BAL) fluids are significantly decreased in patients with acute respiratory distress syndrome (ARDS) and also in patients at risk to develop ARDS. The prominent increase of these proteins in BAL fluids and sputum is diagnostic for pulmonary alveolar proteinosis (PAP). The concentrations of SP-A and SP-D in BAL fluids from patients with idiopathic pulmonary fibrosis (IPF) and interstitial pneumonia with collagen vascular diseases (IPCD) are rather lower than those in healthy controls and the SP-A/phospholipid ratio may be a useful marker of survival prediction. SP-A and SP-D appear in the circulation in specific lung diseases. Their serum concentrations significantly increase in patients with PAP, IPF and IPCD. The successive monitoring of serum levels of SP-A and SP-D may predict the disease activity. The serum SP-A levels increase in patients with ARDS. SP-A is also a marker for lung adenocarcinomas and can be used to differentiate lung adenocarcinomas from other types and metastatic cancers from other origins, and to detect metastasis of lung adenocarcinomas.