Serum Surfactant Protein D as a Biomarker for Measuring Lung Involvement in Obese Patients With Type 2 Diabetes

Lung molecular and histological changes in type 2 diabetic rats and its improvement by high-intensity interval training

BACKGROUND

Type 2 diabetes (T2D) leads to serious respiratory problems. This study investigated the effectiveness of high-intensity interval training (HIIT) on T2D-induced lung injuries at histopathological and molecular levels.

METHODS

Forty-eight male Wistar rats were randomly allocated into control (CTL), Diabetes (Db), exercise (Ex), and Diabetes + exercise (Db + Ex) groups. T2D was induced by a high-fat diet plus (35 mg/kg) of streptozotocin (STZ) administration. Rats in Ex and Db + Ex performed HIIT for eight weeks. Tumor necrosis factor-alpha (TNFα), Interleukin 10 (IL-10), BAX, Bcl2, Lecithin, Sphingomyelin (SPM) and Surfactant protein D (SPD) levels were measured in the bronchoalveolar lavage fluid (BALF) and malondialdehyde (MDA) and total antioxidant capacity (TAC) levels were measured in lung tissue. Lung histopathological alterations were assessed by using H&E and trichrome mason staining.

RESULTS

Diabetes was significantly associated with imbalance in pro/anti-inflammatory, pro/anti-apoptosis and redox systems, and reduced the SPD, lecithin sphingomyelin and alveolar number. Performing HIIT by diabetic animals increased Bcl2 (P < 0.05) and IL10 (P < 0.01) levels as well as surfactants components and TAC (P < 0.05) but decreased fasting blood glucose (P < 0.001), TNFα (P < 0.05), BAX (P < 0.05) and BAX/Bcl2 (P < 0.001) levels as well as MDA (P < 0.01) and MDA/TAC (P < 0.01) compared to the diabetic group. Furthermore, lung injury and fibrosis scores were increased by T2D and recovered in presence of HIIT.

CONCLUSION

These findings suggested that the attenuating effect of HIIT on diabetic lung injury mediated by reducing blood sugar, inflammation, oxidative stress, and apoptosis as well as improving pulmonary surfactants components.

Intravenous surfactant protein D inhibits lipopolysaccharide-induced systemic inflammation

BACKGROUND

Surfactant protein D (SP-D) is an innate host defense protein that clears infectious pathogens from the lung and regulates pulmonary host defense cells. SP-D is also detected in lower concentrations in plasma and many other non-pulmonary tissues. Plasma levels of SP-D increase during infection and other proinflammatory states; however, the source and functions of SP-D in the systemic circulation are largely unknown. We hypothesized that systemic SP-D may clear infectious pathogens and regulate host defense cells in extrapulmonary systems.

METHODS

To determine if SP-D inhibited inflammation induced by systemic lipopolysaccharide (LPS), E.coli LPS was administered to mice via tail vein injection with and without SP-D and the inflammatory response was measured.

RESULTS

Systemic SP-D has a circulating half-life of 6 h. Systemic IL-6 levels in mice lacking the SP-D gene were similar to wild type mice at baseline but were significantly higher than wild type mice following LPS treatment (38,000 vs 29,900 ng/ml for 20 mg/kg LPS and 100,700 vs 73,700 ng/ml for 40 mg/kg LPS). In addition, treating wild type mice with purified intravenous SP-D inhibited LPS induced secretion of IL-6 and TNFα in a concentration dependent manner. Inhibition of LPS induced inflammation by SP-D correlated with SP-D LPS binding suggesting SP-D mediated inhibition of systemic LPS requires direct SP-D LPS interactions.

CONCLUSIONS

Taken together, the above results suggest that circulating SP-D decreases systemic inflammation and raise the possibility that a physiological purpose of increasing systemic SP-D levels during infection is to scavenge systemic infectious pathogens and limit inflammation-induced tissue injury.

CAN PULMONARY SURFACTANT PROTEINS BE RELIABLE INDICATORS OF COVID-19-ASSOCIATED PULMONARY INJURY?

The COVID-19 pandemic is still raging all over the world. New variants of the coronavirus emerge and infect recovered from previous infections, vaccinated, and unvaccinated subjects. One aspect remains unchanged that is the lungs are the main targets of the pandemic coronavirus. This challenging situation requires the search for reliable predictive markers of severe and complicated course of the disease. Serum surfactant proteins are known to correlate with pulmonary injury severity in numerous diseases. Measurement of such protein levels may help timely predict the risk. Surfactant proteins can also be helpful diagnostic purposes in COVID-19.

Liraglutide Improves Forced Vital Capacity in Individuals With Type 2 Diabetes: Data From the Randomized Crossover LIRALUNG Study

To evaluate the effect of liraglutide, a glucagon-like peptide 1 receptor agonist, on pulmonary function and serum levels of surfactant protein D (SP-D) in type 2 diabetes. A double-blind, randomized, crossover, placebo-controlled clinical trial comprising 76 patients with a baseline forced expiratory volume in 1 s <90% of that predicted. Liraglutide was administered for 7 weeks (2 weeks of titration plus 5 weeks at 1.8 mg daily). This short duration was intentional to minimize weight loss as a potential confounding factor. Serum level of SP-D was used as a biomarker of alveolar-capillary barrier integrity. Liraglutide exerted a positive impact on forced vital capacity (FVC) in comparison with placebo (ΔFVC 5.2% of predicted [from 0.8 to 9.6]; P = 0.009). No differences in the other pulmonary variables were observed. Participants under liraglutide treatment also experienced a decrease in serum SP-D (P = 0.038). The absolute change in FVC correlated with final serum SP-D in participants receiving liraglutide (r = -0.313, P = 0.036). Stepwise multivariate regression analysis showed that final serum SP-D independently predicted changes in FVC. In conclusion, liraglutide increased FVC in patients with type 2 diabetes. This effect was associated with a significant decrease of circulating SP-D, thus pointing to a beneficial effect in the alveolar-capillary function.

Clinical characteristics and genetic analysis of a Chinese pedigree of type 2 diabetes complicated with interstitial lung disease

PURPOSE

Diabetes mellitus is a systemic metabolic disorder which may target the lungs and lead to interstitial lung disease. The clinical characteristics and mechanisms of type 2 diabetes mellitus (T2DM) complicated with interstitial lung disease (ILD) have been studied. However, little work has been done to assess genetic contributions to the development of T2DM complicated with ILD.

METHOD

A pedigree of T2DM complicated with ILD was investigated, and the whole genome re-sequencing was performed to identify the genetic variations in the pedigree. According to the literature, the most valuable genetic contributors to the pathogenesis of T2DM complicated with ILD were screened out, and the related cellular functional experiments were also performed.

RESULTS

A large number of SNPs, InDels, SVs and CNVs were identified in eight subjects including two diabetic patients with ILD, two diabetic patients without ILD, and four healthy subjects from the pedigree. After data analysis according to the literature, MUC5B SNP rs2943512 (A > C) was considered to be an important potentially pathogenic gene mutation associated with the pathogenesis of ILD in T2DM patients. In vitro experiments showed that the expression of MUC5B in BEAS-2B cells was significantly up-regulated by high glucose stimulation, accompanied by the activation of ERK1/2 and the increase of IL-1β and IL-6. When silencing MUC5B by RNA interference, the levels of p-ERK1/2 as well as IL-1β and IL-6 in BEAS-2B cells were all significantly decreased.

CONCLUSION

The identification of these genetic variants in the pedigree enriches our understanding of the potential genetic contributions to T2DM complicated with ILD. MUC5B SNP rs2943512 (A > C) or the up-regulated MUC5B in bronchial epithelial cells may be an important factor in promoting ILD inT2DM patients, laying a foundation for future exploration about the pathogenesis of T2DM complicated with ILD.

Understanding the Co-Epidemic of Obesity and COVID-19: Current Evidence, Comparison with Previous Epidemics, Mechanisms, and Preventive and Therapeutic Perspectives

PURPOSE OF REVIEW

A growing body of evidence suggests that obesity and increased visceral adiposity are strongly and independently linked to adverse outcomes and death due to COVID-19. This review summarizes current epidemiologic data, highlights pathogenetic mechanisms on the association between excess body weight and COVID-19, compares data from previous pandemics, discusses why COVID-19 challenges the "obesity paradox," and presents implications in prevention and treatment as well as future perspectives.

RECENT FINDINGS

Data from meta-analyses based on recent observational studies have indicated that obesity increases the risks of infection from SARS-CoV-2, severe infection and hospitalization, admission to the ICU and need of invasive mechanical ventilation (IMV), and the risk of mortality, particularly in severe obesity. The risks of IMV and mortality associated with obesity are accentuated in younger individuals (age ≤ 50 years old). The meta-inflammation in obesity intersects with and exacerbates underlying pathogenetic mechanisms in COVID-19 through the following mechanisms and factors: (i) impaired innate and adaptive immune responses; (ii) chronic inflammation and oxidative stress; (iii) endothelial dysfunction, hypercoagulability, and aberrant activation of the complement; (iv) overactivation of the renin-angiotensin-aldosterone system; (v) overexpression of the angiotensin-converting enzyme 2 receptor in the adipose tissue; (vi) associated cardiometabolic comorbidities; (vii) vitamin D deficiency; (viii) gut dysbiosis; and (ix) mechanical and psychological issues. Mechanistic and large epidemiologic studies using big data sources with omics data exploring genetic determinants of risk and disease severity as well as large randomized controlled trials (RCTs) are necessary to shed light on the pathways connecting chronic subclinical inflammation/meta-inflammation with adverse COVID-19 outcomes and establish the ideal preventive and therapeutic approaches for patients with obesity.

COVID-19 and endocrine and metabolic diseases. An updated statement from the European Society of Endocrinology

BACKGROUND

COVID-19 has completely changed our daily clinical practice as well as our social relations. Many organs and biological systems are involved in SARS-Cov-2 infection, either due to direct virus-induced damage or to indirect effects that can have systemic consequences. Endocrine system is not only an exception but its involvement in COVID-19 is so relevant that an "endocrine phenotype" of COVID-19 has progressively acquired clinical relevance.

AIM

We have been appointed by the European Society of Endocrinology (ESE) to update with the current statement ESE members and the whole endocrine community on the emerging endocrine phenotype of COVID-19 and its implication for the prevention and management of the disease.

CONCLUSIONS

Diabetes has a major role in this phenotype since it is one of the most frequent comorbidities associated with severity and mortality of COVID-19. Careful management including treatment modifications may be required for protecting our patients rather with known diabetes from the most dangerous consequences of COVID-19 or hospitalized with COVID-19, but also in patients with SARS-CoV-2 induced newly onset diabetes. Obesity increases susceptibility to SARS-CoV-2 and the risk for COVID-19 adverse outcome. Adequate nutritional management needs to be granted to patients with obesity or undernourishment in order to limit their increased susceptibility and severity of COVID-19 infection. Lack of vitamin D, hypocalcemia and vertebral fractures have also emerged as frequent findings in the hospitalized COVID-19 population and may negatively impact on the outcome of such patients. Also, in patients with adrenal insufficiency prompt adaptation of glucocorticoid doses may be needed. Moreover, in this updated statement role of sex hormones as well as peculiar pituitary and thyroid aspects of COVID-19 have been included. Finally, in view of the mass vaccination, potential implications for endocrine patients should be considered.

Surfactant protein D concentrations in serum and bronchoalveolar lavage fluid from young healthy horses on pasture and in a barn environment

OBJECTIVE

To evaluate surfactant protein D (SP-D) concentrations in serum and bronchoalveolar lavage fluid (BALF) from young healthy horses on pasture or housed in a typical barn.

ANIMALS

20 young healthy horses.

PROCEDURES

Horses were randomly assigned to 1 of 2 groups (pasture, n = 10; barn, 10), and serum and BALF samples were collected for SP-D determination at baseline (all horses on pasture) and 2 weeks and 4 weeks after the barn group of horses was relocated from the pasture to the barn. Other evaluations included physical and tracheoscopic examinations. Findings were compared within and between groups.

RESULTS

Physical and tracheoscopic examinations, CBC, and serum biochemical analysis did not reveal evidence of respiratory disease, and no significant differences were present within and between groups. Serum SP-D concentrations did not significantly differ within and between groups, but BALF SP-D concentrations were significantly lower for the barn group at 2 weeks but not at 4 weeks, compared with baseline. The BALF SP-D concentration-to-BALF total protein concentration ratio was < 1.5 and did not significantly differ within and between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

A mild decrease was evident in the concentration of SP-D in the BALF collected from young healthy horses after 2 weeks of exposure to a barn environment. The clinical importance of this finding remains to be determined.

Pulmonary function tests in type 2 diabetes: a meta-analysis

OBJECTIVES

The aim of this study was to determine the association between type 2 diabetes (T2D) and pulmonary function tests.

METHODS

After conducting an exhaustive literature search, we performed a meta-analysis. We employed the inverse variance method with a random-effects model to calculate the effect estimate as the mean difference (MD) and 95% confidence interval (CI). We calculated the heterogeneity with the I2 statistic and performed a meta-regression analysis by sex, body mass index (BMI), smoking and geographical region. We also conducted a sensitivity analysis according to the studies' publication date, size of the T2D group and the study quality, excluding the study with the greatest weight in the effect.

RESULTS

The meta-analysis included 66 studies (one longitudinal, two case-control and 63 cross-sectional), with 11 134 patients with T2D and 48 377 control participants. The pooled MD (95% CI) for the predicted percentage of forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), forced expiratory flow at 25-75% of FVC, peak expiratory flow, and diffusing capacity of the lung for carbon monoxide were -7.15 (95% CI -8.27, -6.03; p<0.001), -9.21 (95% CI -11.15, -7.26; p<0.001), -9.89 (95% CI -14.42, -5.36; p<0.001), -9.79 (95% CI -13.42, -6.15; p<0.001) and -7.13 (95% CI -10.62, -3.64; p<0.001), respectively. There was no difference in the ratio of FEV1/FVC (95% CI -0.27; -1.63, 1.08; p=0.69). In all cases, there was considerable heterogeneity. The meta-regression analysis showed that between studies heterogeneity was not explained by patient sex, BMI, smoking or geographical region. The findings were consistent in the sensitivity analysis.

CONCLUSIONS

T2D is associated with impaired pulmonary function, independently of sex, smoking, BMI and geographical region. Longitudinal studies are needed to investigate outcomes for patients with T2D and impaired pulmonary function.

When Two Pandemics Meet: Why Is Obesity Associated with Increased COVID-19 Mortality?

A growing body of evidence indicates that obesity is strongly and independently associated with adverse outcomes of COVID-19, including death. By combining emerging knowledge of the pathological processes involved in COVID-19 with insights into the mechanisms underlying the adverse health consequences of obesity, we present some hypotheses regarding the deleterious impact of obesity on the course of COVID-19. These hypotheses are testable and could guide therapeutic and preventive interventions. As obesity is now almost ubiquitous and no vaccine for COVID-19 is currently available, even a modest reduction in the impact of obesity on mortality and morbidity from this viral infection could have profound consequences for public health.

Endocrine and metabolic aspects of the COVID-19 pandemic

COVID-19 infection has tremendously impacted our daily clinical practice as well as our social living organization. Virtually all organs and biological systems suffer from this new coronavirus infection, either because the virus targets directly specific tissues or because of indirect effects. Endocrine diseases are not an exception and some of endocrine organs are at risk of direct or indirect lesion by COVID-19. Although there is still no evidence of higher predisposition to contract the infection in patients with diabetes and/or obesity, the coexistence of these conditions contributes to a worse prognosis because both conditions confer an impaired immunologic system. Cytokines storm can be amplified by these two latter conditions thereby leading to multisystemic failure and death. Glycaemic control has been demonstrated to be crucial to avoiding long hospital stays, ICU requirement and also prevention of excessive mortality. Endocrine treatment modifications as a consequence of COVID-19 infection are required in a proactive manner, in order to avoid decompensation and eventual hospital admission. This is the case of diabetes and adrenal insufficiency in which prompt increase of insulin dosage and substitutive adrenal steroids through adoption of the sick day's rules should be warranted, as well as easy contact with the health care provider through telematic different modalities. New possible endocrinological targets of COVID-19 have been recently described and warrant a full study in the next future.

Surfactant Protein D Recognizes Multiple Fungal Ligands: A Key Step to Initiate and Intensify the Anti-fungal Host Defense

With limited therapeutic options and associated severe adverse effects, fungal infections are a serious threat to human health. Innate immune response mediated by pattern recognition proteins is integral to host defense against fungi. A soluble pattern recognition protein, Surfactant protein D (SP-D), plays an important role in immune surveillance to detect and eliminate human pathogens. SP-D exerts its immunomodulatory activity via direct interaction with several receptors on the epithelial cells lining the mucosal tracts, as well as on innate and adaptive immune cells. Being a C-type lectin, SP-D shows calcium- and sugar-dependent interactions with several glycosylated ligands present on fungal cell walls. The interactome includes cell wall polysaccharides such as 1,3-β-D-glucan, 1,6-β-D-glucan, Galactosaminogalactan Galactomannan, Glucuronoxylomannan, Mannoprotein 1, and glycosylated proteins such as gp45, gp55, major surface glycoprotein complex (gpA). Recently, binding of a recombinant fragment of human SP-D to melanin on the dormant conidia of Aspergillus fumigatus was demonstrated that was not inhibited by sugars, suggesting a likely protein-protein interaction. Interactions of the ligands on the fungal spores with the oligomeric forms of full-length SP-D resulted in formation of spore-aggregates, increased uptake by phagocytes and rapid clearance besides a direct fungicidal effect against C. albicans. Exogenous administration of SP-D showed significant therapeutic potential in murine models of allergic and invasive mycoses. Altered susceptibility of SP-D gene-deficient mice to various fungal infections emphasized relevance of SP-D as an important sentinel of anti-fungal immunity. Levels of SP-D in the serum or lung lavage were significantly altered in the murine models and patients of fungal infections and allergies. Here, we review the cell wall ligands of clinically relevant fungal pathogens and allergens that are recognized by SP-D and their impact on the host defense. Elucidation of the molecular interactions between innate immune humoral such as SP-D and fungal pathogens would facilitate the development of novel therapeutic interventions.

SURFACTANT PROTEIN D LEVELS WITH OBESITY AND TYPE 2 DIABETES MELLITUS

Surfactant protein D (SP-D) is an important component of pulmonary innate immunity. It is mainly produced by type 2 alveolar and bronchial epithelial cells, but is also found in extra pulmonary tissues and blood. It acts as a primary host defense against inhaled microorganisms. It also enhances adaptive immunity by activating T cells. SP-D deficiency can lead to upper and lower respiratory tract infections. Obesity has reached global epidemic proportions in both adults and children and is associated with numerous co-morbidities and insulin resistance. Type 2 diabetes & obesity are highly associated with recurrent pulmonary & extra pulmonary infections. The primary objective of this study was to determine the association of serum surfactant protein D levels with obesity and type 2 diabetes mellitus.

The Dual Role of Surfactant Protein-D in Vascular Inflammation and Development of Cardiovascular Disease

Cardiovascular disease (CVD) is responsible for 31% of all global deaths. Atherosclerosis is the major cause of cardiovascular disease and is a chronic inflammatory disorder in the arteries. Atherosclerosis is characterized by the accumulation of cholesterol, extracellular matrix, and immune cells in the vascular wall. Recently, the collectin surfactant protein-D (SP-D), an important regulator of the pulmonary immune response, was found to be expressed in the vasculature. Several in vitro studies have examined the role of SP-D in the vascular inflammation leading to atherosclerosis. These studies show that SP-D plays a dual role in the development of atherosclerosis. In general, SP-D shows anti-inflammatory properties, and dampens local inflammation in the vessel, as well as systemic inflammation. However, SP-D can also exert a pro-inflammatory role, as it stimulates C-C chemokine receptor 2 inflammatory blood monocytes to secrete tumor necrosis-factor α and increases secretion of interferon-γ from natural killer cells. In vivo studies examining the role of SP-D in the development of atherosclerosis agree that SP-D plays a proatherogenic role, with SP-D knockout mice having smaller atherosclerotic plaque areas, which might be caused by a decreased systemic inflammation. Clinical studies examining the association between SP-D and cardiovascular disease have reported a positive association between circulatory SP-D level, carotid intima-media thickness, and coronary artery calcification. Other studies have found that circulatory SP-D is correlated with increased risk of both total and cardiovascular disease mortality. Both in vitro, in vivo, and clinical studies examining the relationship between SP-D and CVDs will be discussed in this review.

Lung function measurements in the prediabetes stage: data from the ILERVAS Project

AIMS

Patients with type 2 diabetes have been considered a susceptible group for pulmonary dysfunction. Our aim was to assess pulmonary function on the prediabetes stage.

METHODS

Pulmonary function was assessed in 4,459 non-diabetic subjects, aged between 45 and 70 years, without cardiovascular disease or chronic pulmonary obstructive disease from the ongoing study ILERVAS. A "restrictive spirometric pattern", an "abnormal FEV1" and an "obstructive ventilatory defect" were assessed. Prediabetes was defined by glycosylated hemoglobin (HbA1c) between 5.7 and 6.4% according to the American Diabetes Association criteria.

RESULTS

Population was composed of 52.1% women, aged 57 [53;63] years, a BMI of 28.6 [25.8;31.8] kg/m², and with a prevalence of prediabetes of 29.9% (n = 1392). Subjects with prediabetes had lower forced vital capacity (FVC: 93 [82;105] vs. 96 [84;106], p < 0.001) and lower forced expired volume in the first second (FEV1: 94 [82;107] vs. 96 [84;108], p = 0.011), as well as a higher percentage of the restrictive spirometric pattern (16.5% vs. 13.6%, p = 0.015) and FEV1 < 80% (20.3% vs. 17.2%, p = 0.017) compared to non-prediabetes group. In the prediabetes group, HbA1c was negatively correlated with both pulmonary parameters (FVC: r = - 0.113, p < 0.001; FEV1: r = - 0.079, p = 0.003). The multivariable logistic regression model in the whole population showed that there was a significant and independent association between HbA1c with both restrictive spirometric pattern [OR = 1.42 (1.10-1.83), p = 0.008] and FEV1 < 80% [OR = 1.50 (1.19-1.90), p = 0.001].

CONCLUSIONS

The deleterious effect of type 2 diabetes on pulmonary function appears to be initiated in prediabetes, and it is related to metabolic control. TRIAL REGISTRATION CLINICALTRIALS.GOV: NCT03228459.

Effect of Glucose Improvement on Spirometric Maneuvers in Patients With Type 2 Diabetes: The Sweet Breath Study

OBJECTIVE

Type 2 diabetes exerts a deleterious effect on lung function. However, it is unknown whether an improvement in glycemic control ameliorates pulmonary function.

RESEARCH DESIGN AND METHODS

Prospective interventional study with 60 patients with type 2 diabetes and forced expiratory volume in 1 s (FEV1) ≤90% of predicted. Spirometric maneuvers were evaluated at baseline and after a 3-month period in which antidiabetic therapy was intensified. Those with an HbA_1c reduction of ≥0.5% were considered to be good responders (n = 35).

RESULTS

Good responders exhibited a significant improvement in spirometric values between baseline and the end of the study (forced vital capacity [FVC]: 78.5 ± 12.6% vs. 83.3 ± 14.7%, P = 0.029]; FEV1: 75.6 ± 15.3% vs. 80.9 ± 15.4%, P = 0.010; and peak expiratory flow [PEF]: 80.4 ± 21.6% vs. 89.2 ± 21.0%, P = 0.007). However, no changes were observed in the group of nonresponders when the same parameters were evaluated (P = 0.586, P = 0.987, and P = 0.413, respectively). Similarly, the initial percentage of patients with a nonobstructive ventilatory defect and with an abnormal FEV1 decreased significantly only among good responders. In addition, the absolute change in HbA_1c inversely correlated to increases in FEV1 (r = -0.370, P = 0.029) and PEF (r = -0.471, P = 0.004) in the responders group. Finally, stepwise multivariate regression analysis showed that the absolute change in HbA_1c independently predicted increased FEV1 (R ² = 0.175) and PEF (R ² = 0.323). In contrast, the known duration of type 2 diabetes, but not the amelioration of HbA_1c, was related to changes in forced expiratory flow between 25% and 75% of the FVC.

CONCLUSIONS

In type 2 diabetes, spirometric measurements reflecting central airway obstruction and explosive muscle strength exhibit significant amelioration after a short improvement in glycemic control.

Surfactant Protein D in Respiratory and Non-Respiratory Diseases

Surfactant protein D (SP-D) is a multimeric collectin that is involved in innate immune defense and expressed in pulmonary, as well as non-pulmonary, epithelia. SP-D exerts antimicrobial effects and dampens inflammation through direct microbial interactions and modulation of host cell responses via a series of cellular receptors. However, low protein concentrations, genetic variation, biochemical modification, and proteolytic breakdown can induce decomposition of multimeric SP-D into low-molecular weight forms, which may induce pro-inflammatory SP-D signaling. Multimeric SP-D can decompose into trimeric SP-D, and this process, and total SP-D levels, are partly determined by variation within the SP-D gene, SFTPD. SP-D has been implicated in the development of respiratory diseases including respiratory distress syndrome, bronchopulmonary dysplasia, allergic asthma, and chronic obstructive pulmonary disease. Disease-induced breakdown or modifications of SP-D facilitate its systemic leakage from the lung, and circulatory SP-D is a promising biomarker for lung injury. Moreover, studies in preclinical animal models have demonstrated that local pulmonary treatment with recombinant SP-D is beneficial in these diseases. In recent years, SP-D has been shown to exert antimicrobial and anti-inflammatory effects in various non-pulmonary organs and to have effects on lipid metabolism and pro-inflammatory effects in vessel walls, which enhance the risk of atherosclerosis. A common SFTPD polymorphism is associated with atherosclerosis and diabetes, and SP-D has been associated with metabolic disorders because of its effects in the endothelium and adipocytes and its obesity-dampening properties. This review summarizes and discusses the reported genetic associations of SP-D with disease and the clinical utility of circulating SP-D for respiratory disease prognosis. Moreover, basic research on the mechanistic links between SP-D and respiratory, cardiovascular, and metabolic diseases is summarized. Perspectives on the development of SP-D therapy are addressed.