Serum leptin levels in community acquired pneumonia (CAP) are related to nutritional status and not to acute phase reaction

Obesity, leptin and host defence of Streptococcus pneumoniae: the case for more human research

Pneumococcal pneumonia is the leading cause of community-acquired pneumonia. Obesity is a risk factor for pneumonia. Host factors play a critical role in susceptibility to pulmonary pathogens and outcome from pulmonary infections. Obesity impairs innate and adaptive immune responses, important in the host defence against pneumococcal disease. One area of emerging interest in understanding the complex relationship between obesity and pulmonary infections is the role of the hormone leptin. There is a substantive evidence base supporting the associations between obesity, leptin, pulmonary infections and host defence mechanisms. Despite this, there is a paucity of research that specifically focuses on Streptococcus pneumoniae (pneumococcal) infections, which are the leading cause of community-acquired pneumonia hospitalisations and mortality worldwide. Much of the evidence examining the role of leptin in relation to S. pneumoniae infections has used genetically mutated mice. The purpose of this mini review is to explore the role leptin plays in the host defence of S. pneumoniae in subjects with obesity and posit an argument for the need for more human research.

Leptin in the Respiratory Tract: Is There a Role in SARS-CoV-2 Infection?

Leptin is a pleiotropic adipocytokine involved in several physiologic functions, with a known role in innate and adaptive immunity as well as in tissue homeostasis. Long- and short-isoforms of leptin receptors are widely expressed in many peripheral tissues and organs, such as the respiratory tract. Similar to leptin, microbiota affects the immune system and may interfere with lung health through the bidirectional crosstalk called the “gut-lung axis.” Obesity leads to impaired protective immunity and altered susceptibility to pulmonary infections, as those by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although it is known that leptin and microbiota link metabolism and lung health, their role within the SARS-CoV2 coronavirus disease 2019 (COVID-19) deserves further investigations. This review aimed to summarize the available evidence about: (i) the role of leptin in immune modulation; (ii) the role of gut microbiota within the gut-lung axis in modulating leptin sensitivity; and (iii) the role of leptin in the pathophysiology of COVID-19.

Serum leptin level as a diagnostic and prognostic marker in infectious diseases and sepsis: A comprehensive literature review

BACKGROUND

Infections and sepsis are common causes of morbidity and mortality, with an increasing incidence worldwide. Leptin is involved in the inflammatory process and may modulate the cytokine production, immune cell proliferation and endothelial function. There are conflicting results regarding alterations of leptin levels in infectious diseases and the outcome from sepsis.The aim of the current article is to provide an overview of the medical literature on the correlations between variations of leptin levels and infectious diseases and sepsis.

METHODS

We performed an extensive literature search in PubMed and Google Scholar databases, using keywords to identify articles related to leptin in infectious diseases and sepsis. Searches were referenced using medical subject headings that included "leptin," "adipokines," "sepsis," "infectious diseases," "leptin deficiency," "leptin resistance" or "hyperleptinemia." The language of publication, journal, or country were not included as limitation criteria.Articles or abstracts containing adequate information, such as age, sex, anthropometric indices, clinical presentation, comorbidities, and management were included in the study, whereas articles with insufficient clinical and demographic data were excluded. We assessed the quality of the studies selected.The final review of all databases was conducted on June 18, 2020.

RESULTS

We find the results from the current review to be of great importance due to the possible therapeutic role of leptin analogs in states of leptin deficiency associated with infectious diseases or sepsis.In hyperleptinemia, a therapeutic plan for obtaining leptin neutralization also needs further investigations. This could lead to the reduction of proinflammatory responses.There is a need for further studies to demonstrate the specificity and sensitivity of leptin in the early diagnosis of sepsis and the need to measure serum leptin levels in routine evaluation of the critical patient.

CONCLUSION

The multiple effects of leptin are of growing interest, but further studies are needed to elucidate the role of leptin signalling in infectious diseases and sepsis. Because very few human studies are reported, we recommend the need for further research.Better understanding of the pathophysiology of sepsis and the implication of circulating total leptin in this process could help physicians in managing this life-threatening condition.

Serum Leptin in Hospitalized Community-Acquired Pneumonia Children under the Age of Five Years

Background

Community-acquired pneumonia (CAP) accounts for 19% of the world’s total deaths among all age groups yearly, with highest rates in children less than 5 years. This study is designed to evaluate the serum leptin level in hospitalized children under the age of 5 years with CAP.

Results

This prospective cross-sectional study included CAP children under the age of 5 years. Forty-one patients admitted to pediatric intensive care unit (PICU) and 41 patients admitted to general ward were enrolled. Patients with any other cause that may elevate serum leptin were excluded. Serum leptin was measured on the day of admission. The PICU patients had a significantly higher median serum leptin than that of the ward patients (p < 0.001). C-reactive protein (CRP) level was significantly higher in patients with elevated serum leptin than in patients with normal serum leptin (p = 0.001). There was a significant association between high serum leptin and positive sputum cultures (p < 0.001), particularly cultures growing more than one organism (p < 0.001). There was a positive, weak correlation between serum leptin and length of stay (r = 0.30, p = 0.007). Serum leptin showed good discrimination between PICU admissions and inpatient ward admissions (AUC = 0.777, p < 0.001); at a cut-off value of > 29.6 pg/ml, serum leptin had a sensitivity of 70.7% and a specificity of 87.8%

Conclusion

We may conclude that CAP patients with a serum leptin level above 29.6 pg/ml should be considered for PICU admission.

The Thousand Faces of Leptin in the Lung

Leptin is a pleotropic hormone known to regulate a wide range of systemic functions, from satiety to inflammation. Increasing evidence has shown that leptin and its receptor (ObR) are not only expressed in adipose tissue but also in several organs, including the lungs. Leptin levels were first believed to be elevated only in the lungs of obese patients, and leptin was suspected to be responsible for obesity-related lung complications. Aside from obesity, leptin displays many faces in the respiratory system, independently of body weight, as this cytokine-like hormone plays important physiological roles, from the embryogenic state to maturation of the lungs and the control of ventilation. The leptin-signaling pathway is also involved in immune modulation and cell proliferation, and its dysregulation can lead to the onset of lung diseases. This review article addresses the thousand faces of leptin and its signaling in the lungs under physiological conditions and in disease.

[Modifications of immunity in obesity: The impact on the risk of infection]

Incidence of obesity is constantly rising all over the world; obesity has developed into an important problem of public health. Clinical experience, supported by many clinical trials, shows that obesity constitutes a risk factor for numerous cardiovascular, metabolic, cancer and even infectious diseases. In this revue we summarize the present knowledge on immunological properties and functions of adipose tissue and their modifications in obese subjects, with a bending to a potentially deleterious chronic inflammatory state. We will discuss the negative impact of this chronic inflammation on physiological acute inflammatory reaction during infectious episodes. However, the modifications of anti-infectious immune response in obese subjects are not well known at present and need further investigations.

Acute Systemic Inflammation is Unlikely to Affect Adiponectin and Leptin Synthesis in Humans

Adipose tissue (AT), classically thought to be merely an energy store, has been shown to produce inflammatory and metabolically active cytokines. Recently, adiponectin and leptin, adipokines primarily synthesized by adipocytes, have attracted considerable attention because inflammation has been suggested to modulate adipokine levels. However, the regulation of adiponectin and leptin is complex and the knowledge about their synthesis within the early onset of inflammation is poorly understood. The aim of this study was to investigate if the synthesis of adiponectin and leptin is affected during the early phase of an acute systemic inflammation. Eighteen healthy subjects were allocated to vaccination against Salmonella typhi or to a control group, and adiponectin and leptin concentrations measured in plasma during 24 h. Nine patients, without markers of inflammation, undergoing open heart surgery were investigated before and after the operation by analysis of plasma levels and AT gene expression of adiponectin and leptin. Plasma interleukin (IL)-6 concentrations were measured in both cohorts. Plasma levels of IL-6 were doubled after vaccination and increased 30-fold after open heart surgery. Plasma levels of adiponectin and leptin were unchanged after vaccination whereas adiponectin and leptin tended to decrease after surgery. The gene expression of adiponectin and leptin was unaltered in omental and subcutaneous AT after surgery. Despite the use of two models of stimulated in vivo systemic inflammation, we found no evidence of an early regulation of adiponectin and leptin synthesis, indicating that these two adipokines are not key elements in an acute systemic inflammation in humans.

Leptin, adiponectin and pulmonary diseases

Adipose tissue produces leptin and adiponectin - energy-regulating adipokines that may also play a role in inflammatory pulmonary conditions, as suggested by some murine studies. Leptin and adiponectin and their respective receptors are expressed in the human lung. The association between systemic or airway leptin and asthma in humans is currently controversial, particularly among adults. The majority of the evidence among children however suggests that systemic leptin may be associated with greater asthma prevalence and severity, particularly among prepubertal boys and peripubertal/postpubertal girls. Systemic and airway leptin concentrations may also be disproportionately higher in chronic obstructive pulmonary disease (COPD) patients, particularly among women, and reflect greater airway inflammation and disease severity. Quite like leptin, the association between systemic and airway adiponectin and asthma in humans is also controversial. Some but not all studies, demonstrate that serum adiponectin concentrations are protective against asthma among premenopausal women and peripubertal girls. On the other hand, serum adiponectin concentrations are inversely associated with asthma severity among boys but positively associated among men. Further, systemic and airway adiponectin concentrations are higher in COPD patients than controls, as demonstrated by case-control studies of men. Systemic adiponectin is also positively associated with lung function in healthy adults but inversely associated with lung function in subjects with COPD. It is therefore possible that pro-inflammatory effects of adiponectin dominate under certain physiologic conditions and anti-inflammatory effects under others. The adipokine-lung disease literature has critical gaps that include a lack of adequately powered longitudinal or weight-intervention studies; inadequate adjustment for confounding effect of obesity; and unclear understanding of potential sex interactions. It is also uncertain whether adipokine derangements precede pulmonary disease or are a consequence of it. Future research will determine whether modulation of adipokines, independent of BMI, may allow novel ways to prevent or treat inflammatory pulmonary conditions.

The role of leptin in the respiratory system: an overview

Since its cloning in 1994, leptin has emerged in the literature as a pleiotropic hormone whose actions extend from immune system homeostasis to reproduction and angiogenesis. Recent investigations have identified the lung as a leptin responsive and producing organ, while extensive research has been published concerning the role of leptin in the respiratory system. Animal studies have provided evidence indicating that leptin is a stimulant of ventilation, whereas researchers have proposed an important role for leptin in lung maturation and development. Studies further suggest a significant impact of leptin on specific respiratory diseases, including obstructive sleep apnoea-hypopnoea syndrome, asthma, COPD and lung cancer. However, as new investigations are under way, the picture is becoming more complex. The scope of this review is to decode the existing data concerning the actions of leptin in the lung and provide a detailed description of leptin's involvement in the most common disorders of the respiratory system.