Leptin Action in the Thymus

The Body Fat Percentage Rather Than the BMI Is Associated with the CD4 Count among HIV Positive Japanese Individuals

Maintenance of the cluster of differentiation 4 (CD4) positive lymphocyte count (CD4 count) is important for human immunodeficiency virus (HIV) positive individuals. Although a higher body mass index (BMI) is shown to be associated with a higher CD4 count, BMI itself does not reflect body composition. Therefore, we examined the association of body weight, body composition and the CD4 count, and determined the optimal ranges of CD4 count associated factors in Japanese HIV positive individuals. This cross-sectional study included 338 male patients treated with antiretroviral therapy for ≥12 months. Multiple logistic regression analysis was used to identify factors significantly associated with a CD4 count of ≥500 cells (mm³)⁻¹. The cutoff values of factors for a CD4 ≥ 500 cells (mm³)⁻¹ and cardiovascular disease risk were obtained by receiver operating characteristic curves. Age, body fat percentage (BF%), nadir CD4 count, duration of antiretroviral therapy (ART), years since the HIV-positive diagnosis and cholesterol intake showed significant associations with the CD4 count. The cutoff value of BF% for a CD4 ≥ 500 cells (mm³)⁻¹ and lower cardiovascular disease risk were ≥25.1% and ≤25.5%, respectively. The BF%, but not the BMI, was associated with CD4 count. For the management of HIV positive individuals, 25% appears to be the optimal BF% when considering the balance between CD4 count management and cardiovascular disease risk.

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.

The Teleost Thymus in Health and Disease: New Insights from Transcriptomic and Histopathological Analyses of Turbot, Scophthalmus maximus

The thymus is a primary lymphoid organ that plays a pivotal role in the adaptive immune system. The immunobiology of the thymus in fish is considered to be similar to that of mammals, but it is actually poorly characterized in several cultured teleost species. In particular, while investigations in human and veterinary medicine have highlighted that the thymus can be affected by different pathological conditions, little is known about its response during disease in fish. To better understand the role of the thymus under physiological and pathological conditions, we conducted a study in turbot (Scophthalmus maximus), a commercially valuable flatfish species, combining transcriptomic and histopathological analyses. The myxozoan parasite Enteromyxum scophthalmi, which represents a major challenge to turbot production, was used as a model of infection. The thymus tissues of healthy fish showed overrepresented functions related to its immunological role in T-cell development and maturation. Large differences were observed between the transcriptomes of control and severely infected fish. Evidence of inflammatory response, apoptosis modulation, and declined thymic function associated with loss of cellularity was revealed by both genomic and morphopathological analyses. This study presents the first description of the turbot thymus transcriptome and provides novel insights into the role of this organ in teleosts’ immune responses.

Leptin Functions in Infectious Diseases

Leptin, a pleiotropic protein has long been recognized to play an important role in the regulation of energy homeostasis, metabolism, neuroendocrine function, and other physiological functions through its effects on the central nervous system (CNS) and peripheral tissues. Leptin is secreted by adipose tissue and encoded by the obese (ob) gene. Leptin acts as a central mediator which regulates immunity as well as nutrition. Importantly, leptin can modulate both innate and adaptive immune responses. Leptin deficiency/resistance is associated with dysregulation of cytokine production, increased susceptibility toward infectious diseases, autoimmune disorders, malnutrition and inflammatory responses. Malnutrition induces a state of immunodeficiency and an inclination to death from communicable diseases. Infectious diseases are the disease of poor who invariably suffer from malnutrition that could result from reduced serum leptin levels. Thus, leptin has been placed at the center of many interrelated functions in various pathogenic conditions, such as bacterial, viruses and parasitic infections. We review herein, the recent advances on the role of leptin in malnutrition in pathogenesis of infectious diseases with a particular emphasis on parasitic diseases such as Leishmaniasis, Trypanosomiasis, Amoebiasis, and Malaria.

Prognostic Value of Fetal Thymus Size in Intrauterine Growth Restriction

OBJECTIVES

Our aim was to evaluate the size of the fetal thymus by sonography in pregnancies with intrauterine growth restriction (IUGR) and to search for a possible relationship between a small fetal thymus and adverse perinatal outcomes.

METHODS

The transverse diameter of the fetal thymus was prospectively measured in 150 healthy and 143 IUGR fetuses between 24 and 40 weeks' gestation. The fetuses with IUGR were further divided according to normal or abnormal Doppler assessment of the umbilical and middle cerebral arteries and ductus venosus. Measurements were compared with reference ranges from controls. To determine which perinatal outcomes were independently associated with a small fetal thymus, a multivariate logistic regression analysis was performed.

RESULTS

Thymus size was significantly lower in IUGR fetuses compared to controls (P < .05). Among IUGR fetuses, thymus size was significantly smaller in IUGR fetuses with abnormal Doppler flow compared to normal flow (P < .05). A small thymus in IUGR fetuses was independently associated with early delivery (odds ratio [OR], 1.24; 95% confidence interval [CI], 1.05-1.49; P= .023), respiratory distress syndrome (OR, 1.36; 95% CI, 1.09-1.78; P= .005), early neonatal sepsis (OR, 1.65; 95% CI, 1.11-2.42; P= .001), and a longer stay in the neonatal intensive care unit (OR, 1.33; 95% CI, 1.08-1.71; P = .017).

CONCLUSIONS

Intrauterine growth restriction is associated with fetal thymic involution, and a small fetal thymus is an early indicator of adverse perinatal outcomes in pregnancies complicated by IUGR.

Increased leptin response and inhibition of apoptosis in thymocytes of young rats offspring from protein deprived dams during lactation

We investigated the consequences of mild maternal malnutrition in rat dams, in terms of thymocyte responses and the putative role of leptin. The young progeny of dams submitted to protein deprivation (PD) during lactation showed at 30 days of age lower body and thymus weights, significant alterations in CD4/CD8-defined T cell subsets without modifications in total thymocyte number as well as in proliferative response. Despite, the rats from PD group did not present alterations in leptin circulating levels, the expression of leptin receptor ObRb was enhanced in their thymocytes. This change was accompanied by an increase in leptin signaling response of thymocytes from PD rats, with an increase in JAK2 and STAT3 phosphorylation after leptin stimulation. Thymocytes from PD rats also presented a decreased rate of spontaneous apoptosis when compared to controls. Accordingly, higher expression of anti-apoptotic protein Bcl-2, and lower of pro-apoptotic protein Bax, with no change of pro-apoptotic Bad, and higher pro-caspase 3 content were detected in PD thymocytes. Moreover, thymocytes from PD group exhibited a constitutive higher nuclear content of p65 NF-kB associated to a lower IkB content in the cytoplasm. Finally, although there was no change in ob gene expression in PD thymocytes, a higher mRNA expression for the Ob gene was observed in the thymic microenvironment from PD animals. Taken together, the results show that mild maternal protein deprivation during lactation affects thymic homeostasis, enhancing leptin activity, which in turn protects thymocytes from apoptosis in the young progeny, with possible consequences upon the immune response of these animals in adult life.

Enhancement of hematopoiesis and lymphopoiesis in diet-induced obese mice

A rodent model of diet-induced obesity revealed that obesity significantly altered hematopoietic and lymphopoietic functions in the bone marrow and thymus. C57BL/6 mice were fed a mixed high-fat diet (HFD) of 45% fat or 10% fat diet (lean controls) for 180 d. A sustained increase in the numbers of cells found in bone marrow and thymus of HFD mice occurred from day 90 to day 180. However, with the exception of a 10-18% increase in the proportion of lymphocytes, the composition of monocytes, granulocytes, erythrocytes, and mixed progenitor lineages remained normal in the marrow. Likewise, thymuses of HFD mice increased 30-50% in size compared with controls, with analogous increases in thymocyte numbers. The overall thymus cellular composition remained normal. Although increased blood and lymphatic volume in obese mice would play a role in increased hematopoiesis, there were large and disproportionate increases in blood leukocytes of HFD mice, indicating that homeostasis was not maintained. Leptin, which promotes lymphopoiesis and myelopoiesis, reached 100 ng/mL in sera from HFD mice. Moreover, a three- to sixfold increase in adipocytes in marrow resulted in spiked leptin mRNA expression in bones of HFD mice compared with lean controls. Other cytokines and growth factors did not show any increases in obese marrow. The substantial increase in lymphopoietic and hematopoietic processes in HFD mice indicates that the primary tissues are another facet of the immune system dysregulated by obesity, which was perhaps fostered by higher amounts of leptin in marrow and serum.

Social isolation and depressed mood are associated with elevated serum leptin levels in men but not in women

BACKGROUND

Leptin, involved in energy homeostasis and a predictor of cardiovascular disease, has recently been recognized as mediator in stress reactions. We aimed to explore the association between leptin levels and two stress-related conditions, social isolation and depressed mood, both associated with increased cardiovascular mortality.

METHODS

We analysed leptin levels in 1229 subjects (643 men, 586 women), derived from the population-based MONIKA/KORA study. Standardized questionnaires were used to assess depressive mood and social isolation. In a multiple linear regression adjusted for body weight, age and survey, the association between leptin, social isolation and depressed mood and its interaction was explored in men and women separately. Leptin was then dichotomized and four analyses, adjusted for age, BMI, lifestyle factors, psychosomatic complaints and metabolic variables were performed to compare the risk of elevated leptin levels in the risk groups.

RESULTS

Increased leptin levels were associated with social isolation (p=0.04) and the interaction between social isolation and depressed mood (p=0.02) in men but not in women. In socially isolated and depressed men, leptin levels (mean: 6.07 ng/ml) were significantly increased compared to neither depressed nor isolated men (mean: 4.51 ng/ml, p=0.04). In the multivariate adjusted logistic regression model, the combination of depressed state and social isolation was associated with a 4-fold increased risk (p<0.001) for elevated leptin levels.

CONCLUSION

The finding of elevated leptin levels in socially isolated and depressed men raises the possibility that increased cardiovascular mortality in socially isolated men is partially mediated by hyperleptinemia.

Ursolic acid ameliorates thymic atrophy and hyperglycemia in streptozotocin-nicotinamide-induced diabetic mice

The purpose of this study was to assess the effects of low-dose ursolic acid (UA) on glycemic regulation and immune responses in streptozotocin-nicotinamide (STZ/NA)-induced diabetic mice. Diabetic mice were supplemented with two different doses of UA (0.01 and 0.05%, w/w) or metformin (0.5%, w/w) for 4 weeks. Compared with the untreated diabetic group, UA and metformin significantly improved blood glucose, glycosylated hemoglobin, glucose tolerance, insulin tolerance and plasma leptin levels as well as aminotransferase activity. The plasma and pancreatic insulin concentrations were significantly higher in both UA groups than in the untreated diabetic group. Supplementation with metformin increased the pancreatic insulin level without a change in the plasma insulin level. The relative thymus weights were lower in the untreated diabetic group compared to the non-diabetic group; however, the UA or metformin group had significantly improved thymus weights. Mice receiving UA or metformin supplementation had increased CD4(+)CD8(+) subpopulations in the thymus compared to the untreated diabetic mice. Concanavalin A-stimulated splenic T-lymphocyte proliferation and single-positive (CD4(+) and CD8(+)) subpopulations were significantly higher in the UA-supplemented diabetic groups than in the untreated diabetic group, but lipopolysaccharide-stimulated B-lymphocyte proliferation and the CD19(+) subpopulation were not significantly different among the groups. In the STZ/NA-induced diabetic mice, metformin increased the splenic T-lymphocyte CD4(+) and CD8(+) cell numbers without any change in T-lymphocyte proliferation. Both doses of UA lowered splenic IL-6 levels, whereas metformin increased IFN-γ, IL-6 and TNF-α levels compared to the untreated diabetic mice. These results suggest that low-dose UA may be used as a hypoglycemic agent and immune modulator in non-obese type 2 diabetic mice.

The burden of obesity on infectious disease

The world is now experiencing an epidemic of obesity. Although the effects of obesity on the development of metabolic and cardiovascular problems are well studied, much less is known about the impact of obesity on immune function and infectious disease. Studies in obese humans and with obese animal models have repeatedly demonstrated impaired immune function, including decreased cytokine production, decreased response to antigen/mitogen stimulation, reduced macrophage and dendritic cell function, and natural killer cell impairment. Recent studies have demonstrated that the impaired immune response in the obese host leads to increased susceptibility to infection with a number of different pathogens such as community-acquired tuberculosis, influenza, Mycobacterium tuberculosis, coxsackievirus, Helicobacter pylori and encephalomyocarditis virus. While no specific mechanism has been defined for the decreased immune response to infectious disease in the obese host, several obesity-associated changes such as excessive inflammation, altered adipokine signaling, metabolic changes and even epigenetic regulation could affect the immune response. This review will discuss what is currently known about the relationship between obesity and infectious disease.

Thymic microenvironmental alterations in experimentally induced diabetes

Little is known about the immunologic consequences from endocrine changes observed in diabetes. Since a preserved thymic microenvironment is of critical importance for the T cell development and maturation, we have examined the thymus from alloxan-diabetic mice. An intense thymic atrophy accompanied by changes in histological pattern and in thymocyte subpopulations were observed in diabetic mice. Laminin and fibronectin, which are closely associated with thymocytes maturation, were evaluated, but, only laminin presented an altered distribution and density in thymuses from diabetes group. the expression of fibronectin and laminin receptors was found to be decreased in diabetic mice. There was also intense decrease in the expression of two important chemokines for thymus, CCL25 and CXCL12, and in the CCR9 (CCL25 receptor), but the expression of CXCR4 (CXCL12 receptor) did not drop on cells. However, no significant difference was observed in the in vitro thymocytes migratory capacity from diabetic mice. The results show significant alterations in thymus microenvironment in diabetes and offer insights for studies involving endocrine influences on lymphatic organs and T cell maturation.

Fyn mediates leptin actions in the thymus of rodents

BACKGROUND

Several effects of leptin in the immune system rely on its capacity to modulate cytokine expression and apoptosis in the thymus. Surprisingly, some of these effects are dependent on signal transduction through the IRS1/PI3-kinase, but not on the activation of JAK2. Since all the well known effects of leptin in different cell types and tissues seem to be dependent on JAK2 activation, we hypothesized that, at least for the control of thymic function, another, unknown kinase could mediate the transduction of the leptin signal from the ObR towards the IRS1/PI3-kinase signaling cascade.

METHODOLOGY/PRINCIPAL FINDINGS

Here, by employing immunoblot, real-time PCR and flow citometry we show that the tyrosine kinase, Fyn, is constitutively associated with the ObR in thymic cells. Following a leptin stimulus, Fyn undergoes an activating tyrosine phosphorylation and a transient association with IRS1. All these effects are independent of JAK2 activation and, upon Fyn inhibition, the signal transduction towards IRS1/PI3-kinase is abolished. In addition, the inhibition of Fyn significantly modifies the effects of leptin on thymic cytokine expression.

CONCLUSION/SIGNIFICANCE

Therefore, in the thymus, Fyn acts as a tyrosine kinase that transduces the leptin signal independently of JAK2 activation, and mediates some of the immunomodulatory effects of leptin in this tissue.