Conventional type 1 dendritic cells protect against age-related adipose tissue dysfunction and obesity

From fat storage to immune hubs: the emerging role of adipocytes in coordinating the immune response to infection

Adipose tissue is a rich source of diverse cell populations, including immune cells, adipocytes and stromal cells. Interactions between these different cell types are now appreciated to be critical for maintaining tissue structure and function, by governing processes such as adipogenesis, lipolysis and differentiation of white to beige adipocytes. Interactions between these cells also drive inflammation in obesity, leading to an expansion of adipose tissue immune cells, and the secretion of proinflammatory cytokines from immune cells and from adipocytes themselves. However, in evolutionary terms, obesity is a recent phenomenon, raising the question of why adipocytes evolved to express factors that influence the immune response. Studies of various pathogens indicate that adipocytes are highly responsive to infection, altering their metabolic profiles in a way that can be used to release nutrients and fuel the immune response. In the case of infection with the extracellular parasite Trypanosoma brucei, attenuating the ability of adipocytes to sense the cytokine IL-17 results in a loss of control of the local immune response and an increased pathogen load. Intriguingly, comparisons of the adipocyte response to infection suggest that the immune responses of these cells occur in a pathogen-dependent manner, further confirming their complexity. Here, with a focus on murine adipose tissue, we discuss the emerging concept that, in addition to their canonical function, adipocytes are immune signalling hubs that integrate and disseminate signals from the immune system to generate a local environment conducive to pathogen clearance.

Adipose tissue senescence: Biological changes, hallmarks and therapeutic approaches

Adipose tissue (AT), the largest energy storage reservoir and endocrine organ, plays a crucial role in regulating systemic energy metabolism. As one of the most vulnerable tissues during aging, the plasticity of AT is impaired. With age, AT undergoes redistribution, characterized by expansion of visceral adipose tissue (VAT) and reduction of peripheral subcutaneous adipose tissue (SAT). Additionally, age-related changes in AT include reduced adipogenesis of white adipocytes, decreased proliferation and differentiation capacity of mesenchymal stromal/stem cells (MSCs), diminished thermogenic capacity in brown/beige adipocytes, and dysregulation of immune cells. Specific and sensitive hallmarks enable the monitoring and evaluation of the biological changes associated with aging. In this study, we have innovatively proposed seven characteristic hallmarks of AT senescence, including telomere attrition, epigenetic alterations, genomic instability, mitochondrial dysfunction, disabled macroautophagy, cellular senescence, and chronic inflammation, which are intricately interconnected and mutually regulated. Finally, we discussed anti-aging strategies targeting AT, offering insights into mitigating or delaying metabolic disturbances caused by AT senescence.

Contribution of NKT cells and CD1d-expressing cells in obesity-associated adipose tissue inflammation

Natural killer T (NKT) cell are members of the innate-like T lymphocytes and recognizes lipid antigens presented by CD1d-expressing cells. Obesity-associated inflammation in adipose tissue (AT) leads to metabolic dysfunction, including insulin resistance. When cellular communication is properly regulated among AT-residing immune cells and adipocytes during inflammation, a favorable balance of Th1 and Th2 immune responses is achieved. NKT cells play crucial roles in AT inflammation, influencing the development of diet-induced obesity and insulin resistance. NKT cells interact with CD1d-expressing cells in AT, such as adipocytes, macrophages, and dendritic cells, shaping pro-inflammatory or anti-inflammatory microenvironments with distinct characteristics depending on the antigen-presenting cells. Additionally, CD1d may be involved in the inflammatory process independently of NKT cells. In this mini-review, we provide a brief overview of the current understanding of the interaction between immune cells, focusing on NKT cells and CD1d signaling, which control AT inflammation both in the presence and absence of NKT cells. We aim to enhance our understanding of the mechanisms of obesity-associated diseases.

Orchestration of the Adipose Tissue Immune Landscape by Adipocytes

Obesity is epidemic and of great concern because of its comorbid and costly inflammatory-driven complications. Extensive investigations in mice have elucidated highly coordinated, well-balanced interactions between adipocytes and immune cells in adipose tissue that maintain normal systemic metabolism in the lean state, while in obesity, proinflammatory changes occur in nearly all adipose tissue immune cells. Many of these changes are instigated by adipocytes. However, less is known about obesity-induced adipose-tissue immune cell alterations in humans. Upon high-fat diet feeding, the adipocyte changes its well-known function as a metabolic cell to assume the role of an immune cell, orchestrating proinflammatory changes that escalate inflammation and progress during obesity. This transformation is particularly prominent in humans. In this review, we (a) highlight a leading and early role for adipocytes in promulgating inflammation, (b) discuss immune cell changes and the time course of these changes (comparing humans and mice when possible), and (c) note how reversing proinflammatory changes in most types of immune cells, including adipocytes, rescues adipose tissue from inflammation and obese mice from insulin resistance.

Adipose cDC1s contribute to obesity-associated inflammation through STING-dependent IL-12 production

Obesity is associated with chronic low-grade white adipose tissue (WAT) inflammation that can contribute to the development of insulin resistance in mammals. Previous studies have identified interleukin (IL)-12 as a critical upstream regulator of WAT inflammation and metabolic dysfunction during obesity. However, the cell types and mechanisms that initiate WAT IL-12 production remain unclear. Here we show that conventional type 1 dendritic cells (cDC1s) are the cellular source of WAT IL-12 during obesity through analysis of mouse and human WAT single-cell transcriptomic datasets, IL-12 reporter mice and IL-12p70 protein levels by enzyme-linked immunosorbent assay. We demonstrate that cDC1s contribute to obesity-associated inflammation by increasing group 1 innate lymphocyte interferon-γ production and inflammatory macrophage accumulation. Inducible depletion of cDC1s increased WAT insulin sensitivity and systemic glucose tolerance during diet-induced obesity. Mechanistically, endocytosis of apoptotic bodies containing self-DNA by WAT cDC1s drives stimulator of interferon genes (STING)-dependent IL-12 production. Together, these results suggest that WAT cDC1s act as critical regulators of adipose tissue inflammation and metabolic dysfunction during obesity.

MICa/b-dependent activation of natural killer cells by CD64+ inflammatory type 2 dendritic cells contributes to autoimmunity

Primary Sjögren's syndrome (pSS) is an inflammatory autoimmune disorder largely mediated by type I and II interferon (IFN). The potential contribution of innate immune cells, such as natural killer (NK) cells and dendritic cells (DC), to the pSS pathology remains understudied. Here, we identified an enriched CD16+ CD56hi NK cell subset associated with higher cytotoxic function, as well as elevated proportions of inflammatory CD64+ conventional dendritic cell (cDC2) subtype that expresses increased levels of MICa/b, the ligand for the activating receptor NKG2D, in pSS individuals. Circulating cDC2 from pSS patients efficiently induced activation of cytotoxic NK cells ex vivo and were found in proximity to CD56+ NK cells in salivary glands (SG) from pSS patients. Interestingly, transcriptional activation of IFN signatures associated with the RIG-I/DDX60 pathway, IFN I receptor, and its target genes regulate the expression of NKG2D ligands on cDC2 from pSS patients. Finally, increased proportions of CD64hi RAE-1+ cDC2 and NKG2D+ CD11b+ CD27+ NK cells were present in vivo in the SG after poly I:C injection. Our study provides novel insight into the contribution and interplay of NK and cDC2 in pSS pathology and identifies new potential therapy targets.

Bone marrow immune cells respond to fluctuating nutritional stress to constrain weight regain

Weight regain after weight loss is a major challenge in the treatment of obesity. Immune cells adapt to fluctuating nutritional stress, but their roles in regulating weight regain remain unclear. Here, we identify a stem cell-like CD7+ monocyte subpopulation accumulating in the bone marrow (BM) of mice and humans that experienced dieting-induced weight loss. Adoptive transfer of CD7+ monocytes suppresses weight regain, whereas inducible depletion of CD7+ monocytes accelerates it. These cells, accumulating metabolic memories via epigenetic adaptations, preferentially migrate to the subcutaneous white adipose tissue (WAT), where they secrete fibrinogen-like protein 2 (FGL2) to activate the protein kinase A (PKA) signaling pathway and facilitate beige fat thermogenesis. Nevertheless, CD7+ monocytes gradually enter a quiescent state after weight loss, accompanied by increased susceptibility to weight regain. Notably, administration of FMS-like tyrosine kinase 3 ligand (FLT3L) remarkably rejuvenates CD7+ monocytes, thus ameliorating rapid weight regain. Together, our findings identify a unique bone marrow-derived metabolic-memory immune cell population that could be targeted to combat obesity.

Immunometabolism of dendritic cells in health and disease

Dendritic cells (DCs) are crucial mediators that bridge the innate and adaptive immune responses. Cellular rewiring of metabolism is an emerging regulator of the activation, migration, and functional specialization of DC subsets in specific microenvironments and immunological conditions. DCs undergo metabolic adaptation to exert immunogenic or tolerogenic effects in different contexts. Also, beyond their intracellular metabolic and signaling roles, metabolites and nutrients mediate the intercellular crosstalk between DCs and other cell types, and such crosstalk orchestrates DC function and immune responses. Here, we provide a comprehensive review of the metabolic regulation of DC biology in various contexts and summarize the current understanding of such regulation in directing immune homeostasis and inflammation, specifically with respect to infections, autoimmunity, tolerance, cancer, metabolic diseases, and crosstalk with gut microbes. Understanding context-specific metabolic alterations in DCs may identify mechanisms for physiological and pathological functions of DCs and yield potential opportunities for therapeutic targeting of DC metabolism in many diseases.

Fat and inflammation: adipocyte-myeloid cell crosstalk in atherosclerosis

Adipose tissue inflammation has been implicated in various chronic inflammatory diseases and cancer. Perivascular adipose tissue (PVAT) surrounds the aorta as an extra layer and was suggested to contribute to atherosclerosis development. PVAT regulates the function of endothelial and vascular smooth muscle cells in the aorta and represent a reservoir for various immune cells which may participate in aortic inflammation. Recent studies demonstrate that adipocytes also express various cytokine receptors and, therefore, may directly respond to inflammatory stimuli. Here we will summarize current knowledge on immune mechanisms regulating adipocyte activation and the crosstalk between myeloid cells and adipocytes in pathogenesis of atherosclerosis.

Conventional type 1 dendritic cells protect against gut barrier disruption via maintaining Akkermansia muciniphila in alcoholic steatohepatitis

BACKGROUND AND AIMS

Alcohol-perturbed gut immune homeostasis is associated with the development of alcoholic liver disease (ALD). However, the role of intestinal dendritic cells (DCs) in ALD progression is still unknown. This study aimed to investigate the cellular and molecular mechanisms through which intestinal DCs respond to alcohol exposure and contribute to the pathogenesis of ALD.

APPROACH AND RESULTS

After 8 weeks of alcohol consumption, the number of basic leucine zipper transcription factor ATF-like 3 ( Batf3 )-dependent conventional type 1 DCs (cDC1s) was dramatically decreased in the intestine but not the liver. cDC1 deficient Batf3 knockout mice along with wild-type mice were subjected to chronic-binge ethanol feeding to determine the role of intestinal cDC1s reduction in ALD. cDC1s deficiency exacerbated alcohol-induced gut barrier disruption, bacterial endotoxin translocation into the circulation, and liver injury. Adoptive transfer of cDC1s to alcohol-fed mice ameliorated alcohol-mediated gut barrier dysfunction and liver injury. Further studies revealed that intestinal cDC1s serve as a positive regulator of Akkermansia muciniphila ( A. muciniphila ). Oral administration of A. muciniphila markedly reversed alcoholic steatohepatitis in mice. Mechanistic studies revealed that cDC1s depletion exacerbated alcohol-downregulated intestinal antimicrobial peptides which play a crucial role in maintaining A. muciniphila abundance, by disrupting the IL-12-interferon gamma signaling pathway. Lastly, we identified that intestinal cDC1s were required for the protective role of Lactobacillus reuteri in alcoholic steatohepatitis.

CONCLUSIONS

This study demonstrated that cDC1s protect alcohol-induced liver injury by maintaining A. muciniphila abundance in mice. Targeting cDC1s may serve as a promising therapeutic approach for treating ALD.

Chinese chestnut shell polyphenol extract regulates the JAK2/STAT3 pathway to alleviate high-fat diet-induced, leptin-resistant obesity in mice

Chinese chestnut shell is a by-product of chestnut food processing and is rich in polyphenols. This study sought to investigate the effect of chestnut shell polyphenol extract (CSP) on weight loss and lipid reduction in a 12-week high-fat diet (HFD)-induced murine obesity model. CSP (300 mg per kg body weight) was administered intragastrically daily. AG490, a JAK2 protein tyrosine kinase inhibitor, was also intraperitoneally injected. The results showed that an HFD induced leptin resistance (LR). Compared to corresponding values in the HFD group, CSP treatment improved blood lipid levels, weight, and leptin levels in obese mice (p < 0.01). Additionally, CSP treatment enhanced enzyme activity by improving total antioxidant capacity, attenuating oxidative stress, and reducing fat droplet accumulation and inflammation in the liver, epididymal, and retroperitoneal adipose tissue. CSP also activated the LEPR-JAK2/STAT3-PTP1B-SOCS-3 signal transduction pathway in hypothalamus tissue and improved LR while regulating the expression of proteins related to lipid metabolism (PPARγ, FAS, and LPL) in white adipose tissue in the retroperitoneal cavity. However, the amelioration of lipid metabolism by CSP was dependent on JAK2. Molecular docking simulation further demonstrated the strong binding affinity of procyanidin C1 (-10.3983297 kcal mol^-1) and procyanidin B1 (-9.12686729 kcal mol^-1) to the crystal structure of JAK2. These results suggest that CSP may be used to reduce HFD-induced obesity with potential application as a functional food additive.

Dendritic cells in energy balance regulation

Besides their well-known role in initiating adaptive immune responses, several groups have studied the role of dendritic cells (DCs) in the context of chronic metabolic inflammation, such as in diet-induced obesity (DIO) or metabolic-associated fatty liver disease. DCs also have an important function in maintaining metabolic tissue homeostasis in steady-state conditions. In this review, we will briefly describe the different DC subsets, the murine models available to assess their function, and discuss the role of DCs in regulating energy balance and maintaining tissue homeostasis.

Adipose tissue aging is regulated by an altered immune system

Adipose tissue is a widely distributed organ that plays a critical role in age-related physiological dysfunctions as an important source of chronic sterile low-grade inflammation. Adipose tissue undergoes diverse changes during aging, including fat depot redistribution, brown and beige fat decrease, functional decline of adipose progenitor and stem cells, senescent cell accumulation, and immune cell dysregulation. Specifically, inflammaging is common in aged adipose tissue. Adipose tissue inflammaging reduces adipose plasticity and pathologically contributes to adipocyte hypertrophy, fibrosis, and ultimately, adipose tissue dysfunction. Adipose tissue inflammaging also contributes to age-related diseases, such as diabetes, cardiovascular disease and cancer. There is an increased infiltration of immune cells into adipose tissue, and these infiltrating immune cells secrete proinflammatory cytokines and chemokines. Several important molecular and signaling pathways mediate the process, including JAK/STAT, NFκB and JNK, etc. The roles of immune cells in aging adipose tissue are complex, and the underlying mechanisms remain largely unclear. In this review, we summarize the consequences and causes of inflammaging in adipose tissue. We further outline the cellular/molecular mechanisms of adipose tissue inflammaging and propose potential therapeutic targets to alleviate age-related problems.

Anti-Inflammatory Mechanisms of Dietary Flavones: Tapping into Nature to Control Chronic Inflammation in Obesity and Cancer

Flavones are natural phytochemicals broadly distributed in our diet. Their anti-inflammatory properties provide unique opportunities to control the innate immune system and inflammation. Here, we review the role of flavones in chronic inflammation with an emphasis on their impact on the molecular mechanisms underlying inflammatory diseases including obesity and cancer. Flavones can influence the innate immune cell repertoire restoring the immune landscape. Flavones impinge on NF-κB, STAT, COX-2, or NLRP3 inflammasome pathways reestablishing immune homeostasis. Devoid of adverse side effects, flavones could present alternative opportunities for the treatment and prevention of chronic inflammation that contributes to obesity and cancer.

BATF3 Protects Against Metabolic Syndrome and Maintains Intestinal Epithelial Homeostasis

The intestinal immune system and microbiota are emerging as important contributors to the development of metabolic syndrome, but the role of intestinal dendritic cells (DCs) in this context is incompletely understood. BATF3 is a transcription factor essential in the development of mucosal conventional DCs type 1 (cDC1). We show that Batf3-/- mice developed metabolic syndrome and have altered localization of tight junction proteins in intestinal epithelial cells leading to increased intestinal permeability. Treatment with the glycolysis inhibitor 2-deoxy-D-glucose reduced intestinal inflammation and restored barrier function in obese Batf3-/- mice. High-fat diet further enhanced the metabolic phenotype and susceptibility to dextran sulfate sodium colitis in Batf3-/- mice. Antibiotic treatment of Batf3-/- mice prevented metabolic syndrome and impaired intestinal barrier function. Batf3-/- mice have altered IgA-coating of fecal bacteria and displayed microbial dysbiosis marked by decreased obesity protective Akkermansia muciniphila, and Bifidobacterium. Thus, BATF3 protects against metabolic syndrome and preserves intestinal epithelial barrier by maintaining beneficial microbiota.