Immune response to cold exposure: Role of γδ T cells and TLR2-mediated inflammation.

The mammalian body possesses remarkable adaptability to cold exposure, involving intricate adjustments in cellular metabolism, ultimately leading to thermogenesis. However, cold-induced stress can impact immune response, primarily through noradrenaline-mediated pathways. In our study, we utilized a rat model subjected to short-term or long-term mild cold exposure to investigate systemic immune response during the cold acclimation. To provide human relevance, we included a group of regular cold swimmers in our study. Our research revealed complex relationship between cold exposure, neural signaling, immune response, and thermogenic regulation. One-day cold exposure triggered stress response, including cytokine production in white adipose tissue, subsequently activating brown adipose tissue, and inducing thermogenesis. We further studied systemic immune response, including the proportion of leukocytes and cytokines production. Interestingly, γδ T cells emerged as possible regulators in the broader systemic response, suggesting their possible contribution in the dynamic process of cold adaptation. We employed RNA-seq to gain further insights into the mechanisms by which γδ T cells participate in the response to cold. Additionally, we challenged rats exposed to cold with the Toll-like receptor 2 agonist, showing significant modulation of immune response. These findings significantly contribute to understanding of the physiological acclimation that occur in response to cold exposure.

Dynamic changes of immunocyte subpopulations in thermogenic activation of adipose tissues.

Objectives: The effects of cold exposure on whole-body metabolism in humans have gained increasing attention. Brown or beige adipose tissues are crucial in cold-induced thermogenesis to dissipate energy and thus have the potential to combat metabolic disorders. Despite the immune regulation of thermogenic adipose tissues, the overall changes in vital immune cells during distinct cold periods remain elusive. This study aimed to discuss the overall changes in immune cells under different cold exposure periods and to screen several potential immune cell subpopulations on thermogenic regulation. Methods: Cibersort and mMCP-counter algorithms were employed to analyze immune infiltration in two (brown and beige) thermogenic adipose tissues under distinct cold periods. Changes in some crucial immune cell populations were validated by reanalyzing the single-cell sequencing dataset (GSE207706). Flow cytometry, immunofluorescence, and quantitative real-time PCR assays were performed to detect the proportion or expression changes in mouse immune cells of thermogenic adipose tissues under cold challenge. Results: The proportion of monocytes, naïve, and memory T cells increased, while the proportion of NK cells decreased under cold exposure in brown adipose tissues. Conclusion: Our study revealed dynamic changes in immune cell profiles in thermogenic adipose tissues and identified several novel immune cell subpopulations, which may contribute to thermogenic activation of adipose tissues under cold exposure.

In a model of SAH-induced neurogenic fever, BAT thermogenesis is mediated by erythrocytes and blocked by agonism of adenosine A1 receptors.

Neurogenic fever (NF) after subarachnoid hemorrhage (SAH) is a major cause of morbidity that is associated with poor outcomes and prolonged stay in the neurointensive care unit (NICU). Though SAH is a much more common cause of fever than sepsis in the NICU, it is often a diagnosis of exclusion, requiring significant effort to rule out an infectious source. NF does not respond to standard anti-pyretic medications such as COX inhibitors, and lack of good medical therapy has led to the introduction of external cooling systems that have their own associated problems. In a rodent model of SAH, we measured the effects of injecting whole blood, blood plasma, or erythrocytes on the sympathetic nerve activity to brown adipose tissue and on febrile thermogenesis. We demonstrate that following SAH the acute activation of brown adipose tissue leading to NF, is not dependent on PGE2, that subarachnoid space injection of whole blood or erythrocytes, but not plasma alone, is sufficient to trigger brown adipose tissue thermogenesis, and that activation of adenosine A1 receptors in the CNS can block the brown adipose tissue thermogenic component contributing to NF after SAH. These findings point to a distinct thermogenic mechanism for generating NF, compared to those due to infectious causes, and will hopefully lead to new therapies.

Adiponectin restrains ILC2 activation by AMPK-mediated feedback inhibition of IL-33 signaling.

ILC2s are present in adipose tissue and play a critical role in regulating adipose thermogenesis. However, the mechanisms underlying the activation of adipose-resident ILC2s remain poorly defined. Here, we show that IL-33, a potent ILC2 activator, stimulates phosphorylation of AMPK at Thr172 via TAK1 in primary ILC2s, which provides a feedback mechanism to inhibit IL-33-induced NF-κB activation and IL-13 production. Treating ILC2s with adiponectin or an adiponectin receptor agonist (AdipoRon) activated AMPK and decreased IL-33-NF-κB signaling. AdipoRon also suppressed cold-induced thermogenic gene expression and energy expenditure in vivo. In contrast, adiponectin deficiency increased the ILC2 fraction and activation, leading to up-regulated thermogenic gene expression in adipose tissue of cold-exposed mice. ILC2 deficiency or blocking ILC2 function by neutralization of the IL-33 receptor with anti-ST2 diminished the suppressive effect of adiponectin on cold-induced adipose thermogenesis and energy expenditure. Taken together, our study reveals that adiponectin is a negative regulator of ILC2 function in adipose tissue via AMPK-mediated negative regulation of IL-33 signaling.

Tregs facilitate obesity and insulin resistance via a Blimp-1/IL-10 axis.

Interleukin-10 (IL-10) is a critical cytokine used by immune cells to suppress inflammation. Paradoxically, immune cell-derived IL-10 can drive insulin resistance in obesity by suppressing adipocyte energy expenditure and thermogenesis. However, the source of IL-10 necessary for the suppression of adipocyte thermogenesis is unknown. We show here that CD4+Foxp3+ regulatory T cells (Tregs) are a substantial source of IL-10 and that Treg-derived IL-10 can suppress adipocyte beiging. Unexpectedly, Treg-specific loss of IL-10 resulted in increased insulin sensitivity and reduced obesity in high-fat diet-fed male mice. Mechanistically, we determined that Treg-specific loss of the transcription factor Blimp-1, a driver of IL-10 expression by Tregs, phenocopied the Treg-specific IL-10-deficient mice. Loss of Blimp-1 expression in Tregs resulted in reduced ST2+KLRG1+, IL-10-secreting Tregs, particularly in the white adipose tissue. Blimp-1-deficient mice were protected from glucose intolerance, insulin resistance, and diet-induced obesity, through increased white adipose tissue browning. Taken together, our data show that Blimp-1-regulated IL-10 secretion by Tregs represses white adipose tissue beiging to maintain adipose tissue homeostasis.

A thermogenic fat-epithelium cell axis regulates intestinal disease tolerance.

Disease tolerance, the capacity of tissues to withstand damage caused by a stimulus without a decline in host fitness, varies across tissues, environmental conditions, and physiologic states. While disease tolerance is a known strategy of host defense, its role in noninfectious diseases has been understudied. Here, we provide evidence that a thermogenic fat-epithelial cell axis regulates intestinal disease tolerance during experimental colitis. We find that intestinal disease tolerance is a metabolically expensive trait, whose expression is restricted to thermoneutral mice and is not transferable by the microbiota. Instead, disease tolerance is dependent on the adrenergic state of thermogenic adipocytes, which indirectly regulate tolerogenic responses in intestinal epithelial cells. Our work has identified an unexpected mechanism that controls intestinal disease tolerance with implications for colitogenic diseases.

The essential function of IL-33 in metabolic regulation.

Interleukin-33 (IL-33) is produced by various types of cells under physical or pathological conditions. As a multifunctional partner in health and disease, current evidence reveals that IL-33 also participates in several metabolic processes. IL-33 has been proven to contribute to regulating the activity of ST2+ group 2 innate lymphoid cells and regulatory T cells in adipose, which leads to the shift of insulin sensitivity and glucose clearance in glucose metabolism, thermogenesis, and adipocyte beiging in adipose metabolism. In this review, we briefly summarize the biological characteristics of Il-33 and discuss its regulatory function in glucose and adipose metabolism. By clarifying the underlying mechanism of IL-33, we highlight the crosstalk between immune response and metabolic processes mediated by IL-33.

Toward an Understanding of How Immune Cells Control Brown and Beige Adipobiology.

Immune cells were recently found to have an unexpected involvement in controlling the thermogenic activity of brown and beige adipose tissue. Here, we review how macrophages, eosinophils, type 2 innate lymphoid cells, and T lymphocytes are linked to this process. In particular, the recruitment of alternatively activated macrophages and eosinophils is associated with brown fat activation and white fat browning. Conversely, pro-inflammatory immune cell recruitment represses the thermogenic activity of brown and beige adipose tissues via cytokines that inhibit noradrenergic signaling. Macrophages also influence the noradrenergic tone by degrading norepinephrine locally and by inhibiting sympathetic innervation over time.

Brown-adipose-tissue macrophages control tissue innervation and homeostatic energy expenditure.

Tissue macrophages provide immunological defense and contribute to the establishment and maintenance of tissue homeostasis. Here we used constitutive and inducible mutagenesis to delete the nuclear transcription regulator Mecp2 in macrophages. Mice that lacked the gene encoding Mecp2, which is associated with Rett syndrome, in macrophages did not show signs of neurodevelopmental disorder but displayed spontaneous obesity, which was linked to impaired function of brown adipose tissue (BAT). Specifically, mutagenesis of a BAT-resident Cx3Cr1+ macrophage subpopulation compromised homeostatic thermogenesis but not acute, cold-induced thermogenesis. Mechanistically, malfunction of BAT in pre-obese mice with mutant macrophages was associated with diminished sympathetic innervation and local titers of norepinephrine, which resulted in lower expression of thermogenic factors by adipocytes. Mutant macrophages overexpressed the signaling receptor and ligand PlexinA4, which might contribute to the phenotype by repulsion of sympathetic axons expressing the transmembrane semaphorin Sema6A. Collectively, we report a previously unappreciated homeostatic role for macrophages in the control of tissue innervation. Disruption of this circuit in BAT resulted in metabolic imbalance.

Alternatively activated macrophages do not synthesize catecholamines or contribute to adipose tissue adaptive thermogenesis.

Adaptive thermogenesis is the process of heat generation in response to cold stimulation. It is under the control of the sympathetic nervous system, whose chief effector is the catecholamine norepinephrine (NE). NE enhances thermogenesis through ß3-adrenergic receptors to activate brown adipose tissue and by 'browning' white adipose tissue. Recent studies have reported that alternative activation of macrophages in response to interleukin (IL)-4 stimulation induces the expression of tyrosine hydroxylase (TH), a key enzyme in the catecholamine synthesis pathway, and that this activation provides an alternative source of locally produced catecholamines during the thermogenic process. Here we report that the deletion of Th in hematopoietic cells of adult mice neither alters energy expenditure upon cold exposure nor reduces browning in inguinal adipose tissue. Bone marrow-derived macrophages did not release NE in response to stimulation with IL-4, and conditioned media from IL-4-stimulated macrophages failed to induce expression of thermogenic genes, such as uncoupling protein 1 (Ucp1), in adipocytes cultured with the conditioned media. Furthermore, chronic treatment with IL-4 failed to increase energy expenditure in wild-type, Ucp1-/- and interleukin-4 receptor-α double-negative (Il4ra-/-) mice. In agreement with these findings, adipose-tissue-resident macrophages did not express TH. Thus, we conclude that alternatively activated macrophages do not synthesize relevant amounts of catecholamines, and hence, are not likely to have a direct role in adipocyte metabolism or adaptive thermogenesis.

Non-canonical alternatives: what a macrophage is 4.

Macrophages play pleiotropic, niche-specific roles in all tissues and organs. As immune sentinels, tissue macrophages regulate immune activation and inflammation; in turn, their function is modulated by inflammatory mediators deriving from such activation. Recent papers have established unanticipated roles for interleukin 4 and the alternative activation of tissue macrophages in the organismal response to diverse environmental stressors.

Effect of nucleus tractus solitarius lesions on fever produced by interleukin-1beta.

Partial elimination of vagal sensory afferents by subdiaphragmatic vagal section has variously been reported to eliminate, to reduce, or to have no effect on fever produced by peripheral lipopolysaccharide and interleukin-1beta (IL-1beta). However, to adequately test the idea that vagal afferents convey immune information to the brain, all vagal input to the central nervous system must be eliminated. This was accomplished by bilateral electrolytic lesions of the nucleus tractus solitarius (NTS). Reflex bradycardia evoked by intravenous phenylbiguanide was eliminated in NTS-lesioned rats, verifying the lesion's effectiveness. IL-1beta (2 microg/kg) was given to conscious, unrestrained rats via an indwelling intraperitoneal catheter and produced rapid fever (approximately 1 degree C) with an onset latency of 15 min and peak response at 30 min, with a second, smaller peak at 130 min. NTS lesions attenuated the first fever peak, with a lesser, non-significant effect on the second peak. The thermogenic capacity of NTS-lesioned rats was evaluated using 3 different strategies: (1) thermogenesis evoked by CNS injections of prostaglandin E2, (2) 3 h exposure to a 4 degrees C environment, and (3) heat production of intrascapular brown fat produced by intravenous infusion of the beta3-adrenergic agonist BRL 37344. NTS-lesioned rats were equivalent, or even superior to control animals in their thermogenic response to these non-immune-related stimuli. Therefore, the impaired febrile response of NTS-lesioned rats to IL-1beta cannot be attributed to reduced thermogenic capacity. Finally, these results suggest that fever elicited by intraperitoneal IL-1beta is, at least in part, dependent on the integrity of NTS neurons, but also that mechanisms independent of vagal afferent projections to the NTS must also play a role in immune-to-brain signaling.

Subdiaphragmatic vagotomy blocks interleukin-1beta-induced fever but does not reduce IL-1beta levels in the circulation.

Peripheral interleukin-1beta has been implicated in the initiation of fever responses, yet the pathways by which it influences brain function are still unclear. Sectioning the abdominal vagus has been reported to inhibit fever after intraperitoneal administration of interleukin-1beta, suggesting that vagal afferents participate in signaling the brain to mount a fever response to interleukin-1beta. However, the inhibitory effect of subdiaphragmatic vagotomy could be due to alterations in pharmacokinetics such that the intraperitoneally injected cytokine does not reach the general circulation in sufficient quantities to activate the brain via blood-borne signaling. We measured both fever and plasma levels of interleukin-1beta in vagotomized and sham-operated rats after intraperitoneal administration of 1 microg/kg human recombinant interleukin-1beta to determine whether vagotomy reduces fever and levels of circulating interleukin-1beta after intraperitoneal injection. Plasma levels of human recombinant and endogenous rat interleukin-1beta were measured in separate enzyme-linked immunosorbent assays. While intraperitoneal administration of human recombinant interleukin-1beta elevated plasma levels of this cytokine similarly in vagotomized and sham-operated animals, only sham-operated rats responded with fever. Plasma levels of endogenous rat interleukin-1beta were unchanged by any treatment. These results demonstrate that the blockade of intraperitoneal interleukin-1beta-induced fever after subdiaphragmatic vagotomy cannot be accounted for by alterations of interleukin-1beta levels in the general circulation.