High-Fat Diet Rapidly Modifies Trafficking, Phenotype, and Function of Plasmacytoid Dendritic Cells in Adipose Tissue

Plasmacytoid dendritic cells (pDCs) display an increased abundance in visceral adipose tissue (VAT) of humans with obesity. In the current study, we set out to decipher the molecular mechanisms of their recruitment to VAT and the functional relevance of this process. We observed increased pDC numbers in murine blood, liver, spleen, and VAT after feeding a high-fat diet (HFD) for 3 wk when compared with a standard diet. pDCs were enriched in fat-associated lymphoid clusters representing highly specific lymphoid regions within VAT. HFD led to an enlargement of fat-associated lymphoid clusters with an increased density and migratory speed of pDCs as shown by intravital multiphoton microscopy. For their recruitment into VAT, pDCs employed P-selectin with E-selectin and L-selectin being only critical in response to HFD, indicating that the molecular cues underlying pDC trafficking were dependent on the nutritional state. Subsequent recruitment steps required α₄β₁ and α₄β₇ integrins and engagement of CCR7. Application of fingolimod (FTY720) abrogated egress of pDCs from VAT, indicating the involvement of sphingosine-1-phosphate in this process. Furthermore, HFD altered pDC functions by promoting their activation and type 1 IFN expression. Blocking pDC infiltration into VAT prevented weight gain and improved glucose tolerance during HFD. In summary, a HFD fundamentally alters pDC biology by promoting their trafficking, retention, and activation in VAT, which in turn seems to regulate metabolism.

Type I interferon mediated induction of somatostatin leads to suppression of ghrelin and appetite thereby promoting viral immunity in mice

Loss of appetite (anorexia) is a typical behavioral response to infectious diseases that often reduces body weight. Also, anorexia can be observed in cancer and trauma patients, causing poor quality of life and reduced prospects of positive therapeutic outcomes. Although anorexia is an acute symptom, its initiation and endocrine regulation during antiviral immune responses are poorly understood. During viral infections, plasmacytoid dendritic cells (pDCs) produce abundant type I interferon (IFN-I) to initiate first-line defense mechanisms. Here, by targeted ablation of pDCs and various in vitro and in vivo mouse models of viral infection and inflammation, we identified that IFN-I is a significant driver of somatostatin (SST). Consequently, SST suppressed the hunger hormone ghrelin that led to severe metabolic changes, anorexia, and rapid body weight loss. Furthermore, during vaccination with Modified Vaccinia Ankara virus (MVA), the SST-mediated suppression of ghrelin was critical to viral immune response, as ghrelin restrained the production of early cytokines by natural killer (NK) cells and pDCs, and impaired the clonal expansion of CD8⁺ T cells. Thus, the hormonal modulation of ghrelin through SST and the cytokine IFN-I is fundamental for optimal antiviral immunity, which comes at the expense of calorie intake.

High-fat diet-derived free fatty acids impair the intestinal immune system and increase sensitivity to intestinal epithelial damage

In Japan and other Asian countries, increased fat uptake induced by a westernized diet is thought to be associated with an increased incidence of inflammatory bowel disease, colorectal cancer and food allergies; however, the mechanism for this remains unclear. High-fat diet (HFD)-fed mice are common animal models used to examine the effect of fat intake in vivo. HFDs are reported to exacerbate DSS-induced colitis and intestinal tumorigenesis, but the effect of HFDs on the intestines before disease induction is often overlooked. We found that the intestinal and gut-associated lymphoid tissue (GALT) morphology of HFD-fed mice differed from that of standard diet (SD)-fed mice. To clarify the mechanism by which fat intake increases intestinal diseases, we analyzed the morphological and immunological aspects of the intestines of HFD-fed mice as well as the molecular mechanisms and physiology. Feeding an HFD for 3 weeks induced atrophy of the small intestine, colon and GALT and reduced the number of small intestinal intraepithelial lymphocytes (IELs) and lamina propria lymphocytes (LPLs). Feeding an HFD for only one day reduced the number of small intestinal (SI)-IELs and SI-LPLs. The effect of feeding a 3-week HFD continued for 2 weeks after returning to the SD. The effect of the HFD on the intestinal immune system was independent of the gut microbes. We hypothesized that the cytotoxicity of the abundant HFD-derived free fatty acids in the intestinal lumen impairs the intestinal immune system. Both saturated and unsaturated free fatty acids were toxic to intestinal T-cells in vitro. Orally administering free fatty acids reduced the number of SI-IELs and LPLs. Using a lipase inhibitor to reduce the luminal free fatty acids attenuated the HFD-induced changes in the intestinal immune system, while using a statin to reduce the serum free fatty acids did not. Thus, HFD-induced free fatty acids damaged the intestines; this effect was termed "intestinal lipotoxicity". Because sustained reduction of SI-LPLs after HFD feeding exacerbated indomethacin-induced small intestinal damage, lipotoxicity to the human intestines incurred by consuming a westernized diet in Japan may increase intestinal diseases such as IBD, colorectal cancer or food allergies.

CCL22 controls immunity by promoting regulatory T cell communication with dendritic cells in lymph nodes

Rapp et al. demonstrate that dendritic cells in the lymph node secrete CCL22 to build cell–cell contacts with CCR4-expressing regulatory T cells, leading to immune suppression. Conversely, CCL22 deficiency results in enhanced T cell immunity, shown here in the setting of vaccination, cancer, and inflammatory disease.

Chemokines have crucial roles in organ development and orchestration of leukocyte migration. The chemokine CCL22 is expressed constitutively at high levels in the lymph node, but the functional significance of this expression is so far unknown. Studying a newly established CCL22-deficient mouse, we demonstrate that CCL22 expression by dendritic cells (DCs) promotes the formation of cell–cell contacts and interaction with regulatory T cells (T reg) through their CCR4 receptor. Vaccination of CCL22-deficient mice led to excessive T cell responses that were also observed when wild-type mice were vaccinated using CCL22-deficient DCs. Tumor-bearing mice with CCL22 deficiency showed prolonged survival upon vaccination, and further, CCL22-deficient mice had increased susceptibility to inflammatory disease. In conclusion, we identify the CCL22–CCR4 axis as an immune checkpoint that is crucial for the control of T cell immunity.

Expression of the Phosphatase Ppef2 Controls Survival and Function of CD8+ Dendritic Cells

Apoptotic cell death of Dendritic cells (DCs) is critical for immune homeostasis. Although intrinsic mechanisms controlling DC death have not been fully characterized up to now, experimentally enforced inhibition of DC-death causes various autoimmune diseases in model systems. We have generated mice deficient for Protein Phosphatase with EF-Hands 2 (Ppef2), which is selectively expressed in CD8⁺ DCs, but not in other related DC subtypes such as tissue CD103⁺ DCs. Ppef2 is down-regulated rapidly upon maturation of DCs by toll-like receptor stimuli, but not upon triggering of CD40. Ppef2-deficient CD8⁺ DCs accumulate the pro-apoptotic Bcl-2-like protein 11 (Bim) and show increased apoptosis and reduced competitve repopulation capacities. Furthermore, Ppef2^−/− CD8⁺ DCs have strongly diminished antigen presentation capacities in vivo, as CD8⁺ T cells primed by Ppef2^−/− CD8⁺ DCs undergo reduced expansion. In conclusion, our data suggests that Ppef2 is crucial to support survival of immature CD8⁺ DCs, while Ppef2 down-regulation during DC-maturation limits T cell responses.

Spinal cord injury-induced immunodeficiency is mediated by a sympathetic-neuroendocrine adrenal reflex

Acute spinal cord injury (SCI) causes systemic immunosuppression and life-threatening infections, thought to result from noradrenergic overactivation and excess glucocorticoid release via hypothalamus-pituitary-adrenal axis stimulation. Instead of consecutive hypothalamus-pituitary-adrenal axis activation, we report that acute SCI in mice induced suppression of serum norepinephrine and concomitant increase in cortisol, despite suppressed adrenocorticotropic hormone, indicating primary (adrenal) hypercortisolism. This neurogenic effect was more pronounced after high-thoracic level (Th1) SCI disconnecting adrenal gland innervation, compared with low-thoracic level (Th9) SCI. Prophylactic adrenalectomy completely prevented SCI-induced glucocorticoid excess and lymphocyte depletion but did not prevent pneumonia. When adrenalectomized mice were transplanted with denervated adrenal glands to restore physiologic glucocorticoid levels, the animals were completely protected from pneumonia. These findings identify a maladaptive sympathetic-neuroendocrine adrenal reflex mediating immunosuppression after SCI, implying that therapeutic normalization of the glucocorticoid and catecholamine imbalance in SCI patients could be a strategy to prevent detrimental infections.

Cell division cycle 42 (Cdc42) regulates functions of plasmacytoid dendritic cells

Cell division cycle 42 (Cdc42) is a member of the Rho guanosine triphosphatase family and has pivotal functions in actin organization, cell migration and proliferation. We recently demonstrated that conventional dendritic cells with a Cdc42 deficiency show a significant reduction in lysosomal MHC class II–processing proteins, such as cathepsins, as they are lost by enhanced secretion.

Similarly, we found that Cdc42-deficient plasmacytoid dendritic cells (pDCs) are phenotypically more mature as they up-regulate MHCII and the co-stimulatory molecule CD86. Furthermore, their homing capacity is strongly impaired in steady state causing strongly reduced pDC cell numbers in peripheral organs. In addition, after inflammatory stimulation pDC infiltration is impaired when Cdc42 is absent. We analyzed Cdc42 deficient pDCs in more detail to understand the molecular contribution of Cdc42 in pDC functions and their migratory behavior.

Cdc42-dependent actin dynamics controls maturation and secretory activity of dendritic cells

Cdc42 control of actin dynamics keeps DCs in an immature state, and loss of Cdc42 activity facilitates secretion and rapid up-regulation of intracellular molecules to the cell surface, which shows that Cdc42 contributes to DC immunogenicity by regulating the DC actin cytoskeleton.

Cell division cycle 42 (Cdc42) is a member of the Rho guanosine triphosphatase family and has pivotal functions in actin organization, cell migration, and proliferation. To further study the molecular mechanisms of dendritic cell (DC) regulation by Cdc42, we used Cdc42-deficient DCs. Cdc42 deficiency renders DCs phenotypically mature as they up-regulate the co-stimulatory molecule CD86 from intracellular storages to the cell surface. Cdc42 knockout DCs also accumulate high amounts of invariant chain–major histocompatibility complex (MHC) class II complexes at the cell surface, which cannot efficiently present peptide antigens (Ag’s) for priming of Ag-specific CD4 T cells. Proteome analyses showed a significant reduction in lysosomal MHC class II–processing proteins, such as cathepsins, which are lost from DCs by enhanced secretion. As these effects on DCs can be mimicked by chemical actin disruption, our results propose that Cdc42 control of actin dynamics keeps DCs in an immature state, and cessation of Cdc42 activity during DC maturation facilitates secretion as well as rapid up-regulation of intracellular molecules to the cell surface.

Antigen availability is a critical determinant of in vivo CD8+ T cell-dendritic cell interaction kinetics and initiates effector/memory fate decisions (110.21)

Naïve CD8+ T cells are activated through interactions with antigen (Ag) bearing dendritic cells (DCs) in lymph nodes (LNs). Multiphoton intravital microscopy (MP-IVM) of LNs has demonstrated that T cell activation occurs in three phases. The duration of the initial phase of transient, serial DC-T cell interactions is inversely related to Ag dose. It had been assumed that the second phase, characterized by stable DC-T cell contacts, is necessary for T cell activation, however, here we show that this is not always the case. T cells interacting with DCs presenting Ag at low abundance and with a short half-life did not transition from phase one to phase two, whereas a higher dose of the same Ag yielded robust phase two transition. T cells exposed to either antigenic constellation proliferated and developed equivalent effector functions, including cytokine production and cytotoxicity, over the first two days after priming. There are profound differences in the transcriptome at 12h and 24h after activation between CD8 T cells exposed to antigenic constellations that did or did not promote phase two interactions. Only T cells that had undergone stable contacts gave rise to memory, whereas activating conditions that did not support stable contacts yielded immunological amnesia. T cells can make fate decisions in vivo within a few hours after Ag exposure that result in long-term memory or abortive effector responses and correlate with the kinetics of T cell-DCs interactions.

Intravital two-photon microscopy of immune cells in visceral adipose tissue (173.38)

Obesity causes systemic low-grade inflammation predisposing to metabolic disorders including insulin resistance, diabetes, dyslipidemia and hepatosteatosis and vascular diseases. The unique repertoire of the immune population, in particular T cells and high number of macrophages within the white adipose tissue is an intriguing phenomenon. Previously, it has been shown that both cell types are implicated in adipose-derived inflammation and insulin resistance. We analyzed their cell-cell interactions, and their interactions with adipocytes to see if they result in an increased expression of leukocyte adhesion molecules, chemokines, and cytokines in adipose tissue, thereby enhancing the inflammatory response and affecting insulin signaling. For this purpose, we established the intravital two-photon microscopy on adipose tissue. Furthermore, we compare macrophages and different T cell subtypes in lean and obese mice and describe their localization and proximity to different sized adipocytes, and their migratory behavior within the tissue.

A Distinct Population of Lipid Sensing iNKT Cells are Enriched in Fat and Protect Against Diet-Induced Obesity and Insulin Resistance (176.26)

Immunometabolic interactions regulate adipose tissue inflammation in obesity and subsequent development of insulin resistance. We have shown that human and murine fat contains the largest population of iNKT cells in the body. Fat resident iNKT cells are a distinct subset with unique phenotype and anti-inflammatory cytokine profile. These iNKT cells are depleted in diet-induced obesity and removal of high fat diet results in iNKT restoration in mice and humans. Furthermore, mice lacking iNKT cells have enhanced weight gain, larger adipocyte size, fatty liver infiltration and insulin resistance. Adoptive transfer of iNKT from WT mice into obese mice or activation of remaining iNKT in obese fat by through injection of alpha- galactosylceramide (aGC) had a profound improvement in insulin sensitivity and glucose handling, a decrease in serum triglycerides, leptin, body fat, fatty liver infiltration and proinflammatory macrophage levels. It is therefore beneficial to identify what causes iNKT cells depletion in obesity. We have purified lipids from obese and lean adipose tissue, to identify which, if any, lipid fraction may activate iNKT cells. Early preliminary data suggest that a HFD abherrently activate iNKT cells in obesity, leading to their loss, and subsequent development of insulin resistance. In summary, we have identified a distinct population if iNKT cells in fat and describe a unique role for iNKT in protection against obesity and related metabolic disease. .

[Pediatric hyperglycemic hyperosmolar coma diabeticum: diagnostic evaluation and therapeutic concept]

Hyperglycemic hyperosmolar coma diabeticum (HHS) is a rare phenomenon in pediatric patients. It causes major morbidity and significant mortality. It is characterized by the trias of hyperglycemia, hyperosmolality and absent or mild metabolic acidosis. Major complications include cerebral edema and rhabdomyolysis. Evidence based guidelines for HHS in children are lacking. Based on a literature review we discuss treatment options in pediatric HHS und suggest a therapeutic concept. Appropriate treatment consists of adequate fluid administration and a cautious lowering of the serum glucose level. Patients should be treated on an intensive care unit and monitored closely to avoid complications. Low-dose and late insulin administration seems to be favourable.

Transcriptional signatures of immune cells in aryl hydrocarbon receptor (AHR)-proficient and AHR-deficient mice

The ligand-activated aryl hydrocarbon receptor (AHR) is known to modulate many genes in a highly cell-specific manner, either directly or indirectly via secondary effects. In contrast, little is known about the effects of AHR deficiency on gene expression balance. We compared the transcriptome of CD4 T cells from AHR-/- mice and wild-type mice; 390 genes, many of them immunotypic, were deregulated in AHR-deficient CD4 cells. TCDD-induced transcriptome changes correlated with the AHR expression level in immune cells. However, there was little overlap in AHR-dependent transcripts found in T lineage cells or dendritic cells. Our results demonstrate flexible gene accessibility for the AHR in immune cells. The idea of a universal battery of AHR-responsive genes is not tenable.