Felix M. Wensveen

NK cells link obesity-induced adipose stress to inflammation and insulin resistance

An important cause of obesity-induced insulin resistance is chronic systemic inflammation originating in visceral adipose tissue (VAT). VAT inflammation is associated with the accumulation of proinflammatory macrophages in adipose tissue, but the immunological signals that trigger their accumulation remain unknown. We found that a phenotypically distinct population of tissue-resident natural killer (NK) cells represented a crucial link between obesity-induced adipose stress and VAT inflammation. Obesity drove the upregulation of ligands of the NK cell–activating receptor NCR1 on adipocytes; this stimulated NK cell proliferation and interferon-γ (IFN-γ) production, which in turn triggered the differentiation of proinflammatory macrophages and promoted insulin resistance. Deficiency of NK cells, NCR1 or IFN-γ prevented the accumulation of proinflammatory macrophages in VAT and greatly ameliorated insulin sensitivity. Thus NK cells are key regulators of macrophage polarization and insulin resistance in response to obesity-induced adipocyte stress.

GITR Triggering Induces Expansion of Both Effector and Regulatory CD4+ T Cells In Vivo

Glucocorticoid-induced TNF receptor family-related protein (GITR) is expressed on activated and regulatory T cells, but its role on these functionally opposing cell types is not fully understood. Here we describe that transgenic expression of GITR’s unique ligand (GITRL) induces a prominent increase of both effector and regulatory CD4+ T cells, but not CD8+ T cells. Regulatory T cells from GITRL transgenic mice are phenotypically activated and retain their suppressive capacity. The accumulation of effector and regulatory T cells is not due to enhanced differentiation of naive T cells, but is a direct result of increased proliferation. Functional consequences of increased numbers of both regulatory and effector T cells were tested in an autoimmune model and show that GITR stimulation is protective, as it significantly delays disease induction. These data indicate that GITR regulates the balance between regulatory and effector CD4+ T cells by enhancing proliferation of both populations in parallel.

The “Big Bang” in obese fat: events initiating obesity-induced adipose tissue inflammation

Obesity is associated with the accumulation of pro‐inflammatory cells in visceral adipose tissue (VAT), which is an important underlying cause of insulin resistance and progression to diabetes mellitus type 2 (DM2). Although the role of pro‐inflammatory cytokines in disease development is established, the initiating events leading to immune cell activation remain elusive. Lean adipose tissue is predominantly populated with regulatory cells, such as eosinophils and type 2 innate lymphocytes. These cells maintain tissue homeostasis through the excretion of type 2 cytokines, such as IL‐4, IL‐5, and IL‐13, which keep adipose tissue macrophages (ATMs) in an anti‐inflammatory, M2‐like state. Diet‐induced obesity is associated with the loss of tissue homeostasis and development of type 1 inflammatory responses in VAT, characterized by IFN‐γ. A key event is a shift of ATMs toward an M1 phenotype. Recent studies show that obesity‐induced adipocyte hypertrophy results in upregulated surface expression of stress markers. Adipose stress is detected by local sentinels, such as NK cells and CD8+ T cells, which produce IFN‐γ, driving M1 ATM polarization. A rapid accumulation of pro‐inflammatory cells in VAT follows, leading to inflammation. In this review, we provide an overview of events leading to adipose tissue inflammation, with a special focus on adipose homeostasis and the obesity‐induced loss of homeostasis which marks the initiation of VAT inflammation.

Regulation of immune cell function and differentiation by the NKG2D receptor

NKG2D is one of the most intensively studied immune receptors of the past decade. Its unique binding and signaling properties, expression pattern, and functions have been attracting much interest within the field due to its potent antiviral and anti-tumor properties. As an activating receptor, NKG2D is expressed on cells of the innate and adaptive immune system. It recognizes stress-induced MHC class I-like ligands and acts as a molecular sensor for cells jeopardized by viral infections or DNA damage. Although the activating functions of NKG2D have been well documented, recent analysis of NKG2D-deficient mice suggests that this receptor may have a regulatory role during NK cell development. In this review, we will revisit known aspects of NKG2D functions and present new insights in the proposed influence of this molecule on hematopoietic differentiation.

Apoptosis Threshold Set by Noxa and Mcl-1 after T Cell Activation Regulates Competitive Selection of High-Affinity Clones

The adaptive immune system generates protective T cell responses via a poorly understood selection mechanism that favors expansion of clones with optimal affinity for antigen. Here we showed that upon T cell activation, the proapoptotic molecule Noxa (encoded by Pmaip1) and its antagonist Mcl-1 were induced. During an acute immune response against influenza or ovalbumin, Pmaip1(-/-) effector T cells displayed decreased antigen affinity and functionality. Molecular analysis of influenza-specific T cells revealed persistence of many subdominant clones in the Pmaip1(-/-) effector pool. When competing for low-affinity antigen, Pmaip1(-/-) TCR transgenic T cells had a survival advantage in vitro, resulting in increased numbers of effector cells in vivo. Mcl-1 protein stability was controlled by T cell receptor (TCR) affinity-dependent interleukin-2 signaling. These results establish a role for apoptosis early during T cell expansion, based on antigen-driven competition and survival of the fittest T cells.

Chronic IFN-γ production in mice induces anemia by reducing erythrocyte life span and inhibiting erythropoiesis through an IRF-1/PU.1 axis

Anemia of chronic disease is a complication accompanying many inflammatory diseases. The proinflammatory cytokine IFN-γ has been implicated in this form of anemia, but the underlying mechanism remains unclear. Here we describe a novel mouse model for anemia of chronic disease, in which enhanced CD27-mediated costimulation strongly increases the formation of IFN-γ-producing effector T cells, leading to a progressive anemia. We demonstrate that the anemia in these mice is fully dependent on IFN-γ and that this cytokine reduces both the life span and the formation of red blood cells. Molecular analysis revealed that IFN-γ induces expression of the transcription factors of interferon regulatory factor-1 (IRF-1) and PU.1 in both murine and human erythroid precursors. We found that, on IFN-γ stimulation, IRF-1 binds to the promoter of SPI.1 (PU.1) and induces PU.1 expression, leading to inhibition of erythropoiesis. Notably, down-regulation of either IRF-1 or PU.1 expression is sufficient to overcome IFN-γ-induced inhibition of erythropoiesis. These findings reveal a molecular mechanism by which chronic exposure to IFN-γ induces anemia.

Human CD34-Derived Myeloid Dendritic Cell Development Requires Intact Phosphatidylinositol 3-Kinase–Protein Kinase B–Mammalian Target of Rapamycin Signaling

Dendritic cells (DCs) are composed of different subsets that exhibit distinct functionality in the induction and regulation of immune responses. The myeloid DC subsets, including interstitial DCs and Langerhans cells (LCs), develop from CD34+ hematopoietic progenitors via direct DC precursors or monocytes. The molecular mechanisms regulating DC development are still largely unknown and mostly studied in mice. Phosphatidylinositol 3-kinase (PI3K) regulates multiple processes in myeloid cells. This study investigated the role of PI3K signaling in the development of human CD34-derived myeloid DCs. Pharmacologic inhibition of PI3K or one of its downstream targets mTOR reduced interstitial DC and LC numbers in vitro. Increased activity of this signaling module by introduction of constitutively active protein kinase B (PKB/c-Akt) increased the yields of human DC precursors in vitro as well as in transplanted β2-microglobulin−/− NOD/SCID mice in vivo. Signaling inhibition during differentiation did not affect the acquisition of a DC phenotype, whereas proliferation and survival strongly depended on intact PI3K–PKB–mTOR signaling. Interestingly, however, this pathway became redundant for survival regulation upon terminal differentiation, which was associated with an altered expression of apoptosis regulating genes. Although dispensable for costimulatory molecule expression, the PI3K–PKB–mTOR signaling module was required for other important processes associated with DC function, including Ag uptake, LPS-induced cytokine secretion, CCR7 expression, and T cell stimulation. Thus, PI3K–PKB–mTOR signaling plays a crucial role in the development of functional CD34-derived myeloid DCs. These findings could be used as a strategy to manipulate DC subset distribution and function to regulate immunity.

CD27-CD70 Costimulation Controls T Cell Immunity during Acute and Persistent Cytomegalovirus Infection

ABSTRACT Cytomegaloviruses (CMVs) establish lifelong infections that are controlled in part by CD4+ and CD8+ T cells. To promote persistence, CMVs utilize multiple strategies to evade host immunity, including modulation of costimulatory molecules on infected antigen-presenting cells. In humans, CMV-specific memory T cells are characterized by the loss of CD27 expression, which suggests a critical role of the costimulatory receptor-ligand pair CD27-CD70 for the development of CMV-specific T cell immunity. In this study, the in vivo role of CD27-CD70 costimulation during mouse CMV infection was examined. During the acute phase of infection, the magnitudes of CMV-specific CD4+ and CD8+ T cell responses were decreased in mice with abrogated CD27-CD70 costimulation. Moreover, the accumulation of inflationary memory T cells during the persistent phase of infection and the ability to undergo secondary expansion required CD27-CD70 interactions. The downmodulation of CD27 expression, however, which occurs gradually and exclusively on inflationary memory T cells, is ligand independent. Furthermore, the IL-2 production in both noninflationary and inflationary CMV-specific T cells was dependent on CD27-CD70 costimulation. Collectively, these results highlight the importance of the CD27-CD70 costimulation pathway for the development of CMV-specific T cell immunity during acute and persistent infection.

Mouse Hobit is a homolog of the transcriptional repressor Blimp-1 that regulates NKT cell effector differentiation

The transcriptional repressor Blimp-1 mediates the terminal differentiation of many cell types, including T cells. Here we identified Hobit (Znf683) as a previously unrecognized homolog of Blimp-1 that was specifically expressed in mouse natural killer T cells (NKT cells). Through studies of Hobit-deficient mice, we found that Hobit was essential for the formation of mature thymic NKT cells. In the periphery, Hobit repressed the accumulation of interferon-γ (IFN-γ)-producing NK1.1lo NKT cells at steady state. After antigenic stimulation, Hobit repressed IFN-γ expression, whereas after innate stimulation, Hobit induced granzyme B expression. Thus, reminiscent of the function of Blimp-1 in other lymphocytes, Hobit controlled the maintenance of quiescent, fully differentiated NKT cells and regulated their immediate effector functions.

NKG2D Induces Mcl-1 Expression and Mediates Survival of CD8 Memory T Cell Precursors via Phosphatidylinositol 3-Kinase

Memory formation of activated CD8 T cells is the result of a specific combination of signals that promote long-term survival and inhibit differentiation into effector cells. Much is known about initial cues that drive memory formation, but it is poorly understood which signals are essential during the intermediate stages before terminal differentiation. NKG2D is an activating coreceptor on Ag-experienced CD8 T cells that promotes effector cell functions. Its role in memory formation is currently unknown. In this study, we show that NKG2D controls formation of CD8 memory T cells by promoting survival of precursor cells. We demonstrate that NKG2D enhances IL-15–mediated PI3K signaling of activated CD8 T cells, in a specific phase of memory cell commitment, after activation but before terminal differentiation. This signal is essential for the induction of prosurvival protein Mcl-1 and precursor cell survival. In vivo, NKG2D deficiency results in reduced memory cell formation and impaired protection against reinfection. Our findings show a new role for PI3K and the NKG2D/IL-15 axis in an underappreciated stage of effector to memory cell transition that is essential for the generation of antiviral immunity. Moreover, we provide novel insights how these receptors control both effector and memory T cell differentiation.