Hongmei Fu

Molecular mechanisms of metabolic reprogramming in proliferating cells: implications for T cell-mediated immunity

To engage in proliferation, cells need to increase their biomass and replicate their genome. This process presents a substantial bioenergetic challenge: proliferating cells must increase ATP production and acquire or synthesize raw materials, including lipids, proteins and nucleic acids. To do so, proliferating cells actively reprogramme their intracellular metabolism from catabolic mitochondrial oxidative phosphorylation (OXPHOS) to glycolysis and other anabolic pathways. This metabolic reprogramming, which directs nutrient uptake and metabolism during cell activation and proliferation, is under the control of specific signal transduction pathways. The underlying molecular mechanisms of cell metabolism reprogramming and their relevance to physiology and disease are currently under intense study. Several reports have uncovered the mechanisms of metabolic reprogramming that drive high rates of cell proliferation in cancer. Some recent studies have elucidated the physiological role of metabolic reprogramming during T‐cell activation, differentiation and trafficking, which are potentially relevant to inflammatory disorders. This review describes the impact of metabolic reprogramming on the pathogenesis of cancer and the physiology of T‐cell‐mediated immune responses, with an emphasis on the phosphatidyl inositol 3‐kinase–serine/threonine kinase–mammalian target of rapamycin pathway and the recently discovered metabolic processes regulated by nuclear factor‐κB. These discoveries will hopefully translate into a better understanding of the role of metabolic reprogramming as a key regulator of T‐cell‐mediated immune responses and offer novel, immune‐based therapeutic approaches.

Ig gene-like molecule CD31 plays a nonredundant role in the regulation of T-cell immunity and tolerance

CD31 is an Ig-like molecule expressed by leukocytes and endothelial cells with an established role in the regulation of leukocyte trafficking. Despite genetic deletion of CD31 being associated with exacerbation of T cell-mediated autoimmunity, the contribution of this molecule to T-cell responses is largely unknown. Here we report that tumor and allograft rejection are significantly enhanced in CD31-deficient mice, which are also resistant to tolerance induction. We propose that these effects are dependent on an as yet unrecognized role for CD31-mediated homophilic interactions between T cells and antigen-presenting cells (APCs) during priming. We show that loss of CD31 interactions leads to enhanced primary clonal expansion, increased killing capacity, and diminished regulatory functions by T cells. Immunomodulation by CD31 signals correlates with a partial inhibition of proximal T-cell receptor (TCR) signaling, specifically Zap-70 phosphorylation. However, CD31-deficient mice do not develop autoimmunity due to increased T-cell death following activation, and we show that CD31 triggering induces Erk-mediated prosurvival activity in T cells either in conjunction with TCR signaling or autonomously. We conclude that CD31 functions as a nonredundant comodulator of T-cell responses, which specializes in sizing the ensuing immune response by setting the threshold for T-cell activation and tolerance, while preventing memory T-cell death.

Self-recognition of the endothelium enables regulatory T-cell trafficking and defines the kinetics of immune regulation

Localization of CD4+CD25+Foxp3+ regulatory T (Treg) cells to lymphoid and non-lymphoid tissue is instrumental for the effective control of immune responses. Compared with conventional T cells, Treg cells constitute a minute fraction of the T-cell repertoire. Despite this numeric disadvantage, Tregs efficiently migrate to sites of immune responses reaching an optimal number for the regulation of T effector (Teff) cells. The array and levels of adhesion and chemokine receptor expression by Tregs do not explain their powerful migratory capacity. Here we show that recognition of self-antigens expressed by endothelial cells in target tissue is instrumental for efficient Treg recruitment in vivo. This event relies upon IFN-γ-mediated induction of MHC-class-II molecule expression by the endothelium and requires optimal PI3K p110δ activation by the T-cell receptor. We also show that, once in the tissue, Tregs inhibit Teff recruitment, further enabling a Teff:Treg ratio optimal for regulation.

3-hydroxykynurenine suppresses CD4+ T-cell proliferation, induces T-regulatory-cell development, and prolongs corneal allograft survival

PURPOSE IDO (indoleamine 2,3-dioxygenase) modulates the immune response by depletion of the essential amino acid tryptophan, and IDO overexpression has been shown to prolong corneal allograft survival. This study was conducted to examine the effect of kynurenines, the products of tryptophan breakdown and known to act directly on T lymphocytes, on corneal graft survival. METHODS The effects of kynurenines on T-cell proliferation and death, T-regulatory-cell development, and dendritic cell function, phenotype, and viability were analyzed in vitro. The effect of topical and systemic administration of 3-hydroxykynurenine (3HK) on orthotopic murine corneal allograft survival was examined. RESULTS T-lymphocyte proliferation was inhibited by two of the four different kynurenines: 3HK and 3-hydroxyanthranilic acid (3HAA). This effect was accompanied by significant T-cell death. Neither 3HK nor 3HAA altered dendritic cell function, nor did they induce apoptosis or pathogenicity to corneal endothelial cells. Administration of systemic and topical 3HK to mice receiving a fully mismatched corneal graft resulted in significant prolongation of graft survival (median survival of control grafts, 12 days; of treated, 19 and 15 days, respectively; P < 0.0003). While systemic administration of 3HK was associated with a significant depletion of CD4(+) T, CD8(+) T, and B lymphocytes in peripheral blood, no depletion was found after topical administration. CONCLUSIONS The production of kynurenines, in particular 3HK and 3HAA, may be one mechanism (in addition to tryptophan depletion) by which IDO prolongs graft survival. These molecules have potential as specific agents for preventing allograft rejection in patients at high rejection risk.

Immune modulation in corneal transplantation

Allograft rejection is the most common reason for corneal transplant failure, despite the immunologic privilege of both the graft and the anterior chamber. To prevent corneal allograft rejection, various immunomodulatory strategies have been used in experimental corneal transplantation. These include (1) anti-T-cell receptor and T-cell depletion therapy; (2) manipulation of costimulatory molecule function, including both down-regulation of positive stimulatory molecules and/or up-regulation of inhibitory molecules and overproduction of tumor necrosis factor-related, apoptosis-induced ligand; (3) modulation of cytokine production by reducing proinflammatory cytokines (tumor necrosis factor alpha, interleukin [IL]-12, and IL-1) and/or increasing immunoregulatory cytokines (IL-10 and IL-4); (4) macrophage depletion; and (5) overexpression of the immunomodulatory molecule indoleamine 2,3-dioxygenase. Although these approaches appear promising in animal corneal transplantation models, there has been very little translation of these immunomodulatory approaches in human corneal transplantation.

Hepatocyte Growth Factor Receptor c-Met Instructs T Cell Cardiotropism and Promotes T Cell Migration to the Heart via Autocrine Chemokine Release.

Summary Effector-T-cell-mediated immunity depends on the efficient localization of antigen-primed lymphocytes to antigen-rich non-lymphoid tissue, which is facilitated by the expression of a unique set of “homing” receptors acquired by memory T cells. We report that engagement of the hepatocyte growth factor (HGF) receptor c-Met by heart-produced HGF during priming in the lymph nodes instructs T cell cardiotropism, which was associated with a specialized homing “signature” (c-Met+CCR4+CXCR3+). c-Met signals facilitated T cell recruitment to the heart via the chemokine receptor CCR5 by inducing autocrine CCR5 ligand release. c-Met triggering was sufficient to support cardiotropic T cell recirculation, while CCR4 and CXCR3 sustained recruitment during heart inflammation. Transient pharmacological blockade of c-Met during T cell priming led to enhanced survival of heart, but not skin, allografts associated with impaired localization of alloreactive T cells to heart grafts. These findings suggest c-Met as a target for development of organ-selective immunosuppressive therapies.

Memory T-cell trafficking: new directions for busy commuters

The immune system is unique in representing a network of interacting cells of enormous complexity and yet being based on single cells travelling around the body. The development of effective and regulated immunity relies upon co‐ordinated migration of each cellular component, which is regulated by diverse signals provided by the tissue. Co‐ordinated migration is particularly relevant to the recirculation of primed T cells, which, while performing continuous immune surveillance, need to promptly localize to antigenic sites, reside for a time sufficient to carry out their effector function and then efficiently leave the tissue to avoid bystander damage. Recent advances that have helped to clarify a number of key molecular mechanisms underlying the complexity and efficiency of memory T‐cell trafficking, including antigen‐dependent T‐cell trafficking, the regulation of T‐cell motility by costimulatory molecules, T‐cell migration out of target tissue and fugetaxis, are reviewed in this article.

Obesity-induced metabolic stress leads to biased effector memory CD4+ T cell differentiation via PI3K p110δ/Akt-mediated signals

Summary Low-grade systemic inflammation associated to obesity leads to cardiovascular complications, caused partly by infiltration of adipose and vascular tissue by effector T cells. The signals leading to T cell differentiation and tissue infiltration during obesity are poorly understood. We tested whether saturated fatty acid-induced metabolic stress affects differentiation and trafficking patterns of CD4+ T cells. Memory CD4+ T cells primed in high-fat diet-fed donors preferentially migrated to non-lymphoid, inflammatory sites, independent of the metabolic status of the hosts. This was due to biased CD4+ T cell differentiation into CD44hi-CCR7lo-CD62Llo-CXCR3+-LFA1+ effector memory-like T cells upon priming in high-fat diet-fed animals. Similar phenotype was observed in obese subjects in a cohort of free-living people. This developmental bias was independent of any crosstalk between CD4+ T cells and dendritic cells and was mediated via direct exposure of CD4+ T cells to palmitate, leading to increased activation of a PI3K p110δ-Akt-dependent pathway upon priming.

Arginine depletion as a mechanism for the immune privilege of corneal allografts

The cornea is an immune privileged tissue. Since arginase has been found to modulate T‐cell function by depleting arginine, we investigated the expression of arginase in the cornea and its possible role in immune privilege using a murine transplant model. We found that both the endothelium and epithelium of murine corneas express functional arginase I, capable of down‐regulating T‐cell proliferation in an in vitro culture system. The administration of the specific arginase inhibitor N‐hydroxy‐nor‐L‐Arg to recipient mice resulted in an accelerated rejection of allogeneic C57BL/6 (B6) corneal grafts. In contrast, in vivo blockade of arginase activity had no effect in altering the course of rejection of primary skin grafts that express little, if any, arginase. In addition, the inhibition of arginase did not alter systemic T‐cell proliferation. These data show that arginase is functional in the cornea and contributes to the immune privilege of the eye, and that modulation of arginase contributes to graft survival.

Regulatory T Cell Migration Is Dependent on Glucokinase-Mediated Glycolysis

SUMMARY Migration of activated regulatory T (Treg) cells to inflamed tissue is crucial for their immune‐modulatory function. While metabolic reprogramming during Treg cell differentiation has been extensively studied, the bioenergetics of Treg cell trafficking remains undefined. We have investigated the metabolic demands of migrating Treg cells in vitro and in vivo. We show that glycolysis was instrumental for their migration and was initiated by pro‐migratory stimuli via a PI3K‐mTORC2‐mediated pathway culminating in induction of the enzyme glucokinase (GCK). Subsequently, GCK promoted cytoskeletal rearrangements by associating with actin. Treg cells lacking this pathway were functionally suppressive but failed to migrate to skin allografts and inhibit rejection. Similarly, human carriers of a loss‐of‐function GCK regulatory protein gene—leading to increased GCK activity—had reduced numbers of circulating Treg cells. These cells displayed enhanced migratory activity but similar suppressive function, while conventional T cells were unaffected. Thus, GCK‐dependent glycolysis regulates Treg cell migration. HIGHLIGHTSMigration of regulatory T (Treg) cells requires glycolysisThis is mediated by the enzyme glucokinase induced by a PI3K‐mTORC2 pathwayTreg cells lacking this pathway are unable to localize to inflammatory sitesA loss‐of‐function GCK regulator gene causes enhanced motility of human Treg cells Regulatory T cell localization to inflammatory sites is key to their homeostatic function. Kishore and colleagues demonstrate that Treg cell migration requires the activation of glycolysis by the enzyme glucokinase induced via a Treg cell‐selective PI3K‐mTORC2 pathway.