Metabolic switching and fuel choice during T-cell differentiation and memory development

Metabolic Reprogramming in Resting and Activated Immune Cells

Immune cell activation and proliferation are closely linked to their metabolic programming. These activated immune cells share many features with tumor cells and are capable to respond to stimulations quickly and reprogram their metabolism to fight with invading pathogens. The corresponding changes in metabolism provide immune cells with energy and bio-precursors to match with necessity of immune functions. The major metabolic pathways utilized by immune cells for the purpose of protecting body from invading pathogens are glycolysis, glutaminolysis, fatty acid synthesis and oxidation, and mitochondria oxidative phosphorylation. These pathways play crucial roles in immune cell activation and differentiation. In this review, we describe how immune cells engage in certain metabolic processes according to their functional needs with a focus on T cells and macrophages.

Metabolic Hallmarks of Tumor and Immune Cells in the Tumor Microenvironment

Cytotoxic T lymphocytes and NK cells play an important role in eliminating malignant tumor cells and the number and activity of tumor-infiltrating T cells represent a good marker for tumor prognosis. Based on these findings, immunotherapy, e.g., checkpoint blockade, has received considerable attention during the last couple of years. However, for the majority of patients, immune control of their tumors is gray theory as malignant cells use effective mechanisms to outsmart the immune system. Increasing evidence suggests that changes in tumor metabolism not only ensure an effective energy supply and generation of building blocks for tumor growth but also contribute to inhibition of the antitumor response. Immunosuppression in the tumor microenvironment is often based on the mutual metabolic requirements of immune cells and tumor cells. Cytotoxic T and NK cell activation leads to an increased demand for glucose and amino acids, a well-known feature shown by tumor cells. These close metabolic interdependencies result in metabolic competition, limiting the proliferation, and effector functions of tumor-specific immune cells. Moreover, not only nutrient restriction but also tumor-driven shifts in metabolite abundance and accumulation of metabolic waste products (e.g., lactate) lead to local immunosuppression, thereby facilitating tumor progression and metastasis. In this review, we describe the metabolic interplay between immune cells and tumor cells and discuss tumor cell metabolism as a target structure for cancer therapy. Metabolic (re)education of tumor cells is not only an approach to kill tumor cells directly but could overcome metabolic immunosuppression in the tumor microenvironment and thereby facilitate immunotherapy.

Amino-acid transporters in T-cell activation and differentiation

T-cell-mediated immune responses aim to protect mammals against cancers and infections, and are also involved in the pathogenesis of various inflammatory or autoimmune diseases. Cellular uptake and the utilization of nutrients is closely related to the T-cell fate decision and function. Research in this area has yielded surprising findings in the importance of amino-acid transporters for T-cell development, homeostasis, activation, differentiation and memory. In this review, we present current information on amino-acid transporters, such as LAT1 (l-leucine transporter), ASCT2 (l-glutamine transporter) and GAT-1 (γ-aminobutyric acid transporter-1), which are critically important for mediating peripheral naive T-cell homeostasis, activation and differentiation, especially for Th1 and Th17 cells, and even memory T cells. Mechanically, the influence of amino-acid transporters on T-cell fate decision may largely depend on the mechanistic target of rapamycin complex 1 (mTORC1) signaling. These discoveries remarkably demonstrate the role of amino-acid transporters in T-cell fate determination, and strongly indicate that manipulation of the amino-acid transporter-mTORC1 axis could ameliorate many inflammatory or autoimmune diseases associated with T-cell-based immune responses.

The Emerging Facets of Non-Cancerous Warburg Effect

The Warburg effect (WE), or aerobic glycolysis, is commonly recognized as a hallmark of cancer and has been extensively studied for potential anti-cancer therapeutics development. Beyond cancer, the WE plays an important role in many other cell types involved in immunity, angiogenesis, pluripotency, and infection by pathogens (e.g., malaria). Here, we review the WE in non-cancerous context as a "hallmark of rapid proliferation." We observe that the WE operates in rapidly dividing cells in normal and pathological states that are triggered by internal and external cues. Aerobic glycolysis is also the preferred metabolic program in the cases when robust transient responses are needed. We aim to draw attention to the potential of computational modeling approaches in systematic characterization of common metabolic features beyond the WE across physiological and pathological conditions. Identification of metabolic commonalities across various diseases may lead to successful repurposing of drugs and biomarkers.

Dual Specificity Phosphatase 5 Is Essential for T Cell Survival

The mitogen-activated protein kinase (MAPK) pathway regulates many key cellular processes such as differentiation, apoptosis, and survival. The final proteins in this pathway, ERK1/2, are regulated by dual specificity phosphatase 5 (DUSP5). DUSP5 is a nuclear, inducible phosphatase with high affinity and fidelity for ERK1/2. By regulating the final step in the MAPK signaling cascade, DUSP5 exerts strong regulatory control over a central cellular pathway. Like other DUSPs, DUSP5 plays an important role in immune function. In this study, we have utilized new knockout mouse reagents to explore its function further. We demonstrate that global loss of DUSP5 does not result in any gross phenotypic changes. However, loss of DUSP5 affects memory/effector CD8⁺ T cell populations in response to acute viral infection. Specifically, Dusp5^-/- mice have decreased proportions of short-lived effector cells (SLECs) and increased proportions of memory precursor effector cells (MPECs) in response to infection. Further, we show that this phenotype is T cell intrinsic; a bone marrow chimera model restricting loss of DUSP5 to the CD8⁺ T cell compartment displays a similar phenotype. Dusp5^-/- T cells also display increased proliferation, increased apoptosis, and altered metabolic profiles, suggesting that DUSP5 is a pro-survival protein in T cells.

Metabolic pathways in T cell activation and lineage differentiation

Recent advances in the field of immunometabolism support the concept that fundamental processes in T cell biology, such as TCR-mediated activation and T helper lineage differentiation, are closely linked to changes in the cellular metabolic programs. Although the major task of the intermediate metabolism is to provide the cell with a constant supply of energy and molecular precursors for the production of biomolecules, the dynamic regulation of metabolic pathways also plays an active role in shaping T cell responses. Key metabolic processes such as glycolysis, fatty acid and mitochondrial metabolism are now recognized as crucial players in T cell activation and differentiation, and their modulation can differentially affect the development of T helper cell lineages. In this review, we describe the diverse metabolic processes that T cells engage during their life cycle from naïve towards effector and memory T cells. We consider in particular how the cellular metabolism may actively support the function of T cells in their different states. Moreover, we discuss how molecular regulators such as mTOR or AMPK link environmental changes to adaptations in the cellular metabolism and elucidate the consequences on T cell differentiation and function.

Mitochondrial Superoxide Signaling Contributes to Norepinephrine-Mediated T-Lymphocyte Cytokine Profiles

Norepinephrine (NE) produces multifaceted regulatory patterns in T-lymphocytes. Recently, we have shown that NE utilizes redox signaling as evidenced by increased superoxide (O2●-) causally linked to the observed changes in these cells; however, the source of this reactive oxygen species (ROS) remains elusive. Herein, we hypothesized that the source of increased O2●- in NE-stimulated T-lymphocytes is due to disruption of mitochondrial bioenergetics. To address this hypothesis, we utilized purified mouse splenic CD4+ and CD8+ T-lymphocytes stimulated with NE and assessed O2●- levels, mitochondrial metabolism, cellular proliferation, and cytokine profiles. We demonstrate that the increase in O2●- levels in response to NE is time-dependent and occurs at later points of T-lymphocyte activation. Moreover, the source of O2●- was indeed the mitochondria as evidenced by enhanced MitoSOX Red oxidation as well as abrogation of this signal by the addition of the mitochondrial-targeted O2●--scavenging antioxidant MitoTempol. NE-stimulated T-lymphocytes also demonstrated decreased mitochondrial respiratory capacity, which suggests disruption of mitochondrial metabolism and the potential source of increased mitochondrial O2●-. The effects of NE in regards to redox signaling appear to be adrenergic receptor-dependent as specific receptor antagonists could reverse the increase in O2●-; however, differential receptors regulating these processes were observed in CD4+ versus CD8+ T-lymphocytes. Finally, mitochondrial O2●- was shown to be mechanistic to the NE-mediated T-lymphocyte phenotype as supplementation of MitoTempol could reverse specific changes in cytokine expression observed with NE treatment. Overall, these studies indicate that mitochondrial metabolism and O2●--mediated redox signaling play a regulatory role in the T-lymphocyte response to NE.

Development of Therapeutics That Induce Mitochondrial Biogenesis for the Treatment of Acute and Chronic Degenerative Diseases

Mitochondria have various roles in cellular metabolism and homeostasis. Because mitochondrial dysfunction is associated with many acute and chronic degenerative diseases, mitochondrial biogenesis (MB) is a therapeutic target for treating such diseases. Here, we review the role of mitochondrial dysfunction in acute and chronic degenerative diseases and the cellular signaling pathways by which MB is induced. We then review existing work describing the development and application of drugs that induce MB in vitro and in vivo. In particular, we discuss natural products and modulators of transcription factors, kinases, cyclic nucleotides, and G protein-coupled receptors.

Gain-of-function mutations and immunodeficiency: at a loss for proper tuning of lymphocyte signaling

PURPOSE OF REVIEW

To present recent advances in the discovery and characterization of new immunodeficiency disorders linked to gain-of-function (GOF) mutations in immune signaling molecules. (Figure is included in full-text article.)

RECENT FINDINGS

In the past 2 years, extensive cellular and molecular studies have illuminated the root causes of pathogenesis for several new monogenic primary immunodeficiency disorders (PIDs) linked to GOF mutations in signaling molecules. Here we discuss on two disorders (BENTA and APDS/PASLI) featuring shared clinical presentation (e.g. lymphoproliferation, selective antibody deficiencies, recurrent sinopulmonary infections). These findings highlight an emerging theme: both loss-of-function and gain-of-function mutations in key molecules can disrupt finely tuned immunoreceptor signaling modalities, resulting in the dysregulation of lymphocyte differentiation and impaired adaptive immunity.

SUMMARY

Continued research on the molecular pathogenesis of PIDs defined by hyperactive signaling molecules will better distinguish these and related disorders, and pinpoint tailored therapeutic interventions for 'retuning' the immune response in these patients.

Conditions for the generation of cytotoxic CD4(+) Th cells that enhance CD8(+) CTL-mediated tumor regression

Adoptive cell therapies (ACTs) using tumor-reactive T cells have shown clinical benefit and potential for cancer treatment. While the majority of the current ACT are focused on using CD8⁺ cytotoxic T lymphocytes (CTL), others have shown that the presence of tumor-reactive CD4⁺ T helper (Th) cells can greatly enhance the anti-tumor activity of CD8⁺ CTL. However, difficulties in obtaining adequate numbers of CD4⁺ Th cells through in vitro expansion can limit the application of CD4 Th cells in ACT. This study aims to optimize the culture conditions for mouse CD4 T cells to provide basic information for animal studies of ACT using CD4 T cells. Taking advantage of the antigen-specificity of CD4⁺ Th cells from OT-II transgenic mice, we examined different methodologies for generating antigen-specific CD4⁺ Th1 cells in vitro. We found that cells grown in complete advanced-DMEM/F12 medium supplemented with low-dose IL-2 and IL-7 induced substantial cell expansion. These Th cells were Th1-like, as they expressed multiple Th1-cytokines and exhibited antigen-specific cytotoxicity. In addition co-transfer of these CD4⁺ Th1-like cells with CD8⁺ CTL significantly enhanced tumor regression, leading to complete cure in 80% of mice bearing established B16-OVA. These observations indicate that the CD4⁺ Th1-like cells generated using the method we optimized are functionally active to eliminate their target cells, and can also assist CD8⁺ CTL to enhance tumor regression. The findings of this study provide valuable data for further research into in vitro expansion of CD4⁺ Th1-like cells, with potential applications to cancer treatment involving ACT.

Fine-Tuning of CD8(+) T Cell Mitochondrial Metabolism by the Respiratory Chain Repressor MCJ Dictates Protection to Influenza Virus

Mitochondrial respiration is regulated in CD8(+) T cells during the transition from naive to effector and memory cells, but mechanisms controlling this process have not been defined. Here we show that MCJ (methylation-controlled J protein) acted as an endogenous break for mitochondrial respiration in CD8(+) T cells by interfering with the formation of electron transport chain respiratory supercomplexes. Metabolic profiling revealed enhanced mitochondrial metabolism in MCJ-deficient CD8(+) T cells. Increased oxidative phosphorylation and subcellular ATP accumulation caused by MCJ deficiency selectively increased the secretion, but not expression, of interferon-γ. MCJ also adapted effector CD8(+) T cell metabolism during the contraction phase. Consequently, memory CD8(+) T cells lacking MCJ provided superior protection against influenza virus infection. Thus, MCJ offers a mechanism for fine-tuning CD8(+) T cell mitochondrial metabolism as an alternative to modulating mitochondrial mass, an energetically expensive process. MCJ could be a therapeutic target to enhance CD8(+) T cell responses.

Mitochondria and Antiviral Immunity

Mitochondria are unique dynamic organelles that evolved from free-living bacteria into endosymbionts of mammalian hosts (Sagan 1967; Hatefi 1985). They have a distinct ~16.6 kb closed circular DNA genome coding for 13 polypeptides (Taanman 1999). In addition, a majority of the ~1500 mitochondrial proteins are encoded in the nucleus and transported to the mitochondria (Bonawitz et al. 2006). Mitochondria have two membranes: an outer smooth membrane and a highly folded inner membrane called cristae, which encompasses the matrix that houses the enzymes of the tricarboxylic acid (TCA) cycle and lipid metabolism. The inner mitochondrial membrane houses the protein complexes comprising the electron transport chain (ETC) (Hatefi 1985).

Immunometabolism in Tuberculosis

Immunometabolism, the study of the relationship between bioenergetic pathways and specific functions of immune cells, has recently gained increasing appreciation. In response to infection, activation of the host innate and adaptive immune cells is accompanied by a switch in the bioenergetic pathway from oxidative phosphorylation to glycolysis, a metabolic remodeling known as the Warburg effect, which is required for the production of antimicrobial and pro-inflammatory effector molecules. In this review, we summarize the current understanding of the Warburg effect and discuss its association with the expression of host immune responses in tuberculosis (TB), an infectious disease caused by Mycobacterium tuberculosis (Mtb). We also discuss potential mechanisms underlying the Warburg effect with a focus on the expression and regulation of hypoxia-inducible factor 1 alpha (HIF-1α), the regulatory subunit of HIF-1, a major transcription regulator involved in cellular stress adaptation processes, including energy metabolism and antimicrobial responses. We also propose a novel hypothesis that Mtb perturbs the Warburg effect of immune cells to facilitate its survival and persistence in the host. A better understanding of the dynamics of metabolic states of immune cells and their specific functions during TB pathogenesis can lead to the development of immunotherapies capable of promoting Mtb clearance and reducing Mtb persistence and the emergence of drug resistant strains.

2016: A 'Mitochondria' Odyssey

The integration of the many roles of mitochondria in cellular function and the contribution of mitochondrial dysfunction to disease are major areas of research. Within this realm, the roles of mitochondria in immune defense, epigenetics, and stem cell (SC) development have recently come into the spotlight. With new understanding, mitochondria may bring together these seemingly unrelated fields, a crucial process in treatment and prevention for various diseases. In this review we describe novel findings in these three arenas, discussing the significance of the interplay between mitochondria and the cell nucleus in response to environmental cues. While we optimistically anticipate that further research in these areas can have a profound impact on disease management, we also bring forth some of the key questions and challenges that remain.

Metabolic maintenance of cell asymmetry following division in activated T lymphocytes

Asymmetric cell division (ACD)—the partitioning of cellular components in response to polarizing cues during mitosis—plays roles in differentiation and development¹. ACD is important for the self-renewal of neuroblasts in C. elegans and fertilized zygotes in Drosophila, and participates in the development of mammalian nervous and digestive systems¹. T lymphocytes, upon activation by antigen-presenting cells (APC), can undergo ACD, wherein the daughter cell proximal to the APC is more likely to differentiate into an effector-like T cell and the distal daughter more likely to differentiate into a memory-like T cell². Upon activation and prior to cell division, expression of the transcription factor c-Myc drives metabolic reprogramming, necessary for the subsequent proliferative burst³. We found that during the first division of an activated T cell, c-Myc can sort asymmetrically. Asymmetric amino acid transporter distribution, amino acid content, and TORC1 function correlate with c-Myc expression, and both amino acids and TORC1 activity sustain the differences in c-Myc expression in one daughter over the other. Asymmetric c-Myc levels in daughter T cells affect proliferation, metabolism, and differentiation, and these effects are altered by experimental manipulation of TORC1 activity or Myc expression. Therefore, metabolic signaling pathways cooperate with transcription programs to maintain differential cell fates following asymmetric T cell division.

Expression of bovine interleukin 15 and evaluation of its biological activity in vitro

BACKGROUND/AIM

Recent studies have shown that interleukin-15 (IL-15)is a critical factor for the development and proliferation of CD8(+) memory T cells. The aim of present study is to study the role bovine IL-15 (bIL-15)in activation pathway of bovine CD8(+) T cells if any, which will be useful in designing the adjuvant to increase the duration of immunity of the vaccine preparations.

MATERIALS AND METHODS

Coding region of bIL-15 (489) was amplified from cDNA of lipopolysaccharide-induced bovine peripheral blood mononuclear cells (PBMCs) using gene specific primers and cloned into pcDNA3.1(+). Mature length of bIL-15 was amplified using gene specific primers and cloned into pET32a for expression studies. Expressed fusion protein was purified using Ni-Nitrilotriacetic acid agarose affinity chromatography and analyzed by SDS-Polyacryamide gel electrophoresis (PAGE) and western blotting. Biological activity of purified protein was analyzed by quantitative Polymerase Chain Reaction (qPCR) for an increase in levels of Bcl2, STAT3 and STAT5a using cDNA synthesized from RNA of PBMCs induced with different concentrations of purified bIL-15. Role of IL-15 in inducing memory CD8(+) T cells was analyzed by qPCR for increase in the level of Carnitine Palmitoyl Transferase 1a (CPT1a) using cDNA synthesized from RNA of PBMCs induced with different concentrations of purified bIL-15.

RESULTS

Bovine IL-15 was amplified and analyzed by agarose gel electrophoresis, which showed a specific product of ~490bp, mature sequence was amplified using full-length as a template to get a product of ~350bp. The protein was expressed, purified and analyzed by SDS-PAGE and Western blotting, which showed a specific product of 32kDa. Biological activity of purified bIL-15 fusion protein showed an increase in levels of Bcl2, STAT3 and STAT5a with 5 fold, 9 fold, and 10 fold increases as analyzed by qPCR, respectively. Role of IL-15 in inducing memory T cells showed an increase in expression level of CPT1a at 2.5 fold increase as compared to control cells.

CONCLUSION

Bovine IL-15 has been successfully cloned and expressed in our work, and the biological activity shows that the purified fusion protein is biologically active. As there is an increase in levels of CPT1a an enzyme critical for survival of memory T cells, IL-15 can be used for increase in the memory response, which can be used as an adjuvant with viral vaccines for increasing the immunity.

Arginase 1 is an innate lymphoid-cell-intrinsic metabolic checkpoint controlling type 2 inflammation

Group 2 innate lymphoid cells (ILC2s) regulate tissue inflammation and repair following activation by cell-extrinsic factors including host-derived cytokines. However, the cell-intrinsic metabolic pathways that control ILC2 function are undefined. Here we demonstrate that expression of the enzyme Arginase 1 (Arg1) is a conserved trait of murine and human ILC2s during acute or chronic lung inflammation. Deletion of murine ILC-intrinsic Arg1 abrogated type 2 lung inflammation by restraining ILC2 proliferation and dampening cytokine production. Mechanistically, inhibition of Arg1 enzymatic activity disrupted multiple components of ILC2 metabolic programming by altering arginine catabolism, impairing polyamine biosynthesis and reducing aerobic glycolysis. These data identify Arg1 as a key regulator of ILC2 bioenergetics, controlling proliferative capacity and pro-inflammatory functions that promote type 2 inflammation.

Shortened Intervals during Heterologous Boosting Preserve Memory CD8 T Cell Function but Compromise Longevity

Developing vaccine strategies to generate high numbers of Ag-specific CD8 T cells may be necessary for protection against recalcitrant pathogens. Heterologous prime-boost-boost immunization has been shown to result in large quantities of functional memory CD8 T cells with protective capacities and long-term stability. Completing the serial immunization steps for heterologous prime-boost-boost can be lengthy, leaving the host vulnerable for an extensive period of time during the vaccination process. We show in this study that shortening the intervals between boosting events to 2 wk results in high numbers of functional and protective Ag-specific CD8 T cells. This protection is comparable to that achieved with long-term boosting intervals. Short-boosted Ag-specific CD8 T cells display a canonical memory T cell signature associated with long-lived memory and have identical proliferative potential to long-boosted T cells Both populations robustly respond to antigenic re-exposure. Despite this, short-boosted Ag-specific CD8 T cells continue to contract gradually over time, which correlates to metabolic differences between short- and long-boosted CD8 T cells at early memory time points. Our studies indicate that shortening the interval between boosts can yield abundant, functional Ag-specific CD8 T cells that are poised for immediate protection; however, this is at the expense of forming stable long-term memory.

Different Subsets of T Cells, Memory, Effector Functions, and CAR-T Immunotherapy

This review is focused on different subsets of T cells: CD4 and CD8, memory and effector functions, and their role in CAR-T therapy--a cellular adoptive immunotherapy with T cells expressing chimeric antigen receptor. The CAR-T cells recognize tumor antigens and induce cytotoxic activities against tumor cells. Recently, differences in T cell functions and the role of memory and effector T cells were shown to be important in CAR-T cell immunotherapy. The CD4⁺ subsets (Th1, Th2, Th9, Th17, Th22, Treg, and Tfh) and CD8⁺ memory and effector subsets differ in extra-cellular (CD25, CD45RO, CD45RA, CCR-7, L-Selectin [CD62L], etc.); intracellular markers (FOXP3); epigenetic and genetic programs; and metabolic pathways (catabolic or anabolic); and these differences can be modulated to improve CAR-T therapy. In addition, CD4⁺ Treg cells suppress the efficacy of CAR-T cell therapy, and different approaches to overcome this suppression are discussed in this review. Thus, next-generation CAR-T immunotherapy can be improved, based on our knowledge of T cell subsets functions, differentiation, proliferation, and signaling pathways to generate more active CAR-T cells against tumors.

4-1BB signaling activates glucose and fatty acid metabolism to enhance CD8+ T cell proliferation

4-1BB (CD137) is a strong enhancer of the proliferation of CD8⁺ T cells. Since these cells require increased production of energy and biomass to support their proliferation, we hypothesized that 4-1BB signaling activated glucose and fatty acid metabolism. We found that treatment with agonistic anti-4-1BB mAb promoted the proliferation of CD8⁺ T cells in vitro, increasing their size and granularity. Studies with a glycolysis inhibitor and a fatty acid oxidation inhibitor revealed that CD8⁺ T cell proliferation required both glucose and fatty acid metabolism. Anti-4-1BB treatment increased glucose transporter 1 expression and activated the liver kinase B1 (LKB1)-AMP-activated protein kinase (AMPK)-acetyl-CoA carboxylase (ACC) signaling pathway, which may be responsible for activating the metabolism of glucose and fatty acids. We also examined whether blocking glucose or fatty acid metabolism affected cell cycle progression and the anti-apoptotic effect of 4-1BB signaling. The increase of anti-apoptotic factors and cyclins in response to anti-4-1BB treatment was completely prevented by treating CD8⁺ T cells with the fatty acid oxidation inhibitor, etomoxir, but not with the glycolysis inhibitor, 2-deoxy-D-glucose. We conclude that anti-4-1BB treatment activates glucose and fatty acid metabolism thus supporting the increased demand for energy and biomass, and that fatty acid metabolism plays a crucial role in enhancing the cell cycle progression of anti-CD3-activated CD8⁺ T cells in vitro and the anti-apoptotic effects of 4-1BB signaling on these cells.

Unexpected functions of nuclear factor-κB during germinal center B-cell development: implications for lymphomagenesis

PURPOSE OF REVIEW

B-cell tumors originating from the transformation of germinal center B cells frequently harbor genetic mutations, leading to constitutive activation of the nuclear factor-κB (NF-κB) signaling pathway. The present review highlights recent insights into the roles of separate NF-κB transcription factors in germinal center B-cell development and discusses implications of the results for germinal center lymphomagenesis.

RECENT FINDINGS

Understanding how aberrant NF-κB activation promotes tumorigenesis requires the understanding of the role of NF-κB in the tumor-precursor cells. Despite extensive knowledge on NF-κB biology, the function of this complex signaling pathway in the differentiation of germinal center B cells is largely unknown. The present review will discuss recent findings that revealed distinct roles of separate NF-κB transcription factors during the germinal center reaction in the context of germinal center lymphomagenesis. Most notably, a single NF-κB subunit, c-REL, was found to be required for the maintenance of the germinal center reaction and was associated with the activation of a metabolic program that promotes cell growth.

SUMMARY

Identifying the biological roles of the separate NF-κB transcription factor subunits in germinal center biology will help to better understand the pathogenic consequences of their constitutive activation in B-cell tumors. This knowledge may be exploited for the development of targeted antitumor therapies aimed at inhibiting selectively those components of aberrant NF-κB activity which contribute to pathogenesis.

Starved and Asphyxiated: How Can CD8(+) T Cells within a Tumor Microenvironment Prevent Tumor Progression

Although cancer immunotherapy has achieved significant breakthroughs in recent years, its overall efficacy remains limited in the majority of patients. One major barrier is exhaustion of tumor antigen-specific CD8(+) tumor-infiltrating lymphocytes (TILs), which conventionally has been attributed to persistent stimulation with antigen within the tumor microenvironment (TME). A series of recent studies have highlighted that the TME poses significant metabolic challenges to TILs, which may contribute to their functional exhaustion. Hypoxia increases the expression of coinhibitors on activated CD8(+) T cells, which in general reduces the T cells' effector functions. It also impairs the cells' ability to gain energy through oxidative phosphorylation. Glucose limitation increases the expression of programed cell death protein-1 and reduces functions of activated CD8(+) T cells. A combination of hypoxia and hypoglycemia, as is common in solid tumors, places CD8(+) TILs at dual metabolic jeopardy by affecting both major pathways of energy production. Recently, a number of studies addressed the effects of metabolic stress on modulating CD8(+) T cell metabolism, differentiation, and functions. Here, we discuss recent findings on how different types of metabolic stress within the TME shape the tumor-killing capacity of CD8(+) T cells. We propose that manipulating the metabolism of TILs to more efficiently utilize nutrients, especially during intermittent periods of hypoxia could maximize their performance, prolong their survival and improve the efficacy of active cancer immunotherapy.

The role of metabolic reprogramming in T cell fate and function

T lymphocytes undergo extensive changes in their metabolic properties during their transition through various differentiation states, from naïve to effector to memory or regulatory roles. The cause and effect relationship between metabolism and differentiation is a field of intense investigation. Many recent studies demonstrate the dependency of T cell functional outcomes on metabolic pathways and the possibility of metabolic intervention to modify these functions. In this review, we describe the basic metabolic features of T cells and new findings on how these correlate with various differentiation fates and functions. We also highlight the latest information regarding the main factors that affect T cell metabolic reprogramming.

Dynamic Metabolism in Immune Response

Cell, the basic unit of life depends for its survival on nutrients and thereby energy to perform its physiological function. Cells of lymphoid and myeloid origin are key in evoking an immune response against "self" or "non-self" antigens. The thymus derived lymphoid cells called T cells are a heterogenous group with distinct phenotypic and molecular signatures that have been shown to respond against an infection (bacterial, viral, protozoan) or cancer. Recent studies have unearthed the key differences in energy metabolism between the various T cell subsets, natural killer cells, dendritic cells, macrophages and myeloid derived suppressor cells. While a number of groups are dwelling into the nuances of the metabolism and its role in immune response at various strata, this review focuses on dynamic state of metabolism that is operational within various cellular compartments that interact to mount an effective immune response to alleviate disease state.

TCR signaling to NF-κB and mTORC1: Expanding roles of the CARMA1 complex

Naïve T-cell activation requires signals from both the T-cell receptor (TCR) and the costimulatory molecule CD28. A central mediator of the TCR and CD28 signals is the scaffold protein CARMA1, which functions by forming a complex with partner proteins, Bcl10 and MALT1. A well-known function of the CARMA1 signaling complex is to mediate activation of IκB kinase (IKK) and its target transcription factor NF-κB, thereby promoting T-cell activation and survival. Recent evidence suggests that CARMA1 also mediates TCR/CD28-stimulated activation of the IKK-related kinase TBK1, which plays a role in regulating the homeostasis and migration of T cells. Moreover, the CARMA1 complex connects the TCR/CD28 signals to the activation of mTORC1, a metabolic kinase regulating various aspects of T-cell functions. This review will discuss the mechanism underlying the activation of the CARMA1-dependent signaling pathways and their roles in regulating T-cell functions.

Targeting CD8 T-Cell Metabolism in Transplantation

Infiltration of effector CD8 T cells plays a major role in allograft rejection, and increases in memory and terminally differentiated effector memory CD8 T cells are associated with long-term allograft dysfunction. Alternatively, CD8 regulatory T cells suppress the inflammatory responses of effector lymphocytes and induce allograft tolerance in animal models. Recently, there has been a renewed interest in the field of immunometabolics and its important role in CD8 function and differentiation. The purpose of this review is to highlight the key metabolic pathways involved in CD8 T cells and to discuss how manipulating these metabolic pathways could lead to new immunosuppressive strategies for the transplantation field.

Asymmetric Cell Division in T Lymphocyte Fate Diversification

Immunological protection against microbial pathogens is dependent on robust generation of functionally diverse T lymphocyte subsets. Upon microbial infection, naïve CD4(+) or CD8(+) T lymphocytes can give rise to effector- and memory-fated progeny that together mediate a potent immune response. Recent advances in single-cell immunological and genomic profiling technologies have helped elucidate early and late diversification mechanisms that enable the generation of heterogeneity from single T lymphocytes. We discuss these findings here and argue that one such mechanism, asymmetric cell division, creates an early divergence in T lymphocyte fates by giving rise to daughter cells with a propensity towards the terminally differentiated effector or self-renewing memory lineages, with cell-intrinsic and -extrinsic cues from the microenvironment driving the final maturation steps.

Glutamine-dependent α-ketoglutarate production regulates the balance between T helper 1 cell and regulatory T cell generation

T cell activation requires that the cell meet increased energetic and biosynthetic demands. We showed that exogenous nutrient availability regulated the differentiation of naïve CD4(+) T cells into distinct subsets. Activation of naïve CD4(+) T cells under conditions of glutamine deprivation resulted in their differentiation into Foxp3(+) (forkhead box P3-positive) regulatory T (Treg) cells, which had suppressor function in vivo. Moreover, glutamine-deprived CD4(+) T cells that were activated in the presence of cytokines that normally induce the generation of T helper 1 (TH1) cells instead differentiated into Foxp3(+) Treg cells. We found that α-ketoglutarate (αKG), the glutamine-derived metabolite that enters into the mitochondrial citric acid cycle, acted as a metabolic regulator of CD4(+) T cell differentiation. Activation of glutamine-deprived naïve CD4(+) T cells in the presence of a cell-permeable αKG analog increased the expression of the gene encoding the TH1 cell-associated transcription factor Tbet and resulted in their differentiation into TH1 cells, concomitant with stimulation of mammalian target of rapamycin complex 1 (mTORC1) signaling. Together, these data suggest that a decrease in the intracellular amount of αKG, caused by the limited availability of extracellular glutamine, shifts the balance between the generation of TH1 and Treg cells toward that of a Treg phenotype.

Hypoxia-inducible factors in T lymphocyte differentiation and function. A Review in the Theme: Cellular Responses to Hypoxia

Low oxygen concentrations or hypoxia is a trait common to inflamed tissues. Therefore it is not surprising that pathways of hypoxic stress response, largely governed by hypoxia-inducible factors (HIF), are highly relevant to the proper function of immune cells. HIF expression and stabilization in immune cells can be triggered not only by hypoxia, but also by a variety of stimuli and pathological stresses associated with leukocyte activation and inflammation. In addition to its role as a sensor of oxygen scarcity, HIF is also a major regulator of immune cell metabolic function. Rapid progress is being made in elucidating the roles played by HIF in diverse aspects of both innate and adaptive immunity. Here we discuss a number of breakthroughs that have shed light on how HIF expression and activity impact the differentiation and function of diverse T cell populations. The insights gained from these findings may serve as the foundation for future therapies aimed at fine-tuning the immune response.

Rethinking the paradigm: How comparative studies on fatty acid oxidation inform our understanding of T cell metabolism

The classic paradigm of T cell metabolism posits that activated Teff cells utilize glycolysis to keep pace with increased energetic demands, while resting and Tmem cells rely on the oxidation of fat. In contrast, Teff cells during graft-versus-host disease (GVHD) increase their reliance on oxidative metabolism and, in particular, on fatty acid oxidation (FAO). To explore the potential mechanisms driving adoption of this alternative metabolism, we first review key pathways regulating FAO across a variety of disparate tissue types, including liver, heart, and skeletal muscle. Based upon these comparative studies, we then outline a consensus network of transcriptional and signaling pathways that predict a model for regulating FAO in Teff cells during GVHD. This model raises important implications about the dynamic nature of metabolic reprogramming in T cells and suggests exciting future directions for further study of in vivo T cell metabolism.

Molecular characterization of HCMV-specific immune responses: Parallels between CD8(+) T cells, CD4(+) T cells, and NK cells

CD8(+) T cells are important for immunity against human cytomegalovirus (HCMV). The HCMV-specific CD8(+) T-cell response is characterized by the accumulation of terminally differentiated effector cells that have downregulated the costimulatory molecules CD27 and CD28. These HCMV-specific CD8(+) T cells maintain high levels of cytotoxic molecules such as granzyme B and rapidly produce the inflammatory cytokine IFN-γ upon activation. Remarkably, HCMV-specific CD8(+) T cells are able to persist long term as fully functional effector cells, suggesting a unique differentiation pathway that is distinct from the formation of memory CD8(+) T cells after infection with acute viruses. In this review, we aim to highlight the most recent developments in HCMV-specific CD8(+) T-cell differentiation, maintenance, tissue distribution, metabolism and function. HCMV also induces the differentiation of effector CD4(+) T cells and NK cells, which share characteristics with HCMV-specific CD8(+) T cells. We propose that the overlap in differentiation of NK cells, CD4(+) and CD8(+) T cells after HCMV infection may be regulated by a shared transcriptional machinery. A better understanding of the molecular framework of HCMV-specific CD8(+) T-cell responses may benefit vaccine design, as these cells uniquely combine the capacity to rapidly respond to infection with long-term survival.

REDD1 Is Essential for Optimal T Cell Proliferation and Survival

REDD1 is a highly conserved stress response protein that is upregulated following many types of cellular stress, including hypoxia, DNA damage, energy stress, ER stress, and nutrient deprivation. Recently, REDD1 was shown to be involved in dexamethasone induced autophagy in murine thymocytes. However, we know little of REDD1’s function in mature T cells. Here we show for the first time that REDD1 is upregulated following T cell stimulation with PHA or CD3/CD28 beads. REDD1 knockout T cells exhibit a defect in proliferation and cell survival, although markers of activation appear normal. These findings demonstrate a previously unappreciated role for REDD1 in T cell function.

T cell metabolic reprogramming and plasticity

Upon antigen stimulation, small and quiescent naïve T cells undergo an approximately 24h growth phase followed by rapid proliferation. Depending on the nature of the antigen and cytokine milieu, these proliferating T cells differentiate into distinctive functional subgroups that are essential for appropriate immune defense and regulation. T cells undergo a characteristic metabolic rewiring that fulfills the dramatically increased bioenergetic and biosynthetic demands during the transition between resting, activation and differentiation. Beyond this, T cells are distributed throughout the body and are able to function in a wide range of physio-pathological environments, including some with a dramatic metabolic derangement. As such, T cells must quickly respond to and adapt to fluctuations in environmental nutrient levels. We consider such responsiveness and adaptation in terms of metabolic plasticity, that is, an evolutionarilly selected process which allows T cells to illicit robust immune functions in response to either a continuous or disrupted nutrient supply. In this review, we illustrate the relevant metabolic pathways in T cells and discuss the ability of T cells to change their metabolic substrates in response to changes in the environment.

T cell metabolism drives immunity

Buck et al. discuss the role of lymphocyte metabolism on immune cell development and function.

Lymphocytes must adapt to a wide array of environmental stressors as part of their normal development, during which they undergo a dramatic metabolic remodeling process. Research in this area has yielded surprising findings on the roles of diverse metabolic pathways and metabolites, which have been found to regulate lymphocyte signaling and influence differentiation, function and fate. In this review, we integrate the latest findings in the field to provide an up-to-date resource on lymphocyte metabolism.

Metabolic plasticity of human T cells: Preserved cytokine production under glucose deprivation or mitochondrial restriction, but 2-deoxy-glucose affects effector functions

The strong link between T-cell metabolism and effector functions is well characterized in the murine system but hardly investigated in human T cells. Therefore, we analyzed glycolytic and mitochondrial activity in correlation to function in activated human CD4 and CD8 T cells. Glycolysis was barely detectable upon stimulation but accelerated beyond 24 h, whereas mitochondrial activity was elevated immediately in both T-cell populations. Glucose deprivation or mitochondrial restriction reduced proliferation, had only a transient impact on "on-blast formation" and no impact on viability, IFN-γ, IL-2, IL-4, and IL-10 production, whereas TNF was reduced. Similar results were obtained in bulk T cells and T-cell subsets. Elevated respiration under glucose restriction demonstrated metabolic flexibility. Administration of the glycolytic inhibitor 2-deoxy-glucose suppressed both glycolysis and respiration and exerted a strong impact on cytokine production that persisted for IFN-γ after removal of 2-deoxy-glucose. Taken together, glycolytic or mitochondrial restriction alone compromised proliferation of human T cells, but barely affected their effector functions. In contrast, effector functions were severely affected by 2-deoxy-glucose treatment.

Early Effector CD8 T Cells Display Plasticity in Populating the Short-Lived Effector and Memory-Precursor Pools Following Bacterial or Viral Infection

Naïve antigen-specific CD8 T cells expand in response to infection and can be phenotypically separated into distinct effector populations, which include memory precursor effector cells (MPECs) and short-lived effector cells (SLECs). In the days before the peak of the T cell response, a third population called early effector cells (EECs) predominate the antigen-specific response. However, the contribution of the EEC population to the CD8 T cell differentiation program during an antimicrobial immune response is not well understood. To test if EEC populations were pre-committed to either an MPEC or SLEC fate, we purified EECs from mice infected with Listeria monocytogenes (LM) or vesicular stomatitis virus (VSV), where the relative frequency of each population is known to be different at the peak of the response. Sorted EECs transferred into uninfected hosts revealed that EECs were pre-programmed to differentiate based on early signals received from the distinct infectious environments. Surprisingly, when these same EECs were transferred early into mismatched infected hosts, the transferred EECs could be diverted from their original fate. These results delineate a model of differentiation where EECs are programmed to form MPECs or SLECs, but remain susceptible to additional inflammatory stimuli that can alter their fate.

Essential role of mitochondrial energy metabolism in Foxp3⁺ T-regulatory cell function and allograft survival

Conventional T (Tcon) cells and Foxp3(+) T-regulatory (Treg) cells are thought to have differing metabolic requirements, but little is known of mitochondrial functions within these cell populations in vivo. In murine studies, we found that activation of both Tcon and Treg cells led to myocyte enhancer factor 2 (Mef2)-induced expression of genes important to oxidative phosphorylation (OXPHOS). Inhibition of OXPHOS impaired both Tcon and Treg cell function compared to wild-type cells but disproportionally affected Treg cells. Deletion of Pgc1α or Sirt3, which are key regulators of OXPHOS, abrogated Treg-dependent suppressive function and impaired allograft survival. Mef2 is inhibited by histone/protein deacetylase-9 (Hdac9), and Hdac9 deletion increased Treg suppressive function. Hdac9(-/-) Treg showed increased expression of Pgc1α and Sirt3, and improved mitochondrial respiration, compared to wild-type Treg cells. Our data show that key OXPHOS regulators are required for optimal Treg function and Treg-dependent allograft acceptance. These findings provide a novel approach to increase Treg function and give insights into the fundamental mechanisms by which mitochondrial energy metabolism regulates immune cell functions in vivo.

Airway-Resident Memory CD8 T Cells Provide Antigen-Specific Protection against Respiratory Virus Challenge through Rapid IFN-γ Production

CD8 airway resident memory T (TRM) cells are a distinctive TRM population with a high turnover rate and a unique phenotype influenced by their localization within the airways. Their role in mediating protective immunity to respiratory pathogens, although suggested by many studies, has not been directly proven. This study provides definitive evidence that airway CD8 TRM cells are sufficient to mediate protection against respiratory virus challenge. Despite being poorly cytolytic in vivo and failing to expand after encountering Ag, airway CD8 TRM cells rapidly express effector cytokines, with IFN-γ being produced most robustly. Notably, established airway CD8 TRM cells possess the ability to produce IFN-γ faster than systemic effector memory CD8 T cells. Furthermore, naive mice receiving intratracheal transfer of airway CD8 TRM cells lacking the ability to produce IFN-γ were less effective at controlling pathogen load upon heterologous challenge. This direct evidence of airway CD8 TRM cell-mediated protection demonstrates the importance of these cells as a first line of defense for optimal immunity against respiratory pathogens and suggests they should be considered in the development of future cell-mediated vaccines.

High mitochondrial respiration and glycolytic capacity represent a metabolic phenotype of human tolerogenic dendritic cells

Human dendritic cells (DCs) regulate the balance between immunity and tolerance through selective activation by environmental and pathogen-derived triggers. To characterize the rapid changes that occur during this process, we analyzed the underlying metabolic activity across a spectrum of functional DC activation states, from immunogenic to tolerogenic. We found that in contrast to the pronounced proinflammatory program of mature DCs, tolerogenic DCs displayed a markedly augmented catabolic pathway, related to oxidative phosphorylation, fatty acid metabolism, and glycolysis. Functionally, tolerogenic DCs demonstrated the highest mitochondrial oxidative activity, production of reactive oxygen species, superoxide, and increased spare respiratory capacity. Furthermore, assembled, electron transport chain complexes were significantly more abundant in tolerogenic DCs. At the level of glycolysis, tolerogenic and mature DCs showed similar glycolytic rates, but glycolytic capacity and reserve were more pronounced in tolerogenic DCs. The enhanced glycolytic reserve and respiratory capacity observed in these DCs were reflected in a higher metabolic plasticity to maintain intracellular ATP content. Interestingly, tolerogenic and mature DCs manifested substantially different expression of proteins involved in the fatty acid oxidation (FAO) pathway, and FAO activity was significantly higher in tolerogenic DCs. Inhibition of FAO prevented the function of tolerogenic DCs and partially restored T cell stimulatory capacity, demonstrating their dependence on this pathway. Overall, tolerogenic DCs show metabolic signatures of increased oxidative phosphorylation programing, a shift in redox state, and high plasticity for metabolic adaptation. These observations point to a mechanism for rapid genome-wide reprograming by modulation of underlying cellular metabolism during DC differentiation.

mTORC1 and mTORC2 selectively regulate CD8⁺ T cell differentiation

Activation of mTOR-dependent pathways regulates the specification and differentiation of CD4+ T effector cell subsets. Herein, we show that mTOR complex 1 (mTORC1) and mTORC2 have distinct roles in the generation of CD8+ T cell effector and memory populations. Evaluation of mice with a T cell-specific deletion of the gene encoding the negative regulator of mTORC1, tuberous sclerosis complex 2 (TSC2), resulted in the generation of highly glycolytic and potent effector CD8+ T cells; however, due to constitutive mTORC1 activation, these cells retained a terminally differentiated effector phenotype and were incapable of transitioning into a memory state. In contrast, CD8+ T cells deficient in mTORC1 activity due to loss of RAS homolog enriched in brain (RHEB) failed to differentiate into effector cells but retained memory characteristics, such as surface marker expression, a lower metabolic rate, and increased longevity. However, these RHEB-deficient memory-like T cells failed to generate recall responses as the result of metabolic defects. While mTORC1 influenced CD8+ T cell effector responses, mTORC2 activity regulated CD8+ T cell memory. mTORC2 inhibition resulted in metabolic reprogramming, which enhanced the generation of CD8+ memory cells. Overall, these results define specific roles for mTORC1 and mTORC2 that link metabolism and CD8+ T cell effector and memory generation and suggest that these functions have the potential to be targeted for enhancing vaccine efficacy and antitumor immunity.

Survival of effector CD8+ T cells during influenza infection is dependent on autophagy

The activation and expansion of effector CD8(+) T cells are essential for controlling viral infections and tumor surveillance. During an immune response, T cells encounter extrinsic and intrinsic factors, including oxidative stress, nutrient availability, and inflammation, that can modulate their capacity to activate, proliferate, and survive. The dependency of T cells on autophagy for in vitro and in vivo activation, expansion, and memory remains unclear. Moreover, the specific signals and mechanisms that activate autophagy in T effector cells and their survival are not known. In this study, we generated a novel inducible autophagy knockout mouse to study T cell effector responses during the course of a virus infection. In response to influenza infection, Atg5(-/-) CD8(+) T cells had a decreased capacity to reach the peak effector response and were unable to maintain cell viability during the effector phase. As a consequence of Atg5 deletion and the impairment in effector-to-memory cell survival, mice fail to mount a memory response following a secondary challenge. We found that Atg5(-/-) effector CD8(+) T cells upregulated p53, a transcriptional state that was concomitant with widespread hypoxia in lymphoid tissues of infected mice. The onset of p53 activation was concurrent with higher levels of reactive oxygen species (ROS) that resulted in ROS-dependent apoptotic cell death, a fate that could be rescued by treating with the ROS scavenger N-acetylcysteine. Collectively, these results demonstrate that effector CD8(+) T cells require autophagy to suppress cell death and maintain survival in response to a viral infection.

Evolving perspectives of mTOR complexes in immunity and transplantation

Since the discovery of Rapamycin (RAPA) and its immunosuppressive properties, enormous progress has been made in characterizing the mechanistic target of rapamycin (mTOR). Use of RAPA and its analogues (rapalogs) as anti-rejection agents has been accompanied by extensive investigation of how targeting of mTOR complex 1 (mTORC1), the principal target of RAPA, and more recently mTORC2, affects the function of immune cells, as well as vascular endothelial cells, that play crucial roles in regulation of allograft rejection. While considerable knowledge has accumulated on the function of mTORC1 and 2 in T cells, understanding of the differential roles of these complexes in antigen-presenting cells, NK cells and B cells/plasma cells is only beginning to emerge. Immune cell-specific targeting of mTORC1 or mTORC2, together with use of novel, second generation, dual mTORC kinase inhibitors (TORKinibs) have started to play an important role in elucidating the roles of these complexes and their potential for targeting in transplantation. Much remains unknown about the role of mTOR complexes and the consequences of mTOR targeting on immune reactivity in clinical transplantation. Here we address recent advances in understanding and evolving perspectives of the role of mTOR complexes and mTOR targeting in immunity, with extrapolation to transplantation.

PD-1 alters T-cell metabolic reprogramming by inhibiting glycolysis and promoting lipolysis and fatty acid oxidation

During activation, T cells undergo metabolic reprogramming, which imprints distinct functional fates. We determined that on PD-1 ligation, activated T cells are unable to engage in glycolysis or amino acid metabolism but have an increased rate of fatty acid β-oxidation (FAO). PD-1 promotes FAO of endogenous lipids by increasing expression of CPT1A, and inducing lipolysis as indicated by elevation of the lipase ATGL, the lipolysis marker glycerol and release of fatty acids. Conversely, CTLA-4 inhibits glycolysis without augmenting FAO, suggesting that CTLA-4 sustains the metabolic profile of non-activated cells. Because T cells utilize glycolysis during differentiation to effectors, our findings reveal a metabolic mechanism responsible for PD-1-mediated blockade of T-effector cell differentiation. The enhancement of FAO provides a mechanistic explanation for the longevity of T cells receiving PD-1 signals in patients with chronic infections and cancer, and for their capacity to be reinvigorated by PD-1 blockade.

Activation of T cells results in metabolic reprogramming to favour glycolysis. Here, Patsoukis et al. show that the surface receptor PD-1 inhibits glycolysis and increases the metabolism of lipids, providing a potential mechanism for the blockade of T effector functions but also for the longevity accompanying T cell exhaustion.

Targeting T cell metabolism for therapy

In the past several years a wealth of evidence has emerged illustrating how metabolism supports many aspects of T cell biology, as well as how metabolic changes drive T cell differentiation and fate. We outline developing principles in the regulation of T cell metabolism, and discuss how these processes are affected in settings of inflammation and cancer. In this context we discuss how metabolic pathways might be manipulated for the treatment of human disease, including how metabolism may be targeted to prevent T cell dysfunction in inhospitable microenvironments, to generate more effective adoptive cellular immunotherapies in cancer, and to direct T cell differentiation and function towards non-pathogenic phenotypes in settings of autoimmunity.

Mitochondrial dysfunction in peripheral blood mononuclear cells in pediatric septic shock

OBJECTIVES

Mitochondrial dysfunction in peripheral blood mononuclear cells has been linked to immune dysregulation and organ failure in adult sepsis, but pediatric data are limited. We hypothesized that pediatric septic shock patients exhibit mitochondrial dysfunction within peripheral blood mononuclear cells which in turn correlates with global organ injury.

DESIGN

Prospective observational study.

SETTING

Academic PICU.

PATIENTS

Thirteen pediatric patients with septic shock and greater than or equal to two organ failures and 11 PICU controls without sepsis or organ failure.

INTERVENTIONS

Ex vivo measurements of mitochondrial oxygen consumption and membrane potential (ΔΨm) were performed in intact peripheral blood mononuclear cells on day 1-2 and day 5-7 of septic illness and in controls. The Pediatric Logistic Organ Dysfunction score, inotrope score, and organ failure-free days were determined from medical records.

MEASUREMENTS AND MAIN RESULTS

Spare respiratory capacity, an index of bioenergetic reserve, was lower in septic peripheral blood mononuclear cells on day 1-2 (median, 1.81; interquartile range, 0.52-2.09 pmol O2/s/10 cells) compared with controls (5.55; 2.80-7.21; p = 0.03). Spare respiratory capacity normalized by day 5-7. Patients with sepsis on day 1-2 exhibited a higher ratio of LEAK to maximal respiration than controls (17% vs < 1%; p = 0.047) with normalization by day 5-7 (1%; p = 0.008), suggesting mitochondrial uncoupling early in sepsis. However, septic peripheral blood mononuclear cells exhibited no differences in basal or adenosine triphosphate-linked oxygen consumption or ΔΨm. Oxygen consumption did not correlate with Pediatric Logistic Organ Dysfunction score, inotrope score, or organ failure-free days (all p > 0.05). Although there was a weak overall association between ΔΨm on day 1-2 and organ failure-free days (Spearman ρ = 0.56, p = 0.06), patients with sepsis with normal organ function by day 7 exhibited higher ΔΨm on day 1-2 compared with patients with organ failure for more than 7 days (p = 0.04).

CONCLUSIONS

Mitochondrial dysfunction was present in peripheral blood mononuclear cells in pediatric sepsis, evidenced by decreased bioenergetic reserve and increased uncoupling. Mitochondrial membrane potential, but not respiration, was associated with duration of organ injury.