Summary CD8+ T cells undergo major metabolic changes upon activation, but how metabolism influences the establishment of long-lived memory T (TM) cells after infection remains a key question. We have shown here that CD8+ TM cells, but not effector CD8+ (TE) cells, possessed substantial mitochondrial spare respiratory capacity (SRC). SRC is the extra capacity available in cells to produce energy in response to increased stress or work and as such is associated with cellular survival. We found that interleukin-15 (IL-15), a cytokine critical for CD8+ TM cells, regulated SRC and oxidative metabolism by promoting mitochondrial biogenesis and expression of carnitine palmitoyl transferase (CPT1a), a metabolic enzyme that controls the rate-limiting step to mitochondrial fatty acid oxidation (FAO). These results show how cytokines control the bioenergetic stability of TM cells after infection by regulating mitochondrial metabolism.

Dendritic cells (DCs) are key regulators of innate and acquired immunity. The maturation of DCs is directed by signal transduction events downstream of toll-like receptors (TLRs) and other pattern recognition receptors. Here, we demonstrate that, in mouse DCs, TLR agonists stimulate a profound metabolic transition to aerobic glycolysis, similar to the Warburg metabolism displayed by cancer cells. This metabolic switch depends on the phosphatidyl inositol 3'-kinase/Akt pathway, is antagonized by the adenosine monophosphate (AMP)-activated protein kinase (AMPK), and is required for DC maturation. The metabolic switch induced by DC activation is antagonized by the antiinflammatory cytokine interleukin-10. Our data pinpoint TLR-mediated metabolic conversion as essential for DC maturation and function and reveal it as a potential target for intervention in the control of excessive inflammation and inappropriately regulated immune responses.

The ligation of Toll-like receptors (TLRs) leads to rapid activation of dendritic cells (DCs). However, the metabolic requirements that support this process remain poorly defined. We found that DC glycolytic flux increased within minutes of exposure to TLR agonists and that this served an essential role in supporting the de novo synthesis of fatty acids for the expansion of the endoplasmic reticulum and Golgi required for the production and secretion of proteins that are integral to DC activation. Signaling via the kinases TBK1, IKKε and Akt was essential for the TLR-induced increase in glycolysis by promoting the association of the glycolytic enzyme HK-II with mitochondria. In summary, we identified the rapid induction of glycolysis as an integral component of TLR signaling that is essential for the anabolic demands of the activation and function of DCs.

TLR agonists initiate a rapid activation program in dendritic cells (DCs) that requires support from metabolic and bioenergetic resources. We found previously that TLR signaling promotes aerobic glycolysis and a decline in oxidative phosphorylation (OXHPOS) and that glucose restriction prevents activation and leads to premature cell death. However, it remained unclear why the decrease in OXPHOS occurs under these circumstances. Using real-time metabolic flux analysis, in the present study, we show that mitochondrial activity is lost progressively after activation by TLR agonists in inflammatory blood monocyte-derived DCs that express inducible NO synthase. We found that this is because of inhibition of OXPHOS by NO and that the switch to glycolysis is a survival response that serves to maintain ATP levels when OXPHOS is inhibited. Our data identify NO as a profound metabolic regulator in inflammatory monocyte-derived DCs. (Blood. 2012;120(7):1422-1431) IntroductionDendritic cells (DCs) express TLRs that allow them to detect and respond to pathogen-derived molecules. 1,2 In response to TLR agonists, DCs transition from a resting state to an activated state through a process that that involves the induction of expression of genes encoding a broad array of proteins such as cytokines, chemokines, and costimulatory molecules. 3 Activated DCs play a central role in orchestrating the development of immune responses.Recently, we showed that after exposure to TLR agonists, DCs undergo a striking metabolic transition evident as a pronounced increase in the glycolytic rate. 4 This is highly reminiscent of Warburg metabolism, 5 in which tumor cells preferentially use glycolysis rather than catabolic mitochondrial pathways to conserve and generate metabolic resources to meet the demands of cellular proliferation while still producing sufficient ATP to permit these processes to occur. 6,7 Moreover, the increase in glycolytic rate in DCs was found to be dependent on the PI3K/Akt pathway, 4 which is one of the most commonly mutated signaling pathways in tumors. 8 We reasoned that glycolysis could serve essentially the same purpose in active DCs as it is thought to do in tumors. 4 This view was supported by the fact that glucose restriction inhibits severely the activation and life span of DCs exposed to TLR agonists. 4 However, unlike in most cancers, which continue to consume oxygen at rates comparable to normal tissues despite increased glycolytic rates, 9 activated DCs use significantly less oxygen than do resting DCs. 4 Thus far, the molecular mechanisms underlying mitochondrial impairment in activated DCs, and the metabolic consequences of the loss of mitochondrial function, remain unclear.To address these issues, we have in the present study, undertaken a detailed analysis of mitochondrial function in DCs after TLR stimulation. Using extracellular flux analysis to measure changes in oxygen consumption in real time, we found that 6 hours after stimulation, mitochondrial oxygen consumption was progressively lost due to the ...

Dendritic cells (DCs) are potent inducers of T cell immunity, and autologous DC vaccination holds promise for the treatment of cancers and chronic infectious diseases. In practice, however, therapeutic vaccines of this type have had mixed success. In this article, we show that brief exposure to inhibitors of mechanistic target of rapamycin (mTOR) in DCs during the period that they are responding to TLR agonists makes them particularly potent activators of naive CD8+ T cells and able to enhance control of B16 melanoma in a therapeutic autologous vaccination model in the mouse. The improved performance of DCs in which mTOR has been inhibited is correlated with an extended life span after activation and prolonged, increased expression of costimulatory molecules. Therapeutic autologous vaccination with DCs treated with TLR agonists plus the mTOR inhibitor rapamycin results in improved generation of Ag-specific CD8+ T cells in vivo and improved antitumor immunity compared with that observed with DCs treated with TLR agonists alone. These findings define mTOR as a molecular target for augmenting DC survival and activation, and document a novel pharmacologic approach for enhancing the efficacy of therapeutic autologous DC vaccination.