Futility sustains memory T cells

Summmary

Memory T cells display a distinct metabolic prolife compared to effector T cells. In this issue, O’Sullivan et al. report that memory T cells activate a “futile cycle” of de novo fatty acid synthesis and concurrent fatty acid oxidation to generate ATP for cell survival.

T cells are the primary cells that respond to antigens and thus are central regulators of adaptive immune responses. Aberrant T cell function results in amultitude of pathologies including auto-immunity. There are multiple different types of T cells with different metabolic requirements. Naïve T cells (T_N) rapidly proliferate into effector T cells (T_E) when challenged with an antigen during infection. After the infection is curtailed, the majority of T_E cells undergo cell death (i.e. contraction phase) with a few long-lived memory T cells (T_M) [1]. If a similar infection occurs then T_M cells can be reactivated, rapidly expanding into T_E cells to quickly control the infection.

T_N cells are quiescent cells that catabolize nutrients to generate ATP for cell survival and engage in anabolic functions to maintain homeostasis [2]. By contrast, rapidly proliferating T_E cells uptake nutrients such as glucose to produce ATP, NADPH, as well as de novo lipids and nucleotides-macromolecules necessary to generate two daughter cells [2]. Anabolic functions require ATP and NADPH. T_E cells increase the rate of glycolysis and mitochondrial metabolism to sustain the high anabolic needs of proliferating T_E cells. Glucose and glutamine serves as a primary carbon sources to fuel glycolysis and mitochondrial metabolism to generate metabolites that are precursors for macromolecules biosynthesis [3, 4]. Aside from metabolism simply responding to the anabolic demands of proliferating cells, metabolism also dictates signaling. Notably, the glycolytic enzyme GAPDH and mitochondrial generated reactive oxygen species control effector T cell cytokine production [5, 6]. By contrast, T_M cells are not rapidly proliferating thus do not have high anabolic requirements. However, T_M cells need to efficiently catabolize nutrients to maintain long-term cell survival. A critical question is how these long-lived memory T cells maintain their bioenergetic needs to maintain cell survival.

In this issue of Immunity, O’Sullivan et al. investigated the metabolic pathways that support survival of T_M cells. Previously their laboratory had shown that T_M cell_s display high levels of fatty acid oxidation by mitochondria to produce ATP compared to T_E cells for long-term cell survival [7]. Fatty acid oxidation generates almost 3 times more ATP than glucose oxidation by mitochondria, thus it is robust mechanism to generate ATP. But where do T_M cells acquire fatty acids to conduct fatty acid oxidation? Their original assumption was that T_M cells simply uptake fatty acids from their environment. Conceptually this makes sense since T_M cells reside in adipose rich tissues. Thus, they were surprised that compared with T_E cells, T_M cells obtained significantly less fatty acids from the environment. T_M cells displayed a decrease in the surface expression of CD36, necessary for fatty acid uptake, compared with T_E cells thus providing a potential mechanism for differences in the rate of fatty acid uptake between T_M cells and T_E cells. Fatty acids acquired by T_E cells are stored in lipid droplets therefore are not used to generate ATP by mitochondrial fatty acid oxidation. By contrast, memory T cells do not increase fatty acid uptake even in a setting of increased fatty acid oxidation. Consequently, O’Sullivan et al. questioned if T_M cells do not acquire fatty acids from the environment to increase the rate of fatty acid oxidation, then what is the source of fatty acids to drive the heightened levels of fatty acid oxidation in T_M cells.

The authors reasoned that because T_M cells display no increase in extracellular fatty acid uptake then de novo fatty acid synthesis might provide the necessary fatty acids to generate mitochondrial ATP through fatty acid oxidation. To test this hypothesis, O’Sullivan et al. utilized C75, an inhibitor of the fatty acid synthase-an enzyme necessary fatty acid synthesis, and observed that C75 increased T_M cell death. C75 did not impair T_E cell survival but diminished T_E cell proliferation. These results indicate that de novo fatty acid synthesis is essential for T_M cell survival and T_E cell proliferation. Next, the authors probed whether glucose derived citrate was one of the major cellular carbon source for de novo fatty acid synthesis in both T_M and T_E cells. Glucose generates pyruvate that becomes acetyl-CoA to enter the TCA cycle. Acetyl-CoA and the TCA cycle intermediate oxaloacetate generate citrate, which can be exported into the cytosol for de novo lipogenesis [8]. The cytosolic enzyme ATP-citrate lyase (ACLY) converts citrate into acetyl-CoA and oxaloacetate. The former is a precursor for lipogenesis while the latter is utilized for de novo nucleotide synthesis. Unexpectedly silencing ACLY protein by RNAi did not impair cell survival of T_M cells but did decrease T_E cell proliferation. Hence, it is possible that T_M cells generate cytosolic acetyl-CoA through other metabolites than citrate. Notably, acetate can be converted into to acetyl-CoA by the enzyme acetyl-CoA synthetase. Gut bacteria can generate acetate thus possibly linking microbiome to memory T cells.

At this point, the authors recognized that some of their findings displayed an inherent contradiction. Classically, free fatty acids produced by a cell are quickly diverted in to triglyceride storage in adipocyte-like droplets to prevent toxic effects of high levels of free fatty acids. However, in the case of T_M cells, O’Sullivan and colleagues had observed no appreciable accumulation of lipids droplets. The authors then reasoned that a non-classical pathway of fatty acid storage was being used, and identified increased activity of lysosomal acid lipase (LAL) in T_M cells along with elevated lipid levels localizing to the lysosome. This data strongly suggested that T_M cells in contrast to T_E cells activate a lysosomal-based lipid storage and degradation pathway. Next, using shRNA silencing of LAL in fully activated T cells, the authors showed that T_M cells required LAL to liberate free fatty acids from storage for the robust fatty acid oxidation in the mitochondria. Finally, the authors demonstrated that loss of LAL activity during an immune response greatly reduced the amount of memory T cells produced, which was due a decrease in survival of T_M cells.

Collectively these data support the idea that activation of the “futile cycle” where fatty acid synthesis with concurrent fatty acid oxidation is required for proper maintenance of memory T cells. From a bioenergetic perspective this is an inefficient use of macronutrients because these cells must first use ATP and NADPH to synthesize the fatty acids that ultimately will be used to generate ATP. There are biochemical mechanisms to prevent the “futile cycle” from arising. An early step in fatty acid synthesis is the generation of malonyl-CoA, which prevents fatty acid import into the mitochondria as it increases in concentration [9]. Although O’Sullivan et al. did not decipher the biochemical basis as to how T_M cells evade this regulatory cell, it raises the possibility that long lived cells might utilize the “futile cycle” to maintain their survival. It will be of interest to examine whether certain long lived stem cells, cancer initiating cells, or senescent cells utilize the “futile cycle” to maintain long term cell survival. It is important to note that it is formally possible that memory T cells within a population do not necessarily engage in a “futile cycle” rather they oscillate between fatty acid synthesis and fatty acid oxidation.

Going forward, it will be meaningful to decipher what is the metabolic advantage of engaging the “futile cycle” for memory T cells. Here we speculate two possibilities. First, T_M cells may employ the “futile cycle” to concurrently maintain robust mitochondrial oxidative metabolism and glycolysis. This has the potential advantage of allowing T_M cells the capacity to rapidly use glycolytic and mitochondrial metabolism following a re-encounter with antigen for rapid proliferation and cytokine production. Second, memory T cells might decide that reliance on extracellular fatty acids could be risky since fatty acid levels vacillate depending on the tissues where they reside. Thus, memory cells decide to take up glucose, an abundant nutrient in the blood that is tightly regulated to maintain levels between 5–6 mM, to generate de novo fatty acids for mitochondrial ATP production needed to maintain long term cell survival. Hence, O’Sullivian et al. have not only uncovered an interesting observation for the burgeoning field of immunometabolism but also raised stimulating questions for biochemists to think about regulation of metabolic pathways. Specifically, biochemists will have to incorporate futile metabolic pathways in trying to understand how nutrients fulfill the metabolic demands of cells.

Figure 1.

Following immune exposure, naïve T cells rapidly increase uptake of glucose and glutamine to engage in anabolic metabolism that supports T effector cell proliferation and cytokine production as the effector T cell response subsides, memory T cells engage a “futile cycle” of fatty acid synthesis and subsequent fatty acid oxidation. In this cycle, de novo synthesis of fatty acids are stored in the lysosome where they are liberated by lysosomal acid lipase (LAL) and shuttled into the mitochondria for ATP generation by fatty acid oxidation to sustain prolonged memory cell survival.