Unraveling the Complex Interplay Between T Cell Metabolism and Function.

Metabolism drives function, on both an organismal and a cellular level. In T cell biology, metabolic remodeling is intrinsically linked to cellular development, activation, function, differentiation, and survival. After naive T cells are activated, increased demands for metabolic currency in the form of ATP, as well as biomass for cell growth, proliferation, and the production of effector molecules, are met by rewiring cellular metabolism. Consequently, pharmacological strategies are being developed to perturb or enhance selective metabolic processes that are skewed in immune-related pathologies. Here we review the most recent advances describing the metabolic changes that occur during the T cell lifecycle. We discuss how T cell metabolism can have profound effects on health and disease and where it might be a promising target to treat a variety of pathologies.

Phosphoinositide acyl chain saturation drives CD8+ effector T cell signaling and function.

How lipidome changes support CD8+ effector T (Teff) cell differentiation is not well understood. Here we show that, although naive T cells are rich in polyunsaturated phosphoinositides (PIPn with 3-4 double bonds), Teff cells have unique PIPn marked by saturated fatty acyl chains (0-2 double bonds). PIPn are precursors for second messengers. Polyunsaturated phosphatidylinositol bisphosphate (PIP2) exclusively supported signaling immediately upon T cell antigen receptor activation. In late Teff cells, activity of phospholipase C-γ1, the enzyme that cleaves PIP2 into downstream mediators, waned, and saturated PIPn became essential for sustained signaling. Saturated PIP was more rapidly converted to PIP2 with subsequent recruitment of phospholipase C-γ1, and loss of saturated PIPn impaired Teff cell fitness and function, even in cells with abundant polyunsaturated PIPn. Glucose was the substrate for de novo PIPn synthesis, and was rapidly utilized for saturated PIP2 generation. Thus, separate PIPn pools with distinct acyl chain compositions and metabolic dependencies drive important signaling events to initiate and then sustain effector function during CD8+ T cell differentiation.

Mitochondrial Priming by CD28.

T cell receptor (TCR) signaling without CD28 can elicit primary effector T cells, but memory T cells generated during this process are anergic, failing to respond to secondary antigen exposure. We show that, upon T cell activation, CD28 transiently promotes expression of carnitine palmitoyltransferase 1a (Cpt1a), an enzyme that facilitates mitochondrial fatty acid oxidation (FAO), before the first cell division, coinciding with mitochondrial elongation and enhanced spare respiratory capacity (SRC). microRNA-33 (miR33), a target of thioredoxin-interacting protein (TXNIP), attenuates Cpt1a expression in the absence of CD28, resulting in cells that thereafter are metabolically compromised during reactivation or periods of increased bioenergetic demand. Early CD28-dependent mitochondrial engagement is needed for T cells to remodel cristae, develop SRC, and rapidly produce cytokines upon restimulation-cardinal features of protective memory T cells. Our data show that initial CD28 signals during T cell activation prime mitochondria with latent metabolic capacity that is essential for future T cell responses.

Inflammatory macrophage dependence on NAD+ salvage is a consequence of reactive oxygen species-mediated DNA damage.

The adoption of Warburg metabolism is critical for the activation of macrophages in response to lipopolysaccharide. Macrophages stimulated with lipopolysaccharide increase their expression of nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme in NAD+ salvage, and loss of NAMPT activity alters their inflammatory potential. However, the events that lead to the cells' becoming dependent on NAD+ salvage remain poorly defined. We found that depletion of NAD+ and increased expression of NAMPT occurred rapidly after inflammatory activation and coincided with DNA damage caused by reactive oxygen species (ROS). ROS produced by complex III of the mitochondrial electron-transport chain were required for macrophage activation. DNA damage was associated with activation of poly(ADP-ribose) polymerase, which led to consumption of NAD+. In this setting, increased NAMPT expression allowed the maintenance of NAD+ pools sufficient for glyceraldehyde-3-phosphate dehydrogenase activity and Warburg metabolism. Our findings provide an integrated explanation for the dependence of inflammatory macrophages on the NAD+ salvage pathway.

STAT6 tunes maximum T cell IL-4 production from stochastically regulated Il4 alleles.

T helper 2 (Th2) cells stochastically express from the Il4 locus but it has not been determined whether allelic expression is linked or independent. Here, we provide evidence that alleles are independently activated and inactivated. We compared Il4 locus expression in T cells from hemizygous IL-4 reporter mice in culture and in vivo following exposure to type 2 immunogens. In culture, Il4 alleles had independent, heritable expression probabilities. Modeling showed that in co-expressors, dual allele transcription occurs for only short periods, limiting per-cell mRNA variation in individual cells within a population of Th2 cells. In vivo profiles suggested that early in the immune response, IL-4 output was derived predominantly from single alleles, but co-expression became more frequent over time and were tuned by STAT6, supporting the probabilistic regulation of Il4 alleles in vivo among committed IL-4 producers. We suggest an imprinted probability of expression from individual alleles with a short transcriptional shutoff time controls the magnitude of T cell IL-4 output, but the amount produced per allele is amplified by STAT6 signaling. This form of regulation may be a relevant general mechanism governing cytokine expression.

Fever supports CD8+ effector T cell responses by promoting mitochondrial translation.

Fever can provide a survival advantage during infection. Metabolic processes are sensitive to environmental conditions, but the effect of fever on T cell metabolism is not well characterized. We show that in activated CD8+ T cells, exposure to febrile temperature (39 °C) augmented metabolic activity and T cell effector functions, despite having a limited effect on proliferation or activation marker expression. Transcriptional profiling revealed an up-regulation of mitochondrial pathways, which was consistent with increased mass and metabolism observed in T cells exposed to 39 °C. Through in vitro and in vivo models, we determined that mitochondrial translation is integral to the enhanced metabolic activity and function of CD8+ T cells exposed to febrile temperature. Transiently exposing donor lymphocytes to 39 °C prior to infusion in a myeloid leukemia mouse model conferred enhanced therapeutic efficacy, raising the possibility that exposure of T cells to febrile temperatures could have clinical potential.

Thymic stromal lymphopoietin drives the development of IL-13+ Th2 cells.

T helper 2 (Th2) cells are pivotal in the development of allergy. Allergen exposure primes IL-4+ Th2 cells in lymph node, but production of effector cytokines including IL-5 and IL-13 is thought to require additional signals from antigen and the environment. Here we report that a substantial proportion of naive CD4+ T cells in spleen and lymph node express receptors for the epithelium-derived inflammatory cytokine thymic stromal lymphopoietin (TSLP). Culture of naive CD4+ T cells in anti-(a)CD3, aCD28, and TSLP-supplemented Th2 conditions enabled the development of a unique population of IL-13-single positive (IL-13-SP) CD4+ T cells; TSLP and Th2 conditions were both required for their development. Sorting experiments revealed that IL-13-SP Th2 cells originated from IL-4-negative precursors and coexpressed transcripts for the Th2 cytokines IL-5 and IL-9. In vivo, high TSLP levels acted directly on CD4+ T cells to induce the development of IL-13-SP and IL-4+IL-13+ double-positive populations in lymph node. These cells were phenotypically similar to Th2 effector cells and were CXCR5lowPD1low and expressed low levels of Bcl6 and Il21 transcripts and high levels of Gata3, Il3, and Il5 Our findings suggest a role of TSLP in directly promoting Th2 cell effector function and support the notion of TSLP as a key driver of Th2 inflammation.

Mitochondrial Integrity Regulated by Lipid Metabolism Is a Cell-Intrinsic Checkpoint for Treg Suppressive Function.

Regulatory T cells (Tregs) subdue immune responses. Central to Treg activation are changes in lipid metabolism that support their survival and function. Fatty acid binding proteins (FABPs) are a family of lipid chaperones required to facilitate uptake and intracellular lipid trafficking. One family member, FABP5, is expressed in T cells, but its function remains unclear. We show that in Tregs, genetic or pharmacologic inhibition of FABP5 function causes mitochondrial changes underscored by decreased OXPHOS, impaired lipid metabolism, and loss of cristae structure. FABP5 inhibition in Tregs triggers mtDNA release and consequent cGAS-STING-dependent type I IFN signaling, which induces heightened production of the regulatory cytokine IL-10 and promotes Treg suppressive activity. We find evidence of this pathway, along with correlative mitochondrial changes in tumor infiltrating Tregs, which may underlie enhanced immunosuppression in the tumor microenvironment. Together, our data reveal that FABP5 is a gatekeeper of mitochondrial integrity that modulates Treg function.

Acetate Promotes T Cell Effector Function during Glucose Restriction.

Competition for nutrients like glucose can metabolically restrict T cells and contribute to their hyporesponsiveness during cancer. Metabolic adaptation to the surrounding microenvironment is therefore key for maintaining appropriate cell function. For instance, cancer cells use acetate as a substrate alternative to glucose to fuel metabolism and growth. Here, we show that acetate rescues effector function in glucose-restricted CD8+ T cells. Mechanistically, acetate promotes histone acetylation and chromatin accessibility and enhances IFN-γ gene transcription and cytokine production in an acetyl-CoA synthetase (ACSS)-dependent manner. Ex vivo acetate treatment increases IFN-γ production by exhausted T cells, whereas reducing ACSS expression in T cells impairs IFN-γ production by tumor-infiltrating lymphocytes and tumor clearance. Thus, hyporesponsive T cells can be epigenetically remodeled and reactivated by acetate, suggesting that pathways regulating the use of substrates alternative to glucose could be therapeutically targeted to promote T cell function during cancer.

Polyamines and eIF5A Hypusination Modulate Mitochondrial Respiration and Macrophage Activation.

How cells adapt metabolism to meet demands is an active area of interest across biology. Among a broad range of functions, the polyamine spermidine is needed to hypusinate the translation factor eukaryotic initiation factor 5A (eIF5A). We show here that hypusinated eIF5A (eIF5AH) promotes the efficient expression of a subset of mitochondrial proteins involved in the TCA cycle and oxidative phosphorylation (OXPHOS). Several of these proteins have mitochondrial targeting sequences (MTSs) that in part confer an increased dependency on eIF5AH. In macrophages, metabolic switching between OXPHOS and glycolysis supports divergent functional fates stimulated by activation signals. In these cells, hypusination of eIF5A appears to be dynamically regulated after activation. Using in vivo and in vitro models, we show that acute inhibition of this pathway blunts OXPHOS-dependent alternative activation, while leaving aerobic glycolysis-dependent classical activation intact. These results might have implications for therapeutically controlling macrophage activation by targeting the polyamine-eIF5A-hypusine axis.

IL-4 Is a Key Requirement for IL-4- and IL-4/IL-13-Expressing CD4 Th2 Subsets in Lung and Skin.

Although IL-4 is long associated with CD4 Th2 immune responses, its role in Th2 subset development in non-lymphoid tissues is less clear. We sought to better define IL-4's role in CD4 Th2 responses by using transgenic mice that express a dual IL-4 AmCyan/IL-13 DsRed (IL-4AC/IL-13DR) fluorescent reporter on an IL-4-sufficient or IL-4-deficient background. Using primary Th2 immune response models against house dust mite or Nippostrongylus brasiliensis (Nb) allergens, we examined the requirement for IL-4 by each of the defined Th2 subsets in the antigen draining lymph node, skin, and lung tissues. In the lymph node, a CXCR5+PD-1+ T follicular helper (Tfh) and a CXCR5loPD-1lo Th2 subset could be detected that expressed only IL-4AC but no IL-13DR. The number of IL-4AC+ Tfh cells was not affected by IL-4 deficiency whereas the number of IL-4AC+ Th2 cells was significantly reduced. In the non-lymphoid dermal or lung tissues of allergen primed or Nb-infected mice, three strikingly distinct T cell subsets could be detected that were IL-4AC, or IL-4AC/IL-13DR, or IL-13DR CD4. The IL-4- and IL-4/IL-13-expressing subsets were significantly reduced in IL-4-deficient mice, while the numbers of IL-13-expressing CD4 T cells were not affected by IL-4 deficiency indicating that other factors can play a role in directing the development of this Th2 subtype. Taken together, these data indicate that the appearance of IL-4-expressing Tfh cells in the lymph node is not dependent on IL-4 while the appearance of IL-4-expressing Th2 subsets in the lymph node and IL-4, IL-4/IL-13-expressing Th2 subsets in skin and lung tissues of antigen primed mice is significantly IL-4 dependent.