Ketolysis drives CD8+ T cell effector function through effects on histone acetylation.

Environmental nutrient availability influences T cell metabolism, impacting T cell function and shaping immune outcomes. Here, we identified ketone bodies (KBs)-including ß-hydroxybutyrate (ßOHB) and acetoacetate (AcAc)-as essential fuels supporting CD8+ T cell metabolism and effector function. ßOHB directly increased CD8+ T effector (Teff) cell cytokine production and cytolytic activity, and KB oxidation (ketolysis) was required for Teff cell responses to bacterial infection and tumor challenge. CD8+ Teff cells preferentially used KBs over glucose to fuel the tricarboxylic acid (TCA) cycle in vitro and in vivo. KBs directly boosted the respiratory capacity and TCA cycle-dependent metabolic pathways that fuel CD8+ T cell function. Mechanistically, ßOHB was a major substrate for acetyl-CoA production in CD8+ T cells and regulated effector responses through effects on histone acetylation. Together, our results identify cell-intrinsic ketolysis as a metabolic and epigenetic driver of optimal CD8+ T cell effector responses.

Skip the buffet, for SPARC's sake.

Caloric restriction (CR) reduces inflammation and the incidence of chronic diseases, thereby extending healthspan and lifespan. In this issue of Immunity, Ryu et al. (2022) propose that reduction of SPARC, a matricellular protein, during CR offers beneficial effects by reducing SPARC-driven inflammatory phenotypes in macrophages.

LKB1 controls inflammatory potential through CRTC2-dependent histone acetylation.

Deregulated inflammation is a critical feature driving the progression of tumors harboring mutations in the liver kinase B1 (LKB1), yet the mechanisms linking LKB1 mutations to deregulated inflammation remain undefined. Here, we identify deregulated signaling by CREB-regulated transcription coactivator 2 (CRTC2) as an epigenetic driver of inflammatory potential downstream of LKB1 loss. We demonstrate that LKB1 mutations sensitize both transformed and non-transformed cells to diverse inflammatory stimuli, promoting heightened cytokine and chemokine production. LKB1 loss triggers elevated CRTC2-CREB signaling downstream of the salt-inducible kinases (SIKs), increasing inflammatory gene expression in LKB1-deficient cells. Mechanistically, CRTC2 cooperates with the histone acetyltransferases CBP/p300 to deposit histone acetylation marks associated with active transcription (i.e., H3K27ac) at inflammatory gene loci, promoting cytokine expression. Together, our data reveal a previously undefined anti-inflammatory program, regulated by LKB1 and reinforced through CRTC2-dependent histone modification signaling, that links metabolic and epigenetic states to cell-intrinsic inflammatory potential.

USP15 regulates type I interferon response and is required for pathogenesis of neuroinflammation.

Genes and pathways in which inactivation dampens tissue inflammation present new opportunities for understanding the pathogenesis of common human inflammatory diseases, including inflammatory bowel disease, rheumatoid arthritis and multiple sclerosis. We identified a mutation in the gene encoding the deubiquitination enzyme USP15 (Usp15L749R) that protected mice against both experimental cerebral malaria (ECM) induced by Plasmodium berghei and experimental autoimmune encephalomyelitis (EAE). Combining immunophenotyping and RNA sequencing in brain (ECM) and spinal cord (EAE) revealed that Usp15L749R-associated resistance to neuroinflammation was linked to dampened type I interferon responses in situ. In hematopoietic cells and in resident brain cells, USP15 was coexpressed with, and functionally acted together with the E3 ubiquitin ligase TRIM25 to positively regulate type I interferon responses and to promote pathogenesis during neuroinflammation. The USP15-TRIM25 dyad might be a potential target for intervention in acute or chronic states of neuroinflammation.

Metabolic Profiling Using Stable Isotope Tracing Reveals Distinct Patterns of Glucose Utilization by Physiologically Activated CD8+ T Cells.

Naive CD8+ T cells differentiating into effector T cells increase glucose uptake and shift from quiescent to anabolic metabolism. Although much is known about the metabolism of cultured T cells, how T cells use nutrients during immune responses in vivo is less well defined. Here, we combined bioenergetic profiling and 13C-glucose infusion techniques to investigate the metabolism of CD8+ T cells responding to Listeria infection. In contrast to in vitro-activated T cells, which display hallmarks of Warburg metabolism, physiologically activated CD8+ T cells displayed greater rates of oxidative metabolism, higher bioenergetic capacity, differential use of pyruvate, and prominent flow of 13C-glucose carbon to anabolic pathways, including nucleotide and serine biosynthesis. Glucose-dependent serine biosynthesis mediated by the enzyme Phgdh was essential for CD8+ T cell expansion in vivo. Our data highlight fundamental differences in glucose use by pathogen-specific T cells in vivo, illustrating the impact of environment on T cell metabolic phenotypes.

Interleukin-10 Directly Inhibits CD8+ T Cell Function by Enhancing N-Glycan Branching to Decrease Antigen Sensitivity.

Chronic viral infections remain a global health concern. The early events that facilitate viral persistence have been linked to the activity of the immunoregulatory cytokine IL-10. However, the mechanisms by which IL-10 facilitates the establishment of chronic infection are not fully understood. Herein, we demonstrated that the antigen sensitivity of CD8+ T cells was decreased during chronic infection and that this was directly mediated by IL-10. Mechanistically, we showed that IL-10 induced the expression of Mgat5, a glycosyltransferase that enhances N-glycan branching on surface glycoproteins. Increased N-glycan branching on CD8+ T cells promoted the formation of a galectin 3-mediated membrane lattice, which restricted the interaction of key glycoproteins, ultimately increasing the antigenic threshold required for T cell activation. Our study identified a regulatory loop in which IL-10 directly restricts CD8+ T cell activation and function through modification of cell surface glycosylation allowing the establishment of chronic infection.

Chromatin-binding deubiquitinase MYSM1 acts in haematopoietic progenitors to control dendritic cell development and to program dendritic cell responses to microbial stimulation.

Dendritic cells (DCs) are the most significant antigen presenting cells of the immune system, critical for the activation of naïve T cells. The pathways controlling DC development, maturation, and effector function therefore require precise regulation to allow for an effective induction of adaptive immune response. MYSM1 is a chromatin binding deubiquitinase (DUB) and an activator of gene expression via its catalytic activity for monoubiquitinated histone H2A (H2A-K119ub), which is a highly abundant repressive epigenetic mark. MYSM1 is an important regulator of haematopoiesis in mouse and human, and a systemic constitutive loss of Mysm1 in mice results in a depletion of many haematopoietic progenitors, including DC precursors, with the downstream loss of most DC lineage cells. However, the roles of MYSM1 at the later checkpoints in DC development, maturation, activation, and effector function at present remain unknown. In the current work, using a range of novel mouse models (Mysm1flCreERT2, Mysm1flCD11c-cre, Mysm1DN), we further the understanding of MYSM1 functions in the DC lineage: assessing the requirement for MYSM1 in DC development independently of other complex developmental phenotypes, exploring its role at the later checkpoints in DC maintenance and activation in response to microbial stimulation, and testing the requirement for the DUB catalytic activity of MYSM1 in these processes. Surprisingly, we demonstrate that MYSM1 expression and catalytic activity in DCs are dispensable for the maintenance of DC numbers in vivo or for DC activation in response to microbial stimulation. In contrast, MYSM1 acts via its DUB catalytic activity specifically in haematopoietic progenitors to allow normal DC lineage development, and its loss results not only in a severe DC depletion but also in the production of functionally altered DCs, with a dysregulation of many housekeeping transcriptional programs and significantly altered responses to microbial stimulation.

The energy sensor AMPK regulates T cell metabolic adaptation and effector responses in vivo.

Naive T cells undergo metabolic reprogramming to support the increased energetic and biosynthetic demands of effector T cell function. However, how nutrient availability influences T cell metabolism and function remains poorly understood. Here we report plasticity in effector T cell metabolism in response to changing nutrient availability. Activated T cells were found to possess a glucose-sensitive metabolic checkpoint controlled by the energy sensor AMP-activated protein kinase (AMPK) that regulated mRNA translation and glutamine-dependent mitochondrial metabolism to maintain T cell bioenergetics and viability. T cells lacking AMPKα1 displayed reduced mitochondrial bioenergetics and cellular ATP in response to glucose limitation in vitro or pathogenic challenge in vivo. Finally, we demonstrated that AMPKα1 is essential for T helper 1 (Th1) and Th17 cell development and primary T cell responses to viral and bacterial infections in vivo. Our data highlight AMPK-dependent regulation of metabolic homeostasis as a key regulator of T cell-mediated adaptive immunity.

Implications of cellular metabolism for immune cell migration.

Cell migration is an essential, energetically demanding process in immunity. Immune cells navigate the body via chemokines and other immune mediators, which are altered under inflammatory conditions of injury or infection. Several factors determine the migratory abilities of different types of immune cells in diverse contexts, including the precise co-ordination of cytoskeletal remodelling, the expression of specific chemokine receptors and integrins, and environmental conditions. In this review, we present an overview of recent advances in our understanding of the relationship of each of these factors with cellular metabolism, with a focus on the spatial organization of glycolysis and mitochondria, reciprocal regulation of chemokine receptors and the influence of environmental changes.

Hormonal vitamin D up-regulates tissue-specific PD-L1 and PD-L2 surface glycoprotein expression in humans but not mice.

PD-L1 (programmed death ligand 1) and PD-L2 are cell-surface glycoproteins that interact with programmed death 1 (PD-1) on T cells to attenuate inflammation. PD-1 signaling has attracted intense interest for its role in a pathophysiological context: suppression of anti-tumor immunity. Similarly, vitamin D signaling has been increasingly investigated for its non-classical actions in stimulation of innate immunity and suppression of inflammatory responses. Here, we show that hormonal 1,25-dihydroxyvitamin D (1,25D) is a direct transcriptional inducer of the human genes encoding PD-L1 and PD-L2 through the vitamin D receptor, a ligand-regulated transcription factor. 1,25D stimulated transcription of the gene encoding PD-L1 in epithelial and myeloid cells, whereas the gene encoding the more tissue-restricted PD-L2 was regulated only in myeloid cells. We identified and characterized vitamin D response elements (VDREs) located in both genes and showed that 1,25D treatment induces cell-surface expression of PD-L1 in epithelial and myeloid cells. In co-culture experiments with primary human T cells, epithelial cells pretreated with 1,25D suppressed activation of CD4+ and CD8+ cells and inhibited inflammatory cytokine production in a manner that was abrogated by anti-PD-L1 blocking antibody. Consistent with previous observations of species-specific regulation of immunity by vitamin D, the VDREs are present in primate genes, but neither the VDREs nor the regulation by 1,25D is present in mice. These findings reinforce the physiological role of 1,25D in controlling inflammatory immune responses but may represent a double-edged sword, as they suggest that elevated vitamin D signaling in humans could suppress anti-tumor immunity.

LKB1 and AMPK: central regulators of lymphocyte metabolism and function.

When T cells encounter foreign antigen and appropriate costimulatory signals from professional antigen-presenting cells (APCs), they initiate a coordinated program of rapid proliferation and differentiation, leading to the development of activated T cells with specific effector functions tailored toward pathogen clearance or control. One of the fundamental programs that underpin T-cell proliferation and function is the regulation of cellular metabolism. Recent efforts to identify the signal transduction pathways that regulate T-cell metabolism have led to the identification of liver kinase B1 (LKB1) and AMP-activated protein kinase (AMPK) as key regulators of T-cell metabolism. LKB1 and AMPK are part of an evolutionarily conserved signal transduction pathway that monitors cellular energy status. AMPK senses bioenergetic fluctuations in cells and works in concert with LKB1 to maintain cellular energy homeostasis by promoting catabolic pathways of ATP production and limiting processes that consume ATP. Recent data indicate that LKB1 and AMPK can influence diverse aspects of T-cell biology beyond metabolism, including T-cell development, peripheral T-cell homeostasis, and T-cell effector function. In this review, we focus on the regulation of lymphocyte metabolism by this energy-sensing pathway and discuss its influence on T-cell function.

NRF2-dependent regulation of the prostacyclin receptor PTGIR drives CD8 T cell exhaustion.

The progressive decline of CD8 T cell effector function-also known as terminal exhaustion-is a major contributor to immune evasion in cancer. Yet, the molecular mechanisms that drive CD8 T cell dysfunction remain poorly understood. Here, we report that the Kelch-like ECH-associated protein 1 (KEAP1)-Nuclear factor erythroid 2-related factor 2 (NRF2) signaling axis, which mediates cellular adaptations to oxidative stress, directly regulates CD8 T cell exhaustion. Transcriptional profiling of dysfunctional CD8 T cells from chronic infection and cancer reveals enrichment of NRF2 activity in terminally exhausted (Texterm) CD8 T cells. Increasing NRF2 activity in CD8 T cells (via conditional deletion of KEAP1) promotes increased glutathione production and antioxidant defense yet accelerates the development of terminally exhausted (PD-1+TIM-3+) CD8 T cells in response to chronic infection or tumor challenge. Mechanistically, we identify PTGIR, a receptor for the circulating eicosanoid prostacyclin, as an NRF2-regulated protein that promotes CD8 T cell dysfunction. Silencing PTGIR expression restores the anti-tumor function of KEAP1-deficient T cells. Moreover, lowering PTGIR expression in CD8 T cells both reduces terminal exhaustion and enhances T cell effector responses (i.e. IFN-γ and granzyme production) to chronic infection and cancer. Together, these results establish the KEAP1-NRF2 axis as a metabolic sensor linking oxidative stress to CD8 T cell dysfunction and identify the prostacyclin receptor PTGIR as an NRF2-regulated immune checkpoint that regulates CD8 T cell fate decisions between effector and exhausted states.

13C metabolite tracing reveals glutamine and acetate as critical in vivo fuels for CD8 T cells.

Infusion of 13C-labeled metabolites provides a gold standard for understanding the metabolic processes used by T cells during immune responses in vivo. Through infusion of 13C-labeled metabolites (glucose, glutamine, and acetate) in Listeria monocytogenes-infected mice, we demonstrate that CD8 T effector (Teff) cells use metabolites for specific pathways during specific phases of activation. Highly proliferative early Teff cells in vivo shunt glucose primarily toward nucleotide synthesis and leverage glutamine anaplerosis in the tricarboxylic acid (TCA) cycle to support adenosine triphosphate and de novo pyrimidine synthesis. In addition, early Teff cells rely on glutamic-oxaloacetic transaminase 1 (Got1)-which regulates de novo aspartate synthesis-for effector cell expansion in vivo. CD8 Teff cells change fuel preference over the course of infection, switching from glutamine- to acetate-dependent TCA cycle metabolism late in infection. This study provides insights into the dynamics of Teff metabolism, illuminating distinct pathways of fuel consumption associated with CD8 Teff cell function in vivo.

The histone demethylase KDM5C controls female bone mass by promoting energy metabolism in osteoclasts.

Women experience osteoporosis at higher rates than men. Aside from hormones, the mechanisms driving sex-dependent bone mass regulation are not well-understood. Here, we demonstrate that the X-linked H3K4me2/3 demethylase KDM5C regulates sex-specific bone mass. Loss of KDM5C in hematopoietic stem cells or bone marrow monocytes (BMM) increases bone mass in female but not male mice. Mechanistically, loss of KDM5C impairs the bioenergetic metabolism resulting in impaired osteoclastogenesis. Treatment with the KDM5 inhibitor reduces osteoclastogenesis and energy metabolism of both female mice and human monocytes. Our report details a novel sex-dependent mechanism for bone homeostasis, connecting epigenetic regulation to osteoclast metabolism, and positions KDM5C as a target for future treatment of osteoporosis in women. One-Sentence Summary: KDM5C, an X-linked epigenetic regulator, controls female bone homeostasis by promoting energy metabolism in osteoclasts.

Differential effects of sugar and fat on adipose tissue inflammation.

Obese individuals experience low grade inflammation initiated within their adipose tissue. However, the early events that lead to the release of these inflammatory factors from adipose tissue are poorly characterized. To separate glucose effects from lipid effects on adipose tissue, we used an adipose-specific TXNIP knockout model where excess basal glucose influx into adipocytes led to modest increase in adiposity without using high fat diet. We found an uncoupling of two events that are generally presumed to be coregulated: (1) an increase of adipose tissue macrophage (ATM) number; and (2) pro-inflammatory activation of ATMs. These two events are associated with different triggering signals: elevated free fatty acids output and extracellular matrix remodeling with increased ATM number, whereas decreased adiponectin level with activated ATM. This separation reflects non-overlapping pathways regulated by glucose and lipids in adipocytes, and neither group alone is sufficient to elicit the full inflammatory response in adipose tissue.

The histone demethylase KDM5C controls female bone mass by promoting energy metabolism in osteoclasts.

Women experience osteoporosis at higher rates than men. Aside from hormones, the mechanisms driving sex-dependent bone mass regulation are not well understood. Here, we demonstrate that the X-linked H3K4me2/3 demethylase KDM5C regulates sex-specific bone mass. Loss of KDM5C in hematopoietic stem cells or bone marrow monocytes increases bone mass in female but not male mice. Mechanistically, loss of KDM5C impairs the bioenergetic metabolism, resulting in impaired osteoclastogenesis. Treatment with the KDM5 inhibitor reduces osteoclastogenesis and energy metabolism of both female mice and human monocytes. Our report details a sex-dependent mechanism for bone homeostasis, connecting epigenetic regulation to osteoclast metabolism and positions KDM5C as a potential target for future treatment of osteoporosis in women.

13C metabolite tracing reveals glutamine and acetate as critical in vivo fuels for CD8+ T cells.

Infusion of 13C-labeled metabolites provides a gold-standard for understanding the metabolic processes used by T cells during immune responses in vivo. Through infusion of 13C-labeled metabolites (glucose, glutamine, acetate) in Listeria monocytogenes (Lm)-infected mice, we demonstrate that CD8+ T effector (Teff) cells utilize metabolites for specific pathways during specific phases of activation. Highly proliferative early Teff cells in vivo shunt glucose primarily towards nucleotide synthesis and leverage glutamine anaplerosis in the tricarboxylic acid (TCA) cycle to support ATP and de novo pyrimidine synthesis. Additionally, early Teff cells rely on glutamic-oxaloacetic transaminase 1 (Got1)-which regulates de novo aspartate synthesis-for effector cell expansion in vivo. Importantly, Teff cells change fuel preference over the course of infection, switching from glutamine- to acetate-dependent TCA cycle metabolism late in infection. This study provides insights into the dynamics of Teff metabolism, illuminating distinct pathways of fuel consumption associated with Teff cell function in vivo.

TRIM28 secures skeletal stem cell fate during skeletogenesis by silencing neural gene expression and repressing GREM1/AKT/mTOR signaling axis.

Long bones are generated by mesoderm-derived skeletal progenitor/stem cells (SSCs) through endochondral ossification, a process of sequential chondrogenic and osteogenic differentiation tightly controlled by the synergy between intrinsic and microenvironment cues. Here, we report that loss of TRIM28, a transcriptional corepressor, in mesoderm-derived cells expands the SSC pool, weakens SSC osteochondrogenic potential, and endows SSCs with properties of ectoderm-derived neural crest cells (NCCs), leading to severe defects of skeletogenesis. TRIM28 preferentially enhances H3K9 trimethylation and DNA methylation on chromatin regions more accessible in NCCs; loss of this silencing upregulates neural gene expression and enhances neurogenic potential. Moreover, TRIM28 loss causes hyperexpression of GREM1, which is an extracellular signaling factor promoting SSC self-renewal and SSC neurogenic potential by activating AKT/mTORC1 signaling. Our results suggest that TRIM28-mediated chromatin silencing establishes a barrier for maintaining the SSC lineage trajectory and preventing a transition to ectodermal fate by regulating both intrinsic and microenvironment cues.

Inhibition of the mitochondrial pyruvate carrier simultaneously mitigates hyperinflammation and hyperglycemia in COVID-19.

The relationship between diabetes and coronavirus disease 2019 (COVID-19) is bidirectional: Although individuals with diabetes and high blood glucose (hyperglycemia) are predisposed to severe COVID-19, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection can also cause hyperglycemia and exacerbate underlying metabolic syndrome. Therefore, interventions capable of breaking the network of SARS-CoV-2 infection, hyperglycemia, and hyperinflammation, all factors that drive COVID-19 pathophysiology, are urgently needed. Here, we show that genetic ablation or pharmacological inhibition of mitochondrial pyruvate carrier (MPC) attenuates severe disease after influenza or SARS-CoV-2 pneumonia. MPC inhibition using a second-generation insulin sensitizer, MSDC-0602K (MSDC), dampened pulmonary inflammation and promoted lung recovery while concurrently reducing blood glucose levels and hyperlipidemia after viral pneumonia in obese mice. Mechanistically, MPC inhibition enhanced mitochondrial fitness and destabilized hypoxia-inducible factor-1α, leading to dampened virus-induced inflammatory responses in both murine and human lung macrophages. We further showed that MSDC enhanced responses to nirmatrelvir (the antiviral component of Paxlovid) to provide high levels of protection against severe host disease development after SARS-CoV-2 infection and suppressed cellular inflammation in human COVID-19 lung autopsies, demonstrating its translational potential for treating severe COVID-19. Collectively, we uncover a metabolic pathway that simultaneously modulates pulmonary inflammation, tissue recovery, and host metabolic health, presenting a synergistic therapeutic strategy to treat severe COVID-19, particularly in patients with underlying metabolic disease.