Cutting Edge: Elevated Glycolytic Metabolism Limits the Formation of Memory CD8+ T Cells in Early Life

Inherent metabolic preferences differentially regulate the sensitivity of Th1 and Th2 cells to ribosome-inhibiting antibiotics

Mitochondrial translation is essential to maintain mitochondrial function and energy production. Mutations in genes associated with mitochondrial translation cause several developmental disorders, and immune dysfunction is observed in many such patients. Besides genetic mutations, several antibiotics targeting bacterial ribosomes are well-established to inhibit mitochondrial translation. However, the effect of such antibiotics on different immune cells is not fully understood. Here, we addressed the differential effect of mitochondrial translation inhibition on different subsets of helper T cells (Th) of mice and humans. Inhibition of mitochondrial translation reduced the levels of mitochondrially encoded electron transport chain subunits without affecting their nuclear-encoded counterparts. As a result, mitochondrial oxygen consumption reduced dramatically, but mitochondrial mass was unaffected. Most importantly, we show that inhibition of mitochondrial translation induced apoptosis, specifically in Th2 cells. This increase in apoptosis was associated with higher expression of Bim and Puma, two activators of the intrinsic pathway of apoptosis. We propose that this difference in the sensitivity of Th1 and Th2 cells to mitochondrial translation inhibition reflects the intrinsic metabolic demands of these subtypes. Though Th1 and Th2 cells exhibit similar levels of oxidative phosphorylation, Th1 cells exhibit higher levels of aerobic glycolysis than Th2 cells. Moreover, Th1 cells are more sensitive to the inhibition of glycolysis, while higher concentrations of glycolysis inhibitor 2-deoxyglucose are required to induce cell death in the Th2 lineage. These observations reveal that selection of metabolic pathways for substrate utilization during differentiation of Th1 and Th2 lineages is a fundamental process conserved across species.

Gene Regulatory Programs that Specify Age-Related Differences during Thymocyte Development

T cell development is fundamental to immune system establishment, yet how this development changes with age remains poorly understood. Here, we construct a transcriptional and epigenetic atlas of T cell developmental programs in neonatal and adult mice, revealing the ontogeny of divergent gene regulatory programs and their link to age-related differences in phenotype and function. Specifically, we identify a gene module that diverges with age from the earliest stages of genesis and includes programs that govern effector response and cell cycle regulation. Moreover, we reveal that neonates possess more accessible chromatin during early thymocyte development, likely establishing poised gene expression programs that manifest later in thymocyte development. Finally, we leverage this atlas, employing a CRISPR-based perturbation approach coupled with single-cell RNA sequencing as a readout to uncover a conserved transcriptional regulator, Zbtb20, that contributes to age-dependent differences in T cell development. Altogether, our study defines transcriptional and epigenetic programs that regulate age-specific differences in T cell development.

Update on Early-Life T Cells: Impact on Oral Rotavirus Vaccines

Rotavirus infection continues to be a significant public health problem in developing countries, despite the availability of several vaccines. The efficacy of oral rotavirus vaccines in young children may be affected by significant immunological differences between individuals in early life and adults. Therefore, understanding the dynamics of early-life systemic and mucosal immune responses and the factors that affect them is essential to improve the current rotavirus vaccines and develop the next generation of mucosal vaccines. This review focuses on the advances in T-cell development during early life in mice and humans, discussing how immune homeostasis and response to pathogens is established in this period compared to adults. Finally, the review explores how this knowledge of early-life T-cell immunity could be utilized to enhance current and novel rotavirus vaccines.

Neonatal CD8+ T Cells Resist Exhaustion during Chronic Infection

Chronic viral infections, such as HIV and hepatitis C virus, represent a major public health problem. Although it is well understood that neonates and adults respond differently to chronic viral infections, the underlying mechanisms remain unknown. In this study, we transferred neonatal and adult CD8+ T cells into a mouse model of chronic infection (lymphocytic choriomeningitis virus clone 13) and dissected out the key cell-intrinsic differences that alter their ability to protect the host. Interestingly, we found that neonatal CD8+ T cells preferentially became effector cells early in chronic infection compared with adult CD8+ T cells and expressed higher levels of genes associated with cell migration and effector cell differentiation. During the chronic phase of infection, the neonatal cells retained more immune functionality and expressed lower levels of surface markers and genes related to exhaustion. Because the neonatal cells protect from viral replication early in chronic infection, the altered differentiation trajectories of neonatal and adult CD8+ T cells is functionally significant. Together, our work demonstrates how cell-intrinsic differences between neonatal and adult CD8+ T cells influence key cell fate decisions during chronic infection.

The gene regulatory basis of bystander activation in CD8+ T cells

CD8+ T cells are classically recognized as adaptive lymphocytes based on their ability to recognize specific foreign antigens and mount memory responses. However, recent studies indicate that some antigen-inexperienced CD8+ T cells can respond to innate cytokines alone in the absence of cognate T cell receptor stimulation, a phenomenon referred to as bystander activation. Here, we demonstrate that neonatal CD8+ T cells undergo a robust and diverse program of bystander activation, which corresponds to enhanced innate-like protection against unrelated pathogens. Using a multi-omics approach, we found that the ability of neonatal CD8+ T cells to respond to innate cytokines derives from their capacity to undergo rapid chromatin remodeling, resulting in the usage of a distinct set of enhancers and transcription factors typically found in innate-like T cells. We observed that the switch between innate and adaptive functions in the CD8+ T cell compartment is mediated by changes in the abundance of distinct subsets of cells. The innate CD8+ T cell subset that predominates in early life was also present in adult mice and humans. Our findings provide support for the layered immune hypothesis and indicate that the CD8+ T cell compartment is more functionally diverse than previously thought.

Hurdle or thruster: Glucose metabolism of T cells in anti-tumour immunity

Glucose metabolism is essential for the activation, differentiation and function of T cells and proper glucose metabolism is required to maintain effective T cell immunity. Dysregulation of glucose metabolism is a hallmark of cancer, and the tumour microenvironment (TME2) can create metabolic barriers in T cells that inhibit their anti-tumour immune function. Targeting glucose metabolism is a promising approach to improve the capacity of T cells in the TME. The efficacy of common immunotherapies, such as immune checkpoint inhibitors (ICIs3) and adoptive cell transfer (ACT4), can be limited by T-cell function, and the treatment itself can affect T-cell metabolism. Therefore, understanding the relationship between immunotherapy and T cell glucose metabolism helps to achieve more effective anti-tumour therapy. In this review, we provide an overview of T cell glucose metabolism and how T cell metabolic reprogramming in the TME regulates anti-tumour responses, briefly describe the metabolic patterns of T cells during ICI and ACT therapies, which suggest possible synergistic strategies.

Let-7 enhances murine anti-tumor CD8 T cell responses by promoting memory and antagonizing terminal differentiation

The success of the CD8 T cell-mediated immune response against infections and tumors depends on the formation of a long-lived memory pool, and the protection of effector cells from exhaustion. The advent of checkpoint blockade therapy has significantly improved anti-tumor therapeutic outcomes by reversing CD8 T cell exhaustion, but fails to generate effector cells with memory potential. Here, using in vivo mouse models, we show that let-7 miRNAs determine CD8 T cell fate, where maintenance of let-7 expression during early cell activation results in memory CD8 T cell formation and tumor clearance. Conversely, let-7-deficiency promotes the generation of a terminal effector population that becomes vulnerable to exhaustion and cell death in immunosuppressive environments and fails to reject tumors. Mechanistically, let-7 restrains metabolic changes that occur during T cell activation through the inhibition of the PI3K/AKT/mTOR signaling pathway and production of reactive oxygen species, potent drivers of terminal differentiation and exhaustion. Thus, our results reveal a role for let-7 in the time-sensitive support of memory formation and the protection of effector cells from exhaustion. Overall, our data suggest a strategy in developing next-generation immunotherapies by preserving the multipotency of effector cells rather than enhancing the efficacy of differentiation.

Effects of altered glycolysis levels on CD8+ T cell activation and function

CD8+ T cells are an important component of the body's adaptive immune response. During viral or intracellular bacterial infections, CD8+ T cells are rapidly activated and differentiated to exert their immune function by producing cytokines. Alterations in the glycolysis of CD8+ T cells have an important effect on their activation and function, while glycolysis is important for CD8+ T cell functional failure and recovery. This paper summarizes the importance of CD8+ T cell glycolysis in the immune system. We discuss the link between glycolysis and CD8+ T cell activation, differentiation, and proliferation, and the effect of altered glycolysis on CD8+ T cell function. In addition, potential molecular targets to enhance and restore the immune function of CD8+ T cells by affecting glycolysis and the link between glycolysis and CD8+ T cell senescence are summarized. This review provides new insights into the relationship between glycolysis and CD8+ T cell function, and proposes novel strategies for immunotherapy by targeting glycolysis.

Shaping immunity for life: Layered development of CD8+ T cells

Historically, the immune system was believed to develop along a linear axis of maturity from fetal life to adulthood. Now, it is clear that distinct layers of immune cells are generated from unique waves of hematopoietic progenitors during different windows of development. This model, known as the layered immune model, has provided a useful framework for understanding why distinct lineages of B cells and γδ T cells arise in succession and display unique functions in adulthood. However, the layered immune model has not been applied to CD8+ T cells, which are still often viewed as a uniform population of cells belonging to the same lineage, with functional differences between cells arising from environmental factors encountered during infection. Recent studies have challenged this idea, demonstrating that not all CD8+ T cells are created equally and that the functions of individual CD8+ T cells in adults are linked to when they were created in the host. In this review, we discuss the accumulating evidence suggesting there are distinct ontogenetic subpopulations of CD8+ T cells and propose that the layered immune model be extended to the CD8+ T cell compartment.

mTOR Participates in the Formation, Maintenance, and Function of Memory CD8+T Cells Regulated by Glycometabolism

Memory CD8⁺T cells participate in the fight against infection and tumorigenesis as well as in autoimmune disease progression because of their efficient and rapid immune response, long-term survival, and continuous differentiation. At each stage of their formation, maintenance, and function, the cell metabolism must be adjusted to match the functional requirements of the specific stage. Notably, enhanced glycolytic metabolism can generate sufficient levels of adenosine triphosphate (ATP) to form memory CD8⁺T cells, countering the view that glycolysis prevents the formation of memory CD8⁺T cells. This review focuses on how glycometabolism regulates memory CD8+T cells and highlights the key mechanisms through which the mammalian target of rapamycin (mTOR) signaling pathway affects memory CD8+T cell formation, maintenance, and function by regulating glycometabolism. In addition, different subpopulations of memory CD8⁺T cells exhibit different metabolic flexibility during their formation, survival, and functional stages, during which the energy metabolism may be critical. These findings which may explain why enhanced glycolytic metabolism can give rise to memory CD8⁺T cells. Modulating the metabolism of memory CD8⁺T cells to influence specific cell fates may be useful for disease treatment.

MicroRNA-29 specifies age-related differences in the CD8+ T cell immune response

MicroRNAs (miRNAs) have emerged as critical regulators of cell fate in the CD8+ T cell response to infection. Although there are several examples of miRNAs acting on effector CD8+ T cells after infection, it is unclear whether differential expression of one or more miRNAs in the naive state is consequential in altering their long-term trajectory. To answer this question, we examine the role of miR-29 in neonatal and adult CD8+ T cells, which express different amounts of miR-29 only prior to infection and adopt profoundly different fates after immune challenge. We find that manipulation of miR-29 expression in the naive state is sufficient for age-adjusting the phenotype and function of CD8+ T cells, including their regulatory landscapes and long-term differentiation trajectories after infection. Thus, miR-29 acts as a developmental switch by controlling the balance between a rapid effector response in neonates and the generation of long-lived memory in adults.

A role for metabolism in determining neonatal immune function

Immune responses of neonates differ markedly to those of adults, with skewed cytokine phenotypes, reduced inflammatory properties and drastically diminished memory function. Recent research efforts have started to unravel the role of cellular metabolism in determining immune cell fate and function. For studies in humans, much of the work on metabolic mechanisms underpinning innate and adaptive immune responses by different haematopoietic cell types is in adults. Studies investigating the contribution of metabolic adaptation in the unique setting of early life are just emerging, and much more work is needed to elucidate the contribution of metabolism to neonatal immune responses. Here, we discuss our current understanding of neonatal immune responses, examine some of the latest developments in neonatal immunometabolism and consider the possible role of altered metabolism to the distinctive immune phenotype of the neonate. Understanding the role of metabolism in regulating immune function at this critical stage in life has direct benefit for the child by affording opportunities to maximize immediate and long-term health. Additionally, gaining insight into the diversity of human immune function and naturally evolved immunometabolic strategies that modulate immune function could be harnessed for a wide range of opportunities including new therapeutic approaches.

Developing the right tools for the job: Lin28 regulation of early life T-cell development and function

T cells comprise a functionally heterogeneous cell population that has important roles in the immune system. While T cells are broadly considered to be a component of the antigen-specific adaptive immune response, certain T-cell subsets display innate-like effector characteristics whereas others perform immunosuppressive functions. These functionally diverse T-cell populations preferentially arise at different stages of ontogeny and are tailored to the immunological priorities of the organism over time. Many differences in early life versus adult T-cell phenotypes can be attributed to the cell-intrinsic properties of the distinct progenitors that seed the thymus throughout development. It is becoming clear that Lin28, an evolutionarily conserved, heterochronic RNA-binding protein that is differentially expressed among early life and adult hematopoietic progenitor cells, plays a substantial role in influencing early T-cell development and function. Here, we discuss the mechanisms by which Lin28 shapes the T-cell landscape to protect the developing fetus and newborn. Manipulation of the Lin28 gene regulatory network is being considered as one means of improving hematopoietic stem cell transplant outcomes; as such, understanding the impact of Lin28 on T-cell function is of clinical relevance.

Metabolites in the Tumor Microenvironment Reprogram Functions of Immune Effector Cells Through Epigenetic Modifications

Cellular metabolism of both cancer and immune cells in the acidic, hypoxic, and nutrient-depleted tumor microenvironment (TME) has attracted increasing attention in recent years. Accumulating evidence has shown that cancer cells in TME could outcompete immune cells for nutrients and at the same time, producing inhibitory products that suppress immune effector cell functions. Recent progress revealed that metabolites in the TME could dysregulate gene expression patterns in the differentiation, proliferation, and activation of immune effector cells by interfering with the epigenetic programs and signal transduction networks. Nevertheless, encouraging studies indicated that metabolic plasticity and heterogeneity between cancer and immune effector cells could provide us the opportunity to discover and target the metabolic vulnerabilities of cancer cells while potentiating the anti-tumor functions of immune effector cells. In this review, we will discuss the metabolic impacts on the immune effector cells in TME and explore the therapeutic opportunities for metabolically enhanced immunotherapy.

Immunometabolism and HIV-1 pathogenesis: food for thought

Antiretroviral therapies efficiently block HIV-1 replication but need to be maintained for life. Moreover, chronic inflammation is a hallmark of HIV-1 infection that persists despite treatment. There is, therefore, an urgent need to better understand the mechanisms driving HIV-1 pathogenesis and to identify new targets for therapeutic intervention. In the past few years, the decisive role of cellular metabolism in the fate and activity of immune cells has been uncovered, as well as its impact on the outcome of infectious diseases. Emerging evidence suggests that immunometabolism has a key role in HIV-1 pathogenesis. The metabolic pathways of CD4⁺ T cells and macrophages determine their susceptibility to infection, the persistence of infected cells and the establishment of latency. Immunometabolism also shapes immune responses against HIV-1, and cell metabolic products are key drivers of inflammation during infection. In this Review, we summarize current knowledge of the links between HIV-1 infection and immunometabolism, and we discuss the potential opportunities and challenges for therapeutic interventions.

Building a T cell compartment: how immune cell development shapes function

We are just beginning to understand the diversity of the peripheral T cell compartment, which arises from the specialization of different T cell subsets and the plasticity of individual naive T cells to adopt different fates. Although the progeny of a single T cell can differentiate into many phenotypes following infection, individual T cells are biased towards particular phenotypes. These biases are typically ascribed to random factors that occur during and after antigenic stimulation. However, the T cell compartment does not remain static with age, and shifting immune challenges during ontogeny give rise to T cells with distinct functional properties. Here, we argue that the developmental history of naive T cells creates a 'hidden layer' of diversity that persists into adulthood. Insight into this diversity can provide a new perspective on immunity and immunotherapy across the lifespan.

Neonatal T Cells: A Reinterpretation

Neonatal CD4⁺ and CD8⁺ T cells have historically been characterized as immature or defective. However, recent studies prompt a reinterpretation of the functions of neonatal T cells. Rather than a population of cells always falling short of expectations set by their adult counterparts, neonatal T cells are gaining recognition as a distinct population of lymphocytes well suited for the rapidly changing environment in early life. In this review, I will highlight new evidence indicating that neonatal T cells are not inert or less potent versions of adult T cells but instead are a broadly reactive layer of T cells poised to quickly develop into regulatory or effector cells, depending on the needs of the host. In this way, neonatal T cells are well adapted to provide fast-acting immune protection against foreign pathogens, while also sustaining tolerance to self-antigens.

Tumor Microenvironment: A Metabolic Player that Shapes the Immune Response

Immune cells survey and patrol throughout the body and sometimes take residence in niche environments with distinct cellular subtypes and nutrients that may fluctuate from those in which they matured. Rooted in immune cell physiology are metabolic pathways and metabolites that not only deliver substrates and energy for growth and survival, but also instruct effector functions and cell differentiation. Unlike cancer cells, immune cells are not subject to a "Darwinian evolutionary pressure" that would allow them to adapt to developing tumors but are often irrevocably affected to local nutrient deprivation. Thus, immune cells must metabolically adapt to these changing conditions in order to perform their necessary functions. On the other hand, there is now a growing appreciation that metabolic changes occurring in cancer cells can impact on immune cell functionality and contribute to tumor immune evasion, and as such, there is a considerable and growing interest in developing techniques that target metabolism for immunotherapy. In this review, we discuss the metabolic plasticity displayed by innate and adaptive immune cells and highlight how tumor-derived lactate and tumor acidity restrict immunity. To our knowledge, this review outlines the most recent insights on how tumor microenvironment metabolically instructs immune responsiveness.