Eomes identifies thymic precursors of self-specific memory-phenotype CD8+ T cells

Ndrg3 is a critical regulator of peripheral T cell maturation and homeostasis

To provide protection, anticipatory T cell-dependent immunity is reliant on the generation and maintenance of a naïve T cell repertoire, which is sufficiently diverse to ensure recognition of newly encountered antigens. Therefore, under steady-state conditions, a given individual needs to maintain a large pool of naïve T cells, ready to respond to potential threats. Here, we demonstrate that N-myc downstream-regulated gene 3 (Ndrg3) is essential for naïve T cell stability. Mice with T cell-specific Ndrg3 loss are lymphopenic, with reduced numbers of conventional T cells and natural killer T cells. We show that in the absence of Ndrg3, naïve CD8+ T cells exhibit high rates of both proliferation and apoptosis, phenotypes that could be partially rescued by transgenic expression of a high-avidity T cell receptor. Furthermore, Ndrg3-deficient cells were refractory to interleukin-4, resulting in reduced Eomes induction, and a decreased virtual memory population. Our study therefore identifies Ndrg3 as an unexpected, pleiotropic regulator of T cell homeostasis.

Methionine regulates maternal-fetal immune tolerance and endometrial receptivity by enhancing embryonic IL-5 secretion

Endometrial receptivity and maternal-fetal immune tolerance are two crucial processes for a successful pregnancy. However, the molecular mechanisms of nutrition involved are largely unexplored. Here, we showed that maternal methionine supply significantly improved pregnancy outcomes, which was closely related to interleukin-5 (IL-5) concentration. Mechanistically, methionine induced embryonic IL-5 secretion, which enhanced the conversion of CD4+ T cells to IL-5+ Th2 cells in the uterus, thereby improving maternal-fetal immune tolerance. Meanwhile, methionine-mediated IL-5 secretion activated the nuclear factor κB (NF-κB) pathway and enhanced integrin αvβ3 expression in endometrial cells, which improved endometrial receptivity. Further, methionine strongly influenced the DNA methylation and transcription levels of the transcription factor eomesodermin (Eomes), which bound directly to the IL-5 promoter region and inhibited IL-5 transcription. Methionine modulated IL-5 transcription, maternal-fetal immune tolerance, and endometrial receptivity via its effects on Eomes. This study reveals the crucial functions of methionine and IL-5 and offers a potential nutritional strategy for successful pregnancy.

UFL1 promotes survival and function of virtual memory CD8 T cells

In naïve mice, a fraction of CD8 T cells displaying high affinity for self-MHC peptide complexes develop into virtual memory T (TVM) cells. Due to self-reactivity, TVM cells are exposed to persistent antigenic stimulation, a condition known to induce T cell exhaustion. However, TVM cells do not exhibit characteristics similar to exhausted CD8 T (TEX) cells. Here, we tested the role of the UFL1, E3 ligase of the ufmylation pathway in TVM cells. We show that UFL1 prevents the acquisition of epigenetic, transcriptional, and phenotypic changes in TVM cells that are similar to TEX cells and thus promote their survival and function. UFL1-deficient TVM cells failed to protect mice against Listeria infection. Epigenetic analysis showed higher BATF activity in UFL1-deficient TVM cells. Deletion of BATF and not PD1 decreased inhibitory molecules expression and restored the survival and function of UFL1-deficient TVM cells. Our findings demonstrate a key role of UFL1 in inhibiting the exhaustion of TVM cells and promoting their survival and function.

LCK-co-receptor association ensures T cell lineage fidelity and maximizes epitope-specific TCR diversity

The interaction between the CD4/CD8 co-receptors and LCK (an Src family tyrosine kinase) is thought to augment T cell activation upon recognition of peptide-loaded major histocompatibility complexes (pMHCs). How this interaction influences antigen-specific T cell development is unclear however, as is its impact on naïve and immune antigen-specific T cell repertoires. In mice expressing mutated endogenous LCK unable to bind co-receptors (LCKFREE mice), we show that influenza A virus (IAV)-derived pMHC-specific CD8 and CD4 T cell responses had a significantly narrowed T cell receptor (TCR) repertoire, favoring high-affinity TCRs. This narrowing was established during T cell development and was exacerbated after viral infection. The dissociation of LCK from co-receptors also resulted in the redirection of CD4-fated T cells to the CD8 lineage, with expanded pMHCII-specific cytotoxic CD8 T cells observed after IAV infection. Thus, LCK-co-receptor association is critical for ensuring T cell lineage fidelity and maximizing antigen-specific T cell repertoire diversity.

Targeting the Tumor Microenvironment in EGFR-Mutant Lung Cancer: Opportunities and Challenges

Tyrosine kinase inhibitors (TKIs) have transformed the treatment of epidermal growth factor receptor (EGFR)-mutant non-small cell lung cancer. However, treatment resistance remains a major challenge in clinical practice. The tumor microenvironment (TME) is a complex system composed of tumor cells, immune and non-immune cells, and non-cellular components. Evidence indicates that dynamic changes in TME during TKI treatment are associated with the development of resistance. Research has focused on identifying how each component of the TME interacts with tumors and TKIs to understand therapeutic targets that could address TKI resistance. In this review, we describe how TME components, such as immune cells, fibroblasts, blood vessels, immune checkpoint proteins, and cytokines, interact with EGFR-mutant tumors and how they can promote resistance to TKIs. Furthermore, we discuss potential strategies targeting TME as a novel therapeutic approach.

IL-4 induces CD22 expression to restrain the effector program of virtual memory T cells

Parasitic helminths induce the production of interleukin-4 (IL-4), which causes the expansion of virtual memory CD8+ T cells (TVM cells), a cell subset that contributes to the control of coinfection with intracellular pathogens. However, the mechanisms regulating IL-4-dependent TVM cell activation and expansion remain ill defined. Here, we used single-cell RNA sequencing of CD8+ T cells to identify pathways that control IL-4-dependent TVM cell responses. Gene signature analysis of CD8+ T cells identified a cell cluster marked by CD22, a canonical regulator of B cell activation, as a selective surface marker of IL-4-induced TVM cells. CD22+ TVM cells were enriched for interferon-γ and granzyme A and retained a diverse TCR repertoire while enriched in self-reactive CDR3 sequences. CD22 intrinsically regulated the IL-4-induced CD8+ T cell effector program, resulting in reduced responsiveness of CD22+ TVM cells and regulatory functions to infection and inflammation. Thus, helminth-induced IL-4 drives the expansion and activation of TVM cells that is counterinhibited by CD22.

IL-18 primes T cells with an antigen-inexperienced memory phenotype for proliferation and differentiation into effector cells through Notch signaling

Recent studies have revealed that a subset of CD8+ T cells exhibit innate features and can be activated by cytokines. However, the precise mechanisms underlying the proliferation and differentiation of these cells remain unclear. Here, we demonstrated that CD44highCD8+ T cells in the mouse spleen express functional interleukin-18 (IL-18) receptors, whereas CD44lowCD8+ T cells do not. In response to IL-18 stimulation, these cells activated various metabolic pathways, upregulated the expression of surface molecules, such as c-Kit (CD117), CD25, and PD-1, and induced progression through the G1/S phase in the cell cycle. IL-18-primed cells, expressing a high-affinity receptor for IL-2, exhibited robust proliferation in response to IL-2 and underwent differentiation into effector cells. The splenic CD44highCD8+ T cells exhibited high expression levels of CD122, CD62L, CCR7, and CXCR3, along with CD5, indicating their potential for migration to the lymph nodes, where they could undergo expansion and terminal differentiation into effector cells. Additionally, in a tumor model, administration of IL-18 increased the accumulation of CD8+ T cells in both the lymph nodes and tumors. It is noteworthy that stimulation of CD44highCD8+ T cells with IL-18 upregulated the Notch-1 receptor and c-Myc. Moreover, inclusion of γ-secretase inhibitors attenuated the effect of IL-18 on both proliferation and interferon-γ production in the cells. These results demonstrate that IL-18 primes CD44highCD122highCXCR3highCD62LhighCD8+ T cells for expansion and differentiation into effector cells in a Notch signaling-dependent manner.

Epigenetics behind CD8+ T cell activation and exhaustion

CD8+ T cells play a critical role in specific immunity. In recent years, cell therapy has been emerging rapidly. The specific cytotoxic capabilities of these cells enable them to precisely identify and kill cells presenting specific antigens. This has demonstrated promise in the treatment of autoimmune diseases and cancers, with wide-ranging applications and value. However, in some diseases, such as tumors and chronic infections, T cells may adopt an exhausted phenotype, resulting in a loss of cytotoxicity and limiting their further application. Epigenetics plays a significant role in the differentiation and regulation of gene expression in cells. There is extensive evidence indicating that epigenetic remodeling plays an important role in T cell exhaustion. Therefore, further understanding its role in CD8+ T cell function can provide insights into the programmatic regulation of CD8+ T cells from a genetic perspective and overcome these diseases. We attempted to describe the relationship between the activation, function, and exhaustion mechanisms of CD8+ T cells, as well as epigenetics. This understanding makes it possible for us to address the aforementioned issues.

Developmental trajectory of unconventional T cells of the cynomolgus macaque thymus

As nonhuman primates are immunologically the closest model to humans, a comprehensive understanding of T-cell development in these species is crucial. However, the differentiation pathways in which thymocytes participate, along with their heterogeneity, remain poorly characterized. Using single-cell RNA sequencing, we thoroughly profiled the development of various T-cell lineages in the juvenile cynomolgus monkey thymus, identifying and characterizing 12 distinct thymic cell states or types. Interestingly, we identified two unexpected cell types, an agonist-selected and a memory-like cell population. The agonist-selected cell population expressed genes associated with strong TCR signaling, such as PDCD1, CD5, NFKBID, NFATC1, BCL2L11, and NR4A1 but exhibiting significantly higher PDCD1 expression compared with cells following the conventional developmental pathway. Additionally, we identified a substantial number of memory-like cell populations characterized by high CXCR3 and EOMES expression. Notably, this population also highly expressed the effector-associated markers, GZMK, NKG7, and GNLY, as well as the innate cell-associated markers, ZBTB16, TYROBP, KLRB1, KLRC1, and NCR3. The EOMES + memory-like cell population expressed highly PDCD1, indicating the presence of an agonist-selection footprint. Our findings provide insights into the agonist-selection pathway that allows self-reactive thymocytes to survive thymic selections and differentiate into various unconventional T-cell lineages.

The immunology of systemic lupus erythematosus

Understanding the pathogenesis and clinical manifestations of systemic lupus erythematosus (SLE) has been a great challenge. Reductionist approaches to understand the nature of the disease have identified many pathogenetic contributors that parallel clinical heterogeneity. This Review outlines the immunological control of SLE and looks to experimental tools and approaches that are improving our understanding of the complex contribution of interacting genetics, environment, sex and immunoregulatory factors and their interface with processes inherent to tissue parenchymal cells. Efforts to advance precision medicine in the care of patients with SLE along with treatment strategies to correct the immune system hold hope and are also examined.

Sterile production of interferons in the thymus affects T cell repertoire selection

Type I and III interferons (IFNs) are robustly induced during infections and protect cells against viral infection. Both type I and III IFNs are also produced at low levels in the thymus at steady state; however, their role in T cell development and immune tolerance is unclear. Here, we found that both type I and III IFNs were constitutively produced by a very small number of AIRE+ murine thymic epithelial cells, independent of microbial stimulation. Antigen-presenting cells were highly responsive to thymic IFNs, and IFNs were required for the activation and maturation of thymic type 1 conventional dendritic cells, macrophages, and B cells. Loss of IFN sensing led to reduced regulatory T cell selection, reduced T cell receptor (TCR) repertoire diversity, and enhanced autoreactive T cell responses to self-antigens expressed during peripheral IFN signaling. Thus, constitutive exposure to IFNs in the thymus is required for generating a tolerant and diverse TCR repertoire.

NK-like CD8 T cell: one potential evolutionary continuum between adaptive memory and innate immunity

CD8 T cells are crucial adaptive immune cells with cytotoxicity to fight against pathogens or abnormal self-cells via major histocompatibility complex class I-dependent priming pathways. The composition of the memory CD8 T-cell pool is influenced by various factors. Physiological aging, chronic viral infection, and autoimmune diseases promote the accumulation of CD8 T cells with highly differentiated memory phenotypes. Accumulating studies have shown that some of these memory CD8 T cells also exhibit innate-like cytotoxicity and upregulate the expression of receptors associated with natural killer (NK) cells. Further analysis shows that these NK-like CD8 T cells have transcriptional profiles of both NK and CD8 T cells, suggesting the transformation of CD8 T cells into NK cells. However, the specific induction mechanism underlying NK-like transformation and the implications of this process for CD8 T cells are still unclear. This review aimed to deduce the possible differentiation model of NK-like CD8 T cells, summarize the functions of major NK-cell receptors expressed on these cells, and provide a new perspective for exploring the role of these CD8 T cells in health and disease.

FLT3L-induced virtual memory CD8 T cells engage the immune system against tumors

BACKGROUND

Previous research in FMS-like tyrosine kinase 3 ligands (FLT3L) has primarily focused on their potential to generate dendritic cells (DCs) from bone marrow progenitors, with a limited understanding of how these cells affect CD8 T cell function. In this study, we further investigated the in vivo role of FLT3L for the immunomodulatory capabilities of CD8 T cells.

METHODS

Albumin-conjugated FLT3L (Alb-FLT3L) was generated and applied for translational medicine purposes; here it was used to treat naïve C57BL/6 and OT1 mice for CD8 T cell response analysis. Syngeneic B16ova and E.G7ova mouse models were employed for adoptive cell transfer to evaluate the effects of Alb-FLT3L preconditioning of CD8 T cells on tumor progression. To uncover the underlying mechanisms of Alb-FLT3L modulation, we conducted bulk RNA-seq analysis of the CD44high CD8 T cells. STAT1-deficient mice were used to elucidate the functional roles of Alb-FLT3L in the modulation of T cells. Finally, antibody blockade of type one interferon signaling and in vitro coculture of plasmacytoid DCs (pDCs) with naive CD8 T cells was performed to determine the role of pDCs in mediating regulation of CD44high CD8 T cells.

RESULTS

CD44high CD8 T cells were enhanced in C57BL/6 mice administrated with Alb-FLT3L. These CD8 T cells exhibited virtual memory features and had greater proliferative and effective functions. Notably, the adoptive transfer of CD44high naïve CD8 T cells into C57BL/6 mice with B16ova tumors led to significant tumor regression. RNA-seq analysis of the CD44high naïve CD8 T cells revealed FLT3L to induce CD44high CD8 T cells in a JAK-STAT1 signaling pathway-dependent manner, as supported by results indicating a decreased ability of FLT3L to enhance CD8 T cell proliferation in STAT1-deficient mice as compared to wild-type control mice. Moreover, antibody blockade of type one interferon signaling restricted the generation of FLT3L-induced CD44high CD8 T cells, while CD44 expression was able to be induced in naïve CD8 T cells cocultured with pDCs derived from FLT3L-treated mice. This suggests the crucial role of pDCs in mediating FLT3L regulation of CD44high CD8 T cells.

CONCLUSIONS

These findings provide critical insight and support the therapeutic potential of Alb-FLT3L as an immune modulator in preconditioning of naïve CD8 T cells for cancer immunotherapy.

BRWD1 orchestrates small pre-B cell chromatin topology by converting static to dynamic cohesin

Lymphocyte development consists of sequential and mutually exclusive cell states of proliferative selection and antigen receptor gene recombination. Transitions between each state require large, coordinated changes in epigenetic landscapes and transcriptional programs. How this occurs remains unclear. Here we demonstrate that in small pre-B cells, the lineage and stage-specific epigenetic reader bromodomain and WD repeat-containing protein 1 (BRWD1) reorders three-dimensional chromatin topology to affect the transition between proliferative and gene recombination molecular programs. BRWD1 regulated the switch between poised and active enhancers interacting with promoters, and coordinated this switch with Igk locus contraction. BRWD1 did so by converting chromatin-bound static to dynamic cohesin competent to mediate long-range looping. ATP-depletion revealed cohesin conversion to be the main energetic mechanism dictating dynamic chromatin looping. Our findings provide a new mechanism of cohesin regulation and reveal how cohesin function can be dictated by lineage contextual mechanisms to facilitate specific cell fate transitions.

Establishment of the Myeloid TBX-Code Reveals Aberrant Expression of T-Box Gene TBX1 in Chronic Myeloid Leukemia

T-box genes encode transcription factors, which control developmental processes and promote cancer if deregulated. Recently, we described the lymphoid TBX-code, which collates T-box gene activities in normal lymphopoiesis, enabling identification of members deregulated in lymphoid malignancies. Here, we have extended this analysis to cover myelopoiesis, compiling the myeloid TBX-code and, thus, highlighting which of these genes might be deregulated in myeloid tumor types. We analyzed public T-box gene expression datasets bioinformatically for normal and malignant cells. Candidate T-box-gene-expressing model cell lines were identified and examined by RQ-PCR, Western Blotting, genomic profiling, and siRNA-mediated knockdown combined with RNA-seq analysis and live-cell imaging. The established myeloid TBX-code comprised 10 T-box genes, including progenitor-cell-restricted TBX1. Accordingly, we detected aberrant expression of TBX1 in 10% of stem/progenitor-cell-derived chronic myeloid leukemia (CML) patients. The classic CML cell line K-562 expressed TBX1 at high levels and served as a model to identify TBX1 activators, including transcription factor GATA1 and genomic amplification of the TBX1 locus at 22q11; inhibitors, including BCR::ABL1 fusion and downregulated GNAI2, as well as BMP, FGF2, and WNT signaling; and the target genes CDKN1A, MIR17HG, NAV1, and TMEM38A. The establishment of the myeloid TBX-code permitted identification of aberrant TBX1 expression in subsets of CML patients and cell lines. TBX1 forms an integral part of an oncogenic regulatory network impacting proliferation, survival, and differentiation. Thus, the data spotlight novel diagnostic markers and potential therapeutic targets for this malignancy.

The many faces of the zinc finger protein 335 in brain development and immune system

Zinc finger protein 335 (ZNF335) plays a crucial role in the methylation and, consequently, regulates the expression of a specific set of genes. Variants of the ZNF335 gene have been identified as risk factors for microcephaly in a variety of populations worldwide. Meanwhile, ZNF335 has also been identified as an essential regulator of T-cell development. However, an in-depth understanding of the role of ZNF335 in brain development and T cell maturation is still lacking. In this review, we summarize current knowledge of the molecular mechanisms underlying the involvement of ZNF335 in neuronal and T cell development across a wide range of pre-clinical, post-mortem, ex vivo, in vivo, and clinical studies. We also review the current limitations regarding the study of the pathophysiological functions of ZNF335. Finally, we hypothesize a potential role for ZNF335 in brain disorders and discuss the rationale of targeting ZNF335 as a therapeutic strategy for preventing brain disorders.

The immune checkpoint protein PD-1: Its emerging regulatory role in memory T cells

Immunological memory helps the body rapidly develop immune defense when it re-encounters a bacterial or viral strain or encounters a similar mutation in healthy cells. The immune checkpoint molecule programmed cell death 1 (PD-1) influences memory T cell differentiation. However, the mechanism by which PD-1 regulates the development and maintenance of memory T cells and its impact on memory T cells function remain unclear. In this review, we first discuss the structure and function of PD-1 and then summarize the roles of PD-1 as a marker of tumor memory T cells and in tumor immunotherapy. We also discuss the potential mechanisms through which PD-1 regulates memory T cells development and maintenance during immune diseases such as viral infection-mediated diseases, psoriasis, and rheumatoid arthritis, and list the effects of PD-1 on memory T cells in pregnancy and their function in maternal-fetal immune balance. A complete understanding of how PD-1 influences the development, maintenance, and function of memory T cells will provide new insights into the prevention and treatment of immune-related diseases.

CD1b glycoprotein, a crucial marker of thymocyte development during T cell maturation in cynomolgus monkeys

Phenotypic markers that denote different developmental stages of thymocytes are important for understanding T cell development in the thymus. Here, we show that CD1b is a critical discriminator of thymocyte maturation stage in cynomolgus monkeys. CD1b was expressed by immature thymocytes prior to β-selection, and its expression decreased as cells became fully mature in the thymus. MHC-I expression was lowest at the CD3loCD1b+ immature double-positive (DP) stage, while the ratio of CD1d:MHC-I expression was significantly higher at this stage than at other developmental stages. PLZF was expressed by < 0.2% of thymocytes; most PLZF+ thymocytes were CD3-/loCD1b+ immature DP thymocytes with the potential to produce IL-4. EOMES+ thymocytes, which accounted for > 2% of total thymocytes, were mostly CD3+CD1b- mature thymocytes and predominantly of the CD8 single-positive (SP) lineage. An unconventional CD8+ T cell subset expressing the NKG2AC+CXCR3+ innate-like T cell marker was identified within the EOMES+ CD8 SP lineage; these cells exhibited a memory phenotype. Taken together, these findings show that CD1b is a valuable discriminatory marker of thymocyte development. The data presented herein can be used to characterize the features of PLZF- and EOMES-associated unconventional T cells in the thymus.

A virtual memory CD8+ T cell-originated subset causes alopecia areata through innate-like cytotoxicity

Virtual memory T (TVM) cells are a T cell subtype with a memory phenotype but no prior exposure to foreign antigen. Although TVM cells have antiviral and antibacterial functions, whether these cells can be pathogenic effectors of inflammatory disease is unclear. Here we identified a TVM cell-originated CD44super-high(s-hi)CD49dlo CD8+ T cell subset with features of tissue residency. These cells are transcriptionally, phenotypically and functionally distinct from conventional CD8+ TVM cells and can cause alopecia areata. Mechanistically, CD44s-hiCD49dlo CD8+ T cells could be induced from conventional TVM cells by interleukin (IL)-12, IL-15 and IL-18 stimulation. Pathogenic activity of CD44s-hiCD49dlo CD8+ T cells was mediated by NKG2D-dependent innate-like cytotoxicity, which was further augmented by IL-15 stimulation and triggered disease onset. Collectively, these data suggest an immunological mechanism through which TVM cells can cause chronic inflammatory disease by innate-like cytotoxicity.

TGFβ control of immune responses in cancer: a holistic immuno-oncology perspective

The immune system responds to cancer in two main ways. First, there are prewired responses involving myeloid cells, innate lymphocytes and innate-like adaptive lymphocytes that either reside in premalignant tissues or migrate directly to tumours, and second, there are antigen priming-dependent responses, in which adaptive lymphocytes are primed in secondary lymphoid organs before homing to tumours. Transforming growth factor-β (TGFβ) - one of the most potent and pleiotropic regulatory cytokines - controls almost every stage of the tumour-elicited immune response, from leukocyte development in primary lymphoid organs to their priming in secondary lymphoid organs and their effector functions in the tumour itself. The complexity of TGFβ-regulated immune cell circuitries, as well as the contextual roles of TGFβ signalling in cancer cells and tumour stromal cells, necessitates the use of rigorous experimental systems that closely recapitulate human cancer, such as autochthonous tumour models, to uncover the underlying immunobiology. The diverse functions of TGFβ in healthy tissues further complicate the search for effective and safe cancer therapeutics targeting the TGFβ pathway. Here we discuss the contextual complexity of TGFβ signalling in tumour-elicited immune responses and explain how understanding this may guide the development of mechanism-based cancer immunotherapy.

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.

A single-amino acid substitution in the adaptor LAT accelerates TCR proofreading kinetics and alters T-cell selection, maintenance and function

Mature T cells must discriminate between brief interactions with self-peptides and prolonged binding to agonists. The kinetic proofreading model posits that certain T-cell antigen receptor signaling nodes serve as molecular timers to facilitate such discrimination. However, the physiological significance of this regulatory mechanism and the pathological consequences of disrupting it are unknown. Here we report that accelerating the normally slow phosphorylation of the linker for activation of T cells (LAT) residue Y136 by introducing an adjacent Gly135Asp alteration (LATG135D) disrupts ligand discrimination in vivo. The enhanced self-reactivity of LATG135D T cells triggers excessive thymic negative selection and promotes T-cell anergy. During Listeria infection, LATG135D T cells expand more than wild-type counterparts in response to very weak stimuli but display an imbalance between effector and memory responses. Moreover, despite their enhanced engagement of central and peripheral tolerance mechanisms, mice bearing LATG135D show features associated with autoimmunity and immunopathology. Our data reveal the importance of kinetic proofreading in balancing tolerance and immunity.

Parasitic worms affect virus coinfection: a mechanistic overview

Helminths are parasitic worms that coevolve with their host, usually resulting in long-term persistence through modulating host immunity. The multifarious mechanisms altering the immune system induced by helminths have significant implications on the control of coinfecting pathogens such as viruses. Here, we explore the recent literature to highlight the main immune alterations and mechanisms that affect the control of viral coinfection. Insights from these mechanisms are valuable in the understanding of clinical observations in helminth-prevalent areas and in the design of new therapeutic and vaccination strategies to control viral diseases.

Bystander activation in memory and antigen-inexperienced memory-like CD8 T cells

Antigen-induced memory T cells undergo counterintuitive activation in an antigen-independent manner, which is called bystander response. Although it is well documented that memory CD8⁺ T cells produce IFNγ and upregulate the cytotoxic program upon the stimulation with inflammatory cytokines, there is only rare evidence that this provides an actual protection against pathogens in immunocompetent individuals. One of the reasons might be numerous antigen-inexperienced memory-like T cells that are also capable of the bystander response. Little is known about the bystander protection of memory and memory-like T cells and their redundancies with innate-like lymphocytes in humans because of the interspecies differences and the lack of controlled experiments. However, it has been proposed that IL-15/NKG2D-driven bystander activation of memory T cells drives protection or immunopathology in particular human diseases.

The endogenous repertoire harbors self-reactive CD4+ T cell clones that adopt a follicular helper T cell-like phenotype at steady state

The T cell repertoire of healthy mice and humans harbors self-reactive CD4+ conventional T (Tconv) cells capable of inducing autoimmunity. Using T cell receptor profiling paired with in vivo clonal analysis of T cell differentiation, we identified Tconv cell clones that are recurrently enriched in non-lymphoid organs following ablation of Foxp3+ regulatory T (Treg) cells. A subset of these clones was highly proliferative in the lymphoid organs at steady state and exhibited overt reactivity to self-ligands displayed by dendritic cells, yet were not purged by clonal deletion. These clones spontaneously adopted numerous hallmarks of follicular helper T (TFH) cells, including expression of Bcl6 and PD-1, exhibited an elevated propensity to localize within B cell follicles at steady state, and produced interferon-γ in non-lymphoid organs following sustained Treg cell depletion. Our work identifies a naturally occurring population of self-reactive TFH-like cells and delineates a previously unappreciated fate for self-specific Tconv cells.

Helminth Infection-Induced Increase in Virtual Memory CD8 T Cells Is Transient, Driven by IL-15, and Absent in Aged Mice

CD8 virtual memory T (TVM) cells are Ag-naive CD8 T cells that have undergone partial differentiation in response to common γ-chain cytokines, particularly IL-15 and IL-4. TVM cells from young individuals are highly proliferative in response to TCR and cytokine stimulation but, with age, they lose TCR-mediated proliferative capacity and exhibit hallmarks of senescence. Helminth infection can drive an increase in TVM cells, which is associated with improved pathogen clearance during subsequent infectious challenge in young mice. Given the cytokine-dependent profile of TVM cells and their age-associated dysfunction, we traced proliferative and functional changes in TVM cells, compared with true naive CD8 T cells, after helminth infection of young and aged C57BL/6 mice. We show that IL-15 is essential for the helminth-induced increase in TVM cells, which is driven only by proliferation of existing TVM cells, with negligible contribution from true naive cell differentiation. Additionally, TVM cells showed the greatest proliferation in response to helminth infection and IL-15 compared with other CD8 T cells. Furthermore, TVM cells from aged mice did not undergo expansion after helminth infection due to both TVM cell-intrinsic and -extrinsic changes associated with aging.

Roles of Virtual Memory T Cells in Diseases

Memory T cells that mediate fast and effective protection against reinfections are usually generated upon recognition on foreign Ags. However, a "memory-like" T-cell population, termed virtual memory T (TVM) cells that acquire a memory phenotype in the absence of foreign Ag, has been reported. Although, like innate cells, TVM cells reportedly play a role in first-line defense to bacterial or viral infections, their protective or pathological roles in immune-related diseases are largely unknown. In this review, we discuss the current understanding of TVM cells, focusing on their distinct characteristics, immunological properties, and roles in various immune-related diseases, such as infections and cancers.

Single-cell transcriptional profiling uncovers the association between EOMES+CD8+ T cells and acquired EGFR-TKI resistance

Acquired resistance to tyrosine kinase inhibitors (TKIs) is reportedly inevitable in lung cancers harboring epidermal growth factor receptor (EGFR) mutations, emphasizing the need for novel approaches to predict EGFR-TKI resistance for clinical monitoring and patient management. This study identified a significant increase in eomesodermin (EOMES)+CD8+ T cells in the TKI-resistant patients, which was correlated with poor survival. The increase in EOMES+CD8+ T cells was further confirmed in both tissue samples and peripheral blood of patients with TKIs resistance. The integrated analysis of pseudotime and Gene set variation showed that the increase in EOMES+CD8+ T cells may be attributed to TRM T cell conversion and metabolic reprogramming. Overall, this work suggested an association between the increased number of EOMES+CD8+ T cells and acquired TKI drug resistance, supporting the utility of EOMES+CD8+ T cells as a biomarker for TKI treatment response.

Solely HBsAg intrauterine exposure accelerates HBV clearance by promoting HBs-specific immune response in the mouse pups

Chronic hepatitis B virus (HBV) infection due to perinatal mother-to-infant transmission (MTIT) remains a serious global public health problem. It has been shown that intrauterine exposure to HBV antigens might account for the MTIT-related chronic infection. However, whether hepatitis B surface antigen (HBsAg) intrauterine exposure affected the offspring’s immune response against HBV and MTIT of HBV has not been fully clarified. In this study, we investigated the effects and the potential mechanisms of the HBsAg intrauterine exposure on the persistence of HBV replication using a solely HBsAg intrauterine exposure mice model. Our results revealed that solely HBsAg intrauterine exposure significantly accelerated the clearance of HBV when these mice were hydrodynamically injected with pBB4.5-HBV1.2 plasmids after birth, which may be due to the increased number of HBs-specific CD8⁺ T cells and interferon-gamma secretion in the liver of mice. Mechanismly, HBsAg intrauterine exposure activated antigen-presenting dendritic cells, which led to the generation of antigen-specific cellular immunological memory. Our data provide an important experimental evidence for the activation of neonatal immune response by HBsAg intrauterine exposure.

Virtual Memory CD8+ T Cells: Origin and Beyond

The presence of CD8+ T cells with a memory phenotype in nonimmunized mice has been noted for decades, but it was not until about 2 decades ago that they began to be studied in greater depth. Currently called virtual memory CD8+ T cells, they consist of a heterogeneous group of cells with memory characteristics, without any previous contact with their specific antigens. These cells were identified in mice, but a few years ago, a cell type with characteristics equivalent to the murine ones was described in healthy humans. In this review, we address the different aspects of its biology mainly developed in murine models and what is currently known about its cellular equivalent in humans.

Single-Cell Transcriptomics Reveals Longevity Immune Remodeling Features Shared by Centenarians and Their Offspring

Centenarians, who show mild infections and low incidence of tumors, are the optimal model to investigate healthy aging. However, longevity related immune characteristics has not been fully revealed largely due to lack of appropriate controls. In this study, single-cell transcriptomic analysis of peripheral blood mononuclear cells (PBMCs) derived from seven centenarians (CEN), six centenarians' offspring (CO), and nine offspring spouses or neighbors (Control, age-matched to CO) are performed to investigate the shared immune features between CEN and CO. The results indicate that among all 12 T cell clusters, the cytotoxic-phenotype-clusters (CPC) and the naïve-phenotype-clusters (NPC) significantly change between CEN and ontrol. Compared to Control, both CEN and CO are characterized by depleted NPC and increased CPC, which is dominated by CD8+ T cells. Furthermore, CPC from CEN and CO share enhanced signaling pathways and transcriptional factors associated with immune response, and possesse similar T-cell-receptor features, such as high clonal expansion. Interestingly, rather than a significant increase in GZMK+ CD8 cells during aging, centenarians show accumulation of GZMB+ and CMC1+ CD8 T cells. Collectively, this study unveils an immune remodeling pattern reflected by both quantitative increase and functional reinforcement of cytotoxic T cells which are essential for healthy aging.

Exploring the immunomodulatory role of virtual memory CD8+ T cells: Role of IFN gamma in tumor growth control

Virtual memory CD8⁺ T cells (T_VM) have been described as cells with a memory-like phenotype but without previous antigen (Ag) exposure. T_VM cells have the ability to respond better to innate stimuli rather than by TCR engagement, producing large amounts of interferon gamma (IFNγ) after stimulation with interleukin (IL)-12 plus IL-18. As a result of the phenotypic similarity, T_VM cells have been erroneously included in the central memory T cell subset for many years. However, they can now be discriminated via the CD49d receptor, which is up-regulated only on conventional memory T cells (T_MEM) and effector T cells (T_EFF) after specific cognate Ag recognition by a TCR. In this work we show that systemic expression of IL-12 plus IL-18 induced an alteration in the normal T_VM vs T_MEM/T_EFF distribution in secondary lymphoid organs and a preferential enrichment of T_VM cells in the melanoma (B16) and the pancreatic ductal adenocarcinoma (KPC) tumor models. Using our KPC bearing OT-I mouse model, we observed a significant increase in CD8⁺ T cell infiltrating the tumor islets after IL-12+IL-18 stimulation with a lower average speed when compared to those from control mice. This finding indicates a stronger interaction of T cells with tumor cells after cytokine stimulation. These results correlate with a significant reduction in tumor size in both tumor models in IL-12+IL-18-treated OT-I mice compared to control OT-I mice. Interestingly, the absence of IFNγ completely abolished the high antitumor capacity induced by IL-12+IL-18 expression, indicating an important role for these cytokines in early tumor growth control. Thus, our studies provide significant new information that indicates an important role of T_VM cells in the immune response against cancer.

Self-reactivity of CD8 T-cell clones determines their differentiation status rather than their responsiveness in infections

Mature T cells are selected for recognizing self-antigens with low to intermediate affinity in the thymus. Recently, the relative differences in self-reactivity among individual T-cell clones were appreciated as important factors regulating their fate and immune response, but the role of self-reactivity in T-cell biology is incompletely understood. We addressed the role of self-reactivity in T-cell diversity by generating an atlas of mouse peripheral CD8⁺ T cells, which revealed two unconventional populations of antigen-inexperienced T cells. In the next step, we examined the steady-state phenotype of monoclonal T cells with various levels of self-reactivity. Highly self-reactive clones preferentially differentiate into antigen-inexperienced memory-like cells, but do not form a population expressing type I interferon-induced genes, showing that these two subsets have unrelated origins. The functional comparison of naïve monoclonal CD8⁺ T cells specific to the identical model antigen did not show any correlation between the level of self-reactivity and the magnitude of the immune response.

Clinical predictive value of naïve and memory T cells in advanced NSCLC

Currently, there is no sensitive prognostic biomarker to screen out benefit patients from the non-benefit population in advanced non-small cell lung cancer patients (aNSCLCs). The 435 aNSCLCs and 278 normal controls (NCs) were recruited. The percentages and absolute counts (AC) of circulating naïve and memory T lymphocytes of CD4+ and CD8+ T cells (Tn/Tm) were measured by flow cytometry. The percentage of CD4+ naïve T (Tn), CD8+ Tn, CD8+ T memory stem cell (Tscm), and CD8+ terminal effector T cell decreased obviously. Still, all AC of Tn/Tm of aNSCLCs was significantly lower compared to NCs. Higher AC and percentage of CD4+ Tn, CD8+ Tn, and CD4+ Tscm showed markedly longer median PFS in aNSCLCs. Statistics demonstrated the AC of CD4+ Tn (≥ 3.7 cells/μL) was an independent protective factor for PFS. The analysis of the prognosis of immunotherapy showed the higher AC and percentage of CD4+ Tn and CD4+ Tscm and higher AC of CD8+ Tscm had significantly longer median PFS and the AC of CD4+ Tn (≥ 5.5 cells/μL) was an independent protective factor for PFS. Moreover, higher AC and percentages of Tn/Tm suggested higher disease control rate and lower progressive disease rate. The AC of Tn/Tm showed more regular patterns of impairment and was more relative with the disease progression than percentages in aNSCLCs. AC had a better predictive value than percentages in Tn/Tm for PFS. Notably, the AC of CD4+ Tn was a potential prognostic biomarker for the PFS and efficacy of immunotherapy.

Innate and Innate-like Effector Lymphocytes in Health and Disease

Lymphocytes can be functionally partitioned into subsets belonging to the innate or adaptive arms of the immune system. Subsets of innate and innate-like lymphocytes may or may not express Ag-specific receptors of the adaptive immune system, yet they are poised to respond with innate-like speed to pathogenic insults but lack the capacity to develop classical immunological memory. These lymphocyte subsets display a number of common properties that permit them to integrate danger and stress signals dispatched by innate sensor cells to facilitate the generation of specialized effector immune responses tailored toward specific pathogens or other insults. In this review, we discuss the functions of distinct subsets of innate and innate-like lymphocytes. A better understanding of the mechanisms by which these cells are activated in different contexts, their interactions with other immune cells, and their role in health and disease may inform the development of new or improved immunotherapies.

Adoptive transfer of IL-4 reprogrammed Tc17 cells elicits anti-tumour immunity through functional plasticity

Ability of IL-17-producing CD8+ T cells (Tc17) to transform into cytotoxic anti-tumour effectors makes them a promising candidate for immune effector cell (IEC) therapy. However, key factors regulating Tc17 reprogramming remain poorly defined, hindering translation of Tc17-based IEC use from bench to bedside. We probed the effects of multiple cytokines and underlying signalling pathways on Tc17 cells and identified pivotal role for IL-4 and PI3K/AKT in promoting Tc17 transformation into cytotoxic IFN-γ-producing IECs, an effect dependent on Eomes expression. IL-4 not only triggered Tc17 cytotoxicity, but also induced cell expansion, which significantly improved the antitumour potential of Tc17 cells compared to that of IFN-γ-producing CD8+ T cells (Tc1) in a murine model. Furthermore, IL-4/AKT signalling drove the upregulation of the T-cell receptor-associated transmembrane adaptor 1 (Trat1) in Tc17 cells to promote IL-4-induced T-cell receptor stabilization and Tc17 cytotoxicity. Finally, we proposed a possible procedure to expand human Tc17 from peripheral blood of cancer patients, and confirmed the function of IL-4 in Tc17 reprogramming. Collectively, these results document a novel IL-4/AKT/Eomes/Trat1 axis that promotes expansion and transformation of Tc17 cells into cytotoxic effectors with a therapeutic potential. IL-4 priming of Tc17 cells should be further explored as a cell therapy engineering strategy to generate IECs to augment anti-tumour responses.

Type 2 cytokines in the thymus activate Sirpα+ dendritic cells to promote clonal deletion

The thymus contains a diversity of dendritic cells (DCs) that exist in defined locations and have different antigen-processing and -presenting features. This suggests that they play nonredundant roles in mediating thymocyte selection. In an effort to eliminate SIRPα+ classic DC2 subsets, we discovered that a substantial proportion expresses the surface lectin, CD301b, in the thymus. These cells resemble the CD301b+ type 2 immune response promoting DCs that are present in the skin-draining lymph nodes. Transcriptional and phenotypic comparison to other DC subsets in the thymus revealed that thymic CD301b+ cDCs represent an activated state that exhibits enhanced antigen processing and presentation. Furthermore, a CD301b+ cDC2 subset demonstrated a type 2 cytokine signature and required steady-state interleukin-4 receptor signaling. Selective ablation of CD301b+ cDC2 subsets impaired clonal deletion without affecting regulatory T cells (Treg cells). The T cell receptor α repertoire sequencing confirmed that a cDC2 subset promotes deletion of conventional T cells with minimal effect on Treg cell selection. Together, these findings suggest that cytokine-induced activation of DCs in the thymus substantially enforces central tolerance.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

Control of Memory Phenotype T Lymphocyte Homeostasis: Role of Costimulation

Foxp3⁺ T regulatory cells (Tregs), CD4⁺Foxp3^- T cells, and CD8⁺ T cells are composed of naive phenotype (NP) and memory phenotype (MP) subsets. Ten to 20% of each MP T cell population are cycling (Ki-67⁺) in vivo. We investigated the contribution of costimulatory (CD28) and coinhibitory (CTLA-4, PD-1) receptors on MP T cell homeostatic proliferation in vivo in the mouse. Blockade of CD28-CD80/CD86 signaling completely abolished MP Tregs and profoundly inhibited MP CD4⁺Foxp3^- T cell proliferation, but it did not affect MP CD8⁺ T cell proliferation. Marked enhancement of homeostatic proliferation of MP Tregs and MP CD4⁺Foxp3^- T cells was seen after blocking CTLA4-CD80/CD86 interactions and PD-1-PD-L1/2 interactions, and greater enhancement was seen with blockade of both pathways. The CD28 pathway also played an important role in the expansion of Tregs and MP T cells after treatment of mice with agonistic Abs to members of the TNF receptor superfamily, which can act directly (anti-GITR, anti-OX40, anti-4-1BB) or indirectly (anti-CD40) on T cells. Induction of a cytokine storm by blocking the interaction of NK inhibitory receptors with MHC class I had no effect on Treg homeostasis, enhanced MP CD4⁺ proliferation, and expansion in a CD28-dependent manner, but it enhanced MP CD8⁺ T cell proliferation in a CD28-independent manner. Because MP T cells exert potent biologic effects primarily before the induction of adaptive immune responses, these findings have important implications for the use of biologic agents designed to suppress autoimmune disease or enhance T effector function in cancer that may have negative effects on MP T cells.

Establishment of the TBX-code reveals aberrantly activated T-box gene TBX3 in Hodgkin lymphoma

T-box genes encode transcription factors which control basic processes in development of several tissues including cell differentiation in the hematopoietic system. Here, we analyzed the physiological activities of all 17 human T-box genes in early hematopoiesis and in lymphopoiesis including developing and mature B-cells, T-cells, natural killer (NK)-cells and innate lymphoid cells. The resultant expression pattern comprised six genes, namely EOMES, MGA, TBX1, TBX10, TBX19 and TBX21. We termed this gene signature TBX-code which enables discrimination of normal and aberrant activities of T-box genes in lymphoid malignancies. Accordingly, expression analysis of T-box genes in Hodgkin lymphoma (HL) patients using a public profiling dataset revealed overexpression of EOMES, TBX1, TBX2, TBX3, TBX10, TBX19, TBX21 and TBXT while MGA showed aberrant downregulation. Analysis of T-cell acute lymphoid leukemia patients indicated aberrant overexpression of six T-box genes while no deregulated T-box genes were detected in anaplastic large cell lymphoma patients. As a paradigm we focused on TBX3 which was ectopically activated in about 6% of HL patients analyzed. Normally, TBX3 is expressed in tissues like lung, adrenal gland and retina but not in hematopoiesis. HL cell line KM-H2 expressed enhanced TBX3 levels and was used as an in vitro model to identify upstream regulators and downstream targets in this malignancy. Genomic studies of this cell line showed focal amplification of the TBX3 locus at 12q24 which may underlie its aberrant expression. In addition, promoter analysis and comparative expression profiling of HL cell lines followed by knockdown experiments revealed overexpressed transcription factors E2F4 and FOXC1 and chromatin modulator KDM2B as functional activators. Furthermore, we identified repressed target genes of TBX3 in HL including CDKN2A, NFKBIB and CD19, indicating its respective oncogenic function in proliferation, NFkB-signaling and B-cell differentiation. Taken together, we have revealed a lymphoid TBX-code and used it to identify an aberrant network around deregulated T-box gene TBX3 in HL which promotes hallmark aberrations of this disease. These findings provide a framework for future studies to evaluate deregulated T-box genes in lymphoid malignancies.

The cell-surface 5'-nucleotidase CD73 defines a functional T memory cell subset that declines with age

Memory T cells exhibit considerable diversity that determines their ability to be protective. Here, we examine whether changes in T cell heterogeneity contribute to the age-associated failure of immune memory. By screening for age-dependent T cell-surface markers, we identify CD4 and CD8 memory T cell subsets that are unrelated to previously defined subsets of central and effector memory cells. Memory T cells expressing the ecto-5'-nucleotidase CD73 constitute a functionally distinct subset of memory T cells that declines with age. They resemble long-lived, polyfunctional memory cells but are also poised to display effector functions and to develop into cells resembling tissue-resident memory T cells (T_RMs). Upstream regulators of differential chromatin accessibility and transcriptomes include transcription factors that facilitate CD73 expression and regulate TRM differentiation. CD73 is not just a surrogate marker of these regulatory networks but is directly involved in T cell survival.

Functional heterogeneity and adaptation of naive T cells in response to tonic TCR signals

Mature CD4⁺ and CD8⁺ T cells constitutively experience weak T cell receptor (TCR) stimulation in response to self-antigens, termed tonic (or basal) signaling. How tonic TCR signal strength impacts T cell responses to foreign antigens is an active area of investigation. Such studies rely on surrogate markers of tonic signal strength, including CD5, Ly6C, and transgenic reporters of Nr4a genes. Recent research indicates that strong tonic TCR signal strength influences basal T cell metabolism, effector differentiation, and TCR signal transduction. T cells that experience the strongest tonic TCR signaling exhibit features of T cell activation and negative regulation. These data suggest a model whereby adaptation to tonic signaling has lasting effects that alter T cell activation and differentiation.

Signals for antigen-independent differentiation of memory CD8+ T cells

Conventional CD8+ memory T cells develop upon stimulation with foreign antigen and provide increased protection upon re-challenge. Over the past two decades, new subsets of CD8+ T cells have been identified that acquire memory features independently of antigen exposure. These antigen-inexperienced memory T cells (TAIM) are described under several names including innate memory, virtual memory, and memory phenotype. TAIM cells exhibit characteristics of conventional or true memory cells, including antigen-specific responses. In addition, they show responsiveness to innate stimuli and have been suggested to provide additional levels of protection toward infections and cancer. Here, we discuss the current understanding of TAIM cells, focusing on extrinsic and intrinsic molecular conditions that favor their development, their molecular definitions and immunological properties, as well as their transcriptional and epigenetic regulation.

Effector T cell responses unleashed by regulatory T cell ablation exacerbate oral squamous cell carcinoma

Immune suppression by CD4+FOXP3+ regulatory T (Treg) cells and tumor infiltration by CD8+ effector T cells represent two major factors impacting response to cancer immunotherapy. Using deconvolution-based transcriptional profiling of human papilloma virus (HPV)-negative oral squamous cell carcinomas (OSCCs) and other solid cancers, we demonstrate that the density of Treg cells does not correlate with that of CD8+ T cells in many tumors, revealing polarized clusters enriched for either CD8+ T cells or CD4+ Treg and conventional T cells. In a mouse model of carcinogen-induced OSCC characterized by CD4+ T cell enrichment, late-stage Treg cell ablation triggers increased densities of both CD4+ and CD8+ effector T cells within oral lesions. Notably, this intervention does not induce tumor regression but instead induces rapid emergence of invasive OSCCs via an effector T cell-dependent process. Thus, induction of a T cell-inflamed phenotype via therapeutic manipulation of Treg cells may trigger unexpected tumor-promoting effects in OSCC.

CD8+ Regulatory T Cell - A Mystery to Be Revealed

Regulatory T cells (Treg) are essential to maintain immune homeostasis and prevent autoimmune disorders. While the function and molecular regulation of Foxp3+CD4+ Tregs are well established, much of CD8+ Treg biology remains to be revealed. Here, we will review the heterogenous subsets of CD8+ T cells have been named "CD8+ Treg" and mainly focus on CD122hiLy49+CD8+ Tregs present in naïve mice. CD122hiLy49+CD8+ Tregs, which depends on transcription factor Helios and homeostatic cytokine IL-15, have been established as a non-redundant regulator of germinal center (GC) reaction. Recently, we have demonstrated that TGF-β (Transforming growth factor-β) and transcription factor Eomes (Eomesodermin) are essential for the function and homeostasis of CD8+ Tregs. In addition, we will discuss several open questions regarding the differentiation, function and true identity of CD8+ Tregs as well as a brief comparison between two regulatory T cell subsets critical to control GC reaction, namely CD4+ TFR (follicular regulatory T cells) and CD8+ Tregs.

NR4A3 Mediates Thymic Negative Selection

Central tolerance aims to limit the production of T lymphocytes bearing TCR with high affinity for self-peptide presented by MHC molecules. The accumulation of thymocytes with such receptors is limited by negative selection or by diversion into alternative differentiation, including T regulatory cell commitment. A role for the orphan nuclear receptor NR4A3 in negative selection has been suggested, but its function in this process has never been investigated. We find that Nr4a3 transcription is upregulated in postselection double-positive thymocytes, particularly those that have received a strong selecting signal and are destined for negative selection. Indeed, we found an accumulation of cells bearing a negative selection phenotype in NR4A3-deficient mice as compared with wild-type controls, suggesting that Nr4a3 transcriptional induction is necessary to limit accumulation of self-reactive thymocytes. This is consistent with a decrease of cleaved caspase-3⁺-signaled thymocytes and more T regulatory and CD4⁺Foxp3^-HELIOS⁺ cells in the NR4A3-deficient thymus. We further tested the role for NR4A3 in negative selection by reconstituting transgenic mice expressing the OVA Ag under the control of the insulin promoter with bone marrow cells from OT-I Nr4a3 ^+/+ or OT-I Nr4a3 ^-/- mice. Accumulation of autoreactive CD8 thymocytes and autoimmune diabetes developed only in the absence of NR4A3. Overall, our results demonstrate an important role for NR4A3 in T cell development.

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.

Asymmetric cell division shapes naive and virtual memory T-cell immunity during ageing

Efficient immune responses rely on heterogeneity, which in CD8⁺ T cells, amongst other mechanisms, is achieved by asymmetric cell division (ACD). Here we find that ageing, known to negatively impact immune responses, impairs ACD in murine CD8⁺ T cells, and that this phenotype can be rescued by transient mTOR inhibition. Increased ACD rates in mitotic cells from aged mice restore the expansion and memory potential of their cellular progenies. Further characterization of the composition of CD8⁺ T cells reveals that virtual memory cells (T_VM cells), which accumulate during ageing, have a unique proliferation and metabolic profile, and retain their ability to divide asymmetrically, which correlates with increased memory potential. The opposite is observed for naive CD8⁺ T cells from aged mice. Our data provide evidence on how ACD modulation contributes to long-term survival and function of T cells during ageing, offering new insights into how the immune system adapts to ageing.

Asymmetrical cell division helps to maintain cellular heterogeneity in the T cell compartment. Here the authors examine the differential immune responses built by naive and virtual memory T cells from young and aged individuals, and explore the effect of mTOR inhibition on asymmetrical cell division and memory formation.

CD8+ T-Cell Memory: The Why, the When, and the How

The generation of effective adaptive T-cell memory is a cardinal feature of the adaptive immune system. The establishment of protective T-cell immunity requires the differentiation of CD8⁺ T cells from a naive state to one where pathogen-specific memory CD8⁺ T cells are capable of responding to a secondary infection more rapidly and robustly without the need for further differentiation. The study of factors that determine the fate of activated CD8⁺ T cells into either effector or memory subsets has a long history. The advent of new technologies is now providing new insights into how epigenetic regulation not only impacts acquisition and maintenance of effector function, but also the maintenance of the quiescent yet primed memory state. There is growing appreciation that rather than distinct subsets, memory T-cell populations may reflect different points on a spectrum between the starting naive T-cell population and a terminally differentiated effector CD8⁺ T-cell population. Interestingly, there is growing evidence that the molecular mechanisms that underpin the rapid effector function of memory T cells are also observed in innate immune cells such as macrophages and natural killer (NK) cells. This raises an interesting hypothesis that the memory/effector T-cell state represents a default innate-like response to antigen recognition, and that it is the naive state that is the defining feature of adaptive immunity. These issues are discussed.