Long-lived T follicular helper cells retain plasticity and help sustain humoral immunity

Help me help you: emerging concepts in T follicular helper cell differentiation, identity, and function

Effective high-affinity, long-term humoral immunity requires T cell help provided by a subset of differentiated CD4+ T cells known as T follicular helper (Tfh) cells. Classically, Tfh cells provide contact-dependent help for the generation of germinal centers (GCs) in secondary lymphoid organs (SLOs). Recent studies have expanded the conventional definition of Tfh cells, revealing new functions, new descriptions of Tfh subsets, new factors regulating Tfh differentiation, and new roles outside of SLO GCs. Together, these data suggest that one Tfh is not equivalent to another, helping redefine our understanding of Tfh cells and their biology.

Tissue-resident memory T cells: decoding intra-organ diversity with a gut perspective

Tissue-resident memory T cells (TRM) serve as the frontline of host defense, playing a critical role in protection against invading pathogens. This emphasizes their role in providing rapid on-site immune responses across various organs. The physiological significance of TRM is not just confined to infection control; accumulating evidence has revealed that TRM also determine the pathology of diseases such as autoimmune disorders, inflammatory bowel disease, and cancer. Intensive studies on the origin, mechanisms of formation and maintenance, and physiological significance of TRM have elucidated the transcriptional and functional diversity of these cells, which are often affected by local cues associated with their presence. These were further confirmed by the recent remarkable advancements of next-generation sequencing and single-cell technologies, which allow the transcriptional and phenotypic characterization of each TRM subset induced in different microenvironments. This review first overviews the current knowledge of the cell fate, molecular features, transcriptional and metabolic regulation, and biological importance of TRM in health and disease. Finally, this article presents a variety of recent studies on disease-associated TRM, particularly focusing and elaborating on the TRM in the gut, which constitute the largest and most intricate immune network in the body, and their pathological relevance to gut inflammation in humans.

Biomaterial engineering strategies for B cell immunity modulations

B cell immunity has a penetrating effect on human health and diseases. Therapeutics aiming to modulate B cell immunity have achieved remarkable success in combating infections, autoimmunity, and malignancies. However, current treatments still face significant limitations in generating effective long-lasting therapeutic B cell responses for many conditions. As the understanding of B cell biology has deepened in recent years, clearer regulation networks for B cell differentiation and antibody production have emerged, presenting opportunities to overcome current difficulties and realize the full therapeutic potential of B cell immunity. Biomaterial platforms have been developed to leverage these emerging concepts to augment therapeutic humoral immunity by facilitating immunogenic reagent trafficking, regulating T cell responses, and modulating the immune microenvironment. Moreover, biomaterial engineering tools have also advanced our understanding of B cell biology, further expediting the development of novel therapeutics. In this review, we will introduce the general concept of B cell immunobiology and highlight key biomaterial engineering strategies in the areas including B cell targeted antigen delivery, sustained B cell antigen delivery, antigen engineering, T cell help optimization, and B cell suppression. We will also discuss our perspective on future biomaterial engineering opportunities to leverage humoral immunity for therapeutics.

Acidovorax temperans skews neutrophil maturation and polarizes Th17 cells to promote lung adenocarcinoma development

Change within the intratumoral microbiome is a common feature in lung and other cancers and may influence inflammation and immunity in the tumor microenvironment, affecting growth and metastases. We previously characterized the lung cancer microbiome in patients and identified Acidovorax temperans as enriched in tumors. Here, we instilled A. temperans in an animal model driven by mutant K-ras and Tp53. This revealed A. temperans accelerates tumor development and burden through infiltration of proinflammatory cells. Neutrophils exposed to A. temperans displayed a mature, pro-tumorigenic phenotype with increased cytokine signaling, with a global shift away from IL-1β signaling. Neutrophil to monocyte and macrophage signaling upregulated MHC II to activate CD4+ T cells, polarizing them to an IL-17A+ phenotype detectable in CD4+ and γδ populations (T17). These T17 cells shared a common gene expression program predictive of poor survival in human LUAD. These data indicate bacterial exposure promotes tumor growth by modulating inflammation.

A Trefoil factor 3-Lingo2 axis restrains proliferative expansion of type-1 T helper cells during GI nematode infection

Host defense at the mucosal interface requires collaborative interactions between diverse cell lineages. Epithelial cells damaged by microbial invaders release reparative proteins such as the Trefoil factor family (TFF) peptides that functionally restore barrier integrity. However, whether TFF peptides and their receptors also serve instructive roles for immune cell function during infection is incompletely understood. Here, we demonstrate that the intestinal trefoil factor, TFF3, restrains (T cell helper) TH1 cell proliferation and promotes host-protective type 2 immunity against the gastrointestinal parasitic nematode Trichuris muris. Accordingly, T cell-specific deletion of the TFF3 receptor, leucine-rich repeat and immunoglobulin containing nogo receptor 2 (LINGO2), impairs TH2 cell commitment, allows proliferative expansion of interferon (IFN)g+ cluster of differentiation (CD)4+ TH1 cells and blocks normal worm expulsion through an IFNg-dependent mechanism. This study indicates that TFF3, in addition to its known tissue reparative functions, drives anti-helminth immunity by controlling the balance between TH1/TH2 subsets.

Integrating circulating T follicular memory cells and autoantibody repertoires for characterization of autoimmune disorders

Introduction

Autoimmune diseases are heterogeneous and often lack specific or sensitive diagnostic tests. Increased percentages of CD4+CXCR5+PD1+ circulating T follicular helper (cTfh) cells and skewed distributions of cTfh subtypes have been associated with autoimmunity. However, cTfh cell percentages can normalize with immunomodulatory treatment despite persistent disease activity, indicating the need for identifying additional cellular and/or serologic features correlating with autoimmunity.

Methods

The cohort included 50 controls and 56 patients with autoimmune cytopenias, gastrointestinal, pulmonary, and/or neurologic autoimmune disease. Flow cytometry was used to measure CD4+CXCR5+ T cell subsets expressing the chemokine receptors CXCR3 and/or CCR6: CXCR3+CCR6- Type 1, CXCR3-CCR6- Type 2, CXCR3+CCR6+ Type 1/17, and CXCR3- CCR6+ Type 17 T cells. IgG and IgA autoantibodies were quantified using a microarray featuring 1616 full-length, conformationally intact protein antigens. The 97.5th percentile in the control cohort defined normal limits for T cell subset percentages and total number (burden) of autoantibodies.

Results

This study focused on CD4+CXCR5+ T cells because CXCR5 upregulation occurs after cognate T-B cell interactions characteristic of autoimmune diseases. We refer to these cells as circulating T follicular memory (cTfm) cells to acknowledge the dynamic nature of antigen-experienced CXCR5+ T cells, which encompass progenitors of cTfh or Tfh cells as well as early effector memory T cells that have not yet lost CXCR5. Compared to controls, 57.1% of patients had increased CXCR5+CXCR3+CCR6+ cTfm1/17 and 25% had increased CXCR5+CXCR3-CCR6+ cTfm17 cell percentages. Patients had significantly more diverse IgG and IgA autoantibodies than controls and 44.6% had an increased burden of autoantibodies of either isotype. Unsupervised autoantibody clustering identified three clusters of patients with IgG autoantibody profiles distinct from those of controls, enriched for patients with active autoimmunity and monogenic diseases. An increased percentage of cTfm17 cells was most closely associated with an increased burden of high-titer IgG and IgA autoantibodies. A composite measure integrating increased cTfm1/17, cTfm17, and high-titer IgG and/or IgA autoantibodies had 91.1% sensitivity and 90.9% specificity for identifying patients with autoimmunity. Percentages of cTfm1/17 and cTfm17 percentages and numbers of high-titer autoantibodies in patients receiving immunomodulatory treatment did not differ from those in untreated patients, thus suggesting that measurements of cTfm can complement measurements of other cellular markers affected by treatment.

Conclusions

This study highlights two new approaches for assessing autoimmunity: measuring CD4+CXCR5+ cTfm subsets as well as total burden of autoantibodies. Our findings suggest that these approaches are particularly relevant to patients with rare autoimmune disorders for whom target antigens and prognosis are often unknown.

Durable CD4+ T cell immunity: cherchez la stem

Mammalian stem cells govern development, tissue homeostasis, and regeneration. Following years of study, their functions have been delineated with increasing precision. The past decade has witnessed heightened widespread use of stem cell terminology in association with durable T cell responses to infection, antitumor immunity, and autoimmunity. Interpreting this literature is complicated by the fact that descriptions are diverse and criteria for labeling 'stem-like' T cells are evolving. Working under the hypothesis that conceptual frameworks developed for actual stem cells can be used to better evaluate and organize T cells described to have stem-like features, we outline widely accepted properties of stem cells and compare these to different 'stem-like' CD4+ T cell populations.

Bob1 maintains T follicular helper cells for long-term humoral immunity

Humoral immunity is vital for host protection, yet aberrant antibody responses can trigger harmful inflammation and immune-related disorders. T follicular helper (Tfh) cells, central to humoral immunity, have garnered significant attention for unraveling immune mechanisms. This study shows the role of B-cell Oct-binding protein 1 (Bob1), a transcriptional coactivator, in Tfh cell regulation. Our investigation, utilizing conditional Bob1-deficient mice, suggests that Bob1 plays a critical role in modulating inducible T-cell costimulator expression and cellular respiration in Tfh cells. This regulation maintains the long-term functionality of Tfh cells, enabling their reactivation from central memory T cells to produce antibodies during recall responses. In a bronchial asthma model induced by house dust mite (HDM) inhalation, Bob1 is observed to enhance HDM-specific antibodies, including IgE, highlighting its pivotal function in Tfh cell regulation. Further exploration of Bob1-dependent mechanisms in Tfh cells holds promise for governing protective immunity and addressing immune-related disorders.

A novel memory-like Tfh cell subset is precursor to effector Tfh cells in recall immune responses

T follicular helper (Tfh) cells, essential for germinal center reactions, are not identical, with different phenotypes reported. Whether, when, and how they generate memory cells is still poorly understood. Here, through single-cell RNA-sequencing analysis of CXCR5+Bcl6+ Tfh cells generated under different conditions, we discovered, in addition to PD-1hi effector Tfh cells, a CD62L+PD1low subpopulation. CD62L-expressing Tfh cells developed independently from PD-1+ cells and not in direct contact with B cells. More importantly, CD62L+ Tfh cells expressed memory- and stemness-associated genes, and with better superior long-term survival, they readily generated PD-1hi cells in the recall response. Finally, KLF2 and IL7R, also highly expressed by CD62L+ Tfh cells, were required to regulate their development. Our work thus demonstrates a novel Tfh memory-like cell subpopulation, which may benefit our understanding of immune responses and diseases.

Epithelial organoid supports resident memory CD8 T cell differentiation

Resident Memory T cells (TRM) play a vital role in regional immune defense in barrier organs. Although laboratory rodents have been extensively used to study fundamental TRM biology, poor isolation efficiency, sampling bias and low cell survival rates have limited our ability to conduct TRM-focused high-throughput assays. Here, we engineered a murine vaginal epithelial organoid (VEO)-CD8 T cell co-culture system that supports CD8 TRM differentiation in vitro. The three-dimensional VEOs established from murine adult stem cells resembled stratified squamous vaginal epithelium and induced gradual differentiation of activated CD8 T cells into epithelial TRM. These in vitro generated TRM were phenotypically and transcriptionally similar to in vivo TRM, and key tissue residency features were reinforced with a second cognate-antigen exposure during co-culture. TRM differentiation was not affected even when VEOs and CD8 T cells were separated by a semipermeable barrier, indicating soluble factors' involvement. Pharmacological and genetic approaches showed that TGF-β signaling played a crucial role in their differentiation. We found that the VEOs in our model remained susceptible to viral infections and the CD8 T cells were amenable to genetic manipulation; both of which will allow detailed interrogation of antiviral CD8 T cell biology in a reductionist setting. In summary, we established a robust model which captures bonafide TRM differentiation that is scalable, open to iterative sampling, and can be subjected to high throughput assays that will rapidly add to our understanding of TRM.

Memory T follicular helper cells drive donor-specific antibodies independent of memory B cells and primary germinal center and alloantibody formation

Human leukocyte antigen antibodies are important immunologic mediators of renal allograft loss and are difficult to control. The inability to permanently eliminate donor-specific antibodies (DSA) is partly due to an incomplete understanding of the cellular mechanisms driving alloantibody formation, recurrence, and maintenance. Memory T follicular helper (mTfh) cells rapidly interact with memory B cells upon antigen re-exposure for anamnestic humoral responses, but little is known about Tfh memory in transplantation. We hypothesized that alloreactive mTfh cells form after transplantation and play a critical role in DSA formation following alloantigen re-encounter. To test this hypothesis, we utilized murine skin allograft models to identify and characterize Tfh memory and interrogate its ability to mediate alloantibody responses. We identified alloreactive Tfh memory as a mediator of accelerated humoral alloresponses independent of memory B cells and primary germinal center, or DSA, formation. Furthermore, we demonstrate that mTfh-driven alloantibody formation is susceptible to CD28 costimulation blockade. These findings provide novel insight into a pathologic role for memory Tfh in alloantibody responses and strongly support shifting therapeutic focus from the singular targeting of B cell lineage cells and alloantibodies themselves to multimodal strategies that include inhibition of mTfh cells to treat DSA.

Generation of antigen-specific memory CD4 T cells by heterologous immunization enhances the magnitude of the germinal center response upon influenza infection

Current influenza vaccine strategies have yet to overcome significant obstacles, including rapid antigenic drift of seasonal influenza viruses, in generating efficacious long-term humoral immunity. Due to the necessity of germinal center formation in generating long-lived high affinity antibodies, the germinal center has increasingly become a target for the development of novel or improvement of less-efficacious vaccines. However, there remains a major gap in current influenza research to effectively target T follicular helper cells during vaccination to alter the germinal center reaction. In this study, we used a heterologous infection or immunization priming strategy to seed an antigen-specific memory CD4+ T cell pool prior to influenza infection in mice to evaluate the effect of recalled memory T follicular helper cells in increased help to influenza-specific primary B cells and enhanced generation of neutralizing antibodies. We found that heterologous priming with intranasal infection with acute lymphocytic choriomeningitis virus (LCMV) or intramuscular immunization with adjuvanted recombinant LCMV glycoprotein induced increased antigen-specific effector CD4+ T and B cellular responses following infection with a recombinant influenza strain that expresses LCMV glycoprotein. Heterologously primed mice had increased expansion of secondary Th1 and Tfh cell subsets, including increased CD4+ TRM cells in the lung. However, the early enhancement of the germinal center cellular response following influenza infection did not impact influenza-specific antibody generation or B cell repertoires compared to primary influenza infection. Overall, our study suggests that while heterologous infection/immunization priming of CD4+ T cells is able to enhance the early germinal center reaction, further studies to understand how to target the germinal center and CD4+ T cells specifically to increase long-lived antiviral humoral immunity are needed.

Spatiotemporal development of the human T follicular helper cell response to Influenza vaccination

We profiled blood and draining lymph node (LN) samples from human volunteers after influenza vaccination over two years to define evolution in the T follicular helper cell (TFH) response. We show LN TFH cells expanded in a clonal-manner during the first two weeks after vaccination and persisted within the LN for up to six months. LN and circulating TFH (cTFH) clonotypes overlapped but had distinct kinetics. LN TFH cell phenotypes were heterogeneous and mutable, first differentiating into pre-TFH during the month after vaccination before maturing into GC and IL-10+ TFH cells. TFH expansion, upregulation of glucose metabolism, and redifferentiation into GC TFH cells occurred with faster kinetics after re-vaccination in the second year. We identified several influenza-specific TFH clonal lineages, including multiple responses targeting internal influenza proteins, and show each TFH state is attainable within a lineage. This study demonstrates that human TFH cells form a durable and dynamic multi-tissue network.

CD4+ T cell memory

Specialized subpopulations of CD4+ T cells survey major histocompatibility complex class II-peptide complexes to control phagosomal infections, help B cells, regulate tissue homeostasis and repair or perform immune regulation. Memory CD4+ T cells are positioned throughout the body and not only protect the tissues from reinfection and cancer, but also participate in allergy, autoimmunity, graft rejection and chronic inflammation. Here we provide updates on our understanding of the longevity, functional heterogeneity, differentiation, plasticity, migration and human immunodeficiency virus reservoirs as well as key technological advances that are facilitating the characterization of memory CD4+ T cell biology.

T helper 1 effector memory CD4+ T cells protect the skin from poxvirus infection

Poxvirus infections of the skin are a recent emerging public health concern, yet the mechanisms that mediate protective immunity against these viral infections remain largely unknown. Here, we show that T helper 1 (Th1) memory CD4+ T cells are necessary and sufficient to provide complete and broad protection against poxvirus skin infections, whereas memory CD8+ T cells are dispensable. Core 2 O-glycan-synthesizing Th1 effector memory CD4+ T cells rapidly infiltrate the poxvirus-infected skin microenvironment and produce interferon γ (IFNγ) in an antigen-dependent manner, causing global changes in gene expression to promote anti-viral immunity. Keratinocytes express IFN-stimulated genes, upregulate both major histocompatibility complex (MHC) class I and MHC class II antigen presentation in an IFNγ-dependent manner, and require IFNγ receptor (IFNγR) signaling and MHC class II expression for memory CD4+ T cells to protect the skin from poxvirus infection. Thus, Th1 effector memory CD4+ T cells exhibit potent anti-viral activity within the skin, and keratinocytes are the key targets of IFNγ necessary for preventing poxvirus infection of the epidermis.

Transcriptional programming of CD4+ TRM differentiation in viral infection balances effector- and memory-associated gene expression

After resolution of infection, T cells differentiate into long-lived memory cells that recirculate through secondary lymphoid organs or establish residence in tissues. In contrast to CD8+ tissue-resident memory T cells (TRM), the developmental origins and transcriptional regulation of CD4+ TRM remain largely undefined. Here, we investigated the phenotypic, functional, and transcriptional profiles of CD4+ TRM in the small intestine (SI) responding to acute viral infection, revealing a shared gene expression program and chromatin accessibility profile with circulating TH1 and the progressive acquisition of a mature TRM program. Single-cell RNA sequencing identified heterogeneity among established CD4+ TRM, which were predominantly located in the lamina propria, and revealed a population of cells that coexpressed both effector- and memory-associated genes, including the transcriptional regulators Blimp1, Id2, and Bcl6. TH1-associated Blimp1 and Id2 and TFH-associated Bcl6 were required for early TRM formation and development of a mature TRM population in the SI. These results demonstrate a developmental relationship between TH1 effector cells and the establishment of early TRM, as well as highlighted differences in CD4+ versus CD8+ TRM populations, providing insights into the mechanisms underlying the origins, differentiation, and persistence of CD4+ TRM in response to viral infection.

Treg cells require Izumo1R to regulate γδT cell-driven inflammation in the skin

Izumo1R is a pseudo-folate receptor with an essential role in mediating tight oocyte/spermatozoa contacts during fertilization. Intriguingly, it is also expressed in CD4+ T lymphocytes, in particular Treg cells under the control of Foxp3. To understand Izumo1R function in Treg cells, we analyzed mice with Treg-specific Izumo1r deficiency (Iz1rTrKO). Treg differentiation and homeostasis were largely normal, with no overt autoimmunity and only marginal increases in PD1+ and CD44hi Treg phenotypes. pTreg differentiation was also unaffected. Iz1rTrKO mice proved uniquely susceptible to imiquimod-induced, γδT cell-dependent, skin disease, contrasting with normal responses to several inflammatory or tumor challenges, including other models of skin inflammation. Analysis of Iz1rTrKO skin revealed a subclinical inflammation that presaged IMQ-induced changes, with an imbalance of Rorγ+ γδT cells. Immunostaining of normal mouse skin revealed the expression of Izumo1, the ligand for Izumo1R, electively in dermal γδT cells. We propose that Izumo1R on Tregs enables tight contacts with γδT cells, thereby controlling a particular path of skin inflammation.

An atlas of gene regulatory networks for memory CD4 + T cells in youth and old age

Aging profoundly affects immune-system function, promoting susceptibility to pathogens, cancers and chronic inflammation. We previously identified a population of IL-10-producing, T follicular helper-like cells (" Tfh10 "), linked to suppressed vaccine responses in aged mice. Here, we integrate single-cell ( sc )RNA-seq, scATAC-seq and genome-scale modeling to characterize Tfh10 - and the full CD4 + memory T cell ( CD4 + TM ) compartment - in young and old mice. We identified 13 CD4 + TM populations, which we validated through cross-comparison to prior scRNA-seq studies. We built gene regulatory networks ( GRNs ) that predict transcription-factor control of gene expression in each T-cell population and how these circuits change with age. Through integration with pan-cell aging atlases, we identified intercellular-signaling networks driving age-dependent changes in CD4 + TM. Our atlas of finely resolved CD4 + TM subsets, GRNs and cell-cell communication networks is a comprehensive resource of predicted regulatory mechanisms operative in memory T cells, presenting new opportunities to improve immune responses in the elderly.

T-cell-B-cell collaboration in the lung

Collaboration between T and B cells in secondary lymphoid organs is a crucial component of adaptive immunity, but lymphocytes also persist in other tissues. Recent studies have examined T-cell-B-cell interactions in nonlymphoid tissues such as the lung. CD4+ T- resident helper cells (TRH) remain in the lung after influenza infection and support both resident CD8 T cells and B cells. Multiple lung-resident B-cell subsets (B-resident memory (BRM)) that exhibit spatial and phenotypic diversity have also been described. Though not generated by all types of infection, inducible bronchus-associated lymphoid tissue offers a logical place for T and B cells to interact. Perturbations to BRM and TRH cells elicit effects specific to Immunoglobulin A (IgA) production, an antibody isotype with privileged access to mucosa. Understanding the interplay of lymphocytes in mucosal tissues, which can be insulated from systemic immune responses, may improve the design of future vaccines and therapies.

Toward a general model of CD4+ T cell subset specification and memory cell formation

In the past few decades, a number of transformative discoveries have been made regarding memory CD8+ T cell biology; meanwhile, the CD4+ T cell field has lagged behind this progress. This perspective focuses on CD4+ helper T (Th) cell subset specification and memory cell formation. Here, we argue that the sheer number of Th effector and memory cell subsets and a focus on their differences have been a barrier to a general model of CD4+ memory T cell formation that applies to all immune responses. We highlight a bifurcation model that relies on an IL-2 signal-dependent switch as an explanation for the balanced production of diverse Th memory cells that participate in cell-mediated or humoral immunity in most contexts.

Subset-specific and temporal control of effector and memory CD4+ T cell survival

Following their proliferative expansion and differentiation into effector cells like Th1, Tfh, and T central memory precursors (Tcmp), most effector CD4+ T cells die, while some survive and become memory cells. Here, we explored how Bcl-2 family members controlled the survival of CD4+ T cells during distinct phases of mouse acute LCMV infection. During expansion, we found that Th1 cells dominated the response, downregulated expression of Bcl-2, and did not require Bcl-2 for survival. Instead, they relied on the anti-apoptotic protein, A1 for survival. Similarly, Th17 cells in an EAE model also depended on A1 for survival. However, after the peak of the response, CD4+ effector T cells required Bcl-2 to counteract Bim to aid their transition into memory. This Bcl-2 dependence persisted in established memory CD4+ T cells. Combined, these data show a temporal switch in Bcl-2 family-mediated survival of CD4+ T cells over the course of an immune response. This knowledge can help improve T cell survival to boost immunity and conversely, target pathogenic T cells.

Functional T cells are capable of supernumerary cell division and longevity

Differentiated somatic mammalian cells putatively exhibit species-specific division limits that impede cancer but may constrain lifespans^1-3. To provide immunity, transiently stimulated CD8⁺ T cells undergo unusually rapid bursts of numerous cell divisions, and then form quiescent long-lived memory cells that remain poised to reproliferate following subsequent immunological challenges. Here we addressed whether T cells are intrinsically constrained by chronological or cell-division limits. We activated mouse T cells in vivo using acute heterologous prime-boost-boost vaccinations⁴, transferred expanded cells to new mice, and then repeated this process iteratively. Over 10 years (greatly exceeding the mouse lifespan)⁵ and 51 successive immunizations, T cells remained competent to respond to vaccination. Cells required sufficient rest between stimulation events. Despite demonstrating the potential to expand the starting population at least 10⁴⁰-fold, cells did not show loss of proliferation control and results were not due to contamination with young cells. Persistent stimulation by chronic infections or cancer can cause T cell proliferative senescence, functional exhaustion and death⁶. We found that although iterative acute stimulations also induced sustained expression and epigenetic remodelling of common exhaustion markers (including PD1, which is also known as PDCD1, and TOX) in the cells, they could still proliferate, execute antimicrobial functions and form quiescent memory cells. These observations provide a model to better understand memory cell differentiation, exhaustion, cancer and ageing, and show that functionally competent T cells can retain the potential for extraordinary population expansion and longevity well beyond their organismal lifespan.

Route of self-amplifying mRNA vaccination modulates the establishment of pulmonary resident memory CD8 and CD4 T cells

Respiratory tract resident memory T cells (TRM), typically generated by local vaccination or infection, can accelerate control of pulmonary infections that evade neutralizing antibody. It is unknown whether mRNA vaccination establishes respiratory TRM. We generated a self-amplifying mRNA vaccine encoding the influenza A virus nucleoprotein that is encapsulated in modified dendron-based nanoparticles. Here, we report how routes of immunization in mice, including contralateral versus ipsilateral intramuscular boosts, or intravenous and intranasal routes, influenced influenza-specific cell-mediated and humoral immunity. Parabiotic surgeries revealed that intramuscular immunization was sufficient to establish CD8 TRM in the lung and draining lymph nodes. Contralateral, compared with ipsilateral, intramuscular boosting broadened the distribution of lymph node TRM and T follicular helper cells but slightly diminished resulting levels of serum antibody. Intranasal mRNA delivery established modest circulating CD8 and CD4 T cell memory but augmented distribution to the respiratory mucosa. Combining intramuscular immunizations with an intranasal mRNA boost achieved high levels of both circulating T cell memory and lung TRM. Thus, routes of mRNA vaccination influence humoral and cell-mediated immunity, and intramuscular prime-boosting establishes lung TRM that can be further expanded by an additional intranasal immunization.

Evolution of antigen-specific follicular helper T cell transcription from effector function to memory

Understanding how follicular helper T cells (TFH) regulate the specialization, maturation, and differentiation of adaptive B cell immunity is crucial for developing durable high-affinity immune protection. Using indexed single-cell molecular strategies, we reveal a skewed intraclonal assortment of higher-affinity T cell receptors and the distinct molecular programming of the localized TFH compartment compared with emigrant conventional effector T_H cells. We find a temporal shift in B cell receptor class switch, which permits identification of inflammatory and anti-inflammatory modules of transcriptional programming that subspecialize TFH function before and during the germinal center (GC) reaction. Late collapse of this local primary GC reaction reveals a persistent post-GC TFH population that discloses a putative memory TFH program. These studies define subspecialized antigen-specific TFH transcriptional programs that progressively change with antibody class-specific evolution of high-affinity B cell immunity and a memory TFH transcriptional program that emerges upon local GC resolution.

Heterogeneity, subsets, and plasticity of T follicular helper cells in allergy

Antibody responses are critical for protection against pathogens. However, diseases such as allergic rhinitis or food allergy result from aberrant production of IgE antibodies against otherwise innocuous environmental antigens. The production of allergen-specific IgE requires interaction between B cells and CD4⁺ T cells, and a granular understanding of these interactions is required to develop novel therapies for allergic disease. CD4⁺ T cells are exceptionally heterogeneous in their transcriptional, epigenetic, and proteomic profiles, which poses significant challenges when attempting to define subsets relevant to the study of allergy among a continuum of cells. Defining subsets such as the T follicular helper (T_FH) cell cluster provides a shorthand to understand the functions of CD4⁺ T cells in antibody production and supports mechanistic experimentation for hypothesis-driven discovery. With a focus on allergic disease, this Rostrum article broadly discusses heterogeneity among CD4⁺ T cells and provides a rationale for subdividing T_FH cells into both functional and cytokine-skewed subsets. Further, it highlights the plasticity demonstrated by T_FH cells during the primary response and after recall, and it explores the possibility of harnessing this plasticity to reprogram immunity for therapeutic benefit in allergic disease.

Specific transcriptional programs differentiate ICOS from CD28 costimulatory signaling in human Naïve CD4+ T cells

Costimulatory molecules of the CD28 family play a crucial role in the activation of immune responses in T lymphocytes, complementing and modulating signals originating from the T-cell receptor (TCR) complex. Although distinct functional roles have been demonstrated for each family member, the specific signaling pathways differentiating ICOS- from CD28-mediated costimulation during early T-cell activation are poorly characterized. In the present study, we have performed RNA-Seq-based global transcriptome profiling of anti-CD3-treated naïve CD4+ T cells upon costimulation through either inducible costimulator (ICOS) or CD28, revealing a set of signaling pathways specifically associated with each signal. In particular, we show that CD3/ICOS costimulation plays a major role in pathways related to STAT3 function and osteoarthritis (OA), whereas the CD3/CD28 axis mainly regulates p38 MAPK signaling. Furthermore, we report the activation of distinct immunometabolic pathways, with CD3/ICOS costimulation preferentially targeting glycosaminoglycans (GAGs) and CD3/CD28 regulating mitochondrial respiratory chain and cholesterol biosynthesis. These data suggest that ICOS and CD28 costimulatory signals play distinct roles during the activation of naïve T cells by modulating distinct sets of immunological and immunometabolic genes.

Stem-like T cells and niches: Implications in human health and disease

Recently, accumulating evidence has elucidated the important role of T cells with stem-like characteristics in long-term maintenance of T cell responses and better patient outcomes after immunotherapy. The fate of TSL cells has been correlated with many physiological and pathological human processes. In this review, we described present advances demonstrating that stem-like T (TSL) cells are central players in human health and disease. We interpreted the evolutionary characteristics, mechanism and functions of TSL cells. Moreover, we discuss the import role of distinct niches and how they affect the stemness of TSL cells. Furthermore, we also outlined currently available strategies to generate TSL cells and associated affecting factors. Moreover, we summarized implication of TSL cells in therapies in two areas: stemness enhancement for vaccines, ICB, and adoptive T cell therapies, and stemness disruption for autoimmune disorders.

Id3 expression identifies CD4+ memory Th1 cells

Memory CD4⁺ T cells play a pivotal role in mediating long-term protective immunity, positioning them as an important target in vaccine development. However, multiple functionally distinct helper CD4⁺ T-cell subsets can arise in response to a single invading pathogen, complicating the identification of rare populations of memory precursor cells during the effector phase of infection and memory CD4⁺ T cells following pathogen clearance and the contraction phase of infection. Furthermore, current literature remains unclear regarding whether a single CD4⁺ memory T-cell lineage gives rise to secondary CD4⁺ T helper subsets or if there are unique memory precursor cells within each helper lineage. A majority of T follicular helper (Tfh) cells, which have established memory potential, express Id3, an inhibitor of E protein transcription factors, following acute viral infection. We show that expression of Id3 definitively identified a subset of cells within both the CD4⁺ Tfh and T helper 1 (Th1) lineages at memory time points that exhibited memory potential, with the capacity for significant re-expansion in response to secondary infection. Notably, we demonstrate that a subset of Th1 cells that survive into the memory phase were marked by Id3 expression and possessed the potential for enhanced expansion and generation of both Th1 and Tfh secondary effector cell populations in a secondary response to pathogen. Additionally, these cells exhibited enrichment of key molecules associated with memory potential when compared with Id3^lo Th1 cells. Therefore, we propose that Id3 expression serves as an important marker to indicate multipotent potential in memory CD4⁺ T cells.

The Differentiation and Maintenance of SARS-CoV-2-Specific Follicular Helper T Cells

Upon acute viral infection, virus-specific CD4+ T cells differentiate into either TH1 cells or follicular helper T (TFH) cells. The molecular pathways governing such bimodal cell fate commitment remain elusive. Additionally, effector virus-specific TFH cells further differentiate into corresponding memory population, which confer long-term protection against re-infection of same viruses by providing immediate help to virus-specific memory B cells. Currently, the molecular mechanisms underlying the long-term maintenance of memory TFH cells are largely unknown. In this review, we discuss current understanding of early differentiation of virus-specific effector TFH cells and long-term maintenance of virus-specific memory TFH cells in mouse models of viral infection and patients of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

A CD4+ T cell reference map delineates subtype-specific adaptation during acute and chronic viral infections

CD4⁺ T cells are critical orchestrators of immune responses against a large variety of pathogens, including viruses. While multiple CD4⁺ T cell subtypes and their key transcriptional regulators have been identified, there is a lack of consistent definition for CD4⁺ T cell transcriptional states. In addition, the progressive changes affecting CD4⁺ T cell subtypes during and after immune responses remain poorly defined. Using single-cell transcriptomics, we characterized the diversity of CD4⁺ T cells responding to self-resolving and chronic viral infections in mice. We built a comprehensive map of virus-specific CD4⁺ T cells and their evolution over time, and identified six major cell states consistently observed in acute and chronic infections. During the course of acute infections, T cell composition progressively changed from effector to memory states, with subtype-specific gene modules and kinetics. Conversely, in persistent infections T cells acquired distinct, chronicity-associated programs. By single-cell T cell receptor (TCR) analysis, we characterized the clonal structure of virus-specific CD4⁺ T cells across individuals. Virus-specific CD4⁺ T cell responses were essentially private across individuals and most T cells differentiated into both Tfh and Th1 subtypes irrespective of their TCR. Finally, we showed that our CD4⁺ T cell map can be used as a reference to accurately interpret cell states in external single-cell datasets across tissues and disease models. Overall, this study describes a previously unappreciated level of adaptation of the transcriptional states of CD4⁺ T cells responding to viruses and provides a new computational resource for CD4⁺ T cell analysis.

The human liver microenvironment shapes the homing and function of CD4+ T-cell populations

OBJECTIVE

Tissue-resident memory T cells (TRM) are vital immune sentinels that provide protective immunity. While hepatic CD8+ TRM have been well described, little is known about the location, phenotype and function of CD4+ TRM.

DESIGN

We used multiparametric flow cytometry, histological assessment and novel human tissue coculture systems to interrogate the ex vivo phenotype, function and generation of the intrahepatic CD4+ T-cell compartment. We also used leukocytes isolated from human leukocyte antigen (HLA)-disparate liver allografts to assess long-term retention.

RESULTS

Hepatic CD4+ T cells were delineated into three distinct populations based on CD69 expression: CD69-, CD69INT and CD69HI. CD69HICD4+ cells were identified as tissue-resident CD4+ T cells on the basis of their exclusion from the circulation, phenotypical profile (CXCR6+CD49a+S1PR1-PD-1+) and long-term persistence within the pool of donor-derived leukcoocytes in HLA-disparate liver allografts. CD69HICD4+ T cells produced robust type 1 polyfunctional cytokine responses on stimulation. Conversely, CD69INTCD4+ T cells represented a more heterogenous population containing cells with a more activated phenotype, a distinct chemokine receptor profile (CX3CR1+CXCR3+CXCR1+) and a bias towards interleukin-4 production. While CD69INTCD4+ T cells could be found in the circulation and lymph nodes, these cells also formed part of the long-term resident pool, persisting in HLA-mismatched allografts. Notably, frequencies of CD69INTCD4+ T cells correlated with necroinflammatory scores in chronic hepatitis B infection. Finally, we demonstrated that interaction with hepatic epithelia was sufficient to generate CD69INTCD4+ T cells, while additional signals from the liver microenvironment were required to generate liver-resident CD69HICD4+ T cells.

CONCLUSIONS

High and intermediate CD69 expressions mark human hepatic CD4+ TRM and a novel functionally distinct recirculating population, respectively, both shaped by the liver microenvironment to achieve diverse immunosurveillance.

Providing a Helping Hand: Metabolic Regulation of T Follicular Helper Cells and Their Association With Disease

T follicular helper (Tfh) cells provide support to B cells upon arrival in the germinal center, and thus are critical for the generation of a robust adaptive immune response. Tfh express specific transcription factors and cellular receptors including Bcl6, CXCR5, PD-1, and ICOS, which are critical for homing and overall function. Generally, the induction of an immune response is tightly regulated. However, deviation during this process can result in harmful autoimmunity or the inability to successfully clear pathogens. Recently, it has been shown that Tfh differentiation, activation, and proliferation may be linked with the cellular metabolic state. In this review we will highlight recent discoveries in Tfh differentiation and explore how these cells contribute to functional immunity in disease, including autoimmune-related disorders, cancer, and of particular emphasis, during infection.

Antigen-Presenting B Cells Program the Efferent Lymph T Helper Cell Response

B cells interact with T follicular helper (Tfh) cells in germinal centers (GCs) to generate high-affinity antibodies. Much less is known about how cognate T–B-cell interactions influence Th cells that enter circulation and peripheral tissues. Therefore, we generated mice lacking MHC-II expressing B cells and, by thoracic duct cannulation, analyzed Th cells in the efferent lymph at defined intervals post-immunization. Focusing on gut-draining mesenteric lymph nodes (MLNs), we show that antigen-specific α₄β₇⁺ gut-homing effector Th cells enter the circulation prior to CXCR5⁺PD-1⁺ Tfh-like cells. B cells appear to have no or limited impact on the early generation and egress of gut-homing Th cells but are critical for the subsequent appearance of Tfh-like cells that peak in the lymph before GCs have developed. At this stage, antigen-presenting B cells also reduce the proportion of α₄β₇⁺ Th cells in the MLN and efferent lymph. Furthermore, cognate B-cell interaction drives a broad transcriptional program in Th cells, including IL-4 that is confined to the Tfh cell lineage. The IL-4-producing Tfh-like cells originate from Bcl6⁺ precursors in the LNs and have gut-homing capacity. Hence, B cells program the efferent lymph Th cell response within a limited window of time after antigenic challenge.

Strong influenza-induced TFH generation requires CD4 effectors to recognize antigen locally and receive signals from continuing infection

Influenza infection elicits strong, long-lived protective antibodies, but most current influenza vaccines give weaker, short-lived protection. We noted that live virus is still replicating, making antigen and causing inflammation at 7 d postinfection (dpi), while an inactivated vaccine provides antigen for at most 4 dpi. We show that the generation of key T follicular helper cells (T_FH) requires they recognize antigen locally at 6 dpi in the presence of ongoing viral infection. This creates a checkpoint that restricts T_FH responses to dangerous infections that persist through the checkpoint. Using a live attenuated vaccine, akin to Flumist, we found that adding a second dose at 6 d generated a strong T_FH response, suggesting an approach to improve vaccine strategies.

While influenza infection induces robust, long-lasting, antibody responses and protection, including the T follicular helper cells (T_FH) required to drive B cell germinal center (GC) responses, most influenza vaccines do not. We investigated the mechanisms that drive strong T_FH responses during infection. Infection induces viral replication and antigen (Ag) presentation lasting through the CD4 effector phase, but Ag and pathogen recognition receptor signals are short-lived after vaccination. We analyzed the need for both infection and Ag presentation at the effector phase, using an in vivo sequential transfer model to time their availability. Differentiation of CD4 effectors into T_FH and GC-T_FH required that they recognize Ag locally in the site of T_FH development, at the effector phase, but did not depend on specific Ag-presenting cells (APCs). In addition, concurrent signals from infection were necessary even when sufficient Ag was presented. Providing these signals with a second dose of live attenuated influenza vaccine at the effector phase drove T_FH and GC-T_FH development equivalent to live infection. The results suggest that vaccine approaches can induce strong T_FH development that supports GC responses akin to infection, if they supply these effector phase signals at the right time and site. We suggest that these requirements create a checkpoint that ensures T_FH only develop fully when infection is still ongoing, thereby avoiding unnecessary, potentially autoimmune, responses.

In vivo CRISPR screens reveal a HIF-1α-mTOR-network regulates T follicular helper versus Th1 cells

T follicular helper (Tfh) cells provide signals to initiate and maintain the germinal center (GC) reaction and are crucial for the generation of robust, long-lived antibody responses, but how the GC microenvironment affects Tfh cells is not well understood. Here we develop an in vivo T cell-intrinsic CRISPR-knockout screen to evaluate Tfh and Th1 cells in an acute viral infection model to identify regulators of Tfh cells in their physiological setting. Using a screen of druggable-targets, alongside genetic, transcriptomic and cellular analyses, we identify a function of HIF-1α in suppressing mTORC1-mediated and Myc-related pathways, and provide evidence that VHL-mediated degradation of HIF-1α is required for Tfh development; an expanded in vivo CRISPR screen reveals multiple components of these pathways that regulate Tfh versus Th1 cells, including signaling molecules, cell-cycle regulators, nutrient transporters, metabolic enzymes and autophagy mediators. Collectively, our data serve as a resource for studying Tfh versus Th1 decisions, and implicate the VHL-HIF-1α axis in fine-tuning Tfh generation.

T follicular helper (Tfh) and T help type 1 (Th1) cells both arise from naïve CD4 T cells, but detailed knowledge of their differentiation remains incomplete. Here the authors pursue an in vivo CRISPR screen to identify genes, focusing on druggable targets, regulating Tfh versus Th1 to provide a resource for related studies, while also implicating HIF-1α and VHL in this regulation.

DOCK8-expressing T follicular helper cells newly generated beyond self-organized criticality cause systemic lupus erythematosus

Pathogens including autoantigens all failed to induce systemic lupus erythematosus (SLE). We, instead, studied the integrity of host's immune response that recognized pathogen. By stimulating TCR with an antigen repeatedly to levels that surpass host's steady-state response, self-organized criticality, SLE was induced in mice normally not prone to autoimmunity, wherein T follicular helper (Tfh) cells expressing the guanine nucleotide exchange factor DOCK8 on the cell surface were newly generated. DOCK8⁺Tfh cells passed through TCR re-revision and induced varieties of autoantibody and lupus lesions. They existed in splenic red pulp and peripheral blood of active lupus patients, which subsequently declined after therapy. Autoantibodies and disease were healed by anti-DOCK8 antibody in the mice including SLE-model (NZBxNZW) F1 mice. Thus, DOCK8⁺Tfh cells generated after repeated TCR stimulation by immunogenic form of pathogen, either exogenous or endogenous, in combination with HLA to levels that surpass system's self-organized criticality, cause SLE.

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Autoimmunity seldom takes place under integrated steady-state immune response

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Repeated invasion by pathogen, such as measles virus, is not exceptional but routine in life

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DOCK8+Tfh is generated upon TCR overstimulation by pathogen beyond self-organized criticality

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Newly generated DOCK8+Tfh induces autoantibodies and SLE, i.e., autoimmunity

Engagement of the costimulatory molecule ICOS in tissues promotes establishment of CD8+ tissue-resident memory T cells

Elevated gene expression of the costimulatory receptor Icos is a hallmark of CD8⁺ tissue-resident memory (Trm) T cells. Here, we examined the contribution of ICOS in Trm cell differentiation. Upon transfer into WT mice, Icos^-/- CD8⁺ T cells exhibited defective Trm generation but produced recirculating memory populations normally. ICOS deficiency or ICOS-L blockade compromised establishment of CD8⁺ Trm cells but not their maintenance. ICOS ligation during CD8⁺ T cell priming did not determine Trm induction; rather, effector CD8⁺ T cells showed reduced Trm differentiation after seeding into Icosl^-/- mice. Icos^YF/YF CD8⁺ T cells were compromised in Trm generation, indicating a critical role for PI3K signaling. Modest transcriptional changes in the few Icos^-/- Trm cells suggest that ICOS-PI3K signaling primarily enhances the efficiency of CD8⁺ T cell tissue residency. Thus, local ICOS signaling promotes production of Trm cells, providing insight into the contribution of costimulatory signals in the generation of tissue-resident populations.

Dependence on Bcl6 and Blimp1 drive distinct differentiation of murine memory and follicular helper CD4+ T cells

During the immune response, CD4+ T cells differentiate into distinct effector subtypes, including follicular helper T (Tfh) cells that help B cells, and into memory cells. Tfh and memory cells are required for long-term immunity; both depend on the transcription factor Bcl6, raising the question whether they differentiate through similar mechanisms. Here, using single-cell RNA and ATAC sequencing, we show that virus-responding CD4+ T cells lacking both Bcl6 and Blimp1 can differentiate into cells with transcriptomic, chromatin accessibility, and functional attributes of memory cells but not of Tfh cells. Thus, Bcl6 promotes memory cell differentiation primarily through its repression of Blimp1. These findings demonstrate that distinct mechanisms underpin the differentiation of memory and Tfh CD4+ cells and define the Bcl6-Blimp1 axis as a potential target for promoting long-term memory T cell differentiation.

Principles in Immunology for the Design and Development of Vaccines

Vaccinology has come a long way from early, empirically developed vaccines to modern vaccines rationally designed and produced. Vaccines are meant to cooperate with the human immune system, the later largely unknown in the early years of vaccine development. In the recent years, a tremendous depth of knowledge has been accumulated in the field of immunology that has provided an opportunity to understand the mechanisms of action of the vaccine components. In parallel, our knowledge in microbiology, molecular biology, infectiology, epidemiology, and furthermore in bioinformatics has fostered our understanding of the interaction of microorganisms with the human immune system. Strategies engaged by pathogens strongly determine the targets of a vaccine, which should be formulated to stimulate potent and efficiently protective immune responses. The improved knowledge of immune response mechanisms has facilitated the development of new vaccines with the capacity to selectively address the key pathogenic mechanisms. The primary goal of a vaccine design might no longer be to mimic the pathogen but to identify the relevant processes of the pathogenic mechanisms to be effectively interrupted by a highly specific immune response, eventually surpassing natural limitations. Vaccines have become complex sets of components meant to orchestrate the fine-tuning of the immune processes leading to a lasting and specific immune memory. In addition to antigenic materials, which are comprised of the most critical immunogenic epitopes, adjuvant components are frequently added to induce a favorable immunological activation. Furthermore, for reasons of production and product stability preservatives, stabilizers, inactivators, antibiotics, or diluents could be present, but need to be evaluated. While on the one hand vaccine effectiveness is a primary goal, on the other hand side effects need to be excluded due to safety and tolerability. Further challenges in vaccinology include variability of the vaccinees, the variability of the pathogen, the population-based settings of vaccine application, and the process technology in vaccine production. Vaccine design has become more tailored and in turn has opened up the potential of extending its application to hitherto not accessible complex microbial pathogens plus providing new immunotherapies to tackle diseases such as cancer, Alzheimer's disease, and autoimmune disease. This chapter gives an overview of the key considerations and processes involved in vaccine design and development. It also describes the basic principles of normal immune responses and in their function in defense of infectious agents by vaccination.

CD4+ Memory T-Cell Formation during Type 1 Immune Responses

Naive CD4+ T cells become memory cells after proliferating in response to their cognate major histocompatibility complex class II (MHCII)-bound peptide and passing through an effector cell stage. The process by which CD4+ memory T cells emerge from the effector cell pool, however, is less well understood than in the case of CD8+ T cells. During certain acute infections, naive CD4+ T cells proliferate and differentiate into various forms of type 1 (Th1) and follicular helper (Tfh) effector cells. We review the evidence that about 10% of the cells in each of these subsets survive to become memory cells that resemble their effector cell precursors. The roles that asymmetric cell division, the TCF-1 transcription factor, metabolic activity, reactive oxygen species, and the IL-7 receptor play in the effector to memory cell transition are discussed. We propose a speculative model in which the metabolic activity needed for rapid clonal expansion also generates toxic products that induce apoptosis in most effector cells. Memory cells then arise from the effector cells in each subset that are at the low end of the metabolic activity spectrum.

Vaccine-elicited CD4 T cells prevent the deletion of antiviral B cells in chronic infection

Chronic viral infections subvert protective B cell immunity. An early type I interferon (IFN-I)-driven bias to short-lived plasmablast differentiation leads to clonal deletion, so-called "decimation," of antiviral memory B cells. Therefore, prophylactic countermeasures against decimation remain an unmet need. We show that vaccination-induced CD4 T cells prevented the decimation of naïve and memory B cells in chronically lymphocytic choriomeningitis virus (LCMV)-infected mice. Although these B cell responses were largely T independent when IFN-I was blocked, preexisting T help assured their sustainability under conditions of IFN-I-driven inflammation by instructing a germinal center B cell transcriptional program. Prevention of decimation depended on T cell-intrinsic Bcl6 and Tfh progeny formation. Antigen presentation by B cells, interactions with antigen-specific T helper cells, and costimulation by CD40 and ICOS were also required. Importantly, B cell-mediated virus control averted Th1-driven immunopathology in LCMV-challenged animals with preexisting CD4 T cell immunity. Our findings show that vaccination-induced Tfh cells represent a cornerstone of effective B cell immunity to chronic virus challenge, pointing the way toward more effective B cell-based vaccination against persistent viral diseases.

Selenium-GPX4 axis protects follicular helper T cells from ferroptosis

Follicular helper T (T_FH) cells are a specialized subset of CD4⁺ T cells that essentially support germinal center responses where high-affinity and long-lived humoral immunity is generated. The regulation of T_FH cell survival remains unclear. Here we report that T_FH cells show intensified lipid peroxidation and altered mitochondrial morphology, resembling the features of ferroptosis, a form of programmed cell death that is driven by iron-dependent accumulation of lipid peroxidation. Glutathione peroxidase 4 (GPX4) is the major lipid peroxidation scavenger and is necessary for T_FH cell survival. The deletion of GPX4 in T cells selectively abrogated T_FH cells and germinal center responses in immunized mice. Selenium supplementation enhanced GPX4 expression in T cells, increased T_FH cell numbers and promoted antibody responses in immunized mice and young adults after influenza vaccination. Our findings reveal the central role of the selenium-GPX4-ferroptosis axis in regulating T_FH homeostasis, which can be targeted to enhance T_FH cell function in infection and following vaccination.

Tissue-Resident T Cells in Chronic Relapsing-Remitting Intestinal Disorders

Tissue-resident memory T (TRM) cells critically contribute to the rapid immunoprotection and efficient immunosurveillance against pathogens, particularly in barrier tissues, but also during anti-tumor responses. However, the involvement of TRM cells also in the induction and exacerbation of immunopathologies, notably in chronically relapsing auto-inflammatory disorders, is becoming increasingly recognized as a critical factor. Thus, TRM cells may also represent an attractive target in the management of chronic (auto-) inflammatory disorders, including multiple sclerosis, rheumatoid arthritis, celiac disease and inflammatory bowel diseases. In this review, we focus on current concepts of TRM cell biology, particularly in the intestine, and discuss recent findings on their involvement in chronic relapsing-remitting inflammatory disorders. Potential therapeutic strategies to interfere with these TRM cell-mediated immunopathologies are discussed.

Memory Generation and Re-Activation in Food Allergy

Recent evidence has highlighted the critical role of memory cells in maintaining lifelong food allergies, thereby identifying these cells as therapeutic targets. IgG⁺ memory B cells replenish pools of IgE-secreting cells upon allergen exposure, which contract thereafter due to the short lifespan of tightly regulated IgE-expressing cells. Advances in the detection and highly dimensional analysis of allergen-specific B and T cells from allergic patients have provided insight on their phenotype and function. The newly identified Th2A and Tfh13 populations represent a leap in our understanding of allergen-specific T cell phenotypes, although how these populations contribute to IgE memory responses remains poorly understood. Within, we discuss the mechanisms by which memory B and T cells are activated, integrating knowledge from human systems and fundamental research. We then focus on memory reactivation, specifically, on the pathways of secondary IgE responses. Throughout, we identify areas of future research which will help identify immunotargets for a transformative therapy for food allergy.

Evolving concepts in NAD+ metabolism

NAD(H) and NADP(H) have traditionally been viewed as co-factors (or co-enzymes) involved in a myriad of oxidation-reduction reactions including the electron transport in the mitochondria. However, NAD pathway metabolites have many other important functions, including roles in signaling pathways, post-translational modifications, epigenetic changes, and regulation of RNA stability and function via NAD-capping of RNA. Non-oxidative reactions ultimately lead to the net catabolism of these nucleotides, indicating that NAD metabolism is an extremely dynamic process. In fact, recent studies have clearly demonstrated that NAD has a half-life in the order of minutes in some tissues. Several evolving concepts on the metabolism, transport, and roles of these NAD pathway metabolites in disease states such as cancer, neurodegeneration, and aging have emerged in just the last few years. In this perspective, we discuss key recent discoveries and changing concepts in NAD metabolism and biology that are reshaping the field. In addition, we will pose some open questions in NAD biology, including why NAD metabolism is so fast and dynamic in some tissues, how NAD and its precursors are transported to cells and organelles, and how NAD metabolism is integrated with inflammation and senescence. Resolving these questions will lead to significant advancements in the field.

Metabolic Control of Memory T-Cell Generation and Stemness

The formation of long-lived memory T cells is a critical feature of the adaptive immune response. T cells undergo metabolic reprogramming to establish a functional memory population. While initial studies characterized key metabolic pathways necessary for memory T-cell development, recent findings highlight that metabolic regulation of memory T-cell subsets is diverse. Here we describe the different requirements for metabolic programs and metabolism-related signaling pathways in memory T-cell development. We further discuss the contribution of cellular metabolism to memory T-cell functional reprogramming and stemness within acute and chronic inflammatory environments. Last, we highlight knowledge gaps and propose approaches to determine the roles of metabolites and metabolic enzymes in memory T-cell fate. Understanding how cellular metabolism regulates a functionally diverse memory population will undoubtedly provide new therapeutic insights to modulate protective T-cell immunity in human disease.

CD49d marks Th1 and Tfh-like antigen-specific CD4+ T cells during Plasmodium chabaudi infection

Upon activation, specific CD4+ T cells up-regulate the expression of CD11a and CD49d, surrogate markers of pathogen-specific CD4+ T cells. However, using T-cell receptor transgenic mice specific for a Plasmodium antigen, termed PbT-II, we found that activated CD4+ T cells develop not only to CD11ahiCD49dhi cells, but also to CD11ahiCD49dlo cells during acute Plasmodium infection. CD49dhi PbT-II cells, localized in the red pulp of spleens, expressed transcription factor T-bet and produced IFN-γ, indicating that they were type 1 helper T (Th1)-type cells. In contrast, CD49dlo PbT-II cells resided in the white pulp/marginal zones and were a heterogeneous population, with approximately half of them expressing CXCR5 and a third expressing Bcl-6, a master regulator of follicular helper T (Tfh) cells. In adoptive transfer experiments, both CD49dhi and CD49dlo PbT-II cells differentiated into CD49dhi Th1-type cells after stimulation with antigen-pulsed dendritic cells, while CD49dhi and CD49dlo phenotypes were generally maintained in mice infected with Plasmodium chabaudi. These results suggest that CD49d is expressed on Th1-type Plasmodium-specific CD4+ T cells, which are localized in the red pulp of the spleen, and can be used as a marker of antigen-specific Th1 CD4+ T cells, rather than that of all pathogen-specific CD4+ T cells.