Antigen affinity and antigen dose exert distinct influences on CD4 T-cell differentiation

Hybrid response to SARS-CoV-2 and Neisseria meningitidis C after an OMV-adjuvanted immunization in mice and their offspring

COVID-19, caused by SARS-CoV-2, and meningococcal disease, caused by Neisseria meningitidis, are relevant infectious diseases, preventable through vaccination. Outer membrane vesicles (OMVs), released from Gram-negative bacteria, such as N. meningitidis, present adjuvant characteristics and may confer protection against meningococcal disease. Here, we evaluated in mice the humoral and cellular immune response to different doses of receptor binding domain (RBD) of SARS-CoV-2 adjuvanted by N. meningitidis C:2a:P1.5 OMVs and aluminum hydroxide, as a combined preparation for these pathogens. The immunization induced IgG antibodies of high avidity for RBD and OMVs, besides IgG that recognized the Omicron BA.2 variant of SARS-CoV-2 with intermediary avidity. Cellular immunity showed IFN-γ and IL-4 secretion in response to RBD and OMV stimuli, demonstrating immunologic memory and a mixed Th1/Th2 response. Offspring presented transferred IgG of similar levels and avidity as their mothers. Humoral immunity did not point to the superiority of any RBD dose, but the group immunized with a lower antigenic dose (0.5 μg) had the better cellular response. Overall, OMVs enhanced RBD immunogenicity and conferred an immune response directed to N. meningitidis too.

Augmenting vaccine efficacy: Tailored immune strategy with alum-stabilized Pickering emulsion

BACKGROUND

The achievement of optimal vaccine efficacy is contingent upon the collaborative interactions between T and B cells in adaptive immunity. Although multiple immunization strategies have been proposed, there is a notable scarcity of comprehensive investigations pertaining to enhance immune effects through immune strategy adjustments for individual vaccine.

METHODS

The hierarchically structured aluminum hydroxide microgel-stabilized Pickering emulsion (ASPE) was prepared by ultrasonic method. This study explored the influence of the immune strategy of ASPE to immune responses, including antigen exposure pattern, adjuvants and antigen dosage, and administration interval.

RESULTS

The findings revealed that external antigen adsorption facilitated increased exposure of antigen epitopes, leading to elevated IgG titers and secretion of cytokines such as interferon-gamma (IFN-γ) or interleukin-4 (IL-4). Additionally, even a low dose (1 μg/dose) of antigens of ASPE boosted sufficient neutralizing antibody levels and memory T cells compared to high-dose antigens, which consistent with the adjuvant dosage effect. Furthermore, maintaining a 4-week immunization interval yielded optimal levels of antigen-specific IgG titers in both short-term and long-term scenarios, as compared to intervals of 2, 3, and 5 weeks. A consistent trend was observed in the proliferation of memory B cells, reaching a superior level at the 4-week interval, which could enhance protection against viral re-infection.

CONCLUSION

Tailoring immunization strategies for specific vaccines has emerged as powerful driver in maximizing vaccine efficacy and eliciting robust immune responses, thereby presenting cutting-edge approaches to enhanced vaccination.

Plasmodium infection induces phenotypic, clonal, and spatial diversity among differentiating CD4+ T cells

Naive CD4+ T cells must differentiate in order to orchestrate immunity to Plasmodium, yet understanding of their emerging phenotypes, clonality, spatial distributions, and cellular interactions remains incomplete. Here, we observe that splenic polyclonal CD4+ T cells differentiate toward T helper 1 (Th1) and T follicular helper (Tfh)-like states and exhibit rarer phenotypes not elicited among T cell receptor (TCR) transgenic counterparts. TCR clones present at higher frequencies exhibit Th1 skewing, suggesting that variation in major histocompatibility complex class II (MHC-II) interaction influences proliferation and Th1 differentiation. To characterize CD4+ T cell interactions, we map splenic microarchitecture, cellular locations, and molecular interactions using spatial transcriptomics at near single-cell resolution. Tfh-like cells co-locate with stromal cells in B cell follicles, while Th1 cells in red pulp co-locate with activated monocytes expressing multiple chemokines and MHC-II. Spatial mapping of individual transcriptomes suggests that proximity to chemokine-expressing monocytes correlates with stronger effector phenotypes in Th1 cells. Finally, CRISPR-Cas9 gene disruption reveals a role for CCR5 in promoting clonal expansion and Th1 differentiation. A database of cellular locations and interactions is presented: https://haquelab.mdhs.unimelb.edu.au/spatial_gui/.

Activated B-Cells enhance epitope spreading to support successful cancer immunotherapy

Immune checkpoint therapies (ICT) have transformed the treatment of cancer over the past decade. However, many patients do not respond or suffer relapses. Successful immunotherapy requires epitope spreading, but the slow or inefficient induction of functional antitumoral immunity delays the benefit to patients or causes resistances. Therefore, understanding the key mechanisms that support epitope spreading is essential to improve immunotherapy. In this review, we highlight the major role played by B-cells in breaking immune tolerance by epitope spreading. Activated B-cells are key Antigen-Presenting Cells (APC) that diversify the T-cell response against self-antigens, such as ribonucleoproteins, in autoimmunity but also during successful cancer immunotherapy. This has important implications for the design of future cancer vaccines.

BCG infection dose guides dendritic cell migration and T cell priming in the draining lymph node

In contrast to delayed-type hypersensitivity (DTH) and other hallmark reactions of cell-mediated immunity that correlate with vaccine-mediated protection against Mycobacterium tuberculosis, the contribution of vaccine dose on responses that emerge early after infection in the skin with Bacille Calmette-Guérin (BCG) is not well understood. We used a mouse model of BCG skin infection to study the effect of BCG dose on the relocation of skin Dendritic cells (DCs) to draining lymph node (DLN). Mycobacterium antigen 85B-specific CD4+ P25 T cell-receptor transgenic (P25 TCRTg) cells were used to probe priming to BCG in DLN. DC migration and T cell priming were studied across BCG inocula that varied up to 100-fold (104 to 106 Colony-forming units-CFUs). In line with earlier results in guinea pigs, DTH reaction in our model correlated with BCG dose. Importantly, priming of P25 TCRTg cells in DLN also escalated in a dose-dependent manner, peaking at day 6 after infection. Similar dose-escalation effects were seen for DC migration from infected skin and the accompanying transport of BCG to the DLN. BCG-triggered upregulation of co-stimulatory molecules on migratory DCs was restricted to the first 24 hour after infection and was independent of BCG dose over a 10-fold range (105 to 106 CFUs). The dose seemed to be a determinant of the number of total skin DCs that move to the DLN. In summary, our results support the use of higher BCG doses to detect robust DC migration and T cell priming.

XBB.1.5 monovalent booster improves antibody binding and neutralization against emerging SARS-CoV-2 Omicron variants

The rapid emergence of divergent SARS-CoV-2 variants has led to an update of the COVID-19 booster vaccine to a monovalent version containing the XBB.1.5 spike. To determine the neutralization breadth following booster immunization, we collected blood samples from 24 individuals pre- and post-XBB.1.5 mRNA booster vaccination (∼1 month). The XBB.1.5 booster improved both neutralizing activity against the ancestral SARS-CoV-2 strain (WA1) and the circulating Omicron variants, including EG.5.1, HK.3, HV.1, XBB.1.5 and JN.1. Relative to the pre-boost titers, the XBB.1.5 monovalent booster induced greater total IgG and IgG subclass binding, particular IgG4, to the XBB.1.5 spike as compared to the WA1 spike. We evaluated antigen-specific memory B cells (MBCs) using either spike or receptor binding domain (RBD) probes and found that the monovalent booster largely increases non-RBD cross-reactive MBCs. These data suggest that the XBB.1.5 monovalent booster induces cross-reactive antibodies that neutralize XBB.1.5 and related Omicron variants.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide-major histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs but diverge only over longer (9+ h) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception and establish a framework for understanding T cell responses under diverse contexts.

Unrestrained cleavage of Roquin-1 by MALT1 induces spontaneous T cell activation and the development of autoimmunity

Constitutive activation of the MALT1 paracaspase in conventional T cells of Malt1TBM/TBM (TRAF6 Binding Mutant = TBM) mice causes fatal inflammation and autoimmunity, but the involved targets and underlying molecular mechanisms are unknown. We genetically rendered a single MALT1 substrate, the RNA-binding protein (RBP) Roquin-1, insensitive to MALT1 cleavage. These Rc3h1Mins/Mins mice showed normal immune homeostasis. Combining Rc3h1Mins/Mins alleles with those encoding for constitutively active MALT1 (TBM) prevented spontaneous T cell activation and restored viability of Malt1TBM/TBM mice. Mechanistically, we show how antigen/MHC recognition is translated by MALT1 into Roquin cleavage and derepression of Roquin targets. Increasing T cell receptor (TCR) signals inactivated Roquin more effectively, and only high TCR strength enabled derepression of high-affinity targets to promote Th17 differentiation. Induction of experimental autoimmune encephalomyelitis (EAE) revealed increased cleavage of Roquin-1 in disease-associated Th17 compared to Th1 cells in the CNS. T cells from Rc3h1Mins/Mins mice did not efficiently induce the high-affinity Roquin-1 target IκBNS in response to TCR stimulation, showed reduced Th17 differentiation, and Rc3h1Mins/Mins mice were protected from EAE. These data demonstrate how TCR signaling and MALT1 activation utilize graded cleavage of Roquin to differentially regulate target mRNAs that control T cell activation and differentiation as well as the development of autoimmunity.

T-helper cells flexibility: the possibility of reprogramming T cells fate

Various disciplines cooperate to find novel approaches to cure impaired body functions by repairing, replacing, or regenerating cells, tissues, or organs. The possibility that a stable differentiated cell can reprogram itself opens the door to new therapeutic strategies against a multitude of diseases caused by the loss or dysfunction of essential, irreparable, and specific cells. One approach to cell therapy is to induce reprogramming of adult cells into other functionally active cells. Understanding the factors that cause or contribute to T cell plasticity is not only of clinical importance but also expands the knowledge of the factors that induce cells to differentiate and improves the understanding of normal developmental biology. The present review focuses on the advances in the conversion of peripheral CD4+ T cells, the conditions of their reprogramming, and the methods proposed to control such cell differentiation.

Heterogeneous Tfh cell populations that develop during enteric helminth infection predict the quality of type 2 protective response

T follicular helper (Tfh) cells are an important component of germinal center (GC)-mediated humoral immunity. Yet, how a chronic type 1 versus protective type 2 helminth infection modulates Tfh-GC responses remains poorly understood. Here, we employ the helminth Trichuris muris model and demonstrate that Tfh cell phenotypes and GC are differentially regulated in acute versus chronic infection. The latter failed to induce Tfh-GC B cell responses, with Tfh cells expressing Τ-bet and interferon-γ. In contrast, interleukin-4-producing Tfh cells dominate responses to an acute, resolving infection. Heightened expression and increased chromatin accessibility of T helper (Th)1- and Th2 cell-associated genes are observed in chronic and acute induced Tfh cells, respectively. Blockade of the Th1 cell response by T-cell-intrinsic T-bet deletion promoted Tfh cell expansion during chronic infection, pointing to a correlation between a robust Tfh cell response and protective immunity to parasites. Finally, blockade of Tfh-GC interactions impaired type 2 immunity, revealing the critical protective role of GC-dependent Th2-like Tfh cell responses during acute infection. Collectively, these results provide new insights into the protective roles of Tfh-GC responses and identify distinct transcriptional and epigenetic features of Tfh cells that emerge during resolving or chronic T. muris infection.

Can we predict T cell specificity with digital biology and machine learning?

Recent advances in machine learning and experimental biology have offered breakthrough solutions to problems such as protein structure prediction that were long thought to be intractable. However, despite the pivotal role of the T cell receptor (TCR) in orchestrating cellular immunity in health and disease, computational reconstruction of a reliable map from a TCR to its cognate antigens remains a holy grail of systems immunology. Current data sets are limited to a negligible fraction of the universe of possible TCR-ligand pairs, and performance of state-of-the-art predictive models wanes when applied beyond these known binders. In this Perspective article, we make the case for renewed and coordinated interdisciplinary effort to tackle the problem of predicting TCR-antigen specificity. We set out the general requirements of predictive models of antigen binding, highlight critical challenges and discuss how recent advances in digital biology such as single-cell technology and machine learning may provide possible solutions. Finally, we describe how predicting TCR specificity might contribute to our understanding of the broader puzzle of antigen immunogenicity.

CD4 Effector TCR Avidity for Peptide on APC Determines the Level of Memory Generated

Initial TCR affinity for peptide Ag is known to impact the generation of memory; however, its contributions later, when effectors must again recognize Ag at 5-8 d postinfection to become memory, is unclear. We examined whether the effector TCR affinity for peptide at this "effector checkpoint" dictates the extent of memory and degree of protection against rechallenge. We made an influenza A virus nucleoprotein (NP)-specific TCR transgenic mouse strain, FluNP, and generated NP-peptide variants that are presented by MHC class II to bind to the FluNP TCR over a broad range of avidity. To evaluate the impact of avidity in vivo, we primed naive donor FluNP in influenza A virus-infected host mice, purified donor effectors at the checkpoint, and cotransferred them with the range of peptides pulsed on activated APCs into second uninfected hosts. Higher-avidity peptides yielded higher numbers of FluNP memory cells in spleen and most dramatically in lung and draining lymph nodes and induced better protection against lethal influenza infection. Avidity determined memory cell number, not cytokine profile, and already impacted donor cell number within several days of transfer. We previously found that autocrine IL-2 production at the checkpoint prevents default effector apoptosis and supports memory formation. Here, we find that peptide avidity determines the level of IL-2 produced by these effectors and that IL-2Rα expression by the APCs enhances memory formation, suggesting that transpresentation of IL-2 by APCs further amplifies IL-2 availability. Secondary memory generation was also avidity dependent. We propose that this regulatory pathway selects CD4 effectors of highest affinity to progress to memory.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide major-histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs, but diverge only over longer (9+ hrs) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception, and establish a framework for understanding T cell responses under diverse contexts.

SIGNIFICANCE STATEMENT

To counter diverse pathogens, T cells mount distinct responses to varying peptide-major histocompatibility complex ligands (pMHCs). They perceive the affinity of pMHCs for the T cell receptor (TCR), which reflects its foreignness, as well as pMHC abundance. By tracking signaling responses in single living cells to different pMHCs, we find that T cells can independently perceive pMHC affinity vs dose, and encode this information through the dynamics of Erk and NFAT signaling pathways downstream of the TCR. These dynamics are jointly decoded by gene regulatory mechanisms to produce pMHC-specific activation responses. Our work reveals how T cells can elicit tailored functional responses to diverse threats and how dysregulation of these responses may lead to immune pathologies.

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.

Inflammation durably imprints memory CD4+ T cells

Adaptive immune responses are induced by vaccination and infection, yet little is known about how CD4+ T cell memory differs when primed in these two contexts. Notably, viral infection is generally associated with higher levels of systemic inflammation than is vaccination. To assess whether the inflammatory milieu at the time of CD4+ T cell priming has long-term effects on memory, we compared Spike-specific memory CD4+ T cells in 22 individuals around the time of the participants' third SARS-CoV-2 mRNA vaccination, with stratification by whether the participants' first exposure to Spike was via virus or mRNA vaccine. Multimodal single-cell profiling of Spike-specific CD4+ T cells revealed 755 differentially expressed genes that distinguished infection- and vaccine-primed memory CD4+ T cells. Spike-specific CD4+ T cells from infection-primed individuals had strong enrichment for cytotoxicity and interferon signaling genes, whereas Spike-specific CD4+ T cells from vaccine-primed individuals were enriched for proliferative pathways by gene set enrichment analysis. Moreover, Spike-specific memory CD4+ T cells established by infection had distinct epigenetic landscapes driven by enrichment of IRF-family transcription factors, relative to T cells established by mRNA vaccination. This transcriptional imprint was minimally altered following subsequent mRNA vaccination or breakthrough infection, reflecting the strong bias induced by the inflammatory environment during initial memory differentiation. Together, these data suggest that the inflammatory context during CD4+ T cell priming is durably imprinted in the memory state at transcriptional and epigenetic levels, which has implications for personalization of vaccination based on prior infection history.

IgG4 Antibodies Induced by Repeated Vaccination May Generate Immune Tolerance to the SARS-CoV-2 Spike Protein

Less than a year after the global emergence of the coronavirus SARS-CoV-2, a novel vaccine platform based on mRNA technology was introduced to the market. Globally, around 13.38 billion COVID-19 vaccine doses of diverse platforms have been administered. To date, 72.3% of the total population has been injected at least once with a COVID-19 vaccine. As the immunity provided by these vaccines rapidly wanes, their ability to prevent hospitalization and severe disease in individuals with comorbidities has recently been questioned, and increasing evidence has shown that, as with many other vaccines, they do not produce sterilizing immunity, allowing people to suffer frequent re-infections. Additionally, recent investigations have found abnormally high levels of IgG4 in people who were administered two or more injections of the mRNA vaccines. HIV, Malaria, and Pertussis vaccines have also been reported to induce higher-than-normal IgG4 synthesis. Overall, there are three critical factors determining the class switch to IgG4 antibodies: excessive antigen concentration, repeated vaccination, and the type of vaccine used. It has been suggested that an increase in IgG4 levels could have a protecting role by preventing immune over-activation, similar to that occurring during successful allergen-specific immunotherapy by inhibiting IgE-induced effects. However, emerging evidence suggests that the reported increase in IgG4 levels detected after repeated vaccination with the mRNA vaccines may not be a protective mechanism; rather, it constitutes an immune tolerance mechanism to the spike protein that could promote unopposed SARS-CoV2 infection and replication by suppressing natural antiviral responses. Increased IgG4 synthesis due to repeated mRNA vaccination with high antigen concentrations may also cause autoimmune diseases, and promote cancer growth and autoimmune myocarditis in susceptible individuals.

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.

T cell receptor signaling in the differentiation and plasticity of CD4+ T cells

CD4⁺ T cells are critical components of the adaptive immune system. The T cell receptor (TCR) and co-receptor signaling cascades shape the phenotype and functions of CD4⁺ T cells. TCR signaling plays a crucial role in T cell development, antigen recognition, activation, and differentiation upon recognition of foreign- or auto-antigens. In specific autoimmune conditions, altered TCR repertoire is reported and can predispose autoimmunity with organ-specific inflammation and tissue damage. TCR signaling modulates various signaling cascades and regulates epigenetic and transcriptional regulation during homeostasis and disease conditions. Understanding the mechanism by which coreceptors and cytokine signals control the magnitude of TCR signal amplification will aid in developing therapeutic strategies to treat inflammation and autoimmune diseases. This review focuses on the role of the TCR signaling cascade and its components in the activation, differentiation, and plasticity of various CD4⁺ T cell subsets.

Understanding the development of Th2 cell-driven allergic airway disease in early life

Allergic diseases, including atopic dermatitis, allergic rhinitis, asthma, and food allergy, are caused by abnormal responses to relatively harmless foreign proteins called allergens found in pollen, fungal spores, house dust mites (HDM), animal dander, or certain foods. In particular, the activation of allergen-specific helper T cells towards a type 2 (Th2) phenotype during the first encounters with the allergen, also known as the sensitization phase, is the leading cause of the subsequent development of allergic disease. Infants and children are especially prone to developing Th2 cell responses after initial contact with allergens. But in addition, the rates of allergic sensitization and the development of allergic diseases among children are increasing in the industrialized world and have been associated with living in urban settings. Particularly for respiratory allergies, greater susceptibility to developing allergic Th2 cell responses has been shown in children living in urban environments containing low levels of microbial contaminants, principally bacterial endotoxins [lipopolysaccharide (LPS)], in the causative aeroallergens. This review highlights the current understanding of the factors that balance Th2 cell immunity to environmental allergens, with a particular focus on the determinants that program conventional dendritic cells (cDCs) toward or away from a Th2 stimulatory function. In this context, it discusses transcription factor-guided functional specialization of type-2 cDCs (cDC2s) and how the integration of signals derived from the environment drives this process. In addition, it analyzes observational and mechanistic studies supporting an essential role for innate sensing of microbial-derived products contained in aeroallergens in modulating allergic Th2 cell immune responses. Finally, this review examines whether hyporesponsiveness to microbial stimulation, particularly to LPS, is a risk factor for the induction of Th2 cell responses and allergic sensitization during infancy and early childhood and the potential factors that may affect early-age response to LPS and other environmental microbial components.

Unique roles of co-receptor-bound LCK in helper and cytotoxic T cells

The kinase LCK and CD4/CD8 co-receptors are crucial components of the T cell antigen receptor (TCR) signaling machinery, leading to key T cell fate decisions. Despite decades of research, the roles of CD4-LCK and CD8-LCK interactions in TCR triggering in vivo remain unknown. In this study, we created animal models expressing endogenous levels of modified LCK to resolve whether and how co-receptor-bound LCK drives TCR signaling. We demonstrated that the role of LCK depends on the co-receptor to which it is bound. The CD8-bound LCK is largely dispensable for antiviral and antitumor activity of cytotoxic T cells in mice; however, it facilitates CD8⁺ T cell responses to suboptimal antigens in a kinase-dependent manner. By contrast, the CD4-bound LCK is required for efficient development and function of helper T cells via a kinase-independent stabilization of surface CD4. Overall, our findings reveal the role of co-receptor-bound LCK in T cell biology, show that CD4- and CD8-bound LCK drive T cell development and effector immune responses using qualitatively different mechanisms and identify the co-receptor-LCK interactions as promising targets for immunomodulation.

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.

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.

Tet2 coordinates with Foxo1 and Runx1 to balance T follicular helper cell and T helper 1 cell differentiation

In response to various types of infection, naïve CD4+ T cells differentiate into diverse helper T cell subsets; however, the epigenetic programs that regulate differentiation in response to viral infection remain poorly understood. Demethylation of CpG dinucleotides by Tet methylcytosine dioxygenases is a key component of epigenetic programing that promotes specific gene expression, cellular differentiation, and function. We report that following viral infection, Tet2-deficient CD4+ T cells preferentially differentiate into highly functional germinal center T follicular helper (TFH) cells that provide enhanced help for B cells. Using genome-wide DNA methylation and transcription factor binding analyses, we find that Tet2 coordinates with multiple transcription factors, including Foxo1 and Runx1, to mediate the demethylation and expression of target genes, including genes encoding repressors of TFH differentiation. Our findings establish Tet2 as an important regulator of TFH cell differentiation and reveal pathways that could be targeted to enhance immune responses against infectious disease.

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.

IL-33 acts as a costimulatory signal to generate alloreactive Th1 cells in graft-versus-host disease

Antigen-presenting cells (APCs) integrate signals emanating from local pathology and program appropriate T cell responses. In allogeneic hematopoietic stem cell transplantation (alloHCT), recipient conditioning releases damage-associated molecular patterns (DAMPs) that generate proinflammatory APCs that secrete IL-12, which is a driver of donor Th1 responses, causing graft-versus-host disease (GVHD). Nevertheless, other mechanisms exist to initiate alloreactive T cell responses, as recipients with disrupted DAMP signaling or lacking IL-12 develop GVHD. We established that tissue damage signals are perceived directly by donor CD4⁺ T cells and promoted T cell expansion and differentiation. Specifically, the fibroblastic reticular cell–derived DAMP IL-33 is increased by recipient conditioning and is critical for the initial activation, proliferation, and differentiation of alloreactive Th1 cells. IL-33 stimulation of CD4⁺ T cells was not required for lymphopenia-induced expansion, however. IL-33 promoted IL-12–independent expression of Tbet and generation of Th1 cells that infiltrated GVHD target tissues. Mechanistically, IL-33 augmented CD4⁺ T cell TCR-associated signaling pathways in response to alloantigen. This enhanced T cell expansion and Th1 polarization, but inhibited the expression of regulatory molecules such as IL-10 and Foxp3. These data establish an unappreciated role for IL-33 as a costimulatory signal for donor Th1 generation after alloHCT.

ICOS expression is required for maintenance but not the formation of germinal centers in the spleen in response to P. yoelii infection

Inducible T cell co-stimulator (ICOS) plays a key role in the differentiation and maintenance of follicular helper T (Tfh) cells and thus germinal center (GC) formation. Previously, our lab showed in a Plasmodium chabaudi infection model that Icos^-/- mice were significantly impaired in their ability to form GCs despite a persistent infection and thus a continued antigen (Ag) load. Here, we show that resolution of a primary infection with P. yoelii, was delayed in Icos^-/- mice. This phenotype was associated with a reduction in the accumulation of Tfh-like and GC Tfh cells and an early deficiency in Ag-specific antibody (Ab) production. However, Icos^-/- mice could form GCs, though they were less frequent in number than in wild-type (WT) mice. Nonetheless, the Ag-specific Abs from Icos^-/- mice lacked signs of affinity maturation, suggesting functional defects associated with these GCs. Eventually, these GC structures dissipated more rapidly in Icos^-/- mice than in WT mice. Moreover, the ability of Icos^-/- mice to form these GC structures is not reliant on the high Ag load associated with P. yoelii infections, as GC formation was preserved in Icos^-/- mice treated with atovaquone. Finally, mice were unable to form secondary GCs in the absence of ICOS after re-challenge. Overall, these data demonstrate the necessity of ICOS in the maintenance of Tfh cells, the formation and maintenance of sufficient numbers of functioning GCs, and the ability to generate new GC structures after re-infection with P. yoelii.

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.

Antigen and checkpoint receptor engagement recalibrates T cell receptor signal strength

How T cell receptor (TCR) signal strength modulates T cell function and to what extent this is modified by immune checkpoint blockade (ICB) are key questions in immunology. Using Nr4a3-Tocky mice, we characterized early quantitative and qualitative changes that occur in CD4+ T cells in relation to TCR signaling strength. We captured how dose- and time-dependent programming of distinct co-inhibitory receptors rapidly recalibrates T cell activation thresholds and visualized the immediate effects of ICB on T cell re-activation. Our findings reveal that anti-PD1 immunotherapy leads to an increased TCR signal strength. We defined a strong TCR signal metric of five genes upregulated by anti-PD1 in T cells (TCR.strong), which was superior to a canonical T cell activation gene signature in stratifying melanoma patient outcomes to anti-PD1 therapy. Our study therefore reveals how analysis of TCR signal strength-and its manipulation-can provide powerful metrics for monitoring outcomes to immunotherapy.

Impaired HA-specific T follicular helper cell and antibody responses to influenza vaccination are linked to inflammation in humans

Antibody production following vaccination can provide protective immunity to subsequent infection by pathogens such as influenza viruses. However, circumstances where antibody formation is impaired after vaccination, such as in older people, require us to better understand the cellular and molecular mechanisms that underpin successful vaccination in order to improve vaccine design for at-risk groups. Here, by studying the breadth of anti-haemagglutinin (HA) IgG, serum cytokines, and B and T cell responses by flow cytometry before and after influenza vaccination, we show that formation of circulating T follicular helper (cTfh) cells was associated with high-titre antibody responses. Using Major Histocompatability Complex (MHC) class II tetramers, we demonstrate that HA-specific cTfh cells can derive from pre-existing memory CD4+ T cells and have a diverse T cell receptor (TCR) repertoire. In older people, the differentiation of HA-specific cells into cTfh cells was impaired. This age-dependent defect in cTfh cell formation was not due to a contraction of the TCR repertoire, but rather was linked with an increased inflammatory gene signature in cTfh cells. Together, this suggests that strategies that temporarily dampen inflammation at the time of vaccination may be a viable strategy to boost optimal antibody generation upon immunisation of older people.

Supplementation with Serum-Derived Extracellular Vesicles Reinforces Antitumor Immunity Induced by Cryo-Thermal Therapy

Effective cancer therapies should reshape immunosuppression and trigger antitumor immunity. Previously, we developed a novel cryo-thermal therapy through applying local rapid cooling followed by rapid heating of tumor tissue. It could not only ablate local tumors, but also, subsequently, induce systemic long-term antitumor immunity. Hyperthermia can induce the release of extracellular vesicles (EVs) to stimulate antitumor immunity. We examine whether EVs are released after cryo-thermal therapy and whether they could improve the efficacy of cryo-thermal therapy in the 4T1 model. In this study, serum extracellular vesicles (sEVs) are isolated and characterized 3 h after cryo-thermal therapy of subcutaneous tumors. sEV phagocytosis is observed in vitro and in vivo by using laser confocal microscopy and flow cytometry. After cryo-thermal therapy, sEVs are administered to mice via the tail vein, and changes in immune cells are investigated by using flow cytometry. After cryo-thermal therapy, a large number of sEVs are released to the periphery carrying danger signals and tumor antigens, and these sEVs could be phagocytosed by peripheral blood monocytes and differentiated macrophages. After cryo-thermal therapy, supplementation with sEVs released after treatment promotes the differentiation of myeloid-derived suppressor cells (MDSCs), monocytes into macrophages and CD4⁺ T cells into the Th1 subtype, as well as prolonging the long-term survival of the 4T1 subcutaneous tumor-bearing mice. sEVs released after cryo-thermal tumor treatment could clinically serve as an adjuvant in subsequent cryo-thermal therapy to improve the therapeutic effects on malignant tumors.

In Vivo CD4+ T Cell Differentiation and Function: Revisiting the Th1/Th2 Paradigm

The discovery of CD4⁺ T cell subset-defining master transcription factors and framing of the Th1/Th2 paradigm ignited the CD4⁺ T cell field. Advances in in vivo experimental systems, however, have revealed that more complex lineage-defining transcriptional networks direct CD4⁺ T cell differentiation in the lymphoid organs and tissues. This review focuses on the layers of fate decisions that inform CD4⁺ T cell differentiation in vivo. Cytokine production by antigen-presenting cells and other innate cells influences the CD4⁺ T cell effector program [e.g., T helper type 1 (Th1), Th2, Th17]. Signals downstream of the T cell receptor influence whether individual clones bearing hallmarks of this effector program become T follicular helper cells, supporting development of B cells expressing specific antibody isotypes, or T effector cells, which activate microbicidal innate cells in tissues. These bifurcated, parallel axes allow CD4⁺ T cells to augment their particular effector program and prevent disease.

Understanding Antibody Responses in Early Life: Baby Steps towards Developing an Effective Influenza Vaccine

The immune system of young infants is both quantitatively and qualitatively distinct from that of adults, with diminished responsiveness leaving these individuals vulnerable to infection. Because of this, young infants suffer increased morbidity and mortality from respiratory pathogens such as influenza viruses. The impaired generation of robust and persistent antibody responses in these individuals makes overcoming this increased vulnerability through vaccination challenging. Because of this, an effective vaccine against influenza viruses in infants under 6 months is not available. Furthermore, vaccination against influenza viruses is challenging even in adults due to the high antigenic variability across viral strains, allowing immune evasion even after induction of robust immune responses. This has led to substantial interest in understanding how specific antibody responses are formed to variable and conserved components of influenza viruses, as immune responses tend to strongly favor recognition of variable epitopes. Elicitation of broadly protective antibody in young infants, therefore, requires that both the unique characteristics of young infant immunity as well as the antibody immunodominance present among epitopes be effectively addressed. Here, we review our current understanding of the antibody response in newborns and young infants and discuss recent developments in vaccination strategies that can modulate both magnitude and epitope specificity of IAV-specific antibody.

Upregulation of PD-1 expression on circulating CD8+ but not CD4+ T cells is associated with tuberculosis infection in health care workers

Background

Health care workers (HCWs) are at risk for occupationally acquired Mycobacterium tuberculosis infection and tuberculosis (TB) disease due to repeated exposure to workplace tubercle bacilli. To determine whether continual mycobacterial stimulation correlates with increased expression of inhibitory T cell receptors, here we compared PD-1 receptor expression on surfaces of circulating T cells between naïve (uninfected) HCWs and HCWs with latent TB infection (LTBI).

Result

Data collected from 133 medical workers who met study selection criteria were included in the final analysis. QuantiFERON-TB Gold In-​Tube (QFT-GIT) testing yielded positive results for 32 HCWs, for an overall LTBI rate of 24.1%. Multivariate analysis identified HCW length of service > 15 years as an independent risk factor for a positive QFT-GIT result. In addition, comparisons of blood T cell subgroup profiles between QFT- and QFT+ groups indicated QFT+ subjects possessed greater proportions of mature (TM), transitional memory (TTM) and effector memory (TEM) CD4+ T cell subgroups and lower proportions of naïve T cells (TN). Moreover, the QFT+ group percentage of CD8+ T cells with detectable surface PD-1 was significantly higher than the corresponding percentage for the QFT- group. Meanwhile, no statistical intergroup difference was observed in percentages of CD4+ T cells with detectible surface PD-1.

Conclusions

Our data demonstrated that upregulated PD-1 expression on circulating CD8+, but not CD4+ T cells, was associated with latent TB infection of HCWs. As compared to other hospitals, occupational TB infection risk in our hospital was substantially mitigated by implementation of multitiered infection control measures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00433-9.

Lymphocytes and Trogocytosis-Mediated Signaling

Trogocytosis is the intercellular transfer of membrane and membrane-associated molecules. This underappreciated process has been described in a variety of biological settings including neuronal remodeling, fertilization, viral and bacterial spread, and cancer, but has been most widely studied in cells of the immune system. Trogocytosis is performed by multiple immune cell types, including basophils, macrophages, dendritic cells, neutrophils, natural killer cells, B cells, γδ T cells, and CD4⁺ and CD8⁺ αβ T cells. Although not expressed endogenously, the presence of trogocytosed molecules on cells has the potential to significantly impact an immune response and the biology of the individual trogocytosis-positive cell. Many studies have focused on the ability of the trogocytosis-positive cells to interact with other immune cells and modulate the function of responders. Less understood and arguably equally important is the impact of these molecules on the individual trogocytosis-positive cell. Molecules that have been reported to be trogocytosed by cells include cognate ligands for receptors on the individual cell, such as activating NK cell ligands and MHC:peptide. These trogocytosed molecules have been shown to interact with receptors on the trogocytosis-positive cell and mediate intracellular signaling. In this review, we discuss the impact of this trogocytosis-mediated signaling on the biology of the individual trogocytosis-positive cell by focusing on natural killer cells and CD4⁺ T lymphocytes.

How Does B Cell Antigen Presentation Affect Memory CD4 T Cell Differentiation and Longevity?

Dendritic cells are the antigen presenting cells that process antigens effectively and prime the immune system, a characteristic that have gained them the spotlights in recent years. B cell antigen presentation, although less prominent, deserves equal attention. B cells select antigen experienced CD4 T cells to become memory and initiate an orchestrated genetic program that maintains memory CD4 T cells for life of the individual. Over years of research, we have demonstrated that low levels of antigens captured by B cells during the resolution of an infection render antigen experienced CD4 T cells into a quiescent/resting state. Our studies suggest that in the absence of antigen, the resting state associated with low-energy utilization and proliferation can help memory CD4 T cells to survive nearly throughout the lifetime of mice. In this review we would discuss the primary findings from our lab as well as others that highlight our understanding of B cell antigen presentation and the contributions of the MHC Class II accessory molecules to this outcome. We propose that the quiescence induced by the low levels of antigen presentation might be a mechanism necessary to regulate long-term survival of CD4 memory T cells and to prevent cross-reactivity to autoantigens, hence autoimmunity.

Single cell analysis of host response to helminth infection reveals the clonal breadth, heterogeneity, and tissue-specific programming of the responding CD4+ T cell repertoire

The CD4⁺ T cell response is critical to host protection against helminth infection. How this response varies across different hosts and tissues remains an important gap in our understanding. Using IL-4-reporter mice to identify responding CD4⁺ T cells to Nippostrongylus brasiliensis infection, T cell receptor sequencing paired with novel clustering algorithms revealed a broadly reactive and clonally diverse CD4⁺ T cell response. While the most prevalent clones and clonotypes exhibited some tissue selectivity, most were observed to reside in both the lung and lung-draining lymph nodes. Antigen-reactivity of the broader repertoires was predicted to be shared across both tissues and individual mice. Transcriptome, trajectory, and chromatin accessibility analysis of lung and lymph-node repertoires revealed three unique but related populations of responding IL-4⁺ CD4⁺ T cells consistent with T follicular helper, T helper 2, and a transitional population sharing similarity with both populations. The shared antigen reactivity of lymph node and lung repertoires combined with the adoption of tissue-specific gene programs allows for the pairing of cellular and humoral responses critical to the orchestration of anti-helminth immunity.

Using various “omic” approaches, the CD4⁺ T cell receptor (TCR) repertoire was explored after primary helminth infection. Infection generated a broadly reactive and clonally diverse CD4⁺ T cell response with the most prevalent clonotypes and predicted antigen specificities residing in both the lung and lung-draining lymph nodes. Tissue-specific programming of responding CD4⁺ T cells directed the establishment of committed Tfh and Th2 cells, both critical for driving distinct hallmarks of type-2 inflammation. These datasets help to explore the diverse yet tissue-specific nature of anti-helminth immunity.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4⁺ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

Antigen-dependent multistep differentiation of T follicular helper cells and its role in SARS-CoV-2 infection and vaccination

T follicular helper (Tfh) cells play an essential role in regulating the GC reaction and, consequently, the generation of high‐affinity antibodies and memory B cells. Therefore, Tfh cells are critical for potent humoral immune responses against various pathogens and their dysregulation has been linked to autoimmunity and cancer. Tfh cell differentiation is a multistep process, in which cognate interactions with different APC types, costimulatory and coinhibitory pathways, as well as cytokines are involved. However, it is still not fully understood how a subset of activated CD4⁺ T cells begins to express the Tfh cell‐defining chemokine receptor CXCR5 during the early stage of the immune response, how some CXCR5⁺ pre‐Tfh cells enter the B‐cell follicles and mature further into GC Tfh cells, and how Tfh cells are maintained in the memory compartment. In this review, we discuss recent advances on how antigen and cognate interactions are important for Tfh cell differentiation and long‐term persistence of Tfh cell memory, and how this is relevant to the current understanding of COVID‐19 pathogenesis and the development of potent SARS‐CoV‐2 vaccines.

Lack of Mucosal Cholinergic Innervation Is Associated With Increased Risk of Enterocolitis in Hirschsprung's Disease

BACKGROUND & AIMS

Hirschsprung's disease (HSCR) is a congenital intestinal motility disorder defined by the absence of enteric neuronal cells (ganglia) in the distal gut. The development of HSCR-associated enterocolitis remains a life-threatening complication. Absence of enteric ganglia implicates innervation of acetylcholine-secreting (cholinergic) nerve fibers. Cholinergic signals have been reported to control excessive inflammation, but the impact on HSCR-associated enterocolitis is unknown.

METHODS

We enrolled 44 HSCR patients in a prospective multicenter study and grouped them according to their degree of colonic mucosal acetylcholinesterase-positive innervation into low-fiber and high-fiber patient groups. The fiber phenotype was correlated with the tissue cytokine profile as well as immune cell frequencies using Luminex analysis and fluorescence-activated cell sorting analysis of colonic tissue and immune cells. Using confocal immunofluorescence microscopy, macrophages were identified in close proximity to nerve fibers and characterized by RNA-seq analysis. Microbial dysbiosis was analyzed in colonic tissue using 16S-rDNA gene sequencing. Finally, the fiber phenotype was correlated with postoperative enterocolitis manifestation.

RESULTS

The presence of mucosal nerve fiber innervation correlated with reduced T-helper 17 cytokines and cell frequencies. In high-fiber tissue, macrophages co-localized with nerve fibers and expressed significantly less interleukin 23 than macrophages from low-fiber tissue. HSCR patients lacking mucosal nerve fibers showed microbial dysbiosis and had a higher incidence of postoperative enterocolitis.

CONCLUSIONS

The mucosal fiber phenotype might serve as a prognostic marker for enterocolitis development in HSCR patients and may offer an approach to personalized patient care and new therapeutic options.

Dynamics of the Coreceptor-LCK Interactions during T Cell Development Shape the Self-Reactivity of Peripheral CD4 and CD8 T Cells

Overtly self-reactive T cells are removed during thymic selection. However, it has been recently established that T cell self-reactivity promotes protective immune responses. Apparently, the level of self-reactivity of mature T cells must be tightly balanced. Our mathematical model and experimental data show that the dynamic regulation of CD4- and CD8-LCK coupling establish the self-reactivity of the peripheral T cell pool. The stoichiometry of the interaction between CD8 and LCK, but not between CD4 and LCK, substantially increases upon T cell maturation. As a result, peripheral CD8⁺ T cells are more self-reactive than CD4⁺ T cells. The different levels of self-reactivity of mature CD8⁺ and CD4⁺ T cells likely reflect the unique roles of these subsets in immunity. These results indicate that the evolutionary selection pressure tuned the CD4-LCK and CD8-LCK stoichiometries, as they represent the unique parts of the proximal T cell receptor (TCR) signaling pathway, which differ between CD4⁺ and CD8⁺ T cells.

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Coupling of CD8-LCK but not CD4-LCK increases upon T cell maturation

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Dynamics of coreceptor-LCK coupling stoichiometry establish T cell self-reactivity

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CD8⁺ T cells are more self-reactive than CD4⁺ T cells

Opposing effects of T cell receptor signal strength on CD4 T cells responding to acute versus chronic viral infection

A hallmark of adaptive immunity is CD4 T cells’ ability to differentiate into specialized effectors. A long-standing question is whether T cell receptor (TCR) signal strength can dominantly instruct the development of Th1 and T follicular helper (Tfh) cells across distinct infectious contexts. We characterized the differentiation of murine CD4 TCR transgenic T cells responding to altered peptide ligand lymphocytic choriomeningitis viruses (LCMV) derived from acute and chronic parental strains. We found that TCR signal strength exerts opposite and hierarchical effects on the balance of Th1 and Tfh cells responding to acute versus persistent infection. TCR signal strength correlates positively with Th1 generation during acute but negatively during chronic infection. Weakly activated T cells express lower levels of markers associated with chronic T cell stimulation and may resist functional inactivation. We anticipate that the panel of recombinant viruses described herein will be valuable for investigating a wide range of CD4 T cell responses.

Stepwise Tfh cell differentiation revisited: new advances and long-standing questions

T follicular helper (Tfh) cells play an essential role in germinal center formation and the generation of high-affinity antibodies. Studies have proposed that Tfh cell differentiation is a multi-step process. However, it is still not fully understood how a subset of activated CD4⁺ T cells begin to express CXCR5 during the early stage of the response and, shortly after, how some CXCR5⁺ precursor Tfh (pre-Tfh) cells enter B cell follicles and differentiate further into germinal center Tfh (GC-Tfh) cells while others have a different fate. In this mini-review, we summarize the recent advances surrounding these two aspects of Tfh cell differentiation and discuss related long-standing questions, including Tfh memory.

Immunity to Influenza Infection in Humans

This review discusses the human immune responses to influenza infection with some insights from studies using animal models, such as experimental infection of mice. Recent technological advances in the study of human immune responses have greatly added to our knowledge of the infection and immune responses, and therefore much of the focus is on recent studies that have moved the field forward. We consider the complexity of the adaptive response generated by many sequential encounters through infection and vaccination.

Bcl6-Mediated Transcriptional Regulation of Follicular Helper T cells (TFH)

Follicular helper T cells (T_FH) are essential B cell-help providers in the formation of germinal centers (GCs), affinity maturation of GC B cells, differentiation of high-affinity antibody-producing plasma cells, and production of memory B cells. The transcription factor (TF) B cell lymphoma 6 (Bcl6) is at the center of gene regulation in T_FH biology, including differentiation and function, but how Bcl6 does this, and what additional TFs contribute, remain complex questions. This review focuses on advances in our understanding of Bcl6-mediated gene regulation of T_FH functions, and the modulation of T_FH by other TFs. These advances may have important implications in deciphering how repressor TFs can regulate many immunological cell types. An improved understanding of T_FH biology will likely provide insights into biomedically relevant diseases.

Multiple Levels of Immunological Memory and Their Association with Vaccination

Immunological memory is divided into many levels to counteract the provocations of diverse and ever-changing infections. Fast functions of effector memory and the superposition of both quantitatively and qualitatively plastic anticipatory memory responses together form the walls of protection against pathogens. Here we provide an overview of the role of different B and T cell subsets and their interplay, the parallel and independent functions of the B1, marginal zone B cells, T-independent- and T-dependent B cell responses, as well as functions of central and effector memory T cells, tissue-resident and follicular helper T cells in the memory responses. Age-related limitations in the immunological memory of these cell types in neonates and the elderly are also discussed. We review how certain aspects of immunological memory and the interactions of components can affect the efficacy of vaccines, in order to link our knowledge of immunological memory with the practical application of vaccination.

Modulating the quantity of HIV Env-specific CD4 T cell help promotes rare B cell responses in germinal centers

Immunodominance to nonneutralizing epitopes is a roadblock in designing vaccines against several diseases of high interest. One hypothetical possibility is that limited CD4 T cell help to B cells in a normal germinal center (GC) response results in selective recruitment of abundant, immunodominant B cells. This is a central issue in HIV envelope glycoprotein (Env) vaccine designs, because precursors to broadly neutralizing epitopes are rare. Here, we sought to elucidate whether modulating the quantity of T cell help can influence recruitment and competition of broadly neutralizing antibody precursor B cells at a physiological precursor frequency in response to Env trimer immunization. To do so, two new Env-specific CD4 transgenic (Tg) T cell receptor (TCR) mouse lines were generated, carrying TCR pairs derived from Env-protein immunization. Our results suggest that CD4 T cell help quantitatively regulates early recruitment of rare B cells to GCs.

T resident helper cells promote humoral responses in the lung

Influenza is a deadly and costly infectious disease, even during flu seasons when an effective vaccine has been developed. To improve vaccines against respiratory viruses, a better understanding of the immune response at the site of infection is crucial. After influenza infection, clonally expanded T cells take up permanent residence in the lung, poised to rapidly respond to subsequent infection. Here, we characterized the dynamics and transcriptional regulation of lung-resident CD4⁺ T cells during influenza infection and identified a long-lived, Bcl6-dependent population that we have termed T resident helper (T_RH) cells. T_RH cells arise in the lung independently of lymph node T follicular helper cells but are dependent on B cells, with which they tightly colocalize in inducible bronchus-associated lymphoid tissue (iBALT). Deletion of Bcl6 in CD4⁺ T cells before heterotypic challenge infection resulted in redistribution of CD4⁺ T cells outside of iBALT areas and impaired local antibody production. These results highlight iBALT as a homeostatic niche for T_RH cells and advocate for vaccination strategies that induce T_RH cells in the lung.

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

CD4⁺ memory T cells play an important role in protective immunity and are a key target in vaccine development. Many studies have focused on T central memory (T_cm) cells, whereas the existence and functional significance of long-lived T follicular helper (T_fh) cells are controversial. Here, we show that T_fh cells are highly susceptible to NAD-induced cell death (NICD) during isolation from tissues, leading to their underrepresentation in prior studies. NICD blockade reveals the persistence of abundant T_fh cells with high expression of hallmark T_fh markers to at least 400 days after infection, by which time T_cm cells are no longer found. Using single-cell RNA-seq, we demonstrate that long-lived T_fh cells are transcriptionally distinct from T_cm cells, maintain stemness and self-renewal gene expression, and, in contrast to T_cm cells, are multipotent after recall. At the protein level, we show that folate receptor 4 (FR4) robustly discriminates long-lived T_fh cells from T_cm cells. Unexpectedly, long-lived T_fh cells concurrently express a distinct glycolytic signature similar to trained immune cells, including elevated expression of mTOR-, HIF-1-, and cAMP-regulated genes. Late disruption of glycolysis/ICOS signaling leads to T_fh cell depletion concomitant with decreased splenic plasma cells and circulating antibody titers, demonstrating both unique homeostatic regulation of T_fh and their sustained function during the memory phase of the immune response. These results highlight the metabolic heterogeneity underlying distinct long-lived T cell subsets and establish T_fh cells as an attractive target for the induction of durable adaptive immunity.

Engineering a vector-based pan-Leishmania vaccine for humans: proof of principle

Leishmaniasis is a spectrum of diseases transmitted by sand fly vectors that deposit Leishmania spp. parasites in the host skin during blood feeding. Currently, available treatment options are limited, associated with high toxicity and emerging resistance. Even though a vaccine for human leishmaniasis is considered an achievable goal, to date we still do not have one available, a consequence (amongst other factors) of a lack of pre-clinical to clinical translatability. Pre-exposure to uninfected sand fly bites or immunization with defined sand fly salivary proteins was shown to negatively impact infection. Still, cross-protection reports are rare and dependent on the phylogenetic proximity of the sand fly species, meaning that the applicability of a sand fly saliva-based vaccine will be limited to a defined geography, one parasite species and one form of leishmaniasis. As a proof of principle of a future vector saliva-based pan-Leishmania vaccine, we engineered through a reverse vaccinology approach that maximizes translation to humans, a fusion protein consisting of immunogenic portions of PdSP15 and LJL143, sand fly salivary proteins demonstrated as potential vaccine candidates against cutaneous and visceral leishmaniasis, respectively. The in silico analysis was validated ex vivo, through T cell proliferation experiments, proving that the fusion protein (administered as a DNA vaccine) maintained the immunogenicity of both PdSP15 and LJL143. Additionally, while no significant effect was detected in the context of L. major transmission by P. duboscqi, this DNA vaccine was defined as partially protective, in the context of L. major transmission by L. longipalpis sand flies. Importantly, a high IFNγ response alone was not enough to confer protection, that mainly correlated with low T cell mediated Leishmania-specific IL-4 and IL-10 responses, and consequently with high pro/anti-inflammatory cytokine ratios. Overall our immunogenicity data suggests that to design a potentially safe vector-based pan-Leishmania vaccine, without geographic restrictions and against all forms of leishmaniasis is an achievable goal. This is why we propose our approach as a proof-of principle, perhaps not only applicable to the anti-Leishmania vector-based vaccines’ field, but also to other branches of knowledge that require the design of multi-epitope T cell vaccines with a higher potential for translation.

Strength of tonic T cell receptor signaling instructs T follicular helper cell-fate decisions

T follicular helper (T_FH) cells are critical in adaptive immune responses to pathogens and vaccines; however, what drives the initiation of their developmental program remains unclear. Studies suggest that a T cell antigen receptor (TCR)-dependent mechanism may be responsible for the earliest T_FH cell-fate decision, but a critical aspect of the TCR has been overlooked: tonic TCR signaling. We hypothesized that tonic signaling influences early T_FH cell development. Here, two murine TCR-transgenic CD4⁺ T cells, LLO56 and LLO118, which recognize the same antigenic peptide presented on major histocompatibility complex molecules but experience disparate strengths of tonic signaling, revealed low tonic signaling promotes T_FH cell differentiation. Polyclonal T cells paralleled these findings, with naive Nur77 expression distinguishing T_FH cell potential. Two mouse lines were also generated to both increase and decrease tonic signaling strength, directly establishing an inverse relationship between tonic signaling strength and T_FH cell development. Our findings elucidate a central role for tonic TCR signaling in early T_FH cell-lineage decisions.