In vivo regulation of Bcl6 and T follicular helper cell development

Transcriptional regulation of Tfh dynamics and the formation of immunological synapses

Inside germinal centers (GCs), antigen-specific B cells rely on precise interactions with immune cells and strategic localization between the dark and light zones to clonally expand, undergo affinity maturation, and differentiate into long-lived plasma cells or memory B cells. Follicular helper T (Tfh) cells, the key gatekeepers of GC-dependent humoral immunity, exhibit remarkable dynamic positioning within secondary lymphoid tissues and rely on intercellular interactions with antigen-presenting cells (APCs) during their differentiation and execution of B-cell-facilitating functions within GCs. In this review, we briefly cover the transcriptional regulation of Tfh cell differentiation and function and explore the molecular mechanisms governing Tfh cell motility, their interactions with B cells within GCs, and the impact of their dynamic behavior on humoral responses.

Germinal center versus extrafollicular responses in systemic autoimmunity: Who turns the blade on self?

Spontaneously formed germinal centers (GCs) have been reported in most mouse models of human autoimmune disease and autoimmune patients, and have long been considered a source of somatically-mutated and thus high affinity autoantibodies, but their role in autoimmunity is becoming increasingly controversial, particularly in the context of systemic autoimmune diseases like lupus. On the one hand, there is good evidence that some pathogenic lupus antibodies have acquired somatic mutations that increase affinity for self-antigens. On the other hand, recent studies that have genetically prevented GC formation, suggest that GCs are dispensable for systemic autoimmunity, pointing instead to pathogenic extrafollicular (EF) B-cell responses. Furthermore, several lines of evidence suggest germinal centers may in fact be somewhat protective in the context of autoimmunity. Here we review how some of the conflicting evidence arose, and current views on the role of GCs in autoimmunity, outlining mechanisms by which GC may eliminate self-reactivity. We also discuss recent advances in understanding extrafollicular B cell subsets that participate in autoimmunity.

TGF-β specifies TFH versus TH17 cell fates in murine CD4+ T cells through c-Maf

T follicular helper (TFH) cells are essential for effective antibody responses, but deciphering the intrinsic wiring of mouse TFH cells has long been hampered by the lack of a reliable protocol for their generation in vitro. We report that transforming growth factor-β (TGF-β) induces robust expression of TFH hallmark molecules CXCR5 and Bcl6 in activated mouse CD4+ T cells in vitro. TGF-β-induced mouse CXCR5+ TFH cells are phenotypically, transcriptionally, and functionally similar to in vivo-generated TFH cells and provide critical help to B cells. The study further reveals that TGF-β-induced CXCR5 expression is independent of Bcl6 but requires the transcription factor c-Maf. Classical TGF-β-containing T helper 17 (TH17)-inducing conditions also yield separate CXCR5+ and IL-17A-producing cells, highlighting shared and distinct cell fate trajectories of TFH and TH17 cells. We demonstrate that excess IL-2 in high-density T cell cultures interferes with the TGF-β-induced TFH cell program, that TFH and TH17 cells share a common developmental stage, and that c-Maf acts as a switch factor for TFH versus TH17 cell fates in TGF-β-rich environments in vitro and in vivo.

T-cell help in the germinal center: homing in on the role of IL-21

Interleukin 21 (IL-21) is a pleiotropic cytokine that is overproduced in multiple autoimmune settings. Provision of IL-21 from follicular helper T cells is an important component of T-cell help within germinal centers (GC), and the last few years have seen a resurgence of interest in IL-21 biology in the context of the GC environment. While it has been more than a decade since T cell-derived IL-21 was found to upregulate B-cell expression of the GC master transcription factor B-cell lymphoma 6 (Bcl-6) and to promote GC expansion, several recent studies have collectively delivered significant new insights into how this cytokine shapes GC B-cell selection, proliferation, and fate choice. It is now clear that IL-21 plays an important role in GC zonal polarization by contributing to light zone GC B-cell positive selection for dark zone entry as well as by promoting cyclin D3-dependent dark zone inertial cycling. While it has been established that IL-21 can contribute to the modulation of GC output by aiding the generation of antibody-secreting cells (ASC), recent studies have now revealed how IL-21 signal strength shapes the fate choice between GC cycle re-entry and ASC differentiation in vivo. Both provision of IL-21 and sensitivity to this cytokine are finely tuned within the GC environment, and dysregulation of this pathway in autoimmune settings could alter the threshold for germinal center B-cell selection and differentiation, potentially promoting autoreactive B-cell responses.

Rapid activation of IL-2 receptor signaling by CD301b+ DC-derived IL-2 dictates the outcome of helper T cell differentiation

Effector T helper (Th) cell differentiation is fundamental to functional adaptive immunity. Different subsets of dendritic cells (DCs) preferentially induce different types of Th cells, but the fate instruction mechanism for Th type 2 (Th2) differentiation remains enigmatic, as the critical DC-derived cue has not been clearly identified. Here, we show that CD301b+ DCs, a major Th2-inducing DC subset, drive Th2 differentiation through cognate interaction by 'kick-starting' IL-2 receptor signaling in CD4T cells. Mechanistically, CD40 engagement induces IL-2 production selectively from CD301b+ DCs to maximize CD25 expression in CD4 T cells, which is required specifically for the Th2 fate decision. On the other hand, CD25 in CD301b+ DCs facilitates directed action of IL-2 toward cognate CD4T cells. Furthermore, CD301b+ DC-derived IL-2 skews CD4T cells away from the T follicular helper fate. These results highlight the critical role of DC-intrinsic CD40-IL-2 axis in bifurcation of Th cell fate.

CD4+ T-cell subsets in autoimmune hepatitis: A review

Autoimmune hepatitis (AIH) is a chronic autoimmune liver disease that can lead to hepatocyte destruction, inflammation, liver fibrosis, cirrhosis, and liver failure. The diagnosis of AIH requires the identification of lymphoblast cell interface hepatitis and serum biochemical abnormalities, as well as the exclusion of related diseases. According to different specific autoantibodies, AIH can be divided into AIH-1 and AIH-2. The first-line treatment for AIH is a corticosteroid and azathioprine regimen, and patients with liver failure require liver transplantation. However, the long-term use of corticosteroids has obvious side effects, and patients are prone to relapse after drug withdrawal. Autoimmune diseases are characterized by an imbalance in immune tolerance of self-antigens, activation of autoreactive T cells, overactivity of B cells, and increased production of autoantibodies. CD4+ T cells are key players in adaptive immunity and can secrete cytokines, activate B cells to produce antibodies, and influence the cytotoxicity of CD8+ T cells. According to their characteristics, CD4+ T cells can be divided into different subsets. In this review, we discuss the changes in T helper (Th)1, Th2, Th17, Th9, Th22, regulatory T cell, T follicular helper, and T peripheral helper cells and their related factors in AIH and discuss the therapeutic potential of targeting CD4+ T-cell subsets in AIH.

T follicular helper cells in cancer, tertiary lymphoid structures, and beyond

With the emergence and success of checkpoint blockade immunotherapy, immuno-oncology has primarily focused on CD8 T cells, whose cytotoxic programs directly target tumor cells. However, the limited response rate of current immunotherapy regimens has prompted investigation into other types of tumor-infiltrating immune cells, such as CD4 T cells and B cells, and how they interact with CD8 T cells in a coordinated network. Recent studies have demonstrated the potential therapeutic benefits of CD4 T follicular helper (TFH) cells and B cells in cancer, highlighting the important role of their crosstalk and interactions with other immune cell components in the tumor microenvironment. These interactions also occur in tumor-associated tertiary lymphoid structures (TLS), which resemble secondary lymphoid organs (SLOs) with orchestrated vascular, chemokine, and cellular infrastructures that support the developmental pathways of functional immune cells. In this review, we discuss recent breakthroughs on TFH biology and T cell-B cell interactions in tumor immunology, and their potential as novel therapeutic targets to advance cancer treatment.

Endosome Traffic Modulates Pro-Inflammatory Signal Transduction in CD4+ T Cells-Implications for the Pathogenesis of Systemic Lupus Erythematosus

Endocytic recycling regulates the cell surface receptor composition of the plasma membrane. The surface expression levels of the T cell receptor (TCR), in concert with signal transducing co-receptors, regulate T cell responses, such as proliferation, differentiation, and cytokine production. Altered TCR expression contributes to pro-inflammatory skewing, which is a hallmark of autoimmune diseases, such as systemic lupus erythematosus (SLE), defined by a reduced function of regulatory T cells (Tregs) and the expansion of CD4⁺ helper T (Th) cells. The ensuing secretion of inflammatory cytokines, such as interferon-γ and interleukin (IL)-4, IL-17, IL-21, and IL-23, trigger autoantibody production and tissue infiltration by cells of the adaptive and innate immune system that induce organ damage. Endocytic recycling influences immunological synapse formation by CD4⁺ T lymphocytes, signal transduction from crosslinked surface receptors through recruitment of adaptor molecules, intracellular traffic of organelles, and the generation of metabolites to support growth, cytokine production, and epigenetic control of DNA replication and gene expression in the cell nucleus. This review will delineate checkpoints of endosome traffic that can be targeted for therapeutic interventions in autoimmune and other disease conditions.

A bifurcated role for c-Maf in Th2 and Tfh2 cells during helminth infection

Differences in transcriptomes, transcription factor usage, and function have identified T follicular helper 2 (Tfh2) cells and T helper 2 (Th2) cells as distinct clusters of differentiation 4+",(CD4) T-cell subsets in settings of type-2 inflammation. Although the transcriptional programs driving Th2 cell differentiation and cytokine production are well defined, dependence on these classical Th2 programs by Tfh2 cells is less clear. Using cytokine reporter mice in combination with transcription factor inference analysis, the b-Zip transcription factor c-Maf and its targets were identified as an important regulon in both Th2 and Tfh2 cells. Conditional deletion of c-Maf in T cells confirmed its importance in type-2 cytokine expression by Th2 and Tfh2 cells. However, while c-Maf was not required for Th2-driven helminth clearance or lung eosinophilia, it was required for Tfh2-driven Immunoglobulin E production and germinal center formation. This differential regulation of cell-mediated and humoral immunity by c-Maf was a result of redundant pathways in Th2 cells that were absent in Tfh2 cells, and c-Maf-specific mechanisms in Tfh2 cells that were absent in Th2 cells. Thus, despite shared expression by Tfh2 and Th2 cells, c-Maf serves as a unique regulator of Tfh2-driven humoral hallmarks during type-2 immunity.

AP-1-independent NFAT signaling maintains follicular T cell function in infection and autoimmunity

Coordinated gene expression programs enable development and function of T cell subsets. Follicular helper T (Tfh) cells coordinate humoral immune responses by providing selective and instructive cues to germinal center B cells. Here, we show that AP-1-independent NFAT gene expression, a program associated with hyporesponsive T cell states like anergy or exhaustion, is also a distinguishing feature of Tfh cells. NFAT signaling in Tfh cells, maintained by NFAT2 autoamplification, is required for their survival. ICOS signaling upregulates Bcl6 and induces an AP-1-independent NFAT program in primary T cells. Using lupus-prone mice, we demonstrate that genetic disruption or pharmacologic inhibition of NFAT signaling specifically impacts Tfh cell maintenance and leads to amelioration of autoantibody production and renal injury. Our data provide important conceptual and therapeutic insights into the signaling mechanisms that regulate Tfh cell development and function.

The transcription factor Mef2d regulates B:T synapse-dependent GC-TFH differentiation and IL-21-mediated humoral immunity

Communication between CD4 T cells and cognate B cells is key for the former to fully mature into germinal center-T follicular helper (GC-TFH) cells and for the latter to mount a CD4 T cell-dependent humoral immune response. Although this interaction occurs in a B:T synapse-dependent manner, how CD4 T cells transcriptionally regulate B:T synapse formation remains largely unknown. Here, we report that Mef2d, an isoform of the myocyte enhancer factor 2 (Mef2) transcription factor family, is a critical regulator of this process. In CD4 T cells, Mef2d negatively regulates expression of Sh2d1a, which encodes SLAM-associated protein (SAP), a critical regulator of B:T synapses. We found that Mef2d regulates Sh2d1a expression via DNA binding-dependent transcriptional repression, inhibiting SAP-dependent B:T synapse formation and preventing antigen-specific CD4 T cells from differentiating into GC-TFH cells. Mef2d also impeded IL-21 production by CD4 T cells, an important B cell help signaling molecule, via direct repression of the Il21 gene. In contrast, CD4 T cell-specific disruption of Mef2d led to a substantial increase in GC-TFH differentiation in response to protein immunization, concurrent with enhanced SAP expression. MEF2D mRNA expression inversely correlates with human systemic lupus erythematosus (SLE) patient autoimmune parameters, including circulating TFH-like cell frequencies, autoantibodies, and SLEDAI scores. These findings highlight Mef2d as a pivotal rheostat in CD4 T cells for controlling GC formation and antibody production by B cells.

mTORC2 contributes to systemic autoimmunity

The development of many systemic autoimmune diseases, including systemic lupus erythematosus, is associated with overactivation of the type I interferon (IFN) pathway, lymphopenia and increased follicular helper T (Tfh)-cell differentiation. However, the cellular and molecular mechanisms underlying these immunological perturbations remain incompletely understood. Here, we show that the mechanistic target of rapamycin complex 2 (mTORC2) promotes Tfh differentiation and disrupts Treg homeostasis. Inactivation of mTORC2 in total T cells, but not in Tregs, greatly ameliorated the immunopathology in a systemic autoimmunity mouse model. This was associated with reduced Tfh differentiation, B-cell activation, and reduced T-cell glucose metabolism. Finally, we show that type I IFN can synergize with TCR ligation to activate mTORC2 in T cells, which partially contributes to T-cell lymphopenia. These data indicate that mTORC2 may act as downstream of type I IFN, TCR and costimulatory receptor ICOS, to promote glucose metabolism, Tfh differentiation, and T-cell lymphopenia, but not to suppress Treg function in systemic autoimmunity. Our results suggest that mTORC2 might be a rational target for systemic autoimmunity treatment.

CXCR5+PD-1++ CD4+ T cells colonize infant intestines early in life and promote B cell maturation

Gastrointestinal infections are a major cause for serious clinical complications in infants. The induction of antibody responses by B cells is critical for protective immunity against infections and requires CXCR5+PD-1++ CD4+ T cells (TFH cells). We investigated the ontogeny of CXCR5+PD-1++ CD4+ T cells in human intestines. While CXCR5+PD-1++ CD4+ T cells were absent in fetal intestines, CXCR5+PD-1++ CD4+ T cells increased after birth and were abundant in infant intestines, resulting in significant higher numbers compared to adults. These findings were supported by scRNAseq analyses, showing increased frequencies of CD4+ T cells with a TFH gene signature in infant intestines compared to blood. Co-cultures of autologous infant intestinal CXCR5+PD-1+/-CD4+ T cells with B cells further demonstrated that infant intestinal TFH cells were able to effectively promote class switching and antibody production by B cells. Taken together, we demonstrate that functional TFH cells are numerous in infant intestines, making them a promising target for oral pediatric vaccine strategies.

Precise diagnosis and targeted therapy of nodal T-follicular helper cell lymphoma (T-FHCL)

Nodal T-follicular helper cell lymphoma (T-FHCL) derived from T-follicular helper (Tfh) cell falls into a heterogeneous category of peripheral T-cell lymphoma (PTCL). Due to the limited number of therapeutic regimens and limited first-line efficacy, T-FHCL has a poor prognosis, and there is an urgent need for effective targeted therapies. With advancements in sequencing technologies, especially single-cell sequencing and next-generation sequencing, more specific genetic aberrations characteristic of T-FHCL can be discovered, allowing for precise molecular diagnosis and specific research on novel agents. Many biomarker-targeting agents, used either alone or in combination, have been tested, and they have generally enhanced the therapeutic outcomes of T-FHCL. Histone deacetylase inhibitors achieve significant clinical benefits in the treatment of T-FHCL, especially in combination therapy. Chimeric antigen receptor T-cell (CAR-T-cell) immunotherapies, hematopoietic stem cell transplantation, and other potential agents merit further study.

T cells in ocular autoimmune uveitis: Pathways and therapeutic approaches

Autoimmune uveitis is a non-infectious intraocular condition that affects the uveal tract of the eye and threatens vision if not treated properly. Increasing evidence suggests that activated CD4⁺ T cells are associated with progressive and permanent destruction of photoreceptors in ocular autoimmune diseases. As such, the purpose of this review is to offer an overview of the role of CD4⁺ T cells in autoimmune uveitis as well as a justification for the current development and assessment of innovative autoimmune uveitis medications targeting CD4⁺ T cells. With an emphasis on T helper (Th)17, Th1, and Th2 cells, follicular helper CD4⁺ T cells, and regulatory T cells, this review presents a summary of recent research related to the pathways and signaling that encourage CD4⁺ T cells to develop into specialized effector cells. We also describe immunotherapeutic approaches based on CD4⁺ T cell subsets and their potential as therapeutic agents for autoimmune disorders.

A Bcl6 Intronic Element Regulates T Follicular Helper Cell Differentiation

In response to an intracellular infectious agent, the immune system produces a specific cellular response as well as a T cell–dependent Ab response. Precursor T cells differentiate into effector T cells, including Th1 cells, and T follicular helper (TFH) cells. The latter cooperate with B cells to form germinal centers and induce the formation of Ab-forming plasmacytes. One major focal point for control of T cell differentiation is the transcription factor BCL6. In this study, we demonstrated that the Bcl6 gene is regulated by FOXO1-binding, cis-acting sequences located in a highly conserved region of the first Bcl6 intron. In both mouse and human T cells, deletion of the tandem FOXO1 binding sites increased the expression of BCL6 and enhanced the proportion of TFH cells. These results reveal a fundamental control point for cellular versus humoral immunity.

Type I Interferons Promote Germinal Centers Through B Cell Intrinsic Signaling and Dendritic Cell Dependent Th1 and Tfh Cell Lineages

Type I interferons (IFNs) are essential for antiviral immunity, appear to represent a key component of mRNA vaccine-adjuvanticity, and correlate with severity of systemic autoimmune disease. Relevant to all, type I IFNs can enhance germinal center (GC) B cell responses but underlying signaling pathways are incompletely understood. Here, we demonstrate that a succinct type I IFN response promotes GC formation and associated IgG subclass distribution primarily through signaling in cDCs and B cells. Type I IFN signaling in cDCs, distinct from cDC1, stimulates development of separable Tfh and Th1 cell subsets. However, Th cell-derived IFN-γ induces T-bet expression and IgG2c isotype switching in B cells prior to this bifurcation and has no evident effects once GCs and bona fide Tfh cells developed. This pathway acts in synergy with early B cell-intrinsic type I IFN signaling, which reinforces T-bet expression in B cells and leads to a selective amplification of the IgG2c+ GC B cell response. Despite the strong Th1 polarizing effect of type I IFNs, the Tfh cell subset develops into IL-4 producing cells that control the overall magnitude of the GCs and promote generation of IgG1+ GC B cells. Thus, type I IFNs act on B cells and cDCs to drive GC formation and to coordinate IgG subclass distribution through divergent Th1 and Tfh cell-dependent pathways.

T cell depletion increases humoral response by favoring T follicular helper cells expansion

Antibody-mediated rejection is a major cause of long-term graft loss in kidney transplant patients. T follicular helper (Tfh) cells are crucial for assisting B cell differentiation and are required for an efficient antibody response. Anti-thymocyte globulin (ATG) is a widely used lymphocyte-depleting induction therapy. However, less is known about how ATG affects Tfh cell development and donor-specific antibody (DSA) formation. We observed an increase in circulating Tfh cells at 6 months after kidney transplant in patients who received ATG. Using an NP-OVA immunization model, we found that ATG-treated mice had a higher percentage of Tfh cells, germinal center B cells, and higher titers of antigen-specific antibodies compared to controls. ATG-treated animals had lower levels of IL-2, a known Bcl-6 repressor, but higher levels of IL-21, pSTAT3 and Bcl-6, favoring Tfh differentiation. In a mouse kidney transplant model, ATG-treated recipients showed an increase in Tfh cells, DSA and C4d staining in the allograft. Although ATG was effective in depleting T cells, it favored the expansion of Tfh cells following depletion. Concomitant use of IL-2, tacrolimus, or rapamycin with ATG was essential to control Tfh cell expansion. In summary, ATG depletion favors Tfh expansion, enhancing antibody-mediated response.

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.

Lipid nanoparticles enhance the efficacy of mRNA and protein subunit vaccines by inducing robust T follicular helper cell and humoral responses

Adjuvants are critical for improving the quality and magnitude of adaptive immune responses to vaccination. Lipid nanoparticle (LNP)-encapsulated nucleoside-modified mRNA vaccines have shown great efficacy against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the mechanism of action of this vaccine platform is not well-characterized. Using influenza virus and SARS-CoV-2 mRNA and protein subunit vaccines, we demonstrated that our LNP formulation has intrinsic adjuvant activity that promotes induction of strong T follicular helper cell, germinal center B cell, long-lived plasma cell, and memory B cell responses that are associated with durable and protective antibodies in mice. Comparative experiments demonstrated that this LNP formulation outperformed a widely used MF59-like adjuvant, AddaVax. The adjuvant activity of the LNP relies on the ionizable lipid component and on IL-6 cytokine induction but not on MyD88- or MAVS-dependent sensing of LNPs. Our study identified LNPs as a versatile adjuvant that enhances the efficacy of traditional and next-generation vaccine platforms.

Neoantigen-driven B cell and CD4 T follicular helper cell collaboration promotes anti-tumor CD8 T cell responses

CD4 T follicular helper (TFH) cells support B cells, which are critical for germinal center (GC) formation, but the importance of TFH-B cell interactions in cancer is unclear. We found enrichment of TFH cell transcriptional signature correlates with GC B cell signature and with prolonged survival in individuals with lung adenocarcinoma (LUAD). We further developed a murine LUAD model in which tumor cells express B cell- and T cell-recognized neoantigens. Interactions between tumor-specific TFH and GC B cells, as well as interleukin (IL)-21 primarily produced by TFH cells, are necessary for tumor control and effector CD8 T cell function. Development of TFH cells requires B cells and B cell-recognized neoantigens. Thus, tumor neoantigens can regulate the fate of tumor-specific CD4 T cells by facilitating their interactions with tumor-specific B cells, which in turn promote anti-tumor immunity by enhancing CD8 T cell effector functions.

Enhanced IL-2 in early life limits the development of TFH and protective antiviral immunity

T follicular helper cell (TFH)-dependent antibody responses are critical for long-term immunity. Antibody responses are diminished in early life, limiting long-term protective immunity and allowing prolonged or recurrent infection, which may be important for viral lung infections that are highly prevalent in infancy. In a murine model using respiratory syncytial virus (RSV), we show that TFH and the high-affinity antibody production they promote are vital for preventing disease on RSV reinfection. Following a secondary RSV infection, TFH-deficient mice had significantly exacerbated disease characterized by delayed viral clearance, increased weight loss, and immunopathology. TFH generation in early life was compromised by heightened IL-2 and STAT5 signaling in differentiating naive T cells. Neutralization of IL-2 during early-life RSV infection resulted in a TFH-dependent increase in antibody-mediated immunity and was sufficient to limit disease severity upon reinfection. These data demonstrate the importance of TFH in protection against recurrent RSV infection and highlight a mechanism by which this is suppressed in early life.

T cell activation niches-Optimizing T cell effector function in inflamed and infected tissues

Successful immunity to infection, malignancy, and tissue damage requires the coordinated recruitment of numerous immune cell subsets to target tissues. Once within the target tissue, effector T cells rely on local chemotactic cues and structural cues from the tissue matrix to navigate the tissue, interact with antigen-presenting cells, and release effector cytokines. This highly dynamic process has been "caught on camera" in situ by intravital multiphoton imaging. Initial studies revealed a surprising randomness to the pattern of T cell migration through inflamed tissues, behavior thought to facilitate chance encounters with rare antigen-bearing cells. Subsequent tissue-wide visualization has uncovered a high degree of spatial preference when it comes to T cell activation. Here, we discuss the basic tenants of a successful effector T cell activation niche, taking cues from the dynamics of Tfh positioning in the lymph node germinal center. In peripheral tissues, steady-state microanatomical organization may direct the location of "pop-up" de novo activation niches, often observed as perivascular clusters, that support early effector T cell activation. These perivascular activation niches appear to be regulated by site-specific chemokines that coordinate the recruitment of dendritic cells and other innate cells for local T cell activation, survival, and optimized effector function.

Follicular T cells are clonally and transcriptionally distinct in B cell-driven mouse autoimmune disease

Pathogenic autoantibodies contribute to tissue damage and clinical decline in autoimmune disease. Follicular T cells are central regulators of germinal centers, although their contribution to autoantibody-mediated disease remains unclear. Here we perform single cell RNA and T cell receptor (TCR) sequencing of follicular T cells in a mouse model of autoantibody-mediated disease, allowing for analyses of paired transcriptomes and unbiased TCRαβ repertoires at single cell resolution. A minority of clonotypes are preferentially shared amongst autoimmune follicular T cells and clonotypic expansion is associated with differential gene signatures in autoimmune disease. Antigen prediction using algorithmic and machine learning approaches indicates convergence towards shared specificities between non-autoimmune and autoimmune follicular T cells. However, differential autoimmune transcriptional signatures are preserved even amongst follicular T cells with shared predicted specificities. These results demonstrate that follicular T cells are phenotypically distinct in B cell-driven autoimmune disease, providing potential therapeutic targets to modulate autoantibody development.

Myeloid Mineralocorticoid Receptor Transcriptionally Regulates P-Selectin Glycoprotein Ligand-1 and Promotes Monocyte Trafficking and Atherosclerosis

Objective

MR (mineralocorticoid receptor) activation associates with increased risk of cardiovascular ischemia while MR inhibition reduces cardiovascular-related mortality and plaque inflammation in mouse atherosclerosis. MR in myeloid cells (My-MR) promotes inflammatory cell infiltration into injured tissues and atherosclerotic plaque inflammation by unclear mechanisms. Here, we examined the role of My-MR in leukocyte trafficking and the impact of sex.

Approach and Results

We confirm in vivo that My-MR deletion (My-MR-KO) in ApoE-KO mice decreased plaque size. Flow cytometry revealed fewer plaque macrophages with My-MR-KO. By intravital microscopy, My-MR-KO significantly attenuated monocyte slow-rolling and adhesion to mesenteric vessels and decreased peritoneal infiltration of myeloid cells in response to inflammatory stimuli in male but not female mice. My-MR-KO mice had significantly less PSGL1 (P-selectin glycoprotein ligand 1) mRNA in peritoneal macrophages and surface PSGL1 protein on circulating monocytes in males. In vitro, MR activation with aldosterone significantly increased PSGL1 mRNA only in monocytes from MR-intact males. Similarly, aldosterone induced, and MR antagonist spironolactone inhibited, PSGL1 expression in human U937 monocytes. Mechanistically, aldosterone stimulated MR binding to a predicted MR response element in intron-1 of the PSGL1 gene by ChIP-qPCR. Reporter assays demonstrated that this PSGL1 MR response element is necessary and sufficient for aldosterone-activated, MR-dependent transcriptional activity.

Conclusions

These data identify PSGL1 as a My-MR target gene that drives leukocyte trafficking to enhance atherosclerotic plaque inflammation. These novel and sexually dimorphic findings provide insight into increased ischemia risk with MR activation, cardiovascular protection in women, and the role of MR in atherosclerosis and tissue inflammation.

Diabetes mellitus exacerbates experimental autoimmune myasthenia gravis via modulating both adaptive and innate immunity

BACKGROUND

Diabetes mellitus (DM) is a common concomitant disease of late-onset myasthenia gravis (MG). However, the impacts of DM on the progression of late-onset MG were unclear.

METHODS

In this study, we examined the immune response in experimental autoimmune myasthenia gravis (EAMG) rats with DM or not. The phenotype and function of the spleen and lymph nodes were determined by flow cytometry. The serum antibodies, Tfh cells, and germinal center B cells were determined by ELISA and flow cytometry. The roles of advanced glycation end products (AGEs) in regulating Tfh cells were further explored in vitro by co-culture assays.

RESULTS

Our results indicated clinical scores of EAMG rats were worse in diabetes rats compared to control, which was due to the increased production of anti-R97-116 antibody and antibody-secreting cells. Furthermore, diabetes induced a significant upregulation of Tfh cells and the subtypes of Tfh1 and Tfh17 cells to provide assistance for antibody production. The total percentages of B cells were increased with an activated statue of improved expression of costimulatory molecules CD80 and CD86. We found CD4⁺ T-cell differentiation was shifted from Treg cells towards Th1/Th17 in the DM+EAMG group compared to the EAMG group. In addition, in innate immunity, diabetic EAMG rats displayed more CXCR5 expression on NK cells. However, the expression of CXCR5 on NKT cells was down-regulated with the increased percentages of NKT cells in the DM+EAMG group. Ex vivo studies further indicated that Tfh cells were upregulated by AGEs instead of hyperglycemia. The upregulation was mediated by the existence of B cells, the mechanism of which might be attributed the elevated molecule CD40 on B cells.

CONCLUSIONS

Diabetes promoted both adaptive and innate immunity and exacerbated clinical symptoms in EAMG rats. Considering the effect of diabetes, therapy in reducing blood glucose levels in MG patients might improve clinical efficacy through suppressing the both innate and adaptive immune responses. Additional studies are needed to confirm the effect of glucose or AGEs reduction to seek treatment for MG.

T follicular helper cells: Their development and importance in the context of helminthiasis

The development of T follicular helper cells (Tfh) is a multifactorial process that occurs in multiple stages. After their activation the Tfh cells interact with the B cells to complete their differentiation. During this process, the Tfh cells begin to express canonical molecules such as the transcription factor B-cell lymphoma 6 protein, the CXC chemokine receptors type 5, and the inducible T-cell costimulator, as well as secreting other molecules such as IL-21. This whole process is regulated positively and negatively by several factors so that the best response is offered in the face of diseases of various origins, among them helminthiasis. In this context, the role of circulating Tfh, IL-4 and IgG subtypes is essential for an effective response against these pathogens. In this review, the migration process and the differentiation of Tfh, the regulation, their cell subtypes and the role of Tfh in the context of helminth infections will be addressed.

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.

Re-Programming Autoreactive T Cells Into T-Regulatory Type 1 Cells for the Treatment of Autoimmunity

Systemic delivery of peptide-major histocompatibility complex (pMHC) class II-based nanomedicines can re-program cognate autoantigen-experienced CD4+ T cells into disease-suppressing T-regulatory type 1 (TR1)-like cells. In turn, these TR1-like cells trigger the formation of complex regulatory cell networks that can effectively suppress organ-specific autoimmunity without impairing normal immunity. In this review, we summarize our current understanding of the transcriptional, phenotypic and functional make up of TR1-like cells as described in the literature. The true identity and direct precursors of these cells remain unclear, in particular whether TR1-like cells comprise a single terminally-differentiated lymphocyte population with distinct transcriptional and epigenetic features, or a collection of phenotypically different subsets sharing key regulatory properties. We propose that detailed transcriptional and epigenetic characterization of homogeneous pools of TR1-like cells will unravel this conundrum.

Germinal Center and Extrafollicular B Cell Responses in Vaccination, Immunity, and Autoimmunity

Activated B cells participate in either extrafollicular (EF) or germinal center (GC) responses. Canonical responses are composed of a short wave of plasmablasts (PBs) arising from EF sites, followed by GC producing somatically mutated memory B cells (MBC) and long-lived plasma cells. However, somatic hypermutation (SHM) and affinity maturation can take place at both sites, and a substantial fraction of MBC are produced prior to GC formation. Infection responses range from GC responses that persist for months to persistent EF responses with dominant suppression of GCs. Here, we review the current understanding of the functional output of EF and GC responses and the molecular switches promoting them. We discuss the signals that regulate the magnitude and duration of these responses, and outline gaps in knowledge and important areas of inquiry. Understanding such molecular switches will be critical for vaccine development, interpretation of vaccine efficacy and the treatment for autoimmune diseases.

A review of signaling and transcriptional control in T follicular helper cell differentiation

T follicular helper (Tfh) cells are a critical component of adaptive immunity and assist in optimal Ab-mediated defense. Multiple effector functions of Tfh support germinal center B cell survival, Ab class switching, and plasma cell maturation. In the past 2 decades, the phenotype and functional characteristics of GC Tfh have been clarified allowing for robust studies of the Th subset including activation signals and environmental cues controlling Tfh differentiation and migration during an immune response. A unique, 2-step differentiation process of Tfh has been proposed but the mechanisms underlying transition between unstable Tfh precursors and functional mature Tfh remain elusive. Likewise, newly identified transcriptional regulators of Tfh development have not yet been incorporated into our understanding of how these cells might function in disease. Here, we review the signals and downstream transcription factors that shape Tfh differentiation including what is known about the epigenetic processes that maintain Tfh identity. It is proposed that further evaluation of the stepwise differentiation pattern of Tfh will yield greater insights into how these cells become dysregulated in autoimmunity.

B Cell Response to Vaccination

As one of the most important weapons against infectious diseases, vaccines have saved countless lives since their first use in the late eighteenth century. Antibodies produced by effector B cells upon vaccination play a critical role in mediating protection. The past several decades of research have led to a revolution in our understanding of B cell response to vaccination. Vaccines against SARS-CoV-2 coronavirus were developed at an unprecedented speed to power our global fight against COVID-19 pandemic. Nevertheless, we still face many challenges in the development of vaccines against many other deadly viruses, such as human immunodeficiency virus (HIV) and influenza virus. In this review, we summarize the latest findings on B cell response to vaccination and pathogen infection. We also discuss the current challenges in the field and the potential strategies targeting B cell response to improve vaccine efficacy.Key abbreviations box: BCR: B cell receptor; bNAb: broadly neutralizing antibody; DC: dendritic cells; DZ: dark zone; EF response: extrafollicular response; FDC: follicular dendritic cell; GC: germinal center; HIV: human immunodeficiency virus; IC: immune complex; LLPC: long-lived plasma cell; LZ: light zone; MBC: memory B cell; SLPB: short-lived plasmablast; TFH: T follicular helper cells; TLR: Toll-like receptor.

Dendritic Cell Regulation of T Helper Cells

As the professional antigen-presenting cells of the immune system, dendritic cells (DCs) sense the microenvironment and shape the ensuing adaptive immune response. DCs can induce both immune activation and immune tolerance according to the peripheral cues. Recent work has established that DCs comprise several phenotypically and functionally heterogeneous subsets that differentially regulate T lymphocyte differentiation. This review summarizes both mouse and human DC subset phenotypes, development, diversification, and function. We focus on advances in our understanding of how different DC subsets regulate distinct CD4⁺ T helper (Th) cell differentiation outcomes, including Th1, Th2, Th17, T follicular helper, and T regulatory cells. We review DC subset intrinsic properties, local tissue microenvironments, and other immune cells that together determine Th cell differentiation during homeostasis and inflammation.

Gingival transcriptomics of follicular T cell footprints in progressing periodontitis

Follicular helper T cells (Tfh) cells have been identified in the circulation and in tertiary lymphoid structures in chronic inflammation. Gingival tissues with periodontitis reflect chronic inflammation, so genomic footprints of Tfh cells should occur in these tissues and may differ related to aging effects. Macaca mulatta were used in a ligature-induced periodontitis model [adult group (aged 12-23 years); young group (aged 3-7 years)]. Gingival tissue and subgingival microbiome samples were obtained at matched healthy ligature-induced disease and clinical resolution sites. Microarray analysis examined Tfh genes (n = 54) related to microbiome characteristics documented using 16S MiSeq. An increase in the major transcription factor of Tfh cells, BCL6, was found with disease in both adult and young animals, while master transcription markers of other T cell subsets were either decreased or showed minimal change. Multiple Tfh-related genes, including surface receptors and transcription factors, were also significantly increased during disease. Specific microbiome patterns were significantly associated with profiles indicative of an increased presence/function of Tfh cells. Importantly, unique microbial complexes showed distinctive patterns of interaction with Tfh genes differing in health and disease and with the age of the animals. An increase in Tfh cell responsiveness occurred in the progression of periodontitis, affected by age and related to specific microbial complexes in the oral microbiome. The capacity of gingival Tfh cells to contribute to localized B cell activation and active antibody responses, including affinity maturation, may be critical for controlling periodontal lesions and contributing to limiting and/or resolving the lesions.

Multi-Source Pathways of T Follicular Helper Cell Differentiation

T follicular helper (Tfh) cells participate in humoral immune by promoting inflammation and aiding B cells survival, proliferation, maturation, and generation autoantibodies. The plasticity of Tfh cells enables the immune system to adjust the direction of differentiation according to the degree of the immune response, regulate the germinal center (GC) response and maintain homeostasis. Tfh differentiation involves several signaling factors, including multiple cytokines, receptors, transcription factors and genes. The signal transducer and activator of transcription (STAT) family signaling pathways are crucial for Tfh formation. However, because of the multi-factorial and multi-stage features of Tfh differentiation, every STAT member plays a role in Tfh differentiation, but is not completely depended on. With the gradual recognition of different Tfh subsets (Tfh1, Tfh2, Tfh17), the process of Tfh differentiation can no longer be explained by straight-line derivation models. In this review, we summarize the roles of different STATs in mediating Tfh subsets, analyze the contributions of mutual restraint and cooperation among cytokine-STAT signals to terminal Tfh differentiation, and clarify the multi-source pathways of Tfh differentiation with a three-dimensional illustration.

PSGL-1 Immune Checkpoint Inhibition for CD4+ T Cell Cancer Immunotherapy

Immune checkpoint inhibition targeting T cells has shown tremendous promise in the treatment of many cancer types and are now standard therapies for patients. While standard therapies have focused on PD-1 and CTLA-4 blockade, additional immune checkpoints have shown promise in promoting anti-tumor immunity. PSGL-1, primarily known for its role in cellular migration, has also been shown to function as a negative regulator of CD4+ T cells in numerous disease settings including cancer. PSGL-1 is highly expressed on T cells and can engage numerous ligands that impact signaling pathways, which may modulate CD4+ T cell differentiation and function. PSGL-1 engagement in the tumor microenvironment may promote CD4+ T cell exhaustion pathways that favor tumor growth. Here we highlight that blocking the PSGL-1 pathway on CD4+ T cells may represent a new cancer therapy approach to eradicate tumors.

Functional differentiation and regulation of follicular T helper cells in inflammation and autoimmunity

Follicular T helper (T_FH ) cells are specialized T cells that support B cells, which are essential for humoral immunity. T_FH cells express the transcription factor B-cell lymphoma 6 (Bcl-6), chemokine (C-X-C motif) receptor (CXCR) 5, the surface receptors programmed cell death protein 1 (PD-1) and inducible T-cell costimulator (ICOS), the cytokine IL-21 and other molecules. The activation, proliferation and differentiation of T_FH cells are closely related to dynamic changes in cellular metabolism. In this review, we summarize the progress made in understanding the development and functional differentiation of T_FH cells. Specifically, we focus on the regulatory mechanisms of T_FH cell functional differentiation, including regulatory signalling pathways and the metabolic regulatory mechanisms of T_FH cells. In addition, T_FH cells are closely related to immune-associated diseases, including infections, autoimmune diseases and cancers.

T-B Lymphocyte Interactions Promote Type 1 Diabetes Independently of SLAM-Associated Protein

Signaling lymphocytic activation molecule-associated protein (SAP), a critical intracellular signaling molecule for T-B lymphocyte interactions, drives T follicular helper (Tfh) cell development in germinal centers (GCs). High-affinity islet autoantibodies predict type 1 diabetes (T1D) but do not cause β cell destruction. This paradox intimates Tfh cells as key pathologic effectors, consistent with an observed Tfh signature in T1D. To understand how fully developed Tfh (GC Tfh) contribute to different autoimmune processes, we investigated the role of SAP in T1D and autoantibody-mediated arthritis. Whereas spontaneous arthritis depended on SAP in the autoantibody-mediated K/BxN model, organized insulitis and diabetes onset were unabated, despite a blocked anti-insulin vaccine response in SAP-deficient NOD mice. GC Tfh and GC B cell development were blocked by loss of SAP in K/BxN mice. In contrast, although GC B cell formation was markedly reduced in SAP-deficient NOD mice, T cells with a GC Tfh phenotype were found at disease sites. CXCR3⁺ CCR6^- (Tfh1) subset bias was observed among GC Tfh cells infiltrating the pancreas of NOD mice, which was enhanced by loss of SAP NOD T cells override SAP requirement to undergo activation and proliferation in response to Ag presentation, demonstrating the potential for productive cognate T-B lymphocyte interactions in T1D-prone mice. We find that SAP is essential when autoantibody-driven immune complexes promote inflammation but is not required for effective organ-specific autoimmune attack. Thus, Tfh induced in classic GC reactions are dispensable for T1D, but the autoimmune process in the NOD model retains pathogenic Tfh without SAP.

Tfr-Tfh index: A new predicator for recurrence of hepatocellular carcinoma patients with HBV infection after curative resection

BACKGROUND

T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells were newly identified as the subsets of cluster of CD4+ T cells. As major components of human immune system, they were found in tumor microenvironment and reported to play vital roles in the progression of cancer. But their clinical significance in Hepatocellular carcinoma (HCC) was not elucidated. Thus, this research aimed to investigate their prognostic value in HCC.

MATERIALS AND METHODS

A total of 210 subjects (including 110 HCC patients, 50 chronic hepatitis patients and 50 healthy individuals) were enrolled in the research. Tfh, Tfr cells and Treg cells from peripheral blood were measured by flow cytometry. Receiver operating characteristic (ROC) curves were used to evaluate the diagnostic performance of Tfr-Tfh Index (TTI) in early HCC and relapse status. Its further prognostic valve was assessed by Kaplan-Meier survival estimate and log rank tests.

RESULTS

Tfh cells, Tfr cells, Treg cells and TTI were all higher in HCC patients than in chronic hepatitis patients and healthy control. TTI was found to have positive correlation with the load of HBV. The AUC of TTI for early HCC and relapse status was better than other clinical indices in HBV positive patients. An optimal cutoff point for the TTI stratified the HCC patients into high (>21.96) and low index (≤21.96) groups. High TTI was significantly correlated with recurrence. Univariate and multivariate analyses revealed TTI could be a predictor for recurrence. Moreover, it retained prognostic performance for patients with lower recurrence risk.

CONCLUSION

Our research showed that TTI could be a promising indicator for early recurrence in HCC patients with HBV infection.

Continued Bcl6 Expression Prevents the Transdifferentiation of Established Tfh Cells into Th1 Cells during Acute Viral Infection

T follicular helper (Tfh) cells are crucial for the establishment of germinal centers (GCs) and potent antibody responses. Nevertheless, the T cell-intrinsic factors that are required for the maintenance of already-established Tfh cells and GCs remain largely unknown. Here, we use temporally guided gene ablation in CD4⁺ T cells to dissect the contributions of the Tfh-associated chemokine receptor CXCR5 and the transcription factor Bcl6. Induced ablation of Cxcr5 has minor effects on the function of established Tfh cells, and Cxcr5-ablated cells still exhibit most of the features of CXCR5⁺ Tfh cells. In contrast, continued Bcl6 expression is critical to maintain the GC Tfh cell phenotype and also the GC reaction. Importantly, Bcl6 ablation during acute viral infection results in the transdifferentiation of established Tfh into Th1 cells, thus highlighting the plasticity of Tfh cells. These findings have implications for strategies that boost or restrain Tfh cells and GCs in health and disease.

Transcription tipping points for T follicular helper cell and T-helper 1 cell fate commitment

During viral infection, immune cells coordinate the induction of inflammatory responses that clear infection and humoral responses that promote protection. CD4⁺ T-cell differentiation sits at the center of this axis. Differentiation toward T-helper 1 (Th1) cells mediates inflammation and pathogen clearance, while T follicular helper (Tfh) cells facilitate germinal center (GC) reactions for the generation of high-affinity antibodies and immune memory. While Th1 and Tfh differentiation occurs in parallel, these CD4⁺ T-cell identities are mutually exclusive, and progression toward these ends is determined via the upregulation of T-bet and Bcl6, respectively. These lineage-defining transcription factors act in concert with multiple networks of transcriptional regulators that tip the T-bet and Bcl6 axis in CD4⁺ T-cell progenitors to either a Th1 or Tfh fate. It is now clear that these transcriptional networks are guided by cytokine cues that are not only varied between distinct viral infections but also dynamically altered throughout the duration of infection. Thus, multiple intrinsic and extrinsic factors combine to specify the fate, plasticity, and function of Th1 and Tfh cells during infection. Here, we review the current information on the mode of action of the lineage-defining transcription factors Bcl6 and T-bet and how they act individually and in complex to govern CD4⁺ T-cell ontogeny. Furthermore, we outline the multifaceted transcriptional regulatory networks that act upstream and downstream of Bcl6 and T-bet to tip the differentiation equilibrium toward either a Tfh or Th1 fate and how these are impacted by dynamic inflammatory cues.

B cells are sufficient to prime the dominant CD4+ Tfh response to Plasmodium infection

Arroyo and Pepper demonstrate that interactions with B cells, not dendritic cells, are required for the priming of the CD4⁺ T cell response during Plasmodium infection. This results in a Tfh-biased response as reported by others in both mice and humans.

CD4⁺ T follicular helper (Tfh) cells dominate the acute response to a blood-stage Plasmodium infection and provide signals to direct B cell differentiation and protective antibody expression. We studied antigen-specific CD4⁺ Tfh cells responding to Plasmodium infection in order to understand the generation and maintenance of the Tfh response. We discovered that a dominant, phenotypically stable, CXCR5⁺ Tfh population emerges within the first 4 d of infection and results in a CXCR5⁺ CCR7⁺ Tfh/central memory T cell response that persists well after parasite clearance. We also found that CD4⁺ T cell priming by B cells was both necessary and sufficient to generate this Tfh-dominant response, whereas priming by conventional dendritic cells was dispensable. This study provides important insights into the development of CD4⁺ Tfh cells during Plasmodium infection and highlights the heterogeneity of antigen-presenting cells involved in CD4⁺ T cell priming.

CD8+ T cells are crucial for humoral immunity establishment by SA14-14-2 live attenuated Japanese encephalitis vaccine in mice

The live attenuated SA14-14-2 Japanese encephalitis (JE) vaccine is a historical vaccine that protects against JE. Despite its extensive use, the mechanism of protective immunity conferred by the SA14-14-2 vaccine is not well established. Here, we used mouse models to understand the mechanism of the development of humoral immunity against the vaccine. The vaccine induces robust GC responses within a week postimmunization. In lethal virus challenge, we show that CD4⁺ T cells alone, but not CD8⁺ T cells, are sufficient to confer vaccine-mediated protection. However, the CD4-mediated protection was potentiated in the presence of vaccine-primed CD8⁺ T cells. Employing CD8-deficient mice, we show that both the protective traits of CD4⁺ T cells and the quality of antibody response to the vaccine are impaired in absence of CD8⁺ T cells. We further demonstrate that the poor protective immune response induced by the vaccine in absence of CD8⁺ T cells is mainly due to the impaired differentiation and function of follicular Th cells, leading to suboptimal GC reaction. Our study highlights an unprecedented role of CD8⁺ T cells in the establishment of humoral responses to the vaccine. By elucidating underlying cellular determinants of vaccine-induced protective immunity, our work has implications for rational design of vaccines against JE virus and related flaviviruses.

Low-dose Interleukin-2: Biology and therapeutic prospects in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic aggressive arthritis that is characterized with systemic inflammation response, the production of abnormal antibodies, and persistent synovitis. One of the key mechanisms underlying the pathogenesis of RA is the imbalance of CD4 + T lymphocyte subsets, from T helper (Th) 17 cells and regulatory T (Treg) cells to T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells, which can mediate autoimmune inflammatory response to promote the overproduction of cytokines and abnormal antibodies. Although the treatment of RA has greatly changed due to the discovery of biological agents such as anti-TNF, the remission of it is still not satisfactory, thus, it is urgently required new treatment to realize the sustained remission of RA via restoring the immune tolerance. Interleukin-2 (IL-2) has been discovered to be a pleiotropic cytokine to promote inflammatory response and maintain immune tolerance. Low-dose IL-2 therapy is a driver of the imbalance between autoimmunity and immune tolerance towards immune tolerance, which has been tried to treat various autoimmune diseases. Recent researches show that low-dose IL-2 is a promising treatment for RA. In this review, we summarize the advances understandings in the biology of IL-2 and highlight the impact of the IL-2 pathway on the balance of Th17/Treg and Tfh/Tfr aiming to investigate the role of IL-2-mediated immune tolerance in RA and discuss the application and the therapeutic prospect of low-dose IL-2 in the treatment of RA.

Control of foreign Ag-specific Ab responses by Treg and Tfr

Regulatory T cells (Tregs) expressing the transcription factor Foxp3 play a critical role in the control of immune homeostasis including the regulation of humoral immunity. Recently, it has become clear that a specialized subset of Tregs, T-follicular regulatory cells (Tfr), have a particular role in the control of T-follicular helper (Tfh) cell-driven germinal center (GC) responses. Following similar differentiation signals as received by Tfh, Tfr gain expression of characteristic chemokine receptors and transcription factors such as CXCR5 and BCL6 allowing them to travel to the B-cell follicle and deliver in situ suppression. It seems clear that Tfr are critical for the prevention of autoimmune antibody induction. However, their role in the control of foreign antigen-specific antibody responses appears more complex with various reports demonstrating either increased or decreased antigen-specific antibody responses following inhibition of Tfr function. Due to their recent discovery, our understanding of Tfr formation and function still has many gaps. In this review, we discuss our current knowledge of both Tregs and Tfr in the context of humoral immunity and how these cells might be manipulated in order to better control vaccine responses.

Inhibition of IL-2 responsiveness by IL-6 is required for the generation of GC-TFH cells

Sustained T cell receptor (TCR) stimulation is required for maintaining germinal center T follicular helper (GC-T_FH) cells. Paradoxically, TCR activation induces interleukin-2 receptor (IL-2R) expression and IL-2 production, thereby initiating a feedback loop of IL-2 signaling that normally inhibits T_FH cells. It is unclear how GC-T_FH cells can receive prolonged TCR signaling without succumbing to the detrimental effects of IL-2. Using an influenza infection model, we show here that GC-T_FH cells secreted large amounts of IL-2 but responded poorly to it. To maintain their IL-2 hyporesponsiveness, GC-T_FH cells required intrinsic IL-6 signaling. Mechanistically, we found that IL-6 inhibited up-regulation of IL-2Rβ (CD122) by preventing association of STAT5 with the Il2rb locus, thus allowing GC-T_FH cells to receive sustained TCR signaling and produce IL-2 without initiating a TCR/IL-2 inhibitory feedback loop. Collectively, our results identify a regulatory mechanism that controls the generation of GC-T_FH cells.

Immunogenicity of Protein Therapeutics: A Lymph Node Perspective

The continuous development of molecular biology and protein engineering technologies enables the expansion of the breadth and complexity of protein therapeutics for in vivo administration. However, the immunogenicity and associated in vivo development of antibodies against therapeutics are a major restriction factor for their usage. The B cell follicular and particularly germinal center areas in secondary lymphoid organs are the anatomical sites where the development of antibody responses against pathogens and immunogens takes place. A growing body of data has revealed the importance of the orchestrated function of highly differentiated adaptive immunity cells, including follicular helper CD4 T cells and germinal center B cells, for the optimal generation of these antibody responses. Understanding the cellular and molecular mechanisms mediating the antibody responses against therapeutics could lead to novel strategies to reduce their immunogenicity and increase their efficacy.

Dysregulation of T Follicular Helper Cells in Lupus

Although multiple and overlapping mechanisms are ultimately responsible for the immunopathology observed in patients with systemic lupus erythematosus, autoreactive Abs secreted by autoreactive plasma cells (PCs) are considered to play a critical role in disease progression and immunopathology. Given that PCs derive from the germinal centers (GC), long-term dysregulated GC reactions are often associated with the development of spontaneous autoantibody responses and immunopathology in systemic lupus erythematosus patients. In this review, we summarize the emerging evidence concerning the roles of T follicular helper cells in regulating pathogenic GC and autoreactive PC responses in lupus.

T follicular helper cell heterogeneity: Time, space, and function

T follicular helper (Tfh) cells play a crucial role in orchestrating the humoral arm of adaptive immune responses. Mature Tfh cells localize to follicles in secondary lymphoid organs (SLOs) where they provide help to B cells in germinal centers (GCs) to facilitate immunoglobulin affinity maturation, class-switch recombination, and generation of long-lived plasma cells and memory B cells. Beyond the canonical GC Tfh cells, it has been increasingly appreciated that the Tfh phenotype is highly diverse and dynamic. As naive CD4⁺ T cells progressively differentiate into Tfh cells, they migrate through a variety of microanatomical locations to obtain signals from other cell types, which in turn alters their phenotypic and functional profiles. We herein review the heterogeneity of Tfh cells marked by the dynamic phenotypic changes accompanying their developmental program. Focusing on the various locations where Tfh and Tfh-like cells are found, we highlight their diverse states of differentiation. Recognition of Tfh cell heterogeneity has important implications for understanding the nature of T helper cell identity specification, especially the plasticity of the Tfh cells and their ontogeny as related to conventional T helper subsets.