B cells in T follicular helper cell development and function: separable roles in delivery of ICOS ligand and antigen

Clinical significance of T helper cell subsets in the peripheral blood and bone marrow of patients with multiple myeloma

Background

T helper (Th) cell subsets primarily assist B cells in differentiating into plasma cells in the germinal center. The mechanism of malignant transformation of plasma cells is an important target for the clinical treatment of MM; however, the mechanism remains unclear.

Methods

We collected the peripheral blood (PB) and bone marrow (BM) samples of 33 patients with MM. In addition, the PB was also collected from 25 normal healthy controls (HCs). We analyzed the percentages of Th cell subsets in the PB and BM samples of patients with MM.

Results

Tfh/CD4+ were positively correlated with the proportion of myeloma cells in the BM and PB samples (r = 0.592, P = 0.002 and r = 0.510, P = 0.010 respectively), and showed a strong correlation between the BM and PB samples (r = 0.6559, P = 0.0095). In the PB samples, the percentages of Th2/CD4+ and Tfh2/Tfh cells were significantly lower in patients with MM than in HCs (P = 0.00013 and P = 0.0004, respectively), whereas the percentage of Th17/CD4+ and Tfh17/Tfh was significantly higher in newly diagnosed patients with MM than in HCs (P = 0.0037 and P = 0.03, respectively), and all these cells showed a good predictive value for MM (area under the curve [AUC] 0.781, = 0.792, = 0.837, and 0.723 respectively). In the PB samples, all subsets of PD-1+ICOS- Tfh showed a noticeable downward trend in MM from newly diagnosed to non-remission and remission groups. In contrast, all subsets of PD-1-ICOS+ Tfh increased gradually.

Conclusion

Th cell subsets play an important role in the occurrence and development of MM and may provide a fundamental basis for identifying new immunotherapy targets and prognosis.

A novel microporous biomaterial vaccine platform for long-lasting antibody mediated immunity against viral infection

Current antigen delivery platforms, such as alum and nanoparticles, are not readily tunable, thus may not generate optimal adaptive immune responses. We created an antigen delivery platform by loading lyophilized Microporous Annealed Particle (MAP) with aqueous solution containing target antigens. Upon administration of antigen loaded MAP (VaxMAP), the biomaterial reconstitution forms an instant antigen-loaded porous scaffold area with a sustained release profile to maximize humoral immunity. VaxMAP induced CD4+ T follicular helper (Tfh) cells and germinal center (GC) B cell responses in the lymph nodes similar to Alum. VaxMAP loaded with SARS-CoV-2 spike protein improved the magnitude, neutralization, and duration of anti-receptor binding domain antibodies compared to Alum vaccinated mice. A single injection of Influenza specific HA1-loaded-VaxMAP enhanced neutralizing antibodies and elicited greater protection against influenza virus challenge than HA1-loaded-Alum. Thus, VaxMAP is a platform that can be used to promote adaptive immune cell responses to generate more robust neutralizing antibodies, and better protection upon pathogen challenge.

Peg-IFNα combined with hepatitis B vaccination contributes to HBsAg seroconversion and improved immune function

PURPOSE

The purpose of this study was to investigate immunological variations between a group that received the hepatitis B vaccine and a non-vaccine group. We focused on a cohort that achieved HBsAg seroclearance after Peg-IFNα treatment of CHB.

METHODS

We enrolled twenty-eight individuals who achieved HBsAg seroclearance after Peg-IFNα treatment. They were divided into two groups: a vaccine group (n = 14) and a non-vaccine group (n = 14). We assessed lymphocyte subpopulations, B cell- and T cell-surface costimulatory/inhibitory factors, cytokines and immunoglobulin levels were detected at different time points to explore immune-function differences between both groups.

RESULTS

The seroconversion rate in the vaccine group at 24 weeks post-vaccination was 100%, which was significantly higher (p = 0.006) than that of the non-vaccine group (50%). Additionally, more individuals in the vaccine group exhibited anti-HBs levels exceeding 100 IUs/L and 300 IUs/L compared to the non-vaccine group (p < 0.05). The vaccine group demonstrated significantly increase total B cells and class-switched B cells at 24 weeks and plasma cells, CD80+B cells, Tfh cells, and ICOS+Tfh cell at 12 weeks, compared with baseline levels (p < 0.05). Conversely, Bregs (CD24+CD27+ and CD24+CD38high) decreased significantly at 24 weeks (p < 0.05). None of the above changes were statistically significance in the non-vaccine group (p > 0.05). Total IgG increased significantly in the vaccine group, and IL-2, IL-5, and IL-6 concentrations increased significantly at week 24 (p < 0.05). Differences in various types of cytokines and immunoglobulins in the plasma of the non-vaccine group were not significant (p > 0.05). Anti-HBs titers positively correlated with Th1/Th2 cells at 24 weeks (r = 0.448 and 0.458, respectively, p = 0.022 and 0.019, respectively), and negatively with CD24+CD38highBreg cells (r = -0.402, p = 0.042).

CONCLUSIONS

After achieving HBsAg seroclearance through Peg-IFNα treatment for CHB, administering the hepatitis B vaccine significantly increased anti-HBs-seroconversion rates and antibody levels. We also observed significant immunological differences between the vaccine and non-vaccine groups. Specifically, the vaccine group exhibited significant increases in B cells, plasma cells, and Tfh cells, while Breg levels was significantly lower. These immunological changes are likely conducive to the production of anti-HBs antibodies. However, in the non-vaccine group, the observed changes were not significantlly significant.

CCR9/CXCR5 Co-Expressing CD4 T Cells Are Increased in Primary Sjögren's Syndrome and Are Enriched in PD-1/ICOS-Expressing Effector T Cells

Primary Sjögren's syndrome (pSS) is an autoimmune disease characterised by B cell hyperactivity. CXCR5+ follicular helper T cells (Tfh), CXCR5-PD-1hi peripheral helper T cells (Tph) and CCR9+ Tfh-like cells have been implicated in driving B cell hyperactivity in pSS; however, their potential overlap has not been evaluated. Our aim was to study the overlap between the two CXCR5- cell subsets and to study their PD-1/ICOS expression compared to "true" CXCR5/PD-1/ICOS-expressing Tfh cells. CXCR5- Tph and CCR9+ Tfh-like cell populations from peripheral blood mononuclear cells of pSS patients and healthy controls (HC) were compared using flow cytometry. PD-1/ICOS expression from these cell subsets was compared to each other and to CXCR5+ Tfh cells, taking into account their differentiation status. CXCR5- Tph cells and CCR9+ Tfh-like cells, both in pSS patients and HC, showed limited overlap. PD-1/ICOS expression was higher in memory cells expressing CXCR5 or CCR9. However, the highest expression was found in CXCR5/CCR9 co-expressing T cells, which are enriched in the circulation of pSS patients. CXCR5- Tph and CCR9+ Tfh-like cells are two distinct cell populations that both are enriched in pSS patients and can drive B cell hyperactivity in pSS. The known upregulated expression of CCL25 and CXCL13, ligands of CCR9 and CXCR5, at pSS inflammatory sites suggests concerted action to facilitate the migration of CXCR5+CCR9+ T cells, which are characterised by the highest frequencies of PD-1/ICOS-positive cells. Hence, these co-expressing effector T cells may significantly contribute to the ongoing immune responses in pSS.

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 Regulation between CD4+CXCR5+ Follicular Helper T (Tfh) Cells and CD19+CD24hiCD38hi Regulatory B (Breg) Cells in Gastric Cancer

Purpose

T follicular helper (Tfh) cells and regulatory B (Breg) cells are reported to play essential roles in humoral immunity, especially in inflammation, autoimmune diseases, and cancer. Hence, we sought to investigate the involvement of CXCR5⁺CD4⁺ Tfh cells and CD19⁺CD24^hiCD38^hi Breg cells in gastric cancer.

Methods

The blood samples were obtained from 36 gastric cancer patients and 18 healthy individuals. The percentage of Tfh cells (Tfh%) and Breg cells (Breg%) was detected via flow cytometry, while IL-21, IL-10, and CXCL13 levels were examined with ELISA. The association between them and clinical parameters of patients was also assessed. The in vitro Tfh-B cell coculture experiments were performed for six days, and then, Tfh%, Breg%, and cytokines were valued by flow cytometry and ELISA, respectively.

Results

Tfh%, Breg%, and CXCL13 level were significantly increased among gastric cancer patients. Moreover, higher Tfh% was associated with lymphatic metastasis, patients' worse outcomes and Breg%. Tfh differentiation and CXCL13 were upregulated by cocultured B cells in vitro, while Tfh cells seem to not participate in Breg cell differentiation from B cells.

Conclusion

Altogether, increased Tfh and Breg cells could be involved in immune suppression in gastric cancer. Moreover, B cell may be a potential regulator for Tfh differentiation, while Tfh cells had no significant effects on the regulation of Breg cells.

Composition and regulation of the immune microenvironment of salivary gland in Sjögren’s syndrome

Primary Sjögren’s syndrome (pSS) is a systemic autoimmune disease characterized by exocrine gland dysfunction and inflammation. Patients often have dry mouth and dry eye symptoms, which seriously affect their lives. Improving dry mouth and eye symptoms has become a common demand from patients. For this reason, researchers have conducted many studies on external secretory glands. In this paper, we summarize recent studies on the salivary glands of pSS patients from the perspective of the immune microenvironment. These studies showed that hypoxia, senescence, and chronic inflammation are the essential characteristics of the salivary gland immune microenvironment. In the SG of pSS, genes related to lymphocyte chemotaxis, antigen presentation, and lymphocyte activation are upregulated. Interferon (IFN)-related genes, DNA methylation, sRNA downregulation, and mitochondrial-related differentially expressed genes are also involved in forming the immune microenvironment of pSS, while multiple signaling pathways are involved in regulation. We further elucidated the regulation of the salivary gland immune microenvironment in pSS and relevant, targeted treatments.

Single-dose AAV vector gene immunotherapy to treat food allergy

Immunotherapies for patients with food allergy have shown some success in limiting allergic responses. However, these approaches require lengthy protocols with repeated allergen dosing and patients can relapse following discontinuation of treatment. The purpose of this study was to test if a single dose of an adeno-associated virus (AAV) vector can safely prevent and treat egg allergy in a mouse model. AAV vectors expressing ovalbumin (OVA) under an ubiquitous or liver-specific promoter were injected prior to or after epicutaneous sensitization with OVA. Mice treated with either AAV8-OVA vector were completely protected from allergy sensitization. These animals had a significant reduction in anaphylaxis mediated by a reduction in OVA-specific IgE titers. In mice with established OVA allergy, allergic responses were mitigated only in mice treated with an AAV8-OVA vector expressing OVA from an ubiquitous promoter. In conclusion, an AAV vector with a liver-specific promoter was more effective for allergy prevention, but higher OVA levels were necessary for reducing symptoms in preexisting allergy. Overall, our AAV gene immunotherapy resulted in an expansion of OVA-specific FoxP3⁺ CD4⁺ T cells, an increase in the regulatory cytokine IL-10, and a reduction in the IgE promoting cytokine IL-13.

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

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

ICOS Expression Is Required for Maintenance but Not the Formation of Germinal Centers in the Spleen in Response to Plasmodium yoelii Infection

Inducible T cell costimulator (ICOS) plays a key role in the differentiation and maintenance of follicular helper T (Tfh) cells and, thus, germinal center (GC) formation. Previously, our laboratory showed in a Plasmodium chabaudi infection model that Icos-/- mice were significantly impaired in their ability to form GCs despite persistent infection and, thus, a continued antigen (Ag) load. Here, we show that the resolution of primary infection with Plasmodium 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, although 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 loads 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 rechallenge. 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 reinfection with P. yoelii.

IgD ligation allows peritoneal cavity B cell proliferation

Atypical cytokine production and immune cell subset ratios, particularly those that include high proportions of macrophages, characterize tumor microenvironments (TMEs). TMEs can be modeled by culturing peritoneal cavity (PerC) cells which have a high macrophage to lymphocyte ratio. With TCR or BCR ligation, PerC lymphocyte proliferation is tempered by macrophages. However, PHA (T cells) and anti-CD40 (B cells) are activators that induce proliferation. Herein, we report that ligating IgD, in contrast to IgM, triggers PerC B cell proliferation. IL-4 addition enhanced the IgD response for BALB/c PerC B cells but suppressed that of C57BL/6 mice. Intriguingly, concurrent ligation of IgD and CD3_ε rescued a PerC T cell proliferative response. These results serve to expand the list of targets for promoting cellular and humoral immunity in conditions that model macrophage-rich TMEs.

Cellular and molecular diversity in Sjogren's syndrome salivary glands: Towards a better definition of disease subsets

Primary Sjögren's syndrome (pSS) is a highly heterogeneous disease in terms of clinical presentation ranging from a mild disease localised to the salivary and lacrimal glands, to multiorgan complications of various degrees of severity, finishing with the evolution, in around 5% of pSS patients, to B cell lymphomas most commonly arising in the inflamed salivary glands. Currently, there are poor positive or negative predictors of disease evolution able to guide patient management and treatment at early stages of the diseases. Recent understanding of the pathogenic mechanisms driving immunopathology in pSS, particularly through histological and transcriptomic analysis of minor and parotid salivary gland (SG) biopsies, has highlighted a high degree of cellular and molecular heterogeneity of the inflammatory lesions but also allowed the identification of clusters of patients with similar underlying SG immunopathology. In particular, patients presenting with high degrees of B/T cell infiltration and the formation of ectopic lymphoid structures (ELS) in the SG have been associated, albeit with conflicting results, with higher degree of disease severity and enhanced risk of lymphoma evolution, suggesting that a dysregulated adaptive immune response plays a key role in driving disease manifestations in pSS. Recent data from randomised clinical trials with novel biological therapies in pSS have also highlighted the potential role of SG immunopathology and molecular pathology in stratifying patients for trial inclusion as well as assessing proof of mechanisms in longitudinal SG biopsies before and after treatment. Although significant progress has been made in the understanding of disease pathogenesis and heterogeneity through cellular and molecular SG pathology, further work is needed to validate their clinical utility in routine clinical settings and in randomised clinical trials.

Targeting T Follicular Helper Cells to Control Humoral Allogeneic Immunity

Humoral allogeneic immunity driven by anti-HLA donor-specific antibodies and antibody-mediated rejection (AMR) significantly impede prolonged survival of organ allografts after transplantation. Although the importance of T follicular helper (TFH) cells in controlling antibody responses has been long established, their role in directing donor-specific antibody generation leading to AMR was only recently appreciated in the clinical setting of organ transplantation. In this review, we provide a comprehensive summary of the current knowledge on the biology of human TFH cells as well as their circulating counterparts and describe their pivotal role in driving humoral alloimmunity. In addition, we discuss the intrinsic effects of current induction therapies and maintenance immunosuppressive drugs as well as of biotherapies on TFH cells and provide future directions and novel opportunities of biotherapeutic targeting of TFH cells that have the potential of bringing the prophylactic and curative treatments of AMR toward personalized and precision medicine.

Disruption of Tfh:B Cell Interactions Prevents Antibody-Mediated Rejection in a Kidney Transplant Model in Rats: Impact of Calcineurin Inhibitor Dose

We aimed to investigate the mechanisms of humoral immune activation in ABMR using a MHC-mismatched rat kidney transplant model. We applied low dose cyclosporine A (loCNI) to allow donor-specific antibody (DSA) formation and rejection and high dose cyclosporine A (hiCNI) for non-rejection. DSA and leukocyte subsets were measured by flow cytometry. Germinal centers (GC), T follicular helper cells (Tfh), plasma cells and interleukin-21 (IL-21) expression were analyzed by immunofluorescence microscopy. Expression of important costimulatory molecules and cytokines was measured by qRT-PCR. Allograft rejection was evaluated by a nephropathologist. We found that DSA formation correlated with GC frequency and expansion, and that GC size was linked to the number of activated Tfh. In hiCNI, GC and activated Tfh were virtually absent, resulting in fewer plasma cells and no DSA or ABMR. Expression of B cell activating T cell cytokine IL-21 was substantially inhibited in hiCNI, but not in loCNI. In addition, hiCNI showed lower expression of ICOS ligand and IL-6, which stimulate Tfh differentiation and maintenance. Overall, Tfh:B cell crosstalk was controlled only by hiCNI treatment, preventing the development of DSA and ABMR. Additional strategies targeting Tfh:B cell interactions are needed for preventing alloantibody formation and ABMR.

Helper T Cells in Idiopathic Membranous Nephropathy

Idiopathic membranous nephropathy (IMN) is an autoimmune disease in which the immune system produces an antibody response to its own antigens due to impaired immune tolerance. Although antibodies are derived from plasma cells differentiated by B cells, the T-B cells also contribute a lot to the immune system. In particular, the subsets of helper T (Th) cells, including the dominant subsets such as Th2, Th17, and follicular helper T (Tfh) cells and the inferior subsets such as regulatory T (Treg) cells, shape the immune imbalance of IMN and promote the incidence and development of autoimmune responses. After reviewing the physiological knowledge of various subpopulations of Th cells and combining the existing studies on Th cells in IMN, the role model of Th cells in IMN was explained in this review. Finally, the existing clinical treatment regimens for IMN were reviewed, and the importance of the therapy for Th cells was highlighted.

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.

Unique expansion of IL-21+ Tfh and Tph cells under control of ICOS identifies Sjögren's syndrome with ectopic germinal centres and MALT lymphoma

OBJECTIVES

To explore the relevance of T-follicular-helper (Tfh) and pathogenic peripheral-helper T-cells (Tph) in promoting ectopic lymphoid structures (ELS) and B-cell mucosa-associated lymphoid tissue (MALT) lymphomas (MALT-L) in Sjögren's syndrome (SS) patients.

METHODS

Salivary gland (SG) biopsies with matched peripheral blood were collected from four centres across the European Union. Transcriptomic (microarray and quantitative PCR) analysis, FACS T-cell immunophenotyping with intracellular cytokine detection, multicolor immune-fluorescence microscopy and in situ hybridisation were performed to characterise lesional and circulating Tfh and Tph-cells. SG-organ cultures were used to investigate functionally the blockade of T-cell costimulatory pathways on key proinflammatory cytokine production.

RESULTS

Transcriptomic analysis in SG identified Tfh-signature, interleukin-21 (IL-21) and the inducible T-cell co-stimulator (ICOS) costimulatory pathway as the most upregulated genes in ELS+SS patients, with parotid MALT-L displaying a 400-folds increase in IL-21 mRNA. Peripheral CD4+CXC-motif chemokine receptor 5 (CXCR5)+programmed cell death protein 1 (PD1)+ICOS+ Tfh-like cells were significantly expanded in ELS+SS patients, were the main producers of IL-21, and closely correlated with circulating IgG and reduced complement C4. In the SG, lesional CD4+CD45RO+ICOS+PD1+ cells selectively infiltrated ELS+ tissues and were aberrantly expanded in parotid MALT-L. In ELS+SG and MALT-L parotids, conventional CXCR5+CD4+PD1+ICOS+Foxp3- Tfh-cells and a uniquely expanded population of CXCR5-CD4+PD1hiICOS+Foxp3- Tph-cells displayed frequent IL-21/interferon-γ double-production but poor IL-17 expression. Finally, ICOS blockade in ex vivo SG-organ cultures significantly reduced the production of IL-21 and inflammatory cytokines IL-6, IL-8 and tumour necrosis factor-α (TNF-α).

CONCLUSIONS

Overall, these findings highlight Tfh and Tph-cells, IL-21 and the ICOS costimulatory pathway as key pathogenic players in SS immunopathology and exploitable therapeutic targets in SS.

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.

Impact of low tacrolimus exposure and high tacrolimus intra-patient variability on the development of de novo anti-HLA donor-specific antibodies in kidney transplant recipients

Introduction: De novo donor-specific antibodies (dnDSA) directed against HLA are a major contributing factor to the chronic deterioration of renal allograft function. Several factors, including the degree of HLA matching, younger recipient age, and past sensitization events have been shown to increase the risk for the development of dnDSA. The development of dnDSA is also strongly associated with modifications in the immunosuppressive regimen, non-adherence, and under-immunosuppression.Areas covered: Tacrolimus is widely used after solid organ transplantation (SOT) and in recent years, both a high intra-patient variability in tacrolimus exposure and low tacrolimus exposure have been found to be associated with a higher risk of dnDSA development in kidney transplant recipients. This article provides an overview of current findings published in the recent 5 years regarding the relationship between tacrolimus exposure and variation therein and the development of dnDSA.Expert opinion: In this review, we describe how combining data on tacrolimus intra-patient variability and mean pre-dose concentration may be an effective tool to identify kidney transplant recipients who are at higher risk of developing dnDSA.

Memory B Cells and Long-lived Plasma Cells

The importance of B cell and antibody-mediated immune response in the acute and long-term persistence of transplanted solid organs has become increasingly evident in recent years. A variety of therapeutic innovations target antibodies directed toward HLA or blood groups (ABO) to allow better allocation and posttransplant longevity of organs. Antibodies originate from plasma cells (PCs), which are terminally differentiated B cells. Long-term production and persistence of these antibodies is partly due to fast reactivation of previously generated memory B cells; however, there is increasing evidence that some differentiated PCs can persist independently in the bone marrow for years or even decades, producing specific antibodies or even experiencing regeneration without proliferation without need to be replaced by newly differentiating B cells. This review outlines the currently presumed pathways of differentiation, antibody, and memory generation on both B-cell and PC levels. On this background, current therapeutic concepts for antibody reduction before and after solid organ transplantation are considered, to better understand their mechanisms, possible synergisms, and specific risks. Specific differences in regards to ABO versus HLA antibodies as well as practical relevance for generation of desensitization and posttransplant antibody-directed therapy protocols are discussed.

Factors Affecting Early Antibody Secreting Cell Maturation Into Long-Lived Plasma Cells

Antibody secreting cells (ASCs) are terminally differentiated cells of the humoral immune response and must adapt morphologically, transcriptionally, and metabolically to maintain high-rates of antibody (Ab) secretion. ASCs differentiate from activated B cells in lymph nodes and transiently circulate in the blood. Most of the circulating ASCs undergo apoptosis, but a small fraction of early ASCs migrate to the bone marrow (BM) and eventually mature into long-lived plasma cells (LLPCs). LLPC survival is controlled both intrinsically and extrinsically. Their differentiation and maintenance programs are governed by many intrinsic mechanisms involving anti-apoptosis, autophagy, and metabolism. The extrinsic factors involved in LLPC generation include BM stromal cells, cytokines, and chemokines, such as APRIL, IL-6, and CXCL12. In humans, the BM CD19⁻CD38^hiCD138⁺ ASC subset is the main repository of LLPCs, and our recent development of an in vitro BM mimic provides essential tools to study environmental cues that support LLPC survival and the critical molecular mechanisms of maturation from early minted blood ASCs to LLPCs. In this review, we summarize the evidence of LLPC generation and maintenance and provide novel paradigms of LLPC maturation.

Targeting Antigens to CD180 but Not CD40 Programs Immature and Mature B Cell Subsets to Become Efficient APCs

Targeting Ags to the CD180 receptor activates both B cells and dendritic cells (DCs) to become potent APCs. After inoculating mice with Ag conjugated to an anti-CD180 Ab, B cell receptors were rapidly internalized. Remarkably, all B cell subsets, including even transitional 1 B cells, were programed to process, present Ag, and stimulate Ag-specific CD4⁺ T cells. Within 24-48 hours, Ag-specific B cells were detectable at T-B borders in the spleen; there, they proliferated in a T cell-dependent manner and induced the maturation of T follicular helper (T_FH) cells. Remarkably, immature B cells were sufficient for the maturation of T_FH cells after CD180 targeting: T_FH cells were induced in BAFFR^-/- mice (with only transitional 1 B cells) and not in μMT mice (lacking all B cells) following CD180 targeting. Unlike CD180 targeting, CD40 targeting only induced DCs but not B cells to become APCs and thus failed to efficiently induce T_FH cell maturation, resulting in slower and lower-affinity IgG Ab responses. CD180 targeting induces a unique program in Ag-specific B cells and to our knowledge, is a novel strategy to induce Ag presentation in both DCs and B cells, especially immature B cells and thus has the potential to produce a broad range of Ab specificities. This study highlights the ability of immature B cells to present Ag to and induce the maturation of cognate T_FH cells, providing insights toward vaccination of mature B cell-deficient individuals and implications in treating autoimmune disorders.

ICOS Costimulation Differentially Affects T Cells in Secondary Lymphoid Organs and Inflamed Tissues

B-cell interaction with follicular helper T cells and subsequent differentiation of B cells into high-affinity APCs normally takes place in secondary lymphoid organs. The costimulator ICOS plays a key role in this process and is therefore considered as an attractive target to modulate exaggerated B-cell responses in autoimmune or allergic diseases. Inflamed tissues were recently recognized as additional sites of active T-cell/B-cell interaction. To analyze whether ICOS costimulation is also important there, we employed a mouse airway inflammation model that allows direct comparison of immune reactions in the lung-draining lymph node and the lung tissue as well as assessment of the relative importance of dendritic cells versus B cells as APCs. In both organs, ICOS regulated the pool size of antigen-specific T and B cells and B-cell differentiation into germinal center(-like) cells but not into antibody-secreting cells. In the lymph node, lack of ICOS costimulation drastically reduced the frequency of T follicular helper cells but did not affect production of T-helper cell type 2 (Th2) cytokines. Vice versa in the lung tissue, ICOS did not change PD-1 expression on infiltrating T cells but regulated Th2 cytokine production, a process for which ICOS ligand expression on B cells was of particular importance. Taken together, the results of this study show that ICOS differentially regulates effector T cells in secondary lymphoid organs and inflamed tissues but that blockade of the ICOS pathway is suitable to target T cell-dependent B cell responses at both sites.

Costimulatory blockade molecules and B-cell-mediated immune response: current knowledge and perspectives

There is an urgent need for therapeutic agents that target humoral alloimmunity in solid organ transplantation. This includes sensitized patients with preformed donor-specific human leukocyte antigen antibodies and patients who develop de novo donor-specific antibodies, both of which are associated with acute and chronic antibody-mediated rejection and allograft loss. In the last decade, both experimental and clinical studies highlighted the major impact of costimulation molecules in the control of immune responses both in the field of transplantation and autoimmune disease. Although these molecules have been initially developed to control the early steps of T-cell activation, recent evidence also supports their influence at several steps of the humoral response. In this review, we aim to provide an overview of the current knowledge of the effects of costimulatory blockade agents on humoral responses in both autoimmune and allogeneic contexts. We first present the effects of costimulatory molecules on the different steps of alloantibody production. We then summarize mechanisms and clinical results observed using cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig molecules both in transplantation and autoimmunity. Finally, we present the potential interest and implications of other costimulatory family members as therapeutic targets, with emphasis on combinatorial approaches, for the optimal control of the alloantigen-specific humoral response.

Advances in T follicular helper and T follicular regulatory cells in transplantation immunity

B cells play a crucial role in alloreactivity of organ transplant rejection and graft versus host diseases (GVHD). Over the past decade, it has been well recognized that B-cell infiltration in allografts and de novo donor-specific antibodies (DSA) were strongly associated with severe graft rejection and loss, as well as glucocorticoid resistance. Emerging evidence has demonstrated that Follicular T helper (Tfh) cells are key effectors to promote the proliferation and differentiation of B cells into antibody-producing plasmablasts and memory B cells. T-follicular regulatory (Tfr) cells are a recently recognized cell population that has a negative regulatory role on Tfh cells in the follicle, preventing incessant antibody production. It is still less clear how those humoral immunoreactive cells affect transplant rejection and allograft loss. This review focuses on the production and function of Tfr/Tfh cells in the transplant environment. Better understanding of the functions and mechanisms of Tfr/Tfh cells will help to design new strategies to prevent allograft rejection and prolong graft survival.

Lymph Node Cellular Dynamics in Cancer and HIV: What Can We Learn for the Follicular CD4 (Tfh) Cells?

Lymph nodes (LNs) are central in the generation of adaptive immune responses. Follicular helper CD4 T (Tfh) cells, a highly differentiated CD4 population, provide critical help for the development of antigen-specific B cell responses within the germinal center. Throughout the past decade, numerous studies have revealed the important role of Tfh cells in Human Immunodeficiency Virus (HIV) pathogenesis as well as in the development of neutralizing antibodies post-infection and post-vaccination. It has also been established that tumors influence various immune cell subsets not only in their proximity, but also in draining lymph nodes. The role of local or tumor associated lymph node Tfh cells in disease progression is emerging. Comparative studies of Tfh cells in chronic infections and cancer could therefore provide novel information with regards to their differentiation plasticity and to the mechanisms regulating their development.

Correlates of Follicular Helper Bias in the CD4 T Cell Response to a Retroviral Antigen

CD4+ T cell differentiation is influenced by a plethora of intrinsic and extrinsic factors, providing the immune system with the ability to tailor its response according to specific stimuli. Indeed, different classes of pathogens may induce a distinct balance of CD4+ T cell differentiation programmes. Here, we report an uncommonly strong bias toward follicular helper (Tfh) differentiation of CD4+ T cells reactive with a retroviral envelope glycoprotein model antigen, presented in its natural context during retroviral infection. Conversely, the response to the same antigen, presented in different immunization regimens, elicited a response typically balanced between Tfh and T helper 1 cells. Comprehensive quantitation of variables known to influence Tfh differentiation revealed the closest correlation with the strength of T cell receptor (TCR) signaling, leading to PD-1 expression, but not with surface TCR downregulation, irrespective of TCR clonotypic avidity. In contrast, strong TCR signaling leading to TCR downregulation and induction of LAG3 expression in high TCR avidity clonotypes restrained CD4+ T cell commitment and further differentiation. Finally, stunted Th1 differentiation, correlating with limited IL-2 availability in retroviral infection, provided permissive conditions for Tfh development, suggesting that Tfh differentiation is the default program of envelope-reactive CD4+ T cells.

Humoral immune responses to infection: common mechanisms and unique strategies to combat pathogen immune evasion tactics

Humoral immune responses are crucial for protection against invading pathogens and are the underlying mechanism of protection for most successful vaccines. Our understanding of how humoral immunity develops is largely based on animal models utilizing experimental immunization systems. While these studies have made enormous progress for the field and have defined many of the fundamental principles of B cell differentiation and function, we are only now beginning to appreciate the complexities of humoral immune responses induced by infection. Co-evolution of the adaptive immune system and the pathogenic world has created a diverse array of B cell responses to infections, with both shared and unique strategies. In this review, we consider the common mechanisms that regulate the development of humoral immune responses during infection and highlight recent findings demonstrating the evolution of unique strategies used by either host or pathogen for survival.

STAT4 and T-bet control follicular helper T cell development in viral infections

Follicular helper T (Tfh) cells promote germinal center (GC) B cell survival and proliferation and guide their differentiation and immunoglobulin isotype switching by delivering contact-dependent and soluble factors, including IL-21, IL-4, IL-9, and IFN-γ. IL-21 and IFN-γ are coexpressed by Tfh cells during viral infections, but transcriptional regulation of these cytokines is not completely understood. In this study, we show that the T helper type 1 cell (Th1 cell) transcriptional regulators T-bet and STAT4 are coexpressed with Bcl6 in Tfh cells after acute viral infection, with a temporal decline in T-bet in the waning response. T-bet is important for Tfh cell production of IFN-γ, but not IL-21, and for a robust GC reaction. STAT4, phosphorylated in Tfh cells upon infection, is required for expression of T-bet and Bcl6 and for IFN-γ and IL-21. These data indicate that T-bet is expressed with Bcl6 in Tfh cells and is required alongside STAT4 to coordinate Tfh cell IL-21 and IFN-γ production and for promotion of the GC response after acute viral challenge.

Belatacept Does Not Inhibit Follicular T Cell-Dependent B-Cell Differentiation in Kidney Transplantation

Humoral alloreactivity has been recognized as a common cause of kidney transplant dysfunction. B-cell activation, differentiation, and antibody production are dependent on IL-21⁺CXCR5⁺follicular T-helper (Tfh) cells. Here, we studied whether belatacept, an inhibitor of the costimulatory CD28-CD80/86-pathway, interrupts the crosstalk between Tfh- and B-cells more efficiently than the calcineurin inhibitor tacrolimus. The suppressive effects of belatacept and tacrolimus on donor antigen-driven Tfh-B-cell interaction were functionally studied in peripheral blood mononuclear cells from 40 kidney transplant patients randomized to a belatacept- or tacrolimus-based immunosuppressive regimen. No significant differences in uncultured cells or donor antigen-stimulated cells were found between belatacept- and tacrolimus-treated patients in the CXCR5⁺Tfh cell generation and activation (upregulation of PD-1). Belatacept and tacrolimus in vitro minimally inhibited Tfh-cell generation (by ~6-7%) and partially prevented Tfh-cell activation (by ~30-50%). The proportion of IL-21⁺-activated Tfh-cells was partially decreased by in vitro addition of belatacept or tacrolimus (by ~60%). Baseline expressions and proportions of activated CD86⁺ B-cells, plasmablasts, and transitional B-cells after donor antigen stimulation did not differ between belatacept- and tacrolimus-treated patients. Donor antigen-driven CD86 upregulation on memory B-cells was not fully prevented by adding belatacept in vitro (~35%), even in supratherapeutic doses. In contrast to tacrolimus, belatacept failed to inhibit donor antigen-driven plasmablast formation (~50% inhibition vs. no inhibition, respectively, p < 0.0001). In summary, donor antigen-driven Tfh-B-cell crosstalk is similar in cells obtained from belatacept- and tacrolimus-treated patients. Belatacept is, however, less potent in vitro than tacrolimus in inhibiting Tfh-cell-dependent plasmablast formation.

Death receptor 6 contributes to autoimmunity in lupus-prone mice

Expansion of autoreactive follicular helper T (Tfh) cells is tightly restricted to prevent induction of autoantibody-dependent immunological diseases, such as systemic lupus erythematosus (SLE). Here we show expression of an orphan immune regulator, death receptor 6 (DR6/TNFRSF21), on a population of Tfh cells that are highly expanded in lupus-like disease progression in mice. Genome-wide screening reveals an interaction between syndecan-1 and DR6 resulting in immunosuppressive functions. Importantly, syndecan-1 is expressed specifically on autoreactive germinal centre (GC) B cells that are critical for maintenance of Tfh cells. Syndecan-1 expression level on GC B cells is associated with Tfh cell expansion and disease progression in lupus-prone mouse strains. In addition, Tfh cell suppression by DR6-specific monoclonal antibody delays disease progression in lupus-prone mice. These findings suggest that the DR6/syndecan-1 axis regulates aberrant GC reactions and could be a therapeutic target for autoimmune diseases such as SLE.

Germinal centre (GC) reactions are driven by T follicular helper (Tfh) cells and their dysregulation can cause autoimmune disease. Here the authors show that the orphan receptor DR6 is a Tfh cell marker that binds syndecan-1 on GC B cells driving autoimmunity in lupus-prone mice.

TFH cells progressively differentiate to regulate the germinal center response

Germinal center (GC) B cells undergo affinity selection, which depends on interactions with CD4(+) follicular helper T cells (TFH cells). We found that TFH cells progressed through transcriptionally and functionally distinct stages and provided differential signals for GC regulation. They initially localized proximally to mutating B cells, secreted interleukin 21 (IL-21), induced expression of the transcription factor Bcl-6 and selected high-affinity B cell clones. As the GC response evolved, TFH cells extinguished IL-21 production and switched to IL-4 production, showed robust expression of the co-stimulatory molecule CD40L, and promoted the development of antibody-secreting B cells via upregulation of the transcription factor Blimp-1. Thus, TFH cells in the B cell follicle progressively differentiate through stages of localization, cytokine production and surface ligand expression to 'fine tune' the GC reaction.

ICOS Co-Stimulation: Friend or Foe?

Over the last 15 years, the inducible T cell co-stimulator (ICOS) has been implicated in various immune outcomes, including the induction and regulation of Th1, Th2, and Th17 immunity. In addition to its role in directing effector T cell differentiation, ICOS has also been consistently linked with the induction of thymus-dependent (TD) antibody (Ab) responses and the germinal center (GC) reaction. ICOS co-stimulation, therefore, appears to play a complex role in dictating the course of adaptive immunity. In this article, we summarize the initial characterization of ICOS and its relationship with the related co-stimulatory molecule CD28. We then address the contribution of ICOS in directing an effector T cell response, and ultimately disease outcome, against various bacterial, viral, and parasitic infections. Next, we assess ICOS in the context of TD Ab responses, connecting ICOS signaling to follicular helper T cell differentiation and its role in the GC reaction. Finally, we address the link between ICOS and human autoimmune disorders and evaluate potential therapies aiming to mitigate disease progression by modulating ICOS signaling.

Follicular Helper T Cells in Autoimmunity

The development of multiple disease-relevant autoantibodies is a hallmark of autoimmune diseases. In autoimmune type 1 diabetes (T1D), a variable time frame of autoimmunity precedes the clinically overt disease. The relevance of T follicular helper (TFH) cells for the immune system is increasingly recognized. Their pivotal contribution to antibody production by providing help to germinal center (GC) B cells facilitates the development of a long-lived humoral immunity. Their complex differentiation process, involving various stages and factors like B cell lymphoma 6 (Bcl6), is strictly controlled, as anomalous regulation of TFH cells is connected with immunopathologies. While the adverse effects of a TFH cell-related insufficient humoral immunity are obvious, the role of increased TFH frequencies in autoimmune diseases like T1D is currently highlighted. High levels of autoantigen trigger an excessive induction of TFH cells, consequently resulting in the production of autoantibodies. Therefore, TFH cells might provide promising approaches for novel therapeutic strategies.

Soluble B7-H2 as a novel marker in early evaluation of the severity of acute pancreatitis

The clinical usefulness of soluble B7-H2 (sB7-H2) as an early indicator of acute pancreatitis (AP) remains unclear, so we performed the present study to investigate this issue. For our cohort, we recruited 75 patients with AP, 70 patients with other abdominal sepsis, and 20 healthy control individuals. The sB7-H2 levels of AP patients or healthy control individuals were measured by enzyme-linked immunosorbent assay (ELISA). The sB7-H2 levels in patients with AP rather than other patients with abdominal sepsis were significantly higher than those in healthy controls. Hence, we selected AP to study the clinical significance of sB7-H2 in inflammatory conditions. The sB7-H2 level was positively correlated with the white blood cell (WBC) count and the lactate dehydrogenase (LDH), high-sensitivity C-reactive protein (hs-CRP), and lipopolysaccharide LPS levels (P <.05 for each). Receiver operating characteristic (ROC) analysis revealed that sB7-H2 can distinguish moderately severe acute pancreatitis (MSAP) and severe acute pancreatitis (SAP) from mildly acute pancreatitis (MAP) with 77.8% sensitivity and 80.0% specificity; and that the levels of sB7-H2 also can distinguish SAP from MSAP and MAP with 92.0% sensitivity and 86.0% specificity. The present results indicate that sB7-H2 might be a useful marker in the clinical diagnosis of AP.

BTLA expression declines on B cells of the aged and is associated with low responsiveness to the trivalent influenza vaccine

Virus-neutralizing antibody and B cell responses to influenza A viruses were measured in 35 aged and 28 middle-aged individuals following vaccination with the 2012 and 2013 trivalent inactivated influenza vaccines. Antibody responses to the vaccine strains were lower in the aged. An analysis of B cell subsets by flow cytometry with stains for immunoregulators showed that B cells of multiple subsets from the aged as compared to younger human subjects showed differences in the expression of the co-inhibitor B and T lymphocyte attenuator (BTLA). Expression of BTLA inversely correlated with age and appears to be linked to shifting the nature of the response from IgM to IgG. High BTLA expression on mature B cells was linked to higher IgG responses to the H1N1 virus. Finally, high BTLA expression on isotype switched memory B cells was linked to better preservation of virus neutralizing antibody titers and improved recall responses to vaccination given the following year.

A TRAF-like motif of the inducible costimulator ICOS controls development of germinal center TFH cells via the kinase TBK1

Inducible costimulator (ICOS) signaling fuels the stepwise development of T follicular helper (T_FH) cells. However, a signaling pathway unique to ICOS has not been identified. We show that TANK-binding kinase 1 (TBK1) associates with ICOS via a conserved motif, IProx, which shares homology with tumor necrosis factor receptor (TNFR)-associated factors, TRAF2 and TRAF3. Disruption of this motif abolishes the association with TBK1, thus identifying a TBK1-binding consensus. Mutation of this motif in ICOS, or depletion of TBK1 in T cells severely impaired the differentiation of germinal center (GC) T_FH, B cell and antibody responses, but was dispensable for early T_FH differentiation. These results reveal a novel ICOS-TBK1 signaling pathway that specifies GC T_FH cell commitment.

Development of a novel monoclonal antibody to human inducible co-stimulator ligand (ICOSL): Biological characteristics and application for enzyme-linked immunosorbent assay

ICOSL (B7-H2, CD275), a co-stimulatory molecule of the B7 superfamily, functions as a positive signal in immune response. To investigate whether ICOSL could be released into sera and the possible biological function of soluble ICOS (sICOSL), we generated and characterized a functional anti-human ICOSL monoclonal antibody (mAb), 20B10, and developed a novel enzyme-linked immunosorbent assay (ELISA) based on two anti-human ICOSL antibodies with different epitope specificities. Using the ELISA system, we found that sICOSL in the serum of healthy donors increases in an age-dependent manner and that the matrix metalloproteinase inhibitor (MMPI) could suppress sICOSL production. Together, these data demonstrate that the existence of circulating sICOSL in human serum might play an important role in immunoregulation.

Development of a Novel Functional Monoclonal Antibody to Human CD275: Characterization and Biological Activity

CD275 (B7-H2, ICOSL), a co-stimulatory molecule of the B7 superfamily, plays a critical role in immune response. In this report, a novel mouse anti-human CD275 monoclonal antibody (MAb) was prepared using hybridoma technology, and immunological characteristics of the MAb were determined. The results showed that the MAb (clone 13D11) was IgG2(κ) and bound specifically to human CD275. By mutual competition, we found that the antibody recognized different epitopes of CD275 antigen compared with commercial antibodies and could block ICOS-CD275 interaction. Crosslinking of CD275 with MAb 13D11 markedly blocked ICOS positive signal and inhibited T cell proliferation and cytokine production. In addition, the 13D11 MAb was suitable for indirect ELISA detection. Thus, the MAb against human CD275 with high specificity and different activity would be useful for further study of this molecule.

Local triggering of the ICOS coreceptor by CD11c(+) myeloid cells drives organ inflammation in lupus

The inducible T cell costimulator (ICOS) is a potent promoter of organ inflammation in murine lupus. ICOS stimulates T follicular helper cell differentiation in lymphoid tissue, suggesting that it might drive autoimmunity by enhancing autoantibody production. Yet the pathogenic relevance of this mechanism remains unclear. It is also unknown whether other ICOS-induced processes might contribute to lupus pathology. Here we show that selective ablation of ICOS ligand (ICOSL) in CD11c(+) cells, but not in B cells, dramatically ameliorates kidney and lung inflammation in lupus-prone MRL.Fas(lpr) mice. Autoantibody formation was largely unaffected by ICOSL deficiency in CD11c(+) cells. However, ICOSL display by CD11c(+) cells in inflamed organs had a nonredundant role in protecting invading T cells from apoptosis by elevating activity of the PI3K-Akt signaling pathway, thereby facilitating T cell accrual. These findings reveal a mechanism that locally sustains organ inflammation in lupus.

Circulating follicular helper-like T cells in systemic lupus erythematosus: association with disease activity

OBJECTIVE

To assess circulating follicular helper T (Tfh)-like CD4+ T cells in patients with systemic lupus erythematosus (SLE) and determine their relationship to disease activity.

METHODS

Blood samples from patients with SLE, as well as blood samples from patients with Behçet's disease (BD) and healthy individuals as controls, were analyzed. In all samples, circulating Tfh-like cells were enumerated by flow cytometry, using, as markers, expression of CXCR5, inducible T cell costimulator (ICOS), and programmed death 1 (PD-1) protein, as well as secretion of interleukin-21 (IL-21). The frequency of circulating Tfh-like cells was compared to that of circulating plasmablasts (CD19+IgD-CD38+). In addition, the possible association of circulating Tfh-like cells with the SLE Disease Activity Index (SLEDAI) was evaluated.

RESULTS

The subset of circulating Tfh-like T cells, identified as CXCR5(high) ICOS(high) PD-1(high) , was expanded in the blood of SLE patients compared to controls. Circulating Tfh-like cells were found to produce IL-21 and had lower expression of CCR7 as compared to that in circulating CXCR5(high) central memory T cells, thereby enabling their distinction. Expression of PD-1, but not ICOS or CXCR5, was significantly elevated in circulating Tfh-like cells from SLE patients compared to controls. PD-1 expression among CXCR5(high) circulating Tfh-like cells correlated with the SLEDAI, frequency of circulating plasmablasts, and anti-double-stranded DNA antibody positivity, but not with disease duration or past organ injury; rather, this cell profile appeared to be a reflection of current active disease.

CONCLUSION

Circulating Tfh-like cells are associated with disease activity in SLE, suggesting that their presence indicates abnormal homeostasis of T cell-B cell collaboration, with a causal relationship that is central to disease pathogenesis. These findings also suggest that circulating Tfh-like cells provide a surrogate for aberrant germinal center activity in SLE, and that their PD-1 expression offers a tool for measuring disease activity and monitoring the response to therapies.

IL-4-secreting secondary T follicular helper (Tfh) cells arise from memory T cells, not persisting Tfh cells, through a B cell-dependent mechanism

Humoral immunity requires cross-talk between T follicular helper (Tfh) cells and B cells. Nevertheless, a detailed understanding of this intercellular interaction during secondary immune responses is lacking. We examined this by focusing on the response to a soluble, unadjuvanted, pathogen-derived Ag (soluble extract of Schistosoma mansoni egg [SEA]) that induces type 2 immunity. We found that activated Tfh cells persisted for long periods within germinal centers following primary immunization. However, the magnitude of the secondary response did not appear to depend on pre-existing Tfh cells. Instead, Tfh cell populations expanded through a process that was dependent on memory T cells recruited into the reactive LN, as well as the participation of B cells. We found that, during the secondary response, IL-4 was critical for the expansion of a population of plasmablasts that correlated with increased SEA-specific IgG1 titers. Additionally, following immunization with SEA (but not with an Ag that induced type 1 immunity), IL-4 and IL-21 were coproduced by individual Tfh cells, revealing a potential mechanism through which appropriate class-switching can be coupled to plasmablast proliferation to enforce type 2 immunity. Our findings demonstrate a pivotal role for IL-4 in the interplay between T and B cells during a secondary Th2 response and have significant implications for vaccine design.

ICOS maintains the T follicular helper cell phenotype by down-regulating Krüppel-like factor 2

ICOS signaling is required for inhibition of the transcription factor Klf2, which controls expression of genes expressed by follicular T helper (Tfh) cells. When ICOS signaling is blocked, Tfh cells lose expression of characteristic Tfh genes and revert to an effector phenotype, resulting in disruption of the germinal center response.

The co-stimulators ICOS (inducible T cell co-stimulator) and CD28 are both important for T follicular helper (TFH) cells, yet their individual contributions are unclear. Here, we show that each molecule plays an exclusive role at different stages of TFH cell development. While CD28 regulated early expression of the master transcription factor Bcl-6, ICOS co-stimulation was essential to maintain the phenotype by regulating the novel TFH transcription factor Klf2 via Foxo1. Klf2 directly binds to Cxcr5, Ccr7, Psgl-1, and S1pr1, and low levels of Klf2 were essential to maintain this typical TFH homing receptor pattern. Blocking ICOS resulted in relocation of fully developed TFH cells back to the T cell zone and reversion of their phenotype to non-TFH effector cells, which ultimately resulted in breakdown of the germinal center response. Our study describes for the first time the exclusive role of ICOS and its downstream signaling in the maintenance of TFH cells by controlling their anatomical localization in the B cell follicle.

T follicular helper cell differentiation, function, and roles in disease

Follicular helper T (Tfh) cells are specialized providers of T cell help to B cells, and are essential for germinal center formation, affinity maturation, and the development of most high-affinity antibodies and memory B cells. Tfh cell differentiation is a multistage, multifactorial process involving B cell lymphoma 6 (Bcl6) and other transcription factors. This article reviews understanding of Tfh cell biology, including their differentiation, migration, transcriptional regulation, and B cell help functions. Tfh cells are critical components of many protective immune responses against pathogens. As such, there is strong interest in harnessing Tfh cells to improve vaccination strategies. Tfh cells also have roles in a range of other diseases, particularly autoimmune diseases. Overall, there have been dramatic advances in this young field, but there is much to be learned about Tfh cell biology in the interest of applying that knowledge to biomedical needs.

T-B-cell entanglement and ICOSL-driven feed-forward regulation of germinal centre reaction

The germinal centre (GC) reaction supports affinity-based B-cell competition and generates high-affinity bone-marrow plasma cells (BMPCs). How follicular T-helper (TFH) cells regulate GC selection is not clear. Using competitive mixed chimaera, we show here that, beyond the role in promoting TFH development, ICOSL (inducible T-cell co-stimulator ligand, also known as ICOSLG) is important for individual B cells to competitively participate in the GC reaction and to develop into BMPCs. Using intravital imaging aided by a calcium reporter, we further show that ICOSL promotes an 'entangled' mode of TFH-B-cell interactions, characterized by brief but extensive surface engagement, productive T-cell calcium spikes, and B-cell acquisition of CD40 signals. Reiterated entanglement promotes outer-zone co-localization of outcompeting GC B cells together with TFH cells, affording the former increased access to T-cell help. ICOSL on GC B cells is upregulated by CD40 signals. Such an intercellular positive feedback between contact-dependent help and ICOSL-controlled entanglement promotes positive selection and BMPC development, as evidenced by observations that higher-affinity B-cell receptor variants are enriched in the ICOSL(high) fraction, that numerically disadvantaged ICOSL-deficient GC B cells or BMPCs exhibit strong affinity compensation in competitive chimaera, and that when GC competition proceeds without ICOSL, selection of high-affinity variants in otherwise normal GC reactions is impaired. By demonstrating entanglement as the basic form of GC TFH-B-cell interactions, identifying ICOSL as a molecular linkage between T-B interactional dynamics and positive selection for high-affinity BMPC formation, our study reveals a pathway by which TFH cells control the quality of long-lived humoral immunity.