IL-21 regulates germinal center B cell differentiation and proliferation through a B cell-intrinsic mechanism

IL-21 treatment recovers follicular helper T cells and neutralizing antibody production in respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract infections in children under 1 year. RSV vaccines are currently unavailable, and children suffering from multiple reinfections by the same viral strain fail to develop protective responses. Although RSV-specific antibodies can be detected upon infection, these have limited neutralizing capacity. Follicular helper T (Tfh) cells are specialized in providing signals to B cells and help the production and affinity maturation of antibodies, mainly via interleukin (IL) 21 secretion. In this study, we evaluated whether RSV could inhibit Tfh responses. We observed that Tfh cells fail to upregulate IL-21 production upon RSV infection. In the lungs, RSV infection downregulated the expression of IL-21/interleukin-21 receptor (IL-21R) in Tfh cells and upregulated programmed death-ligand 1 (PD-L1) expression in dendritic cells (DCs) and B cells. PD-L1 blockade during infection recovered IL-21R expression in Tfh cells and increased the secretion of IL-21 in a DC-dependent manner. IL-21 treatment decreased RSV viral load and lung inflammation, inducing the formation of tertiary lymphoid organs in the lung. It also decreased regulatory follicular T cells, and increased Tfh cells, B cells, antibody avidity and neutralization capacity, leading to an overall improved anti-RSV humoral response in infected mice. Passive immunization with purified immunoglobulin G from IL-21-treated RSV-infected mice protected against RSV infection. Our results unveil a pathway by which RSV affects Tfh cells by increasing PD-L1 expression on antigen-presenting cells, highlighting the importance of an IL-21-PD-L1 axis for the generation of protective responses to RSV infection.

BAD inactivation exacerbates rheumatoid arthritis pathology by promoting survival of sublining macrophages

The resistance of synovial sublining macrophages to apoptosis has a crucial role in joint inflammation and destruction in rheumatoid arthritis (RA). However, the underlying mechanism is incompletely understood. Here we report that inactivation of the pro-apoptotic BCL-2 family protein BAD is essential for survival of synovial sublining macrophage in RA. Genetic disruption of Bad leads to more severe joint inflammation and cartilage and bone damage with reduced apoptosis of synovial sublining macrophages in collagen-induced arthritis (CIA) and TNFα transgenic (TNF-Tg) mouse models. Conversely, Bad3SA/3SA mice, in which BAD can no longer be inactivated by phosphorylation, are protected from collagen-induced arthritis. Mechanistically, phosphorylation-mediated inactivation of BAD specifically protects synovial sublining macrophages from apoptosis in highly inflammatory environment of arthritic joints in CIA and TNF-Tg mice, and in patients with RA, thereby contributing to RA pathology. Our findings put forward a model in which inactivation of BAD confers the apoptosis resistance on synovial sublining macrophages, thereby contributing to the development of arthritis, suggesting that BAD may be a potential therapeutic target for RA.

Non-human primates and Leishmania immunity

In the context of infectious diseases, non-human primates (NHP) provide the best animal models of human diseases due to the close phylogenetic relationship and the similar physiology and anatomical systems. Herein, we summarized the contribution of NHP models for understanding the immunity to leishmaniases, which are a group of diseases caused by infection with protozoan parasites of the genus Leishmania and classified as one of the neglected tropical diseases.

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.

IL-21 enhances STAT3/Blimp-1 signaling pathway in B cells and contributes to plasma cell differentiation in newly diagnosed patients with myasthenia gravis

The transcription factor Blimp-1 is necessary for the B cell differentiation toward immunoglobulin-secreting plasma cells. However, the immunopathological mechanisms of Blimp-1 that regulates B cell differentiation remain unclear in MG. The purpose of this study was to perform a quantitative and functional analysis of Blimp-1 in MG. A total of 34 patients with MG (18 ocular MG (OMG) and 16 generalized MG (GMG) and 20 healthy controls (HC) were recruited in this study. CD19⁺ B cells were isolated by positive selection using CD19 beads. The expression of Blimp-1 and p-STAT3 protein in isolated B cells was assessed by Western blot. Plasma cells were analyzed by flow cytometry. Serum IL-21 levels were detected by ELISA. Our data demonstrated that Blimp-1 in peripheral blood B cell of MG patients was significantly increased compared with HC. The increased expression of Blimp-1 was positively associated with clinical severity score (QMGs), plasma cell frequency, and serum IL-21 levels. Furthermore, glucocorticoid (GC) treatment reduced the expression of Blimp-1 and p-STAT3 in B cells, and this change was accompanied with relieved clinical severity, reduced plasma cell frequency, and decreased serum IL-21 levels. In vitro assay demonstrated that IL-21 stimulation upregulated STAT3 phosphorylation, increased Blimp-1 expression in B cells, and promoted plasma cell differentiation, and these processes could be inhibited by dexamethasone or STAT3 inhibitor stattic. This work indicates for the first time that aberrant expression of Blimp-1 exists on B cells and contributes to the plasma cell differentiation in MG patients. Modulation of IL-21/STAT3/Blimp-1 signaling pathway in B cells may be one of the mechanisms of glucocorticoid in the treatment of MG.

CD4 T Helper Cell Subsets and Related Human Immunological Disorders

The immune system plays a critical role in protecting hosts from the invasion of organisms. CD4 T cells, as a key component of the immune system, are central in orchestrating adaptive immune responses. After decades of investigation, five major CD4 T helper cell (Th) subsets have been identified: Th1, Th2, Th17, Treg (T regulatory), and Tfh (follicular T helper) cells. Th1 cells, defined by the expression of lineage cytokine interferon (IFN)-γ and the master transcription factor T-bet, participate in type 1 immune responses to intracellular pathogens such as mycobacterial species and viruses; Th2 cells, defined by the expression of lineage cytokines interleukin (IL)-4/IL-5/IL-13 and the master transcription factor GAΤA3, participate in type 2 immune responses to larger extracellular pathogens such as helminths; Th17 cells, defined by the expression of lineage cytokines IL-17/IL-22 and the master transcription factor RORγt, participate in type 3 immune responses to extracellular pathogens including some bacteria and fungi; Tfh cells, by producing IL-21 and expressing Bcl6, help B cells produce corresponding antibodies; whereas Foxp3-expressing Treg cells, unlike Th1/Th2/Th17/Tfh exerting their effector functions, regulate immune responses to maintain immune cell homeostasis and prevent immunopathology. Interestingly, innate lymphoid cells (ILCs) have been found to mimic the functions of three major effector CD4 T helper subsets (Th1, Th2, and Th17) and thus can also be divided into three major subsets: ILC1s, ILC2s, and ILC3s. In this review, we will discuss the differentiation and functions of each CD4 T helper cell subset in the context of ILCs and human diseases associated with the dysregulation of these lymphocyte subsets particularly caused by monogenic mutations.

Elevated serum IL-21 levels are associated with stable immune status in kidney transplant recipients and a mouse model of kidney transplantation

Allograft rejection after renal transplantation remains a challenge to overcome. Interleukin (IL)-21, a cytokine with pleiotropic effects, maintains immune homeostasis post-transplantation. Here, we report higher levels of IL-21 in kidney transplant recipients with non-rejection (NR) than in recipients with T cell-mediated rejection (TCMR, P < 0.001) and antibody-mediated rejection (ABMR, P = 0.005). We observed a negative correlation between IL-21 and creatinine (Cr) levels (P = 0.016). The receiving operating characteristic (ROC) curve showed a promising diagnostic value of IL-21 to identify acute rejection with an area under the curve (AUC) of 0.822 (P < 0.001). In contrast, exogenous administration of IL-21 accelerated acute rejection in a comparative translational kidney transplant (KT) mouse model. Reduced IL-21 levels in the peripheral blood were observed in KT mice after IL-21 injection. Further analysis revealed that increased IL-21 levels in the spleen induced proliferation of CD4+ T cells and CD19+ B cells after IL-21 treatment. Our findings suggest a critical function of IL-21 in kidney transplantation and the potential involvement of the IL-21/IL-21R pathway in acute rejection management.

Orphan Nuclear Receptor NR2F6 Suppresses T Follicular Helper Cell Accumulation through Regulation of IL-21

CD4 T follicular helper (Tfh) cells are specialized in helping B cells during the germinal center (GC) reaction and ultimately promote long-term humoral immunity. Here we report that loss of the nuclear orphan receptor NR2F6 causes enhanced survival and accumulation of Tfh cells, GC B cells, and plasma cells (PCs) following T cell-dependent immunization. Nr2f6-deficient CD4 T cell dysfunction is the primary cause of cell accumulation. Cytokine expression in Nr2f6-deficient Tfh cells is dysregulated, and Il21 expression is enhanced. Mechanistically, NR2F6 binds directly to the interleukin 21 (IL-21) promoter and a conserved noncoding sequence (CNS) near the Il21 gene in resting CD4⁺ T cells. During Tfh cell differentiation, this direct NR2F6 DNA interaction is abolished. Enhanced Tfh cell accumulation in Nr2f6-deficient mice can be reverted by blocking IL-21R signaling. Thus, NR2F6 is a critical negative regulator of IL-21 cytokine production in Tfh cells and prevents excessive Tfh cell accumulation.

•

Loss of NR2F6 results in accumulation of Tfh, GC B, and plasma cells after immunization

•

Increased GC populations depend on Nr2f6 loss within the CD4 compartment

•

NR2F6 directly binds to several sites within the Il21 promoter and CNS −36

•

NR2F6 restrains Il21 expression in CD4 cells; IL-21R blockade reduces Tfh accumulation

The transcription factor Hhex cooperates with the corepressor Tle3 to promote memory B cell development

Memory B cells (MBCs) are essential for long-lived humoral immunity. However, the transcription factors involved in MBC differentiation are poorly defined. Here, using single-cell RNA sequencing analysis, we identified a population of germinal center (GC) B cells in the process of differentiating into MBCs. Using an inducible CRISPR-Cas9 screening approach, we identified the hematopoietically expressed homeobox protein Hhex as a transcription factor regulating MBC differentiation. The corepressor Tle3 was also identified in the screen and was found to interact with Hhex to promote MBC development. Bcl-6 directly repressed Hhex in GC B cells. Reciprocally, Hhex-deficient MBCs exhibited increased Bcl6 expression and reduced expression of the Bcl-6 target gene Bcl2. Overexpression of Bcl-2 was able to rescue MBC differentiation in Hhex-deficient cells. We also identified Ski as an Hhex-induced transcription factor involved in MBC differentiation. These findings establish an important role for Hhex-Tle3 in regulating the transcriptional circuitry governing MBC differentiation.

DOCK8 is essential for LFA-1-dependent positioning of T follicular helper cells in germinal centers

T follicular helper (Tfh) cell migration into germinal centers (GCs) is essential for the generation of GC B cells and antibody responses to T cell-dependent (TD) antigens. This process requires interactions between lymphocyte function-associated antigen 1 (LFA-1) on Tfh cells and ICAMs on B cells. The mechanisms underlying defective antibody responses to TD antigens in DOCK8 deficiency are incompletely understood. We show that mice selectively lacking DOCK8 in T cells had impaired IgG antibody responses to TD antigens, decreased GC size, and reduced numbers of GC B cells. However, they developed normal numbers of Tfh cells with intact capacity for driving B cell differentiation into a GC phenotype in vitro. Notably, migration of DOCK8-deficient T cells into GCs was defective. Following T cell receptor (TCR)/CD3 ligation, DOCK8-deficient T cells had impaired LFA-1 activation and reduced binding to ICAM-1. Our results therefore indicate that DOCK8 is important for LFA-1-dependent positioning of Tfh cells in GCs, and thereby the generation of GC B cells and IgG antibody responses to TD antigen.

Extracellular Release of Antigen by Dendritic Cell Regurgitation Promotes B Cell Activation through NF-κB/cRel

Dendritic cells (DCs) are professional APCs, which sample Ags in the periphery and migrate to the lymph node where they activate T cells. DCs can also present native Ag to B cells through interactions observed both in vitro and in vivo. However, the mechanisms of Ag transfer and B cell activation by DCs remain incompletely understood. In this study, we report that murine DCs are an important cell transporter of Ag from the periphery to the lymph node B cell zone and also potent inducers of B cell activation both in vivo and in vitro. Importantly, we highlight a novel extracellular mechanism of B cell activation by DCs. In this study, we demonstrate that Ag released upon DC regurgitation is sufficient to efficiently induce early B cell activation, which is BCR driven and mechanistically dependent on the nuclear accumulation of the transcription factor NF-κB/cRel. Thus, our study provides new mechanistic insights into Ag delivery and B cell activation modalities by DCs and a promising approach for targeting NF-κB/cRel pathway to modulate the DC-elicited B cell responses.

Activated inducible co-stimulator-positive programmed cell death 1-positive follicular helper T cells indicate disease activity and severity in ulcerative colitis patients

Inducible co-stimulator-positive (ICOS) and programmed cell death 1-positive (PD-1) are important markers for follicular helper T cells (Tfh); however, their roles and clinical values in ulcerative colitis (UC) remain unknown. In this study, we recruited 68 UC patients and 34 healthy controls. Circulating ICOS⁺ , PD-1⁺ and ICOS⁺ PD-1⁺ Tfh subsets were analyzed by flow cytometry. Twelve active UC patients achieving remission after treatment with 5-aminosalicylic acid were followed-up and Tfh subset changes were analyzed. Serum immunoglobulin (Ig)G, C-reactive protein (CRP), interleukin (IL)-4 and IL-21 levels and B cell subsets were analyzed and Mayo scores were calculated. Correlation analyses were performed between Tfh subsets and the clinical indicators. Receiver operating characteristic (ROC) curves were generated to evaluate the efficiency of Tfh subsets for disease monitoring. We found that levels of ICOS⁺ , PD-1⁺ and ICOS⁺ PD-1⁺ Tfh cells were significantly increased in active UC and significantly decreased when achieving clinical remission. Activated ICOS⁺ PD-1⁺ Tfh cells were positively correlated with serum CRP and Mayo scores. Furthermore, ICOS⁺ PD-1⁺ Tfh cells were significantly correlated with circulating new memory B cells and plasmablasts, as well as serum IgG, IL-4 and IL-21. ROC analyses showed that when ICOS⁺ PD-1⁺ Tfh cells were used in combination with PD-1⁺ Tfh cells, the diagnostic efficacy in distinguishing active UC from stable remission patients was higher than that of any one used alone, with area under curve (AUC) value 0·931. Our findings suggest that increased ICOS⁺ PD-1⁺ Tfh cells are associated with the activation of B cells in the pathogenesis of UC, and may be a potential biomarker for UC disease monitoring.

Regulation of the germinal center and humoral immunity by interleukin-21

Here we review the critical and non-redundant functions of IL-21 in regulating humoral immune responses. We particularly focus on studies in natura—from individuals from inborn errors of immunity that impact on IL-21 production and/or function.

Cytokines play critical roles in regulating the development, survival, differentiation, and function of immune cells. Cytokines exert their function by binding specific receptor complexes on the surface of immune cells and activating intracellular signaling pathways, thereby resulting in induction of specific transcription factors and regulated expression of target genes. While the function of cytokines is often fundamental for the generation of robust and effective immunity following infection or vaccination, aberrant production or function of cytokines can underpin immunopathology. IL-21 is a pleiotropic cytokine produced predominantly by CD4⁺ T cells. Gene-targeting studies in mice, in vitro analyses of human and murine lymphocytes, and the recent discoveries and analyses of humans with germline loss-of-function mutations in IL21 or IL21R have revealed diverse roles of IL-21 in immune regulation and effector function. This review will focus on recent advances in IL-21 biology that have highlighted its critical role in T cell–dependent B cell activation, germinal center reactions, and humoral immunity and how impaired responses to, or production of, IL-21 can lead to immune dysregulation.

Restriction of memory B cell differentiation at the germinal center B cell positive selection stage

Memory B cells (MBCs) are key for protection from reinfection. However, it is mechanistically unclear how germinal center (GC) B cells differentiate into MBCs. MYC is transiently induced in cells fated for GC expansion and plasma cell (PC) formation, so-called positively selected GC B cells. We found that these cells coexpressed MYC and MIZ1 (MYC-interacting zinc-finger protein 1 [ZBTB17]). MYC and MIZ1 are transcriptional activators; however, they form a transcriptional repressor complex that represses MIZ1 target genes. Mice lacking MYC-MIZ1 complexes displayed impaired cell cycle entry of positively selected GC B cells and reduced GC B cell expansion and PC formation. Notably, absence of MYC-MIZ1 complexes in positively selected GC B cells led to a gene expression profile alike that of MBCs and increased MBC differentiation. Thus, at the GC positive selection stage, MYC-MIZ1 complexes are required for effective GC expansion and PC formation and to restrict MBC differentiation. We propose that MYC and MIZ1 form a module that regulates GC B cell fate.

Serine-threonine kinase ROCK2 regulates germinal center B cell positioning and cholesterol biosynthesis

Germinal center (GC) responses require B cells to respond to a dynamic set of intercellular and microenvironmental signals that instruct B cell positioning, differentiation, and metabolic reprogramming. RHO-associated coiled-coil-containing protein kinase 2 (ROCK2), a serine-threonine kinase that can be therapeutically targeted by ROCK inhibitors or statins, is a key downstream effector of RHOA GTPases. Although RHOA-mediated pathways are emerging as critical regulators of GC responses, the role of ROCK2 in B cells is unknown. Here, we found that ROCK2 was activated in response to key T cell signals like CD40 and IL-21 and that it regulated GC formation and maintenance. RNA-Seq analyses revealed that ROCK2 controlled a unique transcriptional program in GC B cells that promoted optimal GC polarization and cholesterol biosynthesis. ROCK2 regulated this program by restraining AKT activation and subsequently enhancing FOXO1 activity. ATAC-Seq (assay for transposase-accessible chromatin with high-throughput sequencing) and biochemical analyses revealed that the effects of ROCK2 on cholesterol biosynthesis were instead mediated via a novel mechanism. ROCK2 directly phosphorylated interferon regulatory factor 8 (IRF8), a crucial mediator of GC responses, and promoted its interaction with sterol regulatory element-binding transcription factor 2 (SREBP2) at key regulatory regions controlling the expression of cholesterol biosynthetic enzymes, resulting in optimal recruitment of SREBP2 at these sites. These findings thus uncover ROCK2 as a multifaceted and therapeutically targetable regulator of GC responses.

T follicular helper cells in germinal center B cell selection and lymphomagenesis

Germinal centers (GCs) are confined anatomic regions where rapidly proliferating B cells undergo somatic mutation and selection and eventual differentiation into memory B cells or long-lived plasma cells. GCs are also the origin of malignancy, namely follicular lymphoma (FL), GC B cell-diffuse large B cell lymphoma (GCB-DLBCL), and Burkitt lymphoma (BL). GC B cell lymphomas maintain their GC transcriptional signatures and sustain many features of the GC microenvironment, including CD4+ T follicular helper (Tfh) cells. Tfh cells are essential for the formation and maintenance of GCs, providing critical helper signals such as CD40L. Large-scale sequencing efforts have led to new insights about the tightly regulated selection mechanisms that are commonly targeted during GC B cell lymphomagenesis. For instance, HVEM, a frequently mutated surface molecule in GC-derived lymphomas, engages the inhibitory receptor BTLA on Tfh cells and loss of HVEM leads to exaggerated T cell help. Here, we review current understanding of how Tfh cells contribute to the selection of GC B cells, with a particular emphasis on how Tfh cell signals may contribute to lymphomagenesis. The possibility of targeting Tfh cells for the treatment of GC-derived lymphomas is discussed.

Serum IL-35 Levels Are Associated With Activity and Progression of Sarcoidosis

Objective: To investigate the relationship of interleukin (IL)-35 with sarcoidosis.

Methods: We enrolled 114 inpatients and outpatients with sarcoidosis at the Shanghai Pulmonary Hospital, and 24 healthy controls between March 2015 and December 2017. Serum and whole blood were collected during the follow-up period. Serum IL-35 levels were detected by ELISA. Proportions of Breg, Tfh, and Treg cells in the peripheral blood were detected using flow cytometry (FCM). The mRNA levels of p35, EBI3, and GAPDH in CD19⁺ cells and CD4⁺ cells were detected by real-time PCR. Sarcoidosis granuloma mice models were established with Propionibacterium acnes (PA) and one group was treated with IL-35 antibodies. Proportions of Breg, Tfh, and Treg cells in the peripheral blood and bronchoalveolar lavage fluid (BALF) were detected by FCM.

Results: The IL-35 levels and the proportions of Breg and Tfh cells in the peripheral blood of patients with active sarcoidosis were significantly higher compared to patients with stable sarcoidosis and healthy controls. Moreover, the IL-35 level in patients with progressive disease was lower than that found at the initial visit. EBI3 and p35 mRNA levels in CD19⁺ cells for patients with active sarcoidosis were significantly higher as compared to patients with stable sarcoidosis and healthy controls, while there were no significant differences in p35 and EBI3 mRNA levels in CD4⁺ cells between the three groups. In the mouse model of sarcoidosis, there were loose granulomata (macrophage accumulation in the bronchial areas and immature granuloma) after intervention with IL-35 antibodies. Meanwhile, the proportions of Breg cells in the peripheral blood and BALF of the model were significantly increased, while the proportion of Treg cells declined significantly. After intervention with IL-35 antibodies, the proportion of Breg cells in the peripheral blood of mice decreased significantly as compared to the mice not exposed to anti-IL-35 antibodies.

Conclusion: IL-35 levels increased significantly in the serum of patients with active sarcoidosis, and lower IL-35 levels were correlated with persistent disease. Serum IL-35 levels might be better correlated with Breg cell functions.

IL-21 and IL-5 coordinately induce surface IgA+ cells

Intestinal IgA is induced by microbes and food antigens. Peyer's patches (PPs) are known as one of the inductive sites for intestinal IgA production. However, the precise mechanism of IgA induction is as yet unknown. IgA secretion was induced from IgD⁺ B cells in vitro by stimulus with lipopolysaccharide in the presence of only retinoic acid (RA) and low doses of TGF-β1. Surface IgA⁺ cells were effectively induced from IgD⁺ B cells in vitro by the mixture of RA and the cytokines TGF-β1, APRIL, IL-5 and IL-21. rIL-21 upregulated surface IgA⁺ but impaired the proliferation of stimulated B cells in the presence of rTGF-β1, RA and rAPRIL, in vitro. The addition of rIL-5 restored the impaired proliferation by rIL-21, resulting in the expansion of IgA⁺ cells. rIL-21 induced the expression of Aicda and Prdm1, and impaired Rel in IgD⁺ B cells. Blockade of IL-21R signaling by a neutralizing mAb in vivo led to lower frequencies of IgA⁺ and IgG2b⁺ cells and lower germinal center B cells in PPs in a homeostatic condition. Although amounts of small intestinal IgA and titers of anti-dsDNA, the major target of intestinal IgA, in these mice were not altered, anti-OVA IgA titers induced by OVA drinking in OVA-specific T-cell receptor (TCR) transgenic mice were decreased. PP-deficient TCR transgenic mice showed diminished anti-OVA IgA induction. Blockade of IL-5R signaling in vivo led to similar results with relatively weaker effects than that of IL-21R mAb administration. These results suggest that IL-21 and IL-5 play cooperative roles in surface expression of IgA in PPs.

Molecular Aspects and Future Perspectives of Cytokine-Based Anti-cancer Immunotherapy

Cytokine-based immunotherapy is a promising field in the cancer treatment, since cytokines, as proteins of the immune system, are able to modulate the host immune response toward cancer cell, as well as directly induce tumor cell death. Since a low dose monotherapy with some cytokines has no significant therapeutic results and a high dose treatment leads to a number of side effects caused by the pleiotropic effect of cytokines, the problem of understanding the influence of cytokines on the immune cells involved in the pro- and anti-tumor immune response remains a pressing one. Immune system cells carry CD makers on their surface which can be used to identify various populations of cells of the immune system that play different roles in pro- and anti-tumor immune responses. This review discusses the functions and specific CD markers of various immune cell populations which are reported to participate in the regulation of the immune response against the tumor. The results of research studies and clinical trials investigating the effect of cytokine therapy on the regulation of immune cell populations and their surface markers are also discussed. Current trends in the development of cancer immunotherapy, as well as the role of cytokines in combination with other therapeutic agents, are also discussed.

TET2 deficiency reprograms the germinal center B cell epigenome and silences genes linked to lymphomagenesis

The TET2 DNA hydroxymethyltransferase is frequently disrupted by somatic mutations in diffuse large B cell lymphomas (DLBCLs), a tumor that originates from germinal center (GC) B cells. Here, we show that TET2 deficiency leads to DNA hypermethylation of regulatory elements in GC B cells, associated with silencing of the respective genes. This hypermethylation affects the binding of transcription factors including those involved in exit from the GC reaction and involves pathways such as B cell receptor, antigen presentation, CD40, and others. Normal GC B cells manifest a typical hypomethylation signature, which is caused by AID, the enzyme that mediates somatic hypermutation. However, AID-induced demethylation is markedly impaired in TET2-deficient GC B cells, suggesting that AID epigenetic effects are partially dependent on TET2. Last, we find that TET2 mutant DLBCLs also manifest the aberrant TET2-deficient GC DNA methylation signature, suggesting that this epigenetic pattern is maintained during and contributes to lymphomagenesis.

Adoptive cellular therapy with T cells expressing the dendritic cell growth factor Flt3L drives epitope spreading and antitumor immunity

Adoptive cell therapies using genetically engineered T cell receptor or chimeric antigen receptor T cells are emerging forms of immunotherapy that redirect T cells to specifically target cancer. However, tumor antigen heterogeneity remains a key challenge limiting their efficacy against solid cancers. Here, we engineered T cells to secrete the dendritic cell (DC) growth factor Fms-like tyrosine kinase 3 ligand (Flt3L). Flt3L-secreting T cells expanded intratumoral conventional type 1 DCs and substantially increased host DC and T cell activation when combined with immune agonists poly (I:C) and anti-4-1BB. Importantly, combination therapy led to enhanced inhibition of tumor growth and the induction of epitope spreading towards antigens beyond those recognized by adoptively transferred T cells in solid tumor models of T cell receptor and chimeric antigen receptor T cell therapy. Our data suggest that augmenting endogenous DCs is a promising strategy to overcome the clinical problem of antigen-negative tumor escape following adoptive cell therapy.

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.

1IFN-α Modulates Memory Tfh Cells and Memory B Cells in Mice, Following Recombinant FMDV Adenoviral Challenge

Follicular helper T (Tfh) cells regulate high-affinity antibody production. Some findings have indicated that Tfh cells could be differentiated into memory cells. Here we have investigated the effects of IFN-α, as an adjuvant, on the generation of memory Tfh cell and memory B cell responses. The data showed that adenoviral vectors expressing: (i) foot-and-mouth disease virus (FMDV) VP1 proteins and porcine IFN-α, or (ii) porcine IFN-α alone, potently enhanced the generation of memory Tfh cells, especially the CCR7 ^{l o} memory Tfh subset. Upon rechallenge with FMD recombinant adenoviral vaccines, IFN-α enhances Tfh cells activity, rapidly upregulating their signature Bcl-6, CXCR5, and IL-21 markers. The results suggest that IFN-α enhances the levels of the transcription factor Bcl-6 within Tfh cells, potentially by regulating STAT1. Additionally, IFN-α substantially increased the number of IgG1⁺ and CD86⁺ memory B cells, which are responsible for inducing the rapid effector functions of memory Tfh cells after vaccine reactivation, establishing the close relationship between memory B cell and memory Tfh cell subsets. In brief, IFN-α enhances the potency of FMD recombinant adenoviral vaccines to induce memory Tfh and memory B cell responses, thus elevating serum antibody titers. IFN-α administration therefore represents an attractive strategy for enhancing responses to vaccination.

Ethanol consumption inhibits TFH cell responses and the development of autoimmune arthritis

Alcohol consumption is a consistent protective factor for the development of autoimmune diseases such as rheumatoid arthritis (RA). The underlying mechanism for this tolerance-inducing effect of alcohol, however, is unknown. Here we show that alcohol and its metabolite acetate alter the functional state of T follicular helper (T_FH) cells in vitro and in vivo, thereby exerting immune regulatory and tolerance-inducing properties. Alcohol-exposed mice have reduced Bcl6 and PD-1 expression as well as IL-21 production by T_FH cells, preventing proper spatial organization of T_FH cells to form T_FH:B cell conjugates in germinal centers. This effect is associated with impaired autoantibody formation, and mitigates experimental autoimmune arthritis. By contrast, T cell independent immune responses and passive models of arthritis are not affected by alcohol exposure. These data clarify the immune regulatory and tolerance-inducing effect of alcohol consumption.

Moderate consumption of alcohol is associated with protection from some autoimmune diseases. Here the authors show that ethanol and its metabolite acetate can protect mice from collagen-induced arthritis and provide evidence that the mechanism of this effect might be via inhibition of the effector function of T follicular helper cells.

B Cell Activation and Response Regulation During Viral Infections

Acute viral infections are characterized by rapid increases in viral load, leading to cellular damage and the resulting induction of complex innate and adaptive antiviral immune responses that cause local and systemic inflammation. Successful antiviral immunity requires the activation of many immune cells, including T cells, natural killer cells, and macrophages. B cells play a unique part through their production of antibodies that can both neutralize and clear viral particles before virus entry into a cell. Protective antibodies are produced even before the first exposure of a pathogen, through the regulated secretion of so-called natural antibodies that are generated even in the complete absence of prior microbial exposure. An early wave of rapidly secreted antibodies from extrafollicular (EF) responses draws on the preexisting naive or memory repertoire of B cells to induce a strong protective response that in kinetics tightly follows the clearance of acute infections, such as with influenza virus. Finally, the generation of germinal centers (GCs) provides long-term protection through production of long-lived plasma cells and memory B cells, which shape and broaden the B cell repertoire for more effective responses following repeat exposures. In this study, we review B cell responses to acute viral infections, primarily influenza virus, from the earliest nonspecific B-1 cell to early, antigen-specific EF responses and finally to GC responses. Throughout, we address known factors that lead to distinct B cell response outcomes and discuss how their functions effect viral clearance, highlighting the critical contributions of each response type to the induction of highly protective antiviral humoral immunity.

Expanded circulating peripheral helper T cells in systemic lupus erythematosus: association with disease activity and B cell differentiation

OBJECTIVE

Peripheral helper T (TPH) cells are a recently identified Th cell subset that promotes B cell differentiation and antibody production in inflamed tissues. This study investigated circulating TPH cells to determine their involvement in systemic lupus erythematosus (SLE).

METHODS

Peripheral blood mononuclear cells collected from SLE patients and healthy individuals were analysed. TPH cells were identified as CD3+CD4+CD45RA-CXCR5- cells with a high expression of PD-1. The frequency, activation status and subsets of TPH cells were evaluated by flow cytometry. The production of IL-21 was assessed by intracellular staining and the association of TPH cells with disease activity and B cell populations was determined.

RESULTS

Circulating TPH cells, identified as CD3+CD4+CD45RA-PD-1highCXCR5- cells were increased in the peripheral blood of SLE patients compared with controls. Circulating TPH cells produced similar amounts of IL-21 compared with follicular Th cells. The expansion and activation of TPH cells were correlated with SLE disease activity. Activated TPH cells, particularly Th1-type TPH cells, were associated with the promotion of B cell differentiation in SLE patients.

CONCLUSION

The association of TPH cells with disease activity suggests the involvement of extrafollicular T-B cell interactions in the pathogenesis of SLE. TPH cells promote autoantibody production in aberrant lymphoid organs and therefore might be a novel therapeutic target in autoantibody-producing disorders.

Immunological mechanism of IgG4-related disease

IgG4-related disease (IgG4-RD) is an immune-mediated inflammatory disorder in multiple organs, characterized by abundant infiltration of IgG4-positive plasmacytes and fibrosis in the involved organs. The precise pathogenic mechanism of IgG4-RD still remains unclear. Aberrant innate and adaptive immunity are considered as the main pathogenesis of IgG4-RD. Recent studies have shown that abnormal adaptive immune responses mediated by T helper type 2 ​cells, regulatory T lymphocytes, CD4+ cytotoxic T lymphocytes, T follicular helper cells, T follicular regulatory cells, PD-1hiCXCR5-peripheral T helper cells and B cell subsets are involved in IgG4-RD. In addition to adaptive immune responses, innate immune responses play pathogenic roles in IgG4-RD. Macrophages, mast cells, basophils, complement, and plasmacytoid dendritic cells are activated to produce various kinds of cytokines in IgG4-RD. This review aims to summarize the most recent knowledge in the pathogenesis of IgG4-RD.

Novel Hemizygous IL2RG p.(Pro58Ser) Mutation Impairs IL-2 Receptor Complex Expression on Lymphocytes Causing X-Linked Combined Immunodeficiency

Hypomorphic IL2RG mutations may lead to milder phenotypes than X-SCID, named variably as atypical X-SCID or X-CID. We report an 11-year-old boy with a novel c. 172C>T;p.(Pro58Ser) mutation in IL2RG, presenting with atypical X-SCID phenotype. We also review the growing number of hypomorphic IL2RG mutations causing atypical X-SCID. We studied the patient's clinical phenotype, B, T, NK, and dendritic cell phenotypes, IL2RG and CD25 cell surface expression, and IL-2 target gene expression, STAT tyrosine phosphorylation, PBMC proliferation, and blast formation in response to IL-2 stimulation, as well as protein-protein interactions of the mutated IL2RG by BioID proximity labeling. The patient suffered from recurrent upper and lower respiratory tract infections, bronchiectasis, and reactive arthritis. His total lymphocyte counts have remained normal despite skewed T and B cells subpopulations, with very low numbers of plasmacytoid dendritic cells. Surface expression of IL2RG was reduced on his lymphocytes. This led to impaired STAT tyrosine phosphorylation in response to IL-2 and IL-21, reduced expression of IL-2 target genes in patient CD4+ T cells, and reduced cell proliferation in response to IL-2 stimulation. BioID proximity labeling showed aberrant interactions between mutated IL2RG and ER/Golgi proteins causing mislocalization of the mutated IL2RG to the ER/Golgi interface. In conclusion, IL2RG p.(Pro58Ser) causes X-CID. Failure of IL2RG plasma membrane targeting may lead to atypical X-SCID. We further identified another carrier of this mutation from newborn SCID screening, lost to closer scrutiny.

Strain specificities in influence of ageing on germinal centre reaction to inactivated influenza virus antigens in mice: Sex-based differences

Considering variability in vaccine responsiveness across human populations, in respect to magnitude and quality, and importance of vaccines in the elderly, the influence of recipient genetic background on the kinetics of age-related changes in the serum IgG antibody responses to seasonal trivalent inactivated split-virus influenza bulk (TIV) was studied in BALB/c and C57BL/6 mice showing quantitative and qualitative differences in this responses in young adult ages. With ageing the total serum IgG response to influenza viruses declined, in a strain-specific manner, so the strain disparity observed in young adult mice (the greater magnitude of IgG response in BALB/c mice) disappeared in aged mice. However, the sexual dimorphisms in this response (more prominent in females of both strains) remained in aged ones. The strain-specific differences in age-related decline in the magnitude of IgG response to TIV correlated with the number of germinal centre (GC) B splenocytes. The age-related decline in GC B cell number was consistent with the decrease in the proliferation of B cells and CD4+ cells in splenocyte cultures upon restimulation with TIV. Additionally, the age-related decrease in the magnitude of IgG response correlated with the increase in follicular T regulatory (fTreg)/follicular T helper (fTh) and fTreg/GC B splenocyte ratios (reflecting decrease in fTh and GC B numbers without changes in fTreg number), and the frequency of CD4+ splenocytes producing IL-21, a key factor in balancing the B cell and fTreg cell activity. With ageing the avidity of virus influenza-specific antibody increased in females of both strains. Moreover, ageing affected IgG2a/IgG1 and IgG2c/IgG1 ratios (reflecting Th1/Th2 balance) in male BALB/c mice and female C57BL/6 mice, respectively. Consequently, differently from young mice exhibiting the similar ratios in male and female mice, in aged female mice of both strains IgG2a(c)/IgG1 ratios were shifted towards a less effective IgG1 response (stimulated by IL-4 cytokines) compared with males. The age-related alterations in IgG subclass profiles in both strains correlated with those in IFN-γ/IL-4 production level ratio in splenocyte cultures restimulated with TIV. These findings stimulate further research to formulate sex-specific strategies to improve efficacy of influenza vaccine in the elderly.

Type 1 diabetes pathogenesis and the role of inhibitory receptors in islet tolerance

Type 1 diabetes (T1D) affects over a million Americans, and disease incidence is on the rise. Despite decades of research, there is still no cure for this disease. Exciting beta cell replacement strategies are being developed, but in order for such approaches to work, targeted immunotherapies must be designed. To selectively halt the autoimmune response, researchers must first understand how this response is regulated and which tolerance checkpoints fail during T1D development. Herein, we discuss the current understanding of T1D pathogenesis in humans, genetic and environmental risk factors, presumed roles of CD4+ and CD8+ T cells as well as B cells, and implicated autoantigens. We also highlight studies in non-obese diabetic mice that have demonstrated the requirement for CD4+ and CD8+ T cells and B cells in driving T1D pathology. We present an overview of central and peripheral tolerance mechanisms and comment on existing controversies in the field regarding central tolerance. Finally, we discuss T cell- and B cell-intrinsic tolerance mechanisms, with an emphasis on the roles of inhibitory receptors in maintaining islet tolerance in humans and in diabetes-prone mice, and strategies employed to date to harness inhibitory receptor signaling to prevent or reverse T1D.

Compromised steady-state germinal center activity with age in nonhuman primates

Age-related reductions in vaccine-induced B cells in aging indicate that germinal centers (GCs), the anatomical site where the development of humoral responses takes place, may lose efficacy with age. We have investigated the baseline follicular and GC composition in nonhuman primates (NHPs) with respect to their age. There was a marked reduction in follicular area in old animals. We found significantly lower normalized numbers of follicular PD1hi CD4 T (Tfh) and proliferating (Ki67hi ) GC B cells with aging, a profile associated with significantly higher numbers of potential follicular suppressor FoxP3hi Lag3hi CD4 T cells. Furthermore, a positive correlation was found between Tfh and follicular CD8 T cells (fCD8) only in young animals. Despite the increased levels of circulating preinflammatory factors in aging, young animals had higher numbers of monocytes and granulocytes in the follicles, a profile negatively associated with numbers of Tfh cells. Multiple regression analysis showed an altered association between GC B cells and other GC immune cell populations in old animals suggesting a differential mechanistic regulation of GC activity in aging. Our data demonstrate defective baseline GC composition in old NHPs and provide an immunological base for further understanding the adaptive humoral responses with respect to aging.

PD-1-expressing MAIT cells from patients with tuberculosis exhibit elevated production of CXCL13

To understand functional role of PD-1-expressing MAIT cells during tuberculosis infection in humans, sorted PD-1⁺ and PD-1^- MAIT cells from pleural effusions of patients with pleural tuberculosis were subjected to transcriptome sequencing. PD-1-expressing MAIT cells were analysed by flow cytometry and their phenotypic and functional features were investigated. Transcriptome sequencing identified 144 genes that were differentially expressed between PD-1⁺ and PD-1^- MAIT cells from tuberculous pleural effusions and CXCL13 was the gene with highest fold difference. The level of PD-1-expressing MAIT cells was associated with extent of TB infection in humans. PD-1-expressing MAIT cells had increased production of CXCL13 and IL-21 as determined by flow cytometry. PD-1^high CXCR5^- MAIT cells were significantly expanded in pleural effusions from patients with pleural tuberculosis as compared with those from peripheral blood of both patients with tuberculosis and healthy controls. Although PD-1^high CXCR5^- MAIT cells from tuberculous pleural effusions had reduced IFN-γ level and increased expression of Tim-3 and GITR, they showed activated phenotype and had higher glucose uptake and lipid content. It is concluded that PD-1-expressing MAIT cells had reduced IFN-γ level but increased production of both CXCL13 and IL-21.

Linking signaling and selection in the germinal center

Germinal centers (GC) are sites of rapid B-cell proliferation in response to certain types of immunization. They arise in about 1 week and can persist for several months. In GCs, B cells differentiate in a unique way and begin to undergo somatic mutation of the Ig V regions at a high rate. GC B cells (GCBC) thus undergo clonal diversification that can affect the affinity of the newly mutant B-cell receptor (BCR) for its driving antigen. Through processes that are still poorly understood, GCBC with higher affinity are selectively expanded while those with mutations that inactivate the BCR are lost. In addition, at various times during the extended GC reaction, some GCBC undergo differentiation into either long-lived memory B cells (MBC) or plasma cells. The cellular and molecular signals that govern these fate decisions are not well-understood, but are an active area of research in multiple laboratories. In this review, we cover both the history of this field and focus on recent work that has helped to elucidate the signals and molecules, such as key transcription factors, that coordinate both positive selection as well as differentiation of GCBC.

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.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

Design and application of single-cell RNA sequencing to study kidney immune cells in lupus nephritis

The immune mechanisms that cause tissue injury in lupus nephritis have been challenging to define. The advent of high-dimensional cellular analyses, such as single-cell RNA sequencing, has enabled detailed characterization of the cell populations present in small biopsy samples of kidney tissue. In parallel, the development of methods that cryopreserve kidney biopsy specimens in a manner that preserves intact, viable cells, has enabled the uniform analysis of tissue samples collected at multiple sites and across many geographic areas and demographic cohorts with high-dimensional platforms. The application of these methods to kidney biopsy samples from patients with lupus nephritis has begun to define the phenotypes of both infiltrating and resident immune cells, as well as parenchymal cells, present in nephritic kidneys. The detection of similar immune cell populations in urine suggests that it might be possible to non-invasively monitor immune activation in kidneys. Once applied to large patient cohorts, these high-dimensional studies might enable patient stratification according to patterns of immune cell activation in the kidney or identify disease features that can be used as surrogate measures of efficacy in clinical trials. Applied broadly across multiple inflammatory kidney diseases, these studies promise to enormously expand our understanding of renal inflammation in the next decade.

B cell memory: building two walls of protection against pathogens

Surviving a single infection often results in lifelong immunity to the infecting pathogen. Such protection is mediated, in large part, by two main B cell memory ‘walls’ — namely, long-lived plasma cells and memory B cells. The cellular and molecular processes that drive the production of long-lived plasma cells and memory B cells are subjects of intensive research and have important implications for global health. Indeed, although nearly all vaccines in use today depend on their ability to induce B cell memory, we have not yet succeeded in developing vaccines for some of the world’s most deadly diseases, including AIDS and malaria. Here, we describe the two-phase process by which antigen drives the generation of long-lived plasma cells and memory B cells and highlight the challenges for successful vaccine development in each phase.

The authors discuss the formation of two main ‘walls’ of B cell memory to protect against pathogen reinfection. The first wall comprises high-affinity antibodies produced by long-lived plasma cells, while the second wall is formed by memory B cells.

T Follicular Helper Cell Subsets and the Associated Cytokine IL-21 in the Pathogenesis and Therapy of Asthma

For many decades, T helper 2 (T_H2) cells have been considered to predominantly regulate the pathogenic manifestations of allergic asthma, such as IgE-mediated sensitization, airway hyperresponsiveness, and eosinophil infiltration. However, recent discoveries have significantly shifted our understanding of asthma from a simple T_H2 cell-dependent disease to a heterogeneous disease regulated by multiple T cell subsets, including T follicular helper (T_FH) cells. T_FH cells, which are a specialized cell population that provides help to B cells, have attracted intensive attention in the past decade because of their crucial role in regulating antibody response in a broad range of diseases. In particular, T_FH cells are essential for IgE antibody class-switching. In this review, we summarize the recent progress regarding the role of T_FH cells and their signature cytokine interleukin (IL)-21 in asthma from mouse studies and clinical reports. We further discuss future therapeutic strategies to treat asthma by targeting T_FH cells and IL-21.

Interleukin (IL)-21 in Inflammation and Immunity During Parasitic Diseases

Parasitic diseases cause significant morbidity and mortality in the developing and underdeveloped countries. No efficacious vaccines are available against most parasitic diseases and there is a critical need for developing novel vaccine strategies for care. IL-21 is a pleiotropic cytokine whose functions in protection and immunopathology during parasitic diseases have been explored in limited ways. IL-21 and its cognate receptor, IL-21R, are highly expressed in parasitized organs of infected humans as well in murine models of the human parasitic diseases. Prior studies have indicated the ability of the IL-21/IL-21R signaling axis to regulate the effector functions (e.g., cytokine production) of T cell subsets by enhancing the expression of T-bet and STAT4 in human T cells, resulting in an augmented production of IFN-γ. Mice deficient for either IL-21 (Il21^−/−) or IL-21R (Il21r^−/−) showed significantly reduced inflammatory responses following parasitic infections as compared with their WT counterparts. Targeting the IL-21/IL-21R signaling axis may provide a novel approach for the development of new therapeutic agents for the prevention of parasite-induced immunopathology and tissue destruction.

Impaired memory B-cell development and antibody maturation with a skewing toward IgE in patients with STAT3 hyper-IgE syndrome

BACKGROUND

Signal transducer and activator of transcription 3 hyper-IgE syndrome (STAT3-HIES) is caused by heterozygous mutations in the STAT3 gene and is associated with eczema, elevated serum IgE, and recurrent infections resembling severe atopic dermatitis, while clinically relevant specific IgE is almost absent.

METHODS

To investigate the impact of STAT3 signaling on B-cell responses, we assessed lymph node and bone marrow, blood B and plasma cell subsets, somatic hypermutations in Ig genes, and in vitro proliferation and antibody production in STAT3-HIES patients and healthy controls.

RESULTS

Lymph nodes of STAT3-HIES patients showed normal germinal center architecture and CD138⁺ plasma cells residing in the paracortex, which expressed IgE, IgG, and IgM but not IgA. IgE⁺ plasma cells were abundantly present in STAT3-HIES bone marrow. Proliferation of naive B cells upon stimulation with CD40L and IL-4 was similar in patients and controls, while patient cells showed reduced responses to IL-21. IgE, IgG1, IgG3 and IgA1 transcripts showed reduced somatic hypermutations. Peripheral blood IgE⁺ memory B-cell frequencies were increased in STAT3-HIES, while other memory B-cell frequencies except for IgG4⁺ cells were decreased.

CONCLUSIONS

Despite impaired STAT3 signaling, STAT3-HIES patients can mount in vivo T-cell-dependent B-cell responses, while circulating memory B cells, except for those expressing IgG4 and IgE, were reduced. Reduced molecular maturation demonstrated the critical need of STAT3 signaling for optimal affinity maturation and B-cell differentiation, supporting the need for immunoglobulin substitution therapy and explaining the high IgE serum level in the majority with absent allergic symptoms.

An overview of T follicular cells in transplantation: spotlight on their clinical significance

Introduction: For late stage organ failure patients, transplantation is the best option to increase life expectancy with a superior quality of life. Unfortunately, after transplantation many patients are at risk of cellular and antibody-mediated rejection (ABMR). The latter is initiated by donor specific antibodies (DSA) which depend on the actions of B cells, T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells that are present in the germinal center of lymphoid organs.Areas covered: In this overview paper, we discuss the biology and function of Tfh and Tfr cells in lymphoid tissues, transplanted organs and their circulating counterparts. We report on their relevance to alloimmunity and on the effects of immunosuppressive drugs on these immunocompetent cell populations.Expert opinion: Growing knowledge about the actions of Tfh and Tfr allows for a better understanding of the immunological mechanisms of ABMR after organ transplantation. This understanding feeds the hypothesis that immunosuppressive drugs targeting the actions of Tfh cells have huge therapeutic potential. This new concept in the treatment of the humoral rejection response will improve graft and patient survival after organ transplantation.

Pregnancy favors circulating IL-21-secreting TFH -like cell recovery in ARV-treated HIV-1-infected women

PROBLEM

Pregnancy appears to favor maternal antibody production. In contrast, by damaging follicular helper T cells (T_FH ), HIV-1 infection compromises protective humoural immune response. Therefore, we aimed to investigate the frequency of different T_FH -like cells in HIV-infected pregnant women (PW) before and after antiretroviral (ARV) therapy.

METHOD OF STUDY

Peripheral blood mononuclear cells, CD4⁺ T and B cells, were obtained from asymptomatic HIV-1-infected non-PW and PW just before and after ARV therapy. In some experiments, healthy HIV-1-negative PW were also tested. The frequency of different T_FH -like cell subsets was determined by flow cytometry. The plasma titers of IgG anti-tetanus toxoid (TT), anti-HBsAg, and anti-gp41 were determined by ELISA. The in vitro production of total IgG, IL-21, and hormones (estrogen and progesterone) was quantified also by ELISA.

RESULTS

Our results demonstrate that antiretroviral (ARV) therapy was more efficient in elevating the percentage of circulating IL-21-secreting T_FH cells in HIV-1-infected pregnant women (PW) than in non-pregnant patients (nPW). Moreover, in co-culture systems, CD4⁺ T cells from ART-treated PW were more efficient in assisting B cells to produce IgG production. The in vivo anti-HBsAg IgG titers after ARV therapy were also significantly higher in PW, and their levels were directly associated with both IL-21⁺ T_FH frequency and plasma concentration of estrogen.

CONCLUSION

In summary, our results suggest that pregnancy favors the recovery of T_FH -like cells after ARV therapy in HIV-1-infected women, which could help these mothers to protect their newborns from infectious diseases by transferring IgG across the placenta.

PD-1hiCXCR5- T peripheral helper cells promote B cell responses in lupus via MAF and IL-21

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by pathologic T cell-B cell interactions and autoantibody production. Defining the T cell populations that drive B cell responses in SLE may enable design of therapies that specifically target pathologic cell subsets. Here, we evaluated the phenotypes of CD4+ T cells in the circulation of 52 SLE patients drawn from multiple cohorts and identified a highly expanded PD-1hiCXCR5-CD4+ T cell population. Cytometric, transcriptomic, and functional assays demonstrated that PD-1hiCXCR5-CD4+ T cells from SLE patients are T peripheral helper (Tph) cells, a CXCR5- T cell population that stimulates B cell responses via IL-21. The frequency of Tph cells, but not T follicular helper (Tfh) cells, correlated with both clinical disease activity and the frequency of CD11c+ B cells in SLE patients. PD-1hiCD4+ T cells were found within lupus nephritis kidneys and correlated with B cell numbers in the kidney. Both IL-21 neutralization and CRISPR-mediated deletion of MAF abrogated the ability of Tph cells to induce memory B cell differentiation into plasmablasts in vitro. These findings identify Tph cells as a highly expanded T cell population in SLE and suggest a key role for Tph cells in stimulating pathologic B cell responses.

Genetic modification of primary human B cells to model high-grade lymphoma

Sequencing studies of diffuse large B cell lymphoma (DLBCL) have identified hundreds of recurrently altered genes. However, it remains largely unknown whether and how these mutations may contribute to lymphomagenesis, either individually or in combination. Existing strategies to address this problem predominantly utilize cell lines, which are limited by their initial characteristics and subsequent adaptions to prolonged in vitro culture. Here, we describe a co-culture system that enables the ex vivo expansion and viral transduction of primary human germinal center B cells. Incorporation of CRISPR/Cas9 technology enables high-throughput functional interrogation of genes recurrently mutated in DLBCL. Using a backbone of BCL2 with either BCL6 or MYC, we identify co-operating genetic alterations that promote growth or even full transformation into synthetically engineered DLBCL models. The resulting tumors can be expanded and sequentially transplanted in vivo, providing a scalable platform to test putative cancer genes and to create mutation-directed, bespoke lymphoma models.

Regulation of B cell responses by distinct populations of CD4 T cells

Maturation of B cells in Germinal Centers (GC) is a hallmark in adaptive immunity and the basis of successful vaccines that protect us against lethal infections. Nonetheless, vaccination efficacy is very much reduced in aged population and against highly mutagenic viruses. Therefore, it is key to understand how B cell selection takes place in GC in order to develop new and fully protective vaccines. The cellular mechanisms that control selection of GC B cells are performed by different T cell populations. On one side, cognate entanglement of B cells with T follicular helper (Tfh) cells through cytokines and co-stimulatory signals promotes survival, proliferation, mutagenesis and terminal differentiation of GC B cells. On the other hand, regulatory T cells have also been reported within GC and interfere with T cell help for antibody production. These cells have been classified as a distinct T cell sub-population called T Follicular regulatory cells (Tfr). In this review, we investigate the phenotype, function and differentiation of these two cell populations. In addition, based on the different functions of these cell subsets, we highlight the open questions surrounding their heterogeneity.

Identification and Characterization of Post-activated B Cells in Systemic Autoimmune Diseases

Autoimmune diseases (AID) such as systemic lupus erythematosus (SLE), primary Sjögren's syndrome (pSS), and rheumatoid arthritis (RA) are chronic inflammatory diseases in which abnormalities of B cell function play a central role. Although it is widely accepted that autoimmune B cells are hyperactive in vivo, a full understanding of their functional status in AID has not been delineated. Here, we present a detailed analysis of the functional capabilities of AID B cells and dissect the mechanisms underlying altered B cell function. Upon BCR activation, decreased spleen tyrosine kinase (Syk) and Bruton's tyrosine kinase (Btk) phosphorylation was noted in AID memory B cells combined with constitutive co-localization of CD22 and protein tyrosine phosphatase (PTP) non-receptor type 6 (SHP-1) along with hyporesponsiveness to TLR9 signaling, a Syk-dependent response. Similar BCR hyporesponsiveness was also noted specifically in SLE CD27⁻ B cells together with increased PTP activities and increased transcripts for PTPN2, PTPN11, PTPN22, PTPRC, and PTPRO in SLE B cells. Additional studies revealed that repetitive BCR stimulation of normal B cells can induce BCR hyporesponsiveness and that tissue-resident memory B cells from AID patients also exhibited decreased responsiveness immediately ex vivo, suggesting that the hyporesponsive status can be acquired by repeated exposure to autoantigen(s) in vivo. Functional studies to overcome B cell hyporesponsiveness revealed that CD40 co-stimulation increased BCR signaling, induced proliferation, and downregulated PTP expression (PTPN2, PTPN22, and receptor-type PTPs). The data support the conclusion that hyporesponsiveness of AID and especially SLE B cells results from chronic in vivo stimulation through the BCR without T cell help mediated by CD40–CD154 interaction and is manifested by decreased phosphorylation of BCR-related proximal signaling molecules and increased PTPs. The hyporesponsiveness of AID B cells is similar to a form of functional anergy.

Neonatal T Follicular Helper Cells Are Lodged in a Pre-T Follicular Helper Stage Favoring Innate Over Adaptive Germinal Center Responses

T follicular helper (T_fh) cells have emerged as a critical limiting factor for controlling the magnitude of neonatal germinal center (GC) reactions and primary vaccine antibody responses. We compared the functional attributes of neonatal and adult T_fh cells at the transcriptomic level and demonstrated that the T_fh cell program is well-initiated in neonates although the T_fh gene-expression pattern (i.e., CXCR5, IL-21, BCL6, TBK1, STAT4, ASCL2, and c-MAF) is largely underrepresented as compared to adult T_fh cells. Importantly, we identified a TH2-bias of neonatal T_fh cells, with preferential differentiation toward short-lived pre-T_fh effector cells. Remarkably, adjuvantation with CpG-ODNs redirect neonatal pre-T_fh cells toward committed GC-T_fh cells, as illustrated by increased expression of T_fh signature genes and reduced expression of TH2-related genes.

Short Communication: Nonprogressive HIV-1 Infection Is Associated with Expansion of IL-21R Expressing Class-Switched Memory B Cells

HIV perturbs the functionality of B cells resulting in defective humoral responses. As efficient humoral immune responses are important in controlling HIV-disease progression, we characterized the memory B cell population for its subsets and their activation (CD38 expression) and functional [interleukin (IL)-21R expression] profile in individuals with nonprogressive [long-term nonprogressors (LTNPs), N = 16] and progressive HIV disease (progressors, N = 16) along with 10 HIV uninfected healthy controls (HCs). The frequencies of total memory B cells were similar in HCs and HIV-infected individuals, whereas the frequencies of unswitched memory B (UMB) cells and CD38⁺ UMB cells were significantly higher in progressors than LTNPs and HCs (p < .03). LTNPs showed higher frequencies of class-switched memory B (SMB) cells and IL-21R expressing SMB cells than seen in progressors (p = .019), which were similar to that seen in HCs. The %UMB cells correlated inversely (p = .0002, r = -0.6053) and %SMB cells correlated positively (p = .0005, r = 0.5804) with CD4 count. IL-21/IL-21R interaction is required for class switching of B cells and differentiation into antibody-secreting plasma cells. The higher expression of IL-21R on class SMB cells from LTNPs might be resulting in efficient plasma cell differentiation and the functional humoral immune response that might be responsible for mounting efficient antibody response against the encountered infections. The more insights in this area might be required to further understand the role of IL-21R expressing class SMB cells in HIV infection.