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

Patients and mice with deficiency in the SNARE protein SYNTAXIN-11 have a secondary B cell defect

SYNTAXIN-11 (STX11) is a SNARE protein that mediates the fusion of cytotoxic granules with the plasma membrane at the immunological synapses of CD8 T or NK cells. Autosomal recessive inheritance of deleterious STX11 variants impairs cytotoxic granule exocytosis, causing familial hemophagocytic lymphohistiocytosis type 4 (FHL-4). In several FHL-4 patients, we also observed hypogammaglobulinemia, elevated frequencies of naive B cells, and increased double-negative DN2:DN1 B cell ratios, indicating a hitherto unrecognized role of STX11 in humoral immunity. Detailed analysis of Stx11-deficient mice revealed impaired CD4 T cell help for B cells, associated with disrupted germinal center formation, reduced isotype class switching, and low antibody avidity. Mechanistically, Stx11-/- CD4 T cells exhibit impaired membrane fusion leading to reduced CD107a and CD40L surface mobilization and diminished IL-2 and IL-10 secretion. Our findings highlight a critical role of STX11 in SNARE-mediated membrane trafficking and vesicle exocytosis in CD4 T cells, important for successful CD4 T cell-B cell interactions. Deficiency in STX11 impairs CD4 T cell-dependent B cell differentiation and humoral responses.

Specialized Tfh cell subsets driving type-1 and type-2 humoral responses in lymphoid tissue

Effective antibody responses are essential to generate protective humoral immunity. Different inflammatory signals polarize T cells towards appropriate effector phenotypes during an infection or immunization. Th1 and Th2 cells have been associated with the polarization of humoral responses. However, T follicular helper cells (Tfh) have a unique ability to access the B cell follicle and support the germinal center (GC) responses by providing B cell help. We investigated the specialization of Tfh cells induced under type-1 and type-2 conditions. We first studied homogenous Tfh cell populations generated by adoptively transferred TCR-transgenic T cells in mice immunized with type-1 and type-2 adjuvants. Using a machine learning approach, we established a gene expression signature that discriminates Tfh cells polarized towards type-1 and type-2 response, defined as Tfh1 and Tfh2 cells. The distinct signatures of Tfh1 and Tfh2 cells were validated against datasets of Tfh cells induced following lymphocytic choriomeningitis virus (LCMV) or helminth infection. We generated single-cell and spatial transcriptomics datasets to dissect the heterogeneity of Tfh cells and their localization under the two immunizing conditions. Besides a distinct specialization of GC Tfh cells under the two immunizations and in different regions of the lymph nodes, we found a population of Gzmk+ Tfh cells specific for type-1 conditions. In human individuals, we could equally identify CMV-specific Tfh cells that expressed Gzmk. Our results show that Tfh cells acquire a specialized function under distinct types of immune responses and with particular properties within the B cell follicle and the GC.

Interferon-γ production by Tfh cells is required for CXCR3+ pre-memory B cell differentiation and subsequent lung-resident memory B cell responses

Lung-resident memory B cells (lung-BRMs) differentiate into plasma cells after reinfection, providing enhanced pulmonary protection. Here, we investigated the determinants of lung-BRM differentiation upon influenza infection. Kinetic analyses revealed that influenza nucleoprotein (NP)-specific BRMs preferentially differentiated early after infection and required T follicular helper (Tfh) cell help. BRM differentiation temporally coincided with transient interferon (IFN)-γ production by Tfh cells. Depletion of IFN-γ in Tfh cells prevented lung-BRM differentiation and impaired protection against heterosubtypic infection. IFN-γ was required for expression of the transcription factor T-bet by germinal center (GC) B cells, which promoted differentiation of a CXCR3+ GC B cell subset that were precursors of lung-BRMs and CXCR3+ memory B cells in the mediastinal lymph node. Absence of IFN-γ signaling or T-bet in GC B cells prevented CXCR3+ pre-memory precursor development and hampered CXCR3+ memory B cell differentiation and subsequent lung-BRM responses. Thus, Tfh-cell-derived IFN-γ is critical for lung-BRM development and pulmonary immunity, with implications for vaccination strategies targeting BRMs.

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

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

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

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

Immune surveillance and humoral immune responses in kidney transplantation - A look back at T follicular helper cells

T follicular helper cells comprise a specialized, heterogeneous subset of immune-competent T helper cells capable of influencing B cell responses in lymphoid tissues. In physiology, for example in response to microbial challenges or vaccination, this interaction chiefly results in the production of protecting antibodies and humoral memory. In the context of kidney transplantation, however, immune surveillance provided by T follicular helper cells can take a life of its own despite matching of human leukocyte antigens and employing the latest immunosuppressive regiments. This puts kidney transplant recipients at risk of subclinical and clinical rejection episodes with a potential risk for allograft loss. In this review, the current understanding of immune surveillance provided by T follicular helper cells is briefly described in physiological responses to contrast those pathological responses observed after kidney transplantation. Sensitization of T follicular helper cells with the subsequent emergence of detectable donor-specific human leukocyte antigen antibodies, non-human leukocyte antigen antibodies their implication for kidney transplantation and lessons learnt from other transplantation "settings" with special attention to antibody-mediated rejection will be addressed.

The human Stat1 gain-of-function T385M mutation causes expansion of activated T-follicular helper/T-helper 1-like CD4 T cells and sex-biased autoimmunity in specific pathogen-free mice

Introduction

Humans with gain-of-function (GOF) mutations in STAT1 (Signal Transducer and Activator of Transcription 1), a potent immune regulator, experience frequent infections. About one-third, especially those with DNA-binding domain (DBD) mutations such as T385M, also develop autoimmunity, sometimes accompanied by increases in T-helper 1 (Th1) and T-follicular helper (Tfh) CD4 effector T cells, resembling those that differentiate following infection-induced STAT1 signaling. However, environmental and molecular mechanisms contributing to autoimmunity in STAT1 GOF patients are not defined.

Methods

We generated Stat1T385M/+ mutant mice to model the immune impacts of STAT1 DBD GOF under specific-pathogen free (SPF) conditions.

Results

Stat1T385M/+ lymphocytes had more total Stat1 at baseline and also higher amounts of IFNg-induced pStat1. Young mutants exhibited expansion of Tfh-like cells, while older mutants developed autoimmunity accompanied by increased Tfh-like cells, B cell activation and germinal center (GC) formation. Mutant females exhibited these immune changes sooner and more robustly than males, identifying significant sex effects of Stat1T385M-induced immune dysregulation. Single cell RNA-Seq (scRNA-Seq) analysis revealed that Stat1T385M activated transcription of GC-associated programs in both B and T cells. However, it had the strongest transcriptional impact on T cells, promoting aberrant CD4 T cell activation and imparting both Tfh-like and Th1-like effector programs.

Discussion

Collectively, these data demonstrate that in the absence of overt infection, Stat1T385M disrupted naïve CD4 T cell homeostasis and promoted expansion and differentiation of abnormal Tfh/Th1-like helper and GC-like B cells, eventually leading to sex-biased autoimmunity, suggesting a model for STAT1 GOF-induced immune dysregulation and autoimmune sequelae in humans.

Evolving cognition of the JAK-STAT signaling pathway: autoimmune disorders and cancer

The Janus kinase (JAK) signal transducer and activator of transcription (JAK-STAT) pathway is an evolutionarily conserved mechanism of transmembrane signal transduction that enables cells to communicate with the exterior environment. Various cytokines, interferons, growth factors, and other specific molecules activate JAK-STAT signaling to drive a series of physiological and pathological processes, including proliferation, metabolism, immune response, inflammation, and malignancy. Dysregulated JAK-STAT signaling and related genetic mutations are strongly associated with immune activation and cancer progression. Insights into the structures and functions of the JAK-STAT pathway have led to the development and approval of diverse drugs for the clinical treatment of diseases. Currently, drugs have been developed to mainly target the JAK-STAT pathway and are commonly divided into three subtypes: cytokine or receptor antibodies, JAK inhibitors, and STAT inhibitors. And novel agents also continue to be developed and tested in preclinical and clinical studies. The effectiveness and safety of each kind of drug also warrant further scientific trials before put into being clinical applications. Here, we review the current understanding of the fundamental composition and function of the JAK-STAT signaling pathway. We also discuss advancements in the understanding of JAK-STAT-related pathogenic mechanisms; targeted JAK-STAT therapies for various diseases, especially immune disorders, and cancers; newly developed JAK inhibitors; and current challenges and directions in the field.

Effects of Low-Level Persistent Infection on Maintenance of Immunity by CD4 T Cell Subsets and Th1 Cytokines

CD4 T cells are required, along with antibodies, for complete protection from blood-stage infection with Plasmodium spp., which cause malaria. Without continuous exposure, as on emigration of people from endemic areas, protection from malaria decays. As in other persistent infections, low-level Plasmodium chabaudi infection protects the host from reinfection at 2 months postinfection, a phenomenon termed premunition. Premunition is correlated with T cell responses, rather than antibody levels. We previously showed that while both effector T cells (Teff) and memory T cells (Tmem) are present after infection, Teff protect better than Tmem. Here, we studied T cell kinetics post-infection by labeling dividing Ifng⁺ T cells with 5-bromo-2'-deoxyuridine (BrdU) in infected Ifng reporter mice. Large drops in specific T cell numbers and Ifng⁺ cells upon clearance of parasites suggest a mechanism for decay of protection. Although protection decays, CD4 Tmem persist, including a highly differentiated CD27^- effector memory (Tem) subset that maintains some Ifng expression. In addition, pretreatment of chronically infected animals with neutralizing antibody to interferon gamma (IFN-γ) or with clodronate liposomes before reinfection decreases premunition, supporting a role for Th1-type immunity to reinfection. A pulse-chase experiment comparing chronically infected to treated animals showed that recently divided Ifng⁺ T cells, particularly IFN-γ⁺ TNF⁺ IL-2^- T cells, are promoted by persistent infection. These data suggest that low-level persistent infection reduces CD4⁺ Tmem and multifunctional Teff survival, but promotes IFN-γ⁺ TNF⁺ IL-2^- T cells and Ifng⁺ terminally differentiated effector T cells, and prolongs immunity.

Ranitidine as an adjuvant regulates macrophage polarization and activates CTLs through the PI3K-Akt2 signaling pathway

Adjuvants are an indispensable component of vaccines, but there are few adjuvants for human vaccines. H2 receptor blockers, inhibiting gastric acid secretion, have immune enhancement effects. Ranitidine (RAN) is a water-soluble H2 receptor blocker, and whether it has an immune-enhancing effect is still unknown. In this study, flow cytometry, western blotting, and immunofluorescence methods were used to analyze whether RAN could activate macrophage polarization to the M1 phenotype in vivo and in vitro. Here, we found that the M1 inflammatory cytokine levels and surface markers in RAW264.7 cells were upregulated by NF-κB activation, possibly through the PI3K-Akt2 signaling pathway, after RAN treatment. Endocytic function was also enhanced by feedback regulation of Akt2/GSK3β/Dynmin1 signaling. Furthermore, to evaluate the adjuvant function of RAN, we used OVA plus RAN as a vaccine to inhibit the growth of B16-OVA tumors in mice. We also found that in the RAN adjuvant group, macrophage polarization to M1, Th1 cell differentiation, and cytotoxic T lymphocyte (CTL) activation were significantly upregulated. The tumor growth of mice was inhibited, and the survival rate of mice was significantly improved. This study provides new evidence for the mechanism by which RAN activates the immune response and is expected to provide a new strategy for the research and development of tumor vaccine adjuvants.

Therapeutic modulation of JAK-STAT, mTOR, and PPAR-γ signaling in neurological dysfunctions

The cytokine-activated Janus kinase (JAK)-signal transducer and activator of transcription (STAT) cascade is a pleiotropic pathway that involves receptor subunit multimerization. The mammalian target of rapamycin (mTOR) is a ubiquitously expressed serine-threonine kinase that perceives and integrates a variety of intracellular and environmental stimuli to regulate essential activities such as cell development and metabolism. Peroxisome proliferator-activated receptor-gamma (PPARγ) is a prototypical metabolic nuclear receptor involved in neural differentiation and axon polarity. The JAK-STAT, mTOR, and PPARγ signaling pathways serve as a highly conserved signaling hub that coordinates neuronal activity and brain development. Additionally, overactivation of JAK/STAT, mTOR, and inhibition of PPARγ signaling have been linked to various neurocomplications, including neuroinflammation, apoptosis, and oxidative stress. Emerging research suggests that even minor disruptions in these cellular and molecular processes can have significant consequences manifested as neurological and neuropsychiatric diseases. Of interest, target modulators have been proven to alleviate neuronal complications associated with acute and chronic neurological deficits. This research-based review explores the therapeutic role of JAK-STAT, mTOR, and PPARγ signaling modulators in preventing neuronal dysfunctions in preclinical and clinical investigations.

Acetylation licenses Th1 cell polarization to constrain Listeria monocytogenes infection

T helper 1 (Th1) immunity is typically viewed as a critical adaptation by vertebrates against intracellular pathogens. Identifying novel targets to enhance Th1 cell differentiation and function is increasingly important for anti-infection immunity. Here, through small-molecule screening focusing on epigenetic modifiers during the in vitro Th1 cell differentiation process, we identified that the selective histone deacetylase 6 (HDAC6) inhibitors ricolinostat and nexturastat A (Nex A) promoted Th1 cell differentiation. HDAC6-depleted mice exhibit elevation of Th1 cell differentiation, and decreased severity of Listeria monocytogenes infection. Mechanistically, HDAC6 directly deacetylated CBP-catalyzed acetylation of signal transducer and activator of transcription 4 (STAT4)-lysine (K) 667 via its enzymatic activity. Acetylation of STAT4-K667 is required for JAK2-mediated phosphorylation and activation of STAT4. Stat4K667R mutant mice lost the ability to normally differentiate into Th1 cells and developed severe Listeria infection. Our study identifies acetylation of STAT4-K667 as an essential signaling event for Th1 cell differentiation and defense against intracellular pathogen infections, and highlights the therapeutic potential of HDAC6 inhibitors for controlling intracellular pathogen infections.

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

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

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

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

Identification of CD4+ T cells with T follicular helper cell characteristics in the pig

T follicular helper (Tfh) cells provide help to germinal center B cells for affinity maturation, class switch and memory formation. Despite these important functions, this subset has not been studied in detail in pigs due to a lack of species-specific antibodies. We investigated putative Tfh cells from lymphoid tissues and blood of healthy pigs by using cross-reactive antibodies for inducible T-cell costimulator (ICOS) and B-cell lymphoma 6 (Bcl-6). In lymph nodes, we identified a CD4⁺ T cell population with an ICOS⁺Bcl-6⁺CD8α⁺ phenotype, reminiscent of human and murine germinal center Tfh cells. Within blood-derived CD4⁺ T cells, sorted ICOS^hiCD25^- and ICOS^dimCD25^dim cells were able to induce the differentiation of CD21⁺IgM⁺ B cells into Ig-secreting plasmablasts. Compared to naïve CD4⁺ T cells, these two phenotypes were 3- to 7-fold enriched for cells expressing the Tfh-related transcripts CD28, CD40LG, IL6R and MAF, as identified by single-cell RNA sequencing. These results provide a first characterization of Tfh cells in swine and confirm their ability to provide B-cell help.

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

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

Development of Tbet- and CD11c-expressing B cells in a viral infection requires T follicular helper cells outside of germinal centers

Tbet+CD11c+ B cells arise during type 1 pathogen challenge, aging, and autoimmunity in mice and humans. Here, we examined the developmental requirements of this B cell subset. In acute infection, T follicular helper (Tfh) cells, but not Th1 cells, drove Tbet+CD11c+ B cell generation through proximal delivery of help. Tbet+CD11c+ B cells developed prior to germinal center (GC) formation, exhibiting phenotypic and transcriptional profiles distinct from GC B cells. Fate tracking revealed that most Tbet+CD11c+ B cells developed independently of GC entry and cell-intrinsic Bcl6 expression. Tbet+CD11c+ and GC B cells exhibited minimal repertoire overlap, indicating distinct developmental pathways. As the infection resolved, Tbet+CD11c+ B cells localized to the marginal zone where splenic retention depended on integrins LFA-1 and VLA-4, forming a competitive memory subset that contributed to antibody production and secondary GC seeding upon rechallenge. Therefore, Tbet+CD11c+ B cells comprise a GC-independent memory subset capable of rapid and robust recall responses.

Anti-Interferon Autoantibodies in Adult-Onset Immunodeficiency Syndrome and Severe COVID-19 Infection

Adult-onset immunodeficiency syndrome due to anti-interferon (IFN)-γ autoantibodies has attracted much attention in recent years. It usually occurs in previously healthy people and usually presents as chronic, recurrent, and hard-to-control infections that can be effectively treated with aggressive antibiotic therapy. Adult-onset immunodeficiency syndrome is also referred to as AIDS-like syndrome. Anti-type I IFN (IFN-I) autoantibodies have been reported to play a significant role in the pathogenesis of coronavirus disease 2019 (COVID-19) and preexisting anti-IFN-I autoantibodies are associated with an increased risk of severe COVID-19. This review summarizes the effects of anti-IFN autoantibodies on the susceptibility and severity of various infectious diseases, including SARS-CoV-2 infection. In addition, we discuss the role of anti-IFN autoantibodies in the pathogenesis of autoimmune diseases that are characterized by recurrent infections.

Ubiquitinated Hepatitis D Antigen-Loaded Microvesicles Induce a Potent Specific Cellular Immune Response to Inhibit HDV Replication in Vivo

Hepatitis D is the most severe form of human viral hepatitis and currently lacks an efficient therapy. Dendritic cell-derived exosomes (Dexs) have been found to induce immune responses capable of eliminating viruses. However, the therapeutic potential of antigen-loaded exosomes in hepatitis D is still unknown. Recently, we designed exosomes loaded with ubiquitinated hepatitis delta virus (HDV) small delta antigen (Ub-S-HDAg) and then treated mice bearing replicating HDV with these exosomes to explore their antiviral effect and mechanism. Mature dendritic cell-derived exosomes (mDexs) were loaded with Ub-S-HDAg and their antivirus function was evaluated in mice with HDV viremia. Furthermore, the proportion of CD8⁺ cells, the ratio of Th1/Th2 cells, the postimmunization levels of cytokines were explored, and the Janus kinases (JAK)/signal transducer and activator of transcription (STAT) pathway was evaluated with a JAK2 inhibitor AG490. In Ub-S-HDAg-Dexs group, the HDV RNA viral load was significantly decreased compared with other groups by CD8⁺ cell enrichment and an increase Th1/Th2 cell ratio. Furthermore, lymphocyte infiltration was increased, while the HDAg level was decreased in mouse liver tissue. However, there were no significant differences in HBV surface antigen (HBsAg), alanine aminotransferase (ALT), or aspartate aminotransferase (AST) levels among the groups. Moreover, p-JAK2, p-STAT1, p-STAT4, STAT1, and STAT4 expression was increased in Ub-S-HDAg-Dexs group. In conclusion, Ub-S-HDAg-Dexs might be a potential immunotherapeutic agent for eradicating HDV by inducing specific cellular immune response via the JAK/STAT pathway. IMPORTANCE Hepatitis D is the most severe viral hepatitis with accelerating the process of liver cirrhosis and increasing the risk of hepatocellular carcinoma. However, there are no effective antiviral drugs. Exosomes derived from mature dendritic cells are used not only as immunomodulators, but also as biological carriers to deliver antigens to induce robust immune response. Based on these properties, exosomes could be used as a biological immunotherapy by enhancing adaptive immune response to inhibit hepatitis D virus replication. Our research may provide a new therapeutic strategy to eradicate HDV in the future.

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

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

The JAK/STAT signaling pathway: from bench to clinic

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway was discovered more than a quarter-century ago. As a fulcrum of many vital cellular processes, the JAK/STAT pathway constitutes a rapid membrane-to-nucleus signaling module and induces the expression of various critical mediators of cancer and inflammation. Growing evidence suggests that dysregulation of the JAK/STAT pathway is associated with various cancers and autoimmune diseases. In this review, we discuss the current knowledge about the composition, activation, and regulation of the JAK/STAT pathway. Moreover, we highlight the role of the JAK/STAT pathway and its inhibitors in various diseases.

The JAK/STAT signaling pathway: from bench to clinic

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway was discovered more than a quarter-century ago. As a fulcrum of many vital cellular processes, the JAK/STAT pathway constitutes a rapid membrane-to-nucleus signaling module and induces the expression of various critical mediators of cancer and inflammation. Growing evidence suggests that dysregulation of the JAK/STAT pathway is associated with various cancers and autoimmune diseases. In this review, we discuss the current knowledge about the composition, activation, and regulation of the JAK/STAT pathway. Moreover, we highlight the role of the JAK/STAT pathway and its inhibitors in various diseases.

Single-cell atlas of splenocytes reveals a critical role of a novel plasma cell‒specific marker Hspa13 in antibody class-switching recombination and somatic hypermutation

Our previous study had shown that member 13 (Hspa13) of heat shock protein family A (Hsp70) promotes plasma cell (PC) production and antibody secretion. To further explore Hspa13 expression and function, we combined single-cell RNA-sequencing and antigen receptor lineage (BCR) analysis to characterize sheep red cell‒primed splenocytes. The single-cell transcriptional profiles revealed that Hspa13 is specifically and highly expressed in PCs. These results suggest that Hspa13 is a novel PC-specific marker. In terms of its function, we found that the CD19^cre-mediated conditional knock-out (cKO) of Hspa13 reduced the expression of Ebi3 and IL-10 in PCs. Ebi3 and IL-10 are important factors in IL-4‒secreting type 2 helper T cell (Th2) activation and differentiation. As expected, we found that the Hspa13 cKO reduced IL‒4-expressing follicular helper T (Tfh2) cells. Finally, the single-cell antigen receptor analysis demonstrated that the Hspa13 cKO reduced the Aicda-mediated antibody class-switching recombination (CSR) and somatic hypermutation (SHM) in germinal centers (GCs) B cells. Altogether, the single-cell atlas of splenocytes revealed a critical indirect role for the novel PC-specific marker Hspa13 in CSR and SHM in GC B cells by promoting Ebi3 and IL-10 expression in PCs to induce IL-4-expressing Tfh2 cells. Further exploration of Hspa13 expression and function will provide valuable clues for how to use Hspa13 in the treatment of autoimmune diseases.

Identification and Characterization of Rift Valley Fever Virus-Specific T Cells Reveals a Dependence on CD40/CD40L Interactions for Prevention of Encephalitis

Rift Valley fever virus (RVFV) is an arbovirus found throughout Africa. It causes disease that is typically mild and self-limiting; however, some infected individuals experience severe manifestations, including hepatitis, encephalitis, or even death. Reports of RVFV encephalitis are notable among immunosuppressed individuals, suggesting a role for adaptive immunity in preventing this severe complication. This phenomenon has been modeled in C57BL/6 mice depleted of CD4 T cells prior to infection with DelNSs RVFV (RVFV containing a deletion of nonstructural protein NSs), resulting in late-onset encephalitis accompanied by high levels of viral RNA in the brain in 30% of animals. In this study, we sought to define the specific type(s) of CD4 T cells that mediate protection from RVFV encephalitis. The viral epitopes targeted by CD4 and CD8 T cells were defined in C57BL/6 mice, and tetramers for both CD4 and CD8 T cells were generated. RVFV-specific CD8 T cells were expanded and of a cytotoxic and proliferating phenotype in the liver following infection. RVFV-specific CD4 T cells were identified in the liver and spleen following infection and phenotyped as largely Th1 or Tfh subtypes. Knockout mice lacking various aspects of pathways important in Th1 and Tfh development and function were used to demonstrate that T-bet, CD40, CD40L, and major histocompatibility complex class II (MHC-II) mediated protection from RVFV encephalitis, while gamma interferon (IFN-γ) and interleukin-12 (IL-12) were dispensable. Virus-specific antibody responses correlated with protection from encephalitis in all mouse strains, suggesting that Tfh/B cell interactions modulate clinical outcome in this model. IMPORTANCE The prevention of RVFV encephalitis requires intact adaptive immunity. In this study, we developed reagents to detect RVFV-specific T cells and provide evidence for Tfh cells and CD40/CD40L interactions as critical mediators of this protection.

Poor CD4+ T Cell Immunogenicity Limits Humoral Immunity to P. falciparum Transmission-Blocking Candidate Pfs25 in Humans

Plasmodium falciparum transmission-blocking vaccines (TBVs) targeting the Pfs25 antigen have shown promise in mice but the same efficacy has never been achieved in humans. We have previously published pre-clinical data related to a TBV candidate Pfs25-IMX313 encoded in viral vectors which was very promising and hence progressed to human clinical trials. The results from the clinical trial of this vaccine were very modest. Here we unravel why, contrary to mice, this vaccine has failed to induce robust antibody (Ab) titres in humans to elicit transmission-blocking activity. We examined Pfs25-specific B cell and T follicular helper (Tfh) cell responses in mice and humans after vaccination with Pfs25-IMX313 encoded by replication-deficient chimpanzee adenovirus serotype 63 (ChAd63) and the attenuated orthopoxvirus modified vaccinia virus Ankara (MVA) delivered in the heterologous prime-boost regimen via intramuscular route. We found that after vaccination, the Pfs25-IMX313 was immunologically suboptimal in humans compared to mice in terms of serum Ab production and antigen-specific B, CD4⁺ and Tfh cell responses. We identified that the key determinant for the poor anti-Pfs25 Ab formation in humans was the lack of CD4⁺ T cell recognition of Pfs25-IMX313 derived peptide epitopes. This is supported by correlations established between the ratio of proliferated antigen-specific CD4⁺/Tfh-like T cells, CXCL13 sera levels, and the corresponding numbers of circulating Pfs25-specific memory B cells, that consequently reflected on antigen-specific IgG sera levels. These correlations can inform the design of next-generation Pfs25-based vaccines for robust and durable blocking of malaria transmission.

T Cell/B Cell Interactions in the Establishment of Protective Immunity

Follicular helper T cells (Tfh) are the T cell subset providing help to B cells for the generation of high-affinity antibodies and are therefore of key interest for the development of vaccination strategies against infectious diseases. In this review, we will discuss how the generation of Tfh cells and their interaction with B cells in secondary lymphoid organs can be optimized for therapeutic purposes. We will summarize different T cell subsets including Tfh-like peripheral helper T cells (Tph) capable of providing B cell help. In particular, we will highlight the novel concept of T cell/B cell interaction in non-lymphoid tissues as an important element for the generation of protective antibodies directly at the site of pathogen invasion.

Booster immunization improves the generation of T follicular helper (Tfh) cells specific to hepatitis B surface antigen (HBsAg) after prenatal HBsAg exposure

Breakthrough infections of hepatitis B virus (HBV) after neonatal vaccination occurred in some adolescents and young adults who were born to mothers with hepatitis B surface antigen (HBsAg). We aimed to determine the impacts of prenatal HBsAg exposure on the generation of T follicular helper (Tfh) cells and antibodies (anti-HBs) specific to HBsAg. To mimic human prenatal HBsAg exposure, we mated female Alb1-HBV (HBV-M) mice with male C57BL/6J mice. Of their first filial generation (F1), HBV-M/F1⁺ expressed HBsAg in liver tissues and blood, and HBV-M/F1^- mice exposed HBsAg in amniotic fluid. At their four weeks old, each HBV-M/F1 mouse was immunized with hepatitis B vaccine containing 5 μg HBsAg subcutaneously. Both HBV-M/F1^- and HBV-M/F1⁺ mice had reduced generation of HBsAg-specific CD4⁺CXCR5⁺PD1⁺ Tfh cells and CD138⁺IgD^- plasma cells in comparison with C57BL/6J mice. Results of coculturing the Tfh cells with B cells that were isolated from different strains of mice indicated that CD4⁺ T cell activation in response to HBsAg was critical for anti-HBs generation after prenatal HBsAg exposure. When interleukin (IL) 21 was supplemented, the generation of HBsAg-specific Tfh and plasma cells in HBV-M/F1^- mice was improved, while supplementation showed little effect in HBV-M/F1⁺ mice. In HBV-M/F1^- mice, HBV vaccine booster improved the generation of Tfh cells and plasma cells, and enhanced anti-HBs production. CONCLUSION: Impaired generation of HBsAg-specific Tfh cells and plasma cells after prenatal HBsAg exposure can be improved by HBV vaccine booster, most likely increasing IL-21 production.

The Role of T Follicular Helper Cells and Interleukin-21 in the Pathogenesis of Inflammatory Bowel Disease

T follicular helper (Tfh) cells represent a novel subset of CD4⁺ T cells which can provide critical help for germinal center (GC) formation and antibody production. The Tfh cells are characterized by the expression of CXC chemokine receptor 5 (CXCR5), programmed death 1 (PD-1), inducible costimulatory molecule (ICOS), B cell lymphoma 6 (BCL-6), and the secretion of interleukin-21 (IL-21). Given the important role of Tfh cells in B cell activation and high-affinity antibody production, Tfh cells are involved in the pathogenesis of many human diseases. Inflammatory bowel disease (IBD) is a group of chronic inflammatory diseases characterized by symptoms such as diarrhea, abdominal pain, and weight loss. Ulcerative colitis (UC) and Crohn's disease (CD) are the most studied types of IBD. Dysregulated mucosal immune response plays an important role in the pathogenesis of IBD. In recent years, many studies have identified the critical role of Tfh cells and IL-21 in the pathogenic process IBD. In this paper, we will discuss the role of Tfh cells and IL-21 in IBD pathogenesis.

Clinical relevance of T follicular helper cells in systemic lupus erythematosus

INTRODUCTION

T helper cells regulate a variety of immune responses and are involved in the pathogenesis of infection, allergy and autoimmune diseases. T follicular helper (Tfh) cells, which induce B cell maturation, play an important role in the production of the extremely diverse autoantibodies found in systemic lupus erythematosus (SLE).

AREA COVERED

We provide an overview of the plasticity and diversity of Tfh cells in humans and their involvement in the pathology and pathogenesis of SLE. Our review outlines the potential of Tfh cells as a therapeutic target for SLE.

EXPERT OPINION

Tfh cells are involved in the pathogenesis of SLE based on their plasticity and diversity. Tfh cell differentiation and function are variably regulated by cytokines (IL-12, interferons, IL-2, etc), co-stimulatory molecules (ICOS, CD40L, OX40, etc), and intracellular signals (JAK-STAT, etc). Elucidation of the mechanisms underlying Tfh cell differentiation and function may lead to the development of new therapies for SLE.

Protein Immunization Induces Memory CD4+ T Cells That Lack Th Lineage Commitment

Acute viral infection generates lineage-committed Th1 and T follicular helper (Tfh) memory cells that recall their lineage-specific functions following secondary challenge with virus. However, the lineage commitment of effector and memory Th cells in vivo following protein vaccination is poorly understood. In this study, we analyzed effector and memory CD4⁺ T cell differentiation in mice (Mus musculus) following adjuvanted glycoprotein immunization compared with acute lymphocytic choriomeningitis virus infection. Glycoprotein immunization induced CXCR5^- non-Tfh effector and memory CD4⁺ T cells that surprisingly had not undergone polarization toward any particular Th cell lineage but had undergone memory differentiation. However, upon challenge with virus, these Th lineage-nonpolarized memory CD4⁺ T cells were able to generate Th1 secondary effector cells, demonstrating their lineage plasticity. In addition, Tfh and memory Tfh cells were generated in response to protein immunization, and these cells differed from infection-induced Tfh cells by their lack of the transcription factor Tbet. Rechallenge experiments demonstrated that viral infection, but not protein immunization, during either the primary or secondary immune response, restricts the recall of Bcl6 expression and the generation of germinal center Tfh cells. Together, these data demonstrate that protein immunization generates a combination of nonpolarized memory cells that are highly plastic and memory Tfh cells that can undergo further Th1-like modulation during a secondary response to viral infection.

Broad phenotypic alterations and potential dysfunction of lymphocytes in individuals clinically recovered from COVID-19

Although millions of patients have clinically recovered from COVID-19, little is known about the immune status of lymphocytes in these individuals. In this study, the peripheral blood mononuclear cells of a clinically recovered (CR) cohort were comparatively analyzed with those of an age- and sex-matched healthy donor cohort. We found that CD8⁺ T cells in the CR cohort had higher numbers of effector T cells and effector memory T cells but lower Tc1 (IFN-γ⁺), Tc2 (IL-4⁺), and Tc17 (IL-17A⁺) cell frequencies. The CD4⁺ T cells of the CR cohort were decreased in frequency, especially the central memory T cell subset. Moreover, CD4⁺ T cells in the CR cohort showed lower programmed cell death protein 1 (PD-1) expression and had lower frequencies of Th1 (IFN-γ⁺), Th2 (IL-4⁺), Th17 (IL-17A⁺), and circulating follicular helper T (CXCR5⁺PD-1⁺) cells. Accordingly, the proportion of isotype-switched memory B cells (IgM⁻CD20^hi) among B cells in the CR cohort showed a significantly lower proportion, although the level of the activation marker CD71 was elevated. For CD3⁻HLA-DR⁻ lymphocytes in the CR cohort, in addition to lower levels of IFN-γ, granzyme B and T-bet, the correlation between T-bet and IFN-γ was not observed. Additionally, by taking into account the number of days after discharge, all the phenotypes associated with reduced function did not show a tendency toward recovery within 4‒11 weeks. The remarkable phenotypic alterations in lymphocytes in the CR cohort suggest that  severe acute respiratory syndrome coronavirus 2 infection profoundly affects lymphocytes and potentially results in dysfunction even after clinical recovery.

CD4+ T cells that help B cells - a proposal for uniform nomenclature

T follicular helper (Tfh) cells cognately guide differentiation of antigen-primed B cells in secondary lymphoid tissues. 'Tfh-like' populations not expressing the canonical Tfh cell transcription factor BCL6 have also been described, which can aid particular aspects of B cell differentiation. Tfh and Tfh-like cells are essential for protective and pathological humoral immunity. These CD4⁺ T cells that help B cells are polarized to produce diverse combinations of cytokines and chemokine receptors and can be grouped into distinct subsets that promote antibodies of different isotype, affinity, and duration, according to the nature of immune challenge. However, unified nomenclature to describe the distinct functional Tfh and Tfh-like cells does not exist. While explicitly acknowledging cellular plasticity, we propose categorizing these cell states into three groups based on phenotype and function, paired with their anatomical site of action.

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

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

Context-dependent roles of B cells during intestinal helminth infection

The current approaches to reduce the burden of chronic helminth infections in endemic areas are adequate sanitation and periodic administration of deworming drugs. Yet, resistance against some deworming drugs and reinfection can still rapidly occur even after treatment. A vaccine against helminths would be an effective solution at preventing reinfection. However, vaccines against helminth parasites have yet to be successfully developed. While T helper cells and innate lymphoid cells have been established as important components of the protective type 2 response, the roles of B cells and antibodies remain the most controversial. Here, we review the roles of B cells during intestinal helminth infection. We discuss the potential factors that contribute to the context-specific roles for B cells in protection against diverse intestinal helminth parasite species, using evidence from well-defined murine model systems. Understanding the precise roles of B cells during resistance and susceptibility to helminth infection may offer a new perspective of type 2 protective immunity.

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.

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

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

Type I Interferon-Activated STAT4 Regulation of Follicular Helper T Cell-Dependent Cytokine and Immunoglobulin Production in Lupus

OBJECTIVE

To assess the role of STAT4 activation in driving pathogenic follicular helper T (Tfh) cell secretion of the cytokines interleukin-21 (IL-21) and interferon-γ (IFNγ) in murine and human lupus.

METHODS

The effect of STAT4-dependent Tfh cell signaling on cytokine production and autoreactive B cell maturation was assessed temporally during the course of lupus in a murine model, with further assessment of Tfh cell gene transcription performed using RNA-Seq technology. STAT4-dependent signaling and cytokine production were also determined in circulating Tfh-like cells in patients with systemic lupus erythematosus (SLE), as compared to cells from healthy control subjects, and correlations with disease activity were assessed in the Tfh-like cells from SLE patients.

RESULTS

IL-21- and IFNγ-coproducing Tfh cells expanded prior to the detection of potentially pathogenic IgG2c autoantibodies in lupus-prone mice. Tfh cells transcriptionally evolved during the course of disease with acquisition of a STAT4-dependent gene signature. Maintenance of Tfh cell cytokine synthesis was dependent upon STAT4 signaling, driven by type I IFNs. Circulating Tfh-like cells from patients with SLE also secreted IL-21 and IFNγ, with STAT4 phosphorylation enhanced by IFNβ, in association with the extent of clinical disease activity.

CONCLUSION

We identified a role for type I IFN signaling in driving STAT4 activation and production of IL-21 and IFNγ by Tfh cells in murine and human lupus. Enhanced STAT4 activation in Tfh cells may underlie pathogenic B cell responses in both murine and human lupus. These data indicate that STAT4 guides pathogenic cytokine and immunoglobulin production in SLE, demonstrating a potential therapeutic target to modulate autoimmunity.

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

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

Modulating Th2 Cell Immunity for the Treatment of Asthma

It is estimated that more than 339 million people worldwide suffer from asthma. The leading cause of asthma development is the breakdown of immune tolerance to inhaled allergens, prompting the immune system's aberrant activation. During the early phase, also known as the sensitization phase, allergen-specific T cells are activated and become central players in orchestrating the subsequent development of allergic asthma following secondary exposure to the same allergens. It is well-established that allergen-specific T helper 2 (Th2) cells play central roles in developing allergic asthma. As such, 80% of children and 60% of adult asthma cases are linked to an unwarranted Th2 cell response against respiratory allergens. Thus, targeting essential components of Th2-type inflammation using neutralizing antibodies against key Th2 modulators has recently become an attractive option for asthmatic patients with moderate to severe symptoms. In addition to directly targeting Th2 mediators, allergen immunotherapy, also known as desensitization, is focused on redirecting the allergen-specific T cells response from a Th2-type profile to a tolerogenic one. This review highlights the current understanding of the heterogeneity of the Th2 cell compartment, their contribution to allergen-induced airway inflammation, and the therapies targeting the Th2 cell pathway in asthma. Further, we discuss available new leads for successful targeting pulmonary Th2 cell responses for future therapeutics.

Micro RNAs in Tfh regulation: Small molecules with a big impact

The germinal center (GC) reactions are critical for the production of high-affinity antibodies that comprise the protective humoral response elicited by infection or vaccination. GCs are initiated through the interaction of B cells with T follicular helper (Tfh) cells. While the transcriptional regulation of Tfh differentiation has been studied in great detail, the impact of micro RNAs (miRNAs) on Tfh development and stability has been harder to address. It was previously shown that a complete deletion of miRNAs biogenesis prevents Tfh differentiation. In this issue of the European Journal of Immunology [Eur. J. Immunol. 2021. 51: 408-413], Zeiträg et al. use an inducible gene deletion approach to reveal that miRNAs are also required for the maintenance of Tfh cells induced following viral infection in mice. These results provide new clues to the regulation of GC responses through Tfh and T follicular regulatory cells.

STAT4 Is Largely Dispensable for Systemic Lupus Erythematosus-like Autoimmune- and Foreign Antigen-Driven Antibody-Forming Cell, Germinal Center, and Follicular Th Cell Responses

Genome-wide association studies identified variants in the transcription factor STAT4 gene and several other genes in the STAT4 signaling pathway, such as IL12A, IL12B, JAK2, and TYK2, which are associated with an increased risk of developing systemic lupus erythematosus (SLE) and other autoimmune diseases. Consistent with the genome-wide association studies data, STAT4 was shown to play an important role in autoimmune responses and autoimmunity development in SLE mouse models. Despite such important role for STAT4 in SLE development in mice and humans, little is known whether and how STAT4 may regulate extrafollicular Ab-forming cell (AFC) and follicular germinal center (GC) responses, two major pathways of autoreactive B cell development and autoantibody production. To our surprise, we found STAT4 to be largely dispensable for promoting autoimmune AFC and GC responses in various autoimmune- and SLE-prone mouse models, which strongly correlated with autoantibody production, and immune complex deposition and immune cell infiltration in the kidney. We further observed that STAT4 deficiency had no effects on AFC, GC, and Ag-specific Ab responses during protein Ag immunization or influenza virus infection. Additionally, CD4+ effector and follicular Th cell responses in autoimmune- and SLE-prone mice and protein Ag-immunized and influenza virus-infected mice were intact in the absence of STAT4. Together, our data demonstrate a largely dispensable role for STAT4 in AFC, GC, and Ab responses in SLE mouse models and in certain foreign Ag-driven responses.

Role of CD4+ T Cells in the Control of Viral Infections: Recent Advances and Open Questions

CD4+ T cells orchestrate adaptive immune responses through their capacity to recruit and provide help to multiple immune effectors, in addition to exerting direct effector functions. CD4+ T cells are increasingly recognized as playing an essential role in the control of chronic viral infections. In this review, we present recent advances in understanding the nature of CD4+ T cell help provided to antiviral effectors. Drawing from our studies of natural human immunodeficiency virus (HIV) control, we then focus on the role of high-affinity T cell receptor (TCR) clonotypes in mediating antiviral CD4+ T cell responses. Last, we discuss the role of TCR affinity in determining CD4+ T cell differentiation, reviewing the at times divergent studies associating TCR signal strength to the choice of a T helper 1 (Th1) or a T follicular helper (Tfh) cell fate.

T cell-expressed microRNAs critically regulate germinal center T follicular helper cell function and maintenance in acute viral infection in mice

Constitutive T cell-intrinsic miRNA expression is required for the differentiation of naïve CD4⁺ T cells into Tfh cells, thus making it difficult to study the role of miRNAs in the maintenance of already established Tfh cells and ongoing germinal center (GC) responses. To overcome this problem, we here used temporally controlled ablation of mature miRNAs specifically in CD4⁺ T cells during acute LCMV infection in mice. T cell-intrinsic miRNA expression was not only critical at early stages of Tfh cell differentiation, but also important for the maintenance of already established Tfh cells. In addition, CD4⁺ T cell-specific ablation of miRNAs resulted in impaired GC B cell responses. Notably, miRNA deficiency also compromised the antigen-specific CD4⁺ T cell compartment, Th1 cells, Treg cells, and Tfr cells. In conclusion, our results highlight miRNAs as important regulators of Tfh cells, thus providing novel insights into the molecular events that govern T cell-B cell interactions and Th cell identity.

The Dynamic Interface of Viruses with STATs

Viruses commonly antagonize the antiviral type I interferon response by targeting signal transducer and activator of transcription 1 (STAT1) and STAT2, key mediators of interferon signaling. Other STAT family members mediate signaling by diverse cytokines important to infection, but their relationship with viruses is more complex. Importantly, virus-STAT interaction can be antagonistic or stimulatory depending on diverse viral and cellular factors. While STAT antagonism can suppress immune pathways, many viruses promote activation of specific STATs to support viral gene expression and/or produce cellular conditions conducive to infection. It is also becoming increasingly clear that viruses can hijack noncanonical STAT functions to benefit infection. For a number of viruses, STAT function is dynamically modulated through infection as requirements for replication change. Given the critical role of STATs in infection by diverse viruses, the virus-STAT interface is an attractive target for the development of antivirals and live-attenuated viral vaccines. Here, we review current understanding of the complex and dynamic virus-STAT interface and discuss how this relationship might be harnessed for medical applications.

IL-12 Blocks Tfh Cell Differentiation during Salmonella Infection, thereby Contributing to Germinal Center Suppression

Germinal centers (GC) are crucial for the formation of long-lived humoral immunity. Many pathogens suppress GC, including Salmonella enterica serovar Typhimurium (STm), but the mechanisms driving suppression remain unknown. We report that neither plasmablasts nor STm-specific B cells are required for GC suppression in mice. Rather, we identify that interleukin-12 (IL-12), but not interferon-γ (IFN-γ), directly suppresses T follicular helper (Tfh) cell differentiation of T cells intrinsically. Administering recombinant IL-12 during nitrophenyl-Chicken Gamma Globulin (NP-CGG) immunization also suppresses Tfh cell differentiation and GC B cells, indicating that IL-12 is sufficient to suppress Tfh cell differentiation independent of STm infection. Recombinant IL-12 induces high levels of T-bet, and T-bet is necessary for Tfh cell suppression. Therefore, IL-12 induced during STm infection in mice contributes to GC suppression via suppression of Tfh cell differentiation. More broadly, these data suggest that IL-12 can tailor the proportions of humoral (Tfh cell) and cellular (T helper type 1 [Th1] cell) immunity to the infection, with implications for IL-12 targeting therapies in autoimmunity and vaccination.