Heparan sulfate regulates IL-21 bioavailability and signal strength that control germinal center B cell selection and differentiation

Tfh cell-derived small extracellular vesicles exacerbate the severity of collagen-induced arthritis by enhancing B-cell responses

Soluble components secreted by Tfh cells are critical for the germinal center responses. In this study, we investigated whether Tfh cells could regulate the B-cell response by releasing small extracellular vesicles (sEVs). Our results showed that Tfh cells promote B-cell differentiation and antibody production through sEVs and that CD40L plays a crucial role in Tfh-sEVs function. In addition, increased Tfh-sEVs were found in mice with collagen-induced arthritis (CIA). Adoptive transfer of Tfh cells significantly exacerbated the severity of CIA; however, the effect of Tfh cells on exacerbating the CIA process was significantly diminished after inhibiting sEVs secretion. Moreover, the levels of plasma Tfh-like-sEVs and CD40L expression on Tfh-like-sEVs in RA patients were significantly higher than those in healthy subjects. In summary, Tfh cell-derived sEVs can enhance the B-cell response, and exacerbate the procession of autoimmune arthritis.

CD4 T Cell-Derived IL-21 Is Critical for Sustaining Plasmodium Infection-Induced Germinal Center Responses and Promoting the Selection of Memory B Cells with Recall Potential

Development of Plasmodium-specific humoral immunity is critically dependent on CD4 Th cell responses and germinal center (GC) reactions during blood-stage Plasmodium infection. IL-21, a cytokine primarily produced by CD4 T cells, is an essential regulator of affinity maturation, isotype class-switching, B cell differentiation, and maintenance of GC reactions in response to many infection and immunization models. In models of experimental malaria, mice deficient in IL-21 or its receptor IL-21R fail to develop memory B cell populations and are not protected against secondary infection. However, whether sustained IL-21 signaling in ongoing GCs is required for maintaining GC magnitude, organization, and output is unclear. In this study, we report that CD4+ Th cells maintain IL-21 expression after resolution of primary Plasmodium yoelii infection. We generated an inducible knockout mouse model that enabled cell type-specific and timed deletion of IL-21 in peripheral, mature CD4 T cells. We found that persistence of IL-21 signaling in active GCs had no impact on the magnitude of GC reactions or their capacity to produce memory B cell populations. However, the memory B cells generated in the absence of IL-21 exhibited reduced recall function upon challenge. Our data support that IL-21 prevents premature cellular dissolution within the GC and promotes stringency of selective pressures during B cell fate determination required to produce high-quality Plasmodium-specific memory B cells. These data are additionally consistent with a temporal requirement for IL-21 in fine-tuning humoral immune memory responses during experimental malaria.

Interleukin-21 as an adjuvant in cancer immunotherapy: Current advances and future directions

Immunotherapy has revolutionized cancer treatment. However, it's well-recognized that a considerable proportion of patients fail to benefit from immunotherapy, and to improve immunotherapy response is clinically urgent. Insufficient immune infiltration and immunosuppressive tumor microenvironments (TME) are main contributors to immunotherapy resistance. Thus sustaining functional self-renewal capacity for immune cells and subverting immune-suppressive signals are potential strategies for boosting the efficacy of immunotherapy. Interleukin-21 (IL-21), a crucial cytokine, which could enhance cytotoxic function of immune cells and reduces immunosuppressive cells enrichment in TME, shows promising orientations as an immunoadjuvant in tumor immunotherapy. This review focuses on IL-21 in cancer treatment, including function and mechanisms of IL-21, preclinical and clinical studies, and future directions for IL-21-assisted therapies.

Regulatory T cells use heparanase to access IL-2 bound to extracellular matrix in inflamed tissue

Although FOXP3+ regulatory T cells (Treg) depend on IL-2 produced by other cells for their survival and function, the levels of IL-2 in inflamed tissue are low, making it unclear how Treg access this critical resource. Here, we show that Treg use heparanase (HPSE) to access IL-2 sequestered by heparan sulfate (HS) within the extracellular matrix (ECM) of inflamed central nervous system tissue. HPSE expression distinguishes human and murine Treg from conventional T cells and is regulated by the availability of IL-2. HPSE-/- Treg have impaired stability and function in vivo, including in the experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis. Conversely, endowing monoclonal antibody-directed chimeric antigen receptor (mAbCAR) Treg with HPSE enhances their ability to access HS-sequestered IL-2 and their ability to suppress neuroinflammation in vivo. Together, these data identify a role for HPSE and the ECM in immune tolerance, providing new avenues for improving Treg-based therapy of autoimmunity.

Cytokines in Follicular Helper T Cell Biology in Physiologic and Pathologic Conditions

Follicular helper T cells (Tfh) play a crucial role in generating high-affinity antibodies (Abs) and establishing immunological memory. Cytokines, among other functional molecules produced by Tfh, are central to germinal center (GC) reactions. This review focuses on the role of cytokines, including IL-21 and IL-4, in regulating B cell responses within the GC, such as differentiation, affinity maturation, and plasma cell development. Additionally, this review explores the impact of other cytokines like CXCL13, IL-10, IL-9, and IL-2 on GC responses and their potential involvement in autoimmune diseases, allergies, and cancer. This review highlights contributions of Tfh-derived cytokines to both protective immunity and immunopathology across a spectrum of diseases. A deeper understanding of Tfh cytokine biology holds promise for insights into biomedical conditions.

Ipsilateral immunization after a prior SARS-CoV-2 mRNA vaccination elicits superior B cell responses compared to contralateral immunization

mRNA vaccines have proven to be pivotal in the fight against COVID-19. A recommended booster, given 3 to 4 weeks post the initial vaccination, can substantially amplify protective antibody levels. Here, we show that, compared to contralateral boost, ipsilateral boost of the SARS-CoV-2 mRNA vaccine induces more germinal center B cells (GCBCs) specific to the receptor binding domain (RBD) and generates more bone marrow plasma cells. Ipsilateral boost can more rapidly generate high-affinity RBD-specific antibodies with improved cross-reactivity to the Omicron variant. Mechanistically, the ipsilateral boost promotes the positive selection and plasma cell differentiation of pre-existing GCBCs from the prior vaccination, associated with the expansion of T follicular helper cells. Furthermore, we show that ipsilateral immunization with an unrelated antigen after a prior mRNA vaccination enhances the germinal center and antibody responses to the new antigen compared to contralateral immunization. These findings propose feasible approaches to optimize vaccine effectiveness.

Memory B cell differentiation from germinal centers

Establishment of humoral immune memory depends on two layers of defense: pre-existing antibodies secreted by long-lived plasma cells; and the antibodies produced by antigen-reactivated memory B cells. Memory B cells can now be considered as a second layer of defense upon re-infection by variant pathogens that have not been cleared by the long-lived plasma cell-mediated defense. Affinity-matured memory B cells are derived from the germinal center (GC) reaction, but the selection mechanism of GC B cells into the memory compartment is still incompletely understood. Recent studies have revealed the critical determinants of cellular and molecular factors for memory B cell differentiation from the GC reaction. In addition, the contribution of antibody-mediated feedback regulation to B cell selection, as exemplified by the B cell response upon COVID-19 mRNA vaccination, has now garnered considerable attention, which may provide valuable implications for future vaccine design.

Systemic immunometabolism and responses to vaccines: insights from T and B cell perspectives

Vaccination stands as the cornerstone in the battle against infectious diseases, and its efficacy hinges on several host-related factors like genetics, age, and metabolic status. Vulnerable populations, such as malnourished individuals, the obese, and the elderly, commonly exhibit diminished vaccine responses and efficacy. While the specific factors contributing to this impairment may vary, these individuals typically display a degree of metabolic dysregulation, thereby underscoring its potential significance as a fundamental determinant of suboptimal vaccine responses. The emerging field of immunometabolism aims to unravel the intricate interplay between immune regulation and metabolic pathways, and recent research has revealed diverse metabolic signatures linked to various vaccine responses and outcomes. In this review, we summarize the major metabolic pathways utilized by B and T cells during vaccine responses, their complex and varied metabolic requirements, and the impact of micronutrients and metabolic hormones on vaccine outcomes. Furthermore, we examine how systemic metabolism influences vaccine responses and the evidence suggesting that metabolic dysregulation in vulnerable populations can lead to impaired vaccine responses. Lastly, we reflect on the challenge of proving causality with respect to the contribution of metabolic dysregulation to poor vaccine outcomes, and highlight the need for a systems biology approach that combines multimodal profiling and mathematical modelling to reveal the underlying mechanisms of such complex interactions.

Germinal centers output clonally diverse plasma cell populations expressing high- and low-affinity antibodies

Germinal centers (GCs) form in lymph nodes after immunization or infection to facilitate antibody affinity maturation and memory and plasma cell (PC) development. PC differentiation is thought to involve stringent selection for GC B cells expressing the highest-affinity antigen receptors, but how this plays out during complex polyclonal responses is unclear. We combine temporal lineage tracing with antibody characterization to gain a snapshot of PCs developing during influenza infection. GCs co-mature B cell clones with antibody affinities spanning multiple orders of magnitude; however, each generates PCs with similar efficiencies, including weak binders. Within lineages, PC selection is not restricted to variants with the highest-affinity antibodies. Differentiation is commonly associated with proliferative expansion to produce "nodes" of identical PCs. Immunization-induced GCs generate fewer PCs but still of low- and high-antibody affinities. We propose that generating low-affinity antibody PCs reflects an evolutionary compromise to facilitate diverse serum antibody responses.

Identification and characterization of a novel heparinase PCHepII from marine bacterium Puteibacter caeruleilacunae

Heparin (HP) and heparan sulfate (HS) are multifunctional polysaccharides widely used in clinical therapy. Heparinases (Hepases) are enzymes that specifically catalyse HP and HS degradation, and they are valuable tools for studying the structure and function of these polysaccharides and for preparing low molecular weight heparins. In this study, by searching the NCBI database, a novel enzyme named PCHepII was discovered in the genome of the marine bacterium Puteibacter caeruleilacuae. Heterologously expressed PCHepII in Escherichia coli (BL21) has high expression levels and good solubility, active in sodium phosphate buffer (pH 7.0) at 20°C. PCHepII exhibits an enzyme activity of 254 mU/mg towards HP and shows weak degradation capacity for HS. More importantly, PCHepII prefers to catalyse the high-sulfated regions of HP and HS rather than the low-sulfated regions. Although PCHepII functions primarily as an endolytic Hepase, it mainly generates disaccharide products during the degradation of HP substrates over time. Investigations reveal that PCHepII exhibits a preference for catalysing the degradation of small substrates, especially HP tetrasaccharides. The catalytic sites of PCHepII include the residues His199, Tyr254, and His403, which play crucial roles in the catalytic process. The study and characterization of PCHepII can potentially benefit research and applications involving HP/HS, making it a promising enzyme.

Accumulation of immune-suppressive CD4 + T cells in aging - tempering inflammaging at the expense of immunity

The 'immune risk profile' has been shown to predict mortality in the elderly, highlighting the need to better understand age-related immune dysfunction. While aging leads to many defects affecting all arms of the immune system, this review is focused on the accrual of immuno-suppressive CD4 + T cell populations, including FoxP3 + regulatory T cells, and subsets of IL-10-producing T follicular helper cells. New data suggest that such accumulations constitute feedback mechanisms to temper the ongoing progressive low-grade inflammation that develops with age, the so-called "inflammaging", and by doing so, how they have the potential to promote healthier aging. However, they also impair effector immune responses, notably to infections, or vaccines. These studies also reinforce the idea that the aged immune system should not be considered as a poorly functional version of the young one, but more as a dynamic system in which CD4 + T cells, and other immune/non-immune subsets, differentiate, interact with their milieu and function differently than in young hosts. A better understanding of these unique interactions is thus needed to improve effector immune responses in the elderly, while keeping inflammaging under control.

IL-21 shapes germinal center polarization via light zone B cell selection and cyclin D3 upregulation

Germinal center (GC) dysregulation has been widely reported in the context of autoimmunity. Here, we show that interleukin 21 (IL-21), the archetypal follicular helper T cell (Tfh) cytokine, shapes the scale and polarization of spontaneous chronic autoimmune as well as transient immunization-induced GC. We find that IL-21 receptor deficiency results in smaller GC that are profoundly skewed toward a light zone GC B cell phenotype and that IL-21 plays a key role in selection of light zone GC B cells for entry to the dark zone. Light zone skewing has been previously reported in mice lacking the cell cycle regulator cyclin D3. We demonstrate that IL-21 triggers cyclin D3 upregulation in GC B cells, thereby tuning dark zone inertial cell cycling. Lastly, we identify Foxo1 regulation as a link between IL-21 signaling and GC dark zone formation. These findings reveal new biological roles for IL-21 within GC and have implications for autoimmune settings where IL-21 is overproduced.

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.

Age-dependent changes in T follicular helper cells shape the humoral immune response to vaccination

Vaccination is an excellent strategy to limit the morbidity and mortality associated with infectious disease. Vaccination creates protective, long-lived antibody-mediated immunity by inducing the germinal centre response, an intricate immune reaction that produces memory B cells and long-lived antibody-secreting plasma cells that provide protection against (re)infection. The magnitude and quality of the germinal centre response declines with age, contributing to poor vaccine-induced immunity in older individuals. T follicular helper cells are essential for the formation and function of the germinal centre response. This review will discuss how age-dependent changes in T follicular helper cells influence the germinal centre response, and the evidence that age-dependent changes need not be a barrier to successful vaccination in the later years of life.

IL-21R signal reprogramming cooperates with CD40 and BCR signals to select and differentiate germinal center B cells

Both B cell receptor (BCR) and CD40 signaling are rewired in germinal center (GC) B cells (GCBCs) to synergistically induce c-MYC and phosphorylated S6 ribosomal protein (p-S6), markers of positive selection. How interleukin-21 (IL-21), a key T follicular helper (TFH)-derived cytokine, affects GCBCs is unclear. Like BCR and CD40 signals, IL-21 receptor (IL-21R) plus CD40 signals also synergize to induce c-MYC and p-S6 in GCBCs. However, IL-21R plus CD40 stimulation differentially affects GCBC fate compared with BCR plus CD40 ligation-engaging unique molecular mechanisms-as revealed by bulk RNA sequencing (RNA-seq), single-cell RNA-seq, and flow cytometry of GCBCs in vitro and in vivo. Whereas both signal pairs induced BLIMP1 in some GCBCs, only the IL-21R/CD40 combination induced IRF4hi/CD138+ cells, indicative of plasma cell differentiation, along with CCR6+/CD38+ memory B cell precursors. These findings reveal a second positive selection pathway in GCBCs, document rewired IL-21R signaling in GCBCs, and link specific TFH- and Ag-derived signals to GCBC differentiation.