CXCR5-Dependent Seeding of Follicular Niches by B and Th Cells Augments Antiviral B Cell Responses

The G allele of SNP rs3922 reduces the binding affinity between IGF2BP3 and CXCR5 correlating with a lower antibody production

Effective vaccines that function through humoral immunity seek to produce high-affinity antibodies. Our previous research identified the single-nucleotide polymorphism rs3922G in the 3'UTR of CXCR5 as being associated with nonresponsiveness to the hepatitis B vaccine. The differential expression of CXCR5 between the dark zone (DZ) and light zone (LZ) is critical for organizing the functional structure of the germinal center (GC). In this study, we report that the RNA-binding protein IGF2BP3 can bind to CXCR5 mRNA containing the rs3922 variant to promote its degradation via the nonsense-mediated mRNA decay pathway. Deficiency of IGF2BP3 leads to increased CXCR5 expression, which results in the disappearance of CXCR5 differential expression between DZ and LZ, disorganized GCs, aberrant somatic hypermutations, and reduced production of high-affinity antibodies. Furthermore, the affinity of IGF2BP3 for the rs3922G-containing sequence is lower than that for the rs3922A counterpart, which may explain the nonresponsiveness to the hepatitis B vaccination. Together, our findings suggest that IGF2BP3 plays a crucial role in the production of high-affinity antibodies in the GC by binding to the rs3922-containing sequence to regulate CXCR5 expression.

Induction of bronchus-associated lymphoid tissue is an early life adaptation for promoting human B cell immunity

Infants and young children are more susceptible to common respiratory pathogens than adults but can fare better against novel pathogens like severe acute respiratory syndrome coronavirus 2. The mechanisms by which infants and young children mount effective immune responses to respiratory pathogens are unknown. Through investigation of lungs and lung-associated lymph nodes from infant and pediatric organ donors aged 0-13 years, we show that bronchus-associated lymphoid tissue (BALT), containing B cell follicles, CD4+ T cells and functionally active germinal centers, develop during infancy. BALT structures are prevalent around lung airways during the first 3 years of life, and their numbers decline through childhood coincident with the accumulation of memory T cells. Single-cell profiling and repertoire analysis reveals that early life lung B cells undergo differentiation, somatic hypermutation and immunoglobulin class switching and exhibit a more activated profile than lymph node B cells. Moreover, B cells in the lung and lung-associated lymph nodes generate biased antibody responses to multiple respiratory pathogens compared to circulating antibodies, which are mostly specific for vaccine antigens in the early years of life. Together, our findings provide evidence for BALT as an early life adaptation for mobilizing localized immune protection to the diverse respiratory challenges during this formative life stage.

BAFF induces CXCR5 expression during B cell differentiation in bone marrow

B cell activating factor (BAFF) plays an important role in antibody production through differentiation and maturation of B cells mainly in secondary lymphoid organs. On the other hand, the role of BAFF in the bone marrow, the primary lymphoid organ of B cell development, has not been well elucidated. Here, effects of BAFF in bone marrow B cell development were examined by using BAFF-deficient mice. When mRNA expression levels of B cell differentiation markers including Cd19, Bcl2, Igμ, Il7r and Cxcr5 were compared between bone marrow of wild-type and BAFF-KO mice, a lower level of Cxcr5 expression was found in the KO mice. Additionally, protein expression of CXCR5 on IgM⁺ cells in the bone marrow was decreased by BAFF deficiency. In vitro studies also confirmed the effect of BAFF on CXCR5 by IgM⁺ cells; culturing bone marrow cells from BAFF-KO mice with BAFF in vitro increased the proportion of CXCR5⁺ cells in IgM⁺ cells compared with non-treated bone marrow cells. In addition, BAFF synergized with TNF-α and IL-6 to increase the expression of CXCR5⁺ on IgM⁺ cells. The BAFF-mediated up-regulation of CXCR5 expression was reproduced by using CD19⁺ cells purified from BAFF-KO bone marrow cells, suggesting that BAFF directly affects B-lineage cells in bone marrow to promote CXCR5 expression. Together, this study suggests that BAFF has an important role in B cell differentiation in bone marrow by directly inducing CXCR5 expression which affect their migration to secondary lymphoid organs.

Stem-like T cells and niches: Implications in human health and disease

Recently, accumulating evidence has elucidated the important role of T cells with stem-like characteristics in long-term maintenance of T cell responses and better patient outcomes after immunotherapy. The fate of TSL cells has been correlated with many physiological and pathological human processes. In this review, we described present advances demonstrating that stem-like T (TSL) cells are central players in human health and disease. We interpreted the evolutionary characteristics, mechanism and functions of TSL cells. Moreover, we discuss the import role of distinct niches and how they affect the stemness of TSL cells. Furthermore, we also outlined currently available strategies to generate TSL cells and associated affecting factors. Moreover, we summarized implication of TSL cells in therapies in two areas: stemness enhancement for vaccines, ICB, and adoptive T cell therapies, and stemness disruption for autoimmune disorders.

High-affinity, neutralizing antibodies to SARS-CoV-2 can be made without T follicular helper cells

T follicular helper (TFH) cells are the conventional drivers of protective, germinal center (GC)–based antiviral antibody responses. However, loss of TFH cells and GCs has been observed in patients with severe COVID-19. As T cell–B cell interactions and immunoglobulin class switching still occur in these patients, noncanonical pathways of antibody production may be operative during SARS-CoV-2 infection. We found that both TFH-dependent and -independent antibodies were induced against SARS-CoV-2 infection, SARS-CoV-2 vaccination, and influenza A virus infection. Although TFH-independent antibodies to SARS-CoV-2 had evidence of reduced somatic hypermutation, they were still high affinity, durable, and reactive against diverse spike-derived epitopes and were capable of neutralizing both homologous SARS-CoV-2 and the B.1.351 (beta) variant of concern. We found by epitope mapping and B cell receptor sequencing that TFH cells focused the B cell response, and therefore, in the absence of TFH cells, a more diverse clonal repertoire was maintained. These data support an alternative pathway for the induction of B cell responses during viral infection that enables effective, neutralizing antibody production to complement traditional GC-derived antibodies that might compensate for GCs damaged by viral inflammation.

CXCR5+CD8+ T Cells Shape Antibody Responses In Vivo Following Protein Immunisation and Peripheral Viral Infection

Crosstalk between T and B cells is crucial for generating high-affinity, class-switched antibody responses. The roles of CD4⁺ T cells in this process have been well-characterised. In contrast, regulation of antibody responses by CD8⁺ T cells is significantly less defined. CD8⁺ T cells are principally recognised for eliciting cytotoxic responses in peripheral tissues and forming protective memory. However, recent findings have identified a novel population of effector CD8⁺ T cells that co-opt a differentiation program characteristic of CD4⁺ T follicular helper (Tfh) cells, upregulate the chemokine receptor CXCR5 and localise to B cell follicles. While it has been shown that CXCR5⁺CD8⁺ T cells mediate the removal of viral reservoirs in the context of follicular-trophic viral infections and maintain the response to chronic insults by virtue of progenitor/stem-like properties, it is not known if CXCR5⁺CD8⁺ T cells arise during acute peripheral challenges in the absence of follicular infection and whether they influence B cell responses in vivo in these settings. Using the ovalbumin-specific T cell receptor transgenic (OT-I) system in an adoptive transfer-immunisation/infection model, this study demonstrates that CXCR5⁺CD8⁺ T cells arise in response to protein immunisation and peripheral viral infection, displaying a follicular-homing phenotype, expression of cell surface molecules associated with Tfh cells and limited cytotoxic potential. Furthermore, studies assessing the B cell response in the presence of OT-I or Cxcr5^-/- OT-I cells revealed that CXCR5⁺CD8⁺ T cells shape the antibody response to protein immunisation and peripheral viral infection, promoting class switching to IgG2c in responding B cells. Overall, the results highlight a novel contribution of CD8⁺ T cells to antibody responses, expanding the functionality of the adaptive immune system.

Durable SARS-CoV-2 B cell immunity after mild or severe disease

Multiple studies have shown loss of severe acute respiratory syndrome coronavirus 2-specific (SARS-CoV-2-specific) antibodies over time after infection, raising concern that humoral immunity against the virus is not durable. If immunity wanes quickly, millions of people may be at risk for reinfection after recovery from coronavirus disease 2019 (COVID-19). However, memory B cells (MBCs) could provide durable humoral immunity even if serum neutralizing antibody titers decline. We performed multidimensional flow cytometric analysis of S protein receptor binding domain-specific (S-RBD-specific) MBCs in cohorts of ambulatory patients with COVID-19 with mild disease (n = 7), and hospitalized patients with moderate to severe disease (n = 7), at a median of 54 days (range, 39-104 days) after symptom onset. We detected S-RBD-specific class-switched MBCs in 13 of 14 participants, failing only in the individual with the lowest plasma levels of anti-S-RBD IgG and neutralizing antibodies. Resting MBCs (rMBCs) made up the largest proportion of S-RBD-specific MBCs in both cohorts. FCRL5, a marker of functional memory on rMBCs, was more dramatically upregulated on S-RBD-specific rMBCs after mild infection than after severe infection. These data indicate that most SARS-CoV-2-infected individuals develop S-RBD-specific, class-switched rMBCs that resemble germinal center-derived B cells induced by effective vaccination against other pathogens, providing evidence for durable B cell-mediated immunity against SARS-CoV-2 after mild or severe disease.

Durable SARS-CoV-2 B cell immunity after mild or severe disease

Multiple studies have shown loss of SARS-CoV-2 specific antibodies over time after infection, raising concern that humoral immunity against the virus is not durable. If immunity wanes quickly, millions of people may be at risk for reinfection after recovery from COVID-19. However, memory B cells (MBC) could provide durable humoral immunity even if serum neutralizing antibody titers decline. We performed multi-dimensional flow cytometric analysis of S protein receptor binding domain (S-RBD)-specific MBC in cohorts of ambulatory COVID-19 patients with mild disease, and hospitalized patients with moderate to severe disease, at a median of 54 (39-104) days after onset of symptoms. We detected S-RBD-specific class-switched MBC in 13 out of 14 participants, including 4 of the 5 participants with lowest plasma levels of anti-S-RBD IgG and neutralizing antibodies. Resting MBC (rMBC) made up the largest proportion of S-RBD-specific class-switched MBC in both cohorts. FCRL5, a marker of functional memory when expressed on rMBC, was dramatically upregulated on S-RBD-specific rMBC. These data indicate that most SARS-CoV-2-infected individuals develop S-RBD-specific, class-switched MBC that phenotypically resemble germinal center-derived B cells induced by effective vaccination against other pathogens, providing evidence for durable B cell-mediated immunity against SARS-CoV-2 after recovery from mild or severe COVID-19 disease.

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

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

Non-human Primate Determinants of Natural Killer Cells in Tissues at Steady-State and During Simian Immunodeficiency Virus Infection

Natural killer (NK) cells play essential roles in immunity to viruses and tumors. Their function is genetically determined but also modulated by environmental factors. The distribution and functional regulation of these cells vary depending on the tissue. NK cell behavior in lymphoid tissues is so far understudied. Non-human primate (NHP) models are essential for the development of therapies and vaccines against human diseases, and access to NHP tissues allows insights into spatial regulations of NK cells. Here, we investigated tissue-specific parameters of NK cells from NHP species, i.e., cynomolgus macaque (Macaca fascicularis), African green monkey (Chlorocebus sabaeus), rhesus macaque (Macaca mulatta), and baboon (Papio anubis). By comprehensive multi-dimensional analysis of NK cells from secondary lymphoid organs, intestinal mucosa, liver, and blood, we identified tissue- and species-specific patterns of NK cell frequencies, phenotypes, and potential activity. Also, we defined the tissue-specific characteristics of NK cells during infection by the simian immunodeficiency virus. Altogether, our results provide a comprehensive anatomic analysis of NK cells in different tissues of primates at steady-state and during a viral infection.

Induction of activated T follicular helper cells is critical for anti-FVIII inhibitor development in hemophilia A mice

The development of neutralizing anti-FVIII antibodies (inhibitors) is a major complication of FVIII protein replacement therapy in patients with hemophilia A (HA). Although multiple lines of evidence indicate that the immune response against FVIII is CD4 T-cell-dependent and many FVIII-derived CD4 epitopes have already been discovered, the role of T follicular helper (TFH) cells in FVIII inhibitor development is unknown. TFH cells, a newly identified subset of CD4 T cells, are characterized by expression of the B-cell follicle-homing receptor CXCR5 and PD-1. In this study, we show for the first time that IV FVIII immunization induces activation and accumulation and/or expansion of PD-1+CXCR5+ TFH cells in the spleen of FVIII-deficient (FVIIInull) mice. FVIII inhibitor-producing mice showed increased germinal center (GC) formation and increased GC TFH cells in response to FVIII immunization. Emergence of TFH cells correlated with titers of anti-FVIII inhibitors. Rechallenge with FVIII antigen elicited recall responses of TFH cells. In vitro FVIII restimulation resulted in antigen-specific proliferation of splenic CD4+ T cells from FVIII-primed FVIIInull mice, and the proliferating cells expressed the TFH hallmark transcription factor BCL6. CXCR5+/+ TFH-cell-specific deletion impaired anti-FVIII inhibitor production, confirming the essential role of CXCR5+/+ TFH cells for the generation of FVIII-neutralizing antibodies. Together, our results demonstrate that the induction of activated TFH cells in FVIIInull mice is critical for FVIII inhibitor development, suggesting that inhibition of FVIII-specific TFH-cell activation may be a promising strategy for preventing anti-FVIII inhibitor formation in patients with HA.

Divergent chemokine receptor expression and the consequence for human IgG4 B cell responses

IgG4 antibodies are unique to humans. IgG4 is associated with tolerance during immunotherapy in allergy, but also with pathology, as in pemphigus vulgaris and IgG4-related disease. Its induction is largely restricted to nonmicrobial antigens, and requires repeated or prolonged antigenic stimulation, for reasons poorly understood. An important aspect in generating high-affinity IgG antibodies is chemokine receptor-mediated migration of B cells into appropriate niches, such as germinal centers. Here, we show that compared to IgG1 B cells, circulating IgG4 B cells express lower levels of CXCR3, CXCR4, CXCR5, CCR6, and CCR7, chemokine receptors involved in GC reactions and generation of long-lived plasma cells. This phenotype was recapitulated by in vitro priming of naive B cells with an IgG4-inducing combination of T_FH /T_H2 cytokines. Consistent with these observations, we found a low abundance of IgG4 B cells in secondary lymphoid tissues in vivo, and the IgG4 antibody response is substantially more short-lived compared to other IgG subclasses in patient groups undergoing CD20⁺ B cell depletion therapy with rituximab. These results prompt the hypothesis that factors needed to form IgG4 B cells restrain at the same time the induction of a robust migratory phenotype that could support a long-lived IgG4 antibody response.

Molecular cues involved in the regulation of B cell dynamics: Assistants of antigen hunting

The ability of a cell to migrate, adhere, and change its morphology is determinant in developing its functions; these capacities reach their maximum relevance in immune cells. For an efficient immune response, immune cells must localize in the right place at the right time; that implies crossing tissue barriers and migrating in the interstitial space of the tissues at high velocities. The dependency on trafficking abilities is even higher for B cells, one of the arms of the adaptive immune system, considering that they must encounter specific antigens for their clonal receptor in the enormous tissue volume of the secondary lymphoid organs (spleen, lymph nodes, Peyer patches). The regulated interplay between cell motility and cell adhesion allows B cells to reach distinct lymphoid tissues and, within them, to explore the stromal cell networks where antigen might be exposed. In this meeting-invited review, I summarize the current knowledge on the molecular cues and mechanisms that shapes B cell dynamics at the initial phase of the humoral immune response, including homeostatic chemoattractants and innate/inflammatory stimuli. I also revised the B cell behavior alterations caused by BCR recognition of antigen and the molecular mechanisms involved.

The COMMD3/8 complex determines GRK6 specificity for chemoattractant receptors

Nakai et al. show that the COMMD3/8 complex functions as an adaptor that selectively recruits GRK6 to chemoattractant receptors and promotes B cell migration and humoral immune responses.

Lymphocyte migration is mediated by G protein–coupled receptors (GPCRs) that respond to chemoattractive molecules. After their activation, GPCRs are phosphorylated by different GPCR kinases (GRKs), which produces distinct functional outcomes through β-arrestins. However, the molecular machinery that targets individual GRKs to activated GPCRs remains elusive. Here, we identified a protein complex consisting of copper metabolism MURR1 domain–containing (COMMD) 3 and COMMD8 (COMMD3/8 complex) as an adaptor that selectively recruits a specific GRK to chemoattractant receptors and promotes lymphocyte chemotaxis. COMMD8, whose stability depended on COMMD3, was recruited to multiple chemoattractant receptors. Deficiency of COMMD8 or COMMD3 impaired B cell migration and humoral immune responses. Using CXC-chemokine receptor 4 (CXCR4) as a model, we demonstrated that the COMMD3/8 complex selectively recruited GRK6 and induced GRK6-mediated phosphorylation of the receptor and activation of β-arrestin–mediated signaling. Thus, the COMMD3/8 complex is a specificity determinant of GRK targeting to GPCRs and represents a point of regulation for immune responses.

Vibrio vulnificus VvhA induces Th1 and Tfh cells to proliferate against Vibrio vulnificus in a mouse model of infection

AIM

To characterize the roles of VvhA in host's acquired immune response to Vibrio vulnificus infection.

MATERIALS & METHODS

The recombinant VvhA fusion protein was used to immunize mice and the anti-VvhA polyclonal antibody was produced in vivo for prophylactic and therapeutic efficacy assay. The roles of VvhA in T helper (Th) cells differentiation were analyzed by vvhA-deleted mutant during the early phase of infection, while the ratio of Th2 and T follicular helper (Tfh) cells were examined in VvhA immunization.

RESULTS

Anti-VvhA antibody exhibited neutralization activity against V. vulnificus. Wild-type strain induced higher level of Th1 cells than the mutant, and the concentrations of IgG2a and IFN-γ were increased during the early phase of infection. The spontaneous development of Tfh was observed in immunized model, and the serum IL-21 was increased.

CONCLUSION

V. vulnificus VvhA elicited cellular and humoral immune responses by Th1 and Tfh cells to provide protection against VvhA.

Heterogeneity of Phenotype and Function Reflects the Multistage Development of T Follicular Helper Cells

T follicular helper cells (Tfh) provide crucial signals for germinal center (GC) formation, but Tfh populations are heterogeneous. While PD1^hi Tfh are important in the GC response, the function of the PD1^lo Tfh-like subset is unknown. We show that these cells, like the PD1^hi GC–Tfh, depend upon B cells; however, their entry to follicles is independent of CXCR5 or cognate interactions with B cells. The differentiation into PD1^hi Tfh is dependent on MHC class II interactions with B cells and requires CXCR5. Our data suggest a Tfh differentiation pathway that is initially B cell-independent, then dependent on non-cognate B cell interactions, and finally following cognate interaction with B cells and CXCR5-ligands allows the formation of GC–Tfh. The PD1^lo Tfh-like cells make early cytokine responses and may represent precursors of CD4 memory cells.

Activated GL7+ B cells are maintained within the inflamed CNS in the absence of follicle formation during viral encephalomyelitis

Central nervous system (CNS) inflammation associated with viral infection and autoimmune disease results in the accumulation of B cells in various differentiation stages. However, the contribution between peripheral and CNS activation remains unclear. During gliatropic coronavirus induced encephalomyelitis, accumulation of protective antibody secreting cells is preceded by infiltration of B cells with a naïve and early differentiation phenotype (Phares et al., 2014). Investigation of the temporal dynamics of B cell activation in draining cervical lymph nodes (CLN) and the CNS revealed that peak CNS infiltration of early activated, unswitched IgD⁺ and IgM⁺ B cells coincided with polyclonal activation in CLN. By contrast, isotype-switched IgG⁺ B cells did not accumulate until peripheral germinal center formation. In the CNS, unswitched B cells were confined to the perivascular space and meninges, with only rare B cell clusters, while isotype-switched B cells localized to parenchymal areas. Although ectopic follicle formation was not observed, more differentiated B cell subsets within the CNS expressed the germinal center marker GL7, albeit at lower levels than CLN counterparts. During chronic infection, CNS IgD^int and IgD^- B cell subsets further displayed sustained markers of proliferation and CD4 T cell help, which were only transiently expressed in the CLN. A contribution of local CD4 T cell help to sustain B cell activation was supported by occasional B cells adjacent to T cells. The results suggest that accumulation of differentiated B cell subsets within the CNS is largely dictated by peripheral activation, but that local events contribute to their sustained activation independent of ectopic follicle formation.

The Role of Lymphatic Niches in T Cell Differentiation

Long-term immunity to many viral and bacterial pathogens requires CD8(+) memory T cell development, and the induction of long-lasting CD8(+) memory T cells from a naïve, undifferentiated state is a major goal of vaccine design. Formation of the memory CD8(+) T cell compartment is highly dependent on the early activation cues received by naïve CD8(+) T cells during primary infection. This review aims to highlight the cellularity of various niches within the lymph node and emphasize recent evidence suggesting that distinct types of T cell activation and differentiation occur within different immune contexts in lymphoid organs.

CXCL13 promotes isotype-switched B cell accumulation to the central nervous system during viral encephalomyelitis

Elevated CXCL13 within the central nervous system (CNS) correlates with humoral responses in several neuroinflammatory diseases, yet its role is controversial. During coronavirus encephalomyelitis CXCL13 deficiency impaired CNS accumulation of memory B cells and antibody-secreting cells (ASC) but not naïve/early-activated B cells. However, despite diminished germinal center B cells and follicular helper T cells in draining lymph nodes, ASC in bone marrow and antiviral serum antibody were intact in the absence of CXCL13. The data demonstrate that CXCL13 is not essential in mounting effective peripheral humoral responses, but specifically promotes CNS accumulation of differentiated B cells.

Cytokines in the Germinal Center Niche

Cytokines are small, secreted, glycoproteins that specifically affect the interactions and communications between cells. Cytokines are produced transiently and locally, acting in a paracrine or autocrine manner, and they are extremely potent, ligating high affinity cell surface receptors to elicit changes in gene expression and protein synthesis in the responding cell. Cytokines produced during the differentiation of T follicular helper (Tfh) cells and B cells within the germinal center (GC) niche play an important role in ensuring that the humoral immune response is robust, whilst retaining flexibility, during the generation of affinity matured antibodies. Cytokines produced by B cells, antigen presenting cells and stromal cells are important for the differentiation of Tfh cells and Tfh cell produced cytokines act both in an autocrine fashion to firm Tfh cell differentiation and in a paracrine fashion to support the differentiation of memory B cells and plasma cells. In this review, we discuss the role of cytokines during the GC reaction with a particular focus on the influence of cytokines on Tfh cells.

p53-dependent expression of CXCR5 chemokine receptor in MCF-7 breast cancer cells

Elevated expression of chemokine receptors in tumors has been reported in many instances and is related to a number of survival advantages for tumor cells including abnormal activation of prosurvival intracellular pathways. In this work we demonstrated an inverse correlation between expression levels of p53 tumor suppressor and CXCR5 chemokine receptor in MCF-7 human breast cancer cell line. Lentiviral transduction of MCF-7 cells with p53 shRNA led to elevated CXCR5 at both mRNA and protein levels. Functional activity of CXCR5 in p53-knockdown MCF-7 cells was also increased as shown by activation of target gene expression and chemotaxis in response to B-lymphocyte chemoattractant CXCL13. Using deletion analysis and site-directed mutagenesis of the cxcr5 gene promoter and enhancer elements, we demonstrated that p53 appears to act upon cxcr5 promoter indirectly, by repressing the activity of NFκB transcription factors. Using chromatin immunoprecipitation and reporter gene analysis, we further demonstrated that p65/RelA was able to bind the cxcr5 promoter in p53-dependent manner and to directly transactivate it when overexpressed. Through the described mechanism, elevated CXCR5 expression may contribute to abnormal cell survival and migration in breast tumors that lack functional p53.

Life of T follicular helper cells

Antibodies are powerful defense tools against pathogens but may cause autoimmune diseases when erroneously directed toward self-antigens. Thus, antibody producing cells are carefully selected, refined, and expanded in a highly regulated microenvironment (germinal center) in the peripheral lymphoid organs. A subset of T cells termed T follicular helper cells (Tfh) play a central role in instructing B cells to form a repertoire of antibody producing cells that provide life-long supply of high affinity, pathogen-specific antibodies. Therefore, understanding how Tfh cells arise and how they facilitate B cell selection and differentiation during germinal center reaction is critical to improve vaccines and better treat autoimmune diseases. In this review, I will summarise recent findings on molecular and cellular mechanisms underlying Tfh generation and function with an emphasis on T cell costimulation.

Rebamipide attenuates autoimmune arthritis severity in SKG mice via regulation of B cell and antibody production

Oxidative stress is involved in the pathophysiology of rheumatoid arthritis (RA). We investigated the therapeutic potential of rebamipide, a gastroprotective agent with a property of reactive oxygen species scavenger, on the development of inflammatory polyarthritis and the pathophysiological mechanisms by which rebamipide might confer anti-arthritic effects in SKG mice, an animal model of RA. Intraperitoneal (i.p.) injection of rebamipide attenuated the severity of clinical and histological arthritis. Rebampide treatment reduced the number of T helper type 1 (Th1), Th2, Th17, inducible T cell co-stimulator (ICOS)(+) follicular helper T (Tfh) transitional type (T2) and mature B cells in the spleen, but increased the number of regulatory T (Treg ), CD19(+) CD1d(high) CD5(high) , CD19(+) CD25(high) forkhead box protein 3 (FoxP3)(+) regulatory B (Breg ) cells, memory B cells, and transitional type 1 (T1) B cells. In addition, flow cytometric analysis revealed significantly decreased populations of FAS(+) GL-7(+) germinal centre B cells and B220(-) CD138(+) plasma cells in the spleens of rebamipide-treated SKG mice compared to controls. Rebamipide decreased germinal centre B cells and reciprocally induced Breg cells in a dose-dependent manner in vitro. Rebamipide-induced Breg cells had more suppressive capacity in relation to T cell proliferation and also inhibited Th17 differentiation from murine CD4(+) T cells. Together, these data show that i.p. administration of rebamipide suppresses arthritis severity by inducing Breg and Treg cells and suppressing Tfh and Th17 cells in a murine model of RA.

Sphingosine-1-phosphate receptor 2 is critical for follicular helper T cell retention in germinal centers

S1PR2 and CXCR5 cooperatively regulate localization of Tfh cells in GCs to support GC responses.

Follicular helper T (Tfh) cells access the B cell follicle to promote antibody responses and are particularly important for germinal center (GC) reactions. However, the molecular mechanisms of how Tfh cells are physically associated with GCs are incompletely understood. We report that the sphingosine-1-phosphate receptor 2 (S1PR2) gene is highly expressed in a subpopulation of Tfh cells that localizes in GCs. S1PR2-deficient Tfh cells exhibited reduced accumulation in GCs due to their impaired retention. T cells deficient in both S1PR2 and CXCR5 were ineffective in supporting GC responses compared with T cells deficient only in CXCR5. These results suggest that S1PR2 and CXCR5 cooperatively regulate localization of Tfh cells in GCs to support GC responses.

Age-dependent hepatic lymphoid organization directs successful immunity to hepatitis B

Hepatitis B virus (HBV) is a major human pathogen that causes immune-mediated hepatitis. Successful immunity to HBV is age dependent: viral clearance occurs in most adults, whereas neonates and young children usually develop chronic infection. Using a mouse model of HBV infection, we sought mechanisms underpinning the age-dependent outcome of HBV and demonstrated that hepatic macrophages facilitate lymphoid organization and immune priming within the adult liver and promote successful immunity. In contrast, lymphoid organization and immune priming was greatly diminished in the livers of young mice, and of macrophage-depleted adult mice, leading to abrogated HBV immunity. Furthermore, we found that CXCL13, which is involved in B lymphocyte trafficking and lymphoid architecture and development, is expressed in an age-dependent manner in both adult mouse and human hepatic macrophages and plays an integral role in facilitating an effective immune response against HBV. Taken together, these results identify some of the immunological mechanisms necessary for effective control of HBV.

CXCR5⁺ T helper cells mediate protective immunity against tuberculosis

One third of the world's population is infected with Mycobacterium tuberculosis (Mtb). Although most infected people remain asymptomatic, they have a 10% lifetime risk of developing active tuberculosis (TB). Thus, the current challenge is to identify immune parameters that distinguish individuals with latent TB from those with active TB. Using human and experimental models of Mtb infection, we demonstrated that organized ectopic lymphoid structures containing CXCR5+ T cells were present in Mtb-infected lungs. In addition, we found that in experimental Mtb infection models, the presence of CXCR5+ T cells within ectopic lymphoid structures was associated with immune control. Furthermore, in a mouse model of Mtb infection, we showed that activated CD4+CXCR5+ T cells accumulated in Mtb-infected lungs and produced proinflammatory cytokines. Mice deficient in Cxcr5 had increased susceptibility to TB due to defective T cell localization within the lung parenchyma. We demonstrated that CXCR5 expression in T cells mediated correct T cell localization within TB granulomas, promoted efficient macrophage activation, protected against Mtb infection, and facilitated lymphoid follicle formation. These data demonstrate that CD4+CXCR5+ T cells play a protective role in the immune response against TB and highlight their potential use for future TB vaccine design and therapy.

Interruption of CXCL13-CXCR5 axis increases upper genital tract pathology and activation of NKT cells following chlamydial genital infection

BACKGROUND

Regulation of immune responses is critical for controlling inflammation and disruption of this process can lead to tissue damage. We reported that CXCL13 was induced in fallopian tube tissue following C. trachomatis infection. Here, we examined the influence of the CXCL13-CXCR5 axis in chlamydial genital infection.

METHODOLOGY AND PRINCIPAL FINDINGS

Disruption of the CXCL13-CXCR5 axis by injecting anti-CXCL13 Ab to BALB/c mice or using Cxcr5-/- mice increased chronic inflammation in the upper genital tract (UGT; uterine horns and oviducts) after Chlamydia muridarum genital infection (GT). Further studies in Cxcr5-/- mice showed an elevation in bacterial burden in the GT and increased numbers of neutrophils, activated DCs and activated NKT cells early after infection. After resolution, we noted increased fibrosis and the accumulation of a variety of T cells subsets (CD4-IFNγ, CD4-IL-17, CD4-IL-10 & CD8-TNFα) in the oviducts. NKT cell depletion in vitro reduced IL-17α and various cytokines and chemokines, suggesting that activated NKT cells modulate neutrophils and DCs through cytokine/chemokine secretion. Further, chlamydial glycolipids directly activated two distinct types of NKT cell hybridomas in a cell-free CD1d presentation assay and genital infection of Cd1d-/- mice showed reduced oviduct inflammation compared to WT mice. CXCR5 involvement in pathology was also noted using single-nucleotide polymorphism analysis in C. trachomatis infected women attending a sub-fertility clinic. Women who developed tubal pathology after a C. trachomatis infection had a decrease in the frequency of CXCR5 SNP +10950 T>C (rs3922).

CONCLUSIONS/SIGNIFICANCE

These experiments indicate that disruption of the CXCL13-CXCR5 axis permits increased activation of NKT cells by type I and type II glycolipids of Chlamydia muridarum and results in UGT pathology potentially through increased numbers of neutrophils and T cell subsets associated with UGT pathology. In addition, CXCR5 appears to contribute to inter-individual differences in human tubal pathology following C. trachomatis infection.

The aged microenvironment contributes to the age-related functional defects of CD4 T cells in mice

CD4 T cells, and especially T follicular helper cells, are critical for the generation of a robust humoral response to an infection or vaccination. Importantly, immunosenescence affects CD4 T-cell function, and the accumulation of intrinsic defects decreases the cognate helper functions of these cells. However, much less is known about the contribution of the aged microenvironment to this impaired CD4 T-cell response. In this study, we have employed a preclinical model to determine whether the aged environment contributes to the defects in CD4 T-cell functions with aging. Using an adoptive transfer model in mice, we demonstrate for the first time that the aged microenvironment negatively impacts at least three steps of the CD4 T-cell response to antigenic stimulation. First, the recruitment of CD4 T cells to the spleen is reduced in aged compared to young hosts, which correlates with dysregulated chemokine expression in the aged organ. Second, the priming of CD4 T cells by DCs is reduced in aged compared to young mice. Finally, naïve CD4 T cells show a reduced transition to a T follicular helper cell phenotype in the aged environment, which impairs the subsequent generation of germinal centers. These studies have provided new insights into how aging impacts the immune system and how these changes influence the development of immunity to infections or vaccinations.

How T cells earn the follicular rite of passage

The discovery that Bcl-6 was the transcriptional regulator of follicular helper T (Tfh) cells completed the recognition of this population as an effector subset specialized in the provision of help to B cells. Improved reagents and recent models that allow tracking of Bcl-6-expressing T cells have revealed that the decision to become a Tfh cell occurs soon after T cells are primed by dendritic cells and start dividing, before interaction with B cells. The latter are important for sustaining Bcl-6 expression. Bcl-6 coordinates a signaling program that changes expression or function of multiple guidance receptors, leading to Tfh cell localization within germinal centers. This program is not unique to CD4(+) helper T cells; FoxP3(+) regulatory T cells and NKT cells co-opt the follicular differentiation pathway to enter the follicle and become specialized follicular cells. This review will focus on recent insights into the early events that determine Tfh cell differentiation.

Cell type-specific DNA methylation at intragenic CpG islands in the immune system

Human and mouse genomes contain a similar number of CpG islands (CGIs), which are discrete CpG-rich DNA sequences associated with transcription start sites. In both species, ∼50% of all CGIs are remote from annotated promoters but, nevertheless, often have promoter-like features. To determine the role of CGI methylation in cell differentiation, we analyzed DNA methylation at a comprehensive CGI set in cells of the mouse hematopoietic lineage. Using a method that potentially detects ∼33% of genomic CpGs in the methylated state, we found that large differences in gene expression were accompanied by surprisingly few DNA methylation changes. There were, however, many DNA methylation differences between hematopoietic cells and a distantly related tissue, brain. Altered DNA methylation in the immune system occurred predominantly at CGIs within gene bodies, which have the properties of cell type-restricted promoters, but infrequently at annotated gene promoters or CGI flanking sequences (CGI "shores"). Unexpectedly, elevated intragenic CGI methylation correlated with silencing of the associated gene. Differentially methylated intragenic CGIs tended to lack H3K4me3 and associate with a transcriptionally repressive environment regardless of methylation state. Our results indicate that DNA methylation changes play a relatively minor role in the late stages of differentiation and suggest that intragenic CGIs represent regulatory sites of differential gene expression during the early stages of lineage specification.

Follicular helper CD4 T cells (TFH)

T cell help to B cells is a fundamental aspect of adaptive immunity and the generation of immunological memory. Follicular helper CD4 T (T(FH)) cells are the specialized providers of B cell help. T(FH) cells depend on expression of the master regulator transcription factor Bcl6. Distinguishing features of T(FH) cells are the expression of CXCR5, PD-1, SAP (SH2D1A), IL-21, and ICOS, among other molecules, and the absence of Blimp-1 (prdm1). T(FH) cells are important for the formation of germinal centers. Once germinal centers are formed, T(FH) cells are needed to maintain them and to regulate germinal center B cell differentiation into plasma cells and memory B cells. This review covers T(FH) differentiation, T(FH) functions, and human T(FH) cells, discussing recent progress and areas of uncertainty or disagreement in the literature, and it debates the developmental relationship between T(FH) cells and other CD4 T cell subsets (Th1, Th2, Th17, iTreg).

Viral persistence redirects CD4 T cell differentiation toward T follicular helper cells

Persistent virus infection drives follicular T helper cell differentiation.

CD4 T cell responses are crucial to prevent and control viral infection; however, virus-specific CD4 T cell activity is considered to be rapidly lost during many persistent viral infections. This is largely caused by the fact that during viral persistence CD4 T cells do not produce the classical Th1 cytokines associated with control of acute viral infections. Considering that CD4 T cell help is critical for both CD8 T cell and B cell functions, it is unclear how CD4 T cells can lose responsiveness but continue to sustain long-term control of persistent viral replication. We now demonstrate that CD4 T cell function is not extinguished as a result of viral persistence. Instead, viral persistence and prolonged T cell receptor stimulation progressively redirects CD4 T cell development away from the Th1 response induced during an acute infection toward T follicular helper cells. Importantly, this sustained CD4 T cell functionality is critical to maintain immunity and ultimately aid in the control of persistent viral infection.

B cell follicles and antigen encounters of the third kind

Defining where and in what form lymphocytes encounter antigen is fundamental to understanding how immune responses occur. Although knowledge of the recognition of antigen by CD4(+) and CD8(+) T cells has advanced greatly, understanding of the dynamics of B cell-antigen encounters has lagged. With the application of advanced imaging approaches, encounters of this third kind are now being brought into focus. Multiple processes facilitate these encounters, from the filtering functions of lymphoid tissues and migration paths of B cells to the antigen-presenting properties of macrophages and follicular dendritic cells. This Review will discuss how these factors work together in the lymph node to ensure efficient and persistent exposure of B cells to diverse forms of antigen and thus effective triggering of the humoral response.

The chemokine receptor CXCR5 is pivotal for ectopic mucosa-associated lymphoid tissue neogenesis in chronic Helicobacter pylori-induced inflammation

Ectopic lymphoid follicles are a key feature of chronic inflammatory autoimmune and infectious diseases, such as rheumatoid arthritis, Sjögren's syndrome, and Helicobacter pylori-induced gastritis. Homeostatic chemokines are considered to be involved in the formation of such tertiary lymphoid tissue. High expression of CXCL13 and its receptor, CXCR5, has been associated with the formation of ectopic lymphoid follicles in chronic infectious diseases. Here, we defined the role of CXCR5 in the development of mucosal tertiary lymphoid tissue and gastric inflammation in a mouse model of chronic H. pylori infection. CXCR5-deficient mice failed to develop organized gastric lymphoid follicles despite similar bacterial colonization density as infected wild-type mice. CXCR5 deficiency altered Th17 responses but not Th1-type cellular immune responses to H. pylori infection. Furthermore, CXCR5-deficient mice exhibited lower H. pylori-specific serum IgG and IgA levels and an overall decrease in chronic gastric immune responses. In conclusion, the development of mucosal tertiary ectopic follicles during chronic H. pylori infection is strongly dependent on the CXCL13/CXCR5 signaling axis, and lack of de novo lymphoid tissue formation attenuates chronic immune responses.

Lymphoid chemokines in the CNS

Lymphoid chemokines, including CCL19, CCL21 and CXCL13, are critical in the development and organization of secondary lymphoid tissues and in the generation of adaptive immune responses. These molecules have also been implicated in the development of ectopic lymphoid structures in the setting of chronic inflammation. Here we review current knowledge on the production of lymphoid chemokines in the central nervous system during both homeostatic conditions and in disease states. Accumulating evidence suggests that constitutive expression of CCL19 plays a critical immunosurveillance role in healthy individuals. In contrast, aberrant induction of CCL19, CCL21 and CXCL13 may support the establishment of chronic autoimmunity and hematopoietic tumors within the CNS.

Understanding the development and function of T follicular helper cells

A fundamental function of T helper (Th) cells is to regulate B-cell proliferation and immunoglobulin class switching, especially in the germinal centers. Th1 and Th2 lineages of CD4(+) T cells have long been considered to play an essential role in helping B cells by promoting the production immunoglobulin G2a (IgG2a) and IgG1/IgE, respectively. Recently, it has become clear that a subset CD4(+) T cells, named T follicular helper (Tfh) cells, is critical to B-cell response induction. In this review, we summarize the latest advances in our understanding of the regulation of Tfh cell differentiation, the relationship of Tfh cells to other CD4(+) T-cell lineages, and the role of Tfh cells in health and disease.

Visualizing B cell capture of cognate antigen from follicular dendritic cells

The prominent display of opsonized antigen by follicular dendritic cells (FDCs) has long favored the view that they serve as antigen-presenting cells for B cells. Surprisingly, however, although B cell capture of antigen from macrophages and dendritic cells has been visualized, acquisition from FDCs has not been directly observed. Using two-photon microscopy, we visualized B cell capture of cognate antigen from FDCs. B cell CXCR5 expression was required, and encounter with FDC-associated antigen could be detected for >1 wk after immunization. B cell-FDC contact times were often brief but occasionally persisted for >30 min, and B cells sometimes acquired antigen together with FDC surface proteins. These observations establish that FDCs can serve as sites of B cell antigen capture, with their prolonged display time ensuring that even rare B cells have the chance of antigen encounter, and they suggest possible information transfer from antigen-presenting cell to B cell.

Induction of autoantibody-mediated spontaneous arthritis critically depends on follicular dendritic cells

Follicular dendritic cells (FDCs) are important for the induction of protective T cell-dependent humoral responses, but their contribution to autoimmunity remains elusive. Here, gene-targeted interruption of FDC development was combined with the K/BxN mouse model of arthritis. We found that FDCs were essential for autoantibody production through two distinct but cooperative functions. In a T cell-independent fashion, FDCs loaded with autoantigen-containing immune complexes supported germinal center (GC) B cell development. Additionally, the integrity of FDC networks was required for the recruitment of arthritogenic follicular helper T cells, a process that drove T-B cell interactions and productive GC reactivity. Importantly, pharmacological interference in the maintenance of FDCs ameliorated disease development, suggesting the FDC as a potential target for dampening autoimmunity.

Form follows function: lymphoid tissue microarchitecture in antimicrobial immune defence

Secondary lymphoid organs (SLOs) are tissues that facilitate the induction of adaptive immune responses. These organs capture pathogens to limit their spread throughout the body, bring antigen-presenting cells into productive contact with their cognate lymphocytes and provide niches for the differentiation of immune effector cells. Therefore, the microanatomy of SLOs defines the ability of an organism to respond to pathogens. SLO microarchitecture is, at the same time, extremely adaptable to environmental changes. In this Review, we discuss recent insights into the function and plasticity of the SLO microenvironment with regards to antimicrobial immune defence.

Generation of T follicular helper cells is mediated by interleukin-21 but independent of T helper 1, 2, or 17 cell lineages

After activation, CD4(+) helper T (Th) cells differentiate into distinct effector subsets. Although chemokine (C-X-C motif) receptor 5-expressing T follicular helper (Tfh) cells are important in humoral immunity, their developmental regulation is unclear. Here we show that Tfh cells had a distinct gene expression profile and developed in vivo independently of the Th1 or Th2 cell lineages. Tfh cell generation was regulated by ICOS ligand (ICOSL) expressed on B cells and was dependent on interleukin-21 (IL-21), IL-6, and signal transducer and activator of transcription 3 (STAT3). However, unlike Th17 cells, differentiation of Tfh cells did not require transforming growth factor beta (TGF-beta) or Th17-specific orphan nuclear receptors RORalpha and RORgamma in vivo. Finally, naive T cells activated in vitro in the presence of IL-21 but not TGF-beta signaling preferentially acquired Tfh gene expression and promoted germinal-center reactions in vivo. This study thus demonstrates that Tfh is a distinct Th cell lineage.

Restoration of lymphoid organ integrity through the interaction of lymphoid tissue-inducer cells with stroma of the T cell zone

The generation of lymphoid microenvironments in early life depends on the interaction of lymphoid tissue-inducer cells with stromal lymphoid tissue-organizer cells. Whether this cellular interface stays operational in adult secondary lymphoid organs has remained elusive. We show here that during acute infection with lymphocytic choriomeningitis virus, antiviral cytotoxic T cells destroyed infected T cell zone stromal cells, which led to profound disruption of secondary lymphoid organ integrity. Furthermore, the ability of the host to respond to secondary antigens was lost. Restoration of the lymphoid microanatomy was dependent on the proliferative accumulation of lymphoid tissue-inducer cells in secondary lymphoid organs during the acute phase of infection and lymphotoxin alpha(1)beta(2) signaling. Thus, crosstalk between lymphoid tissue-inducer cells and stromal cells is reactivated in adults to maintain secondary lymphoid organ integrity and thereby contributes to the preservation of immunocompetence.

Follicular associated T cells and their B-cell helper qualities

The immune system utilizes sophisticated cellular surveillance mechanisms to maintain the integrity of the multicellular host. Adaptive immunosurveillance in particular constitutes a powerful branch of the immune system that houses the capacity to mount exquisitely specific responses against a diverse array of foreign antigens. Central to the development of adaptive immunity is the activation of T and B cells. Upon antigen engagement, T and B cells have been observed to undergo striking changes in their migratory status and distribution within secondary lymphoid organs, a phenomenon that is to a large extent controlled through their altered responsiveness to homeostatic T- and B-zone chemokines. Changes in their chemokine receptor expression and/or sensitivity to their respective ligands assist in bringing rare antigen-specific T and B lymphocytes, dendritic cells and CD4+CD3(-) accessory cells together. Cognate interaction between these cells at the T-B junction can support the generation of extrafollicular foci of antibody producing plasma cells and the formation of germinal centers. Such T-dependent antibody responses are highly dependent on the functional properties and activity of a specialized subset of CXCR5+ICOS+ CD4 T cells referred to as T follicular helper cells (T FH). This review presents an overview of some of the defining characteristics of this subset of T-helper cells and the chemokine receptors and their ligands that help dictate the migratory activity of T(FH) cells within secondary lymphoid organs.

B cell activation state-governed formation of germinal centers following viral infection

Germinal centers are structures that promote humoral memory cell formation and affinity maturation, but the triggers for their development are not entirely clear. Activated extrafollicular B cells can form IgM-producing plasmablasts or enter a germinal center reaction and differentiate into memory or plasma cells, mostly of the IgG isotype. Vesicular stomatitis virus (VSV) induces both types of response, allowing events that promote each of these pathways to be studied. In this work, extrafollicular vs germinal center responses were examined at a cellular level, analyzing VSV-specific B cells in infected mice. We show that VSV-specific germinal centers are transiently formed when insufficient proportions of specific T cell help is available and that strong B cell activation in cells expressing high levels of the VSV-specific BCR promoted their differentiation into early blasts, whereas moderate stimulation of B cells or interaction with Th cells restricted extrafollicular responses and promoted germinal center formation.