Impaired germinal center formation and recall T-cell-dependent immune responses in mice lacking the costimulatory ligand B7-H2

Role of B cells as antigen presenting cells

B cells have been long studied for their role and function in the humoral immune system. Apart from generating antibodies and an antibody-mediated memory response against pathogens, B cells are also capable of generating cell-mediated immunity. It has been demonstrated by several groups that B cells can activate antigen-specific CD4 and CD8 T cells, and can have regulatory and cytotoxic effects. The function of B cells as professional antigen presenting cells (APCs) to activate T cells has been largely understudied. This, however, requires attention as several recent reports have demonstrated the importance of B cells within the tumor microenvironment, and B cells are increasingly being evaluated as cellular therapies. Antigen presentation through B cells can be through antigen-specific (B cell receptor (BCR) dependent) or antigen non-specific (BCR independent) mechanisms and can be modulated by a variety of intrinsic and external factors. This review will discuss the pathways and mechanisms by which B cells present antigens, and how B cells differ from other professional APCs.

Immunofibroblasts regulate LTα3 expression in tertiary lymphoid structures in a pathway dependent on ICOS/ICOSL interaction

Immunofibroblasts have been described within tertiary lymphoid structures (TLS) that regulate lymphocyte aggregation at sites of chronic inflammation. Here we report, for the first time, an immunoregulatory property of this population, dependent on inducible T-cell co-stimulator ligand and its ligand (ICOS/ICOS-L). During inflammation, immunofibroblasts, alongside other antigen presenting cells, like dendritic cells (DCs), upregulate ICOSL, binding incoming ICOS + T cells and inducing LTα3 production that, in turn, drives the chemokine production required for TLS assembly via TNFRI/II engagement. Pharmacological or genetic blocking of ICOS/ICOS-L interaction results in defective LTα expression, abrogating both lymphoid chemokine production and TLS formation. These data provide evidence of a previously unknown function for ICOSL-ICOS interaction, unveil a novel immunomodulatory function for immunofibroblasts, and reveal a key regulatory function of LTα3, both as biomarker of TLS establishment and as first driver of TLS formation and maintenance in mice and humans.

IgD ligation allows peritoneal cavity B cell proliferation

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

The role of ICOS in allergic disease: Positive or Negative?

With the rapid increase in the incidence of allergic diseases, the mechanisms underlying the development of these diseases have received a great deal of attention, and this is particularly true in regard to the role of ICOS in allergic diseases. Current studies have revealed that ICOS affects the functional activity of multiple immune cells that modulate the adaptive immune system. Additionally, ICOS also plays a crucial role in mediating cellular immunity and coordinating the response of the entire immune system, and thus, it plays a role in allergic reactions. However, the ICOS/ICOS-ligand (ICOS-L) axis functions in a dual role during the development of multiple allergic diseases. In this review, we explore the role of ICOS/ICOSL in the context of different immune cells that function in allergic diseases, and we summarize recent advances in their contribution to these diseases.

Inducible Costimulator Contributes to Methicillin-Resistant Staphylococcus aureus Pneumonia

Staphylococcus aureus is a major cause of both community- and healthcare-acquired pneumonias. Inducible costimulator (ICOS) is part of the CD28 family of proteins and is a target for immune checkpoint therapy. We found ICOS highly expressed on activated CD4 cells in response to S. aureus. In the absence of ICOS, mice had improved survival in a pneumonia model with the methicillin-resistant Staphylococcus aureus (MRSA) strain USA300 and significant reductions in bacterial burden in a nonlethal acute pneumonia model. Infected Icos-/- mice had major reductions in several proinflammatory cytokines, neutrophils, inflammatory monocytes, and eosinophils compared to infected wild-type mice, while there was improved expression of CD11c and macrophage receptor with collagenous structure on the surface of alveolar macrophages. Early during infection infected Icos-/- mice had increased numbers of alveolar macrophages and expression of several surface markers on alveolar macrophages and neutrophils. ICOS signaling also contributed to the pathogenesis of the airway pathogens Klebsiella pneumoniae, Pseudomonas aeruginosa, and Streptococcus pneumoniae, and neutralizing antibody to ICOS led to improved clearance of S. aureus from the airway. Our results indicate that ICOS plays a significant role in orchestrating the innate immune response to S. aureus and other airway pathogens, and could be a potential immunomodulatory target to attenuate S. aureus-related immunopathology.

Loss of human ICOSL results in combined immunodeficiency

Primary immunodeficiencies represent naturally occurring experimental models to decipher human immunobiology. We report a patient with combined immunodeficiency, marked by recurrent respiratory tract and DNA-based viral infections, hypogammaglobulinemia, and panlymphopenia. He also developed moderate neutropenia but without prototypical pyogenic infections. Using whole-exome sequencing, we identified a homozygous mutation in the inducible T cell costimulator ligand gene (ICOSLG; c.657C>G; p.N219K). Whereas WT ICOSL is expressed at the cell surface, the ICOSL^N219K mutation abrogates surface localization: mutant protein is retained in the endoplasmic reticulum/Golgi apparatus, which is predicted to result from deleterious conformational and biochemical changes. ICOSL^N219K diminished B cell costimulation of T cells, providing a compelling basis for the observed defect in antibody and memory B cell generation. Interestingly, ICOSL^N219K also impaired migration of lymphocytes and neutrophils across endothelial cells, which normally express ICOSL. These defects likely contributed to the altered adaptive immunity and neutropenia observed in the patient, respectively. Our study identifies human ICOSLG deficiency as a novel cause of a combined immunodeficiency.

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

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

The CD28-B7 Family of Co-signaling Molecules

Immune responses are controlled by the optimal balance between protective immunity and immune tolerance. T-cell receptor (TCR) signals are modulated by co-signaling molecules, which are divided into co-stimulatory and co-inhibitory molecules. By expression at the appropriate time and location, co-signaling molecules positively and negatively control T-cell differentiation and function. For example, ligation of the CD28 on T cells provides a critical secondary signal along with TCR ligation for naive T-cell activation. In contrast, co-inhibitory signaling by the CD28-B7 family is important to regulate immune homeostasis and host defense, as these signals limit the strength and duration of immune responses to prevent autoimmunity. At the same time, microorganisms or tumor cells can use these pathways to establish an immunosuppressive environment to inhibit the immune responses against themselves. Understanding these co-inhibitory pathways will support the development of new immunotherapy for the treatment of tumors and autoimmune and infectious diseases. Here, we introduce diverse molecules belonging to the members of the CD28-B7 family.

DAMP-Inducing Adjuvant and PAMP Adjuvants Parallelly Enhance Protective Type-2 and Type-1 Immune Responses to Influenza Split Vaccination

Recently, it was reported that 2-hydroxypropyl-β-cyclodextrin (HP-β-CyD), a common pharmaceutical additive, can act as a vaccine adjuvant to enhance protective type-2 immunogenicity to co-administered seasonal influenza split vaccine by inducing host-derived damage-associated molecular patterns (DAMPs). However, like most other DAMP-inducing adjuvants such as aluminum hydroxide (Alum), HP-β-CyD may not be sufficient for the induction of protective type-1 (cellular) immune responses, thereby leaving room for improvement. Here, we demonstrate that a combination of HP-β-CyD with a humanized TLR9 agonist, K3 CpG-ODN, a potent pathogen-associated molecular pattern (PAMP), enhanced the protective efficacy of the co-administered influenza split vaccine by inducing antigen-specific type-2 and type-1 immune responses, respectively. Moreover, substantial antigen-specific IgE induction by HP-β-CyD, which can cause an allergic response to immunized antigen was completely suppressed by the addition of K3 CpG-ODN. Furthermore, HP-β-CyD- and K3 CpG-ODN-adjuvanted influenza split vaccination protected the mice against lethal challenge with high doses of heterologous influenza virus, which could not be protected against by single adjuvant vaccines. Further experiments using gene deficient mice revealed the unique immunological mechanism of action in vivo, where type-2 and type-1 immune responses enhanced by the combined adjuvants were dependent on TBK1 and TLR9, respectively, indicating their parallel signaling pathways. Finally, the analysis of immune responses in the draining lymph node suggested that HP-β-CyD promotes the uptake of K3 CpG-ODN by plasmacytoid dendritic cells and B cells, which may contributes to the activation of these cells and enhanced production of IgG2c. Taken together, the results above may offer potential clinical applications for the combination of DAMP-inducing adjuvant and PAMP adjuvant to improve vaccine immunogenicity and efficacy by enhancing both type-2 and type-1 immune responses in a parallel manner.

IL-9 receptor signaling in memory B cells regulates humoral recall responses

Memory B cells (B_mem cells) are the basis of long-lasting humoral immunity. They respond to re-encountered antigens by rapidly producing specific antibodies and forming germinal centers (GCs), a recall response that has been known for decades but remains poorly understood. We found that the receptor for the cytokine IL-9 (IL-9R) was induced selectively on B_mem cells after primary immunization and that IL-9R-deficient mice exhibited a normal primary antibody response but impaired recall antibody responses, with attenuated population expansion and plasma-cell differentiation of B_mem cells. In contrast, there was augmented GC formation, possibly due to defective downregulation of the ligand for the co-stimulatory receptor ICOS on B_mem cells. A fraction of B_mem cells produced IL-9. These findings indicate that IL-9R signaling in B_mem cells regulates humoral recall responses.

TANK-Binding Kinase 1-Dependent Responses in Health and Autoimmunity

The pathogenesis of autoimmune diseases, such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) is driven by genetic predisposition and environmental triggers that lead to dysregulated immune responses. These include the generation of pathogenic autoantibodies and aberrant production of inflammatory cytokines. Current therapies for RA and other autoimmune diseases reduce inflammation by targeting inflammatory mediators, most of which are innate response cytokines, resulting in generalized immunosuppression. Overall, this strategy has been very successful, but not all patients respond, responses can diminish over time and numerous side effects can occur. Therapies that target the germinal center (GC) reaction and/or antibody-secreting plasma cells (PC) potentially provide a novel approach. TANK-binding kinase 1 (TBK1) is an IKK-related serine/threonine kinase best characterized for its involvement in innate antiviral responses through the induction of type I interferons. TBK1 is also gaining attention for its roles in humoral immune responses. In this review, we discuss the role of TBK1 in immunological pathways involved in the development and maintenance of antibody responses, with particular emphasis on its potential relevance in the pathogenesis of humoral autoimmunity. First, we review the role of TBK1 in the induction of type I IFNs. Second, we highlight how TBK1 mediates inducible T cell co-stimulator signaling to the GC T follicular B helper population. Third, we discuss emerging evidence on the contribution of TBK1 to autophagic pathways and the potential implications for immune cell function. Finally, we discuss the therapeutic potential of TBK1 inhibition in autoimmunity.

Common Variable Immunodeficiency and Gastric Malignancies

Common variable immunodeficiency (CVID) is an immunodeficiency disorder with a high incidence of gastrointestinal manifestations and an increased risk of gastric carcinoma and lymphoma. This review discusses the latest advancements into the immunological, clinical and diagnostic aspects of gastric malignancies in patients with CVID. The exact molecular pathways underlying the relationships between CVID and gastric malignancies remain poorly understood. These include genetics, immune dysregulation and chronic infections by Helicobacter pylori. Further studies are needed to better stratify the risk for cancer in these patients, to elaborate surveillance programs aimed at preventing these complications, and to develop new and more effective therapeutic approaches.

ICOS Co-Stimulation: Friend or Foe?

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

ICOS regulates the pool of group 2 innate lymphoid cells under homeostatic and inflammatory conditions in mice

Group 2 innate lymphoid cells (ILC2s) are innate effectors playing an important role in the defense against helminthic infections and in the pathogenesis of allergic inflammation. Cytokines have been identified as the major stimuli driving ILC2 activation and expansion. Conversely, it is unclear whether costimulatory molecules contribute to regulation of ILC2 functions. ILC2s display high expression of inducible T-cell costimulator (ICOS), which belongs to the CD28 superfamily, and which has been shown to control late effector T-cell functions, and is of utmost importance for the humoral immune response. However, the biological function of ICOS expression on ILC2s is unknown. Here, we show that ICOS signaling in mice regulates ILC2 homeostasis independently of T cells and B cells, by promoting proliferation and accumulation of mature ILC2s in lung and intestine. In a model of IL-33-induced airway inflammation, ICOS controls ILC2 activation and eosinophil infiltration in the lung. Our data identify a role of ICOS in innate immunity and indicate that not only cytokines, but also costimulatory pathways such as those involving ICOS, can contribute to regulate the ILC2 pool. Thus, ICOS costimulation blockade, which is currently under clinical evaluation for inhibiting the humoral immune response, could also target innate inflammatory circuits.

The role of metalloproteinase ADAM17 in regulating ICOS ligand-mediated humoral immune responses

Immune cells regulate cell surface receptor expression during their maturation, activation, and motility. Although many of these receptors are regulated largely at the level of expression, protease-mediated ectodomain shedding represents an alternative means of refashioning the surface of immune cells. Shedding is largely attributed to a family of a disintegrin and metalloprotease domain (ADAM) metalloproteases, including ADAM17. Although ADAM17 is well known to contribute to the innate immune response, mainly by releasing TNF-α, much less is known about whether/how this metalloprotease regulates adaptive immunity. To determine whether ADAM17 contributes to regulating adaptive immune responses, we took advantage of ADAM17 hypomorphic (ADAM17(ex/ex)) mice, in which ADAM17 expression is reduced by 90-95% compared with wild-type littermates. In this study, we show that that ADAM17 deficiency results in spleen and lymph node enlargement, as well as increased levels of Ag-specific class-switched Ig production following immunization with OVA together with anti-CD40 mAbs and polyinosinic-polycytidylic acid. Moreover, we demonstrate that the costimulatory ligand ICOS ligand (ICOSL) is selectively downregulated on the surface of B cells in an ADAM17-specific manner, although it is not proteolitically processed by recombinant ADAM17 in vitro. Finally, we show that higher cell surface levels of ICOSL in ADAM17(ex/ex) mice may contribute to the development of excessive Ab responses. Therefore, our data suggest a functional link between ADAM17 and ICOSL in controlling adaptive immune responses.

Attenuation of immune-mediated influenza pneumonia by targeting the inducible co-stimulator (ICOS) molecule on T cells

Inducible Co-stimulator (ICOS) plays a critical role in mediating T cell differentiation and function and is considered a key player in balancing T effector and T regulatory (T_reg) cell responses. Here we show that activation of the ICOS signalling pathway during acute influenza A virus (IAV) infection by application of an agonistic ICOS antibody reduced the frequency of CD8⁺ T cells in the respiratory tract of IAV infected animals and delayed pathogen elimination. In line with this, immune-mediated influenza pneumonia was significantly ameliorated in mice that received ICOS agonist as indicated by significantly reduced alveolar infiltrations and bronchointerstitial pneumonia, while at the same time virus-related pathology remained unaffected. Importantly, ICOS agonist treatment resulted in expansion of CD4⁺Foxp3⁺ T_regs in IAV infected mice, which was associated with elevated levels of the immunosuppressive cytokine IL-10 in the alveolar space. Together, our findings suggest a prominent role of ICOS signaling during acute IAV infection by increasing the T_reg/CD8⁺ T cell ratio with beneficial outcome on immune-mediated pneumonia and underline the suitability of ICOS as potential therapeutic target for immune intervention in those infectious conditions characterized by strong immunopathology rather than virus-mediated cytopathic effects.

Environmentally enriched male mink gain more copulations than stereotypic, barren-reared competitors

Wild carnivores in zoos, conservation breeding centres, and farms commonly live in relatively small, unstimulating enclosures. Under these captive conditions, in a range of species including giant pandas, black-footed ferrets, and European mink, male reproductive abilities are often poor. Such problems have long been hypothesized to be caused by these animals' housing conditions. We show for the first time that rearing under welfare-improving (i.e., highly valued and stress-reducing) environmental enrichments enhances male carnivores' copulatory performance: in mate choice competitions, enriched male American mink (Neovison vison) mated more often than non-enriched males. We screened for several potential mediators of this effect. First was physiological stress and its impact on reproductive physiology; second, stress-mediated changes in morphology and variables related to immunocompetence that could influence male attractiveness; and third, behavioural changes likely to affect social competence, particularly autistic-like excessive routine and repetition ('perseveration') as is reflected in the stereotypies common in captive animals. Consistent with physiological stress, excreted steroid metabolites revealed that non-enriched males had higher cortisol levels and lower androgen levels than enriched conspecifics. Their os penises (bacula) also tended to be less developed. Consistent with reduced attractiveness, non-enriched males were lighter, with comparatively small spleens and a trend to greater fluctuating asymmetry. Consistent with impaired social competence, non-enriched males performed more stereotypic behaviour (e.g., pacing) in their home cages. Of all these effects, the only significant predictor of copulation number was stereotypy (a trend suggesting that low bodyweights may also be influential): highly stereotypic males gained the fewest copulations. The neurophysiological changes underlying stereotypy thus handicap males sexually. We hypothesise that such males are abnormally perseverative when interacting with females. Investigating similar problems in other taxa would be worthwhile, since many vertebrates, wild and domestic, live in conditions that cause stereotypic behaviour and/or impair neurological development.

Deficiency in TNFRSF13B (TACI) expands T-follicular helper and germinal center B cells via increased ICOS-ligand expression but impairs plasma cell survival

Mutations in TNFRSF13B, better known as transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), contribute to common variable immunodeficiency and autoimmunity in humans. How TACI regulates these two opposing conditions is unclear, however. TACI binds the cytokines BAFF and APRIL, and previous studies using gene KO mice indicated that loss of TACI affected only T-cell-independent antibody responses. Here we demonstrate that Taci(-/-) mice have expanded populations of T follicular helper (T(fh)) and germinal center (GC) B cells in their spleens when immunized with T-cell-dependent antigen. The increased numbers of T(fh) and GC B cells in Taci(-/-) mice are largely a result of up-regulation of inducible costimulator (ICOS) ligand on TACI-deficient B cells, given that ablation of one copy of the Icosl allele restores normal levels of T(fh) and GC B cells in Taci(-/-) mice. Interestingly, despite the presence of increased T(fh) and antigen-specific B cells, immunized Taci(-/-) mice demonstrate defective antigen-specific antibody responses resulting from significantly reduced numbers of antibody-secreting cells (ASCs). This effect is attributed to the failure to down-regulate the proapoptotic molecule BIM in Taci(-/-) plasma cells. Ablation of BIM could rescue ASC formation in Taci(-/-) mice, suggesting that TACI is more important for the survival of plasma cells than for the differentiation of these cells. Thus, our data reveal dual roles for TACI in B-cell terminal differentiation. On one hand, TACI modulates ICOS ligand expression and thereby limits the size of T(fh) and GC B-cell compartments and prevents autoimmunity. On the other hand, it regulates the survival of ASCs and plays an important role in humoral immunity.

MODULATING CO-STIMULATION DURING ANTIGEN PRESENTATION TO ENHANCE CANCER IMMUNOTHERAPY

One of the key roles of the immune system is the identification of potentially dangerous pathogens or tumour cells, and raising a wide range of mechanisms to eliminate them from the organism. One of these mechanisms is activation and expansion of antigen-specific cytotoxic T cells, after recognition of antigenic peptides on the surface of antigen presenting cells such as dendritic cells (DCs). However, DCs also process and present autoantigens. Therefore, antigen presentation has to occur in the appropriate context to either trigger immune responses or establishing immunological tolerance. This is achieved by co-stimulation of T cells during antigen presentation. Co-stimulation consists on the simultaneous binding of ligand-receptor molecules at the immunological synapse which will determine the type and extent of T cell responses. In addition, the type of cytokines/chemokines present during antigen presentation will influence the polarisation of T cell responses, whether they lead to tolerance, antibody responses or cytotoxicity. In this review, we will focus on approaches manipulating co-stimulation during antigen presentation, and the role of cytokine stimulation on effective T cell responses. More specifically, we will address the experimental strategies to interfere with negative co-stimulation such as that mediated by PD-L1 (Programmed cell death 1 ligand 1)/PD-1 (Programmed death 1) to enhance anti-tumour immunity.

Comprehensive analysis of CD4+ T cells in the decision between tolerance and immunity in vivo reveals a pivotal role for ICOS

We have established a comprehensive in vivo mouse model for the CD4(+) T cell response to an "innocuous" versus "dangerous" exogenous Ag and developed an in vivo test for tolerance. In this model, specific gene-expression signatures, distinctive upregulation of early T cell-communication molecules, and differential expansion of effector T cells (Teff) and regulatory T cells (Treg) were identified as central correlates of T cell tolerance and T cell immunity. Different from essentially all other T cell-activation molecules, ICOS was found to be induced in the immunity response and not by T cells activated under tolerogenic conditions. If expressed, ICOS did not act as a general T cell costimulator but selectively caused a massive expansion of effector CD4(+) T cells, leaving the regulatory CD4(+) T cell compartment largely undisturbed. Thus, ICOS strongly contributed to the dramatic change in the balance between Ag-specific Teff and Treg from ∼1:1 at steady state to 21:1 at the height of the immune response. This newly defined role for the balance of Teff to Treg, together with its known key function in T cell help for B cells, establishes ICOS as a central mediator of immunity. Given its exceptionally selective induction on CD4(+) T cells under inflammatory, but not tolerogenic, conditions, ICOS emerges as a pivotal effector molecule in the early decision between tolerance and immunity to exogenous Ag.

Growth-factor receptor-bound protein-2 (Grb2) signaling in B cells controls lymphoid follicle organization and germinal center reaction

Grb2 (growth-factor receptor-bound protein-2) is a signaling adaptor that interacts with numerous receptors and intracellular signaling molecules. However, its role in B-cell development and function remains unknown. Here we show that ablation of Grb2 in B cells results in enhanced B-cell receptor signaling; however, mutant B cells do not form germinal centers in the spleen after antigen stimulation. Furthermore, mutant mice exhibit defects in splenic architecture resembling that observed in B-cell-specific lymphotoxin-β-deficient mice, including disruption of marginal zone and follicular dendritic cell networks. We find that grb2(-/-) B cells are defective in lymphotoxin-β expression. Although lymphotoxin can be up-regulated by chemokine CXCL13 and CD40 ligand stimulation in wild-type B cells, elevation of lymphotoxin expression in grb2(-/-) B cells is only induced by anti-CD40 but not by CXCL13. Our results thus define Grb2 as a nonredundant regulator that controls lymphoid follicle organization and germinal center reaction. Loss of Grb2 has no effect on B-cell chemotaxis to CXCL13, indicating that Grb2 executes this function by connecting the CXCR5 signaling pathway to lymphotoxin expression but not to chemotaxis.

The inducible T-cell co-stimulator molecule is expressed on subsets of T cells and is a new marker of lymphomas of T follicular helper cell-derivation

BACKGROUND

T follicular helper (T(FH)) cells reside in the light zone of germinal centers and are considered the cell of origin of angioimmunoblastic T-cell lymphoma. Recently, CXCL13, PD-1 and SAP were described as useful markers for T(FH) cells and angioimmunoblastic T-cell lymphoma but also reported in some peripheral T-cell lymphomas, not otherwise specified.

DESIGN AND METHODS

In the present study the expression pattern of ICOS protein was investigated by immunohistochemistry-based techniques in routine sections of normal lymphoid tissues and 633 human lymphomas.

RESULTS

Cells strongly positive for ICOS were restricted to the light zone of germinal centers and co-expressed T(FH)-associated molecules. In addition, weak to moderate ICOS expression was observed in a small proportion of FOXP3-positive cells. In lymphomas, ICOS expression was confined to angioimmunoblastic T-cell lymphoma (85/86), peripheral T-cell lymphomas of follicular variant (18/18) and a proportion of peripheral T-cell lymphomas, not otherwise specified (24/56) that also expressed other T(FH)-associated molecules.

CONCLUSIONS

ICOS is a useful molecule for identifying T(FH) cells and its restricted expression to angioimmunoblastic T-cell lymphoma and a proportion of peripheral T-cell lymphomas, not otherwise specified (showing a T(FH)-like profile) suggests its inclusion in the antibody panel for diagnosing T(FH)-derived lymphomas. Our findings provide further evidence that the histological spectrum of T(FH)-derived lymphomas is broader than previously assumed.

Expression and function of the inducible costimulator ligand B7-H2 in human airway smooth muscle cells

BACKGROUND

B7-H2 is a ligand for the inducible costimulator (ICOS). The aim of this study was to examine the expression and function of B7-H2 in human airway smooth muscle (ASM) cells and compare them with those of CD40 or OX40 ligand (OX40L).

METHODS

Expression of B7-H2, CD40 and OX40L in ASM cells and their respective counterparts in T cells was analyzed by RT-PCR or flow cytometry. The modulating effect of polyinosinic-polycytidylic acid (poly I:C) on expression of B7-H2, CD40 and OX40L was also examined. The function of these three molecules was evaluated by virtue of adhesion of anti-CD3-activated T cells, IL-6 and IL-8 production and DNA synthesis.

RESULTS

ASM cells constitutively expressed B7-H2, CD40 and OX40L that mediated adhesion of activated T cells expressing ICOS, CD40L and OX40. ASM cells responded to poly I:C with upregulated expression of B7-H2, CD40 and OX40L and displayed enhanced adhesion of activated T cells. Functional analysis performed on untreated ASM cells showed that engagement of B7-H2 with ICOS-Ig clearly induced DNA synthesis, whereas that of CD40 or OX40L with trimeric CD40L or OX40-Ig greatly increased IL-6 and IL-8 production. These responses were enhanced in poly I:C-treated ASM cells.

CONCLUSIONS

The data demonstrate that ASM cells express functionally active B7-H2, CD40 and OX40L and suggest that B7-H2-dependent signaling may play an active role in a proliferative response rather than in cytokine and chemokine production. In addition, the modulation of B7-H2, CD40 and OX40L expression and function by poly I:C may have important implications for the function of virus-infected ASM cells.

Yin-Yang of costimulation: crucial controls of immune tolerance and function

SUMMARY

In addition to signals from the T-cell receptor complex, it has been recognized for many years that a 'second' signal, most notably from CD28, is also important in T-cell activation. In the recent years, many new members of CD28 family as well as the molecules that share structural homology to CD28 ligands CD80 and CD86 have been discovered. Interestingly, some of these proteins function to dampen T-cell activation and regulate the induction of T-cell tolerance. Therefore, positive and negative costimulation are the two sides of the coin to fine tune T-cell receptor signaling to determine the outcome of T-cell receptor engagement-tolerance versus function.

The role of costimulation in antibody deficiencies: ICOS and common variable immunodeficiency

SUMMARY

The identification of mutations in the inducible costimulator (ICOS) gene in nine patients with common variable immunodeficiency (CVID) was a major breakthrough. CVID is a complex, highly heterogeneous primary immunodeficiency disease, and the discovery of these mutations revealed a molecular basis. ICOS belongs to the CD28 family of costimulatory molecules and is expressed exclusively on activated T cells. It has at least three critical functions: germinal center formation, isotype class switching, and the development of memory B cells. The discovery of human ICOS deficiency showed that a monogenic disorder could account for the full spectrum of manifestations seen in childhood and adulthood-onset CVID, including autoimmune, inflammatory, and malignant disease complications, as well as recurrent infections. Moreover, this discovery showed that a disorder which had previously been perceived as a B-cell disease might in fact have its genetic origin in human T cells. In this article, we review the role of ICOS in the mammalian immune system and human disease, as well as the discovery and characteristics of patients with ICOS deficiency. Finally, we also discuss how these 'human knockouts' have contributed to our understanding of ICOS functions and have suggested potential avenues for using therapeutic ICOS manipulation to treat other diseases.

Common variable immunodeficiency

Common variable immunodeficiency is the most prevalent clinically significant antibody deficiency at all ages. The disorder is defined principally by characteristic infection susceptibility with hypogammaglobulinemia and impaired-specific antibody response. Several recent large registry-based studies have defined distinct phenotypic subtypes. Several studies have also correlated specific immunologic markers with these phenotypes. The biochemical or genetic abnormality in the majority of patients remains unknown. Recently, several molecular genetic lesions have been defined. Among these, mutations of inducible costimulator, and CD19, appear to be disease causing by themselves. These account for about 1% of cases. Other mutations or polymorphisms, such as in the human homolog of Escherichia coli MutS 5 (MSH5), and transmembrane activator and calcium mobilizing ligand interactor, seem to be disease associated in 5-10% of patients, but may require additional immunologic abnormalities for full expression of the phenotype, as unaffected heterozygotes have also been described.

The costimulatory molecule ICOS regulates the expression of c-Maf and IL-21 in the development of follicular T helper cells and TH-17 cells

The inducible costimulatory molecule ICOS has been suggested to be important in the development of interleukin 17 (IL-17)-producing helper T cells (T(H)-17 cells) and of follicular helper T cells (T(FH) cells). Here we show that ICOS-deficient mice had no defect in T(H)-17 differentiation but had fewer T(H)-17 cells after IL-23 stimulation and fewer T(FH) cells. We also show that T(FH) cells produced IL-17 and that T(FH) cells in ICOS-deficient mice were defective in IL-17 production. Both T(H)-17 and T(FH) cells had higher expression of the transcription factor c-Maf. Genetic loss of c-Maf resulted in a defect in IL-21 production and fewer T(H)-17 and T(FH) cells. Thus our data suggest that ICOS-induced c-Maf regulates IL-21 production that in turn regulates the expansion of T(H)-17 and T(FH) cells.

Helminth infections associated with multiple sclerosis induce regulatory B cells

OBJECTIVE

To assess the importance of B-cell control during parasite infections in multiple sclerosis (MS) patients.

METHODS

Peripheral blood CD19+ B cells from 12 helminth-infected MS patients, 12 MS patients without infection, 10 patients infected with Trypanosoma cruzi, 8 subjects infected with Paracoccidioides brasiliensis, and 12 healthy control subjects were purified using magnetic cell sorting. Interleukin (IL)-4, IL-6, IL-10, tumor necrosis factor-alpha, lymphotoxin, transforming growth factor-beta, brain-derived neurotrophic factor, and nerve growth factor secretion were evaluated after stimulation with CDw32 L cells and CD40 antibody using enzyme-linked immunosorbent assays. The production of anti-myelin oligodendrocyte glycoprotein IgG and IgM antibodies was evaluated by enzyme-linked immunosorbent spot assays. Cell phenotype was assessed by flow cytometry.

RESULTS

Helminth infections in MS patients created a B-cell population producing high levels of IL-10, dampening harmful immune responses through a mechanism mediated, at least in part, by the ICOS-B7RP-1 pathway. The IL-10-producing B-cell phenotype detected expressed high levels of CD1d and was similar to the one observed in mature naive B2 cells (namely, CD11b(-), CD5(-), CD27(-), and IgD+). Moreover, B cells isolated from helminth-infected MS patients also produced greater amounts of brain-derived neurotrophic factor and nerve growth factor compared with those of normal subjects, T. cruzi-infected subjects, P. brasiliensis-infected subjects, or uninfected MS patients, raising the possibility that these cells may exert a neuroprotective effect on the central nervous system.

INTERPRETATION

Increased production of B-cell-derived IL-10 and of neurotrophic factors are part of the parasite's regulation of host immunity and can alter the course of MS, potentially explaining environmental-related MS suppression observed in areas with low disease prevalence.

ICOS, CD40, and lymphotoxin beta receptors signal sequentially and interdependently to initiate a germinal center reaction

Germinal center (GC) responses to T-dependent Ags require effective collaboration between Th cells, activated B cells, and follicular dendritic cells within a highly organized microenvironment. Studies using gene-targeted mice have highlighted nonredundant molecules that are key for initiating and maintaining the GC niche, including the molecules of the ICOS, CD40, and lymphotoxin (LT) pathways. Signaling through ICOS has multiple consequences, including cytokine production, expression of CD40L on Th cells, and differentiation into CXCR5(+) follicular Th cells, all of which are important in the GC reaction. We have therefore taken advantage of ICOS(-/-) mice to dissect which downstream elements are required to initiate the formation of GC. In the context of a T-dependent immune response, we found that GC B cells from ICOS(-/-) mice express lower levels of LTalphabeta compared with wild-type GC B cells in vivo, and stimulation of ICOS on T cells induces LTalphabeta on B cells in vitro. Administration of agonistic anti-LTbeta receptor Ab was unable to restore the GC response in ICOS(-/-) mice, suggesting that additional input from another pathway is required for optimal GC generation. In contrast, treatment with agonistic anti-CD40 Ab in vivo recovered GC networks and restored LTalphabeta expression on GC B cells in ICOS(-/-) mice, and this effect was dependent on LTbeta receptor signaling. Collectively, these data demonstrate that ICOS activation is a prerequisite for the up-regulation of LTalphabeta on GC B cells in vivo and provide a model for cooperation between ICOS, CD40, and LT pathways in the context of the GC response.

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.

ICOS controls the pool size of effector-memory and regulatory T cells

ICOS is an important regulator of T cell effector function. ICOS-deficient patients as well as knockout mice show severe defects in T cell-dependent B cell responses. Several in vitro and in vivo studies attributed this phenomenon to impaired up-regulation of cell surface communication molecules and cytokine synthesis by ICOS-deficient T cells. However, we now could show with Ag-specific T cells in a murine adoptive transfer system that signaling via ICOS does not significantly affect early T cell activation. Instead, ICOS substantially contributes to the survival and expansion of effector T cells upon local challenge with Ag and adjuvant. Importantly, the observed biological function of ICOS also extends to FoxP3+ regulatory T cells, as can be observed after systemic Ag delivery without adjuvant. In line with these findings, absence of ICOS under homeostatic conditions of nonimmunized mice leads to a reduced number of both effector-memory and FoxP3+ regulatory T cells. Based on these results, we propose a biological role for ICOS as a costimulatory, agonistic molecule for a variety of effector T cells with differing and partly opposing functional roles. This concept may reconcile a number of past in vivo studies with seemingly contradictory results on ICOS function.

Roquin represses autoimmunity by limiting inducible T-cell co-stimulator messenger RNA

Immune responses are normally targeted against microbial pathogens and not self-antigens by mechanisms that are only partly understood. Here we define a newly discovered pathway that prevents autoimmunity by limiting the levels on T lymphocytes of aco-stimulatory receptor, the inducible T-cell co-stimulator(ICOS). In sanroque mice homozygous for an M199R mutation in the ROQ domain of Roquin (also known as Rc3h1), increased Icos expression on T cells causes the accumulation of lymphocytes that is associated with a lupus-like autoimmune syndrome. Roquin normally limits Icos expression by promoting the degradation of Icos messenger RNA.A conserved segment in the unusually long ICOS 3' untranslated mRNA is essential for regulation by Roquin. This segment comprises a 47-base-pair minimal region complementary to T-cell-expressed microRNAs including miR-101, the repressive activity of which is disrupted by base-pair inversions predicted to abrogate miR-101 binding. These findings illuminate a critical post-transcriptional pathway within T cells that regulates lymphocyte accumulation and autoimmunity, and highlights the therapeutic potential of partially antagonising the ICOS pathway.

The expression analysis of ICOS-L on activated T cells and immature dendritic cells as well as malignant B cells and Grave's-disease-derived thyroid tissues by two novel mAbs against human ICOS-L

ICOS-L, a newly identified member of B7 superfamily, plays an important role in immune responses. In this article, we report on two novel mouse anti-human ICOS-L monoclonal antibodies (mAbs) named as 11C4 and 12B11, whose specificities were verified by methods of flow cytometry, western blotting, and epitope competition assay. The two mAbs bound to distinct ICOS-L epitopes on B cells. Interestingly, mAb 11C4 could well recognize ICOS-L molecule on activated T cells and Jurkat cell lines, which is different from commercial anti-ICOS-L mAb (clone number MIH12) and the other mAb 12B11. In addition, we found that the expression of ICOS-L molecule was only detected on the surface of immature monocyte-derived dendritic cells (Mo-DCs) and was sharply decreased after induction of mature Mo-DCs activated by tumor necrosis factor-alpha or CD40. Furthermore, we showed that 11C4 could effectively suppress the maturation of Mo-DCs in vitro as evidenced by the low expression of CD80, CD86, CD83, and human leukocyte antigen-DR, which suggested that ICOS-L may be involved in the maturation of Mo-DCs. Using immunohistochemistry staining with mAb 11C4, the expression of ICOS-L was found in B lymphoma tissues and thyroid tissues from the Grave's disease but not in thyroid adenoma and normal thyroid tissues.

Molecules involved in T-B co-stimulation and B cell homeostasis: possible targets for an immunological intervention in autoimmunity

The deepened knowledge of co-stimulatory mechanisms within the immunological synapse and the emerging biological principles governing B cell homeostasis provide a plethora of new possibilities to selectively block or enhance immune responses. These mechanisms are highly relevant to the development of new treatment modalities for autoimmune diseases. Here we review approaches to antagonise members of the CD28-B7 superfamily as well as the TNF receptor ligand superfamily members, BAFF and APRIL, and their corresponding receptors on B cells (BAFF-R, TACI and BCMA). The proof of principle that such manipulations have indeed profound consequences for the human immune response comes from genetically manipulated mouse models, and, more importantly, from human immunodeficiency syndromes. Thus, the recent discovery of deletions in the ICOS, BAFF-R and TACI genes leading to disturbances in late B cell differentiation and hypogammaglobulinaemia underline the potential impact of targeting these molecules for therapeutic strategies in autoimmune disorders.

B7RP-1-ICOS interactions are required for optimal infection-induced expansion of CD4+ Th1 and Th2 responses

Although initial reports linked the costimulatory molecule ICOS preferentially with the development of Th2 cells, there is evidence that it is not required for protective type 2 immunity to helminths and that it contributes to Th1 and Th2 responses to other parasites. To address the role of ICOS in the development of infection-induced polarized Th cells, ICOS(-/-) mice were infected with Trichuris muris or Toxoplasma gondii. Wild-type mice challenged with T. muris developed Th2 responses and expelled these helminths by day 18 postinfection, whereas ICOS(-/-) mice failed to clear worms and produced reduced levels of type 2 cytokines. However, by day 35 postinfection, ICOS(-/-) mice were able to mount an effective Th2 response and worms were expelled. This delay in protective immunity was associated with a defect in infection-induced increases in the number of activated and proliferating CD4+ T cells. Similarly, following challenge with T. gondii ICOS was required for optimal proliferation by CD4+ T cells. However, the reduced number of activated CD4+ T cells and associated defect in the production of IFN-gamma did not result in increased susceptibility to T. gondii, but rather resulted in decreased CNS pathology during the chronic phase of this infection. Taken together, these data are consistent with a model in which ICOS is not involved in dictating polarity of the Th response but rather regulates the expansion of these subsets.

Differentiation of memory B and T cells

In the past few years progress has been made in understanding the molecular mechanisms that underlie the initial generation, and the ensuing differentiation and maintenance, of humoral and cellular immunity. Although B and T cell immunological memory contribute to protective immunity through fundamentally distinct effector functions, interesting analogies are becoming apparent between the two memory compartments. These include heterogeneity in function, anatomical location and phenotype, which probably relate to differential environmental cues during the early priming events as well as the later differentiation phases. Detailed definition of the molecular and cellular signals involved in the development of immunological memory, and the relative contributions of different memory subsets to protective immunity, remains an important goal.

The ICOS-ligand B7-H2, expressed on human type II alveolar epithelial cells, plays a role in the pulmonary host defense system

The mechanism of immune defense against pathogens in the lung, has so far been poorly understood. Here, we show that human type II alveolar epithelial cells play a key role in defense via interactions between B7 homolog (B7h), also known as ICOS ligand, and its receptor ICOS expressed on activated T cells. The A549 alveolar type II cell line abundantly expresses B7-H2, CD40 and B7-1, but not B7-2 or hGL50. TNF-alpha significantly induced B7-H2 and CD40 expression by A549 cells, but had no effect on B7-1 or B7-2 expression. TNF-alpha-deficient mice exhibited low B7-H2 expression on alveolar epithelial cells in comparison with wild-type mice. Co-culture of TNF-alpha pre-stimulated A549 cells with CD4+ T cells promoted CD154 expression, CD4+ T cell proliferation and cytokine production, especially IFN-gamma. Monocyte-derived TNF-alpha in combination with IFN-gamma and LPS markedly induced B7-H2 expression in A549 cells. This study thus identifies a unique costimulatory pathway via alveolar epithelial type II cells that preferentially affects T helper cell function, implying that alveolar epithelial type II cells play a crucial role in innate immunity in the lung by regulating IFN-gamma-synthesis via B7-H2/ICOS interactions.

Co-stimulatory molecules as potential targets for therapeutic intervention in allergic airway disease

Airway inflammation is a characteristic feature of allergic asthma. Central to the initiation and progression of the inflammatory process are allergen-specific T lymphocytes that attract eosinophils, mast cells, and B cells to the airways by the secretion of specific cytokines. The direction of T cell responses is influenced by co-stimulatory signals that modulate the antigen-specific signal delivered by the T cell receptor. In addition to the prototypic co-stimulatory molecule, CD28, a number of newly identified co-stimulatory molecules and their ligands have now been characterized. Over the past 5 years, the role of these molecules in the pathophysiology of allergen-mediated sensitization and airway inflammation has been extensively studied in animal models of allergic asthma. The aim of this review is to provide a detailed overview on recent studies in mice and preliminary findings in man and to discuss the potential therapeutic and preventive treatment strategies offered by interactions with co-stimulatory molecules for patients with allergic airway diseases.

Expression and function of inducible co-stimulator in patients with systemic lupus erythematosus: possible involvement in excessive interferon-gamma and anti-double-stranded DNA antibody production

Inducible co-stimulator (ICOS) is the third member of the CD28/cytotoxic T-lymphocyte associated antigen-4 family and is involved in the proliferation and activation of T cells. A detailed functional analysis of ICOS on peripheral blood T cells from patients with systemic lupus erythematosus (SLE) has not yet been reported. In the present study we developed a fully human anti-human ICOS mAb (JTA009) with high avidity and investigated the immunopathological roles of ICOS in SLE. JTA009 exhibited higher avidity for ICOS than a previously reported mAb, namely SA12. Using JTA009, ICOS was detected in a substantial proportion of unstimulated peripheral blood T cells from both normal control individuals and patients with SLE. In CD4⁺CD45RO⁺ T cells from peripheral blood, the percentage of ICOS⁺ cells and mean fluorescence intensity with JTA009 were significantly higher in active SLE than in inactive SLE or in normal control individuals. JTA009 co-stimulated peripheral blood T cells in the presence of suboptimal concentrations of anti-CD3 mAb. Median values of [³H]thymidine incorporation were higher in SLE T cells with ICOS co-stimulation than in normal T cells, and the difference between inactive SLE patients and normal control individuals achieved statistical significance. ICOS co-stimulation significantly increased the production of IFN-γ, IL-4 and IL-10 in both SLE and normal T cells. IFN-γ in the culture supernatants of both active and inactive SLE T cells with ICOS co-stimulation was significantly higher than in normal control T cells. Finally, SLE T cells with ICOS co-stimulation selectively and significantly enhanced the production of IgG anti-double-stranded DNA antibodies by autologous B cells. These findings suggest that ICOS is involved in abnormal T cell activation in SLE, and that blockade of the interaction between ICOS and its receptor may have therapeutic value in the treatment of this intractable disease.

Alloimmunity in primate heart recipients with CD154 blockade: evidence for alternative costimulation mechanisms

BACKGROUND

CD154 mediates key facets of humoral and cellular immunity to alloantigens, and is tolerogenic to influenza antigens in primates. Barriers to CD154-based tolerance induction for primate cardiac allografts have not previously been defined.

METHODS

Heterotopic cardiac allograft outcomes in cynomolgus monkeys treated with a CD154 inhibitor, IDEC-131 (n=27), were compared to no treatment (n=4) or cyclosporine A (n=6).

RESULTS

CD154 blockade significantly prolonged median allograft survival, from 6.2 (range 6, 7, n=4) days in untreated controls, to 39 (8,112, n=16) days with intensive monotherapy and 93 (>25, 386; n=3) days with added antithymocyte globulin (ATG), but did not yield tolerance. Alloantibody production was delayed but not prevented by IDEC-131 alone or with ATG, and was exacerbated by infusion of donor bone marrow (n=8). Expression of ICOS was prominent in graft infiltrating lymphocytes, and preceded elaboration of antidonor antibody and vasculopathy.

CONCLUSION

CD154 monotherapy modulates primate cardiac alloimmunity, but does not readily induce tolerance. Targeting alternative costimulation pathways, including ICOS, may facilitate tolerance induction based on CD154 blockade.

Follicular B helper T cells in antibody responses and autoimmunity

T-cell help for B cells is essential for high-affinity antibody responses and B-cell memory. Recently, the identity of a discrete follicular population of T cells that has a crucial role in this process has become clearer. Similar to primed CD4(+) T cells in the tonsils and memory CD4(+) T cells in the peripheral blood, this follicular population of T cells expresses CXC-chemokine receptor 5 (CXCR5). Owing to their distinct homing preferences and helper function, these T cells differ from T helper 1 and T helper 2 cells and have been denoted follicular B helper T cells. Here, we outline the central role of this subset in normal and pathological immune responses.

The role of ICOS in the CXCR5+ follicular B helper T cell maintenance in vivo

ICOS is a new member of the CD28 family of costimulatory molecules that is expressed on activated T cells. Its ligand B7RP-1 is constitutively expressed on B cells. Although the blockade of ICOS/B7RP-1 interaction inhibits T cell-dependent Ab production and germinal center formation, the mechanism remains unclear. We examined the contribution of ICOS/B7RP-1 to the generation of CXCR5+ follicular B helper T (T(FH)) cells in vivo, which preferentially migrate to the B cell zone where they provide cognate help to B cells. In the spleen, anti-B7RP-1 mAb-treated or ICOS-deficient mice showed substantially impaired development of CXCR5+ T(FH) cells and peanut agglutinin+ germinal center B cells in response to primary or secondary immunization with SRBC. Expression of CXCR5 on CD4+ T cells was associated with ICOS expression. Adoptive transfer experiments showed that the development of CXCR5+ T(FH) cells was enhanced by interaction with B cells, which was abrogated by anti-B7RP-1 mAb treatment. The development of CXCR5+ T(FH) cells in the lymph nodes was also inhibited by the anti-B7RP-1 mAb treatment. These results indicated that the ICOS/B7RP-1 interaction plays an essential role in the development of CXCR5+ T(FH) cells in vivo.

Common variable immunodeficiency: the immune system in chaos

Common variable immunodeficiency (CVID) is a heterogeneous disorder that is associated with low serum-immunoglobulin concentrations, defective specific-antibody production and an increased susceptibility to bacterial infections of the respiratory and gastrointestinal tracts. In spite of the identification of genes that are associated with several known primary immunodeficiencies, the basic immunologic and molecular defects of the majority of patients with CVID have remained obscure. Most of the studies aimed at understanding the immunopathogenesis of CVID suggest that this condition is primarily a T-cell disorder, although renewed attention on the genetic linkage and haplotype analysis in families of patients with CVID and on the role of dendritic cells and B cells has revealed several interesting features. This new information should assist in understanding the pathogenesis of CVID and improving the therapeutic strategies.