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1.  A novel ICOS-independent, but CD28- and SAP-dependent, pathway of T cell-dependent, polysaccharide-specific humoral immunity in response to intact Streptococcus pneumoniae versus pneumococcal conjugate vaccine 
Polysaccharide (PS)- and protein-specific murine IgG responses to intact Streptococcus pneumoniae (Pn) are both dependent upon CD4+ T cell help, B7-dependent costimulation, and CD40/CD40-ligand interactions. However, the primary PS-, relative to protein-specific, IgG response terminates more rapidly, requires a shorter period of T cell help and B7-dependent costimulation, and fails to generate memory. In light of the critical role for ICOS/ICOS-ligand interactions in sustaining T cell-dependent Ig responses and promoting germinal center reactions, we hypothesized that this interaction was non-essential for PS-specific IgG responses to Pn. We now demonstrate that ICOS-/-, relative to WT, mice elicit a normal PS-specific IgG isotype response to Pn, despite marked inhibition of both the primary and secondary IgG anti-protein (i.e. PspA, PspC, and PsaA) response. A blocking anti-ICOS-ligand mAb injected during primary Pn immunization inhibits both the primary anti-protein response and the generation of protein-specific memory, but has no effect when injected during secondary immunization. In contrast to Pn, both PS- and protein-specific IgG responses to a pneumococcal conjugate vaccine are inhibited in ICOS-/- mice. ICOS-/- mice immunized with intact Pn or conjugate exhibit nearly complete abrogation in germinal center formation. Finally, although mice that lack the adaptor molecule SAP resemble ICOS-/- mice (and can exhibit decreased ICOS expression), we observe that the PS-, as well as protein-specific IgG responses to both Pn and conjugate are markedly defective in SAP-/- mice. These data define a novel T cell-, SAP-, and B7-dependent, but ICOS-independent, extrafollicular pathway of Ig induction.
PMCID: PMC2893027  PMID: 19050242
Rodent; Bacterial; Antibodies; Transgenic/Knockout mice; Vaccination
2.  CD28 and ICOS play complementary non-overlapping roles in the development of Th2 immunity in vivo 
Cellular immunology  2009;259(2):177-184.
Previous work has shown ICOS can function independently of CD28, but whether either molecule can compensate for the other in vivo is not known. Since ICOS is a potent inducer of Th2 cytokines and linked to allergy and elevated serum IgE in humans, we hypothesized that augmenting ICOS costimulation in murine allergic airway disease may overcome CD28 deficiency. While ICOS was expressed on T cells from CD28−/− mice, Th2-mediated airway inflammation was not induced in CD28−/− mice by increased ICOS costimulation. Further, we determined if augmenting CD28 costimulation could compensate for ICOS deficiency. ICOS−/− mice had a defect in airway eosinophilia that was not overcome by augmenting CD28 costimulation. CD28 costimulation also did not fully compensate for ICOS for antibody responses, germinal center formation or the development of follicular B helper T cells. CD28 and ICOS play complementary non-overlapping roles in the development of Th2 immunity in vivo.
doi:10.1016/j.cellimm.2009.06.013
PMCID: PMC2748173  PMID: 19646680
Costimulation; CD28; ICOS; follicular B helper T cells; Rodent; Th2 Cells; Antibodies; Allergy; Asthma
3.  ICOS Costimulation Expands Th2 Immunity by Augmenting Migration of Lymphocytes to Draining Lymph Nodes1 
The T cell costimulatory molecule ICOS regulates Th2 effector function in allergic airway disease. Recently, several studies with ICOS−/− mice have also demonstrated a role for ICOS in Th2 differentiation. To determine the effects of ICOS on the early immune response, we investigated augmenting ICOS costimulation in a Th2-mediated immune response to Schistosoma mansoni antigens. We found that augmenting ICOS costimulation with B7RP-1-Fc increased the accumulation of T and B cells in the draining lymph nodes post-immunization. Interestingly, the increased numbers were due in part to increased migration of undivided antigen-specific TCR transgenic T cells and surprisingly B cells, as well as non-TCR transgenic T cells. B7RP-1-Fc also increased the levels of the chemokines, CCL21 and CXCL13, in the draining lymph node, suggesting ICOS costimulation contributes to migration by direct or indirect effects on, dendritic cells, stromal cells and high endothelial venules. Further, the effects of B7RP-1-Fc were not dependent on immunization. Our data support a model in which ICOS costimulation augments the pool of lymphocytes in the draining lymph nodes leading to an increase in the frequency of potentially reactive T and B cells.
PMCID: PMC2560985  PMID: 18606653
Costimulation; Th1/Th2 cells; B cells; Rodent; Spleen and Lymph Nodes
4.  The role of ICOS in the development of CD4 T cell help and the reactivation of memory T cells 
European Journal of Immunology  2007;37(7):1796-1808.
We have addressed the role of the inducible costimulator (ICOS) in the development of T cell help for B cells and in the generation, survival and reactivation of memory CD4 T cells and B cells. We find that while T cell help for all antibody isotypes (including IgG2c) is impaired in ICOS knockout (ICOS-KO) mice, the IFN-γ response is little affected, indicating a defect in helper function that is unrelated to cytokine production. In addition, the ICOS-negative T cells do not accumulate in B cell follicles. Secondary (memory), but not primary, clonal proliferation of antigen-specific B cells is impaired in ICOS-KO mice, as is the generation of secondary antibody-secreting cells. Analysis of endogenous CD4 memory cells in ICOS-KO mice, using MHC class II tetramers, reveals normal primary clonal expansion, formation of memory clones and long-term (10 wk) survival of memory cells, but defective expansion upon reactivation in vivo. The data point to a role of ICOS in supporting secondary, memory and effector T cell responses, possibly by influencing cell survival. The data also highlight differences in ICOS dependency of endogenous T cell proliferation in vivo compared to that of adoptively transferred TCR-transgenic T cells.
doi:10.1002/eji.200636661
PMCID: PMC2699381  PMID: 17549732
B cells; Cell differentiation; Costimulation; Memory; T cells
5.  SAP regulates T cell–mediated help for humoral immunity by a mechanism distinct from cytokine regulation 
The Journal of Experimental Medicine  2006;203(6):1551-1565.
X-linked lymphoproliferative disease is caused by mutations affecting SH2D1A/SAP, an adaptor that recruits Fyn to signal lymphocyte activation molecule (SLAM)-related receptors. After infection, SLAM-associated protein (SAP)−/− mice show increased T cell activation and impaired humoral responses. Although SAP−/− mice can respond to T-independent immunization, we find impaired primary and secondary T-dependent responses, with defective B cell proliferation, germinal center formation, and antibody production. Nonetheless, transfer of wild-type but not SAP-deficient CD4 cells rescued humoral responses in reconstituted recombination activating gene 2−/− and SAP−/− mice. To investigate these T cell defects, we examined CD4 cell function in vitro and in vivo. Although SAP-deficient CD4 cells have impaired T cell receptor–mediated T helper (Th)2 cytokine production in vitro, we demonstrate that the humoral defects can be uncoupled from cytokine expression defects in vivo. Instead, SAP-deficient T cells exhibit decreased and delayed inducible costimulator (ICOS) induction and heightened CD40L expression. Notably, in contrast to Th2 cytokine defects, humoral responses, ICOS expression, and CD40L down-regulation were rescued by retroviral reconstitution with SAP-R78A, a SAP mutant that impairs Fyn binding. We further demonstrate a role for SLAM/SAP signaling in the regulation of early surface CD40L expression. Thus, SAP affects expression of key molecules required for T–B cell collaboration by mechanisms that are distinct from its role in cytokine regulation.
doi:10.1084/jem.20052097
PMCID: PMC2118305  PMID: 16754717
6.  TRAF3 regulates the effector function of regulatory T cells and humoral immune responses 
The induction of follicular regulatory T cells and control of germinal center reactions require the adaptor protein Traf3, in part because of Traf3-induced expression of ICOS.
Regulatory T cells (Treg cells) control different aspects of immune responses, but how the effector functions of Treg cells are regulated is incompletely understood. Here we identified TNF receptor–associated factor 3 (TRAF3) as a regulator of Treg cell function. Treg cell–specific ablation of TRAF3 impaired CD4 T cell homeostasis, characterized by an increase in the Th1 type of effector/memory T cells. Moreover, the ablation of TRAF3 in Treg cells resulted in increased antigen-stimulated activation of follicular T helper cells (TFH cells), coupled with heightened formation of germinal centers and production of high-affinity IgG antibodies. Although the loss of TRAF3 did not reduce the overall frequency of Treg cells, it attenuated the antigen-stimulated production of follicular Treg cells (TFR cells). TRAF3 signaling in Treg cells was required to maintain high level expression of inducible co-stimulator (ICOS), which in turn was required for TFR cell generation and inhibition of antibody responses. These findings establish TRAF3 as a mediator of Treg cell function in the regulation of antibody responses and suggest a role for TRAF3 in mediating ICOS expression in Treg cells.
doi:10.1084/jem.20131019
PMCID: PMC3892978  PMID: 24378539
7.  Enhanced Inducible Costimulator Ligand (ICOS-L) Expression on Dendritic Cells in Interleukin-10 Deficiency and Its Impact on T-Cell Subsets in Respiratory Tract Infection 
Molecular Medicine  2013;19(1):346-356.
An association between inducible costimulator ligand (ICOS-L) expression and interleukin (IL)-10 production by dendritic cells (DCs) has been commonly found in infectious disease. DCs with higher ICOS-L expression and IL-10 production are reportedly more efficient in inducing regulatory T cells (Tregs). Here we use the Chlamydia muridarum (Cm) lung infection model in IL-10 knockout (KO) mice to test the relationship between IL-10 production and ICOS-L expression by DCs. We examined ICOS-L expression, the development of T-cell subsets, including Treg, Th17 and Th1 cell, in the background of IL-10 deficiency and its relationship with ICOS-L/ICOS signaling after infection. Surprisingly, we found that the IL-10 KO mice exhibited significantly higher ICOS-L expression by DCs. Moreover, IL-10 KO mice showed lower Tregs but higher Th17 and Th1 responses, but only the Th17 response depended on ICOS signaling. Consistently, most of the Th17 cells were ICOS+, whereas most of the Th1 cells were ICOS− in the infected mice. Furthermore, neutralization of IL-17 in IL-10 KO mice significantly exacerbated lung infection. The data suggest that ICOS-L expression on DC may be negatively regulated by IL-10 and that ICOS-L expression on DC in the presence or absence of IL-10 costimulation may promote Treg or Th17 response, without significant impact on Th1.
doi:10.2119/molmed.2013.00035
PMCID: PMC3883961  PMID: 24100657
8.  ICOS, CD40, and Lymphotoxin β Receptors Signal Sequentially and Interdependently to Initiate a Germinal Center Reaction1 
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 LTαβ compared with wild-type GC B cells in vivo, and stimulation of ICOS on T cells induces LTαβ on B cells in vitro. Administration of agonistic anti-LTβ 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 LTαβ expression on GC B cells in ICOS−/− mice, and this effect was dependent on LTβ receptor signaling. Collectively, these data demonstrate that ICOS activation is a prerequisite for the up-regulation of LTαβ 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.
PMCID: PMC2735203  PMID: 18250437
9.  Role for Inducible Costimulator in Control of Salmonella enterica Serovar Typhimurium Infection in Mice  
Infection and Immunity  2006;74(2):1050-1061.
Inducible costimulator (ICOS) is expressed on activated T cells and plays a key role in sustaining and enhancing the effector function of CD4 T cells. Given the function of this molecule in sustaining T-cell responses, we reasoned that ICOS might play an important role in a prolonged infection model, such as Salmonella infection of mice. To test this hypothesis, wild-type (WT) and ICOS-deficient (ICOS−/−) mice were infected systemically with a Salmonella enterica serovar Typhimurium strain expressing the chicken ovalbumin gene (Salmonella-OVA). ICOS−/− mice exhibited greater splenomegaly than WT mice and showed delayed bacterial clearance. The acquired immune response in this model was slow to develop. Maximal T-cell responses to Salmonella-OVA were detected at 3 weeks postinfection in both WT and ICOS−/− mice. CD4 T-cell-dependent gamma interferon production and a class switch to immunoglobulin G2a were severely reduced in ICOS−/− mice. ICOS−/− mice also exhibited a substantial defect in antigen-specific CD8 T-cell responses. In vitro, the effect of anti-ICOS on CD8 T-cell division was greater when CD8 T cells rather than CD4 T cells expressed ICOS, suggesting that the in vivo effects of ICOS on CD8 T cells could be direct. Taken together, these studies show that ICOS plays a critical role in control of Salmonella infection in mice, with effects on antibody, Th1, and CD8 T-cell responses.
doi:10.1128/IAI.74.2.1050-1061.2006
PMCID: PMC1360312  PMID: 16428752
10.  ICOS-dependent extrafollicular helper T cells elicit IgG production via IL-21 in systemic autoimmunity 
The Journal of Experimental Medicine  2008;205(12):2873-2886.
The role of specialized follicular helper T (TFH) cells in the germinal center has become well recognized, but it is less clear how effector T cells govern the extrafollicular response, the dominant pathway of high-affinity, isotype-switched autoantibody production in the MRL/MpJ-Faslpr (MRLlpr) mouse model of lupus. MRLlpr mice lacking the Icos gene have impaired extrafollicular differentiation of immunoglobulin (Ig) G+ plasma cells accompanied by defects in CXC chemokine receptor (CXCR) 4 expression, interleukin (IL) 21 secretion, and B cell helper function in CD4 T cells. These phenotypes reflect the selective loss of a population of T cells marked by down-regulation of P-selectin glycoprotein ligand 1 (PSGL-1; also known as CD162). PSGL-1lo T cells from MRLlpr mice express CXCR4, localize to extrafollicular sites, and uniquely mediate IgG production through IL-21 and CD40L. In other autoimmune strains, PSGL-1lo T cells are also abundant but may exhibit either a follicular or extrafollicular phenotype. Our findings define an anatomically distinct extrafollicular population of cells that regulates plasma cell differentiation in chronic autoimmunity, indicating that specialized humoral effector T cells akin to TFH cells can occur outside the follicle.
doi:10.1084/jem.20080840
PMCID: PMC2585848  PMID: 18981236
11.  Inducible Costimulator Expression Regulates the Magnitude of Th2-Mediated Airway Inflammation by Regulating the Number of Th2 Cells 
PLoS ONE  2009;4(11):e7525.
Background
Inducible Costimulator (ICOS) is an important regulator of Th2 lymphocyte function and a potential immunotherapeutic target for allergy and asthma. A SNP in the ICOS 5′ promoter in humans is associated with increased atopy and serum IgE in a founder population and increased ICOS surface expression and Th2 cytokine production from peripheral blood mononuclear cells. However, it is unknown if increased ICOS expression contributes to disease progression or is a result of disease pathology.
Methodology/Principal Findings
We developed a mouse model in which ICOS surface expression levels are genetically predetermined to test our hypothesis that genetic regulation of ICOS expression controls the severity of Th2 responses in vivo. Using ICOS+/+ and ICOS+/− mice in a Th2 model of airway inflammation, we found that T cells from the ICOS+/− mice had reduced ICOS expression and decreased Th2-mediated inflammation in vivo. Although the activation status of the T cells did not differ, T cells isolated from the lungs and draining lymph nodes of ICOS+/− mice at the peak of inflammation produced less Th2 cytokines upon stimulation ex vivo. Using 4get mice, which express GFP upon IL-4 transcription, we determined that the decreased Th2 cytokines in ICOS+/− is due to reduced percentage of Th2 cells and not a defect in their ability to produce IL-4.
Conclusion
These data suggest that in both mice and humans, the level of ICOS surface expression regulates the magnitude of the in vivo Th2 response, perhaps by influencing Th2 differentiation.
doi:10.1371/journal.pone.0007525
PMCID: PMC2768787  PMID: 19888475
12.  Constitutive Expression of the B7h Ligand for Inducible Costimulator on Naive B Cells Is Extinguished after Activation by Distinct B Cell Receptor and Interleukin 4 Receptor–mediated Pathways and Can Be Rescued by CD40 Signaling 
The recently described ligand–receptor pair, B7h–inducible costimulator (ICOS), is critical for germinal center formation and antibody responses. In contrast to the induced expression of the related costimulatory ligands B7.1 and B7.2, B7h is constitutively expressed on naive B cells and is surprisingly extinguished after antigen engagement and interleukin (IL)-4 cytokine signaling. Although signaling through both B cell receptor (BCR) and IL-4 receptor (R) converge on the extinction of B7h mRNA levels, BCR down-regulation occurs through Ca2+ mobilization, whereas IL-4R down-regulation occurs through a distinct Stat6-dependent pathway. During antigen-specific B cell activation, costimulation through CD40 signaling can reverse both BCR- and IL-4R–mediated B7h down-regulation. These data suggest that the CD40–CD40 ligand signaling pathway regulates B7h expression on activated B cells and may control whether antigen-activated B cells can express B7h and costimulate cognate antigen–activated T cells through ICOS.
doi:10.1084/jem.20020298
PMCID: PMC2194020  PMID: 12093874
B7RP-1; ICOSL; GL-50; costimulation; CD40L
13.  Plasmacytoid Dendritic Cells Promote Immunosuppression in Ovarian Cancer via ICOS Costimulation of Foxp3+ T-Regulatory Cells 
Cancer research  2012;72(20):5240-5249.
Epithelial ovarian cancer (EOC) is the fifth most common cause of cancer death among women. Despite its immunogenicity, effective antitumor responses are limited, due, in part, to the presence of forkhead box protein 3–positive (Foxp3+) T regulatory (Treg) cells in the tumor microenvironment. However, the mechanisms that regulate the accumulation and the suppressive function of these Foxp3+ Treg cells are poorly understood. Here, we found that the majority of Foxp3+ Treg cells accumulating in the tumor microenvironment of EOCs belong to the subset of Foxp3+ Treg cells expressing inducible costimulator (ICOS). The expansion and the suppressive function of these cells were strictly dependent on ICOS-L costimulation provided by tumor plasmacytoid dendritic cells (pDC). Accordingly, ICOS+Foxp3+Treg cells were found to localize in close vicinity of tumor pDCs, and their number directly correlated with the numbers of pDCs in the tumors. Furthermore, pDCs and ICOS+ Foxp3+Treg cells were found to be strong predictors for disease progression in patients with ovarian cancer, with ICOS+Treg cell subset being a stronger predictor than total Foxp3+Treg cells. These findings suggest an essential role for pDCs and ICOS-L in immunosuppression mediated by ICOS+ Foxp3+ Treg cells, leading to tumor progression in ovarian cancer.
doi:10.1158/0008-5472.CAN-12-2271
PMCID: PMC3652570  PMID: 22850422
14.  Single and Coexpression of CXCR4 and CXCR5 Identifies CD4 T Helper Cells in Distinct Lymph Node Niches during Influenza Virus Infection 
Journal of Virology  2012;86(13):7146-7157.
Influenza virus infection results in strong, mainly T-dependent, extrafollicular and germinal center B cell responses, which provide lifelong humoral immunity against the homotypic virus strain. Follicular T helper cells (TFH) are key regulators of humoral immunity. Questions remain regarding the presence, identity, and function of TFH subsets regulating early extrafollicular and later germinal center B cell responses. This study demonstrates that ICOS but not CXCR5 marks T cells with B helper activity induced by influenza virus infection and identifies germinal center T cells (TGC) as lymph node-resident CD4+ ICOS+ CXCR4+ CXCR5+ PSGL-1lo PD-1hi cells. The CXCR4 expression intensity further distinguished their germinal center light and dark zone locations. This population emerged strongly in regional lymph nodes and with kinetics similar to those of germinal center B cells and were the only TFH subsets missing in influenza virus-infected, germinal center-deficient SAP−/− mice, mice which were shown previously to lack protective memory responses after a secondary influenza virus challenge, thus indicting the nonredundant functions of CXCR4- and CXCR5-coexpressing CD4 helper cells in antiviral B cell immunity. CXCR4-single-positive T cells, present in B cell-mediated autoimmunity and regarded as “extrafollicular” helper T cells, were rare throughout the response, despite prominent extrafollicular B cell responses, revealing fundamental differences in autoimmune- and infection-induced T-dependent B cell responses. While all ICOS+ subsets induced similar antibody levels in vitro, CXCR5-single-positive T cells were superior in inducing B cell proliferation. The regulation of T cell localization, marked by the single and coexpression of CXCR4 and CXCR5, might be an important determinant of TFH function.
doi:10.1128/JVI.06904-11
PMCID: PMC3416343  PMID: 22532671
15.  Interleukin 6 Influences Germinal Center Development and Antibody Production via a Contribution of C3 Complement Component  
The Journal of Experimental Medicine  1998;188(10):1895-1906.
Mice rendered deficient for interleukin (IL) 6 by gene targeting were evaluated for their response to T cell–dependent antigens. Antigen-specific immunoglobulin (Ig)M levels were unaffected whereas all IgG isotypes showed varying degrees of alteration. Germinal center reactions occurred but remained physically smaller in comparison to those in the wild-type mice. This concurred with the observations that molecules involved in initial signaling events leading to germinal center formation were not altered (e.g., B7.2, CD40 and tumor necrosis factor R1). T cell priming was not impaired nor was a gross imbalance of T helper cell (Th) 1 versus Th2 cytokines observed. However, B7.1 molecules, absent from wild-type counterparts, were detected on germinal center B cells isolated from the deficient mice suggesting a modification of costimulatory signaling. A second alteration involved impaired de novo synthesis of C3 both in serum and germinal center cells from IL-6–deficient mice. Indeed, C3 provided an essential stimulatory signal for wild-type germinal center cells as both monoclonal antibodies that interrupted C3-CD21 interactions and sheep anti–mouse C3 antibodies caused a significant decrease in antigen-specific antibody production. In addition, germinal center cells isolated from C3–deficient mice produced a similar defect in isotype production. Low density cells with dendritic morphology were the local source of IL-6 and not the germinal center lymphocytes. Adding IL-6 in vitro to IL-6–deficient germinal center cells stimulated cell cycle progression and increased levels of antibody production. These findings reveal that the germinal center produces and uses molecules of the innate immune system, evolutionarily pirating them in order to optimally generate high affinity antibody responses.
PMCID: PMC2212418  PMID: 9815267
germinal center; interleukin 6; complement; antibody; follicular dendritic cells
16.  4-1BB (CD137) Differentially Regulates Murine In Vivo Protein- and Polysaccharide-Specific Immunoglobulin Isotype Responses to Streptococcus pneumoniae  
Infection and Immunity  2003;71(1):196-204.
4-1BB (CD137) is induced on activated CD4+ and CD8+ T cells and delivers a costimulatory signal upon binding the 4-1BB ligand (4-1BBL) expressed on antigen-presenting cells. Induction of 4-1BB is dependent on activation via the T-cell receptor (TCR) and possibly CD28. It was previously demonstrated that both an in vivo protein (pneumococcal surface protein A [PspA])- and polysaccharide (phosphorylcholine [PC] determinant of teichoic acid)-specific immunoglobulin (Ig) isotype response to Streptococcus pneumoniae was dependent on CD4+ TCRαβ+ T cells and B7-dependent costimulation through CD28. We thus postulated that 4-1BB costimulation would also play a role in regulating the in vivo anti-PspA and anti-PC response to S. pneumoniae. We demonstrate that mice genetically deficient in 4-1BBL elicit a markedly reduced IgM and IgG anti-PC but normal primary and secondary IgG anti-PspA responses to S. pneumoniae relative to those for wild-type mice. However, injection of an agonistic anti-4-1BB monoclonal antibody (MAb), while having no significant effect on the anti-PC response, strongly inhibits the primary anti-PspA response, the generation of PspA-specific memory, and germinal center formation but does not induce a lasting state of tolerance. In contrast, anti-4-1BB MAb has no effect on the anti-PspA response when injected only at the time of secondary immunization. Delay of the addition of anti-4-1BB leads to progressively less inhibition of the primary response up to day 8. This inhibition is independent of CD8+ T cells and is associated with the expansion of CD4+ T cells with an activated phenotype, which is partly dependent on B7-dependent costimulation. These data are the first to suggest a stimulatory role for endogenous 4-1BB-4-1BBL interactions during a humoral immune response to a pathogen and further underscore significant differences in costimulation requirements for an in vivo protein- versus polysaccharide-specific Ig isotype response to an extracellular bacterium.
doi:10.1128/IAI.71.1.196-204.2003
PMCID: PMC143421  PMID: 12496166
17.  Costimulatory molecule ICOS plays a critical role in the development of TH-17 and follicular T-helper cells by regulating c-Maf expression and IL-21 production 
Nature immunology  2008;10(2):167-175.
The inducible costimulatory molecule (ICOS) has been suggested to play an important role in the development of interleukin 17 (IL-17)-producing T helper cells (TH-17 cells) and of follicular helper cells (TFH cells), specialized helper T cells (CD4+CXCR5+ICOShigh) required for antibody class switching and germinal center formation. Here we show that ICOS, while not essential for the differentiation of TH-17 cells, was critical for maintaining effector-memory TH-17 cells as ICOS-deficient mice demonstrated a defect in the expansion of TH-17 cells after IL-23 stimulation. In addition, we found that TFH cells produced IL-17 and that ICOS-deficient mice demonstrated a reduced frequency of TFH with a defect in IL-17 production. Both TH-17 and TFH cells showed increased expression of the transcription factor c-Maf—normally associated with TH2 cells— and that loss of c-Maf results in a defect in IL-21 production, and consequently a defect in the maintenance of IL-23R expression and expansion of TH-17 and TFH cells. These data suggest that c-Maf induced by ICOS regulates IL-21 production that, in turn, regulates expansion of TH-17 cells and TFH cells.
doi:10.1038/ni.1690
PMCID: PMC2742982  PMID: 19098919
18.  Anti-Chlamydial Th17 Responses Are Controlled by the Inducible Costimulator Partially through Phosphoinositide 3-Kinase Signaling 
PLoS ONE  2012;7(12):e52657.
We previously showed that mice deficient in the Inducible Costimulator ligand (ICOSL-KO) develop more severe disease and lung pathology with delayed bacterial clearance upon respiratory infection of Chlamydia muridarum. Importantly, the exacerbation of disease in ICOSL-KO mice was seen despite heightened IFN-γ/Th1 responses, the major defense mechanisms against Chlamydia. To gain insight into the mechanism of ICOS function in this model, we presently analyzed anti-Chlamydia immune responses in mice lacking the entire ICOS (ICOS-KO) versus knock-in mice expressing a mutant ICOS (ICOS-Y181F) that has selectively lost the ability to activate phosphoinositide 3-kinase (PI3K). Like ICOSL-KO mice, ICOS-KO mice showed worse disease with elevated IFN-γ/Th1 responses compared to wild-type (WT) mice. ICOS-Y181F mice developed much milder disease compared to ICOS-KO mice, yet they were still not fully protected to the WT level. This partial protection in ICOS-Y181F mice could not be explained by the magnitude of IFN-γ/Th1 responses since these mice developed a similar level of IFN-γ response compared to WT mice. It was rather IL-17/Th17 responses that reflected disease severity: IL-17/Th17 response was partially impaired in ICOS-Y181F mice compared to WT, but was substantially stronger than that of ICOS-KO mice. Consistently, we found that both polarization and expansion of Th17 cells were partially impaired in ICOS-Y181F CD4 T cells, and was further reduced in ICOS-KO CD4 T cells in vitro. Our results indicate that once the IFN-γ/Th1 response is above a threshold level, the IL-17/Th17 response becomes a limiting factor in controlling Chlamydia lung infection, and that ICOS plays an important role in promoting Th17 responses in part through the activation of PI3K.
doi:10.1371/journal.pone.0052657
PMCID: PMC3527575  PMID: 23285133
19.  ICOS expression by effector T cells influences the ability of regulatory T cells to inhibit anti-chromatin B cell responses in recipient mice 
Journal of autoimmunity  2009;34(4):460-468.
T regulatory cells are critical for the prevention of autoimmunity. Specifically, Treg cells can control anti-chromatin antibody production in vivo, and this correlates with decreased ICOS expression on CD4+ T helper cells. Here we test the significance of high ICOS expression by T effector cells, firstly in terms of the anti-chromatin B cell response, and secondly on the ability of Treg cells to suppress T cell help. We bred CD4+ T cell receptor transgenic mice with mice that carry the Roquinsan/san mutation. The Roquin gene functions to limit ICOS mRNA such that CD4 T cells from mutant mice express elevated ICOS. Using an in vivo model, TS1.Roquinsan/san Th cells were compared with wild-type TS1 Th cells with regard to their ability to help anti-chromatin B cells in the presence or absence of Treg cells. Both TS1 and TS1.Roquinsan/san Th cells induced anti-chromatin IgMa antibodies, but the TS1.Roquinsan/san Th cells resulted in the recovery of more class-switched and germinal center B cells. Neither source of Th cells were capable of inducing long-lived autoantibodies. Treg cells completely suppressed anti-chromatin IgMa antibody production and reduced anti-chromatin B cell recovery induced by TS1 Th cells. Importantly, this suppression was less effective when TS1.Roquinsan/san Th cells were used. Thus, high ICOS levels on effector T cells results in autoimmunity by augmenting the autoreactive B cell response and by dampening the effect of Treg cell suppression.
doi:10.1016/j.jaut.2009.11.016
PMCID: PMC2859982  PMID: 20022728
regulatory T cells; autoreactive B cells; ICOS expression
20.  Plasmacytoid dendritic cells prime IL-10–producing T regulatory cells by inducible costimulator ligand 
Although there is evidence for distinct roles of myeloid dendritic cells (DCs [mDCs]) and plasmacytoid pre-DCs (pDCs) in regulating T cell–mediated adaptive immunity, the concept of functional DC subsets has been questioned because of the lack of a molecular mechanism to explain these differences. In this study, we provide direct evidence that maturing mDCs and pDCs express different sets of molecules for T cell priming. Although both maturing mDCs and pDCs upregulate the expression of CD80 and CD86, only pDCs upregulate the expression of inducible costimulator ligand (ICOS-L) and maintain high expression levels upon differentiation into mature DCs. High ICOS-L expression endows maturing pDCs with the ability to induce the differentiation of naive CD4 T cells to produce interleukin-10 (IL-10) but not the T helper (Th)2 cytokines IL-4, -5, and -13. These IL-10–producing T cells are T regulatory cells, and their generation by ICOS-L is independent of pDC-driven Th1 and Th2 differentiation, although, in the later condition, some contribution from endogenous IL-4 cannot be completely ruled out. Thus, in contrast to mDCs, pDCs are poised to express ICOS-L upon maturation, which leads to the generation of IL-10–producing T regulatory cells. Our findings demonstrate that mDC and pDCs are intrinsically different in the expression of costimulatory molecules that drive distinct types of T cell responses.
doi:10.1084/jem.20061660
PMCID: PMC2118437  PMID: 17200410
21.  Inducible Deletion of CD28 Prior to Secondary Nippostrongylus brasiliensis Infection Impairs Worm Expulsion and Recall of Protective Memory CD4+ T Cell Responses 
PLoS Pathogens  2014;10(2):e1003906.
IL-13 driven Th2 immunity is indispensable for host protection against infection with the gastrointestinal nematode Nippostronglus brasiliensis. Disruption of CD28 mediated costimulation impairs development of adequate Th2 immunity, showing an importance for CD28 during the initiation of an immune response against this pathogen. In this study, we used global CD28−/− mice and a recently established mouse model that allows for inducible deletion of the cd28 gene by oral administration of tamoxifen (CD28−/loxCre+/−+TM) to resolve the controversy surrounding the requirement of CD28 costimulation for recall of protective memory responses against pathogenic infections. Following primary infection with N. brasiliensis, CD28−/− mice had delayed expulsion of adult worms in the small intestine compared to wild-type C57BL/6 mice that cleared the infection by day 9 post-infection. Delayed expulsion was associated with reduced production of IL-13 and reduced serum levels of antigen specific IgG1 and total IgE. Interestingly, abrogation of CD28 costimulation in CD28−/loxCre+/− mice by oral administration of tamoxifen prior to secondary infection with N. brasiliensis resulted in impaired worm expulsion, similarly to infected CD28−/− mice. This was associated with reduced production of the Th2 cytokines IL-13 and IL-4, diminished serum titres of antigen specific IgG1 and total IgE and a reduced CXCR5+ TFH cell population. Furthermore, total number of CD4+ T cells and B220+ B cells secreting Th1 and Th2 cytokines were significantly reduced in CD28−/− mice and tamoxifen treated CD28−/loxCre+/− mice compared to C57BL/6 mice. Importantly, interfering with CD28 costimulatory signalling before re-infection impaired the recruitment and/or expansion of central and effector memory CD4+ T cells and follicular B cells to the draining lymph node of tamoxifen treated CD28−/loxCre+/− mice. Therefore, it can be concluded that CD28 costimulation is essential for conferring host protection during secondary N. brasiliensis infection.
Author Summary
CD28 is an important costimulatory molecule, involved in the activation of naive T cells, enhancing cytokine production, preventing T cell anergy and apoptosis. Furthermore, CD28 plays a crucial role in the organisation of secondary lymphoid tissue by assisting in the recruitment of T cells into the B cell follicles, thus promoting germinal center formation, isotype switching and B cell maturation. The requirement of CD28 costimulatory signalling during recall of memory responses against infections has remained controversial. Hence, here we utilised a mouse model that allowed for inducible deletion of the cd28 gene (CD28−/loxCre+/−) by oral administration of tamoxifen to resolve this controversy. CD28−/− mice and mice given tamoxifen prior to secondary infection failed to expel adult N. brasiliensis worms. This was related to reduced production of the Th2 cytokines IL-13 and IL-4, diminished type 2 antibody titres, and a reduced number of memory CD4+ T cells. In summary, CD28 is crucial for protection against N. brasiliensis secondary infection and plays a key role in the recruitment of TFH cells, memory CD4+ T cells and follicular B cells.
doi:10.1371/journal.ppat.1003906
PMCID: PMC3916406  PMID: 24516382
22.  Protective Effector Memory CD4 T Cells Depend on ICOS for Survival 
PLoS ONE  2011;6(2):e16529.
Memory CD4 T cells play a vital role in protection against re-infection by pathogens as diverse as helminthes or influenza viruses. Inducible costimulator (ICOS) is highly expressed on memory CD4 T cells and has been shown to augment proliferation and survival of activated CD4 T cells. However, the role of ICOS costimulation on the development and maintenance of memory CD4 T cells remains controversial. Herein, we describe a significant defect in the number of effector memory (EM) phenotype cells in ICOS−/− and ICOSL−/− mice that becomes progressively more dramatic as the mice age. This decrease was not due to a defect in the homeostatic proliferation of EM phenotype CD4 T cells in ICOS−/− or ICOSL−/− mice. To determine whether ICOS regulated the development or survival of EM CD4 T cells, we utilized an adoptive transfer model. We found no defect in development of EM CD4 T cells, but long-term survival of ICOS−/− EM CD4 T cells was significantly compromised compared to wild-type cells. The defect in survival was specific to EM cells as the central memory (CM) ICOS−/− CD4 T cells persisted as well as wild type cells. To determine the physiological consequences of a specific defect in EM CD4 T cells, wild-type and ICOS−/− mice were infected with influenza virus. ICOS−/− mice developed significantly fewer influenza-specific EM CD4 T cells and were more susceptible to re-infection than wild-type mice. Collectively, our findings demonstrate a role for ICOS costimulation in the maintenance of EM but not CM CD4 T cells.
doi:10.1371/journal.pone.0016529
PMCID: PMC3041765  PMID: 21364749
23.  An incoherent regulatory network architecture that orchestrates B cell diversification in response to antigen signaling 
B cell receptor signaling controls the expression of IRF-4, a transcription factor required for B cell differentiation. This study shows that IRF-4 regulates divergent B cell fates via a ‘kinetic-control' mechanism that determines the duration of a transient developmental state.
The intensity of signaling through the B cell receptor controls the level of expression of IRF-4, a transcription factor required for B cell differentiation. The rate of IRF-4 production dictates the extent of antibody gene diversification that B cells undergo upon antigen encounter before differentiating into antibody-secreting plasma cells.Computational modeling and experimental analyses substantiate a model, whereby IRF-4 regulates B cell fate trajectories via a ‘kinetic-control' mechanism.Kinetic control is a process by which B cells pass through an obligate state of variable duration that sets the degree of cellular diversification prior to their terminal differentiation.An incoherent regulatory network architecture, within which IRF-4 is embedded, is the basis for realization of kinetic control.
The generation of a diverse set of pathogen-specific antibodies, with differing affinities and effector functions, by B lymphocytes is essential for efficient protection from many microorganisms. Antibody gene diversification in B cells is mediated by two molecular processes termed class-switch recombination and somatic hypermutation (CSR/SHM) (F1A). The former enables the generation of antibodies with the same antigen-binding specificity, but different effector domains, whereas the latter results in a repertoire of antibodies with a range of affinities for a given antigen containing the same effector domain. CSR/SHM occurs in antigen-activated B cells before their terminal differentiation into plasma cells. The transcription factor IRF-4 is required for CSR/SHM as well as plasma-cell differentiation, with its highest levels of expression being necessary for the latter. IRF-4 acts in the context of a network of regulators that include Blimp-1, Pax5, Bach2 and Bcl-6 (F1B). Despite extensive characterization of these individual factors, how the network responds to sensing of antigen by the B cell antigen receptor (BCR, antibody molecule expressed on cell surface) to regulate the extent of antibody gene diversification and plasma-cell differentiation remains to be addressed.
To address this issue, we assemble a computational model. The model reveals two contrasting scenarios that can underlie B cell fate dynamics. In one case, the initial rate of IRF-4 production controls a binary cell fate choice that involves either going to the CSR/SHM state or to the plasma-cell state; the time spent in the CSR state is relatively insensitive to the initial rate of IRF-4 production (herein called ‘basic bistability'). In the other case, IRF-4 drives all cells through a transient CSR/SHM state, but the initial rate of IRF-4 production sets its duration (‘kinetic control'). Both scenarios predict that increasing the initial rate of IRF-4 production favors the generation of plasma cells at the expense of CSR/SHM, but they differ fundamentally with respect to the underlying gene expression patterns.
To distinguish between these two scenarios experimentally, we utilize two different genetic models. The first involves the B1-8i transgenic mouse whose B cells express a rearranged V187.2 VDJ Ig heavy chain gene segment that is specific for the hapten nitrophenol (NP). The second is a newly developed mouse model that allows exogenous control of IRF-4 expression in naive primary B cells using a tet-inducible allele. Using these models, we show that (i) BCR signal strength sets the initial rate of IRF-4 accumulation and (ii) the concentration of IRF-4 is sensed by an incoherent gene regulatory network architecture to regulate the extent of CSR/SHM prior to plasma-cell differentiation. Our results are consistent with the ‘kinetic-control model' in which the levels of BCR-induced IRF-4 expression control the duration of an obligate CSR/SHM state that enables B cell diversification before terminal differentiation into plasma cells. Evidence for the transient CSR/SHM state is corroborated by both patterns of gene expression and the presence of AID-dependent mutations in individual non-switched plasmablasts.
Our results provide a molecular framework for understanding how B cells balance the competing demands for Ig CSR and SHM with the secretion of antibodies during humoral immune responses. The key feature of the network architecture that allows IRF-4 to coordinate the two competing states of gene expression in a temporal manner is that it simultaneously but asymmetrically activates both sides of a bistable mutual repression circuit. Because the two effects of the primary regulator antagonize each other, we describe the circuit as being based on an ‘incoherent' regulatory motif. Other incoherent regulatory motifs in varied biological systems are also associated with the acquisition of transient cell states, and we consider how the kinetic-control mechanism proposed by us could more generally serve to translate developmental cues into elaborate morphogenetic patterns.
The B-lymphocyte lineage is a leading system for analyzing gene regulatory networks (GRNs) that orchestrate distinct cell fate transitions. Upon antigen recognition, B cells can diversify their immunoglobulin (Ig) repertoire via somatic hypermutation (SHM) and/or class switch DNA recombination (CSR) before differentiating into antibody-secreting plasma cells. We construct a mathematical model for a GRN underlying this developmental dynamic. The intensity of signaling through the Ig receptor is shown to control the bimodal expression of a pivotal transcription factor, IRF-4, which dictates B cell fate outcomes. Computational modeling coupled with experimental analysis supports a model of ‘kinetic control', in which B cell developmental trajectories pass through an obligate transient state of variable duration that promotes diversification of the antibody repertoire by SHM/CSR in direct response to antigens. More generally, this network motif could be used to translate a morphogen gradient into developmental inductive events of varying time, thereby enabling the specification of distinct cell fates.
doi:10.1038/msb.2011.25
PMCID: PMC3130558  PMID: 21613984
BCR signal strength; bistability; gene regulatory network; ghost of a fixed point; Irf4
24.  The Tetraspanin Protein CD37 Regulates IgA Responses and Anti-Fungal Immunity 
PLoS Pathogens  2009;5(3):e1000338.
Immunoglobulin A (IgA) secretion by plasma cells in the immune system is critical for protecting the host from environmental and microbial infections. However, the molecular mechanisms underlying the generation of IgA+ plasma cells remain poorly understood. Here, we report that the B cell–expressed tetraspanin CD37 inhibits IgA immune responses in vivo. CD37-deficient (CD37−/−) mice exhibit a 15-fold increased level of IgA in serum and significantly elevated numbers of IgA+ plasma cells in spleen, mucosal-associated lymphoid tissue, as well as bone marrow. Analyses of bone marrow chimeric mice revealed that CD37–deficiency on B cells was directly responsible for the increased IgA production. We identified high local interleukin-6 (IL-6) production in germinal centers of CD37−/− mice after immunization. Notably, neutralizing IL-6 in vivo reversed the increased IgA response in CD37−/− mice. To demonstrate the importance of CD37—which can associate with the pattern-recognition receptor dectin-1—in immunity to infection, CD37−/− mice were exposed to Candida albicans. We report that CD37−/− mice are evidently better protected from infection than wild-type (WT) mice, which was accompanied by increased IL-6 levels and C. albicans–specific IgA antibodies. Importantly, adoptive transfer of CD37−/− serum mediated protection in WT mice and the underlying mechanism involved direct neutralization of fungal cells by IgA. Taken together, tetraspanin protein CD37 inhibits IgA responses and regulates the anti-fungal immune response.
Author Summary
Antibody, or immunoglobulin (Ig), production by plasma cells in the immune system is important for protecting the host from microbial infections. IgA is the most abundant antibody isotype produced in the body. However, the molecular mechanisms underlying the generation of IgA–producing plasma cells remain poorly understood. We now report that the B cell–expressed protein CD37 regulates IgA immune responses, both in steady-state conditions and during infection. We found highly increased levels of IgA in serum and elevated numbers of IgA+ plasma cells in lymphoid tissue of mice that are deficient for CD37 (CD37−/− mice). To demonstrate the importance of CD37 in immunity to infection, CD37−/− mice were exposed to the fungus Candida albicans. C. albicans can cause systemic infection with high mortality in immunocompromised patients. We demonstrate that CD37−/− mice are evidently better protected from infection than wild-type mice, which was dependent on C. albicans–specific IgA antibodies. The underlying mechanism involved direct neutralization of fungal cells by IgA. In summary, the B cell protein CD37 inhibits IgA responses and anti-fungal immunity. This study may contribute to the development of novel immunotherapeutic approaches for invasive fungal disease.
doi:10.1371/journal.ppat.1000338
PMCID: PMC2650281  PMID: 19282981
25.  Ectopic Lymphoid Structures Support Ongoing Production of Class-Switched Autoantibodies in Rheumatoid Synovium 
PLoS Medicine  2009;6(1):e1.
Background
Follicular structures resembling germinal centres (GCs) that are characterized by follicular dendritic cell (FDC) networks have long been recognized in chronically inflamed tissues in autoimmune diseases, including the synovium of rheumatoid arthritis (RA). However, it is debated whether these ectopic structures promote autoimmunity and chronic inflammation driving the production of pathogenic autoantibodies. Anti-citrullinated protein/peptide antibodies (ACPA) are highly specific markers of RA, predict a poor prognosis, and have been suggested to be pathogenic. Therefore, the main study objectives were to determine whether ectopic lymphoid structures in RA synovium: (i) express activation-induced cytidine deaminase (AID), the enzyme required for somatic hypermutation and class-switch recombination (CSR) of Ig genes; (ii) support ongoing CSR and ACPA production; and (iii) remain functional in a RA/severe combined immunodeficiency (SCID) chimera model devoid of new immune cell influx into the synovium.
Methods and Findings
Using immunohistochemistry (IHC) and quantitative Taqman real-time PCR (QT-PCR) in synovial tissue from 55 patients with RA, we demonstrated that FDC+ structures invariably expressed AID with a distribution resembling secondary lymphoid organs. Further, AID+/CD21+ follicular structures were surrounded by ACPA+/CD138+ plasma cells, as demonstrated by immune reactivity to citrullinated fibrinogen. Moreover, we identified a novel subset of synovial AID+/CD20+ B cells outside GCs resembling interfollicular large B cells. In order to gain direct functional evidence that AID+ structures support CSR and in situ manufacturing of class-switched ACPA, 34 SCID mice were transplanted with RA synovium and humanely killed at 4 wk for harvesting of transplants and sera. Persistent expression of AID and Iγ-Cμ circular transcripts (identifying ongoing IgM-IgG class-switching) was observed in synovial grafts expressing FDCs/CD21L. Furthermore, synovial mRNA levels of AID were closely associated with circulating human IgG ACPA in mouse sera. Finally, the survival and proliferation of functional B cell niches was associated with persistent overexpression of genes regulating ectopic lymphoneogenesis.
Conclusions
Our demonstration that FDC+ follicular units invariably express AID and are surrounded by ACPA-producing plasma cells provides strong evidence that ectopic lymphoid structures in the RA synovium are functional and support autoantibody production. This concept is further confirmed by evidence of sustained AID expression, B cell proliferation, ongoing CSR, and production of human IgG ACPA from GC+ synovial tissue transplanted into SCID mice, independently of new B cell influx from the systemic circulation. These data identify AID as a potential therapeutic target in RA and suggest that survival of functional synovial B cell niches may profoundly influence chronic inflammation, autoimmunity, and response to B cell–depleting therapies.
Costantino Pitzalis and colleagues show that lymphoid structures in synovial tissue of patients with rheumatoid arthritis support production of anti-citrullinated peptide antibodies, which continues following transplantation into SCID mice.
Editors' Summary
Background.
More than 1 million people in the United States have rheumatoid arthritis, an “autoimmune” condition that affects the joints. Normally, the immune system provides protection against infection by responding to foreign antigens (molecules that are unique to invading organisms) while ignoring self-antigens present in the body's own tissues. In autoimmune diseases, this ability to discriminate between self and non-self fails for unknown reasons and the immune system begins to attack human tissues. In rheumatoid arthritis, the lining of the joints (the synovium) is attacked, it becomes inflamed and thickened, and chemicals are released that damage all the tissues in the joint. Eventually, the joint may become so scarred that movement is no longer possible. Rheumatoid arthritis usually starts in the small joints in the hands and feet, but larger joints and other tissues (including the heart and blood vessels) can be affected. Its symptoms, which tend to fluctuate, include early morning joint pain, swelling, and stiffness, and feeling generally unwell. Although the disease is not always easy to diagnose, the immune systems of many people with rheumatoid arthritis make “anti-citrullinated protein/peptide antibodies” (ACPA). These “autoantibodies” (which some experts believe can contribute to the joint damage in rheumatoid arthritis) recognize self-proteins that contain the unusual amino acid citrulline, and their detection on blood tests can help make the diagnosis. Although there is no cure for rheumatoid arthritis, the recently developed biologic drugs, often used together with the more traditional disease-modifying therapies, are able to halt its progression by specifically blocking the chemicals that cause joint damage. Painkillers and nonsteroidal anti-inflammatory drugs can reduce its symptoms, and badly damaged joints can sometimes be surgically replaced.
Why Was This Study Done?
Before scientists can develop a cure for rheumatoid arthritis, they need to know how and why autoantibodies are made that attack the joints in this common and disabling disease. B cells, the immune system cells that make antibodies, mature in structures known as “germinal centers” in the spleen and lymph nodes. In the germinal centers, immature B cells are exposed to antigens and undergo two genetic processes called “somatic hypermutation” and “class-switch recombination” that ensure that each B cell makes an antibody that sticks as tightly as possible to just one antigen. The B cells then multiply and enter the bloodstream where they help to deal with infections. Interestingly, the inflamed synovium of many patients with rheumatoid arthritis contains structures that resemble germinal centers. Could these ectopic (misplaced) lymphoid structures, which are characterized by networks of immune system cells called follicular dendritic cells (FDCs), promote autoimmunity and long-term inflammation by driving the production of autoantibodies within the joint itself? In this study, the researchers investigate this possibility.
What Did the Researchers Do and Find?
The researchers collected synovial tissue from 55 patients with rheumatoid arthritis and used two approaches, called immunohistochemistry and real-time PCR, to investigate whether FDC-containing structures in synovium expressed an enzyme called activation-induced cytidine deaminase (AID), which is needed for both somatic hypermutation and class-switch recombination. All the FDC-containing structures that the researchers found in their samples expressed AID. Furthermore, these AID-containing structures were surrounded by mature B cells making ACPAs. To test whether these B cells were derived from AID-expressing cells resident in the synovium rather than ACPA-expressing immune system cells coming into the synovium from elsewhere in the body, the researchers transplanted synovium from patients with rheumatoid arthritis under the skin of a special sort of mouse that largely lacks its own immune system. Four weeks later, the researchers found that the transplanted human lymphoid tissue was still making AID, that the level of AID expression correlated with the amount of human ACPA in the blood of the mice, and that the B cells in the transplant were proliferating.
What Do These Findings Mean?
These findings show that the ectopic lymphoid structures present in the synovium of some patients with rheumatoid arthritis are functional and are able to make ACPA. Because ACPA may be responsible for joint damage, the survival of these structures could, therefore, be involved in the development and progression of rheumatoid arthritis. More experiments are needed to confirm this idea, but these findings may explain why drugs that effectively clear B cells from the bloodstream do not always produce a marked clinical improvement in rheumatoid arthritis. Finally, they suggest that AID might provide a new target for the development of drugs to treat rheumatoid arthritis.
Additional Information.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0060001.
This study is further discussed in a PLoS Medicine Perspective by Rene Toes and Tom Huizinga
The MedlinePlus Encyclopedia has a page on rheumatoid arthritis (in English and Spanish). MedlinePlus provides links to other information on rheumatoid arthritis (in English and Spanish)
The UK National Health Service Choices information service has detailed information on rheumatoid arthritis
The US National Institute of Arthritis and Musculoskeletal and Skin Diseases provides Fast Facts, an easy to read publication for the public, and a more detailed Handbook on rheumatoid arthritis
The US Centers for Disease Control and Prevention has an overview on rheumatoid arthritis that includes statistics about this disease and its impact on daily life
doi:10.1371/journal.pmed.0060001
PMCID: PMC2621263  PMID: 19143467

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