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1.  Central B-Cell Tolerance: Where Selection Begins 
The development of an adaptive immune system based on the random generation of antigen receptors requires a stringent selection process that sifts through receptor specificities to remove those reacting with self-antigens. In the B-cell lineage, this selection process is first applied to IgM+ immature B cells. By using increasingly sophisticated mouse models, investigators have identified the central tolerance mechanisms that negatively select autoreactive immature B cells and prevent inclusion of their antigen receptors into the peripheral B-cell pool. Additional studies have uncovered mechanisms that promote the differentiation of nonautoreactive immature B cells and their positive selection into the peripheral B-cell population. These mechanisms of central selection are fundamental to the generation of a naïve B-cell repertoire that is largely devoid of self-reactivity while capable of reacting with any foreign insult.
Nave B cells must react with foreign insult but lack self-reactivity. Experiments in mice have defined selection processes that determine the entry of new B cells into the peripheral B-cell population.
PMCID: PMC3312675  PMID: 22378602
2.  Studies of lymphocyte reconstitution in a humanized mouse model reveal a requirement of T cells for human B cell maturation 
The hematopoietic humanized mouse (hu-mouse) model is a powerful resource to study and manipulate the human immune system. However, a major and recurrent issue with this model has been the poor maturation of B cells that fail to progress beyond the transitional B cell stage. Interestingly, a similar problem has been reported in transplant patients that receive cord blood stem cells. In this study, we characterize the development of human B and T cells in the lymph nodes (LNs) and spleen of BALB/c-Rag2nullIl2rγnull hu-mice. We find a dominant population of immature B cells in the blood and spleen early, followed by a population of human T cells, coincident with the detection of LNs. Notably, in older mice we observe a major population of mature B cells in LNs and in the spleens of mice with higher T cell frequencies. Moreover, we demonstrate that the T cells are necessary for B cell maturation, as introduction of autologous human T cells expedites the appearance of mature B cells, while in vivo depletion of T cells retards B cell maturation. The presence of the mature B cell population correlates with enhanced IgG and Ag-specific responses to both T-dependent and T-independent challenges, indicating their functionality. These findings enhance our understanding of human B cell development, provide increased details of the reconstitution dynamics of hu-mice, and validates the use of this animal model to study mechanisms and treatments for the similar delay of functional B cells associated with cord blood transplantations.
PMCID: PMC3578183  PMID: 23335750
3.  Immunosuppressive Property within the Streptococcus pneumoniae Cell Wall That Inhibits Generation of T Follicular Helper, Germinal Center, and Plasma Cell Response to a Coimmunized Heterologous Protein 
Infection and Immunity  2013;81(9):3426-3433.
We previously demonstrated that intact, inactivated Streptococcus pneumoniae (unencapsulated strain R36A) inhibits IgG responses to a number of coimmunized soluble antigens (Ags). In this study, we investigated the mechanism of this inhibition and whether other extracellular bacteria exhibited similar effects. No inhibition was observed if R36A was given 24 h before or after immunization with soluble chicken ovalbumin (cOVA), indicating that R36A acts transiently during the initiation of the immune response. Using transgenic cOVA-specific CD4+ T cells, we observed that R36A had no significant effect on T-cell activation (24 h) or generation of regulatory T cells (day 7) and only a modest effect on T-cell proliferation (48 to 96 h) in response to cOVA. However, R36A mediated a significant reduction in the formation of Ag-specific splenic germinal center T follicular helper (GC Tfh) and GC B cells and antibody-secreting cells in the spleen and bone marrow in response to cOVA or cOVA conjugated to 4-hydroxy-3-nitrophenylacetyl hapten (NP-cOVA). Of note, the inhibitory effect of intact R36A on the IgG anti-cOVA response could be reproduced using R36A-derived cell walls. In contrast to R36A, neither inactivated, unencapsulated, intact Neisseria meningitidis nor Streptococcus agalactiae inhibited the OVA-specific IgG response. These results suggest a novel immunosuppressive property within the cell wall of Streptococcus pneumoniae.
PMCID: PMC3754212  PMID: 23817619
4.  Antigen and cytokine receptor signals guide the development of the naïve mature B cell repertoire 
Immunologic research  2013;55(0):231-240.
Immature B cells are generated daily in the bone marrow tissue. More than half of the newly generated immature B cells are autoreactive and bind a self-antigen, while the others are nonautoreactive. A selection process has evolved on the one hand to thwart development of autoreactive immature B cells and, on the other hand, to promote further differentiation of nonautoreactive immature B cells into transitional and mature B cells. These negative and positive selection events are carefully regulated by signals that emanate from the antigen receptor, whether antigen-mediated or tonic, and are influenced by signals that are generated by receptors that bind cytokines, chemokines, and other factors produced in the bone marrow tissue. These signals, therefore, are the predominant driving forces for the generation of a B cell population that is capable of protecting the body from infections while maintaining self-tolerance. Here, we review recent findings from our group and others that describe how tonic antigen receptor signaling and bone marrow cytokines regulate the selection of immature B cells.
PMCID: PMC3924590  PMID: 22941591
B cell development; B cell selection; B cell tolerance; Bone marrow; BCR signaling; Cytokine
5.  Type I IFN enhances follicular B cell contribution to the T cell–independent antibody response 
The Journal of Experimental Medicine  2010;207(7):1485-1500.
Humoral immunity to viruses and encapsulated bacteria is comprised of T cell–independent type 2 (TI-2) antibody responses that are characterized by rapid antibody production by marginal zone and B1 B cells. We demonstrate that toll-like receptor (TLR) ligands influence the TI-2 antibody response not only by enhancing the overall magnitude but also by skewing this response to one that is dominated by IgG isotypes. Importantly, TLR ligands facilitate this response by inducing type I interferon (IFN), which in turn elicits rapid and significant amounts of antigen-specific IgG2c predominantly from FO (follicular) B cells. Furthermore, we show that although the IgG2c antibody response requires B cell–autonomous IFN-α receptor signaling, it is independent of B cell–intrinsic TLR signaling. Thus, innate signals have the capacity to enhance TI-2 antibody responses by promoting participation of FO B cells, which then elaborate effective IgG anti-pathogen antibodies.
PMCID: PMC2901065  PMID: 20566717
6.  S1P3 confers differential S1P migration by autoreactive and non-autoreactive immature B cells and is required for normal B cell development 
European journal of immunology  2010;40(3):688-698.
During B cell development, immature B cell fate is determined by whether the B cell antigen receptor is engaged in the bone marrow. Immature B cells that are non-autoreactive continue maturation and emigrate from the marrow whereas autoreactive immature B cells remain and are tolerized. However, the microenvironment where these events occur and the chemoattractants responsible for immature B cell trafficking within and out of the bone marrow remain largely undefined. Sphingosine 1-phosphate (S1P) is a chemoattractant that directs lymphocyte trafficking and thymocyte egress and in this study we investigated whether S1P contributed to B cell development, egress and positioning within the bone marrow. Our findings show that immature B cells are chemotactic towards S1P but that this response is dependent on antigen receptor specificity: non-autoreactive, but not autoreactive, immature B cells migrate towards S1P and are shown to require S1P3 receptor for this response. Despite this response, S1P3 is shown not to facilitate immature B cell egress but is required for normal B cell development including the positioning of transitional B cells within bone marrow sinusoids. These data indicate that S1P3 signaling directs immature B cells to a bone marrow microenvironment important for both tolerance induction and maturation.
PMCID: PMC2924669  PMID: 20039302
Immature B cells; Autoreactive; Sphingosine 1-phosphate; Migration
Molecular immunology  2007;45(7):1825-1836.
Lsc is a hematopoietic-restricted protein that functions as an effector of Gα12/13-associated G-protein coupled receptors that activates RhoA. In the absence of Lsc leukocytes exhibit impaired migration and B lymphocytes inefficiently resolve integrin-mediated adhesion. Here, we demonstrate that Lsc exists physiologically in primary B lymphocytes as a large molecular weight complex resembling a homo-tetramer. Interfering with the assembly of this large molecular weight Lsc oligomer results in the activation of both Lsc functional activities and leads to cell rounding and inhibition of integrin-mediated adhesion. During cell migration on integrin ligands we find Lsc localizes predominantly toward the rear of migrating cells where we suggest it activates RhoA to resolve integin-mediated adhesion. Together these data demonstrate that Lsc regulates integrin-mediated adhesive events at the trailing edge of migrating cells.
PMCID: PMC2315659  PMID: 18157933
Lsc; oligomerization; RhoA; integrin; adhesion
8.  Dual-reactive B cells are autoreactive and highly enriched in the plasmablast and memory B cell subsets of autoimmune mice 
The Journal of Experimental Medicine  2012;209(10):1797-1812.
Dual–light chain–expressing B cells in autoimmune prone mice increase with age, contribute to the memory and plasma cell compartments, and are autoreactive.
Rare dual-reactive B cells expressing two types of Ig light or heavy chains have been shown to participate in immune responses and differentiate into IgG+ cells in healthy mice. These cells are generated more often in autoreactive mice, leading us to hypothesize they might be relevant in autoimmunity. Using mice bearing Igk allotypic markers and a wild-type Ig repertoire, we demonstrate that the generation of dual-κ B cells increases with age and disease progression in autoimmune-prone MRL and MRL/lpr mice. These dual-reactive cells express markers of activation and are more frequently autoreactive than single-reactive B cells. Moreover, dual-κ B cells represent up to half of plasmablasts and memory B cells in autoimmune mice, whereas they remain infrequent in healthy mice. Differentiation of dual-κ B cells into plasmablasts is driven by MRL genes, whereas the maintenance of IgG+ cells is partly dependent on Fas inactivation. Furthermore, dual-κ B cells that differentiate into plasmablasts retain the capacity to secrete autoantibodies. Overall, our study indicates that dual-reactive B cells significantly contribute to the plasmablast and memory B cell populations of autoimmune-prone mice suggesting a role in autoimmunity.
PMCID: PMC3457739  PMID: 22927551
9.  Sub-chronic exposure to second hand smoke induces airspace leukocyte infiltration and decreased lung elastance 
Exposure to second hand tobacco smoke is associated with the development and/or exacerbation of several different pulmonary diseases in humans. To better understand the possible effects of second hand smoke exposure in humans, we sub-chronically (4 weeks) exposed mice to a mixture of mainstream and sidestream tobacco smoke at concentrations similar to second hand smoke exposure in humans. The inflammatory response to smoke exposures was assessed at the end of this time by enumeration of pulmonary leukocyte infiltration together with measurements of lung elastance and pathology. This response was measured in both healthy wild type (C57BL/6) mice as well as mouse mutants deficient in the expression of Arhgef1 (Arhgef1−/−) that display constitutive pulmonary inflammation and decreased lung elastance reminiscent of emphysema. The results from this study show that sub-chronic second hand smoke exposure leads to significantly increased numbers of airspace leukocytes in both healthy and mutant animals. While sub-chronic cigarette smoke exposure is not sufficient to induce changes in lung architecture as measured by mean linear intercept, both groups exhibit a significant decrease in lung elastance. Together these data demonstrate that even sub-chronic exposure to second hand smoke is sufficient to induce pulmonary inflammation and decrease lung elastance in both healthy and diseased animals and in the absence of tissue destruction.
PMCID: PMC3429071  PMID: 22934051
second hand smoke; inflammation; lung mechanics
10.  Generation of hematopoietic humanized mice in the newborn BALB/c-Rag2nullIl2rγnull mouse model: a multivariable optimization approach 
Clinical immunology (Orlando, Fla.)  2011;140(1):102-116.
Hematopoietic humanized mice generated via transplantation of human hematopoietic stem cells (hHSCs) into immunodeficient mice are a valuable tool for studying development and function of the human immune system. This study was performed to generate a protocol that improves development and quality of humanized mice in the BALB/c-Rag2nullIl2rγnull strain, testing route of injection, in vitro culture and freezing of hHSCs, types of cytokines in the culture, and co-injection of lineage-depleted CD34− cells. Specific hHSC culturing conditions and the addition of support cells were found to increase the frequency, and human hematopoietic chimerism, of humanized mice. The optimized protocol resulted in BALB/c-Rag2nullIl2rγnull humanized mice displaying more consistent human hematopoietic and lymphoid engraftment. Thus, hematopoietic humanized mice generated on a BALB/c immunodeficient background represent a useful model to study the human immune system.
PMCID: PMC3115423  PMID: 21536497
hematopoietic stem cell; human cord blood; hematopoietic chimerism; immunodeficient mouse; cytokine; humanized mouse
11.  Murine B Cell Development and Antibody Responses to Model Antigens Are Not Impaired in the Absence of the TNF Receptor GITR 
PLoS ONE  2012;7(2):e31632.
The Glucocorticoid-Induced Tumor necrosis factor Receptor GITR, a member of the tumor necrosis factor receptor superfamily, has been shown to be important in modulating immune responses in the context of T cell immunity. B lymphocytes also express GITR, but a role of GITR in humoral immunity has not been fully explored. To address this question, we performed studies to determine the kinetics of GITR expression on naïve and stimulated B cells and the capacity of B cells to develop and mount antibody responses in GITR−/− mice. Results of our studies indicate that all mature B cells express GITR on the cell surface, albeit at different levels. Expression of GITR on naïve mature B cells is upregulated by BCR signaling, but is counteracted by helper T cell-related factors and other inflammatory signals in vitro. In line with these findings, expression of GITR on germinal center and memory B cells is lower than that on naïve B cells. However, the expression of GITR is strongly upregulated in plasma cells. Despite these differences in GITR expression, the absence of GITR has no effect on T cell-dependent and T cell-independent antibody responses to model antigens in GITR−/− mice, or on B cell activation and proliferation in vitro. GITR deficiency manifests only with a slight reduction of mature B cell numbers and increased turnover of naïve B cells, suggesting that GITR slightly contributes to mature B cell homeostasis. Overall, our data indicate that GITR does not play a significant role in B cell development and antibody responses to T-dependent and independent model antigens within the context of a GITR-deficient genetic background.
PMCID: PMC3273462  PMID: 22328941
12.  BAFF-R signaling aids the differentiation of immature B cells into transitional B cells following tonic BCR signaling 
BAFF is an important pro-survival cytokine for mature B cells. However, previous studies have shown that the BAFF receptor, BAFF-R, is already expressed at the immature B cell stage, and that the pro-survival protein Bcl-2 does not completely complement the B cell defects resulting from the absence of BAFF-R or BAFF. Thus, we hypothesized that BAFF also functions to aid the differentiation of non-autoreactive immature B cells into transitional B cells and to promote their positive selection. We found that BAFF-R is expressed at higher levels on non-autoreactive than autoreactive immature B cells and that its expression correlates with that of surface IgM and with tonic BCR signaling. Our data indicate that BAFF-R signaling enhances the generation of transitional CD23− B cells in vitro by increasing cell survival. In vivo, however, BAFF-R signaling is dispensable for the generation of CD23− transitional B cells in the bone marrow, but is important for the development of transitional CD23− T1 B cells in the spleen. In addition, we show that BAFF is essential for the differentiation of CD23− into CD23+ transitional B cells both in vitro and in vivo through a mechanism distinct from that mediating cell survival, but requiring tonic BCR signaling. In summary, our data indicate that BAFF-R and tonic BCR signals cooperate to enable non-autoreactive immature B cells to differentiate into transitional B cells and to be positively selected into the naïve B cell repertoire.
PMCID: PMC2950883  PMID: 20861359
13.  Ras activation of Erk restores impaired tonic BCR signaling and rescues immature B cell differentiation 
B cell receptors (BCRs) generate tonic signals critical for B cell survival and early B cell development. To determine whether these signals also mediate the development of transitional and mature B cells, we examined B cell development using a mouse strain in which nonautoreactive immunoglobulin heavy and light chain–targeted B cells express low surface BCR levels. We found that reduced BCR expression translated into diminished tonic BCR signals that strongly impaired the development of transitional and mature B cells. Constitutive expression of Bcl-2 did not rescue the differentiation of BCR-low B cells, suggesting that this defect was not related to decreased cell survival. In contrast, activation of the Ras pathway rescued the differentiation of BCR-low immature B cells both in vitro and in vivo, whereas extracellular signal-regulated kinase (Erk) inhibition impaired the differentiation of normal immature B cells. These results strongly suggest that tonic BCR signaling mediates the differentiation of immature into transitional and mature B cells via activation of Erk, likely through a pathway requiring Ras.
PMCID: PMC2839140  PMID: 20176802
14.  Arhgef1 Is Required by T Cells for the Development of Airway Hyperreactivity and Inflammation 
Rationale: Arhgef1 is an intracellular protein, expressed by hematopoietic cells, that regulates signaling by both G protein–coupled receptors and RhoA, and, consequently, is required for appropriate migration and adhesion of diverse leukocyte populations.
Objectives: To evaluate a possible contribution for Arhgef1 in the development of airway inflammation and airway hyperreactivity.
Methods: Arhgef1-deficient (Arhgef1−/−) and wild-type (WT) mice were sensitized and airway challenged, followed by measurement of airway responsiveness to inhaled methacholine. Inflammation was assessed by several parameters that included flow cytometric analysis and histology. Arhgef1-deficient recipients were reconstituted with WT T lymphocytes before sensitization and challenge, and again measured for airway responsiveness and inflammation. Cytokine production in response to specific antigen was measured in cultures of isolated leukocytes from lung and spleen and compared with the levels generated in lung and spleen explant cultures.
Measurements and Main Results: Arhgef1−/− mice display significantly reduced airway hyperreactivity, Th2 cytokine production, and lung inflammation, despite intact systemic immunity. After airway challenge of Arhgef1−/− mice, antigen-specific T cells were present in mutant lungs, but were found to interact with CD11c+ cells at a significantly reduced frequency. Adoptive transfer of WT T cells into Arhgef1−/− mice restored airway hyperreactivity and inflammation.
Conclusions: These data demonstrate that T cells depend on Arhgef1 to promote lung inflammation. Moreover, a deficiency in Arhgef1 results in reduced T cell–CD11c+ antigen-presenting cell interaction, and likely underscores the inability of Arhgef1−/− mice to mount an adaptive immune response to airway challenge.
PMCID: PMC2049063  PMID: 17463415
airway hyperreactivity; cytokines; lung inflammation; T cells
15.  IL-23-Dependent IL-17 Production Is Essential in Neutrophil Recruitment and Activity in Mouse Lung Defense against Respiratory Mycoplasma pneumoniae Infection 
IL-23 induces IL-17 production in activated CD4+ T cells and participates in host defense against many encapsulated bacteria. However, whether IL-23/IL-17 axis contributes to a Mycoplasma pneumoniae (Mp)-induced lung inflammation (e.g., neutrophils) has not been addressed. Using an acute respiratory Mp infection murine model, we found significantly up-regulated lung IL-23p19 mRNA in the early phase of infection (4 h), and alveolar macrophages were an important cell source of Mp-induced IL-23. We further showed that Mp significantly increased IL-17 protein levels in bronchoalveolar lavage (BAL). Lung gene expression of IL-17, IL-17C and IL-17F was also markedly up-regulated by Mp in vivo. IL-17 and IL-17F were found to be derived mainly from lung CD4+ T cells, and were increased upon IL-23 stimulation in vitro. In vivo blocking of IL-23p19 alone or in combination with IL-23/IL-12p40 resulted in a significant reduction of Mp-induced IL-17 protein and IL-17/IL-17F mRNA expression, which was accompanied by a trend toward reduced lung neutrophil recruitment, BAL neutrophil activity, and Mp clearance. However, IL-23 neutralization had no effect on Mp-induced lung IL-17C mRNA expression. These results demonstrate that IL-17/IL-17F production is IL-23-dependent in an acute Mp infection, and contributes to neutrophil recruitment and activity in lung defense against the infection.
PMCID: PMC1832075  PMID: 17198762
M. pneumoniae; IL-23; IL-17; IL-17F; neutrophil

Results 1-15 (15)