Recombinant CD40 ligand stimulation of murine B cell growth and differentiation: cooperative effects of cytokines

Multifaceted roles for STAT3 in gammaherpesvirus latency revealed through in vivo B cell knockout models

Cancers associated with the oncogenic gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus, are notable for their constitutive activation of the transcription factor signal transducer and activator of transcription 3 (STAT3). To better understand the role of STAT3 during gammaherpesvirus latency and the B cell response to infection, we used the model pathogen murine gammaherpesvirus 68 (MHV68). Genetic deletion of STAT3 in B cells of CD19cre/+Stat3f/f mice reduced peak MHV68 latency approximately sevenfold. However, infected CD19cre/+Stat3f/f mice exhibited disordered germinal centers and heightened virus-specific CD8 T cell responses compared to wild-type (WT) littermates. To circumvent the systemic immune alterations observed in the B cell-STAT3 knockout mice and more directly evaluate intrinsic roles for STAT3, we generated mixed bone marrow chimeric mice consisting of WT and STAT3 knockout B cells. We discovered a dramatic reduction in latency in STAT3 knockout B cells compared to their WT B cell counterparts in the same lymphoid organ. RNA sequencing of sorted germinal center B cells revealed that MHV68 infection shifts the gene signature toward proliferation and away from type I and type II IFN responses. Loss of STAT3 largely reversed the virus-driven transcriptional shift without impacting the viral gene expression program. STAT3 promoted B cell processes of the germinal center, including IL-21-stimulated downregulation of surface CD23 on B cells infected with MHV68 or EBV. Together, our data provide mechanistic insights into the role of STAT3 as a latency determinant in B cells for oncogenic gammaherpesviruses.IMPORTANCEThere are no directed therapies to the latency program of the human gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus. Activated host factor signal transducer and activator of transcription 3 (STAT3) is a hallmark of cancers caused by these viruses. We applied the murine gammaherpesvirus pathogen system to explore STAT3 function upon primary B cell infection in the host. Since STAT3 deletion in all CD19+ B cells of infected mice led to altered B and T cell responses, we generated chimeric mice with both normal and STAT3-deleted B cells. B cells lacking STAT3 failed to support virus latency compared to normal B cells from the same infected animal. Loss of STAT3 impaired B cell proliferation and differentiation and led to a striking upregulation of interferon-stimulated genes. These findings expand our understanding of STAT3-dependent processes that are key to its function as a pro-viral latency determinant for oncogenic gammaherpesviruses in B cells and may provide novel therapeutic targets.

B cell-intrinsic STAT3-mediated support of latency and interferon suppression during murine gammaherpesvirus 68 infection revealed through an in vivo competition model

Cancers associated with the oncogenic gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma herpesvirus, are notable for their constitutive activation of the transcription factor STAT3. To better understand the role of STAT3 during gammaherpesvirus latency and immune control, we utilized murine gammaherpesvirus 68 (MHV68) infection. Genetic deletion of STAT3 in B cells of CD19cre/+Stat3f/f mice reduced peak latency approximately 7-fold. However, infected CD19cre/+Stat3f/f mice exhibited disordered germinal centers and heightened virus-specific CD8 T cell responses compared to WT littermates. To circumvent the systemic immune alterations observed in the B cell-STAT3 knockout mice and more directly evaluate intrinsic roles for STAT3, we generated mixed bone marrow chimeras consisting of WT and STAT3-knockout B cells. Using a competitive model of infection, we discovered a dramatic reduction in latency in STAT3-knockout B cells compared to their WT B cell counterparts in the same lymphoid organ. RNA sequencing of sorted germinal center B cells revealed that STAT3 promotes proliferation and B cell processes of the germinal center but does not directly regulate viral gene expression. Last, this analysis uncovered a STAT3-dependent role for dampening type I IFN responses in newly infected B cells. Together, our data provide mechanistic insight into the role of STAT3 as a latency determinant in B cells for oncogenic gammaherpesviruses.

Regulatory B cells, A to Z

B cells play a central role in the immune system through the production of antibodies. During the past two decades, it has become increasingly clear that B cells also have the capacity to regulate immune responses through mechanisms that extend beyond antibody production. Several types of human and murine regulatory B cells have been reported that suppress inflammatory responses in autoimmune disease, allergy, infection, transplantation, and cancer. Key suppressive molecules associated with regulatory B-cell function include the cytokines IL-10, IL-35, and TGF-β as well as cell membrane-bound molecules such as programmed death-ligand 1, CD39, CD73, and aryl hydrocarbon receptor. Regulatory B cells can be induced by a range of different stimuli, including microbial products such as TLR4 or TLR9 ligands, inflammatory cytokines such as IL-6, IL-1β, and IFN-α, as well as CD40 ligation. This review provides an overview of our current knowledge on regulatory B cells. We discuss different types of regulatory B cells, the mechanisms through which they exert their regulatory functions, factors that lead to induction of regulatory B cells and their role in the alteration of inflammatory responses in different diseases.

Permissive selection followed by affinity-based proliferation of GC light zone B cells dictates cell fate and ensures clonal breadth

Affinity maturation depends on how efficiently germinal centers (GCs) positively select B cells in the light zone (LZ). Positively selected GC B cells recirculate between LZs and dark zones (DZs) and ultimately differentiate into plasmablasts (PBs) and memory B cells (MBCs). Current understanding of the GC reaction presumes that cMyc-dependent positive selection of LZ B cells is a competitive affinity-dependent process; however, this cannot explain the production of GC-derived lower-affinity MBCs or retention of GC B cells with varied affinities. Here, by combining single-cell/bulk RNA sequencing and flow cytometry, we identified and characterized temporally and functionally distinct positively selected cMyc⁺ GC B cell subpopulations. cMyc⁺ LZ B cell subpopulations enriched with either higher- or lower-affinity cells diverged soon after permissive positive selection. The former subpopulation contained PB precursors, whereas the latter comprised less proliferative MBC precursors and future DZ entrants. The overall affinity of future DZ entrants was enhanced in the LZ through preferential proliferation of higher-affinity cells. Concurrently, lower-affinity cells were retained in GCs and protected from apoptosis. These findings redefine positive selection as a dynamic process generating three distinct B cell fates and elucidate how positive selection ensures clonal diversity for broad protection.

B regulatory cells in allergy

B cells have classically been recognized for their unique and indispensable role in the production of antibodies. Their potential as immunoregulatory cells with anti-inflammatory functions has received increasing attention during the last two decades. Herein, we highlight pioneering studies in the field of regulatory B cell (Breg) research. We will review the literature on Bregs with a particular focus on their role in the regulation of allergic inflammation.

Distinct modes of action of CD40L and adaptive cytokines IL-2, IL-4/13, IL-10 and IL-21 on rainbow trout IgM+ B cells

In mammals, conventional B (B2) cells are activated within lymphoid follicles through a close relationship with T follicular helper (Tfh) cells. The interaction between CD40 expressed on B cells and its ligand (CD40L) expressed on Tfh cells is a key signal that regulates the formation of germinal centers (GCs), B cell survival, proliferation and differentiation to plasma cells (PCs) or memory cells. Additionally, certain soluble cytokines produced by T cells also strongly condition the outcome of this interaction. Despite the many differences found between fish B cells and mammalian B2 cells, and the lack of conventional GCs, rainbow trout IgM⁺ B cells have been shown to be stimulated by CD40L, however, whether cytokines commonly produced by T cells can further modulate this response has never been addressed to date. Thus, in this study, we determined the effects of recombinant rainbow trout adaptive cytokines interleukin 2B (IL-2B), IL-4/13A, IL-4/13B, IL-10 and IL-21 (cytokines known to activate B cells in mammals) on splenic IgM⁺ B cells alone or in combination with CD40L. We studied how these cytokines and CD40L cooperated to promote IgM⁺ B cell survival, proliferation and IgM secretion. The results obtained provide valuable information for the first time in teleost fish on how different T cell signals cooperate to activate B cells in the absence of GCs.

Genetic engineering in primary human B cells with CRISPR-Cas9 ribonucleoproteins

Genome editing in human cells with targeted nucleases now enables diverse experimental and therapeutic genome engineering applications, but extension to primary human B cells remains limited. Here we report a method for targeted genetic engineering in primary human B cells, utilizing electroporation of CRISPR-Cas9 ribonucleoproteins (RNPs) to introduce gene knockout mutations at protein-coding loci with high efficiencies that in some cases exceeded 80%. Further, we demonstrate knock-in editing of targeted nucleotides with efficiency exceeding 10% through co-delivery of oligonucleotide templates for homology directed repair. We delivered Cas9 RNPs in two distinct in vitro culture systems to achieve editing in both undifferentiated B cells and activated B cells undergoing differentiation, reflecting utility in diverse experimental conditions. In summary, we demonstrate a powerful and scalable research tool for functional genetic studies of human B cell biology that may have further applications in engineered B cell therapeutics.

Anti-CD83 promotes IgG1 isotype switch in marginal zone B cells in response to TI-2 antigen

CD83 is a transmembrane glycoprotein that is rapidly up-regulated on activated B cells. Although CD83 itself is incapable to transduce intracellular signaling, it acts as a negative regulator of B cell function. We have recently described that a single application of anti-CD83 antibody results in dramatically enhanced production of antigen-specific IgG1 but not other isotypes upon immunization of mice with the TI-2 model antigen (Ag) NIP-Ficoll. This effect was mediated by the binding of anti-CD83 to CD83 on the surface of B cells themselves. In the current study we show that administration of anti-CD83 enhances IgG1-production independent of IL-4. Application of anti-CD83 does not alter the proliferation and general expansion of NIP-specific B cells. In the presence of anti-CD83, immunized mice develop normal frequencies of plasmablasts in response to NIP-Ficoll of which an increased number produces IgG1. These cells localize in extrafollicular foci in the spleen of immunized mice and originate from the marginal zone B cell pool. Taken together, our results indicate that CD83 engagement in vivo does not generally enhance B cell activation but selectively promotes IgG1 class switch in marginal zone B cells in response to TI-2 Ag.

Myeloid-derived suppressor cells regulate T cell and B cell responses during autoimmune disease

MDSCs are a heterogeneous group of myeloid cells that suppress T cell activity in cancer and autoimmune disease. The effect of MDSCs on B cell function is not clear. Using the CIA model of autoimmune disease, we found an increase in M-MDSCs in the periphery of WT mice with CIA compared with naïve mice. These MDSCs were absent from the periphery of CCR2(-/-) mice that developed exacerbated disease. M-MDSCs, isolated from immunized mice, inhibited autologous CD4(+) T cell proliferation. The M-MDSC-mediated suppression of T cell proliferation was NO and IFN-γ dependent but IL-17 independent. Furthermore, we demonstrated for the first time that M-MDSCs from CIA mice also inhibited autologous B cell proliferation and antibody production. The suppression of B cells by M-MDSCs was dependent on the production of NO and PGE2 and required cell-cell contact. Administration of M-MDSCs rescued CCR2(-/-) mice from the exacerbated CIA phenotype and ameliorated disease in WT mice. Furthermore, adoptive transfer of M-MDSCs reduced autoantibody production by CCR2(-/-) and WT mice. In summary, M-MDSCs inhibit T cell and B cell function in CIA and may serve as a therapeutic approach in the treatment of autoimmune arthritis.

Cytokine-Mediated Regulation of Plasma Cell Generation: IL-21 Takes Center Stage

During our life, we are surrounded by continuous threats from a diverse range of invading pathogens. Our immune system has evolved multiple mechanisms to efficiently deal with these threats so as to prevent them from causing disease. Terminal differentiation of mature B cells into plasma cells (PC) – the antibody (Ab) secreting cells of the immune system – is critical for the generation of protective and long-lived humoral immune responses. Indeed, efficient production of antigen (Ag)-specific Ab by activated B cells underlies the success of most currently available vaccines. The mature B-cell pool is composed of several subsets, distinguished from one according to size, surface marker expression, location, and Ag exposure, and they all have the capacity to differentiate into PCs. For a B-cell to acquire the capacity to produce Abs, it must undergo an extensive differentiation process driven by changes in gene expression. Two broad categories of Ags exist that cause B-cell activation and differentiation: T cell dependent (TD) or T cell independent (TI). In addition to the B-cell subset and nature of the Ag, it is important to consider the cytokine environment that can also influence how B-cell differentiation is achieved. Thus, while many cytokines can induce Ab-secretion by B cells after activation with mimics of TD and TI stimuli in vitro, they can have different efficacies and specificities, and can often preferentially induce production of one particular Ig isotype over another. Here, we will provide an overview of in vitro studies (mouse and human origin) that evaluated the role of different cytokines in inducing the differentiation of distinct B-cell subsets to the PC lineage. We will place particular emphasis on IL-21, which has emerged as the most potent inducer of terminal B-cell differentiation in humans. We will also focus on the role of IL-21 and defects in B-cell function and how these contribute to human immunopathologies such as primary immunodeficiencies and B-cell mediated autoimmune conditions.

Protein kinase R is a novel mediator of CD40 signaling and plays a critical role in modulating immunoglobulin expression during respiratory syncytial virus infection

Effective immunoglobulin responses play a vital role in protection against most pathogens. However, the molecular mediators and mechanisms responsible for signaling and selective expression of immunoglobulin types remain to be elucidated. Previous studies in our laboratory have demonstrated that protein kinase R (PKR) plays a crucial role in IgE responses to double-stranded RNA (dsRNA) in vitro. In this study, we show that PKR plays a critical role in IgG expression both in vivo and in vitro. PKR(-/-) mice show significantly altered serum IgG levels during respiratory syncytial virus (RSV) infection. IgG2a expression is particularly sensitive to a lack of PKR and is below the detection level in mock- or RSV-infected PKR(-/-) mice. Interestingly, we show that upon activation by anti-CD40 and gamma interferon (IFN-γ), B cells from PKR(-/-) mice show diminished major histocompatibility complex class II (MHC II), CD80, and CD86 levels on the cell surface compared to wild-type (WT) mice. Our data also show that PKR is necessary for optimal expression of adhesion molecules, such as CD11a and ICAM-1, that are necessary for homotypic aggregation of B cells. Furthermore, in this report we demonstrate for the first time that upon CD40 ligation, PKR is rapidly phosphorylated and activated, indicating that PKR is an early and novel downstream mediator of CD40 signaling pathways.

Modulation of factor VIII-specific memory B cells

The development of inhibitory antibodies against factor VIII (FVIII) is the major complication in patients with haemophilia A who are treated with FVIII products. Memory B cells play an essential role in maintaining established antibody responses. Upon re-exposure to the same antigen, they are rapidly re-stimulated to proliferate and differentiate into antibody-secreting plasma cells (ASC) that secrete high-affinity antibodies. It is, therefore, reasonable to believe that memory B cells have to be eradicated or inactivated for immune tolerance induction therapy to be successful in patients with haemophilia A and FVIII inhibitors. The aim of our studies was the development of strategies to prevent FVIII-specific memory B cells from becoming re-stimulated. We established a 6-day in vitro culture system that enabled us to study the regulation of FVIII-specific murine memory-B-cell re-stimulation. We tested the impact of the blockade of co-stimulatory interactions, of different concentrations of FVIII and of ligands for toll-like receptors (TLR). The blockade of B7-CD28 and CD40-CD40 ligand interactions prevented FVIII-specific murine memory B cells from becoming re-stimulated by FVIII in vitro and in vivo. Furthermore, high concentrations of FVIII blocked re-stimulation of FVIII-specific murine memory B cells. Triggering of TLR7 amplified re-stimulation by low concentrations of FVIII and prevented blockade by high concentrations of FVIII. We conclude that we defined modulators that either amplify or inhibit the re-stimulation of FVIII-specific murine memory B cells. Currently, we are investigating whether the same modulators operate in patients with haemophilia A and FVIII inhibitors.

Differential B-lymphocyte regulation by CD40 and its viral mimic, latent membrane protein 1

CD40 plays a vital role in humoral immunity, via its potent and multifaceted function as an activating receptor of various immune cells, most notably B lymphocytes. The Epstein-Barr virus-encoded transforming protein latent membrane protein 1 (LMP1) serves as a functional mimic of CD40 signals to B cells but lacks key regulatory controls that restrain CD40 signaling. This allows LMP1 to activate B cells in an abnormal manner that can contribute to the pathogenesis of human B-cell lymphoma and autoimmune disease. This review focuses upon a comparative analysis of CD40 versus LMP1 functions and mechanisms of action in B lymphocytes, discussing how this comparison can provide valuable information on both how CD40 signaling is normally regulated and how LMP1 disrupts the normal CD40 pathways, which can provide information of value to therapeutic design.

The in vitro derivation of phenotypically mature and diverse B cells from immature spleen and bone marrow precursors

The capacity of immature B cells of the spleen and bone marrow to differentiate in vitro into cells representing mature end stage cells was investigated using B-cell activating factor belonging to the TNF family (BAFF) and Notch pathway activators. Immature splenic and bone marrow B cells were found, in the presence of both of these activators, to mature into cells with follicular mature (FM) and marginal zone (MZ) cell phenotypes. Such cells were functionally responsive to B-cell-specific activation. The derivation in vitro of cells with an MZ phenotype was more robust from CD23(-) populations than CD23(+) immature/transitional B cells, suggesting a direct immature/T1 B cell to MZ cell differentiation pathway. Transcript analysis of the in vitro-derived B-cell populations demonstrated expression profiles similar to maturing B cells in vivo. FACS-purified populations of B220(+)CD19(+)CD21(-)CD23(-) cells from bone marrow of 2-wk-old mice gave rise to populations of CD21(+)CD23(-) cells with MZ cell phenotypes as well as CD21(+)CD23(+) cells with FM cell phenotypes in percentages similar to those found in vivo. These data suggest that the commitment to an MZ and FM B cell phenotype is set prior to immature B-cell release from the marrow.

CD28 and inducible costimulator (ICOS) signalling can sustain CD154 expression on activated T cells

The biological outcome of receptor-mediated signalling often depends on the duration of engagement. Because CD40 signalling is controlled by the regulated expression of its ligand, CD154, the mechanisms that regulate CD154 expression probably determine the strength and duration of CD40 signalling. Here, we demonstrate that CD154 expression on the surface of mouse CD4 T cells can be separated into an early phase, occurring between 0 and 24 hr after T-cell activation, and a later extended phase, occurring after 24 hr. The early phase of CD154 expression did not require costimulation and was probably influenced by the strength of T-cell receptor (TCR) signalling alone. However, later CD154 expression was highly dependent on costimulation through either CD28 or inducible costimulator (ICOS). Although CD28 signalling interleukin (IL)-2 secretion, ICOS not, suggesting that costimulation enhance CD154 expression independently of IL-2 production. In fact, anti-CD28 treatment could still induce late-phase CD154 on anti-CD3-stimulated CD4 T cells expressing a mutated form of CD28 that not lead to the induction of IL-2. However, this CD154 induction was somewhat weaker than that of wild-type CD28-expressing cells, suggesting that direct signalling and IL-2-mediated signalling co-operatively responsible for the levels of CD154 induced by CD28. Finally, we show that the second phase of CD154 expression negatively regulated B-cell terminal differentiation and antibody secretion. These results demonstrate that TCR signalling and costimulation each regulate different phases of CD154 expression and control the biological outcome of CD40 signalling on B cells.

The 'short' history of regulatory B cells

The maintenance of tolerance is the sine qua non of a sophisticated regulatory apparatus to prevent or dampen overzealous immune responses. In addition to the ability of B cells to prime and activate the immune system, B cells with regulatory function (Bregs) have been identified in experimental models of autoimmunity, infections, and cancer, supporting the notion that, similar to regulatory T cells (Tregs), Breg-mediated suppression is an important means for the maintenance of peripheral tolerance. This regulatory function appears to be directly mediated by the production of IL-10 and/or TGFbeta and by the ability of B cells to interact with pathogenic T cells to inhibit harmful immune responses. The identification of their existence is of great relevance to the understanding of autoimmune diseases and to the development of new therapeutic strategies.

Divergent effects of BAFF on human memory B cell differentiation into Ig-secreting cells

B cell-activating factor belonging to the TNF family (BAFF) plays a critical role in B cell maturation, yet its precise role in B cell differentiation into Ig-secreting cells (ISCs) remains unclear. In this study, we find that upon isolation human naive and memory B (MB) cells have prebound BAFF on their surface, whereas germinal center (GC) B cells lack detectable levels of prebound BAFF. We attribute their lack of prebound BAFF to cell activation, because we demonstrate that stimulation of naive and MB cells results in the loss of prebound BAFF. Furthermore, the absence of prebound BAFF on GC B cells is not related to a lack of BAFF-binding receptors or an inability to bind exogenous BAFF. Instead, our data suggest that accessibility to soluble BAFF is limited within GCs, perhaps to prevent skewing of the conventional B cell differentiation program. In support of this concept, whereas BAFF significantly enhances ISC differentiation in response to T cell-dependent activation, we report for the first time the ability of BAFF to considerably attenuate ISC differentiation of MB cells in response to CpG stimulation, a form of T cell-independent activation. Our data suggest that BAFF may be providing regulatory signals during specific T cell-independent events, which protect the balance between MB cells and ISCs outside GCs. Taken together, these data define a complex role for BAFF in humoral immune responses and show for the first time that BAFF can also play an inhibitory role in B cell differentiation.

Characterization of a phenotypically unique population of CD13+ dendritic cells resident in the spleen

Immature dendritic cells (DCs) resident in bovine spleens represent a distinct CD11a(+) CD11c(+) CD13(+) CD172(+) CD205(+) population compared to those circulating in peripheral blood or trafficking via afferent lymph. Upon cytokine-induced maturation, splenic DCs both efficiently present antigen in the stimulation of allogeneic lymphocyte proliferation and recall antigen-specific responses.

Anti-CD23

CD23, the low-affinity immunoglobulin (Ig)E receptor (FcepsilonRII), is widely distributed on the surface of various human cells. CD23 mediates numerous IgE-related immune responses (including allergen focusing) by enhancing IgE antigen complex presentation, regulating IgE synthesis, influencing cell differentiation and growth of both B- and T-cells, and stimulating production of pro-inflammatory mediators from monocytes/macrophages, eosinophils, and even airway smooth muscle cells. Both membrane and soluble CD23 play an important role in allergic reactions. Cellular contacts and cytokines modulate its expression in a concerted manner as needed for allergic reactions. Expression of CD23 and soluble CD23 has been associated with allergic diseases. Targeting CD23 with monoclonal antibody (MAb) is a promising candidate therapy in allergic diseases. A newly developed agent known as Lumiliximab, which is an anti-CD23 MAb (Lumiliximab), was demonstrated to be a well-tolerated agent in a phase I clinical trial (a placebo-controlled study with allergic asthma). Adverse events were mild, and no relationship was apparent between the dose of Lumilixilab and the frequency, severity, or type of event. Sustained and dose-dependent decreases in mean serum total IgE concentrations were noted. The serum half-life of Lumilixilab increased from 2 to 10 d with increasing doses. Blocking antigen presentation, preventing costimulation signals, and reducing production of pro-inflammatory mediators are some of the potential mechanisms involved for anti-CD23 activity. Although the safety and clinical efficacy of Lumilixilab in allergic asthma and rhinitis require confirmation, the observed data imply that anti-CD23 is a promising candidate therapy option for future treatment of allergic diseases.

A role for c-fos/activator protein 1 in B lymphocyte terminal differentiation

Expression of B lymphocyte-induced maturation protein 1 (Blimp-1) transcription factor is essential for promoting B cell differentiation into plasma cells. However, a critical transcription factor for Blimp-1 expression in activated B cells is unclear. When splenic B cells were stimulated with CD40 ligand (CD40L) and IL-4, terminal differentiation was induced in the B cells from c-fos transgenic (H2-c-fos) mice but barely in those from control littermates and from c-fos-deficient mice. AP-1 family and Blimp-1 mRNAs were transiently induced in the control B cells, and overexpression of c-Fos induced a sufficient amount of Blimp-1 for terminal differentiation in the H2-c-fos B cells. When normal and c-fos-deficient B cells were stimulated with LPS, a sufficient amount of Blimp-1 for terminal differentiation was induced in those B cells. However, expression of c-fos/AP-1 family mRNAs in LPS-stimulated normal B cells was similar to that of normal B cells stimulated with CD40L and IL-4. EMSA and chromatin immunoprecipitation assays using the AP-1-binding DNA sequence in the murine Blimp-1 promoter region demonstrated that AP-1-binding activity in nuclear protein of LPS-stimulated normal B cells was prolonged more than that in normal B cells stimulated with CD40L and IL-4. Furthermore, the percentage of CD138(+) B cells within germinal center B cells in the spleen and the number of Ab-forming cells in the bone marrow of H2-c-fos mice was larger than that of control mice 12 days after immunization. Thus, although c-Fos is not essential for Blimp-1 expression, c-Fos/AP-1 positively regulates Blimp-1 expression and terminal differentiation of activated B cells.

Ets-1 deficiency leads to altered B cell differentiation, hyperresponsiveness to TLR9 and autoimmune disease

It has been shown that mice with a targeted mutation in the Ets-1 gene exhibit increased B cell terminal differentiation to IgM-secreting plasma cells. Here, we show that mice, formerly described to lack Ets-1 protein, actually express low levels of an internally deleted Ets-1 protein. Mice harboring this Ets-1 hypomorphic allele possess very few marginal zone B cells and have increased expression of activation markers on follicular B cells. Adoptive transfer experiments indicate that this activated phenotype can be reversed upon transfer of Ets-1-deficient B cells to a wild-type host, suggesting a role for B cell-extrinsic factors in regulating the activated state. Supporting this observation, the reverse transfer experiment of wild-type B cells into an Ets-1-deficient host resulted in increased expression of activation markers on the transferred B cells. However, there are also cell-intrinsic changes in Ets-1-deficient B cells as demonstrated by their increased differentiation to plasma cells in vitro in response to stimulation with cytosine-phosphate-guanine DNA sequence-containing oligodeoxynucleotide [CpG DNA, a Toll-like receptor (TLR) 9 ligand]. Consistent with the activated phenotype and increased terminal differentiation of Ets-1-deficient B cells, Ets-1 mutant mice develop autoimmune disease. Hence, our studies establish Ets-1 as an important regulator of peripheral B cell differentiation and B cell responses to TLR9 activation.

Loss of CD154 impairs the Th2 extrafollicular plasma cell response but not early T cell proliferation and interleukin-4 induction

Ligation of CD40 by CD4 T cells through CD154 is key both to germinal centre induction and follicular T-dependent Ig class switching, but its requirement for aspects of T cell priming and extrafollicular antibody responses is less clear. Here comparison of the T helper (Th) type 2 response in lymph nodes from wild-type mice and CD154-deficient mice after immunization with alum-precipitated antigen reveals selective effects of this immunodeficiency. The timing and magnitude of the early interleukin (IL)-4 induction and proliferation in T cells of the T zone were unaltered by CD154 deficiency. As expected, germinal centres were not induced. Additionally the T-dependent extrafollicular antibody response, which initially requires T cell help but expands without further T cell involvement, was severely curtailed. The median number of extrafollicular antigen-specific plasma cells was 370-fold lower in CD154-deficient mice. Of these plasma cells the median proportion that had switched to IgG1 was <5%, while in wild-type mice the proportion was 89%. Surprisingly, some CD154-deficient lymph nodes showed substantial switching to IgG1. Commensurately, increases in gamma1 germline transcripts and Blimp-1 mRNA were observed, albeit significantly lower than in controls, but activation-induced cytidine deaminase mRNA was undetectable in CD154-deficient mice. These experiments demonstrate that the acquisition of some T cell priming characteristics can be CD154-independent; in contrast, T-dependent extrafollicular responses require CD154. Thus functional CD154 ligation during the first encounter of T cells and B cells in the T zone is critical for follicular and extrafollicular antibody responses.

The SH3-SAM adaptor HACS1 is up-regulated in B cell activation signaling cascades

HACS1 is a Src homology 3 and sterile alpha motif domain–containing adaptor that is preferentially expressed in normal hematopoietic tissues and malignancies including myeloid leukemia, lymphoma, and myeloma. Microarray data showed HACS1 expression is up-regulated in activated human B cells treated with interleukin (IL)-4, CD40L, and anti–immunoglobulin (Ig)M and clustered with genes involved in signaling, including TNF receptor–associated protein 1, signaling lymphocytic activation molecule, IL-6, and DEC205. Immunoblot analysis demonstrated that HACS1 is up-regulated by IL-4, IL-13, anti-IgM, and anti-CD40 in human peripheral blood B cells. In murine spleen B cells, Hacs1 can also be up-regulated by lipopolysaccharide but not IL-13. Induction of Hacs1 by IL-4 is dependent on Stat6 signaling and can also be impaired by inhibitors of phosphatidylinositol 3-kinase, protein kinase C, and nuclear factor κB. HACS1 associates with tyrosine-phosphorylated proteins after B cell activation and binds in vitro to the inhibitory molecule paired Ig-like receptor B. Overexpression of HACS1 in murine spleen B cells resulted in a down-regulation of the activation marker CD23 and enhancement of CD138 expression, IgM secretion, and Xbp-1 expression. Knock down of HACS1 in a human B lymphoma cell line by small interfering ribonucleic acid did not significantly change IL-4–stimulated B cell proliferation. Our study demonstrates that HACS1 is up-regulated by B cell activation signals and is a participant in B cell activation and differentiation.

Preventing restimulation of memory B cells in hemophilia A: a potential new strategy for the treatment of antibody-dependent immune disorders

Memory B cells are responsible for the rapidly emerging antibody response after antigen reexposure. The signals required for the restimulation of memory B cells have not been fully explained. We used a murine model of anti–factor VIII (FVIII) antibody responses in hemophilia A to study the requirements for the restimulation of FVIII-specific memory B cells and their differentiation into anti-FVIII antibody-producing cells. We were particularly interested in the significance of activated T cells and costimulatory interactions. Our results indicate that the restimulation of FVIII-specific memory B cells is strictly dependent on interactions with activated T cells. These activated T cells can be specific for either FVIII or third-party antigens. Restimulation by T cells specific for third-party antigens requires the presence of FVIII, indicating that signals induced by B-cell receptor (BCR) triggering and by interactions with activated T cells are important. The blockade of B7-1 or B7-2 as well as the blockade of CD40L inhibits the restimulation and differentiation of FVIII-specific memory B cells in vitro and in vivo. The interference with inducible costimulator–inducible costimulator ligand (ICOS-ICOSL) interactions, however, does not cause any modulation. As expected, the production of anti-FVIII antibodies by plasma cells is not dependent on any of the costimulatory interactions tested.

Evidence from the generation of immunoglobulin G–secreting cells that stochastic mechanisms regulate lymphocyte differentiation

Naive B lymphocytes undergo isotype switching and develop into immunoglobulin-secreting cells to generate the appropriate class and amount of antibody necessary for effective immunity. Although this seems complex, we report here that the generation of immunoglobulin G-secreting cells from naive precursors is highly predictable. The probabilities of isotype switching and development into secreting cells change with successive cell divisions and interleave independently. Cytokines alter the probability of each differentiation event, while leaving intact their independent assortment. As a result, cellular heterogeneity arises automatically as the cells divide. Stochastic division-linked regulation of heterogeneity challenges the conventional paradigms linking distinct phenotypes to unique combinations of signals and has the potential to simplify our concept of immune complexity considerably.

The biological outcome of CD40 signaling is dependent on the duration of CD40 ligand expression: reciprocal regulation by interleukin (IL)-4 and IL-12

CD40 ligand (CD154) expression on activated T cells can be separated into an early TCR-dependent phase, which occurs between 0 and 24 h after activation, and a later extended phase, which occurs after 24 h and is reciprocally regulated by the cytokines IL-4 and IL-12. IL-4 represses, whereas IL-12 sustains CD154 expression. Consistent with this, Th1, but not Th2, cells express CD154 for extended periods. Differences in the duration of CD154 expression have important biological consequences because sustained, but not transient, expression of CD154 on activated T cells can prevent B cell terminal differentiation. Thus, the differential ability of Th cells to sustain CD154 expression is an important part of their helper function and should influence the activities of other CD40-expressing cell types.

Cross-linking surface Ig delays CD40 ligand- and IL-4-induced B cell Ig class switching and reveals evidence for independent regulation of B cell proliferation and differentiation

T cells stimulate B cells to divide and differentiate by providing activating signals in the form of inducible membrane-bound molecules and secreted cytokines. Provision of these signals in vitro reproduces many of the consequences of T-B collaboration in the absence of any form of Ag stimulation. Although clearly not obligatory, Ag signals appear to play an important regulatory role in numerous aspects of the B cell response. To examine directly the effect of an Ag signal, naive B cells were stimulated in the presence of rCD40 ligand, with or without IL-4 in the presence or absence of different anti-Ig mAbs. Anti-Ig mAbs exerted variable effects on the B cell division rate, from enhancement to no effect to inhibition. In contrast, all anti-Ig mAbs tested inhibited division-linked isotype switching to IgG1 and IgE. Thus, B cell Ag receptor ligands could modify the rates of B cell expansion and class switching independently. The ability of anti-Ig reagents to modify class switching suggests the B cell Ag receptor may play an important role in the selection of Ig isotypes during T cell-dependent humoral immune responses to Ags of different physical structure.

Short-circuiting long-lived humoral immunity by the heightened engagement of CD40

Agonistic alpha CD40 Ab's have been shown to be potent immune adjuvants for both cell- and humoral-mediated immunity. While enhancing short-lived humoral immunity, the administration of a CD40 agonist during thymus-dependent immune responses ablates germinal center formation, prematurely terminates the humoral immune response, blocks the generation of B cell memory, and prevents the generation of long-lived bone marrow plasma cells. Interestingly, some of these effects of heightened CD40 engagement could be mimicked by enhancing the magnitude of antigen-specific T cell help. Taken together, these studies demonstrate that as the magnitude of CD40 signaling intensifies, the fate of antigen-reactive B cells can be dramatically altered. These are the first studies to describe the multifaceted function of CD40 in determining the fate of antigen-reactive B cells and provide novel insights into how CD40 agonists can short-circuit humoral immunity.

A factor of inducing IgE from a filarial parasite is an agonist of human CD40

Immune responses to parasitic helminth are usually characterized by quite mysterious phenomena: dominance of Th2-like immunity and antigen-nonspecific IgE secretion. We previously purified a factor from Dirofilaria immitis that induces antigen-nonspecific IgE in rats and named it DiAg. In the presence of IL-4, DiAg induces mouse B cells to secrete IgE, which is antigen-nonspecific polyclonal antibody. We investigated the biochemical characteristics of DiAg as a factor of inducing IgE in this study. Recombinant DiAg (rDiAg) with interleukin (IL)-4 induced IgE synthesis in highly purified human normal B cells in vitro cell culture systems. The addition of recombinant human soluble CD40 IgG fusion protein (rsCD40-Ig) inhibited induction of IgE synthesis by rDiAg with IL-4. Monocyte cells were stimulated with rDiAg and recombinant human soluble CD40L (rsCD40L); IL-12 and TNF-alpha were induced. The addition of rsCD40-Ig to THP-1 cells activated with rDiAg and rsCD40L inhibited the production of IL-12. rDiAg bound to the monocyte cell membrane fraction and recombinant human soluble CD40; this binding of rDiAg was competitively inhibited by addition of rsCD40L. Moreover, in CD40-deficient mice, IgE production and MLN-B cell proliferation by rDiAg were completely absent. Based on these results, we concluded that DiAg is an agonist of CD40.

Tubular lesions and tubular cell adhesion molecules for the prognosis of lupus nephritis

BACKGROUND

To assess the prognostic value of tubular lesions and cell adhesion molecules' expression, a retrospective study with immunohistochemistry was performed on 152 patients presenting lupus nephritis from January 1985 to December 1999.

METHODS

The following clinical parameters were recorded: age, sex, race, time of systemic lupus erythematosus (SLE) diagnosis, time of the biopsy, proteinuria, creatininemia, and renal function at the end of follow-up. All biopsies were re-evaluated according to a tubular grading, an inflammatory grading, the percentage of sclerosed glomeruli, the percentage of crescents, and the current WHO classification. Immunohistochemistry was performed with anti-CD40, anti-CD44, and anti-intercellular adhesion molecule-1 (anti-ICAM-1) antibodies.

RESULTS

Patients were 136 women (89.5%) and 16 men with a mean age of 31.2 years +/- 12.8 at the time of biopsy. The mean follow-up period was 94.3 months +/- 64.1. Eighty-eight biopsies (58%) showed various degrees of tubular atrophy. Males (P = 0.001) and tubular grading (P = 0.0001) were linked with renal survival in univariate and multivariate analysis. CD40 (P = 0.01) and ICAM-1 (P = 0.001) tubular expressions were linked with renal survival. ICAM-1 tubular expression provided additional information for the prognosis of the patients with biopsies showing tubular atrophy (P = 0.005) or not (P = 0.05).

CONCLUSIONS

Our study shows that tubular lesions are good indicator of lupus nephritis outcome. Furthermore, tubular expression of cell adhesion molecules like ICAM-1 and CD40 also serves to predict the outcome.

Analysis of the CD40 and CD28 pathways on alloimmune responses by CD4+ T cells in vivo

BACKGROUND

Blockade of the CD40 and CD28 pathways is a powerful strategy to inhibit CD4-mediated alloimmune responses. In this study, we examine the relative roles of the CD40 and CD28 pathways on CD4-mediated allograft rejection responses, and further characterize the role of these pathways on CD4+ T-cell activation, priming for cytokine production, and cell proliferation in response to alloantigen in vivo.

METHODS

BALB/c skin allografts were transplanted onto C57BL/6 Rag 1-/- recipients reconstituted with CD4 cells from CD28-/- or CD40L-/- donors. The popliteal lymph node assay was used to study the role of these pathways on CD4-cell activation and priming in vivo. To investigate the role of CD40 and CD28 blockade on CD4-cell proliferation, the fluorescein dye carboxyfluorescein diacetate succinimidyl ester was used. We performed heterotopic cardiac transplantation using CD40-/- mice to evaluate the role of CD40 on donor versus recipient cells in CD4-mediated rejection.

RESULTS

B6 Rag 1-/- recipients reconstituted with CD28-/- CD4+ T cells acutely rejected allografts (median survival time 15 days), whereas recipients reconstituted with CD40L-/- CD4+ T cells had significantly prolonged survival of BALB/c skin grafts (MST 71 days). CD40L blockade was equivalent to or inferior to CD28 blockade in inhibition of in vivo CD4-cell activation, priming for cytokine production, and proliferation responses to alloantigen. BALB/c recipients depleted of CD8 cells promptly rejected donor B6 CD40-/- cardiac allografts, whereas B6 CD40-/- recipients depleted of CD8 cells had significantly prolonged survival of BALB/c wild-type cardiac allografts.

CONCLUSIONS

The CD40/CD40L pathway, but not the CD28/B7 pathway, is critical for CD4-mediated rejection responses, however, the responsible mechanisms remain unclear.

Differential expression of cannabinoid CB(2) receptor mRNA in mouse immune cell subpopulations and following B cell stimulation

Cannabinoid CB(2) receptor is reported to be expressed in varying amounts in different human immune subpopulations. To examine the expression pattern of CB(2) in the mouse, immune cell subpopulations were purified and studied by semiquantitative Reverse Transcription-Polymerase Chain Reaction (RT-PCR). CB(2) mRNA was most abundant in splenic B cells, followed by macrophages and T cells. Furthermore, CB(2) was expressed in thioglycollate-elicited peritoneal macrophages, but not in resident peritoneal macrophages. In addition to these studies on receptor expression at basal activity, CB(2) mRNA expression was also studied following immune cell activation. Bacterial lipopolysaccharide stimulation downregulated CB(2) mRNA expression in splenocyte cultures in a dose-response manner, while stimulation through cluster of differentiation 40 (CD40) using anti-CD40 antibody upregulated the response and costimulation with interleukin-4 attenuated the anti-CD40 response. These results demonstrate that CB(2) mRNA expression differs among mouse immune subpopulations similar to what is observed in human immune cells. Furthermore, the results suggest that the signaling pathways activated by lipopolysaccharide and anti-CD40 might have different effects on CB(2) mRNA expression.

B cells activated via CD40 and IL-4 undergo a division burst but require continued stimulation to maintain division, survival and differentiation

T cell stimulation of B cell proliferation during T-B collaboration requires membrane-bound stimulatory ligands, such as CD40 ligand and the secretion of soluble cytokines, such as IL-4. Nevertheless, it remains unclear whether T cell contact is required to provoke each consecutive B cell division, or whether B cells divide in a T cell-free burst following the initial stimulation. To test this, naive B cells were cultured with anti-CD40 monoclonal antibody (mAb) and IL-4 and, after various times, these stimuli were removed and the cells re-cultured with or without further stimulation. Following stimulus removal, B cells were able to continue proliferating, with the size of the B cell burst dependent on the strength of the initial anti-CD40 mAb stimulus. Furthermore, in the absence of activating signals from anti-CD40 and/or IL-4, re-cultured B cells died rapidly. In addition, B cells undergoing a stimulus-free division burst could switch to IgG1. Thus, maximal B cell proliferation, differentiation and survival may require continued, although not necessarily consecutive, cognate interactions with T cells. These results suggest that antigen persistence and T cell help are necessary to sustain B cell proliferation and differentiation in vivo.

Murine B1 B cells require IL-5 for optimal T cell-dependent activation

T helper cell-driven activation of murine B cells has been shown to depend upon CD40-CD40 ligand (CD40L) interactions and a defined set of cytokines. These observations are primarily based on the use of conventional B cells obtained from the spleen. Therefore, it is presently unclear whether all mature B cell subsets found in the mouse have an equal dependence upon CD40-CD40L interactions and use the same T cell-derived cytokines. The present study tested the response of splenic follicular and marginal zone as well as peritoneal B2 and B1 B cells to Th cell stimulation. Splenic and peritoneal B cell subsets were sort purified based on CD23 expression, and cultured with rCD40L and cytokines or Th2 cells. The results demonstrate that follicular, marginal zone, and peritoneal B2 B cells require CD40-CD40L interactions and preferentially use IL-4 for optimal proliferation, differentiation, and isotype switching. In contrast, peritoneal B1 B cells use IL-5 in conjunction with CD40-CD40L interactions for maximal Th cell-dependent responses. Furthermore, B1 B cells are capable of proliferating, differentiating, and isotype switching in the absence of CD40-CD40L interactions. B1 B cells are able to respond to Th2 clones in the presence of anti-CD40L mAb as well as to Th2 clones derived from CD40L(-/-) mice. The CD40-CD40L-independent response of B1 B cells is attributable to the presence of both IL-4 and IL-5, and may explain the residual Ab response to T cell-dependent Ags in CD40L- or CD40-deficient mice, and in X-linked hyper-IgM (X-HIM) patients.

Commitment of B lymphocytes to a plasma cell fate is associated with Blimp-1 expression in vivo

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that is sufficient to trigger terminal differentiation in the B cell lymphoma BCL-1. In this study, we have determined the expression pattern of Blimp-1 in vivo in primary and secondary lymphoid organs of humans and immunized mice. Blimp-1 is expressed in plasma cells derived from either a T-independent or T-dependent response in plasma cells that have undergone isotype switching and those resulting from secondary immunization. Blimp-1 is also present in long-lived plasma cells residing in the bone marrow. However, Blimp-1 was not detected in memory B cells. This expression pattern provides further evidence of a critical role for Blimp-1 in plasma cell development, supporting earlier studies in cultured lines. Significantly, Blimp-1 was also found in a fraction (4-15%) of germinal center B cells in murine spleen and human tonsils. Blimp-1 expression in the germinal center is associated with an interesting subset of cells with a phenotype intermediate between germinal center B cells and plasma cells. In the mouse, Blimp-1(+) germinal center B cells peak at day 12 postimmunization and disappear soon thereafter. They are not apoptotic, some are proliferating, they express germinal center markers peanut agglutinin or CD10 but not Bcl-6, and most express CD138 (syndecan-1), IRF4, and cytoplasmic Ig. Together, these data support a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.

Lymphocytes lacking I kappa B-alpha develop normally, but have selective defects in proliferation and function

NF-kappaB has been implicated in the development, activation, and function of B and T lymphocytes. We have evaluated the in vivo effects of deletion of IkappaB-alpha, a major inhibitor of NF-kappaB, on lymphocyte development, proliferation, and function. To elucidate the long term role of IkappaB-alpha in lymphocytes, fetal liver cells of 14.5-day-old IkappaB-alpha(-/-) or wild-type embryos were transplanted into irradiated recombinase-activating gene-2-deficient mice. Within 4 wk, the IkappaB-alpha(-/-) fetal liver cells reconstitute mature B and T cell populations in the recipients comparable to those produced by wild-type fetal liver cells. However, the proliferative responses of IkappaB-alpha(-/-) B cells are enhanced, whereas those of IkappaB-alpha(-/-) T cells are reduced. The levels of IgG1, IgG2a, IgA, and IgE produced by IkappaB-alpha(-/-) B cells are elevated relative to those produced by IkappaB-alpha(+/+) or IkappaB-alpha(+/-). Moreover, the specific immune responses to OVA and the generation of germinal centers are impaired in recipients of IkappaB-alpha(-/-) fetal liver cells. These results indicate that IkappaB-alpha plays a vital role in signal transduction pathways regulating lymphocyte proliferation and also in the production of specific Ig isotypes.

The role of CD40L in T cell-dependent nitric oxide production by murine macrophages

Upon activation, T cells express CD40L, a member of the TNF cytokine superfamily, which serves as a ligand for CD40 on antigen presenting cells, including dendritic cells, B cells, and macrophages. While initial studies on the function of CD40 focused its role in the regulation of B cell activation, more recent studies have indicated that CD40 ligation may be critical for the initiation of T cell-dependent macrophage activation, including stimulation of nitric oxide production. However, the relative contribution of the CD40 pathway in macrophage nitric oxide production during T-dependent immune responses remains unclear. We have found that while CD40 ligation of macrophages can stimulate nitric oxide production, disruption of CD40 signaling during a T cell-mediated alloimmune response has no appreciable effect on nitric oxide production. If the T cell alloimmune response is restricted to CD4 cells, CD40L blockade has only a minimal effect on nitric oxide production. Rather, IFNgamma, produced by alloactivated T cells, seems to be a necessary 'first' signal for nitric oxide production, while TNFalpha and CD40L each provide independent 'second' signals. Finally, we demonstrate that CD40L stimulates macrophage NO production independent from autocrine TNFalpha stimulation. These results suggest that macrophage nitric oxide production during a T-dependent immune response requires IFNgamma production by CD4 cells whereas TNFalpha and CD40L can each provide important functionally overlapping 'second' signals to costimulate nitric oxide production, though neither is required.

The lymphoid-specific cofactor OBF-1 is essential for the expression of a V(H) promoter/HS1,2 enhancer-linked transgene in late B cell development

Mice deficient for the lymphoid-specific cofactor OBF-1 display reduced levels of IgG, IgA and IgE. To examine whether the lowered immunoglobulin expression is linked to reduced activity of IgH cis-regulatory elements, OBF-1(-/-) mice were crossed with mice expressing transgenes driven by a V(H) or beta-globin promoter linked to the HS1,2 enhancer. Here we show that OBF-1 is essential for the induced expression of a V(H) promoter-linked transgene, in contrast to a beta-globin promoter-dependent transgene, in LPS/IL-4 or CD40-stimulated splenic B cells. Furthermore, impaired transgene expression is observed in OBF-1(-/-) peritoneal B cells. This deficiency may be linked to OBF-1, as peritoneal cells from normal mice express OBF-1 protein constitutively. Our data link OBF-1 to IgH gene expression in late B lymphoid development.

Switching to IgG3, IgG2b, and IgA Is Division Linked and Independent, Revealing a Stochastic Framework for Describing Differentiation

LPS was used to induce switching of B cells to IgG3 and, in the presence of TGF-β, to IgG2b and IgA. Switching to all three isotypes increased with division number according to a consistent relationship that was independent of time in culture. The mode of activation altered the relationship with division, as CD40 ligand increased switching to IgA and decreased switching to IgG2b and IgG3 when measured per division. This division-linked switching behavior could be described by Gaussian probability distributions centered around a mean division number. The divisions at which switching to IgG3 and IgG2b occurred overlapped, raising the possibility that the two switching mechanisms were linked. However, when IgG3+ and IgG3− B cells were sorted and placed back in culture, they switched to IgG2b at an equivalent rate, indicating that alternative switching decisions were made independently within a single cell. As a consequence, isotype switching could be predicted at the population level by standard probability laws. Therefore, division number provides a framework for a stochastic description of differentiation that may be widely applicable.

Recombinant human CD40 ligand inhibits simian immunodeficiency virus replication: a role for interleukin- 16

CD40 ligand (CD40L), expressed on activated T cells, binds its receptor, CD40, on dendritic cells, B cells, and monocytes/ macrophages. Human immunodeficiency virus (HIV)-infected individuals exhibit normal B-cell CD40 expression but diminished expression of CD40L on CD4 + T cells. Thus, we studied recombinant human CD40L (huCD40L) in an in vitro rhesus macaque model of acquired immunodeficiency syndrome (AIDS). huCD40L induced peripheral blood mononuclear cell (PBMC) proliferation independent of mitogenic cytokines and led to a 70% reduction in p27 production by simian immunodeficiency virus (SIV) mac239 infected PBMCs (P < 0.05). Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed reduced expression of SIV gag and increased expression of interleukin (IL)-16 mRNA. Supernatants from huCD40L-stimulated PBMC and control cultures contained similar amounts of IL-16, suggesting an intracellular antiviral effect by IL-16. Phytohemagglutinin (PHA)-stimulated PBMCs similarly cultured with huCD40L showed only slight increases in chemokine production (P > 0.05). These results suggest that huCD40L inhibits replication (antigen and mRNA production) of SIVmac239. This response involves huCD40L induction of IL16 mRNA expression and appears to be independent of beta-chemokines.

P, Martínez-M. JA, Morales-A. S, Copín SG, Marcos MAR, Gaspar ML. Differential Involvement of the Transcription Factor Blimp-1 in T Cell-Independent and -Dependent B Cell Differentiation to Plasma Cells

Along humoral immune responses, different stimuli drive the differentiation of B lymphocytes to Ig-secreting plasma cells in discrete microenvironments. The Blimp-1 transcription factor is up-regulated early during the transition of mature B cells to IgM-secreting plasma cells. In the present study, we have examined the requirement of Blimp-1 in plasma cell formation after both T cell-independent (LPS) and -dependent (CD40 + IL-4, Th cell lines) stimulation of spleen B cells. B lymphocyte-induced maturation protein (Blimp-1) was expressed early after in vitro LPS stimulation, mainly in a population of IgM+Syndecan+CD43+ preplasma cells. In contrast, the BSAP transcription factor expressed in mature B cells was down-regulated during the differentiation to plasma cells. Treatment of these cultures with Blimp-1-specific antisense phosphorothioate oligonucleotides suppressed both Blimp-1 protein levels and the emergence of IgM+Syndecan+ cells and plasma cells. However, T-B cell cocultures of spleen B cells from C3H/HeJ (H-2k) mice and syngeneic autoreactive SR.10 Th2 cells submitted to the anti-Blimp-1 therapy did not show any significant reduction in IgM- and IgG1-secreting plasma cell formation. Spleen B cells treated with anti-CD40 mAb + IL-4 differentiated to IgG1-secreting cells without significant transcription of the Blimp-1 gene; anti-Blimp-1 treatment subsequently did not have any effect in the later cultures. Altogether, these results suggest that Blimp-1 transcription factor specifically promotes T cell-independent B cell differentiation to plasma cells, probably at preplasma cell stages. In contrast, T cell-dependent plasma cell formation likely evolves through Blimp-1-independent pathways.

In vitro system for immunoglobulin class switching using BHK cells transfected with murine recombinant CD40 ligand

To develop an in vitro system for mouse immunoglobulin (Ig) class switching, the expression vector of murine CD40 ligand (CD40L) which is expressed on T cells was transfected to BHK cells. By using culture plates coated with the BHK cells expressing the recombinant CD40L, Ig class switching of splenic B cells was examined. The CD40L mRNA was cloned from splenic T cells of BALB/mice activated with anti-CD3 antibody in vitro. As the No.593 base in the open reading frame sequence of the CD40L from BALB/c spleen differed from T to G, when compared with the known sequence from C57BL/6, one of the BALB/c-derived clones was reconstructed to the known CD40L by site-directed mutagenesis. Splenic B cells from BALB/c were induced secretion of Ig isotypes, IgM, IgG1 and IgE when cultured on two types of BHK cells, the transfected BHK cells with a CD40L clone from BALB/c and the transfected BHK cells with the reconstructed CD40L clone, in the presence of IL-4. However, when splenic B cells from C57BL/6 were cultured on the same systems, the B cells produced Ig isotypes, IgM, IgG1, IgG2a, IgG2b, IgG3 and IgE. In the similar experiments using the transfected BHK cells with a empty vector and the normal BHK cells, none of B cells produced any Ig isotypes other than IgM. These results indicate that Ig class switching of murine B cells can be induced by using these two types of CD40L-expressing BHK cells in vitro.

TCR Vaccines Against T Cell Lymphoma: QS-21 and IL-12 Adjuvants Induce a Protective CD8+ T Cell Response

Tumor-specific TCR can serve as an effective target for active immunotherapy of T cell malignancies. Using the murine T cell tumor model C6VL, vaccination with C6VL TCR protected mice from a subsequent lethal dose of tumor cells. This study characterizes the immune mechanisms involved in the tumor protection, and the influence of immunologic adjuvants in inducing a protective immune response. Immune responses induced by TCR vaccines formulated with various adjuvants: QS-21, IL-12, SAF-1, CD40L, and GM-CSF were compared. QS-21, IL-12, and SAF-1 biased the humoral immune response toward Th1-type, reflected by the induction of IgG2a and IgG2b anti-C6VL TCR Abs. CD40L and GM-CSF exclusively produced IgG1 Abs, reflecting a Th2-type immune response. In our tumor model system, only vaccines containing adjuvants that induced a Th1-type immune response favored tumor protection. Furthermore, we demonstrated that CD8+ T cells were necessary and sufficient for tumor protection using anti-CD8 mAb depletion and adoptive cell transfer experiments. Transfer of hyperimmune serum containing anti-C6VL TCR Abs into naïve mice had modest anti-tumor effects and was not sufficient to prevent tumor growth. TCR-vaccinated B cell-deficient mice were not protected against C6VL tumor, and tumor protection was not completely restored after hyperimmune serum transfer. Thus, B cells may serve as important APCs in inducing a protective immune response. Based on these results future TCR vaccines should be designed to maintain native TCR conformation, as well as induce a strong Th1-type immune response.

Interaction of B cells with activated T cells reduces the threshold for CD40-mediated B cell activation

CD154-CD40 interactions are of central importance for the induction of antibody responses to T-dependent antigens. Since most anti-CD40 mAb are only weak B cell mitogens, it is believed that under physiological conditions, signals through CD40 synergize with those from other receptors on B cells to induce B cell activation. We show here that the interaction of either normal B cells, or those from CBA/N (xid) mice, with CD3-activated primary T cells in whole spleen cell cultures markedly reduces the threshold for B cell activation via CD40. Hence, these pre-activated cells undergo vigorous proliferation when stimulated with either optimal or suboptimal concentrations of weakly mitogenic anti-CD40 mAb, or with soluble CD40 ligand. Blocking experiments indicate that the establishment of this priming effect requires stimulation via CD40 itself, plus T cell-derived IL-2. In support of this concept, only CD3/CD28-pre-activated, but not CD3-pre-activated T cells induce this effect, unless the co-cultures of B cells with the latter T cells are supplemented with IL-2. Although B cells activated in this fashion do express higher levels of CD40 than naive cells, we believe that this is insufficient to explain the observed dramatic effects on their proliferative capacity. Rather we propose that T cell-dependent B cell activation induces fundamental changes in the signalling machinery invoked by ligation of CD40. It is likely that this amplification loop could play an important role during the initiation of antibody responses to T-dependent antigens, when activated CD4 T cells only express low levels of CD154.

CD23 and allergic pulmonary inflammation: potential role as an inhibitor

CD23, a receptor for immunoglobulin E, is expressed at increased levels in asthmatic and atopic individuals and has been associated with disorders characterized by chronic inflammation. Using an established murine model, we employed several complementary strategies to investigate the role of CD23 in allergic pulmonary inflammation and airway hyperresponsiveness (AHR). Specifically, these approaches included the modulation of CD23 function in vivo by administration of anti-CD23 monoclonal antibody (mAb) or Fab fragments to wild-type mice and the analysis of CD23-deficient mice. Administration of anti-CD23 mAb, but not anti-CD23 Fab fragments, produced attenuation of pulmonary inflammation, AHR, and CD8(+) T-cell activation. On the basis of a model that the anti-CD23 mAb transduces, whereas the Fab fragment inhibits, CD23 signaling, these results suggest that CD23 negatively regulates pulmonary inflammation and AHR. This hypothesis is supported by our observation that CD23-deficient mice developed increased inflammation and AHR after sensitization and challenge with allergen. Together, these results indicate that CD23 negatively regulates pulmonary inflammation and airway hyperreactivity.

Differential requirements for CD28 and CD40 ligand in the induction of experimental autoimmune myasthenia gravis

The interactions of CD28-B7 and CD40-CD40 ligand (CD40L) pathways in T cell costimulation and autoimmune disease are incompletely understood. We sought to address this issue by investigation of the genesis of acetylcholine receptor (AChR)-induced antibody-mediated experimental autoimmune myasthenia gravis (EAMG) in CD28- and CD40L-deficient mice (CD28-/-, CD40L-/-). Compared to wild-type mice, the CD28-/- mice became less susceptible, and CD40L-/- mice were completely resistant to EAMG induction. Analysis of T helper functions, reflected by cytokine responses, revealed a switch to a Th1 profile in CD28-/- mice. Consistently, levels of serum AChR-specific antibodies of the IgG1 isotype were decreased in CD28-/- mice. In the CD40L-/- mice, both Th1 and Th2 cytokine responses were diminished, and T cell-dependent AChR-reactive B cell responses were more severely impaired than in the CD28-/- mice. Thus, CD28 and CD40L are differentially required for induction of EAMG.

Evidence for a Critical Role for IL-2 in CD40-Mediated Activation of Naive B Cells by Primary CD4 T Cells

The interaction of CD40 on B cells with the CD40 ligand (CD40L) on preactivated CD4 T cells is critical for the initiation of T-dependent Ab responses. It is believed that signals via CD40 synergize with cytokines (e.g., IL-4 and IL-5) to drive B cell activation. However, primary T cells preactivated via CD3 alone cannot induce B cell proliferation; we have shown previously that costimulation of T cells via CD3 and CD28 stabilizes the expression of the CD40L, which we propose contributes to their capacity to act as competent helper-effector cells. Here we show that an additional, critical reason why CD3-stimulated CD40L-bearing T cells are incompetent helper cells is because they secrete insufficient IL-2. In contrast, CD28/CD3-activated T cells induce B cells to become IL-2 responsive via a combination of CD40L and IL-2-mediated signals, and these two stimuli subsequently drive B cell proliferation and IgM secretion. We therefore propose that T cells must first encounter Ag in conjunction with CD80/86 on APCs. This leads to the stable expression of CD40L and maximal secretion of IL-2, which together render primary T cells competent to activate B cells in an IL-2-dependent fashion.