Antibodies to murine CD40 stimulate normal B lymphocytes but inhibit proliferation of B lymphoma cells

Harnessing the potential of CD40 agonism in cancer therapy

CD40 is a member of the tumor necrosis factor (TNF) receptor superfamily of receptors expressed on a variety of cell types. The CD40-CD40L interaction gives rise to many immune events, including the licensing of dendritic cells to activate CD8+ effector T cells, as well as the facilitation of B cell activation, proliferation, and differentiation. In malignant cells, the expression of CD40 varies among cancer types, mediating cellular proliferation, apoptosis, survival and the secretion of cytokines and chemokines. Agonistic human anti-CD40 antibodies are emerging as an option for cancer treatment, and early-phase clinical trials explored its monotherapy or combination with radiotherapy, chemotherapy, immune checkpoint blockade, and other immunomodulatory approaches. In this review, we present the current understanding of the mechanism of action for CD40, along with results from the clinical development of agonistic human CD40 antibodies in cancer treatment (selicrelumab, CDX-1140, APX005M, mitazalimab, 2141-V11, SEA-CD40, LVGN7409, and bispecific antibodies). This review also examines the safety profile of CD40 agonists in both preclinical and clinical settings, highlighting optimized dosage levels, potential adverse effects, and strategies to mitigate them.

Molecular Cytogenetic Characterization Identified the Murine B-Cell Lymphoma Cell Line A-20 as a Model for Sporadic Burkitt's Lymphoma

Here, we report the first molecular cytogenetic characterization of the BALB/cAnN mouse derived B-cell non-Hodgkin lymphoma (B-cell NHL) cell lines A-20. Even though previously used as a model for testing of, for example, dexametason, up to present, no data in the genetic properties of A-20 were available. The present study closed this gap and provides evidence that A-20 is a model for B-cell NHL subgroup sporadic Burkitt's lymphoma. C-myc oncogene is involved in a translocation and copy number alterations as gain of murine 14q material could be observed. Interestingly, the cell line showed the karyotype 39,X,-X or -Y,t(2;15)(qE5;qD2),del(6)(qB3qC3),del(9)(qA3qA4),dup(14)(qE1qE4) in ~95% of the cells, being exceptionally stable for cell lines being established 38 years ago. Still, ~5% of the cells showed polyploidization followed by chromothripsis. It remains to be determined if this can be observed also in other cell lines, just has not been reported yet, and/or if it is a unique feature of A-20. Overall, finally here, the necessary genetic data to identify A-20 as a model for human sporadic Burkitt's lymphoma are provided.

Design of CD40 agonists and their use in growing B cells for cancer immunotherapy

CD40 stimulation has produced impressive results in early-stage clinical trials of patients with cancer. Further progress will be facilitated by a better understanding of how the CD40 receptor becomes activated and the subsequent functions of CD40-stimulated immune cells. This review focuses on two aspects of this subject. The first is the recent recognition that signaling by CD40 is initiated when the receptors are induced to cluster within the membrane of responding cells. This requirement for CD40 clustering explains the stimulatory effects of certain anti-CD40 antibodies and the activity of many-trimer, but not one-trimer, forms of CD40 ligand (CD40L, CD154). The second topic is the use of these CD40 activators to expand B cells ("CD40-B cells"). As antigen-presenting cells (APCs), CD40-B cells are as effective as dendritic cells, with the important difference that CD40 B cells can be induced to proliferate in vitro, whereas DCs proliferate poorly if at all. As a result, the use of CD40-B cells as antigen-presenting cells (APCs) promises to streamline the generation of anti-tumor CD8(+) T cells for the adoptive cell therapy (ACT) of cancer.

Cross-linking of CD80 and CD86 Diminishes Expression of CD54 on EBV-transformed B Cells through Inactivation of RhoA and Ras

Background

Epstein Barr virus (EBV) infected B cells are transformed into lymphoblastoid cell lines. Some researchers suggested some a few similarities between this process and carcinogenesis. We observed the expression of CD80 and CD86, co-stimulatory molecules on EBV-transformed B cells and changes of CD54 expression after stimulation of CD80 and CD86.

Methods

CD80 and CD86 were stimulated using anti-CD80 and anti-CD86 monoclonal antibodies. To assess apoptosis and surface protein expression, flow cytometric analysis was performed. Intracellular signal molecules were evaluated by RT-PCR and immunoblot. Morphology and localization of proteins were examined using inverted or confocal microscope.

Results

Cross-linking of CD80 and CD86 induced apoptosis and interfered with proliferation of EBV-transformed B cells, and dispersion of clumped cells. We also examined that their stimulation induced ROS accumulation and reduced CD54 expression. Interestingly, we observed that CD80 and CD86 diminished the expression of CD54 in different methods. Both CD80 and CD86 down-regulated activation of focal adhesion kinase. CD80 stimulus inhibited CD54 expression through mainly RhoA inactivation, while CD86 down-regulated Ras and JNK phosphorylation.

Conclusion

These results suggest that co-stimulatory CD80 and CD86 molecules, expressed EBV-transformed B cells, may play a role in apoptosis and cell adhesion.

CD154-CD40 interactions in the control of murine B cell hematopoiesis

Interactions between CD40 and CD154 play a very important role in control of immune responses, including the delivery of T cell help to B cells and other APCs. Thus far, there has been no role postulated for CD40-CD154 interactions in hematopoiesis. We show here that CD40 is expressed on murine pro-B cells and that its ligation enhances pro-B cell proliferation in vitro and in vivo. In addition, CD154 mRNA is present in the BM. Moreover, we show that a deficiency in CD154 expression has effects on B cell hematopoiesis. Aged, CD154-deficient mice have significantly lower levels of B hematopoietic subsets downstream of pro-B cells in the BM. In addition, B lineage cells reconstitute more slowly following BMT into CD154-deficient recipients. We hypothesize that CD154 is expressed by radio-resistant cells in the BM and plays a role in fine-tuning B cell hematopoiesis.

Immunomodulation for inhibitors in hemophilia A: the important role of Treg cells

Approximately 25-30% of the hemophilia A patients develop inhibitory antibodies against Factor VIII (FVIII) following protein-replacement therapy. This problem is also thought to occur following gene-replacement therapy. Recently, many approaches have been investigated to modulate FVIII-specific immune responses in either protein-replacement or gene therapy hemophilia A mouse models. Several promising protocols have been demonstrated to successfully prevent or modulate the formation of anti-FVIII antibodies, including methods to manipulate antigen presentation, development of less immunogenic FVIII proteins, or formulations or gene therapy protocols to evade immune responses, as well as immunomodulation strategies to target either T- and/or B-cell responses. Most of these successful protocols involve the induction of activated Treg cells to create a regulatory immune environment during tolerance induction. Innovative strategies to overcome pre-existing anti-FVIII immune responses and induce long-term tolerance in primed subjects still need to be developed.

CD40mAb adjuvant induces a rapid antibody response that may be beneficial in post-exposure prophylaxis

Active vaccination can be effective as a post-exposure prophylaxis, but the rapidity of the immune response induced, relative to the incubation time of the pathogen, is critical. We show here that CD40mAb conjugated to antigen induces a more rapid specific antibody response than currently used immunological adjuvants, alum and monophosphoryl lipid A.

Therapeutic interventions targeting CD40L (CD154) and CD40: the opportunities and challenges

CD40 was originally identified as a receptor on B-cells that delivers contact-dependent T helper signals to B-cells through interaction with CD40 ligand (CD40L, CD154). The pivotal role played by CD40-CD40L interaction is illustrated by the defects in B-lineage cell development and the altered structures of secondary lymphoid tissues in patients and engineered mice deficient in CD40 or CD40L. CD40 signaling also provides critical functions in stimulating antigen presentation, priming of helper and cytotoxic T-cells and a variety of inflammatory reactions. As such, dysregulations in the CD40-CD40L costimulation pathway are prominently featured in human diseases ranging from inflammatory conditions to systemic autoimmunity and tissue-specific autoimmune diseases. Moreover, studies in CD40-expressing cancers have provided convincing evidence that the CD40-CD40L pathway regulates survival of neoplastic cells as well as presentation of tumor-associated antigens to the immune system. Extensive research has been devoted to explore CD40 and CD40L as drug targets. A number of anti-CD40L and anti-CD40 antibodies with diverse biological effects are in clinical development for treatment of cancer and autoimmune diseases. This chapter reviews the role of CD40-CD40L costimulation in disease pathogenesis, the characteristics of therapeutic agents targeting this pathway and status of their clinical development.

CTL-promoting effects of CD40 stimulation outweigh B cell-stimulatory effects resulting in B cell elimination and disease improvement in a murine model of lupus

CD40/CD40L signaling promotes both B cell and CTL responses in vivo, the latter being beneficial in tumor models. Because CTL may also limit autoreactive B cell expansion in lupus, we asked whether an agonist CD40 mAb would exacerbate lupus due to B cell stimulation or would improve lupus due to CTL promotion. These studies used an induced model of lupus, the parent-into-F1 model in which transfer of DBA/2 splenocytes into B6D2F1 mice induces chronic lupus-like graft-vs-host disease (GVHD). Although agonist CD40 mAb treatment of DBA-->F1 mice initially exacerbated B cell expansion, it also strongly promoted donor CD8 T cell engraftment and cytolytic activity such that by 10 days host B cells were eliminated consistent with an accelerated acute GVHD. CD40 stimulation bypassed the requirement for CD4 T cell help for CD8 CTL possibly by licensing dendritic cells (DC) as shown by the following: 1) greater initial activation of donor CD8 T cells, but not CD4 T cells; 2) earlier activation of host DC; 3) host DC expansion that was CD8 dependent and CD4 independent; and 4) induction of acute GVHD using CD4-depleted purified DBA CD8+ T cells. A single dose of CD40 mAb improved lupus-like renal disease at 12 wk, but may not suffice for longer periods consistent with a need for continuing CD8 CTL surveillance. These results demonstrate that in the setting of lupus-like CD4 T cell-driven B cell hyperactivity, CTL promotion is both feasible and beneficial and the CTL-promoting properties of CD40 stimulation outweigh the B cell-stimulatory properties.

Transient blockade of the inducible costimulator pathway generates long-term tolerance to factor VIII after nonviral gene transfer into hemophilia A mice

Formation of inhibitory antibodies is a common problem encountered in clinical treatment for hemophilia. Human factor VIII (hFVIII) plasmid gene therapy in hemophilia A mice also leads to strong humoral responses. We demonstrate that short-term therapy with an anti-ICOS monoclonal antibody to transiently block the inducible costimulator/inducible costimulator ligand (ICOS/ICOSL) signaling pathway led to sustained tolerance to hFVIII in hFVIII plasmid-treated hemophilia A mice and allowed persistent, high-level FVIII functional activity (100%-300% of normal). Anti-ICOS treatment resulted in depletion of ICOS(+)CD4(+) T cells and activation of CD25(+)Foxp3(+) Tregs in the peripheral blood, spleen, and lymph nodes. CD4(+) T cells from anti-ICOS-treated mice did not proliferate in response to hFVIII stimulation and produced high levels of regulatory cytokines, including interleukin-10 and transforming growth factor-beta. Moreover, CD4(+)CD25(+) Tregs from tolerized mice adoptively transferred dominant tolerance in syngeneic hFVIII plasmid-treated hemophilia A mice and reduced the production of antibodies against FVIII. Anti-ICOS-treated mice tolerized to hFVIII generated normal primary and secondary antibody responses after immunization with the T-dependent antigen, bacteriophage Phix 174, indicating maintenance of immune competency. Our data indicate that transient anti-ICOS monoclonal antibody treatment represents a novel single-agent immunomodulatory strategy to overcome the immune responses against transgene product after gene therapy.

CD40-mediated enhancement of immune responses against three forms of influenza vaccine

There is potential for influenza A infections to cause massive morbidity and mortality. Vaccination may be the primary defence against pandemic influenza, and potential pandemic'flu vaccines may be produced conventionally, in embryonated eggs, or as recombinant protein or synthetic peptide vaccines. However the vaccines are produced, the supply may be limiting, and it will be important to enhance the immunogenicity of the vaccines as much as possible. We have shown that conjugation to CD40 binding antibody is a very efficient way of enhancing immune responses against model antigens, but were interested in assessing the effectiveness of this system using influenza vaccines. We produced conjugates of CD40 monoclonal antibody (mAb) and isotype control with three potential influenza vaccines: a peptide-based vaccine containing T- and B-cell epitopes from virus haemagglutinin; a whole, killed virus vaccine; and a commercially produced split virus vaccine. CD40 mAb conjugates in each case were more immunogenic, but the adjuvant effect of CD40 conjugation was greatest with the split vaccine, where antibody responses were enhanced by several hundred-fold after a single immunization, and lymphocyte proliferation in response to antigen in vitro was also strongly enhanced.

BAFF and LPS cooperate to induce B cells to become susceptible to CD95/Fas-mediated cell death

Microorganisms with pathogen-associated molecular patterns (PAMP) activate B cells directly by binding to TLR and also indirectly by inducing APC to release cytokines such as BAFF that promote B cell survival. We found that murine B cells activated concomitantly with LPS (TLR-4 ligand) and BAFF are protected from spontaneous apoptosis, but are more susceptible to Fas/CD95-mediated cell death. This increased susceptibility to Fas-induced apoptosis is associated with a dramatic coordinated up-regulation of Fas/CD95 and IRF-4 expression through a mechanism mediated, at least in part, by inhibition of the MEK/ERK pathway. Up-regulation of Fas/CD95 by BAFF is restricted to B cells activated through TLR-4, but not through TLR-9, BCR or CD40. TLR ligands differ in the BAFF family receptors (R) they induce on B cells: BAFF-R is increased by the TLR4 ligand, LPS, but not by the TLR9 ligand, CpG-containing oligodeoxynucleotides, which, in contrast, strongly up-regulates transmembrane activator and CAML interactor (TACI). This suggests the up-regulation of Fas by BAFF is mediated by BAFF-R and not by TACI. Consistently, APRIL, which binds to TACI and B cell maturation antigen but not BAFF-R, did not enhance Fas expression on LPS-activated B cells. Increased susceptibility to Fas-mediated killing of B cells activated with LPS and BAFF may be a fail-safe mechanism to avoid overexpansion of nonspecific or autoreactive B cells.

The Sjögren’s Syndrome-Associated Autoantigen Ro52 Is an E3 Ligase That Regulates Proliferation and Cell Death

Patients affected by Sjögren's syndrome and systemic lupus erythematosus (SLE) carry autoantibodies to an intracellular protein denoted Ro52. Although the serologic presence of Ro52 autoantibodies is used clinically for diagnostic purposes, the function of the protein or why it is targeted as an autoantigen in several rheumatic conditions has not been elucidated. In this study, we show that the expression of Ro52 is significantly increased in PBMC of patients with Sjögren's syndrome and SLE, and demonstrate that Ro52 is a RING-dependent E3 ligase involved in ubiquitination. Overexpression of Ro52, but not of Ro52 lacking the RING domain, in a mouse B cell line lead to decreased growth in steady state and increased cell death after activation via the CD40 pathway. The role of Ro52 in activation-mediated cell death was further confirmed as a reduction in Ro52 expression restored cell viability. These findings suggest that the increased expression of the Ro52 autoantigen in patients may be directly involved in the reduced cellular proliferation and increased apoptotic cell death observed in Sjögren's syndrome and SLE, and may thus contribute to the autoantigenic load and induction of autoimmune B and T cell responses observed in rheumatic patients.

Preclinical antilymphoma activity of a humanized anti-CD40 monoclonal antibody, SGN-40

SGN-40 is a humanized IgG1 antihuman CD40 that is currently in a phase I clinical trial for the treatment of multiple myeloma. As surface CD40 expression on B-lineage cells is maintained from pro-B cells to plasma cells, SGN-40 may be applicable to treatment of other B-cell neoplasias, including non-Hodgkin's lymphoma. In this study, we examined potential in vitro and in vivo anti-B-lineage lymphoma activity of SGN-40. Recombinant SGN-40 was expressed and purified from Chinese hamster ovary cells and characterized based on binding affinity, specificity, and normal B-cell stimulation. The ability of SGN-40 to target neoplastic B cells was examined in vitro by proliferation inhibition, cytotoxicity, and antibody-dependent cell cytotoxicity assays and in vivo by human lymphoma xenograft models. Recombinant SGN-40 showed high affinity, Kd of approximately 1 nmol/L, and specific binding to CD40. Whereas SGN-40 was a weak agonist in stimulating normal B-cell proliferation in the absence of IL-4 and CD40L, it delivered potent proliferation inhibitory and apoptotic signals to, and mediated antibody-dependent cytotoxicity against, a panel of high-grade B-lymphoma lines. These in vitro antilymphoma effects were extended to disseminated and s.c. xenograft CD40 tumor models. In these xenograft models, the antitumor activity of SGN-40 was comparable with that of rituximab. The preclinical in vitro and in vivo antilymphoma activity of SGN-40 observed in this study provides a rationale for the clinical testing of SGN-40 in the treatment of CD40+ B-lineage lymphomas.

Conservation of biological properties of the CD40 ligand, CD154 in a non-mammalian vertebrate

Signals delivered by the CD40 ligand, CD154, have crucial roles in immune responses in mammals, being required for development of germinal centres, maturation of T-dependent antibody responses, and generation of B-cell memory. To determine whether these functions were conserved in a non-mammalian species, a putative chicken CD 154 cDNA was used to make an oligomeric fusion protein, and to raise monoclonal antibodies. The antibodies detected surface expression on activated T-cells. The fusion protein detected expression of a receptor on B-cells, thrombocytes and macrophages. Biological effects of the fusion protein included induction of NO synthesis in a macrophage cell line, enhancement of splenic B-cell survival, and induction of apoptosis in a bursal lymphoma cell line. These observations demonstrated substantial functional equivalence with mammalian CD 154 and thus provided evidence for the early evolutionary emergence of the set of functions associated with this molecule, and its central role in the vertebrate immune system.

Activation-induced cell death of aggressive histology lymphomas by CD40 stimulation: induction of bax

CD40 is present on both normal and neoplastic B-lineage cells. CD40 stimulation of normal B cells has been shown to promote normal growth and differentiation, whereas aggressive histology B lymphomas are growth inhibited. The inhibition of neoplastic B-cell growth is believed to occur via activation-induced cell death in which stimuli that typically promote the growth of normal cells prevent the growth of their neoplastic counterparts. We show here that CD40 stimulation using either a soluble recombinant human CD40 ligand (srhCD40L) or anti-CD40 monoclonal antibody resulted in apoptosis of human Burkitt lymphoma cell lines. Additional studies examining the mechanism of CD40-mediated death revealed an increase in bax messenger RNA with a subsequent increase in Bax protein in the mitochondria of the treated cells. In vitro exposure of the cells to bax antisense oligonucleotides resulted in a significant decline in Bax protein levels and partial protection from CD40-mediated death, indicating that induction of Bax was at least one mechanism underlying this inhibitory effect of CD40 stimulation on lymphomas. When immunodeficient mice bearing Burkitt lymphoma were treated with srhCD40L, significant increases in survival were observed indicating a direct antitumor effect as a result of CD40 stimulation in vivo. Overall, these results demonstrate that CD40 ligation of aggressive histology B-lymphoma cells results in inhibition both in vitro and in vivo and thus may be of potential clinical use in their treatment.

Population depletion activates autonomous CD154-dependent survival in biopsylike Burkitt lymphoma cells

Population size is governed through cells reacting to a variety of intrinsic and extrinsic cues. Tumors, while liberated from many of the homeostatic constraints placed on physiologic counterparts, can nonetheless remain subject to both social and environmental control. Burkitt lymphoma cells faithful to the biopsy phenotype were used to model the reliance of the colony, if any, on an inbuilt population sensor. Below a normally suicidal threshold number of cells, low picomolar quantities of exogenous CD40 ligand (CD40L/CD154) were found to sustain the clone without the discernible shift in phenotype that accompanies high CD40L encounter. Although CD154 was undetectable in populous cultures, message was induced as numbers became limiting. Correspondingly, attempts to neutralize endogenous CD40L activity failed to perturb cells at optimal densities but resulted in their marked decline as the critical threshold was approached. These data reveal an auto-inducible survival mechanism seemingly regulated through the monitoring of population size, a process somewhat akin to that of "quorum sensing" among gram-negative bacteria in which diffusible molecules provide a means of communication to coordinate gene expression with population density. This process could be activated as cells discern depletions in their community or when deprived of signals otherwise furnished within an appropriate environmental niche.

Distinct role of CD80 and CD86 in the regulation of the activation of B cell and B cell lymphoma

To date, not much has been known regarding the role of CD80 and CD86 molecules in signaling of B cells. The CD28/CTLA4 ligands, CD80 (B7-1) and CD86 (B7-2), are expressed on the surface of freshly isolated splenic B cells, and their expression is up-regulated by lipopolysaccharides. In the present study, we have investigated whether signaling via CD80/CD86 could alter the proliferation and immunoglobulin synthesis of B cells. Splenic B cells were stimulated with lipopolysaccharides in the presence of anti-B7-1 (16-10A1) and anti-B7-2 (GL1) monoclonal antibodies (mAbs). Exciting features observed during the study were that cross-linking of CD86 with GL1 enhanced the proliferation and production of IgG1 and IgG2a isotypes. In contrast, anti-B7-1 (16-10A1) mAb could efficiently block the proliferation and production of IgG1 and IgG2a. Furthermore, GL1 mAb could also induce the secretion of IgG isotypes from B cell lymphomas. Importantly, 16-10A1 could retard the growth of lymphomas and favored the up-regulation of pro-apoptotic molecules caspase-3, caspase-8, Fas, FasL, Bak, and Bax and down-regulation of anti-apoptotic molecule Bcl-x(L). In contrast, GL1 augmented the level of anti-apoptotic molecules Bcl-w and Bcl-x(L) and decreased the levels of pro-apoptotic molecule caspase-8, thereby providing a novel insight into the mechanism whereby triggering through CD80 and CD86 could deliver regulatory signals. Thus, this study is the first demonstration of a distinct signaling event induced by CD80 and CD86 molecules in B cell lymphoma. Finally, the significance of the finding is that CD80 provided negative signal for the proliferation and IgG secretion of normal B cells and B cell lymphomas. In contrast, CD86 encouraged the activity of B cells.

Does CD40 ligation induce B cell negative selection?

Binding of CD154 to its receptor, CD40, provides costimulation for mature B cell activation and differentiation in response to Ag receptor signals. In mice, early B cell precursors express CD40, but its function at this stage is unknown. We examined the effects of CD40 ligation during B cell ontogeny in transgenic mice constitutively expressing CD154 on B cells (kappaEP-CD154). Precursors beyond pro-B cells were absent in adult bone marrow but were increased in the fetal liver. Newborn kappaEP-CD154 mice had largely increased numbers of peripheral B cells, which were CD154+, and that 36 h after birth expressed high surface levels of CD23 and MHC class II, resembling activated mature B cells. Nevertheless, kappaEP-CD154 mice were hypogammaglobulinemic, indicating that the expanded population of apparently activated B cells was nonfunctional. Further analysis revealed that soon after birth, kappaEP-CD154 mice-derived B cells became CD5+/Fas+, after which progressively decreased in the periphery in a CD154-CD40-dependent manner. These results indicate that CD40 ligation during B cell ontogeny induces negative selection characterized by either hyporesponsiveness or an arrest in maturation depending on the time of analysis and the anatomic site studied.

Enforced and prolonged CD40 ligand expression triggers autoantibody production in vivo

CD40, a glycoprotein expressed on B lymphocytes plays an important role in B cell development, growth and differentiation. The ligand for the CD40 is a 39-kDa glycoprotein (CD154) expressed on the surface of activated T lymphocytes and is essential for thymus-dependent humoral immunity. The expression of CD154 is tightly regulated and its transient expression reduces the chances of potentially deleterious bystander activation of B cells. Stimulation through CD40 has been studied in vitro by using antibodies against CD40, by membranes of activated T cells or lately, by CD154 transfected cells. In this work we have evaluated the outcome of CD40-CD40 ligand interaction in vitro and in vivo by using CD154-transfected L929 cells. In vitro assays showed that CD154-L929 cells can induce on B cells: IL-4-dependent proliferation, up-regulation of CD23, CD54 and class II molecules and can also rescue WEHI-231 B cell lymphoma from anti-IgM-induced apoptosis. Interestingly, in vivo assays revealed that when CD154-L929 cells were inoculated into the spleen, mice developed a strong but transient production of anti-erythrocyte autoantibodies. Through B lymphocyte activation with CD154-transfected L929 cells both in vitro and in vivo, our data reveal that enforced and prolonged expression of CD40 ligand overcomes the tightly regulated mechanisms of B cell activation, triggering the production of autoantibodies. This system might be used to evaluate the early steps of an autoimmune response and the role of CD40-CD154 in the induction of primary responses in vivo.

Lack of CD40-dependent B-cell proliferation in B lymphocytes isolated from patients with persistent polyclonal B-cell lymphocytosis

Persistent B-cell lymphocytosis (PPBL) is a haematological disorder diagnosed primarily in adult female smokers that is characterized by a polyclonal increase in peripheral blood B lymphocytes and a moderate elevation of serum IgM. B lymphocyte-associated cellular abnormalities, such as the occurrence of multi-lobed nuclei, increased bcl2/Ig gene rearrangements and the identification of an extra long-arm chromosome (i3)(q10) in the B-cell population, indicate that PPBL could be part of a multi-step process leading to the emergence of a malignant B lymphoproliferation. However, the resulting impact on cellular functional properties remains to be elucidated. Our goal was to address that aspect via the study of B-cell activity following stimulation through CD40, a key molecule of the tumour necrosis factor receptor superfamily involved in B lymphocyte development. In contrast to normal B cells, PPBL B lymphocytes were unable to respond to the proliferative signal delivered in vitro by CD40, indicating a defect in the CD40 activation pathway. Polymerase chain reaction amplification and sequencing of the receptor as well as FACScan analysis of patient B lymphocytes dismissed the possibility of a defect in either CD40 structure or expression. Moreover, Western blot analysis of tyrosine phosphorylation, an early event in the CD40-signalling cascade, was similar in patients and controls, leading to the conclusion that the defect affecting B lymphocytes in PPBL patients is probably located downstream of that signalling cascade.

CD45 is required for CD40-induced inhibition of DNA synthesis and regulation of c-Jun NH2-terminal kinase and p38 in BAL-17 B cells

Stimulation of B cell antigen receptor (BCR) may induce proliferation, differentiation, or apoptosis, depending upon the maturational stage of the cell and the presence or absence of signals transmitted via coreceptors. One such signal is delivered via CD40; for instance, ligation of CD40 rescues B cells from BCR-induced apoptosis. Here we show that, in contrast to WEHI-231 cells, CD40 ligation did not reverse BCR-induced growth inhibition in the BAL-17 mature B cell line and CD40 ligation itself inhibited proliferation. This inhibitory signaling was not observed in CD45-deficient cells. Further analyses demonstrate that transfection of dominant-negative form of SEK1 or treatment with SB203580 strongly reduced CD40-induced inhibition of BAL-17 proliferation, suggesting a requirement for c-Jun NH2-terminal kinase and p38 in CD40-induced inhibition of proliferation. Interestingly, CD40-initiated activation of c-Jun NH2-terminal kinase and p38 was enhanced and sustained in CD45-deficient cells, and these phenotypes were reversed by transfecting CD45 gene. However, CD40-mediated induction of cell surface molecules was not affected in CD45-deficient cells. Taken collectively, these results suggest that CD45 exerts a decisive effect on selective sets of CD40-mediated signaling pathways, dictating B cell fate.

Role of CD40 in a T cell-mediated negative regulation of Ig production

To investigate the possible role of CD40 in a negative regulation of Ig production, we used the mouse Ig allotype suppression model. T splenocytes from IGH(a/a) mice are able in vivo to totally and chronically inhibit the production of IgG(2a)(b) (IgG2a from the IGH(b) haplotype). Accordingly, postnatal transfer of IGH(a/a) T splenocytes into histocompatible IGH(a/b) F(1) or congenic IGH(b/b) mice leads to a characteristic IgG(2a)(b) suppression. The helper action of anti-IgG(2a)(b) CD4(+) T cells is required for the recruitment of anti-IgG(2a)(b) CD8(+) T suppression effectors. The latter use perforin (pore-forming protein, Pfp)- and/or Fas-dependent cytotoxic pathways to continuously eliminate B cells recently committed to IgG(2a)(b) production. In the present study we first showed that in vivo agonistic anti-CD40 mAb treatment of IGH(a/a) mice, deprived of their CD4(+) T cell compartment, could bypass the help of Ig allotype-specific CD4(+) T cells and generate CD8(+) T effector cells able to strongly inhibit IgG(2a)(b) production. This result demonstrates the usefulness of CD40 triggering in setting up an immune regulatory mechanism. Furthermore, with regard to the suppression-effector mechanism, we demonstrated that B cell CD40 expression was required for full suppression establishment via the Fas-dependent pathway. Indeed, IGH(a/a) PFP(degrees/degrees) T cells (using exclusively the Fas pathway) induced full IgG(2a)(b) suppression against IGH(b/b) CD40(+/+) B cells, but only partial inhibition of IgG(2a)(b) production against IGH(b/b) CD40(degrees/degrees) B cells. This finding provides the first demonstration of direct involvement of B cell CD40 expression in in vivo negative control of an Ig production.

CD40 ligation alters the cell cycle of differentiating keratinocytes

CD40 is expressed in normal human keratinocytes, especially in the basal cell layer. We have recently reported that CD40 ligation strongly inhibits keratinocyte proliferation and induces their differentiation. In this study, the CD40 pathway that prevents keratinocyte growth was investigated. We first reported that interferon-gamma treatment potentiated the CD40-mediated inhibition of keratinocyte proliferation. CD40-CD40 ligand interactions, in the presence or absence of interferon-gamma, neither enhanced spontaneous keratinocyte apoptosis, nor did it enhance apoptosis induced by various agents. More importantly, we showed that CD40 signaling altered the keratinocyte cell cycle, as demonstrated by a decreasing number of cells in the G1 and S phases and an accumulation in G2/M phase of the cell cycle. Furthermore, western blot analysis of cell cycle regulatory proteins, showed a decrease in cyclin A and E expression in CD40-activated keratinocytes. Collectively, these results indicate that CD40 ligation inhibits keratinocyte renewal by a mechanism independent of cell apoptosis and that modulation of the keratinocyte cell cycle is an additional outcome of CD40 signaling.

Proliferative response of mantle cell lymphoma cells stimulated by CD40 ligation and IL-4

Mantle cell lymphoma (MCL) is a tumor of intermediate-size, IgM+, IgD+ B cells derived from the mantle zone of the germinal center. Little is known about its specific immunologic features or responsiveness to T cell-derived signals. In this work, we evaluated the proliferation and cell cycle properties of freshly isolated MCL cells after CD40 ligation, in the absence and presence of interleukin 4 (IL-4). In each MCL case examined, there was a marked growth-enhancing effect of these two stimuli characterized by improved viability, augmented expression of Ki-67, and induction of the proliferating cell nuclear antigen (PCNA). Cyclin D1 was expressed throughout the cell cycle in MCL cells induced to enter S phase. From these investigations, we conclude that the biology of MCL B lymphocytes is affected by CD154 (CD40 ligand) and IL-4, two signals usually provided by CD4+ T cells. The capacity to manipulate the activation and cell cycle state of MCL cells by these specific immunological stimuli may be exploited to confer susceptibility to chemotherapy agents and develop novel therapies in this disease.

CD40 signals apoptosis through FAN-regulated activation of the sphingomyelin-ceramide pathway

The possibility that the sphingomyelin (SM)-ceramide pathway is activated by CD40, a transmembrane glycoprotein belonging to the tumor necrosis factor receptor superfamily and that plays a critical role in the regulation of immune responses has been investigated. We demonstrate that incubation of Epstein-Barr virus-transformed lymphoid cells with an anti-CD40 antibody acting as an agonist results in the stimulation of a neutral sphingomyelinase, hydrolysis of cellular SM, and concomitant ceramide generation. In addition, SM degradation was observed in acid sphingomyelinase-deficient cells, as well as after ligation by soluble CD40 ligand. The anti-CD40 antibody, as well as the soluble CD40 ligand induced a decrease in thymidine incorporation and morphological features of apoptosis, which were mimicked by cell-permeant or bacterial sphingomyelinase-produced ceramides. Stable expression of a dominant-negative form of the FAN protein (factor associated with neutral sphingomyelinase activation), which has been reported to mediate tumor necrosis factor-induced activation of neutral sphingomyelinase, significantly inhibited CD40 ligand-induced sphingomyelinase stimulation and apoptosis of transformed human fibroblasts. Transformed fibroblasts from FAN knockout mice were also protected from CD40-mediated cell death. Finally, anti-CD40 antibodies were able to co-immunoprecipitate FAN in control fibroblasts but not in cells expressing the dominant-negative form of FAN, indicating interaction between CD40 and FAN. Altogether, these results strongly suggest that CD40 ligation can activate via FAN a neutral sphingomyelinase-mediated ceramide pathway that is involved in the cell growth inhibitory effects of CD40.

Differential Responses to CD40 Ligation Among Burkitt Lymphoma Lines That Are Uniformly Responsive to Epstein-Barr Virus Latent Membrane Protein 1

Ligation of CD40 on the surface of B cells induces multiple phenotypic effects, many of which are mimicked by the EBV latent membrane protein 1 (LMP1) through its interaction with downstream components of the CD40 signaling pathway. Because the effects of LMP1 have been most closely studied in human Burkitt Lymphoma (BL) cell lines retaining a tumor biopsy-like phenotype in vitro, we have examined the response of a panel of such lines to CD40 ligation. Two distinct patterns of response were observed that were unrelated to the surface level of CD40 or to the EBV genome status of the lines. Following exposure to either CD40-specific mAbs or the soluble trimeric ligand (sCD40L), high responder (HR) lines showed rapid aggregation, activation of NF-κB, up-regulation of cell surface markers ICAM-1/CD54 and Fas/CD95, and growth inhibition. Aggregation was seen at lower doses than those required to elicit the other effects. By contrast, low responder (LR) lines showed no detectable response to CD40 mAbs, while their responses to sCD40L were limited to activation of NF-κB and up-regulation of CD95 only. However, in transfection experiments, LMP1 uniformly induced the full spectrum of phenotypic effects in both HR and LR lines. We conclude that some BL cell lines show a highly restricted response to CD40 ligation but remain fully susceptible to LMP1.

Thymic lymphoproliferative disease after successful correction of CD40 ligand deficiency by gene transfer in mice

Inherited deficiency of the CD40 ligand (X-linked hyper-IgM syndrome) is characterized by failure of immunoglobulin isotype switching and severe defects of cell-mediated immunity. To test the potential for gene transfer therapy to correct this disorder, we transduced murine bone marrow or thymic cells with a retroviral vector containing the cDNA for the murine CD40 ligand (CD40L) and injected them into CD40L-/- mice. Even low-level, constitutive expression of the transgene stimulated humoral and cellular immune functions in these mice. With extended follow-up, however, 12 of 19 treated mice developed T-lymphoproliferative disorders, ranging from polyclonal increases of lymphoblasts to overt monoclonal T-lymphoblastic lymphomas that involved multiple organs. Our findings show that constitutive (rather than tightly regulated), low-level expression of CD40L can produce abnormal proliferative responses in developing T lymphocytes, apparently through aberrant interaction between CD40L+ and TCRalphabeta+CD40+ thymocytes. Current methods of gene therapy may prove inappropriate for disorders involving highly regulated genes in essential positions in proliferative cascades.

STAT6 Is Required for IL-4-Induced Germline Ig Gene Transcription and Switch Recombination

Transcription of the germline Cγ1 and Cε Ig genes is believed to be a necessary prerequisite for isotype switching to IgG1 and IgE, respectively. IL-4 stimulation and ligation of CD40 can each independently induce low level germline γ1 and ε transcription in murine B cells. Together these signals act synergistically to promote high level germline transcription and are normally required for T-dependent isotype switching to IgG1 and IgE. The STAT6 transcription factor has been suggested to play a critical role in IL-4-induced activation of germline Cγ1 and Cε genes. To directly assess the role of STAT6 in IL-4R- and CD40-mediated germline transcription and switching, we have analyzed these events in splenic B cells from STAT6-deficient mice. Our results demonstrate that IL-4 does not induce detectable levels of germline γ1 or ε transcripts in STAT6-deficient B cells. Germline transcript expression induced by CD40 stimulation alone is unaffected, but synergism between CD40- and IL-4R-mediated signals is completely ablated. Switch recombination to Sγ1, as measured by digestion-circularization PCR, is dramatically reduced in STAT6-deficient B cells stimulated with CD40 ligand plus IL-4. Similarly, germline γ1 transcript expression and switch recombination to Sγ1 are also impaired in STAT6-deficient B cells stimulated with IL-4, IL-5, and anti-IgD Abs conjugated to dextran, a model for T-independent type II responses. These results directly demonstrate a critical role for STAT6 in the IL-4-mediated activation of germline Ig gene transcription and switch recombination in nontransformed B cells.

Arrest of B lymphocyte terminal differentiation by CD40 signaling: mechanism for lack of antibody-secreting cells in germinal centers

Despite extensive research, the role of CD40 signaling in B cell terminal differentiation remains controversial. Here we show that CD40 engagement arrests B cell differentiation prior to plasma cell formation. This arrest is manifested at a molecular level as a reduction in mRNA levels of secretory immunoglobulin gene products such as mu(s) and J chain as well as the loss of the transcriptional regulator BLIMP-1. Furthermore, the inhibition of B cell differentiation by CD40 engagement could not be overcome by either mitogens or cytokines, but could be reversed by antibodies that interfere with the CD40/gp39 interaction. These data suggest that secretory immunoglobulin is not produced by B cells that are actively engaged by gp39-expressing T cells.

Detection of low-affinity adhesion ligands by linking recombinant cell adhesion molecules in uniform orientation to a fluorescently labelled dextran molecule by means of hexahistidine tagging: the case of multimeric CD40

Cell-cell interactions involve highly polyvalent associations between receptors on adjacent cells. In order to mimic this process, we have prepared a highly polyvalent form of CD40 attached to a dextran backbone. This was accomplished by engineering a hexahistidine tag on the C-terminus of the CD40 and binding, in a uniform orientation, up to 100 molecules of hexahistidine CD40 by metal chelation to a single fluorescently tagged dextran molecule. The advantage of this 'multimeric' CD40 is that it would be expected to bind to any counterstructure with a significantly higher avidity compared to monomeric CD40. The multimeric CD40 bound with high affinity to stably transfected mouse fibroblasts expressing CD40L. The multimeric ligand also bound to the activated T cell clone, D10, but did not bind to resting cells, showing that it bound to the physiological ligand. Using this system, we found no evidence to support the claim [Heath et al., 1993. Cell. Immunol. 152, 468.] that the A20 cells have a counterstructure for CD40, and propose that the high binding of CD40 observed in this study may have been due to an exposed hexahistidine tag on the molecule. This multimeric technology has considerable potential for detecting low-affinity interactions between cell adhesion receptors and ligands. The uniform orientation of the molecules on the dextran is an advantage over previous systems and permits the preparation of heterogeneous, multimeric ligands which more closely mimic the conditions at the cell surface.

CD40 ligation impedes lymphoblastoid B cell proliferation and S-phase entry

The CD40 cell surface antigen and member of the tumor necrosis factor (TNF) receptor superfamily is expressed in many cell types, including normal and neoplastic B cells. Signaling through CD40 induces B cell proliferation, differentiation and, in some circumstances, protects the B cell from apoptosis. Lymphoblastoid cells (LCLs) resemble the malignant B cells that comprise the Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disorders, in that the cells bear a highly activated phenotype and, unlike most other EBV positive tumor cells, express the majority of latent EBV genes. In this study, we use assays of cell viability, proliferation, cell cycle and apoptosis to demonstrate that ligation of the CD40 receptor in EBV-transformed LCLs inhibits their growth. The process does not involve apoptosis, but is characterized by reduced S-phase entry from G0/G1. A better understanding of the negative effects of CD40 ligation in these cells may offer clues for the development of novel therapies in EBV-related B cell disorders.

Role for CD40-mediated activation of c-Rel and maintenance of c-myc RNA levels in mitigating anti-IgM-induced growth arrest

CD40 crosslinking on B cells activates NF-kappaB and stress-activated protein kinase (SAPK) pathways. Since CD40 crosslinking rescues WEHI 231 B cells from anti-IgM-induced apoptosis, those pathways were likely candidates to be involved. Indeed, both signaling cascades predominated in anti-IgM-treated WEHI 231 cells, treated concurrently with anti-CD40 to rescue them from apoptosis. Crosslinking of CD40 activated the NF-kappaB proteins c-Rel and p50, but had no influence on their cytoplasmic steady state level. However, in contrast to-and even in the presence of-anti-IgM-mediated signals, engagement of CD40 resulted in a prolonged nuclear translocation of c-Rel, thereby allowing the formation of active NF-kappaB complexes. Consistent with this, the upstream regulatory element of the c-myc promoter, known to be regulated by NF-kappaB, was differently regulated after BCR ligation vs BCR plus CD40 crosslinking. The level of c-myc RNA was rapidly downregulated after BCR engagement, but persistent in the presence of CD40 signaling.

A function for CD2 on murine B cells?

CD2 is expressed on murine B cells, probably as a result of a chromosomal translocation event during speciation. There are no activating antibodies to mouse CD2, but when activated non-specifically, only T cells up-regulate their expression of CD2. We investigated the expression and function of CD2 on B cells using mice transgenic for human CD2 under the control of a modified version of the autologous promoter/enhancer that included the immunoglobulin enhancer. This construction directs expression of human CD2 to the B cell compartment as well as to the T cell compartment. In this paper, we confirm that activating pairs of anti-CD2 monoclonal antibodies are mitogenic for mouse T cells transgenic for human CD2. In contrast, mouse B cells that express similar amounts of human CD2 are not stimulated to proliferate by equivalent doses of these antibodies. We were also unable to show any functional consequence for these B cells as a result of CD2 ligation.

CD40/CD40 ligand interactions in normal, reactive and malignant lympho-hematopoietic tissues

CD40 is a 48 Kd integral membrane protein expressed by cells of B cells, origin, dentritic cells, monocytes, epithelial cells, endothelial cells and tumor cells including carcinomas, B cell lymphomas/leukemias and Hodgkin and Reed-Sternberg (HRS) cells of Hodgkin's disease (HD). CD40 has been clustered as a member of the nerve growth factor (NGF)/tumor necrosis factor (TNF) receptor superfamily with the corresponding counterstructure, the CD40 ligand (L) being mainly expressed by activated CD4+ T cells, but also some activated CD8+ T cells, basophils, eosinophils, mast cells and stromal cells. CD40L shares significant amino acid homology with TNF particularly in its extracellular domain ("TNF homology region") and is therefore viewed as a member of the TNF ligand superfamily. Binding of CD40L+ T cells to CD40+ B cells is thought to play a major role in T cell-dependent B cell activation, B cell proliferation, Ig isotype switching, memory B cell formation and rescue of B cells from apoptotic death in germinal centers. Mutations of the CD40L gene have been associated with the X-linked hyper-IgM immunodeficiency syndrome, pointing to the critical role of the CD40/CD40L interaction in the T cell-B cell interplay. Accordingly, expression of CD40 by human lympho-hematopoietic tumors has been shown in most of the B cell neoplasias, H-RS cells and HD and some carcinomas. In contrast, CD40L+ tumor cells are almost invariably restricted to CD4+/CD8- T cell lymphomas. Overall, functional CD40/CD40L interactions appear to be critical for cellular activation signals during immune responses and neoplastic tumor cell growth. The understanding of the biology of CD40L has improved our diagnostic and therapeutic repertoire in the management of several human diseases, including CD40+ tumors.

Vaccine strategies in non-Hodgkin's lymphomas

As described above, the most recent advances in anti-idiotype vaccination strategies have gone hand in hand with recent developments in molecular biology and other forms of cancer therapy. The techniques that are currently available in antibody engineering will greatly facilitate protein production and purification and will reduce the time and effort needed to produce the patient specific vaccines. Cytokine (gene) therapy has extensively been studied in cancer treatment and cancer vaccination and some therapeutic strategies are currently being evaluated in clinical trials (Bubenik, 1996). Combination therapy of idiotypic vaccination with cytokine therapy has recently been explored with promising results. The main focus so far has been on GM-CSF and IL-2, although other cytokines might prove to be efficient in stimulating different effector arms of the immune system. The nature of the immune response mounted by the host against the tumour and the mechanisms by which the tumour cells escape the effector functions of the immune system are not yet fully known. A better knowledge of the nature of B-cell lymphomas and the relation to the patient's immune system will therefore benefit the further development of the therapeutic strategies. Further research will provide us with a better view of how to break the immune tolerance and of which components of the immune system have to be targeted in order to obtain optimal therapeutic results.

Negative signaling in B cells by surface immunoglobulins

Cross-linking of surface immunoglobulins generates negative signals that cause B-cell death unless appropriate rescue signals are provided. Surface IgM is the main transducer of the negative signaling, but surface IgD and IgG may also transduce negative signaling when cross-linked intensively. In the surface IgM+, IgD+ human malignant B lymphoma cell lines B104 and DND-39, cross-linking of surface IgM by anti-IgM antibodies induced cell death. Anti-IgM antibody-induced B104 cell death was inhibited by stimulation with alpha- and beta-interferons but not stimulation with anti-CD40 antibody or IL-4, whereas anti-IgM antibody-induced DND-39 cell death was inhibited by stimulation with anti-CD40 antibody but not stimulation with alpha- and beta-interferons. Anti-IgM antibody-stimulated B104 cells had morphologic features compatible with necrosis, whereas anti-IgM antibody-stimulated DND-39 cells showed morphologic features of apoptosis. CD11a/CD54-dependent cell adhesion induced by stimulation with anti-CD40 antibody was involved in anti-CD40 antibody-mediated inhibition of anti-IgM antibody-induced DND-39 cells. In normal human mature B cells, cross-linking of surface IgM induced different signaling consequences, including DNA synthesis or cell division (positive signaling) or cell cycle arrest or death (negative signaling). In this system, too, CD40-transduced signal inhibited anti-IgM antibody-induced negative signaling, and CD11a/CD54-dependent cell adhesion played a role in the rescue process. It is suggested that quantitatively different intensities of surface IgM cross-linking induce qualitatively different signaling consequences; relatively weak cross-linking may induce DNA synthesis; moderate cross-linking may induce DNA synthesis with cell cycle arrest at the G2/M interphase; and intense cross-linking may induce apoptotic cell death. The reasons for this difference are not yet known. Further elucidation of the molecular mechanisms responsible for surface IgM-mediated negative signaling and its rescue signaling may contribute toward development of therapy for allergic disorders by artificial modulation of specific immunoglobulin production.

Increased inhibition of proliferation of human B cell lymphomas following ligation of CD40, and either CD19, CD20, CD95 or surface immunoglobulin

Non-Hodgkin's (NHL) B cell lymphomas are growth-inhibited by ligation of their CD40 molecules. This inhibition is not absolute in that approximately 50% of the cells are not inhibited. We conducted studies to see if other signals that have been reported to inhibit B cell lymphoma growth could be used in combination with anti-CD40 signaling to completely inhibit growth. Ligation of surface immunoglobulin (Ig), CD19, CD20, CD37 or CD95 with soluble antibody did not affect growth of the panel of NHL cells examined. Ligation of CD20, CD19 or CD95 was inhibitory for some NHL cell lines if the primary antibody was crosslinked with a secondary antibody. Combining anti-CD40 with anti-CD19, anti-CD20, or anti-Ig resulted in increased inhibition past that produced by anti-CD40 alone. The additive effect of anti-CD40 and other antibodies to selected surface markers was not observed in all NHL cell lines. Crosslinking of CD95 was also growth inhibitory for the majority of the NHL, and when combined with anti-CD40 under conditions that afforded crosslinking of the two receptors, increased inhibition was seen in three of the NHL cell lines. We found that cAMP or sodium butyrate (NaB) were also effective at inhibiting growth of the NHL cells; this was a profound inhibition (approaching 100%) compared to the 50% inhibition seen with anti-CD40 treatment. The potential for anti-CD40 and either cAMP or NaB to be additive was tested and not found to be the case. The ability to inhibit proliferation of the NHL was very dynamic with some antibody combinations being either inhibitory for multiple cells, not having an effect at all, or in some cases being stimulatory. This suggests that the NHL may represent unique stages of B cells that might serve as a model system which could be developed to precisely categorize patient NHL.

Molecular, structural, and biological characteristics of the tumor necrosis factor ligand superfamily

The tumor necrosis factor receptor superfamily at present consists of ten different transmembrane (type I) glycoproteins with characteristic limited sequence homology for the cysteine-rich repeats in the extracellular domain. In parallel the tumor necrosis factor ligand super-family has been recognized by discovery of ligands for all members of the receptor superfamily. These molecules are also transmembrane (type II) glycoproteins, with the exception of lymphotoxin-alpha which is the only entirely secreted protein of the tumor necrosis factor-like proteins. Several members of the ligand superfamily, including tumor necrosis factor and CD95L also exist in a biologically active soluble form. The tumor necrosis factor ligand superfamily contains at present ten different proteins. In addition, NGFR p75 binds to a second family of proteins (neurotrophins). These nerve growth factor-like dimeric soluble molecules are basic neurotrophic factors and the five members (NGF, BDNF, NT-3, NT-4, NT-5) are not related to the tumor necrosis factor superfamily ligands. The members of the tumor necrosis factor ligand superfamily (TNF, LT-alpha, LT-beta, CD27L, CD30L, CD40L, CD95L, 4-IBB, OX40L, TRAIL) share common biological activities, but some properties are shared by only some ligands, while others are unique. The diverse biological activities triggered through tumor necrosis factor receptors have been linked to the regulation of cellular activation, including immune responses and inflammatory reactions, but also with the pathology of a series of human diseases.

Tumor dormancy and cell signaling. II. Antibody as an agonist in inducing dormancy of a B cell lymphoma in SCID mice

Tumor dormancy can be induced in a murine B cell lymphoma (BCL1) by immunizing BALB/c mice with the tumor immunoglobulin (Ig) before tumor cell challenge. In this report, we have investigated the immunological and cellular mechanisms underlying the induction of dormancy. BCL1 tumor cells were injected into SCID mice passively immunized with antibody against different epitopes on IgM or IgD with or without idiotype (Id)-immune T lymphocytes. Results indicate that antibody to IgM is sufficient to induce a state of dormancy. Antibodies against other cell surface molecules including IgD and CD44 (Pgp1) had no effect on tumor growth. Id-immune T cells by themselves also had no effect on tumor growth in SCID mice. However, simultaneous transfer of anti-Id and Id-immune T cells enhanced both the induction and duration of the dormant state. In vitro studies indicated that antibody to IgM induced apoptosis within several hours and cell cycle arrest by 24 h. Hyper cross-linking increased apoptosis. The Fc gamma RII receptor played little or no role in the negative signaling. Antibodies that did not negatively signal in vitro did not induce dormancy in vivo. The results suggest that anti-IgM plays a decisive role in inducing tumor dormancy to BCL1 by acting as an agonist of IgM-mediated signal transduction pathways.

Involvement of CRAF1, a relative of TRAF, in CD40 signaling

CD40 is a receptor on the surface of B lymphocytes, the activation of which leads to B cell survival, growth, and differentiation. A yeast two-hybrid screen identified a gene, CRAF1, encoding a protein that interacts directly with the CD40 cytoplasmic tail through a region of similarity to the tumor necrosis factor-alpha (TNF-alpha) receptor-associated factors. Overexpression of a truncated CRAF1 gene inhibited CD40-mediated up-regulation of CD23. A region of CRAF1 was similar to the TNF-alpha receptor-associated factors TRAF1 and TRAF2 and so defined a shared TRAF-C domain that was necessary and sufficient for CD40 binding and homodimerization. The CRAF1 sequence also predicted a long amphipathic helix, a pattern of five zinc fingers, and a zinc ring finger. It is likely that other members of the TNF receptor superfamily use CRAF-related proteins in their signal transduction processes.

Protection against Fas-dependent Th1-mediated apoptosis by antigen receptor engagement in B cells

Cytotoxic CD4+ Th1-cells induce cell death by triggering a Fas-dependent apoptotic pathway. Potential targets include activated B cells, but it is not known whether the mode of B-cell stimulation influences susceptibility to Th1-mediated cytotoxicity. Here we report that CD40-ligand-stimulated B cells were extremely sensitive, whereas anti-IgM-stimulated B cells were resistant, to Fas-mediated apoptosis. B cells stimulated by both CD40L and anti-IgM were not susceptible to cytolysis, demonstrating that anti-IgM-mediated protection is an active, dominant process. Resistance to Th1-mediated cytotoxicity was similarly observed in CD40L-stimulated 3-83 (anti-H-2Kk,b) transgenic B cells co-cultured with H-2Kk or H-2Kb (but not H-2Kd) splenocytes. These results indicate that B cells can participate in regulating their own destruction. Protection against Fas-dependent apoptosis afforded by immunoglobulin-receptor engagement may constitute a fail-safe mechanism that eliminates bystander B cells activated by CD40L-expressing T cells, but ensures survival of antigen-specific B cells.

CD40 and its ligand: central players in B lymphocyte survival, growth, and differentiation

This article brings together information garnered over the last 10 years on the B cell-associated CD40 surface glycoprotein. The discovery in 1992 of a natural counterstructure, the CD40 ligand (CD40L), which can be rapidly induced on T-cells has hastened research and increased interest in this area dramatically. It has become clear that CD40 and its ligand are central players in B cell responses to T-dependent antigens. The capacity of CD40 to deliver signals for B cell growth, differentiation and survival is covered together with discussion of the signal transduction pathways which operate to bring about these changes. Future issues surrounding this important receptor-ligand pair are discussed.

The role of CD40 in the regulation of humoral and cell-mediated immunity

The dynamic and reciprocal communication between T helper (Th) cells and B cells appears to rely on the provision of multiple signals. The first is antigen specific and is mediated by the interaction between the T-cell receptor (TCR) and antigen bound to the major histocompatibility complex (MHC). The subsequent signals are provided by the binding of accessory molecules such as CD28 and CD40 to their respective ligands. Here, Fiona Durie and colleagues discuss the co-stimulatory role of the interaction between CD40 on B cells and CD40 ligand (CD40L, gp39) on T cells, and review evidence that suggests blocking this interaction may induce T-cell tolerance.