Identification of a source of biologically active CD40 ligand

A potential novel cancer immunotherapy: Agonistic anti-CD40 antibodies

CD40, a novel immunomodulatory cancer therapy target, is expressed by B cells, macrophages, and dendritic cells (DCs) and mediates cytotoxic T cell priming through the CD40 ligand. Some tumors show promising responses to monotherapy or combination therapy with agonistic anti-CD40 antibodies. The development of improved anti-CD40 antibodies makes CD40 activation an innovative strategy in cancer immunotherapy. In this review, we trace the history of CD40 research and summarize preclinical and clinical findings. We emphasize the ongoing development of improved anti-CD40 antibodies and explore strategies for effective combination therapies. Guided by predictive biomarkers, future research should identify patient populations benefiting the most from CD40 activation.

A tetrameric form of CD40 ligand with potent biological activities in both mouse and human primary B cells

CD40 ligand (CD40 L) expressed by activated T cells interacts with CD40 on B cells and triggers B cell survival, proliferation and differentiation. Deficiency in CD40 L or CD40 in humans causes hyper IgM syndrome due to a defect in T-B interaction that is essential for Ig gene class switch recombination (CSR). CD40 L belongs to the tumor necrosis factor family and normally forms a homotrimer on the cell surface, which is important for its biological activity. To generate a multimeric CD40 L that can be used to stimulate both mouse and human B cells, we fused the extracellular domain of mouse CD40 L, which is known to also bind human CD40, with streptavidin (SA) that forms a stable tetramer under physiological conditions. As expected, 293 T cells transiently transfected with an SA-CD40 L expression vector secreted tetrameric SA-CD40 L in the culture supernatant. The secreted SA-CD40 L exhibited > 25-fold stronger activities in inducing the survival, activation and proliferation of both mouse and human primary B cells than did an agonistic anti-mouse or anti-human CD40 antibody. In the presence of IL-4, SA-CD40 L also induced efficient CSR and plasma cell differentiation in both mouse and human B cells. Moreover, administration of SA-CD40 L in mice induced activation and proliferation of spleen B cells in vivo. These results demonstrate that the SA-CD40 L fusion protein generated in the present study recapitulates the function of membrane-bound trimeric CD40 L and has potent biological activities in both mouse and human primary B cells.

CD30/CD30 Ligand and CD40/CD40 Ligand in Malignant Lymphoid Disorders

CD30 and CD40 are members of the tumor necrosis factor (TNF) receptor family. These two receptors have pleiotropic biologic functions including induction of apoptosis and enhancing cell survival. This review will discuss the pattern of expression of these receptors in malignant lymphoid disorders and their prospective ligands. Understanding issues related to these two ligands and their receptors in lymphoid malignancies may help to improve the classification of these diseases and could open the doors for new treatment strategies.

CD40L and Its Receptors in Atherothrombosis-An Update

CD40L (CD154), a member of the tumor necrosis factor superfamily, is a co-stimulatory molecule that was first discovered on activated T cells. Beyond its fundamental role in adaptive immunity-ligation of CD40L to its receptor CD40 is a prerequisite for B cell activation and antibody production-evidence from more than two decades has expanded our understanding of CD40L as a powerful modulator of inflammatory pathways. Although inhibition of CD40L with neutralizing antibodies has induced life-threatening side effects in clinical trials, the discovery of cell-specific effects and novel receptors with distinct functional consequences has opened a new path for therapies that specifically target detrimental properties of CD40L. Here, we carefully evaluate the signaling network of CD40L by gene enrichment analysis and its cell-specific expression, and thoroughly discuss its role in cardiovascular pathologies with a specific emphasis on atherosclerotic and thrombotic disease.

Binding Studies of TNF Receptor Superfamily (TNFRSF) Receptors on Intact Cells

Ligands of the tumor necrosis factor superfamily (TNFSF) interact with members of the TNF receptor superfamily (TNFRSF). TNFSF ligand-TNFRSF receptor interactions have been intensively evaluated by many groups. The affinities of TNFSF ligand-TNFRSF receptor interactions are highly dependent on the oligomerization state of the receptor, and cellular factors (e.g. actin cytoskeleton and lipid rafts) influence the assembly of ligand-receptor complexes, too. Binding studies on TNFSF ligand-TNFRSF receptor interactions were typically performed using cell-free assays with recombinant fusion proteins that contain varying numbers of TNFRSF ectodomains. It is therefore not surprising that affinities determined for an individual TNFSF ligand-TNFRSF interaction differ sometimes by several orders of magnitude and often do not reflect the ligand activity observed in cellular assays. To overcome the intrinsic limitations of cell-free binding studies and usage of recombinant receptor domains, we performed comprehensive binding studies with Gaussia princeps luciferase TNFSF ligand fusion proteins for cell-bound TNFRSF members on intact cells at 37 °C. The affinities of the TNFSF ligand G. princeps luciferase-fusion proteins ranged between 0.01 and 19 nm and offer the currently most comprehensive and best suited panel of affinities for in silico studies of ligand-receptor systems of the TNF family.

The CD40-CD154 co-stimulation pathway mediates innate immune injury in adriamycin nephrosis

BACKGROUND

Blockade of CD40-CD40 ligand (CD154) interactions protects against renal injury in adriamycin nephropathy (AN) in immunocompetent mice. To investigate whether this protection relied on adaptive or cognate immunity, we tested the effect of CD40-CD154 blockade in severe combined immunodeficient (SCID) mice.

METHODS

SCID mice were divided into three groups: normal, AN + hamster IgG (ADR+IgG group) and AN + anti-CD154 antibody (MR1) (ADR+MR1 group). AN was induced by tail vein injection of 5.2 mg/kg of adriamycin (ADR). Hamster IgG (control Ab) or MR1 was administered intraperitoneally on days 5, 7, 9 and 11 after ADR injection. Histological and functional data were collected 4 weeks after ADR injection. In vitro experiments tested the effect of soluble and cell-bound CD154 co-cultured with CD40-expressing cells [macrophages, mesangial cells and renal tubular epithelial cells (RTEC)].

RESULTS

All experimental animals developed nephropathy. Compared to the ADR+IgG group, ADR+MR1 animals had significantly less histological injury (glomerulosclerosis and tubular atrophy) and functional injury (creatinine clearance). Kidneys of ADR+MR1 animals had significantly less macrophage infiltration than those of ADR+IgG animals. Interestingly, expression of CD40 and CD41 (a platelet-specific marker) was significantly less in ADR+MR1 animals compared to ADR+IgG animals. In vitro, CD154 blockade significantly attenuated upregulation of CCL2 gene expression by RTEC stimulated by activated macrophage-conditioned medium. In contrast, platelet-induced upregulation of macrophage and mesangial cell proinflammatory cytokine gene expression were not CD154-dependent.

CONCLUSION

CD40-CD154 blockade has a significant innate renoprotective effect in ADR nephrosis. This is potentially due to inhibition of macrophage-derived soluble CD154.

Anti-CD40 monoclonal antibody

Normal, bi-directional interactions between CD 40 and its natural ligand CD 154 (CD 40 ligand) are central to the generation of both T cell-dependent, humoral immune responses and cytotoxic T-cell responses. CD 40 is expressed on a broad range of hematological and epithelial malignancies. The development of monoclonal antibodies directed against CD 40 allows effective targeting of malignant cells through multiple mechanisms that include the recruitment of immune effector mechanisms such as complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity, direct anti-proliferative effects on neoplastic cells and, importantly, by the activation of tumor-targeted cellular cytotoxicity. This review provides the background to the early clinical trial data that are now beginning to emerge for this potentially exciting new treatment approach.

Exploration of factors affecting the onset and maturation course of follicular lymphoma through simulations of the germinal center

Genetic mutations frequently observed in human follicular lymphoma (FL) B-cells result in aberrant expression of the anti-apoptotic protein bcl-2 and surface immunoglobulins (Igs) which display one or more novel variable (V) region N-glycosylation motifs. In the present study, we develop a simulation model of the germinal center (GC) to explore how these mutations might influence the emergence and clonal expansion of key mutants which provoke FL development. The simulations employ a stochastic method for calculating the cellular dynamics, which incorporates actual IgV region sequences and a simplified hypermutation scheme. We first bring our simulations into agreement with experimental data for well-characterized normal and bcl-2(+) anti-hapten GC responses in mice to provide a model for understanding how bcl-2 expression leads to permissive selection and memory cell differentiation of weakly competitive B-cells. However, as bcl-2 expression in the GC alone is thought to be insufficient for FL development, we next monitor simulated IgV region mutations to determine the emergence times of key mutants displaying aberrant N-glycosylation motifs recurrently observed in human FL IgV regions. Simulations of 26 germline V(H) gene segments indicate that particular IgV regions have a dynamical selective advantage by virtue of the speed with which one or more of their key sites can generate N-glycosylation motifs upon hypermutation. Separate calculations attribute the high occurrence frequency of such IgV regions in FL to an ability to produce key mutants at a fast enough rate to overcome stochastic processes in the GC that hinder clonal expansion. Altogether, these simulations characterize three pathways for FL maturation through positively selected N-glycosylations, namely, via one of two key sites within germline V(H) region gene segments, or via a site in the third heavy chain complementarity-determining region (CDR-H3) that is generated from VDJ recombination.

CD40-CD40 ligand interactions in oxidative stress, inflammation and vascular disease

CD40 ligand (CD40L) and its receptor CD40 participate in numerous inflammatory pathways that contribute to multiple pathophysiological processes. A role for CD40-CD40L interactions has been identified in atherosclerosis, and such interactions are known to destabilize atherosclerotic plaques by inducing the expression of cytokines, chemokines, growth factors, matrix metalloproteinases and pro-coagulant factors. The CD40-CD40L interaction has also been implicated in immune system disorders. Recent studies have suggested that CD40-CD40L interactions regulate oxidative stress and affect various signaling pathways in both the immunological and cardiovascular systems. Here, we discuss the emerging role of CD40-CD40L-mediated processes in oxidative stress, inflammatory pathways and vascular diseases. Understanding the roles and regulation of CD40-CD40L-mediated oxidative signaling in immune and non-immune cells could facilitate the development of therapeutics targeting diverse inflammatory diseases.

CD40-CD40 ligand interactions in human microglia induce CXCL8 (interleukin-8) secretion by a mechanism dependent on activation of ERK1/2 and nuclear translocation of nuclear factor-kappaB (NFkappaB) and activator protein-1 (AP-1)

CXCL8 is a CXC chemokine that recruits leukocytes to sites of inflammation. Expression of CXCL8 in the CNS has been demonstrated in neuroinflammatory diseases, including human immunodeficiency virus (HIV-1) encephalitis, but the mechanism of secretion of this chemokine is not fully understood. CD40 is a 50-kDa protein on the surface of microglia, and we have previously shown that it is increased in expression in HIV-1-infected brain tissue as well as by interferon-gamma (IFNgamma) in tissue culture. We examined the expression and regulation of CXCL8 in cultured human fetal microglia after ligation of CD40 with soluble trimeric CD40 ligand (sCD40L) as well as the expression of CXCL8 on microglia in HIV encephalitic brain tissue sections. Treatment of cultured microglia with IFNgamma + sCD40L resulted in significant induction of CXCL8. This expression was mediated by activation of the ERK1/2 MAPK pathway, as demonstrated by ELISA and Western blot using a specific inhibitor (U0126). Gel shift analyses demonstrated that NFkappaB and AP-1, but not C/EBPbeta, mediate microglial CXCL8 production. We also found increased colocalization of CXCL8 with CD68/CD40-positive cells in HIV encephalitic brain tissue compared with HIV-infected nonencephalitic and normal tissue. Thus, CD40-CD40L interactions facilitate chemokine expression, leading to the influx of inflammatory cells into the CNS. These events can lead to the pathology that is associated with neuroinflammatory diseases.

Platelets as immune cells: bridging inflammation and cardiovascular disease

Beyond an eminent role in hemostasis and thrombosis, platelets are characterized by expert functions in assisting and modulating inflammatory reactions and immune responses. This is achieved by the regulated expression of adhesive and immune receptors on the platelet surface and by the release of a multitude of secretory products including inflammatory mediators and cytokines, which can mediate the interaction with leukocytes and enhance their recruitment. In addition, platelets are characterized by an enormous surface area and open canalicular system, which in concert with specialized recognition receptors may contribute to the engulfment of serum components, antigens, and pathogens. Platelet-dependent increases in leukocyte adhesion may not only account for an exacerbation of atherosclerosis, for arterial repair processes, but also for lymphocyte trafficking during adaptive immunity and host defense. This review compiles a selection of platelet-derived tools for bridging inflammation and vascular disease and highlights the molecular key components governing platelet-mediated mechanisms operative in immune surveillance, vascular remodeling, and atherosclerosis.

Potential use of CD40 ligand for immunotherapy of childhood B-cell precursor acute lymphoblastic leukaemia

Around 20% of children affected by B-cell precursor acute lymphoblastic leukaemia (BCP-ALL) still experience a recurrence of the disease after diagnosis, despite a significant improvement in the cure rate (80%). Moreover, standard therapies have high and often unacceptable acute and chronic organ toxicity, with an increased risk for secondary malignancies. Therefore, new strategies are needed to improve overall survival and decrease treatment-associated morbidity. Recent in-vitro and in-vivo studies have demonstrated that CD40 engagement improves tumour immunogenicity and, consequently, generates a strong antitumour immune response. The CD40-CD40 ligand (CD40L) system is of pivotal importance in the immune response via interactions between T cells and antigen-presenting cells. The general aim of this chapter is to review the feasibility of developing cellular strategies to increase childhood BCP-ALL immunogenicity, and the potential use of CD40L as a new strategy to induce an antileukaemia immune response in BCP-ALL.

Emerging applications of the tumor necrosis factor family of ligands and receptors in cancer therapy

Abnormalities of the tumor necrosis factor (TNF) family members have been linked to several human diseases, including cancer. Novel treatment strategies for cancer are emerging based on an understanding of the function of TNF family members. The advantage of these strategies is their potential to selectively target cancer cells, while sparing normal cells. Combining these new strategies with currently available treatments such as chemotherapy and radiation therapy is under investigation, with promising results. However, because some TNF family members are toxic to normal mammalian cells when administered systemically, only a few TNF family members have potential therapeutic value. This concise review focuses on the clinical implications of four TNF family members for cancer treatment: CD30/CD30 ligand, CD40/CD40 ligand, receptor activator of nuclear factor-kappaB (RANK)/RANK ligand, and TNF-related apoptosis-inducing ligand (TRAIL) Apo-2L/TRAIL receptors.

Prospects for CD40-directed experimental therapy of human cancer

CD40, a member of the tumor necrosis factor receptor (TNF-R) family, is a surface receptor best known for its capacity to initiate multifaceted activation signals in normal B cells and dendritic cells (DCs). CD40-related treatment approaches have been considered for the experimental therapy of human leukemias, lymphomas, and multiple myeloma, based on findings that CD40 binding by its natural ligand (CD40L), CD154, led to growth modulation of malignant B cells. Recent studies also exploited the selective expression of the CD40 receptor on human epithelial and mesenchymal tumors but not on most normal, nonproliferating epithelial tissues. Ligation of CD40 on human breast, ovarian, cervical, bladder, non small cell lung, and squamous epithelial carcinoma cells was found to produce a direct growth-inhibitory effect through cell cycle blockage and/or apoptotic induction with no overt side effects on their normal counterparts. CD154 treatment also heightened tumor rejection immune responses through DC activation, and by increasing tumor immunogenicity through up-regulation of costimulatory molecule expression and cytokine production of epithelial cancer cells. These immunopotentiating features can produce a "bystander effect" through which the CD40-negative tumor subset is eliminated by activated tumor-reactive cytotoxic T cells. However, the potential risk of systemic inflammation and autoimmune consequences remains a concern for systemic CD154-based experimental therapy. The promise of CD154 as a tumor therapeutic agent to directly modulate tumor cell growth, and indirectly activate antitumor immune response, may depend on selective and/or restricted CD154 expression within the tumor microenvironment. This may be achieved by inoculating cancer vaccines of autologous cancer cells that have been transduced ex vivo with CD154, as documented by recently clinical trials. This review summarizes recent findings on CD154 recombinant protein- and gene therapy-based tumor treatment approaches, and examines our understanding of the multifaceted molecular mechanisms of CD154-CD40 interactions.

CD40-CD40L interactions induce chemokine expression by human microglia: implications for human immunodeficiency virus encephalitis and multiple sclerosis

CD40 is a protein on microglia that is up-regulated with interferon (IFN)-gamma and is engaged by CD40L, found on CD4+ T cells, B cells, and monocytes. These interactions may be important in central nervous system inflammatory diseases. Microglia have been shown to be a source of chemokines, whose expression plays a key role in central nervous system pathologies. We examined the expression of CD40 on microglia in human immunodeficiency virus (HIV) encephalitic brain, and the effects of CD40-CD40L interactions on the expression of chemokines by cultured microglia. We found significantly increased numbers of CD40-positive microglia in HIV-infected brain tissue. Treatment of cultured microglia with IFN-gamma and CD40L increased expression of several chemokines. IFN-gamma- and CD40L-induced MCP-1 protein was mediated by activation of the ERK1/2 MAPK pathway, and Western blot analysis demonstrated phosphorylation of ERK1/2 upon stimulation of microglia. In contrast, IFN-gamma- and CD40L-induced IP-10 protein production was mediated by the p38 MAPK pathway. Our data suggest a mechanism whereby CD40L+ cells can induce microglia to secrete chemokines, amplifying inflammatory processes seen in HIV encephalitis and multiple sclerosis, and implicate CD40-CD40L interactions as a target for interventional strategies.

Soluble CD40 in the serum of healthy donors, patients with chronic renal failure, haemodialysis and chronic ambulatory peritoneal dialysis (CAPD) patients

CD40 and its ligand CD40L are key players in T cell-B cell interaction and T cell-antigen-presenting cell (APC) interaction. Inhibition of CD40-CD40L interaction leads to severe humoral and cellular immunodeficiency. In this study we examined the presence of soluble CD40 (sCD40) in the serum of haemodialysis (HD) patients, CAPD patients, chronic renal failure (CRF) patients and healthy donors in order to evaluate the possible involvement of CD40 in uraemic immunodeficiency. Soluble CD40 was detected in the serum of healthy donors (n = 41) with a mean of 0.14 +/- 0.12 ng/ml and in the urine of healthy donors with a mean of 1.80 +/- 0.74 ng/ml. Soluble CD40 was highly elevated in all patients with impaired renal function. HD patients (n = 22) had up to 100-fold elevated sCD40 levels with a mean concentration of 8.32 +/- 4.11 ng/ml, whereas CAPD patients (n = 10) had considerably lower levels of sCD40 with a mean of 3.58 +/- 2.40 ng/ml. A strong correlation between sCD40 and serum creatinine levels was noted in CRF patients (n = 66). The highly elevated levels of sCD40 may point to the involvement of CD40 and its ligand CD40L in the clinical manifestation of uraemic immunodeficiency.

Incorporation of an isoleucine zipper motif enhances the biological activity of soluble CD40L (CD154)

Recent progress in the understanding of immune function indicates that the interaction of CD40L with its receptor, CD40, plays a pivotal role in both humoral immunity and cell-mediated defense against pathogens. Functional studies of this interaction on both dendritic cells and malignant cells have demonstrated that CD40L also plays an important role in immune surveillance and anti-tumor immunity. CD40L exists in nature predominantly as a membrane-anchored molecule. To develop CD40L as a potential therapeutic, it is important to optimize soluble forms of this molecule that could be used in a clinical setting. Several reports have shown that soluble forms of CD40L, like CD40 antibodies, are biologically active. In the present report we demonstrate that the incorporation of an isoleucine zipper trimerization motif significantly enhances the biological activity of soluble CD40L.

CD30 is a CD40-inducible molecule that negatively regulates CD40-mediated immunoglobulin class switching in non-antigen-selected human B cells

We used our monoclonal model of germinal center maturation, CL-01 B cells, to investigate the role of CD30 in human B cell differentiation. CL-01 cells are IgM+ IgD+ CD30+ and switch to IgG, IgA, and IgE when exposed to CD40L and IL-4. Switching is hampered by CD30 coengagement, possibly through interference with the CD40-mediated NF-kappaB-dependent transcriptional activation of downstream C(H) genes. The physiological relevance of this phenomenon is emphasized by similar CD30-mediated effects in naive B cells. Expression of CD30 by these cells is induced by CD40L but is inhibited by B cell receptor coengagement and/or exposure to IL-6 and IL-12. Our data suggest that CD30 critically regulates the CD40-mediated differentiation of non-antigen-selected human B cells.

Molecular cloning and characterization of the mouse tag7 gene encoding a novel cytokine

Cloning of the mouse tag7 gene encoding a novel cytokine is described. The Tag7 protein consists of 182 amino acids. Genomic organization of the tag7 gene and its promoter region remind those of the genes of the tumor necrosis factor locus, although the tag7 gene is not linked to this locus. The gene is located on chromosome 7 at the area that corresponds to band 7A3, which has genetic linkage with lupus-like disease in mouse models. tag7 transcription is essential for lymphoid organs. It is also detected in certain areas of lungs, brain, and intestine and in some tumors. Tag7 protein is detectable in both cell-associated and soluble forms. The soluble form of Tag7 triggers apoptosis in mouse L929 cells in vitro and does not involve NF-kappaB activation. The relationship between Tag7 and tumor necrosis factor family of ligands is discussed.

CD40 Ligand/CD40 Stimulation Regulates the Production of IFN-γ from Human Peripheral Blood Mononuclear Cells in an IL-12- and/or CD28-Dependent Manner

CD40 ligand (CD40L)/CD40 costimulation is an important regulator of Th1 responses. Two mechanisms by which CD40L/CD40 stimulation may enhance IFN-γ are via direct induction of IL-12 and augmentation of the expression of costimulatory molecules such as B7 from APCs. We examined the ability of CD40L/CD40 stimulation to regulate the production of IFN-γ through IL-12 and/or CD28 costimulation from human PBMCs stimulated with T cell-specific stimuli. The roles of exogenous and endogenous CD40L/CD40 stimulation were evaluated using a trimeric soluble CD40L agonist (CD40T) and an anti-CD40L Ab, respectively. The presence of CD40T in cultures increased the production of IL-12 and IFN-γ from PBMCs stimulated with varying amounts of PHA. The mechanism, however, by which CD40T enhanced IFN-γ varied according to the level of T cell activation. Under maximal stimulatory conditions (PHA, 1/100), an IL-12-dependent pathway was dominant. At relatively low levels of T cell stimulation (PHA, 1/500 and 1/1000), however, an additional IL-12-independent CD28-dependent pathway was elucidated. We further studied the role of exogenous CD28 stimulation in regulating the production of IFN-γ. The enhancement of IFN-γ production induced by direct CD28 stimulation was primarily dependent on endogenous IL-12 or CD40L/CD40 stimulation. Together, these data suggest that the production of IFN-γ involves a complex interaction between two interdependent, yet distinct, costimulatory pathways and provide evidence that CD40T may be an effective adjuvant for the enhancement of responses.

CD40 and its ligand in cell-mediated immunity

CD40 and its ligand (gp39, CD40L, TBAM) is central to the control of thymus-dependent humoral immunity. However, in recent years it has become evident that CD40 signaling also is critical in the development of cell-mediated immune responses. How CD40 regulates cell-mediated immunity is discussed.

The role of CD40-CD40 ligand interactions in suppression of human B cell responsiveness by CD4+ T cells

Although human T cells have been shown to regulate humoral immune responses by directly inhibiting B cells, the precise sequelae for the mechanism of suppression have not yet been delineated. The present study was therefore examined to explore the nature of T cell-B cell collaboration to suppress B cell responses. Special attention was directed to the roles of Fas (CD95)-Fas ligand (FasL) interactions and CD40-CD40 ligand (CD40L) interactions. The suppressive activity was assessed by the effects of mitomycin C untreated CD4+ T cells (control CD4+ T cells) activated by immobilized anti-CD3 for 72 h on the production of IgM and IgG of B cells stimulated for 72 h with immobilized anti-CD3-activated mitomycin C treated CD4+ T cells. In this model system, B cells stimulated with anti-CD3-activated CD4+ T cells have been shown to express functional Fas receptor. Thus, anti-Fas mAb CH11 inhibited the production of IgM and IgG induced by anti-CD3-activated mitomycin C treated CD4+ T cells in a manner that was completely reversed by a neutralizing anti-Fas mAb ZB4. However, neither ZB4 nor anti-FasL mAb reversed the suppression of B cell responses by anti-CD3-activated control CD4+ T cells. Anti-CD40L mAb inhibited the production of IgM and IgG stimulated with anti-CD3-activated mitomycin C treated CD4+ T cells when it was added at the initiation of cultures. By contrast, anti-CD40L mAb markedly reversed the suppression of B cell production of IgM and IgG by anti-CD3-activated control CD4+ T cells when it was added after 72 h from the initiation of cultures. Consistently, the extent and intensity of CD40L on anti-CD3-stimulated CD4+ T cells declined upon treatment with mitomycin C in parallel with the loss of suppressive activities on B cell responses. These results indicate that signals achieved by direct interactions through CD40-CD40L exert bidirectional effects on the outcome of humoral immune responses depending on the state of activation of B cells and on the extent of CD40 ligation. Moreover, the data suggest that CD40-CD40L interactions rather than Fas-FasL interactions may play more critical roles in direct cellular collaboration between B cells and anti-CD3 stimulated CD4+ T cells to prevent the extension of responses of inappropriately activated B cells.

Functional B cell abnormalities in HIV type 1 infection: role of CD40L and CD70

Early in HIV-1 infection, B cell responses to T cell-dependent antigens are impaired. In addition to the receptor-ligand pair CD40/CD40L, CD27/CD70 also appears to be involved in T cell-dependent B cell stimulation. We have shown that CD70+ B cells are the main producers of Ig when stimulated in a T cell-dependent manner, and that CD70 upregulation is dependent on interaction of CD40L on T cells with CD40 on B cells. We confirm here that B cells from HIV-infected individuals are impaired in T cell-dependent Ig production in vitro. This dysfunction could partly be restored by adding allogeneic T cells to the culture. In contrast, IgG production induced by CD40 MAb, IgM MAb, and IL-10 was in the normal range. In line with this, CD70 upregulation on B cells from HIV-infected individuals was impaired after stimulation in vitro by activated T cells but not after stimulation with CD40 MAb and IgM MAb. Furthermore, CD40L expression was decreased on CD4+ T cells after stimulation in vitro. Finally, CD70 expression on freshly isolated B cells from HIV-infected individuals was decreased, and low CD70 expression correlated with low IgG production after T cell-dependent stimulation. In conclusion, our data strongly suggest that impaired B cell responses to T cell-dependent Ag in HIV-1 infection are due to a defect in T cells.

Modulation of soluble CD40 ligand bioactivity with anti-CD40 antibodies

The B cell surface molecule CD40 may be activated either by its ligand CD40L or by anti-CD40 antibodies. In this study, five new anti-CD40 monoclonal antibodies (MAb) were characterized. Bioactivity of the MAb was assessed using a receptor hybrid consisting of the extracellular domain of CD40 and the intracellular domain of the p55 TNF receptor as a model for CD40 activation. Two agonistic MAb were able to enhance the activation of this CD40 hybrid CD40L. These MAb bound to an epitope that was not located within the CD40L-binding region indicating that activation of CD40 occurs epitope-independent. A second pair of ligand mimetic anti-CD40 MAb which appeared to bind to the CD40L binding site decreased CD40L bioactivity. With regard to ligand mimetic effects binding of the CD40L epitope was not of advantage. Combining anti-CD40 MAb with different epitope specificities or cross linking anti-CD40 MAB with secondary antibodies enhanced ligand mimetic effects. These data clearly show that ligand or antibody-mediated receptor aggregation is the major mechanism by which CD40 is activated. Furthermore, our data support that an aggregate of activated receptors is favorable in regard to CD40 activation.

Identification of amino acid residues important for ligand binding to Fas

The interaction of Fas (CD95), a member of the tumor necrosis factor receptor (TNFR) family, and its ligand (FasL) triggers programmed cell death (apoptosis) and is involved in the regulation of immune responses. Although the Fas-FasL interaction is conserved across species barriers, little is currently known about the molecular details of this interaction. Our aim was to identify residues in Fas that are important for ligand binding. With the aid of a Fas molecular model, candidate amino acid residues were selected in the Fas extracellular domain 2 (D2) and D3 and subjected to serine-scanning mutagenesis to produce mutant Fas molecules in the form of Ig fusion proteins. The effects of these mutations on FasL binding was examined by measuring the ability of these proteins to inhibit FasL-mediated apoptosis of Jurkat cells and bind FasL in ELISA and BIAcore assays. Mutation of two amino acids, R86 and R87 (D2), to serine totally abolished the ability of Fas to interact with its ligand, whereas mutants K84S, L90S, E93S (D2), or H126S (D3) showed reduced binding compared with wild-type Fas. Two mutants (K78S and H95S) bound FasL comparably to wild type. Therefore, the binding of FasL involves residues in two domains that correspond to positions critical for ligand binding in other family members (TNFR and CD40) but are conserved between murine and human Fas.

Heteromultimeric complexes of CD40 ligand are present on the cell surface of human T lymphocytes

CD40 ligand (CD40L), a 33-kDa type II membrane glycoprotein expressed primarily on activated CD4+ T lymphocytes, is responsible for the helper function of T cells on resting B cells in a non-antigen-dependent, non-major histocompatability complex-restricted fashion. Interaction of CD40L with its receptor CD40 induces proliferation of and isotype switching in B lymphocytes. Recently we solved the x-ray structure of recombinant soluble CD40L and showed that, similar to other members of the tumor necrosis factor family, CD40L indeed exists as a trimer. We now report that, under normal physiological conditions, CD40L molecules exist as heteromultimeric complexes. These CD40L complexes, made of the full length and smaller fragments of CD40L, are present on the cell surface of T lymphocytes and are capable of interacting with CD40 molecule. A prominent fragment with a mass of 31 kDa accounts for as much as half of the CD40L on the surface of Jurkat cells. N-terminal sequence data revealed that this fragment lacks the cytoplasmic tail. A minor 18-kDa fragment of CD40L was also characterized which lacks the cytoplasmic tail, transmembrane region, and stalk region of the extracellular domain. The presence of CD40L heteromultimeric variants implies an additional regulation of the functional activity of this ligand complex.

The SCID but not the RAG-2 gene product is required for S mu-S epsilon heavy chain class switching

We have investigated the capacity of precursor B cells from normal (BDF1) and V(D)J recombinase-deficient (RAG-27) or defective (SCID) mice to be induced by a CD40-specific monoclonal antibody and IL-4 to epsilon H chain gene transcription and to S mu-S epsilon switch recombination. In differentiating precursor B cells from all three strains of mice, the development of similar numbers of CD19+, CD23+, CD40+, and MHC class II+ expressing B lineage cells and similar levels of epsilon H chain gene transcription were induced. Efficient S mu-S epsilon switching occurred in normal and RAG-2-deficient, but not in SCID, precursor B cells. Thus, the transcription of the epsilon H chain is independent of the RAG-2 and the SCID gene product, while the S mu-S epsilon switch recombination requires the SCID gene-encoded DNA-dependent protein kinase, but not the RAG-2 protein.

Ligation of CD40 with soluble CD40 ligand reverses anti-immunoglobulin-mediated negative signalling in murine B lymphoma cell lines but not in immature B cells from neonatal mice

Ligation of surface immunoglobulin (sIg) on certain murine B-lymphoma lines has been shown to initiate a programme leading to growth arrest and death of the cells by apoptosis. The cell lines WEHI 231 and CH33 which respond in this way to receptor cross-linking have phenotypic characteristics resembling those of immature normal B cells, and their responses have been taken to model those responsible for clonal deletion or anergy. Cross-linking of sIg on normal neonatal B cells has also been shown to inhibit their responsiveness to polyclonal activators. We have examined the ability of various co-stimuli to modify the response of growth-inhibitable B lymphoma lines to sIg cross-linking. Our findings indicate that cell-cell contact between cells of the WEHI 231 or CH33 lines and activated T cells rescues these cells from growth arrest and apoptosis. Cell-free supernatants from some T-cell lines were also protective although recombinant IL-4 had no effect. Analysis of the most effective signals and timing for inducing this protection suggested that it might, in part, be mediated by CD40 ligand (CD40L) expressed on or secreted by activated T cells. Using a soluble recombinant CD40L-CD8 fusion protein we have now shown that co-ligation of CD40 is sufficient to rescue WEHI231 and CH33 cells from anti-Ig-induced apoptosis. In contrast, the inhibitory effect of anti-Ig antibodies on the lipopolysaccharide (LPS)-driven proliferation of neonatal B cells was not relieved by co-ligation of CD40 with CD40L. These findings bring into question the usefulness of 'immature' B-cell lines as models for tolerance induction.

The transmembrane form of tumor necrosis factor is the prime activating ligand of the 80 kDa tumor necrosis factor receptor

The 60 kDa tumor necrosis factor receptor (TNFR60) is regarded as the major signal transducer of TNF-induced cellular responses, whereas the signal capacity and role of the 80 kDa TNFR (TNFR80) remain largely undefined. We show here that the transmembrane form of TNF is superior to soluble TNF in activating TNFR80 in various systems such as T cell activation, thymocyte proliferation, and granulocyte/macrophage colony-stimulating factor production. Intriguingly, activation of TNFR80 by membrane TNF can lead to qualitatively different TNF responses such as rendering resistant tumor cells sensitive to TNF-mediated cytotoxicity. This study demonstrates that the diversity of TNF effects can be controlled through the differential sensitivity of TNFR80 for the two forms of TNF and suggests an important physiological role for TNFR80 in local inflammatory responses.

The role of gp39 (CD40L) in immunity

The function of CD40 and its ligand (CD40L;gp39) has provided a molecular understanding for cognate interactions and the basis for hyper IgM syndrome. Studies using an antibody that neutralizes gp39 function in vivo have demonstrated that interactions between CD40 and gp39 are essential for primary and secondary thymus-dependent humoral immune responses, as well as for the generation of memory B cells. In addition, gp39 is critical for B cells to upregulate important costimulatory molecules and acquire professional antigen-presenting cell status. If B cells do not receive the gp39 signal, they are rendered tolerogenic for T cells. Finally, gp39-CD40 interactions are important in thymic education since blockade of this interaction interrupts negative selection.

Recombinant soluble trimeric CD40 ligand is biologically active

CD40 ligand (CD40L) is expressed on the surface of activated CD4+ T cells, basophils, and mast cells. Binding of C40L to its receptor, CD40, on the surface of B cells stimulates B cell proliferation, adhesion and differentiation. A preparation of soluble, recombinant CD40L (Tyr-45 to Leu-261), containing the full-length 29-kDa protein and two smaller fragments of 18 and 14 kDa, has been shown to induce differentiation of B cells derived either from normal donors or from patients with X-linked hyper-IgM syndrome (Durandy, A., Schiff, C., Bonnefoy, J.-Y., Forveille, M., Rousset, F., Mazzei, G., Milili, M., and Fischer, A. (1993) Eur. J. Immunol. 23, 2294-2299). We have now purified each of these fragments to homogeneity and show that only the 18-kDa fragment (identified as Glu-108 to Leu-261) is biologically active. When expressed in recombinant form, the 18-kDa protein exhibited full activity in B cell proliferation and differentiation assays, was able to rescue of B cells from apoptosis, and bound soluble CD40. Sucrose gradient sedimentation shows that the 18-kDa protein sediments as an apparent homotrimer, a result consistent with the proposed trimeric structure of CD40L. This demonstrates that a soluble CD40L can stimulate CD40 in a manner indistinguishable from the membrane-bound form of the protein.

Physiological enzymatic cleavage of leukocyte membrane molecules

Certain membrane molecules are enzymatically cleaved from the cell surface and then released into the extracellular medium in the form of soluble fragments. This process, commonly initiated by cell stimulation, may regulate the surface expression of such molecules, and may also be responsible for the production of their soluble forms in vivo. Here, Vladimír Bazil provides an overview of the molecules that are cleaved from cells, focusing particularly on leukocyte receptors. In addition, he discusses the mechanisms and putative enzymes involved in this process, as well as the potential physiological significance of such events.

A cytotoxic CD40/p55 tumor necrosis factor receptor hybrid detects CD40 ligand on herpesvirus saimiri-transformed T cells

The B cell activation molecule CD40 and the p55 tumor necrosis factor receptor (p55TNFR) belong to the same family of structurally conserved proteins. We constructed a chimeric receptor consisting of the CD40 extracellular and transmembrane domains and the p55TNFR intracellular domain. This receptor hybrid retained the biological activity and the ligand specificity of the respective wild-type receptor domains. Thus it exerted a marked cytotoxic effect in three different transfected cell lines after activation not only with anti-CD40 antibody but also with CD40 ligand (CD40L) in soluble and membrane-bound forms. Using hybrid-transfected baby hamster kidney cells we demonstrated that herpesvirus saimiri-transformed human CD4+ T lymphocytes constitutively express bioactive CD40 ligand on their surface. The hybrid receptor-based assay was highly specific for CD40 activating reagents and more sensitive than an assay measuring CD40-mediated B cell rescue from apoptosis. Hence CD40/p55TNFR transfectants may be useful for dissecting CD40L-mediated events in T-B cell interactions, and also to detect a defective CD40L molecule in putative hyper-IgM syndrome patients.

Hyper IgM syndrome associated with defective CD40-mediated B cell activation

Recent studies show that most patients with X-linked hyper IgM syndrome have defects in the gene for CD40 ligand. We evaluated 17 unrelated males suspected of having X-linked hyper IgM syndrome. Activated T cells from 13 of the 17 patients failed to bind a soluble CD40 construct. In these patients, the sequence of CD40 ligand demonstrated mutations. By contrast, T cells from the remaining four patients exhibited normal binding to the CD40 construct. Sequencing of the cDNA for CD40 ligand from these patients did not show mutations. The possibility that hyper IgM syndrome in these four patients was due to abnormalities in the B cell response to CD40-mediated signals was examined. Peripheral blood lymphocytes were stimulated with anti-CD40 alone, IL4 alone or anti-CD40 plus IL4. In comparison with B cells from controls or patients with hyper IgM syndrome and mutant CD40 ligand, B cells from the patients with hyper IgM syndrome and normal CD40 ligand were defective in their ability to secrete IgE (P < 0.02) or express activation markers, CD25 and CD23 (P < 0.02) in response to stimulation with anti-CD40. The failure of these B cells to respond to CD40-mediated activation could not be attributed to a generalized deficiency in B cell activation because IL4 induced normal up-regulation of CD23 and CD25 expression. These findings indicate that hyper IgM syndrome may result from defects in expression of CD40 ligand by activated T cells or defects in CD40-mediated signal transduction in B cells.

Ineffective expression of CD40 ligand on cord blood T cells may contribute to poor immunoglobulin production in the newborn

A major feature of the immature immune system in the newborn is its inability to produce significant levels of immunoglobulins other than IgM in response to antigens. It has recently been demonstrated that interaction of the CD40 molecule on B cells with the CD40 ligand (CD40L) on activated T cells is pivotal for immunoglobulin switching. In view of these findings, we have tested cord blood mononuclear cells (CBMC) for expression of CD40L. Our data clearly demonstrate that freshly isolated CBMC do not express significant levels of CD40L upon in vitro activation; this defect is intrinsic to CD4+ cord blood lymphocytes. In vitro priming of CBMC with phytohemagglutinin and interleukin-2 for several days induced a conversion from the "naive" to the "memory" phenotype (as assessed by expression of CD45 isoforms) and led to substantial CD40L expression upon appropriate restimulation. These data indicate that ineffective CD40L expression might represent a major factor for reduced immunoglobulin production in the neonate, and suggest that antigenic exposure in vivo leads to changes in the genetic program, enabling T cells to express CD40L.

Interleukin 10 induces B lymphocytes from IgA-deficient patients to secrete IgA

We have previously shown that human B lymphocytes cultured in the CD40 system, composed of an anti-CD40 mAb presented by a CD32-transfected fibroblastic cell line, proliferate but do not secrete antibodies. However, the addition of particles of Staphylococcus aureus Cowan (SAC) induces B cell differentiation even in the absence of exogenous cytokines (CD40/SAC system). Additionally, B lymphocytes cultured in the CD40 system in the presence of human IL-10, produce IgM, IgG, and IgA, and Ig levels are further increased by SAC. Here, we have studied the capacity of peripheral blood lymphocytes from patients with IgA deficiency (IgA-D) to secrete Igs, particularly IgA after CD40 triggering. Peripheral blood mononuclear cells (PBMNC) from IgA-D patients cultured in the CD40/SAC system produced IgM and IgG, but not IgA. The addition of IL-10 to the cultures, enhanced the production of IgM and IgG and most strikingly induced the production of high amounts of IgA. The addition of IL-10 to PBMNC from IgA-D patients activated through CD40 alone resulted in the production of IgA. Thus, SAC and anti-CD40 mAb stimulate B cells to differentiate into cells secreting IgG and IgM whereas IL-10 plays a central role in inducing B cells from IgA-D patients to differentiate into IgA secreting cells.

Expression of APO-1 (CD95), a member of the NGF/TNF receptor superfamily, in normal and neoplastic colon epithelium

APO-1 is a 48-kDa cell-membrane protein identical to the Fas antigen now designated CD95. It is a member of the NGF/TNF receptor superfamily. Anti-APO-1 monoclonal antibody induces apoptosis in a variety of cell types expressing this antigen. We immunohistochemically investigated APO-1 expression in normal colon mucosa, 20 adenomas, 258 colon carcinomas and 10 liver metastases and carried out in vitro studies using a panel of colon-carcinoma cell lines. Immunohistochemically, APO-1 was regularly expressed at the basolateral membrane of normal colon epithelia. In a minor fraction of colon adenomas and in 39.1% of colon carcinomas APO-1 expression was diminished and in 48.1% of carcinomas, predominantly of the non-mucinous type, APO-1 expression was completely abrogated. The normal level of APO-1 in carcinomas was correlated with the mucinous type. Reduced/lost APO-1 expression was more frequent in rectal carcinomas. Complete loss of APO-1 was more frequent in tumors that had already metastasized. APO-1 expression in liver metastases essentially corresponded to that of the primary tumors. Comparative analysis with data from previous studies revealed that the mode of APO-1 expression is correlated with that of HLA-A,B,C./beta 2m, HLA-DR, HLA-D-associated invariant chain and of the secretory component. Surface expression of APO-1 was heterogeneous in colon-carcinoma cell lines; SW480 expressed considerable amounts of APO-1 on all cells, while HT-29 constitutively did less so and only in a minority of cells. Surface density of APO-1 and the fraction of positive cells in HT-29 was enhanced by interferon-gamma (IFN-gamma) and, additively, by tumor necrosis factor-alpha (TNF-alpha), whereas in SW480 APO-1 expression was not modulated by these cytokines. We conclude that neoplastic transformation of colon epithelium often leads to a loss of the physiologic, high level of surface APO-1 by giving rise either to a stable lack of APO-1 or to an IFN-gamma/TNF-alpha-sensitive phenotype of inducible APO-1 expression.

Molecular control of B lymphocyte growth and differentiation

During antigen driven immune responses, antigen-specific naive B lymphocytes undergo a cascade of events including activation, expansion, mutations, isotype switch, selections and differentiation into either antibody secreting plasma cells or memory B cells. These antigen-dependent events, which we propose to call immunopoiesis, occur in different areas of secondary lymphoid organs, as well as other nonlymphoid organs. B cells interact with antigens and numerous cell types (T cells, dendritic cells, follicular dendritic cells and macrophages) through numerous cell surface molecules and cytokines. B cells costimulated through their antigen receptor and cytokines such as interleukin 2 (IL-2), IL-4 and IL-10 undergo limited proliferation and differentiation into immunoglobulin (Ig) secreting cells. In contrast, crosslinking of the B cell CD40 antigen, a member of the tumor necrosis factor (TNF) receptor family, results in major cellular activation further modulated by cytokines. In particular, IL-4 and IL-13 permit establishment of long-term factor-dependent B cell lines, as well as isotype switch towards the production of IgE and IgG4. Addition of IL-10 to CD40-activated B cells results in limited proliferation and remarkable differentiation into plasma cells. IL-10 also participates in isotype switch towards IgG1, IgG3 and IgA. The ligand for CD40, a member of the TNF family, is transiently expressed on activated T cells, and interrupted CD40/CD40-L interactions result in profoundly altered humoral immune responses.

B cells regulate expression of CD40 ligand on activated T cells by lowering the mRNA level and through the release of soluble CD40

The expression of CD40 ligand (CD40L) on activated T cells (CD4+ T cell clone MT9) is diminished when the T cells are cultured in the presence of B cells. This effect, observed both with normal tonsil B cells and with the B cell line JY, was detected after 6 h and sustained at least until 18 h of co-culture. Analysis of mRNA showed that CD40L mRNA levels were not modified after 6 h, but were significantly down-regulated after 18 h of co-culture with B cells. Although CD40L expression could not be detected by a CD40-Fc chimera, the molecule was still expressed at the membrane as shown with a polyclonal antiserum against CD40L (anti-TRAP). In addition, T cells activated in the presence of B cells were stained by a polyclonal antiserum against CD40, without the appearance of CD40 mRNA. These results indicated that a soluble form of CD40 (sCD40) bound to the expressed CD40L on T cells. The existence of sCD40 was confirmed by detection of sCD40 in B cell supernatants using a specific enzyme-linked immunosorbent assay. Collectively, these data show that B cells can regulate the expression of CD40L on activated T cells at least by two different mechanisms.

Purification and characterization of the Fas-ligand that induces apoptosis

Fas is a 45-kD cell surface protein belonging to the tumor necrosis factor/nerve growth factor receptor family, and transduces the signal for apoptosis. The cytotoxic T lymphocyte (CTL) hybridoma, PC60-d10S requires the presence of Fas on target cells to induce cytolysis in target cells. This CTL cell line was weakly but specifically stained by a chimeric protein that consisted of the extracellular domain of mouse Fas and the Fc portion of human immunoglobulin G1 (mFas-Fc). Moreover, mFas-Fc inhibited the cytotoxic activity of PC60-d10S. Sublines of d10S that were stained intensively by mFas-Fc were isolated by repetitive fluorescence-activated cell sorter sorting. A cell-surface protein of about 40 kD was specifically precipitated by mFas-Fc from the lysates of these sublines. This protein was homogeneously purified by sequential affinity chromatographies using mFas-Fc and concanavalin A beads. The purified protein exhibited cytotoxic activity against cells expressing Fas but not to the cells which do not express Fas. These results indicated that the 40-kD membrane glycoprotein expressed on PC60-d10S cells is the Fas-ligand that induces the apoptotic signal by binding to Fas.