Requirement of the extracellular cysteine at position six for CD40/CD40 dimer formation and CD40-induced IL-8 expression

Atypical TNF-TNFR superfamily binding interface in the GITR-GITRL complex for T cell activation

Glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR) is a critical regulatory molecule in modulation of T cell immune responses. Here we report the mouse GITR (mGITR) and mGITR ligand (mGITRL) complex structure and find that the binding interface of mGITR and mGITRL is distinct from the typical tumor necrosis factor superfamily (TNFSF)/TNF receptor superfamily (TNFRSF) members. mGITR binds to its ligand with a single domain, whereas the binding interface on mGITRL is located on the side, which is distal from conserved binding sites of TNFSF molecules. Mutational analysis reveals that the binding interface of GITR/GITRL in humans is conserved with that in the mouse. Substitution of key interacting D93-I94-V95 (DIV) in mGITR with the corresponding K93-F94-S95 (KFS) in human GITR enables cross-recognition with human GITRL and cross-activation of receptor signaling. The findings of this study substantially expand our understanding of the interaction of TNFSF/TNFRSF superfamily molecules and can benefit the future design of biologics by targeting GITR/GITRL.

Limited Cross-Linking of 4-1BB by 4-1BB Ligand and the Agonist Monoclonal Antibody Utomilumab

Monoclonal antibodies (mAbs) targeting the co-stimulatory molecule 4-1BB are of interest for tumor immunotherapy. We determined the complex structures of human 4-1BB with 4-1BB ligand (4-1BBL) or utomilumab to elucidate the structural basis of 4-1BB activation. The 4-1BB/4-1BBL complex displays a typical TNF/TNFR family binding mode. The structure of utomilumab/4-1BB complex shows that utomilumab binds to dimeric 4-1BB with a distinct but partially overlapping binding area with 4-1BBL. Competitive binding analysis demonstrates that utomilumab blocks the 4-1BB/4-1BBL interaction, indicating the interruption of ligand-mediated signaling. The binding profiles of 4-1BBL and utomilumab to monomeric or dimeric 4-1BB indicate limited cross-linking of 4-1BB molecules. These findings provide mechanistic insight into the binding of 4-1BB with its ligand and its agonist mAb, which may facilitate the future development of anti-4-1BB biologics for tumor immunotherapy.

Involvement of the cytoplasmic cysteine-238 of CD40 in its up-regulation of CD23 expression and its enhancement of TLR4-triggered responses

CD40, a member of the tumor necrosis factor receptor superfamily, plays a key role in both adaptive and innate immunity. Engagement of CD40 with its natural trimeric ligand or with cross-linked antibodies results in disulfide-linked CD40 (dl-CD40) homodimer formation, a process mediated by the cysteine-238 residues of the cytoplasmic tail of CD40. The present study was designed to elucidate the biological relevance of cysteine-238-mediated dl-CD40 homodimers to the expression of CD23 on B cells and to investigate its possible involvement in the innate response. Our results indicate that cysteine-238-mediated dl-CD40 homodimerization is required for CD40-induced activation of PI3-kinase/Akt signaling and the subsequent CD23 expression, as inhibition of dl-CD40 homodimer formation through a point mutation-approach specifically impairs these responses. Interestingly, cysteine-238-mediated dl-CD40 homodimers are also shown to play a crucial role in Toll-like receptor 4-induced CD23 expression, further validating the importance of this system in bridging innate and adaptive immune responses. This process also necessitates the activation of the PI3-kinase/Akt cascade. Thus, our results highlight new roles for CD40 and cysteine-238-mediated CD40 homodimers in cell biology and identify a potential new target for therapeutic strategies against CD40-associated chronic inflammatory diseases.

Enhancement of Rituximab-induced cell death by the physical association of CD20 with CD40 molecules on the cell surface

CD20 is an attractive therapeutic target given the success of its monoclonal antibody, Rituximab, in the treatment of B-cell malignancies and B-cell-mediated autoimmune diseases. Treatment with Rituximab causes a rapid depletion of B cells and a decrease in disease symptoms. Despite the clinical efficiency of Rituximab, its mechanism of action is not completely understood. In this study, we aimed at further investigating the Rituximab-induced cell death and the factors affecting such responses. Our results indicate that Rituximab-induced cell death depends on the nature of the cells and levels of CD20 expression on the cell surface. Coexpression of CD20 with CD40, a member of the TNF receptor family that is known to be physically associated with CD20 on the cell surface, enhances the apoptotic response induced by Rituximab. Inhibiting the formation of CD40 disulfide-bound-homodimers, a process required for some CD40 signaling, further enhances Rituximab-induced cell death. Cell death induced by anti-CD40 mAb is also upregulated by the presence of CD20, suggesting a bidirectional influence of the CD20/CD40 association. Moreover, treating cells with both anti-CD20 and anti-CD40 antibodies improves the cell death response induced by a single-agent treatment. These results highlight the role of the CD20/CD40 association in triggering B-cell depletion and may pave the way for an alternative more efficient therapeutic strategy in treating B-cell-mediated disorders.

High molecular weight complex analysis of Epstein-Barr virus Latent Membrane Protein 1 (LMP-1): structural insights into LMP-1's homo-oligomerization and lipid raft association

LMP-1 is a constitutively active Tumor Necrosis Factor Receptor analog encoded by Epstein-Barr virus. LMP-1 activation correlates with oligomerization and raft localization, but direct evidence of LMP-1 oligomers is limited. We report that LMP-1 forms multiple high molecular weight native LMP-1 complexes when analyzed by BN-PAGE, the largest of which are enriched in detergent resistant membranes. The largest of these high molecular weight complexes are not formed by purified LMP-1 or by loss of function LMP-1 mutants. Consistent with these results we find a dimeric form of LMP-1 that can be stabilized by disulfide crosslinking. We identify cysteine 238 in the C-terminus of LMP-1 as the crosslinked cysteine. Disulfide crosslinking occurs post-lysis but the dimer can be crosslinked in intact cells with membrane permeable crosslinkers. LMP-1/C238A retains wild type LMP-1 NF-κB activity. LMP-1's TRAF binding, raft association and oligomerization are associated with the dimeric form of LMP-1. Our results suggest the possibility that the observed dimeric species results from inter-oligomeric crosslinking of LMP-1 molecules in adjacent core LMP-1 oligomers.

Cysteine-rich domain 1 of CD40 mediates receptor self-assembly

The activation of CD40 on B cells, macrophages, and dendritic cells by its ligand CD154 (CD40L) is essential for the development of humoral and cellular immune responses. CD40L and other TNF superfamily ligands are noncovalent homotrimers, but the form under which CD40 exists in the absence of ligand remains to be elucidated. Here, we show that both cell surface-expressed and soluble CD40 self-assemble, most probably as noncovalent dimers. The cysteine-rich domain 1 (CRD1) of CD40 participated to dimerization and was also required for efficient receptor expression. Modelization of a CD40 dimer allowed the identification of lysine 29 in CRD1, whose mutation decreased CD40 self-interaction without affecting expression or response to ligand. When expressed alone, recombinant CD40-CRD1 bound CD40 with a K(D) of 0.6 μM. This molecule triggered expression of maturation markers on human dendritic cells and potentiated CD40L activity. These results suggest that CD40 self-assembly modulates signaling, possibly by maintaining the receptor in a quiescent state.

CD40 pathway activation reveals dual function for macrophages in human endometrial cancer cell survival and invasion

Reproductive malignancies are a major cause of cancer death in women worldwide. CD40 is a TNF receptor family member, which upon activation may mediate tumor regression. However, despite the great potential of CD40 agonists, their use as a therapeutic option for reproductive cancers has never been investigated. Because CD40 ligation is a potent pathway of macrophage activation, an in vitro model of pro-inflammatory type-1 (Mϕ-1) and anti-inflammatory type-2 (Mϕ-2) macrophages was developed to determine whether and how macrophage CD40 pathway activation might influence endometrial tumor cell behavior. Analysis of tumor growth kinetic in the endometrial cancer xenograft model indicates that, when injected once into the growing tumors, CD40-activated Mϕ-1 greatly reduced, while CD40-activated Mϕ-2 increased tumor size when compared to control isotype-activated Mϕ-1 and Mϕ-2, respectively. In vitro assays indicated that CD40-activated Mϕ-2 increased cell viability but failed to promote cell invasion. CD40-activated Mϕ-1, in contrast, decreased cell survival but greatly increased cell invasion in tumor cells less susceptible to cell death by apoptosis; they also induced the expression of some pro-inflammatory genes, such as IL-6, LIF, and TNF-α, known to be involved in tumor promotion and metastasis. The presence of IFN-γ is minimally required for CD40-activated Mϕ-1 to promote tumor cell invasion, a process that is mediated in part through the activation of the PI3K/Akt2 signaling pathway in tumor cells. From these results, we speculate that some functions of CD40 in tumor-associated Mϕs might limit the therapeutic development of CD40 agonists in endometrial cancer malignancies.

Autoantibodies specific to a peptide of β2-glycoprotein I cross-react with TLR4, inducing a proinflammatory phenotype in endothelial cells and monocytes

β(2)-glycoprotein I (β(2)GPI) is the major antigenic target for antiphospholipid Abs. Anti-β(2)GPI Abs are a heterogeneous population of Igs targeting all domains of the molecule. Abs specific to β(2)GPI domain I are strongly associated with thrombosis and obstetric complications. In the present study, we sought to understand the possible pathogenic mechanism for this subset of anti-β(2)GPI Abs, investigating their potential cross-reactivity with other self-proteins involved in inflammatory or coagulant events. We compared the amino acid sequence of the β(2)GPI domain I with human proteins in a protein databank and identified a peptide sharing 88% identity with an epitope of human TLR4. A high percentage of patients with antiphospholipid syndrome (41%) and systemic lupus erythematosus (50%) presented serum IgG specific to this peptide. Anti-β(2)GPI peptide Abs binding the TLR4 were able to induce NF-κB activation in HEK293 cells that were stably transfected with the TLR4 gene. Anti-β(2)GPI peptide Abs induced activation of TLR4 and triggered interleukin-1 receptor-associated kinase phosphorylation and NF-κB translocation, promoting VCAM expression on endothelial cells and TNF-α release by monocytes. In conclusion, our observations suggest a novel pathogenic mechanism in the TLR4 stimulation by anti-β(2)GPI peptide Abs that links adaptive immune responses with innate immunity in antiphospholipid syndrome and systemic lupus erythematosus.

CD40-mediated cell death requires TRAF6 recruitment

CD40 has an important role in T cell-B cell interaction which rescues B lymphocytes from undergoing apoptosis. However, various studies have demonstrated that CD40 can also play a direct role in the induction of specific cell death and thus in the inhibition of tumour cell proliferation. Our previous studies showed that CD40-mediated cell death was independent of caspases and required no de novo protein synthesis. Knowing that CD40 signaling is mediated by its association with several intracellular effectors, including members of TNFR-associated factors (TRAFs) family, the goal of the present study is to investigate the mechanisms involved in the induction of cell death by CD40. Our data reveals that CD40-mediated cell death required lysosomal membrane permeabilization and the subsequent cathepsin B release. In addition, CD40 homodimer formation, a phenomenon known to be necessary for some CD40-mediated signals, was shown to negatively regulate cell death induced by CD40. Moreover, using HEK293 cells ectopically expressing CD40 deficient in TRAF binding, we showed that CD40-mediated apoptosis occurred in the absence of TRAF2 and TRAF3 association, but was significantly reduced when CD40 was deficient in its TRAF6 binding. Therefore, by outlining the role of lysosomal pathways and intracellular effectors, namely TRAF6 in CD40-mediated cell death, our study identifies new targets for anti-cancer therapy.

BAFF-R promotes cell proliferation and survival through interaction with IKKbeta and NF-kappaB/c-Rel in the nucleus of normal and neoplastic B-lymphoid cells

BLyS and its major receptor BAFF-R have been shown to be critical for development and homeostasis of normal B lymphocytes, and for cell growth and survival of neoplastic B lymphocytes, but the biologic mechanisms of this ligand/receptor-derived intracellular signaling pathway(s) have not been completely defined. We have discovered that the BAFF-R protein was present in the cell nucleus, in addition to its integral presence in the plasma membrane and cytoplasm, in both normal and neoplastic B cells. BAFF-R interacted with histone H3 and IKKbeta in the cell nucleus, enhancing histone H3 phosphorylation through IKKbeta. Nuclear BAFF-R was also associated with NF-kappaB/c-Rel and bound to NF-kappaB targeted promoters including BLyS, CD154, Bcl-xL, IL-8, and Bfl-1/A1, promoting the transcription of these genes. These observations suggested that in addition to activating NF-kappaB pathways in the plasma membrane, BAFF-R also promotes normal B-cell and B-cell non-Hodgkin lymphoma (NHL-B) survival and proliferation by functioning as a transcriptional regulator through a chromatin remodeling mechanism(s) and NF-kappaB association. Our studies provide an expanded conceptual view of the BAFF-R signaling, which should contribute a better understanding of the physiologic mechanisms involved in normal B-cell survival and growth, as well as in the pathophysiology of aggressive B-cell malignancies and autoimmune 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.

Requirement of Oxidation-dependent CD40 Homodimers for CD154/CD40 Bidirectional Signaling

It is well established that the CD154/CD40 interaction is required for T cell-dependent B cell differentiation and maturation. However, the early molecular and structural mechanisms that orchestrate CD154 and CD40 signaling at the T cell/APC contact site are not well understood. We demonstrated that CD40 engagement induces the formation of disulfide-linked (dl) CD40 homodimers that predominantly associate with detergent-resistant membrane microdomains. Mutagenesis and biochemical analyses revealed that (a) the integrity of the detergent-resistant membranes is necessary for dl-CD40 homodimer formation, (b) the cytoplasmic Cys(238) of CD40 is the target for the de novo disulfide oxidation induced by receptor oligomerization, and (c) dl-CD40 homodimer formation is required for CD40-induced interleukin-8 secretion. Stimulation of CD154-positive T cells with staphylococcal enterotoxin E superantigen that mimics nominal antigen in initiating cognate T cell/APC interaction revealed that dl-CD40 homodimer formation is required for interleukin-2 production by T cells. These findings indicate that dl-CD40 homodimer formation has a physiological role in regulating bidirectional signaling.

Nuclear CD40 interacts with c-Rel and enhances proliferation in aggressive B-cell lymphoma

CD40 is an integral plasma membrane-associated member of the TNF receptor family that has recently been shown to also reside in the nucleus of both normal B cells and large B-cell lymphoma (LBCL) cells. However, the physiological function of CD40 in the B-cell nucleus has not been examined. In this study, we demonstrate that nuclear CD40 interacts with the NF-kappaB protein c-Rel, but not p65, in LBCL cells. Nuclear CD40 forms complexes with c-Rel on the promoters of NF-kappaB target genes, CD154, BLyS/BAFF, and Bfl-1/A1, in various LBCL cell lines. Wild-type CD40, but not NLS-mutated CD40, further enhances c-Rel-mediated Blys promoter activation as well as proliferation in LBCL cells. Studies in normal B cells and LBCL patient cells further support a nuclear transcriptional function for CD40 and c-Rel. Cooperation between nuclear CD40 and c-Rel appears to be important in regulating cell growth and survival genes involved in lymphoma cell proliferation and survival mechanisms. Modulating the nuclear function of CD40 and c-Rel could reveal new mechanisms in LBCL pathophysiology and provide potential new targets for lymphoma therapy.

Nuclear localization in the biology of the CD40 receptor in normal and neoplastic human B lymphocytes

CD40 is a tumor necrosis factor (TNF) receptor superfamily, (TNFR; TNFRSF-5) member, that initiates important signaling pathways mediating cell growth, survival, and differentiation in B-lymphocytes. Although CD40 has been extensively studied as a plasma membrane-associated growth factor receptor, we demonstrate here that CD40 is present not only in the plasma membrane and cytoplasm but also in the nucleus of normal and neoplastic B-lymphoid cells. Confocal microscopy showed that transfected CD40-green fluorescent fusion protein entered B-cell nuclei. The CD40 protein contains a nuclear localization signal sequence that, when mutated, blocks entry of CD40 into the nucleus through the classic karyopherins (importins-alpha/beta) pathway. Nuclear fractionation studies revealed the presence of CD40 protein in the nucleoplasm fraction of activated B cells, and chromatin immunoprecipitation assays demonstrated that CD40 binds to and stimulates the BLyS/BAFF promoter, another TNF family member (TNFSF-13B) involved in cell survival in the B cell lineage. Like other nuclear growth factor receptors, CD40 appears to be a transcriptional regulator and is likely to play a larger and more complex role than previously demonstrated in regulating essential growth and survival pathways in B-lymphocytes.