Cutting edge: molecular mechanisms of synergy between CD40 and the B cell antigen receptor: role for TNF receptor-associated factor 2 in receptor interaction

Co-stimulation of CD28/CD40 signaling molecule potentiates CAR-T cell efficacy and stemness

CD19 chimeric antigen receptor T (CD19CAR-T) cells have achieved promising outcomes in relapsed/refractory B cell malignancies. However, recurrences occur due to the loss of CAR-T cell persistence. We developed dual T/B cell co-stimulatory molecules (CD28 and CD40) in CAR-T cells to enhance intense tumoricidal activity and persistence. CD19.28.40z CAR-T cells promoted pNF-κB and pRelB downstream signaling while diminishing NFAT signaling upon antigen exposure. CD19.28.40z CAR-T cells demonstrated greater proliferation, which translated into effective anti-tumor cytotoxicity in long-term co-culture assay. Repetitive weekly antigen stimulation unveiled continuous CAR-T cell expansion while preserving central memory T cell subset and lower expression of exhaustion phenotypes. The intrinsic genes underlying CD19.28.40z CAR-T cell responses were compared with conventional CARs and demonstrated the up-regulated genes associated with T cell proliferation and memory as well as down-regulated genes related to apoptosis, exhaustion, and glycolysis pathway. Enrichment of genes toward T cell stemness, particularly SELL, IL-7r, TCF7, and KLF2, was observed. Effective and continuing anti-tumor cytotoxicity in vivo was exhibited in both B cell lymphoblastic leukemia and B cell non-Hodgkin lymphoma xenograft models while demonstrating persistent T cell memory signatures. The functional enhancement of CD37.28.40z CAR-T cell activities against CD37+ tumor cells was further validated. The modification of dual T/B cell signaling molecules remarkably maximized the efficacy of CAR-T cell therapy.

CD22L Conjugation to Insulin Attenuates Insulin-Specific B Cell Activation

Pancreatic islet-reactive B lymphocytes promote Type 1 diabetes (T1D) by presenting an antigen to islet-destructive T cells. Teplizumab, an anti-CD3 monoclonal, delays T1D onset in patients at risk, but additional therapies are needed to prevent the disease entirely. Therefore, bifunctional molecules were designed to selectively inhibit T1D-promoting anti-insulin B cells by conjugating a ligand for the B cell inhibitory receptor CD22 (i.e., CD22L) to insulin, which permit these molecules to concomitantly bind to anti-insulin B cell receptors (BCRs) and CD22. Two prototypes were synthesized: 2:2 insulin-CD22L conjugate on a 4-arm PEG backbone, and 1:1 insulin-CD22L direct conjugate. Transgenic mice (125TgSD) expressing anti-insulin BCRs provided cells for in vitro testing. Cells were cultured with constructs for 3 days, then assessed by flow cytometry. Duplicate wells with anti-CD40 simulated T cell help. A 2-insulin 4-arm PEG control caused robust proliferation and activation-induced CD86 upregulation. Anti-CD40 further boosted these effects. This may indicate that BCR-cross-linking occurs when antigens are tethered by the PEG backbone as soluble insulin alone has no effect. Addition of CD22L via the 2:2 insulin-CD22L conjugate restored B cell properties to that of controls without an additional beneficial effect. In contrast, the 1:1 insulin-CD22L direct conjugate significantly reduced anti-insulin B cell proliferation in the presence of anti-CD40. CD22L alone had no effect, and the constructs did not affect the WT B cells. Thus, multivalent antigen constructs tend to activate anti-insulin B cells, while monomeric antigen-CD22L conjugates reduce B cell activation in response to simulated T cell help and reduce pathogenic B cell numbers without harming normal cells. Therefore, monomeric antigen-CD22L conjugates warrant futher study and may be promising candidates for preclinical trials to prevent T1D without inducing immunodeficiency.

Mimicking the brain: Epstein-Barr virus and foreign agents as drivers of neuroimmune attack in multiple sclerosis

T cells have an essential role in adaptive immunity against pathogens and cancer, but failure of thymic tolerance mechanisms can instead lead to escape of T cells with the ability to attack host tissues. Multiple sclerosis (MS) occurs when structures such as myelin and neurons in the central nervous system (CNS) are the target of autoreactive immune responses, resulting in lesions in the brain and spinal cord which cause varied and episodic neurological deficits. A role for autoreactive T cell and antibody responses in MS is likely, and mounting evidence implicates Epstein-Barr virus (EBV) in disease mechanisms. In this review we discuss antigen specificity of T cells involved in development and progression of MS. We examine the current evidence that these T cells can target multiple antigens such as those from pathogens including EBV and briefly describe other mechanisms through which viruses could affect disease. Unravelling the complexity of the autoantigen T cell repertoire is essential for understanding key events in the development and progression of MS, with wider implications for development of future therapies.

Multiple mechanisms for TRAF3-mediated regulation of the T cell costimulatory receptor GITR

Tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) plays context-specific roles in multiple receptor-mediated signaling pathways in different cell types. Mice lacking TRAF3 in T cells display defective T-cell-mediated immune responses to immunization and infection and demonstrate defective early signaling via the TCR complex. However, the role of TRAF3 in the function of GITR/TNFRSF18, an important costimulatory member of the TNFR superfamily, is unclear. Here we investigated the impact of T cell TRAF3 status on both GITR expression and activation of specific kinases in the GITR signaling pathway in T cells. Our results indicate that TRAF3 negatively regulates GITR functions in several ways. First, expression of GITR protein was elevated in TRAF3-deficient T cells, resulting from both transcriptional and posttranslational regulation that led to greater GITR transcript levels, as well as enhanced GITR protein stability. TRAF3 associated with T cell GITR in a manner dependent upon GITR ligation. TRAF3 also inhibited several events of the GITR mediated early signaling cascade, in a manner independent of recruitment of phosphatases, a mechanism by which TRAF3 inhibits signaling through several other cytokine receptors. These results add new information to our understanding of GITR signaling and function in T cells, which is relevant to the potential use of GITR to enhance immune therapies.

Composite CD79A/CD40 co-stimulatory endodomain enhances CD19CAR-T cell proliferation and survival

Adoptively transferred CD19 chimeric antigen receptor (CAR) T cells have led to impressive clinical outcomes in B cell malignancies. Beyond induction of remission, the persistence of CAR-T cells is required to prevent relapse and provide long-term disease control. To improve CAR-T cell function and persistence, we developed a composite co-stimulatory domain of a B cell signaling moiety, CD79A/CD40, to induce a nuclear translocating signal, NF-κB, to synergize with other T cell signals and improve CAR-T cell function. CD79A/CD40 incorporating CD19CAR-T cells (CD19.79a.40z) exhibited higher NF-κB and p38 activity upon CD19 antigen exposure compared with the CD28 or 4-1BB incorporating CD19CAR-T cells (CD19.28z and CD19.BBz). Notably, we found that CD19.79a.40z CAR-T cells continued to suppress CD19⁺ target cells throughout the co-culture assay, whereas a tendency for tumor growth was observed with CD19.28z CAR-T cells. Moreover, CD19.79a.40z CAR-T cells exhibited robust T cell proliferation after culturing with CD19⁺ target cells, regardless of exogenous interleukin-2. In terms of in vivo efficiency, CD19.79a.40z demonstrated superior anti-tumor activity and in vivo CAR-T cell proliferation compared with CD19.28z and CD19.BBz CD19CAR-T cells in Raji-inoculated mice. Our data demonstrate that the CD79A/CD40 co-stimulatory domain endows CAR-T cells with enhanced proliferative capacity and improved anti-tumor efficacy in a murine model.

TNF receptor-associated factor 3 restrains B-cell receptor signaling in normal and malignant B cells

TRAF3 has diverse signaling functions, which vary by cell type. Uniquely in B lymphocytes, TRAF3 inhibits homeostatic survival. Highlighting the role of TRAF3 as a tumor suppressor, loss-of-function TRAF3 mutations are associated with human B-cell malignancies, while B-cell-specific deletion of TRAF3 in mice leads to autoimmunity and lymphoma development. The role of TRAF3 in inhibiting noncanonical NF-κB activation, CD40 and BAFF-R signaling to B cells is well documented. In contrast, TRAF3 enhances many T-cell effector functions, through associating with and enhancing signaling by the T-cell receptor (TCR)-CD28 complex. The present study was designed to determine the role of TRAF3 in signaling via the B-cell antigen receptor (BCR). The BCR is crucial for antigen recognition, survival, proliferation, and antibody production, and defects in BCR signaling can promote abnormal survival of malignant B cells. Here, we show that TRAF3 is associated with both CD79B and the BCR-activated kinases Syk and Btk following BCR stimulation. BCR-induced phosphorylation of Syk and additional downstream kinases was increased in TRAF3^−/− B cells, with regulation observed in both follicular and marginal zone B-cell subsets. BCR stimulation of TRAF3^−/− B cells resulted in increased surface expression of MHC-II, CD80, and CD86 molecules. Interestingly, increased survival of TRAF3^−/− primary B cells was resistant to inhibition of Btk, while TRAF3-deficient malignant B-cell lines showed enhanced sensitivity. TRAF3 serves to restrain normal and malignant BCR signaling, with important implications for its role in normal B-cell biology and abnormal survival of malignant B cells.

Epstein-Barr Functional Mimicry: Pathogenicity of Oncogenic Latent Membrane Protein-1 in Systemic Lupus Erythematosus and Autoimmunity

Systemic lupus erythematosus (SLE) and other autoimmune diseases are propelled by immune dysregulation and pathogenic, disease-specific autoantibodies. Autoimmunity against the lupus autoantigen Sm is associated with cross-reactivity to Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1). Additionally, EBV latent membrane protein-1 (LMP1), initially noted for its oncogenic activity, is an aberrantly active functional mimic of the B cell co-stimulatory molecule CD40. Mice expressing a transgene (Tg) for the mCD40-LMP1 hybrid molecule (containing the cytoplasmic tail of LMP1) have mild autoantibody production and other features of immune dysregulation by 2-3 months of age, but no overt autoimmune disease. This study evaluates whether exposure to the EBV molecular mimic, EBNA-1, stimulates antigen-specific and concurrently-reactive humoral and cellular immunity, as well as lupus-like features. After immunization with EBNA-1, mCD40-LMP1 Tg mice exhibited enhanced, antigen-specific, cellular and humoral responses compared to immunized WT congenic mice. EBNA-1 specific proliferative and inflammatory cytokine responses, including IL-17 and IFN-γ, were significantly increased (p<0.0001) in mCD40-LMP1 Tg mice, as well as antibody responses to amino- and carboxy-domains of EBNA-1. Of particular interest was the ability of mCD40-LMP1 to drive EBNA-1 associated molecular mimicry with the lupus-associated autoantigen, Sm. EBNA-1 immunized mCD40-LMP1 Tg mice exhibited enhanced proliferative and cytokine cellular responses (p<0.0001) to the EBNA-1 homologous epitope PPPGRRP and the Sm B/B' cross-reactive sequence PPPGMRPP. When immunized with the SLE autoantigen Sm, mCD40-LMP1 Tg mice again exhibited enhanced cellular and humoral immune responses to both Sm and EBNA-1. Cellular immune dysregulation with EBNA-1 immunization in mCD40-LMP1 Tg mice was accompanied by enhanced splenomegaly, increased serum blood urea nitrogen (BUN) and creatinine levels, and elevated anti-dsDNA and antinuclear antibody (ANA) levels (p<0.0001 compared to mCD40 WT mice). However, no evidence of immune-complex glomerulonephritis pathology was noted, suggesting that a combination of EBV and genetic factors may be required to drive lupus-associated renal disease. These data support that the expression of LMP1 in the context of EBNA-1 may interact to increase immune dysregulation that leads to pathogenic, autoantigen-specific lupus inflammation.

Regulatory B cells and advances in transplantation

The effects of B cell subsets with regulatory activity on the immune response to an allograft have evoked increasing interest. Here, we summarize the function and signaling of regulatory B cells (Bregs) and their potential effects on transplantation. These cells are able to suppress the immune system directly via ligand-receptor interactions and indirectly by secretion of immunosuppressive cytokines, particularly IL-10. In experimental animal models, the extensively studied IL-10-producing B cells have shown unique therapeutic advantages in the transplant field. In addition, adoptive transfer of B cell subsets with regulatory activity may reveal a new approach to prolonging allograft survival. Recent clinical observations on currently available therapies targeting B cells have revealed that Bregs play an important role in immune tolerance and that these cells are expected to become a new target of immunotherapy for transplant-related diseases.

TRAF3 as a Multifaceted Regulator of B Lymphocyte Survival and Activation

The adaptor protein TNF receptor-associated factor 3 (TRAF3) serves as a powerful negative regulator in multiple aspects of B cell biology. Early in vitro studies in transformed cell lines suggested the potential of TRAF3 to inhibit signaling by its first identified binding receptor, CD40. However, because the canonical TRAF3 binding site on many receptors also mediates binding of other TRAFs, and whole-mouse TRAF3 deficiency is neonatally lethal, an accurate understanding of TRAF3's specific functions was delayed until conditional TRAF3-deficient mice were produced. Studies of B cell-specific TRAF3-deficient mice, complemented by investigations in normal and malignant mouse and human B cells, reveal that TRAF3 has powerful regulatory roles that are unique to this TRAF, as well as functions context-specific to the B cell. This review summarizes the current state of knowledge of these roles and functions. These include inhibition of signaling by plasma membrane receptors, negative regulation of intracellular receptors, and restraint of cytoplasmic NF- κB pathways. TRAF3 is also now known to function as a resident nuclear protein, and to impact B cell metabolism. Through these and additional mechanisms TRAF3 exerts powerful restraint upon B cell survival and activation. It is thus perhaps not surprising that TRAF3 has been revealed as an important tumor suppressor in B cells. The many and varied functions of TRAF3 in B cells, and new directions to pursue in future studies, are summarized and discussed here.

Tespa1 Deficiency Dampens Thymus-Dependent B-Cell Activation and Attenuates Collagen-Induced Arthritis in Mice

Thymocyte-expressed, positive selection-associated 1 (Tespa1) plays an important role in both T cell receptor (TCR)-driven thymocyte development and in the FcεRI-mediated activation of mast cells. Herein, we show that lack of Tespa1 does not impair B cell development but dampens the in vitro activation and proliferation of B cells induced by T cell-dependent (TD) antigens, significantly reduces serum antibody concentrations in vivo, and impairs germinal center formation in both aged and TD antigen-immunized mice. We also provide evidence that dysregulated signaling in Tespa1-deficient B cells may be linked to CD40-induced TRAF6 degradation, and subsequent effects on 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-2 (PLCγ2) phosphorylation, MAPK activation, and calcium influx. Furthermore, we demonstrate that Tespa1 plays a critical role in pathogenic B cells, since Tespa1-deficient chimeric mice showed a lower incidence and clinical disease severity of collagen-induced arthritis. Overall, our study demonstrates that Tespa1 is essential for TD B cell responses, and suggests an important role for Tespa1 during the development of autoimmune arthritis.

The oncogenic membrane protein LMP1 sequesters TRAF3 in B-cell lymphoma cells to produce functional TRAF3 deficiency

Loss-of-function mutations in genes encoding the signaling protein tumor necrosis factor receptor-associated factor 3 (TRAF3) are commonly found in human B-cell malignancies, especially multiple myeloma and B-cell lymphoma (BCL). B-cell TRAF3 deficiency results in enhanced cell survival, elevated activation receptor signaling, and increased activity of certain transcriptional pathways regulating expression of prosurvival proteins. A recent analysis of TRAF3 protein staining of ∼300 human BCL tissue samples revealed that a higher proportion of samples expressing the oncogenic Epstein-Barr virus-encoded protein latent membrane protein 1 (LMP1) showed low/negative TRAF3 staining than predicted. LMP1, a dysregulated mimic of the CD40 receptor, binds TRAF3 more effectively than CD40. We hypothesized that LMP1 may sequester TRAF3, reducing its availability to inhibit prosurvival signaling pathways in the B cell. This hypothesis was addressed via 2 complementary approaches: (1) comparison of TRAF3-regulated activation and survival-related events with relative LMP1 expression in human BCL lines and (2) analysis of the impact upon such events in matched pairs of mouse BCL lines, both parental cells and subclones transfected with inducible LMP1, either wild-type LMP1 or a mutant LMP1 with defective TRAF3 binding. Results from both approaches showed that LMP1-expressing B cells display a phenotype highly similar to that of B cells lacking TRAF3 genes, indicating that LMP1 can render B cells functionally TRAF3 deficient without TRAF3 gene mutations, a finding of significant relevance to selecting pathway-targeted therapies for B-cell malignancies.

TRAF3 as a powerful and multitalented regulator of lymphocyte functions

This review summarizes the current state of knowledge regarding the roles of the signaling adapter protein tumor necrosis factor receptor (TNFR)-associated factor 3 in regulating the functions of B and T lymphocytes. In B lymphocytes, TNFR-associated factor 3 inhibits signaling by TNFR superfamily receptors, Toll-like receptors, and interleukin-6R. In contrast, signaling to B cells by the virally encoded oncogenic protein latent membrane protein 1 is promoted by TNFR-associated factor 3. An important B cell-specific role for TNFR-associated factor 3 is the inhibition of homeostatic survival, directly relevant to the common occurrence of TNFR-associated factor 3 mutations in human B cell malignancies. TNFR-associated factor 3 was recently found to be a resident nuclear protein in B cells, where it interacts with and inhibits gene expression mediated by the cAMP response element-binding protein transcription complex, including expression of the prosurvival protein myeloid leukemia cell differentiation protein 1. In T lymphocytes, TNFR-associated factor 3 is required for normal signaling by the T cell antigen receptor, while inhibiting signaling by the interleukin-2 receptor. Cytoplasmic TNFR -associated factor 3 restrains nuclear factor-κB2 activation in both T and B cells. Clinical implications and future directions for the study of this context-dependent signaling regulator are discussed.

TRAF3, ubiquitination, and B-lymphocyte regulation

The signaling adapter protein tumor necrosis factor receptor (TNFR)-associated factor 3 (TRAF3) is both modified by and contributes to several types of ubiquitination events. TRAF3 plays a variety of context-dependent regulatory roles in all types of immune cells. In B lymphocytes, TRAF3 contributes to regulation of signaling by members of both the TNFR superfamily and innate immune receptors. TRAF3 also plays a unique cell type-specific and critical role in the restraint of B-cell homeostatic survival, a role with important implications for both B-cell differentiation and the pathogenesis of B-cell malignancies. This review focuses upon the relationship between ubiquitin and TRAF3, and how this contributes to multiple functions of TRAF3 in the regulation of signal transduction, transcriptional activation, and effector functions of B lymphocytes.

CD40-TRAF Signaling Upregulates CX3CL1 and TNF-α in Human Aortic Endothelial Cells but Not in Retinal Endothelial Cells

CD40, CX3CL1 and TNF-α promote atheroma and neointima formation. CD40 and TNF-α are also central to the development of diabetic retinopathy while CX3CL1 may play a role in the pathogenesis of this retinopathy. The purpose of this study was to examine whether CD40 ligation increases CX3CL1 and TNF-α protein expression in human endothelial cells from the aorta and retina. CD154 (CD40 ligand) upregulated membrane-bound and soluble CX3CL1 in human aortic endothelial cells. CD154 triggered TNF-α production by human aortic endothelial cells. TNF Receptor Associated Factors (TRAF) are key mediators of CD40 signaling. Compared to human aortic endothelial cells that express wt CD40, cells that express CD40 with a mutation that prevents TRAF2,3 recruitment, or CD40 with a mutation that prevents TRAF6 recruitment exhibited a profound inhibition of CD154-driven upregulation of membrane bound and soluble CX3CL1 as well as of TNF-α secretion. While both CD154 and TNF-α upregulated CX3CL1 in human aortic endothelial cells, these stimuli could act independently of each other. In contrast to human aortic endothelial cells, human retinal endothelial cells did not increase membrane bound or soluble CX3CL1 expression or secrete TNF-α in response to CD154 even though CD40 ligation upregulated ICAM-1 and CCL2 in these cells. Moreover, TNF-α did not upregulate CX3CL1 in retinal endothelial cells. In conclusion, CD40 ligation increases CX3CL1 protein levels and induces TNF-α production in endothelial cells. However, endothelial cells are heterogeneous in regards to these responses. Human aortic but not retinal endothelial cells upregulated CX3CL1 and TNF-α in response to CD40 ligation, as well as upregulated CX3CL1 in response to TNF-α. These dissimilarities may contribute to differences in regulation of inflammation in large vessels versus the retina.

Blockade of CD40-TRAF2,3 or CD40-TRAF6 is sufficient to inhibit pro-inflammatory responses in non-haematopoietic cells

Inhibition of the CD40-CD154 pathway controls inflammatory disorders. Unfortunately, administration of anti-CD154 monoclonal antibodies causes thromboembolism. Blockade of signalling downstream of CD40 may represent an approach to treat CD40-driven inflammatory disorders. Blocking tumour necrosis factor receptor-associated factor 6 (TRAF6) signalling downstream of CD40 in MHC II(+) cells diminishes inflammation. However, CD40-TRAF6 blockade may cause immunosuppression. We examined the role of CD40-TRAF2,3 and CD40-TRAF6 signalling in the development of pro-inflammatory responses in human non-haematopoietic and monocytic cells. Human aortic endothelial cells, aortic smooth muscle cells, renal proximal tubule epithelial cells, renal mesangial cells and monocytic cells were transduced with retroviral vectors that encode wild-type CD40, CD40 with a mutation that prevents TRAF2,3 recruitment (ΔT2,3), TRAF6 recruitment (ΔT6) or both TRAF2,3 plus TRAF6 recruitment (ΔT2,3,6). Non-haematopoietic cells that expressed CD40 ΔT2,3 exhibited marked inhibition in CD154-induced up-regulation of vascular cell adhesion molecule 1, intercellular adhesion molecule 1 (ICAM-1), monocyte chemotactic protein 1 (MCP-1), tissue factor and matrix metalloproteinase 9. Similar results were obtained with cells that expressed CD40 ΔT6. Although both mutations impaired ICAM-1 up-regulation in monocytic cells, only expression of CD40 ΔT6 reduced MCP-1 and tissue factor up-regulation in these cells. Treatment of endothelial and smooth muscle cells with cell-permeable peptides that block CD40-TRAF2,3 or CD40-TRAF6 signalling impaired pro-inflammatory responses. In contrast, while the CD40-TRAF2,3 blocking peptide did not reduce CD154-induced dendritic cell maturation, the CD40-TRAF6 blocking peptide impaired this response. Hence, preventing CD40-TRAF2,3 or CD40-TRAF6 interaction inhibits pro-inflammatory responses in human non-haematopoietic cells. In contrast to inhibition of CD40-TRAF6 signalling, inhibition of CD40-TRAF2,3 signalling did not impair dendritic cell maturation. Blocking CD40-TRAF2,3 signalling may control CD40-CD154-dependent inflammatory disorders.

Proinflammatory responses induced by CD40 in retinal endothelial and Müller cells are inhibited by blocking CD40-Traf2,3 or CD40-Traf6 signaling

PURPOSE

The cell surface receptor CD40 is required for the development of retinopathies induced by diabetes and ischemia/reperfusion. The purpose of this study was to identify signaling pathways by which CD40 triggers proinflammatory responses in retinal cells, since this may lead to pharmacologic targeting of these pathways as novel therapy against retinopathies.

METHODS

Retinal endothelial and Müller cells were transduced with vectors that encode wild-type CD40 or CD40 with mutations in sites that recruit TNF receptor associated factors (TRAF): TRAF2,3 (ΔT2,3), TRAF6 (ΔT6), or TRAF2,3 plus TRAF6 (ΔT2,3,6). Cells also were incubated with CD40-TRAF2,3 or CD40-TRAF6 blocking peptides. We assessed intercellular adhesion molecule-1 (ICAM-1), CD40, monocyte chemoattractant protein-1 (MCP-1), VEGF, and prostaglandin E₂ (PGE₂) by fluorescence-activated cell sorting (FACS), ELISA, or mass spectrometry. Mice (B6 and CD40(-/-)) were made diabetic using streptozotocin. The MCP-1 mRNA was assessed by real-time PCR.

RESULTS

The CD40-mediated ICAM-1 upregulation in endothelial and Müller cells was markedly inhibited by expression of CD40 ΔT2,3 or CD40 ΔT6. The CD40 was required for MCP-1 mRNA upregulation in the retina of diabetic mice. The CD40 stimulation of endothelial and Müller cells enhanced MCP-1 production that was markedly diminished by CD40 ΔT2,3 or CD40 ΔT6. Similar results were obtained in cells incubated with CD40-TRAF2,3 or CD40-TRAF6 blocking peptides. The CD40 ligation upregulated PGE₂ and VEGF production by Müller cells, that was inhibited by CD40 ΔT2,3 or CD40 ΔT6. All cellular responses tested were obliterated by expression of CD40 ΔT2,3,6.

CONCLUSIONS

Blockade of a single CD40-TRAF pathway was sufficient to impair ICAM-1, MCP-1, PGE₂, and VEGF upregulation in retinal endothelial and/or Müller cells. Blockade of CD40-TRAF signaling may control retinopathies.

Hepatitis B vaccine responsiveness and clinical outcomes in HIV controllers

Background

Hepatitis B virus (HBV) vaccine responsiveness is associated with reduced risk of AIDS or death in HIV-infected individuals. Although HIV controllers (HIC) typically have favorable immunologic and clinical characteristics compared to non-controllers, vaccine responsiveness has not been studied.

Methods and Findings

In the U.S. Military HIV Natural History Study, HBV vaccine response was defined as antibody to hepatitis B surface antigen (anti-HBs) ≥10 IU/L after last vaccination. For determination of vaccine responsiveness, HIC (n = 44) and treatment-naïve non-controllers (n = 476) were not on highly active antiretroviral therapy (HAART) when vaccinated while treated non-controllers (n = 284) received all HBV vaccine doses during viral load (VL)-suppressive HAART. Progression to AIDS or death was also compared for all HIC (n = 143) and non-controllers (n = 1566) with documented anti-HBs regardless of the timing of HBV vaccination. Positive vaccine responses were more common in HIC (65.9%) compared to HAART-naïve non-controllers (36.6%; P<0.001), but similar to non-controllers on HAART (59.9%; P = 0.549). Factors associated with vaccine response for HIC compared to HAART-naïve non-controllers include HIC status (OR 2.65, 95% CI 1.23–5.89; P = 0.014), CD4 count at last vaccination (OR 1.28, 1.15–1.45 for every 100 cells/uL; P<0.001), and number of vaccine doses administered (OR 0.56, 0.35–0.88; P = 0.011). When HIC were compared to non-controllers on HAART, only CD4 count at last vaccination was significant (OR 1.23, 1.1–1.38 for every 100 cells/uL; P<0.001). The rate of AIDS or death per 100 person/years for HIC compared to non-controllers was 0.14 (95% CI 0–0.76) versus 0.98 (95% CI 0.74–1.28) for vaccine responders and 0 (95% CI 0–2.22) versus 4.11 (95% CI 3.38–4.96) for non-responders, respectively.

Conclusions

HIC have improved HBV vaccine responsiveness compared to treatment-naïve non-controllers, but similar to those on VL-suppressive HAART. Progression to AIDS or death can be predicted by HBV vaccine responder status for non-controllers, however these events are rarely observed in HIC.

Combinatorial signaling through TLR-2 and CD86 augments activation and differentiation of resting B cells

B cells are an integral component in mounting humoral immune responses and they are also crucial in programming T cell mediated immunity. Usually, B cell activation is initiated by recognition of antigen through B cell receptor (BCR), followed by its processing and presentation to T cells. But very little is known about BCR independent activation of B cells. Now, there is an increasing body of evidence indicating the combinatorial effect of innate and adaptive immune components in modulating the functions of B cells. In this study, we demonstrate the activation of resting B cells (RB) by simultaneous involvement of Toll like Receptor-2 (TLR-2) and costimulatory molecule, CD86. Interestingly, these B cells exhibited significant level of activation and proliferation. Furthermore, this process of activation leads to the differentiation of RB cells, preferably into marginal zone precursors (CD19(+)IgD(hi)IgM(hi)CD21/35(hi)CD23(hi)) in a shorter time window and showed increased secretion of IgG isotype. These RB cells also showed enhanced antigen uptake capacity. These observations were also substantiated by microarray gene expression results, which strengthen the notion that combinatorial signaling through innate and adaptive immune components enhances B cell mediated immune response. Thus, the present study elucidates a novel BCR independent B cell activation mechanism that links TLR-2 and CD86. Hence signaling of TLRs in conjunction with costimulatory molecules will substantially help in bolstering humoral immune response, which can be extrapolated to formulate vaccination strategies for diseases involving B cell-mediated immunity.

Local administration of soluble CD40:Fc to the salivary glands of non-obese diabetic mice does not ameliorate autoimmune inflammation

Objective

CD40–CD154 (CD40 ligand) interaction in the co-stimulatory pathway is involved in many (auto)immune processes and both molecules are upregulated in salivary glands of Sjögren’s syndrome (SS) patients. Interference within the CD40 pathway has ameliorated (auto)inflammation in a number of disease models. To test the potential role of the CD40 pathway in loss of gland function and inflammation in SS, an inhibitor of CD40-CD154 interaction was overexpressed in the salivary glands (SGs) of a spontaneous murine model of SS; the Non-Obese Diabetic (NOD) mouse.

Materials and Methods

At different disease stages an adeno associated viral vector encoding CD40 coupled to a human Fc domain (CD40:Fc) was injected locally into the SGs of NOD mice. Delivery was confirmed by PCR. The overall effect on local inflammation was determined by assessment of the focus score (FS), quantification of infiltrating cell types, immunoglobulin levels, and microarray analysis. The effect on SG function was determined by measuring stimulated salivary flow.

Results

CD40:Fc was stably expressed in the SG of NOD mice, and the protein was secreted into the blood stream. Microarray analysis revealed that expression of CD40:Fc affected the expression of many genes involved in regulation of the immune response. However, FS, infiltrating cell types, immunoglobulin levels, and salivary gland output were similar for treated and control mice.

Discussion

Although endogenous CD40 is expressed in SG inflammatory foci in the SG of NOD mice, the expression of soluble CD40:Fc did not lead to reduced overall inflammation and/or improved salivary gland function. These data indicate possible redundancy of the CD40 pathway in the SG and suggests that targeting CD40 alone may not be sufficient to alter the disease phenotype.

Roles of tumor necrosis factor receptor associated factor 3 (TRAF3) and TRAF5 in immune cell functions

A large and diverse group of receptors utilizes the family of cytoplasmic signaling proteins known as tumor necrosis factor receptor (TNFR)-associated factors (TRAFs). In recent years, there has been a resurgence of interest and exploration of the roles played by TRAF3 and TRAF5 in cellular regulation, particularly in cells of the immune system, the cell types of focus in this review. This work has revealed that TRAF3 and TRAF5 can play diverse roles for different receptors even in the same cell type, as well as distinct roles in different cell types. Evidence indicates that TRAF3 and TRAF5 play important roles beyond the TNFR-superfamily (SF) and viral mimics of its members, mediating certain innate immune receptor and cytokine receptor signals, and most recently, signals delivered by the T-cell receptor (TCR) signaling complex. Additionally, much research has demonstrated the importance of TRAF3-mediated cellular regulation via its cytoplasmic interactions with additional signaling proteins. In particular, we discuss below evidence for the participation by TRAF3 in a number of the regulatory post-translational modifications involving ubiquitin that are important in various signaling pathways.

Enhanced Toll-like receptor (TLR) responses of TNFR-associated factor 3 (TRAF3)-deficient B lymphocytes

The key role of TRAF6 in TLR signaling pathways is well known. More recent evidence has implicated TRAF3 as another TRAF family member important to certain TLR responses of myeloid cells. Previous studies demonstrate that TRAF3 functions are highly context-dependent, displaying receptor and cell-type specificity. We thus examined the TLR responses of TRAF3(-/-)mouse B lymphocytes to test the hypothesis that TRAF3 plays distinct roles in such responses, depending on cell type. TRAF3(-/-) DC are known to have a defect in type 1 IFN production and here, showed diminished production of TNF and IL-10 and unaltered IL-6. In marked contrast, TRAF3(-/-) B cells made elevated amounts of TNF and IL-6 protein, as well as IL-10 and IP-10 mRNA, in response to TLR ligands. Also, in contrast to TRAF3(-/-) DC, the type 1 IFN pathway was elevated in TRAF3(-/-) B cells. Increased early responses of TRAF3(-/-) B cells to TLR signals were independent of cell survival or proliferation but associated with elevated canonical NF-κB activation. Additionally, TRAF3(-/-) B cells displayed enhanced TLR-mediated expression of AID and Ig isotype switching. Thus, TRAF3 plays varied and cell type-specific, biological roles in TLR responses.

Syk mediates BCR- and CD40-signaling integration during B cell activation

CD40 is essential for optimal B cell activation. It has been shown that CD40 stimulation can augment BCR-induced B cell responses, but the molecular mechanism(s) by which CD40 regulates BCR signaling is poorly understood. In this report, we attempted to characterize the signaling synergy between BCR- and CD40-mediated pathways during B cell activation. We found that spleen tyrosine kinase (Syk) is involved in CD40 signaling, and is synergistically activated in B cells in response to BCR/CD40 costimulation. CD40 stimulation alone also activates B cell linker (BLNK), Bruton tyrosine kinase (Btk), and Vav-2 downstream of Syk, and significantly enhances BCR-induced formation of complex consisting of, Vav-2, Btk, BLNK, and phospholipase C-gamma2 (PLC-γ2) leading to activation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase, Akt, and NF-κB required for optimal B cell activation. Therefore, our data suggest that CD40 can strengthen BCR-signaling pathway and quantitatively modify BCR signaling during B cell activation.

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

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

A BAFF-R mutation associated with non-Hodgkin lymphoma alters TRAF recruitment and reveals new insights into BAFF-R signaling

The cytokine B cell activating factor (BAFF) and its receptor, BAFF receptor (BAFF-R), modulate signaling cascades critical for B cell development and survival. We identified a novel mutation in TNFRSF13C, the gene encoding human BAFF-R, that is present in both tumor and germline tissue from a subset of patients with non-Hodgkin lymphoma. This mutation encodes a His159Tyr substitution in the cytoplasmic tail of BAFF-R adjacent to the TRAF3 binding motif. Signaling through this mutant BAFF-R results in increased NF-κB1 and NF-κB2 activity and increased immunoglobulin production compared with the wild-type (WT) BAFF-R. This correlates with increased TRAF2, TRAF3, and TRAF6 recruitment to His159Tyr BAFF-R. In addition, we document a requirement for TRAF6 in WT BAFF-R signaling. Together, these data identify a novel lymphoma-associated mutation in human BAFF-R that results in NF-κB activation and reveals TRAF6 as a necessary component of normal BAFF-R signaling.

Roles of the TRAF2/3 binding site in differential B cell signaling by CD40 and its viral oncogenic mimic, LMP1

The EBV protein, latent membrane protein 1 (LMP1), is a functional mimic of the cellular receptor CD40, but signals to B lymphocytes in an amplified and sustained manner compared with CD40. LMP1 contributes to the development of B cell lymphoma in immunosuppressed patients, and may exacerbate flares of certain autoimmune diseases. The cytoplasmic domain of LMP1 binds the signaling adaptor TRAF2 with lower avidity than the cytoplasmic domain of CD40, and TRAF2 is needed for CD40-mediated degradation of TRAFs 2 and 3. LMP1 doesn't induce TRAF degradation, and employs TRAF3 as a positive mediator of cell signaling, whereas CD40 signals are inhibited by TRAF3. We thus tested the hypothesis that relative affinity for TRAF2, and/or distinct sequence differences in the TRAF2/3 binding sites of CD40 vs LMP1, controls the disparate ways in which CD40 and LMP1 use TRAFs 2 and 3, and their distinct signaling characteristics. CD40 and LMP1 mutants in which the TRAF binding site sequences were swapped were examined, testing TRAF binding and degradation, and induction of B cell activation. Results revealed that TRAF binding affinity and TRAF binding site sequence dictate a distinct subset of CD40 vs LMP1 signaling properties. Examination of TRAF binding, degradation, cytokine production, IgM secretion, and the activation of c-Jun kinase and NF-kappaB revealed that some events are dictated by TRAF binding site sequences, others are partially regulated, and still others are independent of the TRAF binding site sequence.

Functional roles for T cell CD40 in infection and autoimmune disease: the role of CD40 in lymphocyte homeostasis

CD40 stimulation on monocytes/macrophages, dendritic cells, and B-lymphocytes has been the subject of much study. It is well recognized that activation of CD40 on antigen presenting cells by its ligand, CD154, expressed on T-lymphocytes, contributes to the pro-inflammatory response necessary for eradication of infection, yet pathological in autoimmunity. However, there is evidence that CD40 is also expressed on T-lymphocytes and can act as a costimulatory molecule. While the exact role of CD40 on CD8 T cells remains controversial, it does appear to contribute to the adaptive immune response against infection. CD40 on CD4 T cells, on the other hand, plays a functional role in the autoimmune disease process. Further dissection of the exact nature and role of CD40 in T cell activation could lead the way to more effective vaccines and novel therapeutics for autoimmune diseases.

Cell Defence and Survival

Central to immune defence mechanisms is the role of transcription factor nuclear factor kappa B (NF-kB). This is a complex biochemical topic with ever more controls revealed. NF-kB determines the production of proinflammatory cytokines and chemokines. Pharmacologists step in with possible means of control. Other systems involved in defence include the cyclooxygenase 2 (Cox-2) enzyme and perioxisome proliferator-activated receptors. Insulin receptor activation needs to be seen in context. The mTOR system directs uptake of nutrients by cells. mTOR is suppressed by rapamycin, whose usage is now quite considerable in the control of transplant rejection.

A novel polymorphism of the human CD40 receptor with enhanced function

CD40 signaling is critical for innate and adaptive immunity against pathogens, and the cytoplasmic domain of CD40 is highly conserved both within and between species. A novel missense single nucleotide polymorphism (SNP) in the cytoplasmic domain of CD40 at position 227 (P227A) was identified, which resides on a conserved ancestral haplotype highly enriched in persons of Mexican and South American descent. Functional studies indicated that signaling via human (h) CD40-P227A stably expressed in several B-cell lines led to increased phosphorylation of c-Jun, increased secretion of the pro-inflammatory cytokines interleukin (IL)-6 and TNF-alpha, and increased Ig production, compared with wild-type hCD40. Cooperation between hCD40-P227A signaling and B-cell receptor (BCR)- or Toll-like receptor 9 (TLR9)-mediated signaling was also enhanced, resulting in elevated and synergistic production of IL-6 and Ig. We have thus identified a novel genetic variant of hCD40 with a gain-of-function immune phenotype.

TRAF proteins in CD40 signaling

The tumor necrosis factor receptor (TNFR) superfamily molecule CD40 is expressed by a wide variety of cell types following activation signals, and constitutively on B lymphocytes, macrophages, and dendritic cells. CD40 signals to cells stimulate kinase activation, gene expression, production of a antibody and a variety of cytokines, expression or upregulation of surface molecules, and protection or promotion of apoptosis. Initial steps in CD40-mediated signal cascades involve the interactions of CD40 with various members of the TNFR-associated factor (TRAF) family of cytoplasmic proteins. This review summarizes current understanding of the nature of these interactions, and how they induce and regulate CD40 functions.

Physiological roles and mechanisms of signaling by TRAF2 and TRAF5

RAF2 and TRAF5 are closely related members of the TRAF family of proteins. They are important signal transducers for a wide range of TNF receptor superfamily members, including TNFR1, TNFR2, CD40 and other lymphocyte costimulatory receptors, RANK/TRANCE-R, EDAR, LTbetaR, LMP-1 and IRE1. TRAF2 andTRAF5 therefore regulate diverse physiological roles, ranging from T and B cell signaling and inflammatory responses to organogenesis and cell survival. The major pathways mediated by TRAF2 and TRAF5 are the classical and alternative pathways of NF-kappaB activation, and MAPK and JNK activation. TRAF2 is heavily regulated by ubiquitin signals, and many of the signaling functions of TRAF2 are mediated through its RING domain and likely its own role as an E3 ubiquitin ligase.

A costimulatory function for T cell CD40

CD40 plays a significant role in the pathogenesis of inflammation and autoimmunity. B cell CD40 directly activates cells, which can result in autoantibody production. T cells can also express CD40, with an increased frequency and amount of expression seen in CD4(+) T lymphocytes of autoimmune mice, including T cells from mice with collagen-induced arthritis. However, the mechanisms of T cell CD40 function have not been clearly defined. To test the hypothesis that CD40 can serve as a costimulatory molecule on T lymphocytes, CD40(+) T cells from collagen-induced arthritis mice were examined in parallel with mouse and human T cell lines transfected with CD40. CD40 served as effectively as CD28 in costimulating TCR-mediated activation, including induction of kinase and transcription factor activities and production of cytokines. An additional enhancement was seen when both CD40 and CD28 signals were combined with AgR stimulation. These findings reveal potent biologic functions for T cell CD40 and suggest an additional means for amplification of autoimmune responses.

Rapid CD40-mediated rescue from CD95-induced apoptosis requires TNFR-associated factor-6 and PI3K

The activation molecule CD40 and the death receptor CD95/Fas play important roles in regulating B cells so that effective antimicrobial immunity occurs without autoimmunity. CD40 signaling increases CD95 expression, sensitizing cells to apoptosis, but sustained CD40 signals rescue B cells from CD95 killing. Here we describe a mechanism of early CD40-mediated rescue from CD95-induced apoptosis in B cells. Maximal rescue was achieved when CD40 signals were given within 1-2 h of initiating CD95 apoptosis. CD40 signaling did not block association of Fas-associated death domain-containing protein with CD95, but decreased CD95-induced activation of caspases 3 and 8. Rapid CD40 rescue did not require NF-kappaB activation and was independent of de novo protein synthesis, but was dependent upon active PI3 K. Signaling via a CD40 mutant that does not bind TNFR-associated factor (TRAF)1, TRAF2, and TRAF3 rescued B cells from CD95-induced apoptosis. TRAF1/2/3-independent rescue was confirmed in B cell lines made deficient in these TRAF molecules by gene targeting. In contrast, CD40 rescue was completely abrogated in TRAF6-deficient B cells, which showed reduced activation of Akt in response to CD40 engagement. These results reveal a new rapid mechanism to balance B cell activation and apoptosis.

beta-TrCP binding and processing of NF-kappaB2/p100 involve its phosphorylation at serines 866 and 870

Processing of the NF-kappaB2 precursor protein p100 is a major step in noncanonical NF-kappaB signaling. This signaling step requires the NF-kappaB inducing kinase (NIK) and its downstream kinase, IkappaB kinase alpha (IKKalpha). We show here that p100 undergoes phosphorylation at serines 866, 870, and possibly 872, in cells stimulated with noncanonical NF-kappaB stimuli or transfected with NIK and IKKalpha. Phosphorylation of this serine cluster creates a binding site for beta-TrCP, the receptor subunit of the beta-TrCP(SCF) ubiquitin ligase. Mutation of either serine 866 or serine 870 abolishes the beta-TrCP recruitment and ubiquitination of p100. The functional significance of p100 phosphorylation is further supported by the finding that this molecular event occurs in a NIK- and IKKalpha-dependent manner. Additionally, induction of p100 phosphorylation can be blocked by a protein synthesis inhibitor, suggesting the requirement of de novo protein synthesis. These data suggest that p100 processing involves its phosphorylation at specific terminal serines, which form a binding site for beta-TrCP thereby regulating p100 ubiquitination.

Cooperation between TNF Receptor-Associated Factors 1 and 2 in CD40 Signaling

TNFR-associated factor 1 (TRAF1) is unique among the TRAF family, lacking most zinc-binding features, and showing marked up-regulation following activation signals. However, the biological roles that TRAF1 plays in immune cell signaling have been elusive, with many reports assigning contradictory roles to TRAF1. The overlapping binding site for TRAFs 1, 2, and 3 on many TNFR superfamily molecules, together with the early lethality of mice deficient in TRAFs 2 and 3, has complicated the quest for a clear understanding of the functions of TRAF1. Using a new method for gene targeting by homologous recombination in somatic cells, we produced and studied signaling by CD40 and its viral oncogenic mimic, latent membrane protein 1 (LMP1) in mouse B cell lines lacking TRAF1, TRAF2, or both TRAFs. Results indicate that TRAFs 1 and 2 cooperate in CD40-mediated activation of the B cell lines, with a dual deficiency leading to a markedly greater loss of function than that of either TRAF alone. In the absence of TRAF1, an increased amount of TRAF2 was recruited to lipid rafts, and subsequently, more robust degradation of TRAF2 and TRAF3 was induced in response to CD40 signaling. In contrast, LMP1 did not require either TRAFs 1 or 2 to induce activation. Taken together, our findings indicate that TRAF1 and TRAF2 cooperate in CD40 but not LMP1 signaling and suggest that cellular levels of TRAF1 may play an important role in modulating the degradation of TRAF2 and TRAF3 in response to signals from the TNFR superfamily.

Differential regulation of CD40-mediated TNF receptor-associated factor degradation in B lymphocytes

Engagement of CD40 on murine B cells by its ligand CD154 induces the binding of TNFR-associated factors (TRAFs) 1, 2, 3, and 6, followed by the rapid degradation of TRAFs 2 and 3. TRAF degradation occurs in response to signaling by other TNFR superfamily members, and is likely to be a normal regulatory component of signaling by this receptor family. In this study, we found that receptor-induced TRAF degradation limits TRAF2-dependent CD40 signals to murine B cells. However, TRAFs 1 and 6 are not degraded in response to CD40 engagement, despite their association with CD40. To better understand the mechanisms underlying differential TRAF degradation, mixed protein domain TRAF chimeras were analyzed in murine B cells. Chimeras containing the TRAF2 zinc (Zn) domains induced effective degradation, if attached to a TRAF domain that binds to the PXQXT motif of CD40. However, the Zn domains of TRAF3 and TRAF6 could not induce degradation in response to CD40, regardless of the TRAF domains to which they were attached. Our data indicate that TRAF2 serves as the master regulator of TRAF degradation in response to CD40 signaling, and this function is dependent upon both the TRAF Zn domains and receptor binding position.

Simultaneous targeting of CD3 on T cells and CD40 on B or dendritic cells augments the antitumor reactivity of tumor-primed lymph node cells

To date, molecular targets chosen for Ab activation to generate antitumor effector cells have been confined on T cells, such as TCR/CD3, CD28, CD137 (4-1BB), CD134 (OX40), and inducible costimulator. In this report we investigated the immune function of murine tumor-draining lymph node (TDLN) cells after simultaneous Ab targeting of CD3 on T cells and CD40 on APCs. Anti-CD3 plus anti-CD40-activated TDLN cells secreted significantly higher amounts of IFN-gamma, but less IL-10, compared with anti-CD3-activated cells. In adoptive immunotherapy, ligation of CD3 and CD40 resulted in the generation of more potent effector cells in mediating tumor regression. Freshly harvested TDLN cells were composed of approximately 60% CD3+ T cells, 30-35% CD19+ B cells, 5% CD11c+ dendritic cells (DC), and few CD14+ or NK cells (each <3%). CD40 was distributed predominantly on B cells and DCs. Cell depletion indicated that simultaneous targeting was toward CD3 on T cells and CD40 on APCs, respectively. Elimination of APCs completely abrogated the augmented antitumor responses induced by anti-CD40. Either B cell or DC removal partially, but significantly, reduced the therapeutic efficacy conferred by CD40 engagement. Furthermore, the immunomodulation function of anti-CD40 was associated with its capability to increase IL-12 secretion while inhibiting IL-4 production. Our study establishes a role for CD40 expressed on B cells or DCs in the costimulation of TDLN cells. Eliciting antitumor activity via simultaneous targeting of CD3 on T cells and CD40 on APCs is relevant for the design of effective T cell-based cancer immunotherapy.

TNF receptor (TNFR)-associated factor (TRAF) 3 serves as an inhibitor of TRAF2/5-mediated activation of the noncanonical NF-kappaB pathway by TRAF-binding TNFRs

TNF family members and their receptors contribute to increased gene expression for inflammatory processes and intracellular cascades leading to programmed cell death, both via activation of NF-kappaB. TNF receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins binding to various receptors of the TNFR family. In an attempt to delineate the role of individual TRAFs, we compared NF-kappaB activation by CD40(wt) and CD40 mutants with different TRAF recruitment patterns. Recognized only recently, NF-kappaB signaling occurs at least via two different pathways. Each pathway results in nuclear translocation of two different Reldimers, the canonical p50/RelA and the noncanonical p52/RelB. Here, we show that via TRAF6, CD40 mediates only the activation of the canonical NF-kappaB pathway. Via TRAF2/5, CD40 activates both the canonical and the noncanonical NF-kappaB pathways. We observed that TRAF3 specifically blocked the NF-kappaB activation via TRAF2/5. This inhibitory effect of TRAF3 depends on the presence of an intact zinc finger domain. Paradoxically, suppression of TRAF2/5-mediated NF-kappaB activation by TRAF3 resulted in enhanced transcriptional activity of TRAF6-mediated canonical NF-kappaB emanating from CD40. We also observed that 12 TNFR family members (p75TNFR, LTbetaR, RANK, HVEM, CD40, CD30, CD27, 4-1BB, GITR, BCMA, OX40, and TACI) are each capable of activating the alternative NF-kappaB pathway and conclude that TRAF3 serves as a negative regulator of this pathway for all tested receptors.

B Cell Selection and Susceptibility to Autoimmunity

Autoreactive B cells arise routinely as part of the naive B cell repertoire. The immune system employs several mechanisms in an attempt to silence these autoreactive cells before they achieve immunocompetence. The BCR plays a central role in B cell development, activation, survival, and apoptosis, and thus is a critical component of the regulation of both protective and autoreactive B cells. The strength of signal mediated by the BCR is determined by numerous factors, both B cell intrinsic and B cell extrinsic. Perturbations in the molecules that regulate the BCR signal strength or that activate pathways that engage in cross talk with the BCR-mediated signaling pathways can lead to the aberrant survival and activation of autoreactive B cells. In this review, we will discuss the some newly identified genetic loci and factors that modulate the BCR signal transduction pathway and, therefore, the regulation of autoreactive B cells. We will also provide evidence for a model of autoreactivity in which a reduction in the strength of the BCR signal allows the survival and the modulation of a naive B cell repertoire replete with autoreactivity.

An atypical tumor necrosis factor (TNF) receptor-associated factor-binding motif of B cell-activating factor belonging to the TNF family (BAFF) receptor mediates induction of the noncanonical NF-kappaB signaling pathway

BAFF receptor (BAFFR) is a member of the TNF receptor (TNFR) superfamily that regulates the survival and maturation of B cells. BAFFR exerts its signaling function by inducing activation of NF-kappaB, although the underlying mechanism has not been well defined. By using a chimeric BAFFR, we show that BAFFR preferentially induces the noncanonical NF-kappaB signaling pathway. This specific function of BAFFR is mediated by a sequence motif, PVPAT, which is homologous to the TRAF-binding site (PVQET) present in CD40, a TNFR known to induce both the canonical and noncanonical NF-kappaB pathways. Mutation of this putative TRAF-binding motif within BAFFR abolishes its interaction with TRAF3 as well as its ability to induce noncanonical NF-kappaB. Interestingly, modification of the PVPAT sequence to the typical TRAF-binding sequence, PVQET, is sufficient to render the BAFFR capable of inducing strong canonical NF-kappaB signaling. Further, this functional acquisition of the modified BAFFR is associated with its stronger and more rapid association with TRAF3. These findings suggest that the PVPAT sequence of BAFFR not only functions as a key signaling motif of BAFFR but also determines its signaling specificity in the induction of the noncanonical NF-kappaB pathway.

Roles of TNF Receptor-Associated Factor 3 in Signaling to B Lymphocytes by Carboxyl-Terminal Activating Regions 1 and 2 of the EBV-Encoded Oncoprotein Latent Membrane Protein 1

TNFR-associated factor (TRAF)3, an adaptor protein that binds the cytoplasmic domains of both CD40 and the EBV-encoded oncoprotein latent membrane protein (LMP)1, is required for positive signaling by LMP1 but not CD40 in B lymphocytes. The present study further investigated how TRAF3 participates in LMP1 signaling. We found that TRAF3 mediates signaling both through direct interactions with the C-terminal activating region (CTAR)1 of LMP1 and through indirect interactions with the CTAR2 region of LMP1 in mouse B cells. Notably, our results demonstrated that the CTAR2 region appears to inhibit the recruitment of TRAF1 and TRAF2 to membrane rafts by the CTAR1 region. Additionally, the absence of TRAF2 in B cells resulted in only a modest reduction in CTAR1-mediated signals and no detectable effect on CTAR2-mediated signals. CTAR1 and CTAR2 cooperated to achieve the robust signaling activity of LMP1 when recruited to the same membrane microdomains in B cells. Interestingly, TRAF3 deficiency completely abrogated the cooperation between CTAR1 and CTAR2, supporting the hypothesis that TRAF3 participates in the physical interaction between CTAR1 and CTAR2 of LMP1. Together, our findings highlight the central importance of TRAF3 in LMP1-mediated signaling, which is critical for EBV persistent infection and EBV-associated pathogenesis.

Role of tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2) in distinct and overlapping CD40 and TNF receptor 2/CD120b-mediated B lymphocyte activation

Members of the tumor necrosis factor receptor (TNFR) family play a variety of roles in the regulation of lymphocyte activation. An important TNFR family member for B cell activation is CD40. CD40 signals stimulate B cell TNF-alpha secretion, which subsequently signals via TNFR2 (CD120b) to enhance B cell activation. Although the function of the pro-apoptotic and pro-inflammatory receptor TNFR1 (CD120a) has been the subject of much research, less is understood about the distinct contributions of CD120b to cell activation and how it stimulates downstream events. Members of the tumor necrosis factor receptor family bind various members of the cytoplasmic adapter protein family, the tumor necrosis factor receptor-associated factors (TRAFs), during signaling. Both CD40 and CD120b bind TNF receptor-associated factor 2 (TRAF2) upon ligand stimulation. Wild type and TRAF2-deficient B cells expressing CD40 or the hybrid molecule (human) CD40 (mouse)-CD120b were examined. CD40- and CD120b-mediated IgM secretion were partly TRAF2-dependent, but only CD40 required TRAF2 for c-Jun N-terminal kinase activation. CD40 and CD120b used primarily divergent mechanisms to activate NF-kappaB, exemplifying how TNFR family members can use diverse mechanisms to mediate similar downstream events.

The multifaceted roles of TRAFs in the regulation of B-cell function

Tumour-necrosis factor receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins that are important in lymphocyte activation and apoptosis. Many studies of TRAFs have used models of exogenous overexpression by non-lymphoid cells. However, the actions of TRAFs present at normal levels in lymphoid cells often differ considerably from those that have been established in non-lymphocyte overexpression models. As I discuss here, information obtained from studying these molecules in physiological settings in B cells reveals that they have several roles, which are both unique and overlapping. These include activation of kinases and transcription factors, and interactions with other signalling proteins, culminating in the induction or inhibition of biological functions.

Engagement of the B-cell antigen receptor (BCR) allows efficient transduction of ZAP-70-positive primary B-CLL cells by recombinant adeno-associated virus (rAAV) vectors

Engagement of the B-cell antigen receptor (BCR) by crosslinking of the surface immunoglobulin (sIg) homodimer was studied for recombinant adeno-associated virus (rAAV)-mediated gene transfer into B-cell chronic lymphocytic leukaemia (B-CLL) cells. Leukemic cells obtained from 20 patients were stimulated with anti-sIg-directed antibodies and transduced with rAAV vectors coding for enhanced green fluorescent protein (EGFP) (AAV/EGFP) or CD40L (AAV/CD40L). Transduction of B-CLL cells was enhanced after BCR engagement compared to unstimulated controls (P=0.0356). BCR crosslinking induced a significant, dose- and time-dependent upregulation of heparan sulfate proteoglycan (HSPG), the primary receptor for AAV, on B-CLL cells (mean: 38.2 versus 1.7%; P=0.0006). A correlation of HSPG expression after BCR crosslinking with transduction efficiency by AAV/EGFP (P=0.0153) and AAV/CD40L (P=0.0347) was observed. High expression of zeta-associated protein 70 (ZAP-70) in B-CLL cells correlated with a better transduction efficiency by AAV/EGFP (P<0.0001) and AAV/CD40L (P=0.002), respectively: 48 h after transduction of ZAP-70-positive samples, transgene expression was seen in a mean of 33.8% (s.e.m. 3.7%) and 28.9% (s.e.m. 6.7%) of cells, respectively, and could be specifically blocked by heparin, a soluble competitor of HSPG (P<0.0001). In summary, engagement of the BCR on ZAP-70 positive B-CLL cells allows efficient rAAV-mediated gene delivery.

Regulation of the NF-kappaB-inducing kinase by tumor necrosis factor receptor-associated factor 3-induced degradation

The NF-kappaB family of transcription factors plays a pivotal role in regulation of diverse biological processes, including immune responses, cell growth, and apoptosis. Activation of NF-kappaB is mediated by both canonical and noncanonical signaling pathways. Although the canonical pathway has been extensively studied, the mechanism mediating the noncanonical pathway is still poorly understood. Recent studies have identified the NF-kappaB-inducing kinase (NIK) as a key component of the noncanonical pathway of NF-kappaB activation; however, how the signaling function of NIK is regulated remains unknown. We report here that one important mechanism of NIK regulation is through its dynamic interaction with the tumor necrosis factor receptor-associated factor 3 (TRAF3). TRAF3 physically associates with NIK via a specific sequence motif located in the N-terminal region of NIK; this molecular interaction appears to target NIK for degradation by the proteasome. Interestingly, induction of noncanonical NF-kappaB signaling by extracellular signals involves degradation of TRAF3 and the concomitant enhancement of NIK expression. These results suggest that induction of noncanonical NF-kappaB signaling may involve the rescue of NIK from TRAF3-mediated negative regulation.

A novel interaction between protein kinase D and TNF receptor-associated factor molecules regulates B cell receptor-CD40 synergy

Signaling by Ag to the B cell Ag receptor (BCR) is enhanced by several cooperating signals, including several provided by B-T cell interactions. One of these, CD40, provides critical signals for B cell differentiation, isotype switching, and B cell memory. The molecular mechanisms by which BCR and CD40 signals synergize are not well understood. Although the BCR and CD40 share certain signaling pathways, we hypothesized that unique signals provided by each could provide mutual enhancement of their signaling pathways. The BCR, but not CD40, activates protein kinase D (PKD), while CD40, but not the BCR, employs the TNFR-associated factor (TRAF) adapter proteins in signaling. In this study, we show that genetic or pharmacologic inhibition of BCR-mediated PKD activation in B lymphocytes abrogated the synergy between the CD40 and the BCR, as measured by activation of Ig and cytokine secretion. Interestingly, the role of PKD was dependent upon the association of CD40 with TRAF2, and was inhibited by the binding of TRAF3, revealing a novel functional link between these two classes of signaling molecules.

Synergistic B Cell Activation by CD40 and the B Cell Antigen Receptor

Optimal activation of B-lymphocytes depends both upon expression of various cell surface receptors and adequate integration of signaling pathways. This requires signals generated upon recognition of antigen by the B lymphocyte antigen receptor (BCR) as well as additional signals provided by cognate interaction with T helper cells, including the CD40-CD154 interaction. Engagement of both the BCR and CD40 results in synergistic activation of B cells. Previous studies identified tumor necrosis factor receptor-associated factor (TRAF)-2 and TRAF3 in the CD40-signaling pathway together with BCR-activated protein kinase D (PKD) as important cooperative factors in this synergy. To better understand the role of these factors in bridging the BCR and CD40 signaling pathways, BCR signal regulation of TRAF function was examined. Results show that phosphorylation of TRAF2 is increased upon BCR but not CD40 engagement and that of the potentially phosphorylated residues of TRAF2, tyrosine 484 is crucial for BCR-CD40 synergy. Additionally, wild type or constitutively active Bruton's tyrosine kinase (Btk) enhanced, whereas the xid mutant form of Btk prevented, BCR-CD40 synergy. These effects were dependent upon TRAF2 and PKD activity. These findings suggest a model in which Btk contributes to the enhancement of the CD40 response by TRAF2 in a PKD-dependent manner.

Signaling and protein associations of a cell permeable CD40 complex in B cells

Signaling through the CD40 receptor activates diverse molecular pathways in a variety of immune cell types. To study CD40 signaling complexes in B cells, we produced soluble CD40 cytoplasmic domain multimers that translocate across cell membranes and engage intracellular CD40 signaling pathways. As visualized by fluorescence microscopy, rapid transduction of recombinant Antennapedia-isoleucine zipper (Izip)-CD40 cytoplasmic domain fusion protein (Antp-CD40) occurred in both the DND39 B cell line and human tonsillar B cells. Upon cellular entry, Antp-CD40 activated NF-kappaB-dependent transcription, induced proteolytic processing of p100 to the p52/NF-kappaB2 subunit, and increased expression of CD80 and CD54 on the surface of B cells. Antp-CD40 transduction of B cells did not, however, activate detectable levels of p38 mitogen-activated protein kinase or c-Jun N-terminal kinase and did not up-regulate CD95 expression. Analysis of Antp-CD40 complexes recovered from transduced B cells revealed that Antp-CD40 associated with endogenous TRAF3 and Ku proteins. Multimerization of Antp-CD40, or extensive clustering of transmembrane CD40, diminished the disruptive effect of the T254A mutation in the TRAF2/3 binding site of the CD40 cytoplasmic domain. Taken together, these results indicate that Antp-CD40 mimics some of the natural CD40 signaling pathways in B cells by assembling partially functional signaling intermediates that do not require plasma membrane localization. We present a novel approach for delivering pre-activated, soluble receptor cytoplasmic domains into cells and recovering intact signaling complexes for molecular analysis.