NF-kappa B is required for surface Ig-induced Fas resistance in B cells

Pevonedistat, a Nedd8-activating enzyme inhibitor, sensitizes neoplastic B-cells to death receptor-mediated apoptosis

While death receptor ligands (Fas and TRAIL) kill chemoresistant tumor cell lines, related therapies have limited clinical efficacy as single agents. Death receptor signaling is modulated by nuclear factor-κB (NFκB), a family of transcription factors which are constitutively active in B-cell malignancies. We and others have shown that pevonedistat, an investigational inhibitor of the NEDD8-activating enzyme, abrogates NFκB activity in B-cell neoplasia. Here we demonstrate that diffuse large B-cell lymphoma, particularly activated B-cell type, and primary chronic lymphocytic leukemia cells are re-sensitized to extrinsic apoptosis by pevonedistat. Pevonedistat enhanced caspase-8 processing following death receptor ligation, and downmodulated cFLIP, a NFκB-regulated protease-deficient caspase homolog. However, treatment with pevonedistat did not modulate death-inducing signaling complex in neoplastic B-cells, suggesting that they were sensitized to death ligands through the mitochondrial pathway. Our data provide rationale for further development of pharmacologic agents including pevonedistat in strategies which enhance death receptor signaling in lymphoid malignancies.

CD72 Stimulation Modulates Anti-IgM Induced Apoptotic Signaling through the Pathway of NF-κB, c-Myc and p27Kip1

Engagement of mIgM induces G1 arrest and apoptosis in immature B cells. The biochemical mechanism(s) regulating the cell death process are poorly understood. Cross-linking of CD72 (a B cell co-receptor) with anti-CD72 antibody was shown to protect B cells from apoptosis. We investigated the molecular mechanism involved in apoptosis preventing signaling mediated by CD72 ligation using a derivative (WEHIdelta) of the WEHI231 cell line which is representative of immature B cells. Apoptotic WEHIdelta cells following cross-linking of mIgM demonstrate a dramatic loss of c-Myc protein after transient up-regulation. In contrast, pre-ligation of CD72 was able to sustain c-Myc expression after transient up-regulation. Cross-linking of mIgM of WEHIdelta cells causes accumulation of the Cdk inhibitor, p27(Kip1). CD72 pre-ligation was shown to inhibit the accumulation of p27(Kip1) protein. Moreover, NF-kappaB activity was not suppressed in WEHIdelta cells after mIgM cross-linking when the cells were pre-treated with anti-CD72 antibody. These results strongly suggest that the apoptosis preventing signal evoked by CD72 ligation is delivered through the pathway of NF-kappaB, c-Myc, p27(Kip1) and cyclin.

Aberrant B cell selection and activation in systemic lupus erythematosus

The detrimental role of B lymphocytes in systemic lupus erythematosus (SLE) is evident from the high levels of pathogenic antinuclear autoantibodies (ANAs) found in SLE patients. Affirming this causative role, additional antibody-independent roles of B cells in SLE were appreciated. In recent years, many defects in B cell selection and activation have been identified in murine lupus models and SLE patients that explain the increased emergence and persistence of autoreactive B cells and their lowered activation threshold. Therefore, clinical trials with B cell depletion regimens in SLE patients were initiated but disappointingly the efficacy of B cell depleting agents proved to be limited. Remarkably however, a major breakthrough in SLE therapy was accomplished by blocking B cell survival factors rather then eliminating B cells. This surprising finding indicates that although SLE is a B cell-driven disease, the amplifying crosstalk between B cells and other cells of the immune system likely evokes the observed tolerance breakdown in B cells. Moreover, this implies that intelligent interception of pro-inflammatory loops rather then selectively silencing B cells will be key to the development of new SLE therapies. In this review, we will not only highlight the intrinsic B cell defects that facilitate the persistence of autoreactive B cells and their activation, but in addition we will focus on B cell extrinsic signals derived from T cells and innate immune cells that lower the activation threshold for B cells.

The Fas/CD95 Receptor Regulates the Death of Autoreactive B Cells and the Selection of Antigen-Specific B Cells

Cell death receptors have crucial roles in the regulation of immune responses. Here we review recent in vivo data confirming that the Fas death receptor (TNFSR6) on B cells is important for the regulation of autoimmunity since the impairment of only Fas function on B cells results in uncontrolled autoantibody production and autoimmunity. Fas plays a role in the elimination of the non-specific and autoreactive B cells in germinal center, while during the selection of antigen-specific B cells different escape signals ensure the resistance to Fas-mediated apoptosis. Antigen-specific survival such as BCR or MHCII signal or coreceptors (CD19) cooperating with BCR inhibits the formation of death inducing signaling complex. Antigen-specific survival can be reinforced by antigen-independent signals of IL-4 or CD40 overproducing the anti-apoptotic members of the Bcl-2 family proteins.

The ubiquitin modifying enzyme A20 restricts B cell survival and prevents autoimmunity

A20 is a ubiquitin modifying enzyme that restricts NF-kappaB signals and protects cells against tumor necrosis factor (TNF)-induced programmed cell death. Given recent data linking A20 (TNFAIP3) with human B cell lymphomas and systemic lupus erythematosus (SLE), we have generated mice bearing a floxed allele of Tnfaip3 to interrogate A20's roles in regulating B cell functions. A20-deficient B cells are hyperresponsive to multiple stimuli and display exaggerated NF-kappaB responses to CD40-induced signals. Mice expressing absent or hypomorphic amounts of A20 in B cells possess elevated numbers of germinal center B cells, autoantibodies, and glomerular immunoglobulin deposits. A20-deficient B cells are resistant to Fas-mediated cell death, probably due to increased expression of NF-kappaB-dependent antiapoptotic proteins such as Bcl-x. These findings show that A20 can restrict B cell survival, whereas A20 protects other cells from TNF-induced cell death. Our studies demonstrate how reduced A20 expression predisposes to autoimmunity.

Plasmacytoid dendritic cells enhance mortality during lethal influenza infections by eliminating virus-specific CD8 T cells

Previous studies have shown that the reduction in CD8 T cell immunity observed during high-dose influenza A virus (IAV) infection is mediated via lymph node (LN) dendritic cells (DCs) that express Fas ligand (FasL) and drive FasL-Fas (DC-T)-induced apoptosis. However, the specific DC subset(s) within the LN and the additional factors required for DC-mediated elimination of IAV-specific CD8 T cells remain unknown. In this paper, we demonstrate that plasmacytoid DCs (pDCs), which downregulate FasL during sublethal, but not lethal, IAV infection, accumulate to greater numbers within the LNs of lethal dose-infected mice. Further our findings show that pDCs from lethal, but not sublethal, dose IAV infections drive elimination of Fas(+) CD8 T cells and that this elimination occurs only in the absence of TCR recognition of IAV peptide-MHC class I complexes. Together, these results suggest that pDCs play a heretofore unknown deleterious role during lethal dose IAV infections by limiting the CD8 T cell response.

Fas apoptosis inhibitory molecule enhances CD40 signaling in B cells and augments the plasma cell compartment

Fas apoptosis inhibitory molecule (FAIM) was cloned as a mediator of Fas resistance that is highly evolutionarily conserved but contains no known effector motifs. In this study, we report entirely new functions of FAIM that regulate B cell signaling and differentiation. FAIM acts to specifically enhance CD40 signaling for NF-kappaB activation, IRF-4 expression, and BCL-6 down-regulation in vitro, but has no effect on its own or in conjunction with LPS or anti-Ig stimulation. In keeping with its effects on IRF-4 and BCL-6, FAIM overexpression augments the plasma cell compartment in vivo. These results indicate that FAIM is a new player on the field of B cell differentiation and acts as a force multiplier for a series of events that begins with CD40 engagement and ends with plasma cell differentiation.

The many roles of FAS receptor signaling in the immune system

FAS belongs to the subgroup of the tumor necrosis factor receptor (TNF-R) family that contains an intracellular "death domain" and triggers apoptosis. Its physiological ligand FASL is a member of the TNF cytokine family. Studies with mutant mice and cells from human patients have shown that FAS plays critical roles in the immune system, including the killing of pathogen-infected cells and the death of obsolete and potentially dangerous lymphocytes. Fas thereby functions as a guardian against autoimmunity and tumor development. FAS triggers apoptosis through FADD-mediated recruitment and activation of caspase-8. In certain cells such as hepatocytes, albeit not lymphocytes, FAS-induced apoptosis requires amplification through proteolytic activation of the proapoptotic BCL-2 family member BID. Curiously, several components of the FAS signaling machinery have been implicated in nonapoptotic processes, including cellular activation, differentiation, and proliferation. This review describes current understanding of Fas-induced apoptosis signaling and proposes experimental strategies for future advances.

Treatment with cystamine reduces apoptosis in liver from NZB/W F1 mice

Increased population with hepatic diseases and apoptosis were found in patients with SLE and implicated in the pathogenesis of SLE. Since cystamine has been demonstrated to be beneficial to NZB/W F1 mice in our previous report, this study intends to investigate the effects of cystamine in liver from NZB/W F1 mice. Decreased apoptosis was detected in liver from NZB/W F1 mice given cystamine as compared to those given PBS by TUNEL and caspase-3 activity assay. Fas-dependent apoptotic proteins including Fas, cleaved caspase-8 and tBid were reduced in liver from NZB/W F1 mice given cystamine as compared to those given PBS. Additionally, the mitochondria-dependent apoptotic proteins including cytochrome c and Apaf-1 were reduced in liver from NZB/W F1 mice given cystamine as compared to those given PBS. Moreover, increased BCL-2 protein was observed in liver from both mice. Notably, increased NF-kappaB protein was detected in liver from NZB/W F1 mice given cystamine as compared to those given PBS. These experimental results suggest the effect of cystamine in reducing apoptosis in liver from NZB/W F1 mice through Fas-dependent and mitochondrial-dependent pathways. The phosphorylation of NF-kappaB (p65) could be a possible mechanism involving anti-apoptotic effects of cystamine in liver from NZB/W F1 mice.

MALT1 directs B cell receptor-induced canonical nuclear factor-kappaB signaling selectively to the c-Rel subunit

NF-kappaB (Rel) transcription factors control physiological and pathological immune cell function. The scaffold proteins Bcl-10 and MALT1 couple antigen-receptor signals to the canonical NF-kappaB pathway and are pivotal in lymphomagenesis. Here we found that Bcl-10 and MALT1 differentially regulated B cell receptor-induced activation of RelA and c-Rel. Bcl-10 was essential for recruitment of the kinase IKK into lipid rafts for the activation of RelA and c-Rel, for blocking apoptosis and for inducing division after B cell receptor ligation. In contrast, MALT1 participated in survival signaling but was not involved in IKK recruitment or activation and was dispensable for RelA induction and proliferation. MALT1 selectively activated c-Rel to control a distinct subprogram. Our results provide mechanistic insights into B cell receptor-induced survival and proliferation signals and demonstrate the selective control of c-Rel in the canonical NF-kappaB pathway.

B cell receptor (BCR) cross-talk: CD40 engagement creates an alternate pathway for BCR signaling that activates I kappa B kinase/I kappa B alpha/NF-kappa B without the need for PI3K and phospholipase C gamma

BCR signaling is propagated by a series of intermediaries and eventuates in NF-kappaB activation, among other outcomes. Interruption of several mediators that constitute the signalosome, such as PI3K and phospholipase Cgamma2, completely blocks BCR signaling for NF-kappaB. We show here that this accepted, conventional paradigm is, in fact, limited to naive B cells. CD40L treatment reprograms normal B cells such that a novel, alternate pathway for BCR signaling is created. Through this alternate pathway BCR triggering induces nuclear NF-kappaB without the need for PI3K or for phospholipase Cgamma2. Induction of NF-kappaB via the alternate pathway is accompanied by IkappaB kinase beta (IKKbeta) phosphorylation, IkappaBalpha phosphorylation, and IkappaBalpha degradation, and inhibition of IKKbeta blocked IkappaBalpha degradation. Several key events in the conventional pathway, including early protein tyrosine phosphorylation, were unimpeded by generation of the alternate pathway which appears to operate in parallel, rather than in competition, with classical BCR signaling. These results demonstrate cross-talk between CD40 and BCR, such that the requirements for BCR signaling are altered by prior B cell exposure to CD40L. The alternate BCR signaling pathway bypasses multiple signalosome elements and terminates in IKKbeta activation.

Dual regulation of BCR-mediated growth inhibition signaling by CD72

CD72 has been reported to regulate BCR-mediated signals both positively and negatively. SHP-1 and Grb2 bind, respectively, to ITIM1 and ITIM2 of CD72. We generated transformed B cell lines with an immature phenotype following J2 virus infection of splenocytes from CD72(-/-) and wild-type (Wt) mice. The transformed lines were infected with retroviral vectors carrying Tyr (Y) to Phe (F) substitutions in the ITIM sequences (ITIM1 mutated: Y7/F; ITIM2 mutated: Y39/F; and both ITIM mutated: Y7,39/F). Cross-linking of the BCR induced growth inhibition in transfectants expressing Wt CD72, but this response was less sensitive in transfectants with Y7,39/F. The Y7/F transfectants demonstrated the least sensitive response. We were not able to obtain transfectants with Y39/F, suggesting that CD72 associated with SHP-1, but not with Grb2, delivers a strong negative signal. Pre-ligation of CD72, which induces dephosphorylation of the molecule, partially rescued the Wt transfectants from growth inhibition, leading to a growth response profile similar to that of Y7,39/F transfectants. These results suggest that ITIM1/SHP-1 delivers a very strong negative signal that is down-modulated by signals through ITIM2/Grb2, leading to delivery of an attenuated negative signal. Thus, pre-ligation of CD72 results in the manifestation of an ostensible positive signal.

B cell receptor (BCR) cross-talk: CD40 engagement enhances BCR-induced ERK activation

Bystander B cells may be initially stimulated through CD40, which enhances susceptibility to Fas-mediated apoptosis, before encountering Ag, which produces Fas resistance. A key issue in this process is to what extent CD40 cross-talk might affect subsequent BCR signaling. It has previously been shown that CD40 engagement bypasses or mitigates the need for Bruton's tyrosine kinase in subsequent BCR signaling for NF-kappaB activation. However, the full extent of the effects of CD40 on BCR signaling has not been delineated. In the present study we evaluated the possibility that CD40-mediated cross-talk also affects another principal outcome of BCR signaling: MAPK activation. We found that prior stimulation of primary murine B cells with CD40L markedly enhanced the level of ERK and JNK (but not p38 MAPK) phosphorylation produced by subsequently added anti-Ig Ab, and much, but not all, of this enhancement was independent of PI3K and phospholipase C. CD40L treatment similarly enhanced BCR-induced MAPK kinase (MEK) phosphorylation, and MEK was required for enhancement of ERK. Although BCR-induced c-Raf phosphorylation was also enhanced by prior CD40L treatment, c-Raf was not required for MEK/ERK phosphorylation. These results identify a novel system of receptor cross-talk between CD40 and BCR and indicate that the effects of CD40 engagement on subsequent BCR stimulation spread beyond NF-kappaB to involve the MAPK pathway.

Microarray and real-time RT-PCR analyses of differential human gene expression patterns induced by severe acute respiratory syndrome (SARS) coronavirus infection of Vero cells

Vero E6 African green monkey kidney cells are highly susceptible to infection with the newly emerging severe acute respiratory syndrome coronavirus (SARS-CoV), and they are permissive for rapid viral replication, with resultant cytopathic effects. We employed cDNA microarray analysis to characterize the cellular transcriptional responses of homologous human genes at 12 h post-infection. Seventy mRNA transcripts belonging to various functional classes exhibited significant alterations in gene expression. There was considerable induction of heat shock proteins that are crucial to the immune response mechanism. Modified levels of several transcripts involved in pro-inflammatory and anti-inflammatory processes exemplified the balance between opposing forces during SARS pathogenesis. Other genes displaying altered transcription included those associated with host translation, cellular metabolism, cell cycle, signal transduction, transcriptional regulation, protein trafficking, protein modulators, and cytoskeletal proteins. Alterations in the levels of several novel transcripts encoding hypothetical proteins and expressed sequence tags were also identified. In addition, transcription of apoptosis-related genes DENN and hIAP1 was upregulated in contrast to FAIM. Elevated Mx1 expression signified a strong host response to mediate antiviral resistance. Also expressed in infected cells was the C-terminal alternative splice variant of the p53 tumor suppressor gene encoding a modified truncated protein that can influence the activity of wild-type p53. We observed the interplay between various mechanisms to favor virus multiplication before full-blown apoptosis and the triggering of several pathways in host cells in an attempt to eliminate the pathogen. Microarray analysis identifies the critical host–pathogen interactions during SARS-CoV infection and provides new insights into the pathophysiology of SARS.

Delivery of bioactive molecules into the cell: the Trojan horse approach

In recent years, vast amounts of data on the mechanisms of neural de- and regeneration have accumulated. However, only in disproportionally few cases has this led to efficient therapies for human patients. Part of the problem is to deliver cell death-averting genes or gene products across the blood-brain barrier (BBB) and cellular membranes. The discovery of Antennapedia (Antp)-mediated transduction of heterologous proteins into cells in 1992 and other "Trojan horse peptides" raised hopes that often-frustrating attempts to deliver proteins would now be history. The demonstration that proteins fused to the Tat protein transduction domain (PTD) are capable of crossing the BBB may revolutionize molecular research and neurobiological therapy. However, it was only recently that PTD-mediated delivery of proteins with therapeutic potential has been achieved in models of neural degeneration in nerve trauma and ischemia. Several groups have published the first positive results using protein transduction domains for the delivery of therapeutic proteins in relevant animal models of human neurological disorders. Here, we give an extensive review of peptide-mediated protein transduction from its early beginnings to new advances, discuss their application, with particular focus on a critical evaluation of the limitations of the method, as well as alternative approaches. Besides applications in neurobiology, a large number of reports using PTD in other systems are included as well. Because each protein requires an individual purification scheme that yields sufficient quantities of soluble, transducible material, the neurobiologist will benefit from the experiences of other researchers in the growing field of protein transduction.

Phosphatidylinositol 3-Kinase-Dependent Mitogen-Activated Protein/Extracellular Signal-Regulated Kinase Kinase 1/2 and NF-κB Signaling Pathways Are Required for B Cell Antigen Receptor-Mediated Cyclin D2 Induction in Mature B Cells

Phosphatidylinositol 3-kinase (PI-3K) has been linked to promitogenic responses in splenic B cells following B cell Ag receptor (BCR) cross-linking; however identification of the signaling intermediates that link PI-3K activity to the cell cycle remains incomplete. We show that cyclin D2 induction is blocked by the PI-3K inhibitors wortmannin and LY294002, which coincides with impaired BCR-mediated mitogen-activated protein/extracellular signal-related kinase kinase (MEK)1/2 and p42/44ERK phosphorylation on activation residues. Cyclin D2 induction is virtually absent in B lymphocytes from mice deficient in the class I(A) PI-3K p85alpha regulatory subunit. In contrast to studies with PI-3K inhibitors, which inhibit all classes of PI-3Ks, the p85alpha regulatory subunit is not required for BCR-induced MEK1/2 and p42/44ERK phosphorylation, suggesting the contribution of another PI-3K family members in MEK1/2 and p42/44ERK activation. However, p85alpha(-/-) splenic B cells are defective in BCR-induced IkappaB kinase beta and IkappaBalpha phosphorylation. We demonstrate that NF-kappaB signaling is required for cyclin D2 induction via the BCR in normal B cells, implicating a possible link with the defective IkappaB kinase beta and IkappaBalpha phosphorylation in p85alpha(-/-) splenic B cells and their ability to induce cyclin D2. These results indicate that MEK1/2-p42/44ERK and NF-kappaB pathways link PI-3K activity to Ag receptor-mediated cyclin D2 induction in splenic B cells.