HIV-1, HTLV-1 and normal T-cell growth: transcriptional strategies and surprises

HTLV-1 infection of donor-derived T cells might promote acute graft-versus-host disease following liver transplantation

Acute graft versus host disease (aGVHD) is a rare, but severe complication of liver transplantation (LT). It is caused by the activation of donor immune cells in the graft against the host shortly after transplantation, but the contributing pathogenic factors remain unclear. Here we show that human T cell lymphotropic virus type I (HTLV-1) infection of donor T cells is highly associated with aGVHD following LT. The presence of HTLV-1 in peripheral blood and tissue samples from a discovery cohort of 7 aGVHD patients and 17 control patients is assessed with hybridization probes (TargetSeq), mass cytometry (CyTOF), and multiplex immunohistology (IMC). All 7 of our aGVHD patients display detectable HTLV-1 Tax signals by IMC. We identify donor-derived cells based on a Y chromosome-specific genetic marker, EIF1AY. Thus, we confirm the presence of CD4⁺Tax⁺EIF1AY⁺ T cells and Tax⁺CD68⁺EIF1AY⁺ antigen-presenting cells, indicating HTLV-1 infection of donor immune cells. In an independent cohort of 400 patients, we verify that HTLV-1 prevalence correlates with aGVHD incidence, while none of the control viruses shows significant associations. Our findings thus provide new insights into the aetio-pathology of liver-transplantation-associated aGVHD and raise the possibility of preventing aGVHD prior to transplantation.

Bromodomain and extraterminal (BET) protein inhibition suppresses human T cell leukemia virus 1 (HTLV-1) Tax protein-mediated tumorigenesis by inhibiting nuclear factor κB (NF-κB) signaling

The etiology of human T cell leukemia virus 1 (HTLV-1)-mediated adult T cell leukemia is associated with the ability of viral oncoprotein Tax to induce sustained NF-κB activation and the expression of many NF-κB target genes. Acetylation of the RelA subunit of NF-κB and the subsequent recruitment of bromodomain-containing factor Brd4 are important for the expression of NF-κB target genes in response to various stimuli. However, their contributions to Tax-mediated NF-κB target gene expression and tumorigenesis remain unclear. Here we report that Tax induced the acetylation of lysine 310 of RelA and the binding of Brd4 to acetylated RelA to facilitate Tax-mediated transcriptional activation of NF-κB. Depletion of Brd4 down-regulated Tax-mediated NF-κB target gene expression and cell proliferation. Inhibiting the interaction of Brd4 and acetylated RelA with the bromodomain extraterminal protein inhibitor JQ1 suppressed the proliferation of Tax-expressing rat fibroblasts and Tax-positive HTLV-1-infected cells and Tax-mediated cell transformation and tumorigenesis. Moreover, JQ1 attenuated the Tax-mediated transcriptional activation of NF-κB, triggering the polyubiquitination and proteasome-mediated degradation of constitutively active nuclear RelA. Our results identify Brd4 as a key regulator for Tax-mediated NF-κB gene expression and suggest that targeting epigenetic regulators such as Brd4 with the bromodomain extraterminal protein inhibitor might be a potential therapeutic strategy for cancers and other diseases associated with HTLV-1 infection.

Socio-demographic and epidemiological characteristics associated with human immunodeficiency virus type I (HIV-1) infection in HIV-1-exposed but uninfected individuals, and in HIV-1-infected patients from a southern Brazilian population

The ability to control human immunodeficiency virus type 1 (HIV-1) infection and progression of the disease is regulated by host and viral factors. This cross-sectional study describes the socio-demographic and epidemiological characteristics associated with HIV-1 infection in 1,061 subjects attended in Londrina and region, south of Brazil: 136 healthy individuals (Group 1), 147 HIV-1-exposed but uninfected individuals (Group 2), 161 HIV-1-infected asymptomatic patients (Group 3), and 617 patients with AIDS (Group 4). Data were obtained by a standardized questionnaire and serological tests. The age of the individuals ranged from 15.1 to 79.5 years, 54.0% and 56.1% of the Groups 3 and 4 patients, respectively, were men. The major features of groups 2, 3, and 4 were a predominance of education level up to secondary school (55.8%, 60.2% and 62.4%, respectively), sexual route of exposure (88.4%, 87.0% and 82.0%, respectively), heterosexual behavior (91.8%, 75.2% and 83.7%, respectively), and previous sexually transmitted diseases (20.4%, 32.5%, and 38.1%, respectively). The patients with AIDS showed the highest rates of seropositivity for syphilis (25.6%), of anti-HCV (22.3%), and anti-HTLV I/II obtained by two serological screening tests (6.2% and 6.8%, respectively). The results documenting the predominant characteristics for HIV-1 infection among residents of Londrina and region, could be useful for the improvement of current HIV-1 prevention, monitoring and therapeutic programs targeted at this population.

Compared to mycophenolate mofetil, rapamycin induces significant changes on growth factors and growth factor receptors in the early days post-kidney transplantation

BACKGROUND

The new immunosuppressive drug Rapamycin (Rapa) is endowed with a mechanism of action that is distinct from that of calcineurin inhibitors. It has been claimed that Rapa does not significantly modulate either the cytokine expression or the transcription of several growth factors in mitogen-activated T cells. Previously, we reported that fine-needle aspiration biopsy (FNAB) sample cultures synthesize a large array of cytokines, and some of them may be powerful predictors of acute rejection in renal transplants. We hypothesized that Rapa may induce significant changes on cytokine production by FNAB sample cultures and on serum cytokine receptors when compared to other immunosuppressive drugs.

METHODS

Kidney transplants treated with CsA-Rapa-Pred (Rapa group) were compared with transplants treated with CsA-mycophenolate mofetil-Pred (MMF group). They were studied on day 7 posttransplantation, and they remained rejection free for at least the first 6 months. FNAB samples were cultured and the supernatants were collected at 48 hr of incubation and analyzed by ELISA for interleukin 1 receptor antagonist (IL-1ra), IL-2, IL-6, IL-10, IL-18, monocyte chemotactic protein 1 (MCP-1), soluble tumor necrosis factor I, and transforming growth factor (TGF)-beta(1). The soluble receptors for IL-1, IL-2, IL-6, and tumor necrosis factor alpha, together with IL-2 and IL-18 were also measured in serum.

RESULTS

Significant differences were observed when comparing Rapa with the MMF group. IL-18 and TGF-beta(1) synthesis were up-regulated, whereas IL-6 and MCP-1 were down-regulated in FNAB sample cultures. The Rapa group showed a significant down-regulation of each cytokine receptor and of IL-2 in serum.

CONCLUSIONS

Rapa was associated with a decreased synthesis of primarily monocyte-derived cytokines and enhanced production of TGF-beta(1), which in an appropriate cytokine milieu may promote allograft tolerance. The down-regulation of cytokine receptors and IL-2 may be associated with a depressed immune response towards the kidney allograft.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Autoantibodies to TNFalpha in HIV-1 infection: prospects for anti-cytokine vaccine therapy

Tumor necrosis factor alpha (TNFalpha) is a proinflammatory cytokine principally involved in the activation of lymphocytes in response to viral infection. TNFalpha also stimulates the production of other cytokines, activates NK cells and potentiates cell death and/or lysis in certain models of viral infection. Although TNFalpha might be expected to be a protective component of an antiviral immune response, several lines of evidence suggest that TNFalpha and other virally-induced cytokines actually may contribute to the pathogenesis of HIV infection. Based on the activation of HIV replication in response to TNFalpha, HIV appears to have evolved to take advantage of host cytokine activation pathways. Antibodies to TNFalpha are present in the serum of normal individuals as well as in certain autoimmune disorders, and may modulate disease progression in the setting of HIV infection. We examined TNFalpha-specific antibodies in HIV-infected non-progressors and healthy seronegatives; anti-TNFalpha antibody levels are significantly higher in GRIV seropositive slow/non-progressors (N = 120, mean = 0.24), compared to seronegative controls (N= 12, mean = 0.11). TNFalpha antibodies correlated positively with viral load, (P = 0.013, r = 0.282), and CD8+ cell count (P = 0.03, r = 0.258), and inversely with CD4+ cell count (P = 0.003, r = - 0.246), percent CD4+ cells (P = 0.008, r = -0.306), and CD4 :CD8 ratio (P = 0.033, r = - 0.251). TNFalpha antibodies also correlated positively with antibodies to peptides corresponding to the CD4 binding site of gp160 (P = 0.001, r = 0.384), the CD4 identity region (P = 0.016, r = 0.29), the V3 loop (P = 0.005, r = 0.34), and the amino terminus of Tat (P = 0.001, r = 0.395); TNFalpha antibodies also correlated positively with antibodies to Nef protein (P = 0.008, r = 0.302). The production of anti-TNFalpha antibodies appears to be an adaptive response to HIV infection and suggests the potential utility of modified cytokine vaccines in the treatment of HIV infections as well as AIDS-related and unrelated autoimmune and CNS disorders.

Activation of IKKalpha and IKKbeta through their fusion with HTLV-I tax protein

Human T-cell leukemia virus type I (HTLV-I) Tax protein persistently stimulates the activity of IkappaB kinase (IKK), resulting in constitutive activation of the transcription factor NF-kappaB. Tax activation of IKK requires physical interaction of this viral protein with the IKK regulatory subunit, IKKgamma. The Tax/IKKgamma interaction allows Tax to engage the IKK catalytic subunits, IKKalpha and IKKbeta, although it remains unclear whether this linker function of IKKgamma is sufficient for supporting the Tax-specific IKK activation. To address this question, we have examined the sequences of IKKgamma required for modulating the Tax/IKK signaling. We demonstrate that when fused to Tax, a small N-terminal fragment of IKKgamma, containing its minimal IKKalpha/beta-binding domain, is sufficient for bringing Tax to and activating the IKK catalytic subunits. Disruption of the IKKalpha/beta-binding activity of this domain abolishes its function in modulating the Tax/IKK signaling. We further demonstrate that direct fusion of Tax to IKKalpha and IKKbeta leads to activation of these kinases. These findings suggest that the IKKgamma-directed Tax/IKK association serves as a molecular trigger for IKK activation.

Reovirus-induced apoptosis requires activation of transcription factor NF-kappaB

Reovirus infection induces apoptosis in cultured cells and in vivo. To identify host cell factors that mediate this response, we investigated whether reovirus infection alters the activation state of the transcription factor nuclear factor kappa B (NF-kappaB). As determined in electrophoretic mobility shift assays, reovirus infection of HeLa cells leads to nuclear translocation of NF-kappaB complexes containing Rel family members p50 and p65. Reovirus-induced activation of NF-kappaB DNA-binding activity correlated with the onset of NF-kappaB-directed transcription in reporter gene assays. Three independent lines of evidence indicate that this functional form of NF-kappaB is required for reovirus-induced apoptosis. First, treatment of reovirus-infected HeLa cells with a proteasome inhibitor prevents NF-kappaB activation following infection and substantially diminishes reovirus-induced apoptosis. Second, transient expression of a dominant-negative form of IkappaB that constitutively represses NF-kappaB activation significantly reduces levels of apoptosis triggered by reovirus infection. Third, mutant cell lines deficient for either the p50 or p65 subunits of NF-kappaB are resistant to reovirus-induced apoptosis compared with cells expressing an intact NF-kappaB signaling pathway. These findings indicate that NF-kappaB plays a significant role in the mechanism by which reovirus induces apoptosis in susceptible host cells.

Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Biochemical coupling of transcription factor NF-kappaB to antigen and co-stimulatory receptors is required for the temporal control of T-cell proliferation. In contrast to its transitory activation during normal growth-signal transduction, NF-kappaB is constitutively deployed in T-cells transformed by the type 1 human T-cell leukemia virus (HTLV-1). This viral/host interaction is mediated by the HTLV-1-encoded Tax protein, which has potent oncogenic properties. As reviewed here, Tax activates NF-kappaB primarily via a pathway leading to the chronic phosphorylation and degradation of IkappaBalpha, a cytoplasmic inhibitor of NF-kappaB. To access this pathway, Tax associates stably with a cytokine-inducible IkappaB kinase (IKK), which contains both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. Unlike their transiently induced counterparts in cytokine-treated cells, Tax-associated forms of IKKalpha and IKKbeta are persistently activated in HTLV-1-infected T cells. Acquisition of the deregulated IKK phenotype is contingent on the presence of IKKgamma, which functions as a molecular adaptor in the assembly of pathologic Tax/IkappaB kinase complexes. These findings highlight a key mechanistic role for IKK in the Tax/NF-kappaB signaling axis and define new intracellular targets for the therapeutic control of HTLV-1-associated disease.

IKKgamma serves as a docking subunit of the IkappaB kinase (IKK) and mediates interaction of IKK with the human T-cell leukemia virus Tax protein

The tax gene product of human T-cell leukemia virus type I induces activation of transcription factor NF-kappaB, which contributes to deregulated expression of various cellular genes. Tax expression triggers persistent phosphorylation and degradation of the NF-kappaB inhibitory proteins IkappaBalpha and IkappaBbeta, resulting in constitutive nuclear expression of NF-kappaB. Recent studies demonstrate that Tax activates the IkappaB kinase (IKK), although the underlying mechanism remains unclear. In this report, we show that Tax physically interacts with a regulatory component of the IKK complex, the NF-kappaB essential modulator or IKKgamma (NEMO/IKKgamma). This molecular interaction appears to be important for recruiting Tax to the IKK catalytic subunits, IKKalpha and IKKbeta. Expression of NEMO/IKKgamma greatly promotes binding of Tax to IKKalpha and IKKbeta and stimulates Tax-mediated IKK activation. Interestingly, a mutant form of Tax defective in IKK activation exhibited a markedly diminished level of NEMO/IKKgamma association. These findings suggest that the physical interaction of Tax with NEMO/IKKgamma may play an important role in Tax-mediated IKK activation.

Human T-cell leukemia virus type 1 Tax induction of NF-kappaB involves activation of the IkappaB kinase alpha (IKKalpha) and IKKbeta cellular kinases

Tax corresponds to a 40-kDa transforming protein from the pathogenic retrovirus human T-cell leukemia virus type 1 (HTLV-1) that activates nuclear expression of the NF-kappaB/Rel family of transcription factors by an unknown mechanism. Tax expression promotes N-terminal phosphorylation and degradation of IkappaB alpha, a principal cytoplasmic inhibitor of NF-kappaB. Our studies now demonstrate that HTLV-1 Tax activates the recently identified cellular kinases IkappaB kinase alpha (IKKalpha) and IKKbeta, which normally phosphorylate IkappaB alpha on both of its N-terminal regulatory serines in response to tumor necrosis factor alpha (TNF-alpha) and interleukin-1 (IL-1) stimulation. In contrast, a mutant of Tax termed M22, which does not induce NF-kappaB, fails to activate either IKKalpha or IKKbeta. Furthermore, endogenous IKK enzymatic activity was significantly elevated in HTLV-1-infected and Tax-expressing T-cell lines. Transfection of kinase-deficient mutants of IKKalpha and IKKbeta into either human Jurkat T or 293 cells also inhibits NF-kappaB-dependent reporter gene expression induced by Tax. Similarly, a kinase-deficient mutant of NIK (NF-kappaB-inducing kinase), which represents an upstream kinase in the TNF-alpha and IL-1 signaling pathways leading to IKKalpha and IKKbeta activation, blocks Tax induction of NF-kappaB. However, plasma membrane-proximal elements in these proinflammatory cytokine pathways are apparently not involved since dominant negative mutants of the TRAF2 and TRAF6 adaptors, which effectively block signaling through the cytoplasmic tails of the TNF-alpha and IL-1 receptors, respectively, do not inhibit Tax induction of NF-kappaB. Together, these studies demonstrate that HTLV-1 Tax exploits a distal part of the proinflammatory cytokine signaling cascade leading to induction of NF-kappaB. The pathological alteration of this cytokine pathway leading to NF-kappaB activation by Tax may play a central role in HTLV-1-mediated transformation of human T cells, clinically manifested as the adult T-cell leukemia.

Involvement of NF-AT in type I human T-cell leukemia virus Tax-mediated Fas ligand promoter transactivation

Human T-cell leukemia virus type I (HTLV-I)-infected T-cells constitutively express surface Fas ligand (FasL), which may serve as a mechanism of viral pathogenesis. HTLV-I induces transcription of FasL gene through the viral transactivator Tax, although the underlying molecular mechanism remains unclear. In the present study, we have analyzed both the cis-activating element and transactivating factors involved in Tax activation of the FasL promoter. We show that the 486-base pair upstream region of the human FasL gene is sufficient for Tax-mediated transactivation in Jurkat T-cells. Interestingly, a palindromic DNA sequence (GGAAACTTCC) covering the consensus NF-ATp binding site (GGAAA), is required for Tax activation of FasL promoter. The involvement of NF-AT in this transactivation event is suggested by the finding that Tax fails to activate the FasL promoter in a Jurkat T-cell line defective in capacitative calcium entry, a signaling mechanism involved in NF-AT activation. Furthermore, activation of FasL promoter by Tax is largely attenuated in the nonlymphoid F9 embryonal and COS kidney cells deficient in NF-ATp expression. DNA-protein interaction assays reveal that the NF-AT-like motif in the FasL promoter is bound by both NF-ATp and NF-AT4 in Jurkat T-cells expressing Tax. The binding of NF-ATp, although not NF-AT4, to this enhancer also occurs along with HTLV-I-mediated infection of human peripheral blood T-cells. Furthermore, exogenously transfected NF-ATp binds to the NF-AT-like enhancer and participates in Tax-mediated FasL promoter transactivation. These results suggest an important role for proteins of the NF-AT family in HTLV-I Tax-mediated FasL gene transactivation.

NF-kappaB-inducing kinase and IkappaB kinase participate in human T-cell leukemia virus I Tax-mediated NF-kappaB activation

The tax gene product of human T-cell leukemia virus I induces aberrant expression of various cellular genes, which contributes to transformation of host cells. Induction of many Tax target genes is mediated through transcription factor NF-kappaB. Here we show that Tax triggers activation of cellular protein kinases, IkappaB kinase alpha (IKKalpha) and IKKbeta, which phosphorylate the NF-kappaB inhibitory protein IkappaB alpha, resulting in its degradation and NF-kappaB activation. Constitutive IKK activation occurs in both Tax-transfected and human T-cell leukemia virus I-infected T cells. We further demonstrate that Tax-mediated NF-kappaB signaling also requires the NF-kappaB-inducing kinase (NIK). Consistently, inactive forms of either IKKs or NIK attenuate Tax-mediated NF-kappaB activation. Therefore, Tax activates NF-kappaB by targeting cellular signaling molecules, including both IKKs and NIK.

B cell-driven HIV type 1 expression in T cells: an essential role of CD86 costimulatory molecule

During HIV-1 infection, HIV-1 is sequestered and actively replicates within lymphoid organs, mainly in areas essential for antigen-specific T-B interactions. We investigated whether cognate T-B interactions not only drive humoral response to HIV-1 but also enhance viral replication. Costimulation of in vitro HIV-1-infected tonsillar T cells with autologous or allogeneic activated B cells increased both viral replication and T cell proliferation. Addition of CD86 MAb to cocultures inhibited most p24 (84 +/- 12%, n = 13) and IL-2 (99 +/- 2%, n = 6) production, decreased T cell proliferation by 46 +/- 15% (n = 13), and decreased TNF-alpha and IFN-gamma production by 67 +/- 17% (n = 6) and 53 +/- 6% (n = 6), respectively. In contrast, CD80 MAb, which strongly inhibited IL-2 production (77 +/- 10%, n = 6), moderately downregulated p24 and TNF-alpha production (29 +/- 21%, n = 13 and 34 +/- 10%, n = 6, respectively) and did not decrease T cell proliferation (8 +/- 10%, n = 13) or IFN-gamma production (14 +/- 13%, n = 6). We thus showed that B cells deliver a potent CD86/CD28 costimulatory signal that induces T cell proliferation and simultaneously enhances HIV-1 replication. CD86+ B cells, mainly localized within the light zone of germinal centers, might thus favor active in situ replication of HIV-1 in response to each new challenge by T-dependent antigens.

Expansion of clonotypic T-cell populations in the peripheral blood of asymptomatic Gran Chaco Amerindians infected with HTLV-IIB

Peripheral blood mononuclear cells from asymptomatic HTLV-II-infected and uninfected Gran Chaco Amerindians were analyzed using polymerase chain reaction (PCR) for expansions of T-cell receptor (TCR) V-beta gene clonotypes. Analyses were performed using primer pairs designed to identify expanded T-cell familial clonotypes based on their unique TCR beta gene rearrangements. Of the 30 HTLV-IIB-positive samples tested, five showed evidence of V-beta clonotypic T-cell expansion. Of the five expansions, two were monoclonotypic and the remaining three were oligoclonotypic. In comparison, 30 HTLV-II-negative Amerindians showed no evidence of clonotypic T-cell expansion. Amplified DNA from one of the monoclonotypic samples was subsequently cloned and sequenced and was found to have uniform variable/ diversity/joining sequences confirming its unique monoclonal T-cell expansion. This method of detecting clonal TCR beta gene rearrangements has the advantage over traditional Southern blot techniques of being more sensitive and specific even with suboptimal specimens. The prognostic significance of clonotypic T-cell expansion in a group such as the HTLV-II-infected Gran Chaco Amerindians remains to be determined.

Constitutive dephosphorylation and activation of a member of the nuclear factor of activated T cells, NF-AT1, in Tax-expressing and type I human T-cell leukemia virus-infected human T cells

The tax gene product of the type I human T-cell leukemia virus (HTLV-I) transactivates interleukin-2 (IL-2) gene through activation of an enhancer termed CD28 responsive element (CD28RE). Tax activation of the CD28RE is partially mediated by a member of the nuclear factor of activated T cells, NF-AT1. We have previously shown that NF-AT1 is constitutively active in Jurkat T cells stably transfected with the Tax cDNA, although the underlying molecular mechanism and physiological relevance of this finding remain unclear. In this report, we demonstrate that the active form of NF-AT1 is also present in the nuclei of HTLV-I-transformed T cells that express the Tax protein. Interestingly, the constitutive activation of NF-AT1 in these T cells is associated with its dephosphorylation. Furthermore, the dephosphorylated NF-AT1 can be rapidly rephosphorylated when the cells are incubated with cyclosporin A, an immunosuppressant inhibiting the serine/threonine phosphatase calcineurin. These results suggest that activation of NF-AT1 in Tax-expressing and HTLV-I-transformed T cells results from its dephosphorylation, which in turn may be due to deregulation of calcineurin.

Costimulation of cytokine gene expression in T cells by the human T leukemia/lymphotropic virus type 1 trans activator Tax

Many cell signals such as CD28 and CD4 binding can costimulate cytokine gene expression in activated T cells. We have found that the human T leukemia/lymphotropic virus type 1 viral protein Tax can also strongly costimulate expression of interleukin-2 (IL-2), IL-3, and granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA in T cells activated with the phorbol ester phorbol myristate acetate (PMA) and calcium ionophore, which can mimic activation through the antigen specific T-cell receptor. Reporter constructs also showed strong synergy between both stimuli and showed that Tax and the PMA-Ca2+ ionophore act through different regions of the IL-2 and GM-CSF genes. Furthermore, the Tax-responsive regions (TxRR) from both GM-CSF and IL-2 respond to costimulation through the CD28 surface receptor. The GM-CSF and IL-2 TxRRs showed significantly higher levels of NF-kappaB/rel binding, following induction by Tax, compared with that of the PMA-Ca2+ ionophore with only Tax capable of inducing c-Rel binding to a Consensus kappaB element within the GM-CSF TxRR. Tax protein mutants, however, showed that a pathway(s) other than NF-kappaB/rel induction could also cooperate with the PMA-Ca2+ ionophore to activate the GM-CSF and IL-2 genes. This high-level costimulation by Tax, through multiple pathways, may be important in the early stages of leukemia and in the nervous system disorder tropical spastic paraparesis.

Persistent activation of NF-kappa B/Rel by human T-cell leukemia virus type 1 tax involves degradation of I kappa B beta

Activation of the eukaryotic NF-kappaB/Rel transcription factors by various cytokines and mitogens is a transient event, reflecting the fact that these inducers trigger the degradation and resynthesis of the dynamic NF-kappaB/Rel inhibitor IkappaBalpha. However, the tax gene product of the human T-cell leukemia virus type 1 (HTLV-1) is known to induce the persistent nuclear expression of various NF-kappaB/Rel factors, especially the c-Rel proto-oncoprotein, although the underlying mechanism remains unclear. In the present study, we demonstrate that Tax induces the degradation Of IkappaBbeta, another NF-kappaB/Rel cytoplasmic inhibitor that differs from IkappaBalpha in signal responses. Unlike that observed with IkappaBalpha, the degradation Of IkappaBbeta is not associated with its rapid resynthesis, apparently because of the failure of Tax to stimulate IkappaBbeta gene transcription. Thus, expression of Tax in Jurkat T cells leads to the gradual depletion of IkappaBbeta, which is correlated with the induction of c-Rel-containing kappaB binding complexes. Remarkably, in the three HTLV-1-infected T-cell lines investigated, little or no detectable amount of IkappaBbeta was found. We further demonstrate that Tax is able to override the cytoplasmic retention of c-Rel by 1kappaBbeta in transiently transfected cells. Together, these studies suggest that Tax-mediated inactivation Of IkappaBbeta may play a role in the persistent nuclear expression of c-Rel induced by HTLV-I infection.

Inactivation of IkappaBbeta by the tax protein of human T-cell leukemia virus type 1: a potential mechanism for constitutive induction of NF-kappaB

In resting T lymphocytes, the transcription factor NF-kappaB is sequestered in the cytoplasm via interactions with members of the I kappa B family of inhibitors, including IkappaBalpha and IkappaBbeta. During normal T-cell activation, IkappaBalpha is rapidly phosphorylated, ubiquitinated, and degraded by the 26S proteasome, thus permitting the release of functional NF-kappaB. In contrast to its transient pattern of nuclear induction during an immune response, NF-kappaB is constitutively activated in cells expressing the Tax transforming protein of human T-cell leukemia virus type I (HTLV-1). Recent studies indicate that HTLV-1 Tax targets IkappaBalpha to the ubiquitin-proteasome pathway. However, it remains unclear how this viral protein induces a persistent rather than transient NF-kappaB response. In this report, we provide evidence that in addition to acting on IkappaBalpha, Tax stimulates the turnover Of IkappaBbeta via a related targeting mechanism. Like IkappaBalpha, Tax-mediated breakdown of IkappaBbeta in transfected T lymphocytes is blocked either by cell-permeable proteasome inhibitors or by mutation Of IkappaBbeta at two serine residues present within its N-terminal region. Despite the dual specificity of HTLV-1 Tax for IkappaBalpha and IkappaBbeta at the protein level, Tax selectively stimulates NF-kappaB-directed transcription of the IkappaBalpha gene. Consequently, IkappaBbeta protein expression is chronically downregulated in HTLV-1-infected T lymphocytes. These findings with IkappaBbeta provide a potential mechanism for the constitutive activation of NF-kappaB in Tax-expressing cells.

Non-Hodgkin's lymphoma

Lymphoid neoplasia is a complex area comprising multiple diseases with varied pathology, treatment, and outcome. The non-Hodgkin's lymphomas are reviewed here. Non-Hodgkin's lymphomas, collectively, represent the sixth most common cancer in the United States as well as the sixth most common cause of cancer deaths. The overall incidence of non-Hodgkin's lymphoma has risen steadily over the past four decades. Although some of this is attributable to human immunodeficiency virus (HIV)-associated lymphoma, HIV-associated disease accounts for only a small part of the increase in lymphoma. As our knowledge of normal as well as neoplastic lymphoid development has expanded on the basis of histopathology as well as adjunct cellular and molecular techniques, multiple classifications have been proposed to take these into account. The clinical relevance to our understanding of non-Hodgkin's lymphoma is the concept that various lymphoid cancers are counterparts of stages of normal lymphoid development. Stages of lymphoid development in terms of cell surface markers and immunoglobulin gene rearrangements have been well characterized. These are particularly applicable to the early B-cell development, which is antigen-independent and occurs in the bone marrow. Diseases correlating with these stages are largely acute lymphocytic and lymphoblastic leukemia/lymphoma and high-grade lymphomas, such as Burkitt's lymphomas. Much has been learned recently about subsequent antigen-dependent B-cell development in secondary lymphoid organs to improve our understanding of the corresponding stages of B-cell neoplasia. Many of these stages correlate with more recently described entities such as mantle cell and marginal zone lymphomas. Histologic study remains crucial in determining the subtype of NHLs, whereas immunohistochemistry, surface phenotype, and molecular studies are useful in selected cases. Although some lymphoma classifications may be better in terms of understanding the lymphoma biology, the working formulation remains useful to guide clinical decision making. Lymphomas classified as low grade are considered incurable with standard therapy when diagnosed, as is usual, at advanced stages. Different subtypes may have different median survivals, but the goal has typically been palliation, whereas experimental approaches are clearly needed. Intermediate and high-grade lymphomas are potentially curable with aggressive combination chemotherapy. Recent evidence suggests that CHOP chemotherapy is as effective as more complex regimens. Still, 40% to 50% of patients are cured. Prognostic factor analysis has allowed separation of subgroups with much better survival in whom CHOP is adequate versus those with much poorer survival in whom experimental approaches are rational. Additional subtypes of lymphomas have been described and characterized since the working formulation was developed, including mucosa-associated lymphoid tissue tumors (MALT-oma), mantle zone lymphoma, anaplastic large cell lymphoma and AILD-like T-cell lymphoma. Approaches to these entities are still being optimized. Newer approaches, including high-dose therapy with stem cell support, biologic agents, and newer chemotherapeutic agents are discussed, as are special situations such as localized lymphoma of certain sites and lymphoma in immunosuppressed patients.

Herpesvirus Saimiri encodes a new cytokine, IL-17, which binds to a novel cytokine receptor

Herpesvirus Saimiri gene 13 (HVS13) exhibits 57% identity with the predicted sequence of a T cell-derived molecule termed CTLA8. Recombinant HVS13 and CTLA8 stimulate transcriptional factor NF-kappa B activity and interleukin-6 (IL-6) secretion in fibroblasts, and costimulate T cell proliferation. An HVS13.Fc fusion protein was used to isolate a cDNA encoding a novel receptor that also binds CTLA8. This receptor is unrelated to previously identified cytokine receptor families. A recombinant soluble receptor inhibited T cell proliferation and IL-2 production induced by PHA, concanavalin A (conA), and anti-TCR MAb. These results define CTLA8 and HVS13 as novel cytokines that bind to a novel cytokine receptor. We propose to call these molecules IL-17, vIL-17, and IL-17R, respectively.

Coupling of a signal response domain in I kappa B alpha to multiple pathways for NF-kappa B activation

The eukaryotic transcription factor NF-kappa B plays a central role in the induced expression of human immunodeficiency virus type 1 and in many aspects of the genetic program mediating normal T-cell activation and growth. The nuclear activity of NF-kappa B is tightly regulated from the cytoplasmic compartment by an inhibitory subunit called I kappa B alpha. This cytoplasmic inhibitor is rapidly phosphorylated and degraded in response to a diverse set of NF-kappa B-inducing agents, including T-cell mitogens, proinflammatory cytokines, and viral transactivators such as the Tax protein of human T-cell leukemia virus type 1. To explore these I kappa B alpha-dependent mechanisms for NF-kappa B induction, we identified novel mutants of I kappa B alpha that uncouple its inhibitory and signal-transducing functions in human T lymphocytes. Specifically, removal of the N-terminal 36 amino acids of I kappa B alpha failed to disrupt its ability to form latent complexes with NF-kappa B in the cytoplasm. However, this deletion mutation prevented the induced phosphorylation, degradative loss, and functional release of I kappa B alpha from NF-kappa B in Tax-expressing cells. Alanine substitutions introduced at two serine residues positioned within this N-terminal regulatory region of I kappa B alpha also yielded constitutive repressors that escaped from Tax-induced turnover and that potently inhibited immune activation pathways for NF-kappa B induction, including those initiated from antigen and cytokine receptors. In contrast, introduction of a phosphoserine mimetic at these sites rectified this functional defect, a finding consistent with a causal linkage between the phosphorylation status and proteolytic stability of this cytoplasmic inhibitor. Together, these in vivo studies define a critical signal response domain in I kappa B alpha that coordinately controls the biologic activities of I kappa B alpha and NF-kappa B in response to viral and immune stimuli.

Anti-immunoglobulin and anti-CD40 stimulation induces CD25 expression by resting human tonsillar B lymphocytes

In this report, the effect of ligation of a number of B-cell surface molecules upon expression of CD25, the 55-kDa inducible component of the IL-2 receptor complex found on T and B lymphocytes, is reported. IL-4 is the only cytokine apparently capable of promoting CD25 expression in human high-density quiescent tonsillar B cells; neither IL-10 nor IL-13 could induce CD25 expression. Cross-linking of the antigen receptors or CD40 with antibody elicited CD25 expression in a dose-dependent manner. Stimulation with anti-CD40 promoted CD25 expression in approximately 25% of B cells, while anti-Ig caused 80% or more of cells to become CD25+. In experiments where the stimuli were used in combination, some additive effects upon CD25 expression were noted, but no obvious synergistic effects could be detected.

Transactivation of cellular genes by human retroviruses

We have focused this chapter on interactions with two of the best characterized transregulatory genes, tax for HTLV-I/II and Tat for HIV-1. Both genes illustrate the complex interplay between retroviral regulatory genes and cellular gene regulation. In both instances a viral gene of relatively straightforward function in the viral context appears to cause extensive dysregulation of cellular genes, either directly or as a consequence of altered cellular differentiation. Understanding this viral/cellular gene cross-talk may elucidate mechanisms leading to malignant transformation autoimmune disease and to neurologic and paraneoplastic complications such as hypercalcemia for HTLV-I/II, as well as the pathogenesis of immune dysfunction and opportunistic malignancy in HIV-I/II-infected individuals. An understanding of functional mechanisms of these transregulatory viral genes will undoubtedly afford better explanations for the myriad manifestations of retroviral infection.

Human T-cell leukemia virus type I Tax activation of NF-kappa B/Rel involves phosphorylation and degradation of I kappa B alpha and RelA (p65)-mediated induction of the c-rel gene

The tax gene product of human T-cell leukemia virus type I (HTLV-I) is a potent transcriptional activator that both stimulates viral gene expression and activates an array of cellular genes involved in T-cell growth. Tax acts indirectly by inducing or modifying the action of various host transcription factors, including members of the NF-kappa B/Rel family of enhancer-binding proteins. In resting T cells, many of these NF-kappa B/Rel factors are sequestered in the cytoplasm by various ankyrin-rich inhibitory proteins, including I kappa B alpha. HTLV-I Tax expression leads to the constitutive nuclear expression of biologically active NF-kappa B and c-Rel complexes; however, the biochemical mechanism(s) underlying this response remains poorly understood. In this study, we demonstrate that Tax-stimulated nuclear expression of NF-kappa B in both HTLV-I-infected and Tax-transfected human T cells is associated with the phosphorylation and rapid proteolytic degradation of I kappa B alpha. In contrast to prior in vitro studies, at least a fraction of the phosphorylated form of I kappa B alpha remains physically associated with the NF-kappa B complex in vivo but is subject to rapid degradation, thereby promoting the nuclear translocation of the active NF-kappa B complex. We further demonstrate that Tax induction of nuclear c-Rel expression is activated by the RelA (p65) subunit of NF-kappa B, which activates transcription of the c-rel gene through an intrinsic kappa B enhancer element. In normal cells, the subsequent accumulation of nuclear c-Rel acts to inhibit its own continued production, indicating the presence of an autoregulatory loop. However, the pathologic action HTLV-I Tax leads to the deregulated and sustained nuclear expression of both NF-kappa B and c-Rel, a response that may contribute to HTLV-I-induced T-cell transformation.

Human T-cell leukemia virus type I Tax activation of NF-kappa B/Rel involves phosphorylation and degradation of I kappa B alpha and RelA (p65)-mediated induction of the c-rel gene

The tax gene product of human T-cell leukemia virus type I (HTLV-I) is a potent transcriptional activator that both stimulates viral gene expression and activates an array of cellular genes involved in T-cell growth. Tax acts indirectly by inducing or modifying the action of various host transcription factors, including members of the NF-kappa B/Rel family of enhancer-binding proteins. In resting T cells, many of these NF-kappa B/Rel factors are sequestered in the cytoplasm by various ankyrin-rich inhibitory proteins, including I kappa B alpha. HTLV-I Tax expression leads to the constitutive nuclear expression of biologically active NF-kappa B and c-Rel complexes; however, the biochemical mechanism(s) underlying this response remains poorly understood. In this study, we demonstrate that Tax-stimulated nuclear expression of NF-kappa B in both HTLV-I-infected and Tax-transfected human T cells is associated with the phosphorylation and rapid proteolytic degradation of I kappa B alpha. In contrast to prior in vitro studies, at least a fraction of the phosphorylated form of I kappa B alpha remains physically associated with the NF-kappa B complex in vivo but is subject to rapid degradation, thereby promoting the nuclear translocation of the active NF-kappa B complex. We further demonstrate that Tax induction of nuclear c-Rel expression is activated by the RelA (p65) subunit of NF-kappa B, which activates transcription of the c-rel gene through an intrinsic kappa B enhancer element. In normal cells, the subsequent accumulation of nuclear c-Rel acts to inhibit its own continued production, indicating the presence of an autoregulatory loop. However, the pathologic action HTLV-I Tax leads to the deregulated and sustained nuclear expression of both NF-kappa B and c-Rel, a response that may contribute to HTLV-I-induced T-cell transformation.

Serum response factor has functional roles both in indirect binding to the CArG box and in the transcriptional activation function of human T-cell leukemia virus type I Tax

We previously reported that Tax1 of human T-cell leukemia virus type I interacts directly with serum response factor (SRF) and that Tax1 activates the transcription of several cellular immediate-early genes through the SRF binding site (CArG box). This activation required the transcriptional activation function of Tax1, identified as an activity of GALTax (a chimeric Tax1 with the yeast transcription factor GAL4) at the GAL4-binding site. In this study, we examined whether SRF plays a role in the transcriptional activation function of Tax1. Expression of Tax1 suppressed the GALTax activity at the GAL4 site as a result of squelching, and the suppressed activity was recovered by the overexpression of SRF, suggesting that SRF is a factor that is required for GALTax activity and that is inhibited by competition with Tax1. The expression of antisense SRF RNA specifically inhibited GALTax activity to less than 20%. Deletion of the Tax1 interaction domain of SRF at its C terminus converted SRF from an activator of GALTax to an inhibitor. These results suggest that SRF is an essential component of the transcriptional activation of Tax1 in addition to a mediator of CArG box binding.

A novel NF-kappa B complex containing p65 homodimers: implications for transcriptional control at the level of subunit dimerization

The predominant inducible form of the NF-kappa B transcription factor is a heteromeric complex containing two Rel-related DNA-binding subunits, termed p65 and p50. Prior transfection studies have shown that when these p65 and p50 subunits are expressed independently as stable homodimers, p65 stimulates kappa B-directed transcription, whereas p50 functions as a kappa B-specific repressor. While authentic p50 homodimers (previously termed KBF1) have been detected in nuclear extracts from nontransfected cells, experimental evidence supporting the existence of p65 homodimers in vivo was lacking. We now provide direct biochemical evidence for the presence of an endogenous pool of inducible p65 homodimers in intact human T cells. As with the prototypical NF-kappa B p50-p65 heterodimer, this novel p65 homodimeric form of NF-kappa B is functionally sequestered in the cytoplasm but rapidly appears in the nuclear compartment following cellular stimulation. Site-directed mutagenesis studies indicate that the homodimerization function of p65 is dependent upon the presence of cysteine 216 and a conserved recognition motif for protein kinase A (RRPS; amino acids 273 to 276), both of which reside within a 91-amino-acid segment of the Rel homology domain that mediates self-association. In contrast, mutations at these two sites do not affect heterodimerization of p65 with p50 or its functional interaction with I kappa B alpha. These later findings indicate that neither homo- nor heterodimer formation is an absolute prerequisite for I kappa B alpha recognition of p65. Taken together with prior in vivo transcription studies, these results suggest that the biological activities of p65 and p50 homodimers are independently regulated, thereby providing an integrated and flexible control mechanism for the rapid activation and repression of NF-kappa B/Rel-directed gene expression.

A novel NF-kappa B complex containing p65 homodimers: implications for transcriptional control at the level of subunit dimerization

The predominant inducible form of the NF-kappa B transcription factor is a heteromeric complex containing two Rel-related DNA-binding subunits, termed p65 and p50. Prior transfection studies have shown that when these p65 and p50 subunits are expressed independently as stable homodimers, p65 stimulates kappa B-directed transcription, whereas p50 functions as a kappa B-specific repressor. While authentic p50 homodimers (previously termed KBF1) have been detected in nuclear extracts from nontransfected cells, experimental evidence supporting the existence of p65 homodimers in vivo was lacking. We now provide direct biochemical evidence for the presence of an endogenous pool of inducible p65 homodimers in intact human T cells. As with the prototypical NF-kappa B p50-p65 heterodimer, this novel p65 homodimeric form of NF-kappa B is functionally sequestered in the cytoplasm but rapidly appears in the nuclear compartment following cellular stimulation. Site-directed mutagenesis studies indicate that the homodimerization function of p65 is dependent upon the presence of cysteine 216 and a conserved recognition motif for protein kinase A (RRPS; amino acids 273 to 276), both of which reside within a 91-amino-acid segment of the Rel homology domain that mediates self-association. In contrast, mutations at these two sites do not affect heterodimerization of p65 with p50 or its functional interaction with I kappa B alpha. These later findings indicate that neither homo- nor heterodimer formation is an absolute prerequisite for I kappa B alpha recognition of p65. Taken together with prior in vivo transcription studies, these results suggest that the biological activities of p65 and p50 homodimers are independently regulated, thereby providing an integrated and flexible control mechanism for the rapid activation and repression of NF-kappa B/Rel-directed gene expression.

Modulation of interleukin 2 high affinity binding to human T cells by a pyrimidodiazepine insect metabolite

An insect metabolite containing the little known pyrimido[4,5-b][1,4]diazepine ring system has been found to act as an effective mimic of tetrahydrobiopterin in its ability to modulate the affinity of interleukin 2 (IL-2) for its receptors on human T cells. Semi-empirical molecular orbital calculations reveal that while tetrahydrobiopterin has considerable flexibility, the pyrimidodiazepine has rather few conformational options and offers a useful model for exploring the nature of the pterin binding site.

Characterization of the human interleukin-2 receptor beta-chain gene promoter: regulation of promoter activity by ets gene products

The interleukin-2 receptor (IL-2R) beta chain (IL-2R beta) is an essential signaling component of high- and intermediate-affinity IL-2Rs. Our laboratory previously reported that a DNA fragment containing 857 bp of 5'-flanking sequence of the human IL-2R beta gene exhibited promoter activity. We have now further characterized the promoter and delineated cis-acting regulatory regions. The region downstream of -363 is critical for basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and contains at least three enhancer-like regions. Among them, the -56 to -34 enhancer was the most potent and had high-level activity in two T-cell lines but not in nonlymphoid HeLaS3 and MG63 cells. This enhancer contains a GGAA Ets binding site which bound two Ets family proteins, Ets-1 and GA-binding protein in vitro. Mutation of the Ets motif strongly diminished both promoter and enhancer activities. We conclude that this Ets binding site plays a key role in regulating basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and may also contribute to tissue-specific expression of the IL-2R beta gene.

Characterization of the human interleukin-2 receptor beta-chain gene promoter: regulation of promoter activity by ets gene products

The interleukin-2 receptor (IL-2R) beta chain (IL-2R beta) is an essential signaling component of high- and intermediate-affinity IL-2Rs. Our laboratory previously reported that a DNA fragment containing 857 bp of 5'-flanking sequence of the human IL-2R beta gene exhibited promoter activity. We have now further characterized the promoter and delineated cis-acting regulatory regions. The region downstream of -363 is critical for basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and contains at least three enhancer-like regions. Among them, the -56 to -34 enhancer was the most potent and had high-level activity in two T-cell lines but not in nonlymphoid HeLaS3 and MG63 cells. This enhancer contains a GGAA Ets binding site which bound two Ets family proteins, Ets-1 and GA-binding protein in vitro. Mutation of the Ets motif strongly diminished both promoter and enhancer activities. We conclude that this Ets binding site plays a key role in regulating basal and phorbol myristate acetate-inducible IL-2R beta promoter activity and may also contribute to tissue-specific expression of the IL-2R beta gene.

Cross-coupling of the NF-kappa B p65 and Fos/Jun transcription factors produces potentiated biological function

NF-kappa B and AP-1 represent distinct mammalian transcription factors that target unique DNA enhancer elements. The heterodimeric NF-kappa B complex is typically composed of two DNA binding subunits, NF-kappa B p50 and NF-kappa B p65, which share structural homology with the c-rel proto-oncogene product. Similarly, the AP-1 transcription factor complex is comprised of dimers of the c-fos and c-jun proto-oncogene products or of closely related proteins. We now demonstrate that the bZIP regions of c-Fos and c-Jun are capable of physically interacting with NF-kappa B p65 through the Rel homology domain. This complex of NF-kappa B p65 and Jun or Fos exhibits enhanced DNA binding and biological function via both the kappa B and AP-1 response elements including synergistic activation of the 5' long terminal repeat of the human immunodeficiency virus type 1. These findings support a combinatorial mechanism of gene regulation involving the unexpected cross-coupling of two different classes of transcription factors to form novel protein complexes exhibiting potentiated biological activity.

Functional and physical associations between NF-kappa B and C/EBP family members: a Rel domain-bZIP interaction

NF-kappa B and C/EBP represent distinct families of transcription factors that target unique DNA enhancer elements. The heterodimeric NF-kappa B complex is composed of two subunits, a 50- and a 65-kDa protein. All members of the NF-kappa B family, including the product of the proto-oncogene c-rel, are characterized by their highly homologous approximately 300-amino-acid N-terminal region. This Rel homology domain mediates DNA binding, dimerization, and nuclear targeting of these proteins. C/EBP contains the bZIP region, which is characterized by two motifs in the C-terminal half of the protein: a basic region involved in DNA binding and a leucine zipper motif involved in dimerization. The C/EBP family consist of several related proteins, C/EBP alpha, C/EBP beta, C/EBP gamma, and C/EBP delta, that form homodimers and that form heterodimers with each other. We now demonstrated the unexpected cross-coupling of members of the NF-kappa B family three members of the C/EBP family. NF-kappa B p65, p50, and Rel functionally synergize with C/EBP alpha, C/EBP beta, and C/EBP delta. This cross-coupling results in the inhibition of promoters with kappa B enhancer motifs and in the synergistic stimulation of promoters with C/EBP binding sites. These studies demonstrate that NF-kappa B augments gene expression mediated by a multimerized c-fos serum response element in the presence of C/EBP. We show a direct physical association of the bZIP region of C/EBP with the Rel homology domain of NF-kappa B. The cross-coupling of NF-kappa B with C/EBP highlights a mechanism of gene regulation involving an interaction between distinct transcription factor families.

Functional and physical associations between NF-kappa B and C/EBP family members: a Rel domain-bZIP interaction

NF-kappa B and C/EBP represent distinct families of transcription factors that target unique DNA enhancer elements. The heterodimeric NF-kappa B complex is composed of two subunits, a 50- and a 65-kDa protein. All members of the NF-kappa B family, including the product of the proto-oncogene c-rel, are characterized by their highly homologous approximately 300-amino-acid N-terminal region. This Rel homology domain mediates DNA binding, dimerization, and nuclear targeting of these proteins. C/EBP contains the bZIP region, which is characterized by two motifs in the C-terminal half of the protein: a basic region involved in DNA binding and a leucine zipper motif involved in dimerization. The C/EBP family consist of several related proteins, C/EBP alpha, C/EBP beta, C/EBP gamma, and C/EBP delta, that form homodimers and that form heterodimers with each other. We now demonstrated the unexpected cross-coupling of members of the NF-kappa B family three members of the C/EBP family. NF-kappa B p65, p50, and Rel functionally synergize with C/EBP alpha, C/EBP beta, and C/EBP delta. This cross-coupling results in the inhibition of promoters with kappa B enhancer motifs and in the synergistic stimulation of promoters with C/EBP binding sites. These studies demonstrate that NF-kappa B augments gene expression mediated by a multimerized c-fos serum response element in the presence of C/EBP. We show a direct physical association of the bZIP region of C/EBP with the Rel homology domain of NF-kappa B. The cross-coupling of NF-kappa B with C/EBP highlights a mechanism of gene regulation involving an interaction between distinct transcription factor families.

Cellular signalling mechanisms in B lymphocytes

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Inhibition of cellular activation of retroviral replication by CD8+ T cells derived from non-human primates

To test the hypothesis that CD8+ T cells inhibit viral replication at the level of cellular activation, an Epstein-Barr virus (EBV)-transformed cell line (FEc1) from a simian immunodeficiency virus (SIV)-seropositive sooty mangabey monkey was transfected with a human CD4 gene and shown to be replication-competent for HIV-1, HIV-2 and SIV. Utilizing a dual-chamber culture system, it was found that inhibition of viral replication can be mediated by a soluble factor. The FEc1 cell line was transiently transfected with an LTR-driven CAT reporter gene. It was found that autologous CD8+ T cells markedly inhibited CAT activity. Furthermore, co-transfection of the FEc1 cell line with an LTR-driven tat plasmid and LTR-CAT was able to quantitatively mitigate the suppressive effect. Thus, this inhibition appears to be directed at cellular mechanisms of viral transcription. Control transfections with an LTR-driven CAT plasmid with a mutation at the NFkB binding site yielded no CAT activity, suggesting that most viral replication as measured by CAT activity is dependent, to a large extent, upon cellularly derived NFkB binding proteins.

Identification of alternatively spliced mRNAs encoding potential new regulatory proteins in cattle infected with bovine leukemia virus

The polymerase chain reaction was used to detect and characterize low-abundance bovine leukemia virus (BLV) mRNAs. In infected cattle we could detect spliced mRNA with a splice pattern consistent with a Tax/Rex mRNA, as well as at least four alternatively spliced RNAs. Two of the alternatively spliced mRNAs encoded hitherto unrecognized BLV proteins, designated RIII and GIV. The Tax/Rex and alternatively spliced mRNAs could be detected at their highest levels in BLV-infected cell cultures; the next highest levels were found in samples from calves experimentally infected at 6 weeks postinoculation. Alternatively spliced mRNAs were also expressed, albeit at lower levels, in naturally infected animals; they were detected by a nested polymerase chain reaction. Interestingly, the GIV mRNA was specifically detected in naturally infected cows with persistent lymphocytosis and in two of five calves at 6 months after experimental infection with BLV. Furthermore, the calf with the strongest signal for GIV had the highest lymphocyte counts. These data may suggest a correlation between expression of the GIV product and development of persistent lymphocytosis. Some of the donor and acceptor sites in the alternatively spliced mRNAs were highly unusual. The biological mechanisms and significance of such a choice of unexpected splice sites are currently unknown.