Costimulatory regulation of T cell function

Cbl and Cbl-b in T-cell regulation

Genetic studies indicate that Cbl and Cbl-b, two highly homologous adaptor proteins, are involved in the negative regulation of thymocyte development and peripheral T-cell activation, respectively. The recent identification of Cbl proteins as RING-type E3 ubiquitin ligases might provide insights into their distinct immune regulatory functions, involving the targeting of different substrates for ubiquitination. The structural similarity and ubiquitous expression of Cbl and Cbl-b suggest, however, that they might also have overlapping functions in setting the thresholds for thymocyte selection and mature T-cell signaling.

A T cell-specific enhancer of the human CD40 ligand gene

We observed that the human CD40 ligand (CD40L) gene 5'-flanking region conferred weak promoter activity in activated CD4 T cells, suggesting that additional regions are required for optimal CD40L gene transcription. We therefore examined a 3'-flanking segment of the CD40L gene, which contained a putative NF-kappaB/Rel cis-element, for its ability to enhance CD40L promoter function. This segment augmented CD40L promoter activity in an orientation-independent manner in CD4 T-lineage cells but not in human B cell or monocyte cell lines. Mapping of CD4 T-lineage cell nuclei identified a DNase I-hypersensitive site in the flanking region near the NF-kappaB/Rel sequence, suggesting a transcriptional regulatory role. This was further supported by truncation analysis and site-directed mutagenesis, which indicated that the CD40L 3'-flanking NF-kappaB/Rel cis-element was critical for enhancer function. Electrophoretic mobility shift assays showed that the cis-element preferentially bound the p50 form of the NF-kappaB1 gene contained in human T cell nuclear protein extracts. This binding also appeared to occur in vivo in CD4 T cells based on chromatin immunoprecipitation assays using NF-kappaB p50-specific antiserum. Together, these results suggest that the CD40L gene 3'-flanking region acts as a T cell-specific classical transcriptional enhancer by a NF-kappaB p50-dependent mechanism.

The role of IRF-4 in B and T cell activation and differentiation

Appropriate activation and differentiation of lymphocytes are critical for effective immune responses. These processes are normally guided by exposure of lymphocytes to different stimuli, which need to be appropriately integrated in order for lymphocytes to proceed along their activation and differentiation pathways. Although the early steps in lymphocyte activation have been studied extensively, the downstream effectors of these activation pathways and the basic mechanisms employed by lymphocytes to integrate the information provided by different activation stimuli are not fully characterized. Interferon (IFN) regulatory factor-4 (IRF-4) is a recently described member of the IRF family of transcription factors whose expression is largely restricted to lymphocytes. Genetic studies have indicated that IRF-4 is critical for the function of mature T and B cells. Here we review the role of IRF-4 as a downstream effector and potentially an integrator of lymphocyte responses.

Improving immune reconstitution while preventing graft-versus-host disease in allogeneic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of choice for many hematologic malignancies or inherited disorders. Ex vivo T-cell depletion (TCD) of the graft and post-transplantation immunosuppression efficiently prevent the development of graft-versus-host disease (GVHD). However, the consequence of these nonspecific approaches is a long-lasting immunodeficiency associated with increased disease relapse, graft rejection, and reactivation of viral infections. Donor lymphocyte infusion, to treat leukemic relapse after allogeneic HSCT, can cause severe GVHD. Several strategies are being optimized to specifically inactivate anti-host T cells while preserving antileukemic or antimicrobial immunocompetence, based on ex vivo or in vivo elimination of anti-host T cells or on the modulation of their anti-host activity.

B7-1 and B7-2 co-stimulatory molecules are required for mercury-induced autoimmunity

B7-1 (CD80) and B7-2 (CD86) molecules on antigen presenting cells play important roles in providing co-stimulatory signals required for activation and expansion of autoreactive T cells. Moreover, some reports have suggested that these molecules may have distinct functions in the differentiation of Th1 and Th2 cells. Mercury-induced autoimmunity in H-2s mice is characterized by lymphoproliferation of T and B cells, serum increases in IgG1 and IgE and production of antinucleolar antibodies (ANoA). The mechanisms responsible for the various manifestations of this syndrome have yet to be elucidated. To examine the contributions of B7 co-stimulatory molecules to this model, susceptible mice were treated with antibodies to B7-1, B7-2, or both during the development of mercury-induced autoimmunity. The combination of anti-B7-1 and anti-B7-2 antibodies prevented Hg-induced disease in H-2s mice. Additionally, single anti-B7-1 antibody treatment was sufficient to prevent Hg-induced ANoA production, but not IgG1 and IgE hypergammaglobulinaemia. Further, single antibody treatment with anti-B7-2 resulted in a partial reduction of ANoA titres but had no significant effect on total serum IgG1 and IgE levels. Taken together, these results indicate that B7-1 and B7-2 molecules are critical for the development of Hg-induced autoimmunity and suggest that the different manifestations of the syndrome are regulated by independent mechanisms.

T cell infiltration of the prostate induced by androgen withdrawal in patients with prostate cancer

Manipulations capable of breaking host tolerance to induce tissue-specific T cell-mediated inflammation are of central importance to tumor immunotherapy and our understanding of autoimmunity. We demonstrate that androgen ablative therapy induces profuse T cell infiltration of benign glands and tumors in human prostates. T cell infiltration is readily apparent after 7-28 days of therapy and is comprised predominantly of a response by CD4+ T cells and comparatively fewer CD8+ T cells. Also, T cells within the treated prostate exhibit restricted TCR Vbeta gene usage, consistent with a local oligoclonal response. Recruitment/activation of antigen-presenting cells in treated prostate tissues may contribute to local T cell activation. The induction of T cell infiltration in prostate tissues treated with androgen ablation may have implications for the immunotherapeutic treatment of prostate cancer as well as other hormone-sensitive malignancies, including breast carcinoma.

Cytotoxic T lymphocyte antigen 4 and CD28 modulate cell surface raft expression in their regulation of T cell function

Coreceptors CD28 and cytotoxic T lymphocyte antigen (CTLA)-4 have opposing effects on TcR/CD3 activation of T cells. While CD28 enhances and CTLA-4 inhibits activation, the underlying molecular basis of these effects has yet to be established. In this context, ganglioside and cholesterol enriched membrane microdomains (rafts, GEMs) serve as centers of signaling in T cells. Although CD28 can promote TcR/raft colocalization, evidence is lacking on whether the surface expression of membrane rafts can be targeted by CTLA-4 in its modulation of T cell responses. In this study, we demonstrate that both CD28 and CTLA-4 profoundly alter the surface expression of membrane rafts during T cell activation. While CD28 increased expression and the number of peripheral T cells induced to express surface rafts in response to TcR ligation, CTLA-4 potently inhibited both TcR and TcR x CD28 induced raft expression on the surface of T cells. Consistent with this, CD28 increased the presence of the linker of activated T cells (LAT) in purified membrane rafts, while CTLA-4 coligation effectively blocked this increase. Further, the reversal of the CTLA-4 block with CD3/CD28 ligation was accompanied by an increase in surface raft expression and associated LAT. Our observations demonstrate for the first time that CTLA-4 targets the release of rafts to the surface of T cells, and provides a mechanism for the opposing effects of CD28 and CTLA-4 on costimulation.

Therapeutic intervention with inhibitors of co-stimulatory pathways in autoimmune disease

Many humanized antibodies and fusion proteins targeting T-cell co-stimulatory molecules are now in late-stage clinical development (phase II, phase III) or have recently completed phase III clinical trials. Both Amevive, an LFA-3-Ig fusion protein targeting CD2, and Xanelim, a humanized anti-CD11a antibody, have shown efficacy in pivotal phase III trials in patients with plaque psoriasis. These new medicines are poised to enter clinical use in 2002.

Site-specific immune response to implanted gliomas

OBJECT

Immunotherapy for glioblastoma has been uniformly ineffective. The immunological environment of the brain, with its low expression of major histocompatibility complex (MHC) molecules and limited access for inflammatory cells and humoral immune effectors due to the blood-brain barrier (BBB), may contribute to the failure of immunotherapy. The authors hypothesize that brain tumors are protected from immune surveillance by an intact BBB at early stages of development. To investigate the immunological characteristics of early tumor growth, the authors compared the host response to a glioma implanted into the brain and into subcutaneous tissue.

METHODS

Samples of tumors growing in the brain or subcutaneously in rats were obtained for 7 consecutive days and were examined immunohistochemically for MHC Class I & II molecules, and for CD4 and CD8 lymphocyte markers. Additionally, B7-1 costimulatory molecule expression and lymphocyte-specific apoptosis were examined.

CONCLUSIONS

On Days 3 and 4 after implantation, brain tumors displayed significantly lower MHC Class II expression and lymphocytic infiltration (p < 0.05). After Day 5, however, no differences were detected. The MHC Class II expressing cells within the brain tumors appeared to be infiltrating microglia. Minimal B7-1 expression combined with lymphocyte-specific apoptosis were detected in both brain and subcutaneous tumors. Low MHC Class II expression and low lymphocytic infiltration at early time points indicate the importance of the immunologically privileged status of the brain during early tumor growth. These characteristics disappeared at later time points, possibly because the increasing perturbation of the BBB alters the specific immunological environment of the brain. The lack of B7-1 expression combined with lymphocyte apoptosis indicates clonal anergy of glioma-infiltrating lymphocytes regardless of implantation site.

Calcium signaling through the beta 2-cytoplasmic domain of LFA-1 requires intracellular elements of the T cell receptor complex

The beta(2) integrin LFA-1 is an important cell-cell adhesion receptor of the immune system. Evidence suggests that the molecule also participates in signaling and co-stimulatory function. We show here that clustering of the intracellular domain of the beta(2) chain but not of the alpha(L)- or beta(1)-cytoplasmic domains, respectively, triggers intracellular Ca(2+) mobilization in Jurkat cells. A beta(2)-specific NPXF motif, located in the C-terminal portion of the beta(2) tail, is required for Ca(2+) signaling, and we show that this motif is important for the induction of allo-specific target cell lysis by cytotoxic T cells in vitro. Significantly, the Ca(2+)-signaling capacity of the beta(2) integrin is abrogated in T cells that do not express the T cell receptor but may be reconstituted by co-expression of the T cell receptor-zeta chain. Our data suggest a specific function of the cytoplasmic domain of the beta(2) integrin chain in T cell signaling.

CD80(B7.1) and CD86(B7.2) do not have distinct roles in setting the Th1/Th2 balance in autoimmunity in rats

Some data suggest that the interaction between CD28 and CD80 (B7.1) stimulates Th1-responses and that CD28 and CD86 (B7.2) stimulates Th2-responses, however this is controversial. We addressed this issue by using mercuric chloride (HgCl2)-induced autoimmunity in Brown Norway (BN) rats as a highly polarized Th2 model and experimental autoimmune encephalomyelitis (EAE) in Lewis rats as a highly polarized Th1 model. Monoclonal antibodies (MoAbs) to CD80 and CD86, given singly, had little effect in either model, however when given together they almost completely suppressed the HgCl2-induced autoimmunity: the peak immunoglobulin (Ig)E concentration was 3.25 microg/ml in treated animals versus 2770 microg/ml in controls (P < 0.0001); caecal vasculitis was suppressed with a median vasculitis score of 0 in treated animals versus 6 in controls (P < 0.0001); and new germinal centre formation was significantly suppressed. A combination of the antibodies also markedly reduced the severity of clinical EAE; from a median aggregate clinical score of 9 to 3 (P = 0.02) and delayed the onset from a median of 12.5 days to 16 days after immunization (P = 0.006). We have demonstrated profound suppression of both Th1 and Th2-driven autoimmunity in rats by a combination of anti-CD80 and CD86, but have been unable to demonstrate any clear differential effects.

A dual-function DNA vaccine encoding carcinoembryonic antigen and CD40 ligand trimer induces T cell-mediated protective immunity against colon cancer in carcinoembryonic antigen-transgenic mice

A carcinoembryonic Ag (CEA)-based DNA vaccine encoding both CEA and CD40 ligand trimer achieved effective tumor-protective immunity against murine colon carcinoma in CEA-transgenic mice by activating both naive T cells and dendritic cells. Peripheral T cell tolerance to CEA was broken in a prophylactic model by this novel, dual-function DNA vaccine, whose efficacy was further enhanced by boosts with a recombinant Ab-IL-2 fusion protein (huKS1/4-IL-2). These conclusions are supported by four lines of evidence. First, a lethal challenge of MC38-CEA-KS Ag murine colon carcinoma cells was for the first time completely rejected in 100% of experimental animals treated by oral gavage of this DNA vaccine carried by attenuated Salmonella typhimurium, followed by five boosts with huKS1/4-IL-2. Second, specific activation of dendritic cells was indicated by their marked up-regulation in expression of costimulatory molecules B7.1 (CD80), B7.2 (CD86), and ICAM-1. Third, a decisive increase over control values was observed in both MHC class I Ag-restricted cytotoxicity of CTLs from successfully vaccinated mice and secretion of proinflammatory cytokines IFN-gamma and IL-12. Fourth, activation of CTLs was augmented, as indicated by up-regulation of activity markers LFA-1, CD25, CD28, and CD69. Taken together, these results suggest that a dual-function DNA vaccine encoding CEA and CD40 ligand trimer combined with tumor-targeted IL-2 may be a promising strategy for the rational development of DNA-based cancer vaccines for future clinical applications.

The dual role of CTLA-4 in Leishmania infection

The role of CTLA-4 in inducing the production of transforming growth factor beta (TGF-beta) from T cells during a Leishmania infection has only recently been recognized. However, CTLA-4 and TGF-beta affect T helper cells differently, depending on the maturation. This review discusses the data obtained from different experimental models and demonstrates that CTLA-4 is a target molecule for vaccination and therapy against leishmaniasis.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

Synergism of cytotoxic T lymphocyte-associated antigen 4 blockade and depletion of CD25(+) regulatory T cells in antitumor therapy reveals alternative pathways for suppression of autoreactive cytotoxic T lymphocyte responses

Therapeutic efficacy of a tumor cell-based vaccine against experimental B16 melanoma requires the disruption of either of two immunoregulatory mechanisms that control autoreactive T cell responses: the cytotoxic T lymphocyte-associated antigen (CTLA)-4 pathway or the CD25(+) regulatory T (Treg) cells. Combination of CTLA-4 blockade and depletion of CD25(+) Treg cells results in maximal tumor rejection. Efficacy of the antitumor therapy correlates with the extent of autoimmune skin depigmentation as well as with the frequency of tyrosinase-related protein 2(180-188)-specific CTLs detected in the periphery. Furthermore, tumor rejection is dependent on the CD8(+) T cell subset. Our data demonstrate that the CTL response against melanoma antigens is an important component of the therapeutic antitumor response and that the reactivity of these CTLs can be augmented through interference with immunoregulatory mechanisms. The synergism in the effects of CTLA-4 blockade and depletion of CD25(+) Treg cells indicates that CD25(+) Treg cells and CTLA-4 signaling represent two alternative pathways for suppression of autoreactive T cell immunity. Simultaneous intervention with both regulatory mechanisms is therefore a promising concept for the induction of therapeutic antitumor immunity.

An important role of CDK inhibitor p18(INK4c) in modulating antigen receptor-mediated T cell proliferation

The inhibitors of cyclin-dependent kinase (CDK) 4 (INK4) bind CDK4/6 to prevent their association with D-cyclins and G(1) cell cycle initiation and progression. We report here that among the seven CDK inhibitors, p18(INK4c) played an important role in modulating TCR-mediated T cell proliferation. Loss of p18(INK4c) in T cells led to hyperproliferation in response to CD3 stimulation. p18(INK4c)-null mice developed lymphoproliferative disorder and T cell lymphomas. Expression of IL-2, IL-2R-alpha, and the major G(1) cell cycle regulatory proteins was not altered in p18-null T cells. Both FK506 and rapamycin efficiently inhibited proliferation of p18-null T cells. In activated T cells, p18(INK4c) remained constant, and preferentially associated with and inhibited CDK6 but not CDK4. We propose that p18(INK4c) sets an inhibitory threshold in T cells and one function of CD28 costimulation is to counteract the p18(INK4c) inhibitory activity on CDK6-cyclin D complexes. The p18(INK4c) protein may provide a novel target to modulate T cell immunity.

Single-cell analysis of signal transduction in CD4 T cells stimulated by antigen in vivo

Flow cytometry was used to study signaling events in individual CD4 T cells after antigen recognition in the body. Phosphorylation of c-jun and p38 mitogen-activated protein kinase was detected within minutes in all antigen-specific CD4 T cells in secondary lymphoid tissues after injection of peptide antigen into the bloodstream. The remarkable rapidity of this response correlated with the finding that most naive T cells are in constant contact with dendritic antigen-presenting cells. Contrary to predictions from in vitro experiments, antigen-induced c-jun and p38 mitogen-activated protein kinase phosphorylation did not depend on CD28 signals and was insensitive to inhibition by cyclosporin A. Our results highlight the efficiency of the in vivo immune response and underscore the need to verify which signaling pathways identified in vitro actually operate under physiological conditions.

Role of CD47 in the induction of human naive T cell anergy

We recently reported that CD47 ligation inhibited IL-2 release by umbilical cord blood mononuclear cells activated in the presence of IL-12, but not IL-4, preventing the induction of IL-12Rbeta(2) expression and the acquisition of Th1, but not the Th2 phenotype. Here we show that in the absence of exogenous cytokine at priming, CD47 ligation of umbilical cord blood mononuclear cells promotes the development of hyporesponsive T cells. Naive cells were treated with CD47 mAb for 3 days, expanded in IL-2 for 9-12 days, and restimulated by CD3 and CD28 coengagement. Effector T cells generated under these conditions were considered to be anergic because they produced a reduced amount of IL-2 at the single-cell level and displayed an impaired capacity 1) to proliferate, 2) to secrete Th1/Th2 cytokines, and 3) to respond to IL-2, IL-4, or IL-12. Moreover, CD47 mAb strongly suppressed IL-2 production and IL-2Ralpha expression in primary cultures and IL-2 response of activated naive T cells. Induction of anergy by CD47 mAb was IL-10 independent, whereas inclusion of IL-2 and IL-4, but not IL-7, at priming fully restored T cell activation. Furthermore, CD28 costimulation prevented induction of anergy. Thus, CD47 may represent a potential target to induce anergy and prevent undesired Th0/Th1 responses such as graft vs host diseases, allograft rejection, or autoimmune diseases.

Proteolysis-independent regulation of PI3K by Cbl-b-mediated ubiquitination in T cells

Cbl-b, a ring-type E3 ubiquitin protein ligase, is implicated in setting the threshold of T lymphocyte activation. The p85 regulatory subunit of phosphatidylinositol 3 kinase (PI3K) was identified as a substrate for Cbl-b. We have shown that Cbl-b negatively regulated p85 in a proteolysis-independent manner. Cbl-b is involved in the recruitment of p85 to CD28 and T cell antigen receptor zeta through its E3 ubiquitin ligase activity. The enhanced activation of Cbl-b(-/-) T cells was suppressed by the inhibition of PI3K. The results suggest a proteolysis-independent function for Cbl-b in the modification of protein recruitment.

Roles of I-E molecule and CD28 costimulation in induction of suppression by staphylococcal enterotoxin B in vivo

Exposure to bacterial superantigens leads to the induction of a subsequent state of immune hyporesponsiveness. Using a transwell coculture system, a previous report demonstrated that splenocytes from staphylococcal enterotoxin B (SEB)-injected BALB/c mice secreted soluble mediators to suppress the proliferative response of naive syngeneic splenocytes to SEB stimulation. We show in the present study that, in contrast to the suppressive effect induced by SEB in BALB/c (H-2(d) haplotype), MRL(+/+), and MRL-lpr/lpr (H-2(k)) mice, SEB-primed splenocytes from I-E(-) strains such as B6, B10, A. BY (H-2(b)), and A.SW (H-2(s)) mice failed to inhibit the CD25 expression and the proliferative activity of their syngeneic naive responder splenocytes. Further results revealed that the SEB-primed cells from BALB/c, but not B6, mice inhibited the CD25 expression and proliferation of naive responder cells from either BALB/c or B6 mice, indicating the critical regulatory role of the effector cells. Unlike SEB, staphylococcal enterotoxin A induced profound suppression in both BALB/c and B6 mice. Moreover, the suppressive competence of SEB-primed splenocytes was diminished in CD28-deficient BALB/c mice. Taken together, our results indicate that when SEB is used as a stimulator in vivo, both the I-E molecule and CD28 costimulation are required for the induction of regulatory cells bearing suppressive activity.

Nature and function of gastrointestinal antigen-presenting cells

Antigen-presenting cells (APCs) of the human gut are heterogeneous, including both macrophages, a variety of dendritic cells and B cells. They are found both in gut-associated lymphoid tissue and in the mucosal lamina propria, especially beneath the surface epithelium. APCs have diverse phenotypes and therefore probably different functions in various locations; their expression levels of a variety of costimulatory molecules are most likely important for immunological decision making of stimulated T cells, either locally in the gut or in regional lymph nodes to which migrating APCs (dendritic cells) carry antigen. Thus, APCs are involved in active immunity as well as in induction of oral tolerance. However, their precise role in food allergy remains to be defined.

Receptor proximity, not intermolecular orientation, is critical for triggering T-cell activation

Engagement of antigen receptors on the surface of T-cells with peptides bound to major histocompatibility complex (MHC) proteins triggers T-cell activation in a mechanism involving receptor oligomerization. Receptor dimerization by soluble MHC oligomers is sufficient to induce several characteristic activation processes in T-cells including internalization of engaged receptors and up-regulation of cell surface proteins. In this work, the influence of intermolecular orientation within the activating receptor dimer was studied. Dimers of class II MHC proteins coupled in a variety of orientations and topologies each were able to activate CD4+ T-cells, indicating that triggering was not dependent on a particular receptor orientation. In contrast to the minimal influence of receptor orientation, T-cell triggering was affected by the inter-molecular distance between MHC molecules, and MHC dimers coupled through shorter cross-linkers were consistently more potent than those coupled through longer cross-linkers. These results are consistent with a mechanism in which intermolecular receptor proximity, but not intermolecular orientation, is the key determinant for antigen-induced CD4+ T-cell activation.

A retro-inverso peptide mimic of CD28 encompassing the MYPPPY motif adopts a polyproline type II helix and inhibits encephalitogenic T cells in vitro

Complete activation of T cells requires two signals: an Ag-specific signal delivered via the TCR by the peptide-MHC complex and a second costimulatory signal largely provided by B7:CD28/CTLA-4 interactions. Previous studies have shown that B7 blockade can either ameliorate experimental autoimmune encephalomyelitis by interfering with CD28 signaling or exacerbate the disease by concomitant blockade of CTLA-4 interaction. Therefore, we developed a functional CD28 mimic to selectively block B7:CD28 interactions. The design, synthesis, and structural and functional properties of the CD28 free peptide, the end group-blocked CD28 peptide, and its retro-inverso isomer are shown. The synthetic T cell-costimulatory receptor peptides fold into a polyproline type II helical structure commonly seen in regions of globular proteins involved in transient protein-protein interactions. The binding determinants of CD28 can be transferred onto a short peptide mimic of its ligand-binding region. The CD28 peptide mimics effectively block the expansion of encephalitogenic T cells in vitro suggesting the potential usefulness of the peptides for the treatment of autoimmune disease conditions requiring down-regulation of T cell responses.

Novel CD28-responsive enhancer activated by CREB/ATF and AP-1 families in the human interleukin-2 receptor alpha-chain locus

The interaction of interleukin-2 (IL-2) with its receptor (IL-2R) critically regulates the T-cell immune response, and the alpha chain CD25/IL-2Ralpha is required for the formation of the high-affinity receptor. Tissue-specific, inducible expression of the IL-2Ralpha gene is regulated by at least three positive regulatory regions (PRRI, PRRII, and PRRIII), but none responded to CD28 engagement in gene reporter assays although CD28 costimulation strongly amplifies IL-2Ralpha gene transcription. By DNase I hypersensitivity analysis, we have identified a novel TCR-CD3- and CD28-responsive enhancer (CD28rE) located 8.5 kb 5' of the IL-2Ralpha gene. PRRIV/CD28rE contains a functional CRE/TRE element required for CD28 signaling. The T-cell-specific, CD28-responsive expression of the IL-2Ralpha gene appears controlled through PRRIV/CD28rE by cooperation of CREB/ATF and AP-1 family transcription factors.

Isolation and characterization of sea urchin egg lipid rafts and their possible function during fertilization

Specialized membrane microdomains called rafts are thought to play a role in many types of cell-cell interactions and signaling. We have investigated the possibility that sea urchin eggs contain these specialized membrane microdomains and if they play a role in signal transduction at fertilization. A low density, TX-100 insoluble membrane fraction, typical of lipid rafts, was isolated by equilibrium gradient centrifugation. This raft fraction contained proteins distinct from cytoskeletal complexes. The fraction was enriched in tyrosine phosphorylated proteins and contained two proteins known to be involved in signaling during egg activation (an egg Src-type kinase and PLC gamma). This fraction was further characterized as a prototypical raft fraction by the release of proteins in response to in vitro treatment of the rafts with the cholesterol binding drug, methyl-beta-cyclodextrin (M beta CD). Furthermore, treatment of eggs with M beta CD inhibited fertilization, suggesting that egg lipid rafts play a physiological role in fertilization. Mol. Reprod. Dev. 59:294-305, 2001.

Pathways for self-tolerance and the treatment of autoimmune diseases

Antigen delivers both immunogenic and tolerogenic signals to lymphocytes. The outcome of antigen exposure represents a complex integration of the timing of antigen binding with signals from many other immunogenic and tolerogenic costimulatory pathways. A road map of these signalling pathways is only beginning to be charted, revealing the mechansim of action and limitations of current immunotherapeutic agents and the points of attack for new agents. Ciclosporin and tacrolimus interfere with tolerogenic signals from antigen in addition to blocking immunogenic signals, thus preventing active establishment of tolerance. Corticosteroids inhibit a key immunogenic pathway, NFkappaB, and more specific inhibitors of this pathway may allow tolerance to be actively established while immune responses are blocked. New experimental therapies aim to mimic tolerogenic antigen signals by chronically stimulating antigen receptors with antigen or antibodies to the receptor, or aim to block costimulatory pathways involving CD40 ligand, B7, or interleukin 2. Obtaining the desired response with these strategies is unpredictable because many of these signals have both tolerogenic and immunogenic roles. The cause of autoimune diseases has been determined for several rare monogenic disorders, revealing inherited deficiencies in tolerogenic costimulatory pathways such as FAS. Common autoimmune disorders may have a biochemically related pathogenesis.

Immune reconstitution after allogeneic marrow transplantation compared with blood stem cell transplantation

Allogeneic peripheral blood stem cell grafts contain about 10 times more T and B cells than marrow grafts. Because these cells may survive in transplant recipients for a long time, recipients of blood stem cells may be less immunocompromised than recipients of marrow. Immune reconstitution was studied in 115 patients randomly assigned to receive either allogeneic marrow or filgrastim-mobilized blood stem cell transplantation. Between day 30 and 365 after transplantation, counts of most lymphocyte subsets were higher in the blood stem cell recipients. The difference was most striking for CD4 T cells (about 4-fold higher counts for CD45RA(high) CD4 T cells and about 2-fold higher counts for CD45RA(low/-)CD4 T cells; P <.05). On assessment using phytohemagglutinin and herpesvirus antigen-stimulated proliferation, T cells in the 2 groups of patients appeared equally functional. Median serum IgG levels were similar in the 2 groups. The rate of definite infections after engraftment was 1.7-fold higher in marrow recipients (P =.001). The rate of severe (inpatient treatment required) definite infections after engraftment was 2.4-fold higher in marrow recipients (P =.002). The difference in the rates of definite infections was greatest for fungal infections, intermediate for bacterial infections, and lowest for viral infections. Death associated with a fungal or bacterial infection occurred between day 30 and day 365 after transplantation in 9 marrow recipients and no blood stem cell recipients (P =.008). In conclusion, blood stem cell recipients have higher lymphocyte-subset counts and this appears to result in fewer infections. (Blood. 2001;97:3380-3389)

Augmentation in expression of activation-induced genes differentiates memory from naive CD4+ T cells and is a molecular mechanism for enhanced cellular response of memory CD4+ T cells

In an attempt to understand the molecular basis for the immunological memory response, we have used cDNA microarrays to measure gene expression of human memory and naive CD4+ T cells at rest and after activation. Our analysis of 54,768 cDNA clones provides the first glimpse into gene expression patterns of memory and naive CD4+ T cells at the genome-scale and reveals several novel findings. First, memory and naive CD4+ T cells expressed similar numbers of genes at rest and after activation. Second, we have identified 14 cDNA clones that expressed higher levels of transcripts in memory cells than in naive cells. Third, we have identified 135 (130 known genes and 5 expressed sequence tags) up-regulated and 68 (42 known genes and 26 expressed sequence tags) down-regulated cDNA clones in memory CD4+ T after in vitro stimulation with anti-CD3 plus anti-CD28. Interestingly, the increase in mRNA levels of up-regulated genes was greater in memory than in naive CD4+ T cells after in vitro stimulation and was higher with anti-CD3 plus anti-CD28 than with anti-CD3 alone in both memory and naive CD4+ T cells. Finally, the changes in expression of actin and cytokine genes identified by cDNA microarrays were confirmed by Northern and protein analyses. Together, we have identified approximately 200 cDNA clones whose expression levels changed after activation and suggest that the level of expression of up-regulated genes is a molecular mechanism that differentiates the response of memory from naive CD4+ T cells.

A viral protein that selectively downregulates ICAM-1 and B7-2 and modulates T cell costimulation

Kaposi's sarcoma-associated (KS-associated) herpesvirus (KSHV) is a B-lymphotropic agent linked to AIDS-related lymphoproliferative disorders and KS. We and others have earlier identified two viral genes, K3 and K5, that encode endoplasmic reticulum proteins that downregulate surface MHC-I chains by enhancing their endocytosis. Here we have examined the ability of these proteins to influence the disposition of other host surface proteins implicated in immune recognition and activation. We report that K5, but not K3, expression in BJAB cells dramatically reduces ICAM-1 and B7-2 surface expression; B7-1 expression is unaffected. This K5-induced reduction can be reversed by coexpression of a dominant negative mutant of dynamin, indicating that the loss of ICAM and B7-2 surface expression is due to their enhanced endocytosis. This downregulation is functionally significant, because K5-transfected B cells show substantial impairment in their ability to induce T cell activation. K5 is thus the first example of a viral modulator of immunological synapse formation and T cell costimulation. We propose that its expression reduces T cell responses to KSHV-infected B cells early in infection, thereby diminishing antiviral cytokine release and the production of stimulatory signals for CTL generation.

Involvement of hematopoietic progenitor kinase 1 in T cell receptor signaling

Hematopoietic progenitor kinase 1 (HPK1), a mammalian Ste20-related serine/threonine protein kinase, is a hematopoietic-specific upstream activator of the c-Jun N-terminal kinase. Here, we provide evidence to demonstrate the involvement of HPK1 in T cell receptor (TCR) signaling. HPK1 was activated and tyrosine-phosphorylated with similar kinetics following TCR/CD3 or pervanadate stimulation. Co-expression of protein-tyrosine kinases, Lck and Zap70, with HPK1 led to HPK1 activation and tyrosine phosphorylation in transfected mammalian cells. Upon TCR/CD3 stimulation, HPK1 formed inducible complexes with the adapters Nck and Crk with different kinetics, whereas it constitutively interacted with the adapters Grb2 and CrkL in Jurkat T cells. Interestingly, HPK1 also inducibly associated with linker for activation of T cells (LAT) through its proline-rich motif and translocated into glycolipid-enriched microdomains (also called lipid rafts) following TCR/CD3 stimulation, suggesting a critical role for LAT in the regulation of HPK1. Together, these results identify HPK1 as a new component of TCR signaling. T cell-specific signaling molecules Lck, Zap70, and LAT play roles in the regulation of HPK1 during TCR signaling. Differential complex formation between HPK1 and adapters highlights the possible involvement of HPK1 in multiple signaling pathways in T cells.

CD43 potentiates CD3-induced proliferation of murine intestinal intraepithelial lymphocytes

The involvement of CD43 in cell proliferation of murine intestinal intraepithelial lymphocytes (IEL) has been studied in in vitro CD3-stimulated cell cultures. In the presence of either IL-2 or IL-15, CD3 stimulation of IEL resulted in low levels of proliferation as measured by thymidine incorporation, whereas no proliferation occurred upon CD3 stimulation in the absence of cytokines. The combination of both cytokines to IEL cultures synergistically enhanced CD3-induced proliferation by approximately threefold that of cultures supplemented with either cytokine alone. Most importantly, however, proliferation of IEL was significantly greater when CD3 stimulation occurred in conjunction with CD43 triggering, indicating that CD43 functions as a coactivational signal for murine IEL. These findings indicate that a spectrum of potential proliferative responses exist among murine IEL depending on the types and combinations of signals received, and that because under normal conditions murine IEL are largely devoid of CD28 expression, a classical T-cell coactivational molecule, the capacity for high-level IEL proliferation may reside with CD43.

Face off--the interplay between activating and inhibitory immune receptors

The function of leukocytes is regulated by the integration of positive and negative signals received through cell surface receptors. Related receptors with similar extracellular domains and often binding the same ligands can transmit either inhibitory or activating signals. Studies are beginning to reveal how these 'paired receptors' control immune functions.

4-1BBL enhances anti-tumor responses in the presence or absence of CD28 but CD28 is required for protective immunity against parental tumors

A20 is an aggressive BALB/c B cell lymphoma that, despite its expression of B7-2, rapidly forms tumors in syngeneic mice. We have generated A20 transfectants expressing elevated levels of B7-2 (A20/B7-2high) or 4-1BBL (A20/4-1BBL(low,mod,high)) and found that mice which were able to reject the A20/B7-2 or A20/4-1BBL transfectants were also resistant to subsequent systemic challenge with the parental cell line. To assess whether the effectiveness of 4-1BBL in enhancing anti-tumor immunogenicity was dependent on additional signals from B7-CD28 interaction, we injected the A20 variants into BALB/c CD28(-/-) mice. We found that CD28(-/-) mice were able to reject the A20/4-1BBL variants while A20/B7-2 cells formed tumors. However, when the A20/4-1BBL resistant CD28(-/-) mice were systemically challenged with the A20 parental line, tumors formed rapidly. Upon restimulation in vitro, splenocytes from A20/4-1BBL immunized CD28(+/+) mice were able to kill parental tumors whereas splenocytes from CD28(-/-) mice showed a reduction in CTL activity against A20 or A20/4-1BBL targets. Examination of cytokine production by the immunized animals indicated that the CD28(-/-) splenocytes secreted substantially less IL-2 as well as reduced levels of IFN-gamma compared with their CD28(+/+) counterparts. Thus, 4-1BBL expressing tumors are capable of priming CTL responses against 4-1BBL transfected as well as parental tumors in the absence of CD28. However, in the absence of CD28 signaling, the production of cytokines and particularly IL-2 was lower, resulting in a weaker CTL recall response and reduced ability to survive challenge with parental tumor.

Stimulatory effects of B7-related protein-1 on cellular and humoral immune responses in mice

Inducible costimulator (ICOS) and B7-related protein-1 (B7RP-1) constitute a receptor-ligand pair involved in T cell costimulation. In this study, the stimulatory effects of B7RP-1 on cellular and humoral immune responses were investigated giving mice a construct with the extracellular domain of murine B7RP-1 fused with human IgG1 Fc (B7RP-1-Fc). B7RP-1-Fc stimulated contact hypersensitivity (CH) given near either the time of sensitization or challenge with oxazolone. When given near challenge time, B7RP-1-Fc stimulated CH more than a construct containing the extracellular domain of murine B7.2 and Fc (B7.2-Fc). B7RP-1-Fc increased the number of cells in lymph nodes draining the skin sensitized with oxazolone, especially activated T cells. B7RP-1-Fc also increased the ability of the cells in these lymph nodes to induce CH when transfused into naive mice. B7RP-1-Fc stimulated the production of anti-keyhole limpet hemocyanin (KLH) Ab, increasing anti-KLH IgG, IgG2a, and IgE, whereas B7.2-Fc did not affect this production. B7RP-1-Fc also increased the number of cells in lymph nodes draining the skin immunized with KLH and their production of IFN-gamma, IL-4, and IL-10 in response to KLH. Finally, B7RP-1-Fc increased the presence of eosinophils in the bronchoalveolar lavage and lungs of mice sensitized and challenged with OVA so to mount an asthmatic reaction. B7RP-1-Fc stimulates both cellular and humoral immune responses in vivo by increasing number and function of T and B cells reacting to Ag exposure.

Molecular controls of antigen receptor clustering and autoimmunity

Cellular organization of the cytoskeleton, assembly of intracellular signaling complexes and movement of membrane receptors into supramolecular activation complexes (SMACs) are crucial prerequisites for lymphocyte activation and function. Full T-cell activation requires costimulatory signals in addition to antigen-mediated signals. Costimulatory signals facilitate T-cell activation by inducing SMAC formation, resulting in sustained signal transduction, cell-cycle progression and cytokine production. The guanine nucleotide exchange factor Vav1 and the Wiscott-Aldrich syndrome protein (WASP) regulate the actin cytoskeleton in T cells and also regulate SMAC formation. In mice lacking the E3 ubiquitin ligase Cbl-b, the Vav-WASP signaling pathway is active in the absence of costimulation resulting in deregulated cytoskeletal reorganization, enhanced priming and expansion of autoreactive T cells, and the development of autoimmunity. This review discusses the role of Cbl-b, Vav and WASP in the regulation of SMAC formation and the implications for the maintenance of tolerance and the development of autoimmunity.

Molecular therapeutics of liver disease

Fundamental advances in biomedical research will revolutionize the prevention and treatment of liver disease during the early twenty-first century. Recent progress in gene-, cell-, and recombinant protein-based therapeutics will contribute to this revolution, although formidable obstacles currently prevent the clinical application of these novel therapies. Eventually, these obstacles will be overcome, and molecular therapeutics of liver disease will become a clinical reality. As a result, the new millennium will be a very interesting time to practice hepatology.

ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

Co-stimulation and counter-stimulation: lipid raft clustering controls TCR signaling and functional outcomes

T cell receptor (TCR) antigen recognition induces the formation of a specialized 'immunological synapse' at the T cell : antigen presenting cell (APC) junction. This junction is generated by the recruitment and exclusion of particular proteins from the contact area and is required for T cell activation. We and others have hypothesized that lipid raft/non-raft partitioning provides a molecular basis for protein sorting which organizes the TCR, co-stimulators, signal transducers and the actin cytoskeleton at the T cell : APC interface. Here we discuss the emerging paradigm that co-stimulators induce the directional transport and clustering of lipid rafts at the T cell : APC interface, thus generating platform(s) specialized for processive and sustained TCR signal transduction and T cell activation. We also discuss recent data implicating the involvement of 'counter-stimulators' and other negative regulators which prevent optimal raft clustering at the TCR contact site and, thus, facilitate T cell inactivation and tolerance induction.

The expanding world of co-stimulation: the two-signal model revisited

The crucial role for CD28, its homolog CTLA-4 and their binding partners B7-1 and B7-2 in the generation of effective T-cell responses has been well documented. Recently, two new pairs of the CD28/B7 families were identified. The ability of these molecules to regulate T-cell expansion and effector function and the dynamic integration of the co-stimulatory and T-cell receptor signals are just beginning to be explored. Understanding these processes will be crucial for designing clinically relevant approaches to manipulate the adaptive immune system.

Membrane lipid microdomains and the role of PKCtheta in T cell activation

Productive T cell activation depends on the assembly of a highly ordered and compartmentalized immunological synapse or supramolecular activation complex (SMAC). Reorganization of the actin cytoskeleton and clustering of specialized membrane microdomains, or lipid rafts, occur early following TCR/CD3 and costimulatory receptor ligation. Many key signaling molecules localize in lipid raft patches during T cell activation. Lipid raft reorganization is required for T cell activation, where it plays an apparently important role in stabilizing the T cell synapse. Here we review recent evidence supporting the role of lipid rafts in T cell activation. Particular emphasis is placed on the coupling of protein kinase C-theta(PKCtheta), which is selectively expressed in T cells and is known to function as an essential signal for T cell activation, and lipid rafts.

Interleukin-10 induces macrophage apoptosis and expression of CD16 (FcgammaRIII) whose engagement blocks the cell death programme and facilitates differentiation

The development of monocytes into macrophages is regulated by helper T cells (Th) cells and, vice versa, the differentiation of Th cells into Th1 and Th2 is regulated by macrophages. Herein we examined the role of the Th2 cytokine, interleukin-10 (IL-10), on the development of macrophages. IL-10 is known to block the expression of antigen-presenting major histocompatibility complex (MHC) II and of costimulatory B7 molecules but it induces the expression of FcRs, especially the FcgammaRIII (CD16). The expression of CD16 enables the macrophage to carry out antibody-dependent cell-mediated cytotoxicity (ADCC) functions. However, this differentiation step is largely undercut by the capacity of IL-10 to induce macrophage apoptosis before the process of differentiation ensues. We found that the negative effect of IL-10 on the survival of macrophages is reversed in an environment that contains immunoglobulin G (IgG). IgG, especially when immune complexed with antigen, stimulates CD16 to transmit survival signals in macrophages which enable them to complete the differentiation process into CD16+ cells. Thus, IL-10 suppresses macrophage accumulation in healthy tissues where IgG is absent, and facilitates macrophage accumulation and differentiation in tissues that contain IgG, for example inflamed tissues or tissues that present autoreactive antibodies.

Costimulation of CD8alphabeta T cells by NKG2D via engagement by MIC induced on virus-infected cells

NKG2D is an activating receptor that stimulates innate immune responses by natural killer cells upon engagement by MIC ligands, which are induced by cellular stress. Because NKG2D is also present on most CD8alphabeta T cells, it may modulate antigen-specific T cell responses, depending on whether MIC molecules--distant homologs of major histocompatibility complex (MHC) class I with no function in antigen presentation--are induced on the surface of pathogen-infected cells. We found that infection by cytomegalovirus (CMV) resulted in substantial increases in MIC on cultured fibroblast and endothelial cells and was associated with induced MIC expression in interstitial pneumonia. MIC engagement of NKG2D potently augmented T cell antigen receptor (TCR)-dependent cytolytic and cytokine responses by CMV-specific CD28- CD8alphabeta T cells. This function overcame viral interference with MHC class I antigen presentation. Combined triggering of TCR-CD3 complexes and NKG2D induced interleukin 2 production and T cell proliferation. Thus NKG2D functioned as a costimulatory receptor that can substitute for CD28.

Cutting edge: two distinct mechanisms lead to impaired T cell homeostasis in Janus kinase 3- and CTLA-4-deficient mice

Cytokine receptor signaling and costimulatory receptor signaling play distinct roles in T cell activation. Nonetheless, deficiencies in either of these pathways lead to seemingly similar phenotypes of impaired T cell homeostasis. A dramatic expansion of CD4(+) peripheral T cells with an activated phenotype has been observed in both Janus kinase (Jak) 3-deficient and CTLA-4-deficient mice. Despite these similarities, the mechanisms driving T cell expansion may be distinct. To address this possibility, we examined the TCR repertoire of peripheral T cells in Jak3(-/-) and CTLA-4(-/-) mice using complementarity-determining region 3 spectratype analysis. Interestingly, a restricted and highly biased TCR repertoire was observed in the Jak3(-/-) T cells, strongly supporting a role for foreign Ag in the activation and expansion of these cells. In contrast, CTLA-4(-/-) T cells had a diverse and unbiased TCR repertoire, suggestive of a universal, Ag-independent mechanism of activation and expansion. These findings provide insight into the diverse mechanisms controlling T cell homeostasis.