Role of 4-1BB in immune responses

T cell costimulation by the TNF ligand BAFF

The TNF ligand family member BAFF (B cell activating factor belonging to the TNF family, also called Blys, TALL-1, zTNF-4, or THANK) is an important survival factor for B cells [corrected]. In this study, we show that BAFF is able to regulate T cell activation. rBAFF induced responses (thymidine incorporation and cytokine secretion) of T cells, suboptimally stimulated through their TCR. BAFF activity was observed on naive, as well as on effector/memory T cells (both CD4+ and CD8+ subsets), indicating that BAFF has a wide function on T cell responses. Analysis of the signal transduced by BAFF into T cells shows that BAFF has no obvious effect on T cell survival upon activation, but is able to deliver a complete costimulation signal into T cells. Indeed, BAFF is sufficient to induce IL-2 secretion and T cell division, when added to an anti-TCR stimulation. This highlights some differences in the BAFF signaling pathway in T and B cells. In conclusion, our results indicate that BAFF may play a role in the development of T cell responses, in addition to its role in B cell homeostasis.

Lipopolysaccharide modulation of dendritic cells is insufficient to mature dendritic cells to generate CTLs from naive polyclonal CD8+ T cells in vitro, whereas CD40 ligation is essential

Many cytotoxic CD8+ T cell responses are dependent on the interactions between CD40 ligand on the helper CD4+ T cell and CD40 on the APC. Although CD40 triggering of dendritic cells (DC) has been shown to mature the DC by increasing the level of expression of costimulatory molecules and inducing IL-12 secretion, the precise mechanisms by which CD40-CD40 ligand interactions allow DC to drive CTL responses remain unknown. We have used an in vitro model in which naive polyclonal CD8+ T cells can be activated by bone marrow-derived DC to investigate factor(s) that are responsible for this CD40-dependent generation of CTLs. DC modulated with agonistic anti-CD40 mAb (aCD40) are able to generate Ag-specific CTL responses while DC modulated with the microbial stimulus LPS alone do not. We compared the Ag-presenting capacity, levels of costimulatory molecules, and release of cytokines and chemokines of DC modulated with aCD40 to that of DC modulated by LPS. None of the factors assayed account for the unique capacity of anti-CD40-matured DC to drive CTL but this model provides a simplified system for further investigation. Although we attempted to use an LPS-free system for these studies, we are unable to rule out the possibility that very low levels of endotoxin (<20 pg/ml) may synergize with CD40 ligation in the generation of CTLs.

Intraepithelial lymphocytes: exploring the Third Way in immunology

Locally resident intraepithelial lymphocytes (IELs) are primarily T cells with potent cytolytic and immunoregulatory capacities, which they use to sustain epithelial integrity. Here, we consider that most IEL compartments comprise a variable mixture of two cell types: T cells primed to conventional antigen in the systemic compartment and T cells with ill-defined reactivities and origins, whose properties seem to place them mid-way between the adaptive and innate immune responses. We review the capacity of IELs to limit the dissemination of infectious pathogens and malignant cells and to control the infiltration of epithelial surfaces by systemic cells. An improved characterization of IELs would seem essential if we are to understand how immune responses and immunopathologies develop at body surfaces.

Progressive depletion of peripheral B lymphocytes in 4-1BB (CD137) ligand/I-Ealpha)-transgenic mice

Interaction of 4-1BB (CD137) and its ligand (4-1BBL) is thought to positively regulate cell-mediated and humoral immune responses. We have prepared transgenic mouse strains that express 4-1BBL cDNA under the control of MHC class II I-Ealpha promoter. The 4-1BBL-transgenic mice show progressive splenomegaly and selective depletion of B220(+) B cells accompanied with low levels of circulating IgG and defective humoral responses to Ag challenge. In addition, splenocytes from the transgenic mice fail to provide stimulation for allogeneic T cells in both lymphoproliferative and CTL responses in vitro, whereas their T cells remain functionally normal. Our results reveal unexpected functions of 4-1BBL in the regulation of humoral immune responses and Ag presentation.

Death-inducing tumour necrosis factor (TNF) superfamily ligands and receptors are transcribed in human placentae, cytotrophoblasts, placental macrophages and placental cell lines

Human placentae and two of the cell types in placentae (cytotrophoblasts and macrophages) were examined by RT-PCR for transcripts of the eight TNF superfamily ligands known to induce death of activated immune cells, tumour cells, and virus-infected cells (TNFalpha, LT alpha, LT beta, FasL, TRAIL, TWEAK, LIGHT, 4-1BBL). Transcripts for all ligands were detected in term placenta but LT alpha and 4-1BBL were not detected in first trimester placenta. Although term cytotrophoblasts contained mRNAs specific for TNF alpha, LT alpha, TWEAK, and 4-1BBL, messages encoding LT beta, FasL, TRAIL, and LIGHT were absent. In term placental macrophages, messages for all ligands except 4-1BBL were present. Transcripts for the 14 receptors to which the ligands bind, six of which contain death-domains (TNFR1, Fas, DR3, DR4, DR5, DR6), were also identified using RT-PCR. Term and first trimester placentae contained transcripts for all receptors except 4-1BB. Although term cytotrophoblasts lacked receptor mRNA encoding 4-1BB and OPG, term placental macrophages lacked DcR1 and OPG. Detection of nearly all the death-inducing TNF superfamily ligands and their receptors in human placentae implies that these powerful cytokines contribute to programmed or activated cell death in this organ.

Recombinant adenovirus coexpressing covalent peptide/MHC class II complex and B7-1: in vitro and in vivo activation of myelin basic protein-specific T cells

Previous studies have demonstrated that an MHC class II molecule with an antigenic peptide genetically fused to its beta-chain is capable of presenting this peptide to CD4(+) T cells. We hypothesized that covalent peptide/class II complex may direct the accessory molecules to exert their function specifically onto T cells in a TCR-guided fashion. To test this hypothesis, we generated several recombinant adenoviruses expressing covalent myelin basic protein peptide/I-A(u) complex (MBP(1-11)/I-A(u)) and the costimulatory molecule B7-1. Functional studies demonstrated that adenovirus-infected cells are capable of activating an MBP(1-11)-specific T cell hybridoma. Coexpression of the B7-1 molecule and MBP(1-11)/I-A(u) by the same adenovirus leads to synergy in T cell activation elicited by virus-infected cells. Furthermore, studies in syngeneic mice infected with the various adenoviruses revealed that MBP(1-11)-specific T cells are specifically activated by the coexpression of B7-1 and MBP(1-11)/I-A(u) in vivo. In conclusion, the coexpression of the covalent peptide/class II complex and accessory molecules by the same adenovirus provides a unique strategy to modulate the epitope-specific T cell response in a TCR-guided fashion. This approach may be applicable to investigate the roles of other accessory molecules in the engagement of the TCR class II molecule by substituting B7-1 with other accessory molecules in the recombinant adenovirus.

4-1BB ligand induces cell division, sustains survival, and enhances effector function of CD4 and CD8 T cells with similar efficacy

A costimulatory member of the TNFR family, 4-1BB, is expressed on activated T cells. Although some reports have suggested that 4-1BB is primarily involved in CD8 T cell activation, in this report we demonstrate that both CD4 and CD8 T cells respond to 4-1BB ligand (4-1BBL) with similar efficacy. CD4 and CD8 TCR transgenic T cells up-regulate 4-1BB, OX40, and CD27 and respond to 4-1BBL-mediated costimulation during a primary response to peptide Ag. 4-1BBL enhanced proliferation, cytokine production, and CTL effector function of TCR transgenic T cells. To compare CD4 vs CD8 responses to 4-1BBL under similar conditions of antigenic stimulation, we performed MLRs with purified CD4 or CD8 responders from CD28(+/+) and CD28(-/-) mice. We found that CD8 T cells produced IL-2 and IFN-gamma in a 4-1BBL-dependent manner, whereas under the same conditions the CD4 T cells produced IL-2 and IL-4. 4-1BBL promoted survival of CD4 and CD8 T cells, particularly at late stages of the MLR. CD4 and CD8 T cells both responded to anti-CD3 plus s4-1BBL with a similar cytokine profile as observed in the MLR. CD4 and CD8 T cells exhibited enhanced proliferation and earlier cell division when stimulated with anti-CD3 plus anti-CD28 compared with anti-CD3 plus 4-1BBL, and both subsets responded comparably to anti-CD3 plus 4-1BBL. These data support the idea that CD28 plays a primary role in initial T cell expansion, whereas 4-1BB/4-1BBL sustains both CD4 and CD8 T cell responses, as well as enhances cell division and T cell effector function.

B lymphocyte activation by contact-mediated interactions with T lymphocytes

T cell dependent B lymphocyte activation requires interactions between numerous receptor-ligand pairs on the two cell types. Recently, advances have been made both in understanding how these various signals regulate B cell effector functions and in identifying many new receptor-ligand pairs that contribute to the regulation of B cell function by T lymphocytes.

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.

Differential clonal expansion of CD4 and CD8 T cells in response to 4-1BB ligation: contribution of 4-1BB during inflammatory responses

Cell surface proteins of the tumor necrosis factor (TNF) family of receptors have been intimately involved in inducing T cell death. A feature of these family members that is less well studied is their ability to rescue T cells from apoptosis. One such member is 4-1BB; an activation induced surface receptor on CD4 and CD8 T cells. This study demonstrates that the costimulatory effects of 4-1BB, which was found to enhance clonal expansion, required cross-linking of the receptor. The survival of the activated CD8 T cells following expansion was not associated with an increase in Bcl-2 expression. Provided that 4-1BB signaling was present, the amplification of activated CD8 T cell growth in vivo was independent of CD28 ligation. In vivo clonal expansion of activated CD4 T cells, however, was not as responsive to 4-1BB cross-linking. Moreover, 4-1BB-induced expansion was comparable to that mediated by LPS which can incite multiple costimulatory signals. Furthermore, LPS-mediated growth and survival of superantigen (SAg) stimulated T cells appeared to be partially dependent on interactions between 4-1BB and 4-1BB ligand (4-1BBL).

Ligation of 4-1BB (CDw137) regulates graft-versus-host disease, graft-versus-leukemia, and graft rejection in allogeneic bone marrow transplant recipients

4-1BB is expressed on activated CD4(+) and CD8(+) T cells; its ligand, 4-1BB ligand is expressed on APCs. Despite expression on both T cell subpopulations, 4-1BB has been reported to predominantly affect CD8(+) T cell responses. By quantifying graft-vs-host disease alloresponses in vivo, we demonstrate that both CD4(+) and CD8(+) T cell-mediated alloresponses are regulated by 4-1BB/4-1BB ligand interactions to approximately the same extent. 4-1BB receptor-facilitated CD4(+) T cell-mediated alloresponses were partly CD28 independent. In two distinct marrow graft rejection systems, host CD8(+) and CD4(+) T cells each separately contributed to host anti-donor T cell-mediated allograft rejection. alpha 4-1BB mAb increased the graft-vs-leukemia effect of a suboptimal number of donor splenocytes given later post bone marrow transplantation by bolstering allogeneic responses resulting in leukemia elimination. In summary, 4-1BB ligation is a potent regulator of CD4(+) and CD8(+) T cell-mediated allogeneic responses in vivo. Modifying the ligation of 4-1BB represents a new approach to altering the graft-vs-host disease and graft-vs-leukemia effects of allogeneic T cells post bone marrow transplantation.

T-lymphocyte coactivator molecules

T-cell recognition and activation occurs within a specialized area of contact known as the immunologic synapse, localized to areas of glycolipid-enriched membrane microdomains. Within this area, T-cell activation is dependent not only upon specific recognition of peptide antigen embedded within molecules of the major histocompatibility complex, but also on a variety of costimulatory receptors and interactions. Engagement of T-cell receptor (TCR) with antigen alone will induce T-cell unresponsiveness; ligation of the coreceptor CD28 will prevent the induction of unresponsiveness. Novel costimulatory molecules belonging to both the CD28 and TNF/TNFR superfamilies have recently been identified. These receptors appear to act at different stages of T-cell differentiation and activation, have been shown to play a role in promoting different T-cell effector functions, and are important for B-cell differentiation and function.

CD137 activation abrogates granulocyte-macrophage colony-stimulating factor-mediated anti-apoptosis in neutrophils

CD137 (ILA / 4-1BB) is a member of the TNF / NGF receptor family, and has previously been suggested to be involved in T cell activation and differentiation. Here, we demonstrate that blood neutrophils from control individuals and patients with cystic fibrosis express CD137 mRNA and surface protein. In contrast, lung neutrophils derived from patients with cystic fibrosis did not express detectable CD137 levels. Such CD137-deficient neutrophils could also be generated from normal neutrophils by TNF-alpha stimulation in vitro. TNF-alpha was found to be highly expressed in epithelial cells from cystic fibrosis but not normal lungs, suggesting that TNF-alpha might account for reduced neutrophil CD137 levels under inflammatory conditions in vivo. To investigate whether CD137 is involved in the regulation of apoptosis, neutrophils were activated with functional anti-CD137 antibody in the presence or absence of different neutrophil survival factors in vitro. Activation of CD137 abrogated GM-CSF-mediated anti-apoptosis in normal but not in CD137-deficient neutrophils. Moreover, G-CSF- and IFN-gamma-mediated neutrophil anti-apoptosis was not affected by anti-CD137 antibody treatment. In conclusion, these data suggest that CD137 activation may limit GM-CSF-mediated anti-apoptosis of neutrophils. The absence of this anti-inflammatory mechanism in inflammatory responses might be associated with massive neutrophil accumulation and consequent tissue damage.

Role of TNF receptor-associated factor 2 and p38 mitogen-activated protein kinase activation during 4-1BB-dependent immune response

4-1BB is a costimulatory member of the TNFR family, expressed on activated CD4(+) and CD8(+) T cells. Previous results showed that 4-1BB-mediated T cell costimulation is CD28-independent and involves recruitment of TNFR-associated factor 2 (TRAF2) and activation of the stress-activated protein kinase cascade. Here we describe a role for the p38 mitogen-activated protein kinase (MAPK) pathway in 4-1BB signaling. Aggregation of 4-1BB alone induces p38 activation in a T cell hybridoma, whereas, in normal T cells, p38 MAPK is activated synergistically by immobilized anti-CD3 plus immobilized 4-1BB ligand. 4-1BB-induced p38 MAPK activation is inhibited by the p38-specific inhibitor SB203580 in both a T cell hybridoma and in murine T cells. T cells from TRAF2 dominant-negative mice are impaired in 4-1BB-mediated p38 MAPK activation. A link between TRAF2 and the p38 cascade is provided by the MAPK kinase kinase, apoptosis-signal-regulating kinase 1. A T cell hybrid transfected with a kinase-dead apoptosis-signal-regulating kinase 1 fails to activate p38 MAPK in response to 4-1BB signaling. To assess the role of p38 activation in an immune response, T cells were stimulated in an MLR in the presence of SB203580. In a primary MLR, SB203580 blocked IL-2, IFN-gamma, and IL-4 secretion whether the costimulatory signal was delivered via 4-1BB or CD28. In contrast, following differentiation into Th1 or Th2 cells, p38 inhibition blocked IL-2 and IFN-gamma without affecting IL-4 secretion. Nevertheless, IL-4 secretion by Th2 cells remained costimulation-dependent. Thus, critical T cell signaling events diverge following Th1 vs Th2 differentiation.

A humanized anti--4-1BB monoclonal antibody suppresses antigen-induced humoral immune response in nonhuman primates

The interaction of 4-1BB and its ligand plays an important role in the regulation of T-cell-mediated immune responses. In this study, the authors examined the effect of a humanized anti--4-1BB monoclonal antibody (H4B4) on ovalbumin-induced immune responses in baboons. Previously, a mouse monoclonal antibody, 4B4 against the human 4-1BB molecule, was generated and characterized. Based on this antibody, a humanized version of 4B4 monoclonal antibody was constructed and the resultant antibody, H4B4, showed full recovery of the binding activity of the original antibody 4B4: a 1.5-fold increase in affinity for 4-1BB. In addition, H4B4 mediated antibody-dependent cellular cytotoxicity of activated human peripheral blood T cells and CEM cells in a dose-dependent manner. Weekly administration of H4B4 at doses of 1 or 4 mg/kg could suppress immunoglobulin G production against ovalbumin. This was not a result of the overall immune suppression, because the numbers of B and T cells and the total immunoglobulin G production were not altered during treatment with H4B4. These findings suggest that treatment with H4B4 may be a valid therapeutic approach to control unwanted immune responses in persons with autoimmune diseases.

CD27 is required for generation and long-term maintenance of T cell immunity

The Traf-linked tumor necrosis factor receptor family member CD27 is known as a T cell costimulatory molecule. We generated CD27-/- mice and found that CD27 makes essential contributions to mature CD4+ and CD8+ T cell function: CD27 supported antigen-specific expansion (but not effector cell maturation) of naïve T cells, independent of the cell cycle-promoting activities of CD28 and interleukin 2. Primary CD4+ and CD8+ T cell responses to influenza virus were impaired in CD27-/- mice. Effects of deleting the gene encoding CD27 were most profound on T cell memory, reflected by delayed response kinetics and reduction of CD8+ virus-specific T cell numbers to the level seen in the primary response. This demonstrates the requirement for a costimulatory receptor in the generation of T cell memory.

Host T cells resist graft-versus-host disease mediated by donor leukocyte infusions

Delayed lymphocyte infusions (DLIs) are used to treat relapse occurring post bone marrow transplantation (BMT) and to increase the donor chimerism in recipients receiving nonmyeloablative conditioning. As compared with donor lymphocytes given early post-BMT, DLIs are associated with a reduced risk of graft-vs-host disease (GVHD). The mechanism(s) responsible for such resistance have remained incompletely defined. We now have observed that host T cells present 3 wk after lethal total body irradiation, at the time of DLI, contribute to DLI-GVHD resistance. The infusion of donor splenocytes on day 0, a time when host bone marrow (BM)-derived T cells are absent, results in greater expansion than later post-BMT when host and donor BM-derived T cells coexist. Selective depletion of host T cells with anti-Thy1 allelic mAb increased the GVHD risk of DLI, indicating that a Thy1(+) host T cell regulated DLI-GVHD lethality. The conditions by which host T cells are required for optimal DLI resistance were determined. Recipients unable to express CD28 or 4-1BB were as susceptible to DLI-GVHD as anti-Thy1 allelic mAb-treated recipients, indicating that CD28 and 4-1BB are critical to DLI-GVHD resistance. Recipients deficient in both perforin and Fas ligand but not individually were highly susceptible to DLI-GVHD. Recipients that cannot produce IFN-gamma were more susceptible to DLI-GVHD, whereas those deficient in IL-12 or p55 TNFRI were not. Collectively, these data indicate that host T cells, which are capable of generating antidonor CTL effector cells, are responsible for the impaired ability of DLI to induce GVHD. These same mechanisms may limit the efficacy of DLI in cancer therapy under some conditions.

T lymphocyte costimulatory molecules in host defense and immunologic diseases

BACKGROUND

Costimulation is an essential component for the optimal induction of T cell-mediated immune responses. Manipulation of the costimulatory pathway with antibodies or genetically-engineered fusion proteins is an important strategy to treat immune-related diseases including allergy, asthma, transplantation and cancer. Recent advances have revealed several new costimulatory molecules, and the functional characteristics of each costimulatory pathway are now becoming clearer.

LEARNING OBJECTIVES

In this review, we summarize basic outlines of the costimulatory systems in terms of molecular structure, expression kinetics and immunological function. We further discuss involvement and therapeutic manipulation of costimulation in several clinical diseases.

DATA SOURCE

The MEDLINE database was used to review the literature related to costimulation.

CONCLUSION

Costimulatory pathways play an essential role in the activation and regulation of T cell immune responses and the induction of T cell tolerance. Therapeutic manipulation of the costimulatory system demonstrates beneficial effects to treat immunological diseases in murine models as well as some clinical situations.

T cells can use either T cell receptor or CD28 receptors to absorb and internalize cell surface molecules derived from antigen-presenting cells

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)-peptide-major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell-APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

Engagement of the OX-40 receptor in vivo enhances antitumor immunity

The OX-40 receptor (OX-40R), a member of the TNFR family, is primarily expressed on activated CD4+ T lymphocytes. Engagement of the OX-40R, with either OX-40 ligand (OX-40L) or an Ab agonist, delivers a strong costimulatory signal to effector T cells. OX-40R+ T cells isolated from inflammatory lesions in the CNS of animals with experimental autoimmune encephalomyelitis are the cells that respond to autoantigen (myelin basic protein) in vivo. We identified OX-40R+ T cells within primary tumors and tumor-invaded lymph nodes of patients with cancer and hypothesized that they are the tumor-Ag-specific T cells. Therefore, we investigated whether engagement of the OX-40R in vivo during tumor priming would enhance a tumor-specific T cell response. Injection of OX-40L:Ig or anti-OX-40R in vivo during tumor priming resulted in a significant improvement in the percentage of tumor-free survivors (20-55%) in four different murine tumors derived from four separate tissues. This anti-OX-40R effect was dose dependent and accentuated tumor-specific T cell memory. The data suggest that engagement of the OX-40R in vivo augments tumor-specific priming by stimulating/expanding the natural repertoire of the host's tumor-specific CD4+ T cells. The identification of OX-40R+ T cells clustered around human tumor cells in vivo suggests that engagement of the OX-40R may be a practical approach for expanding tumor-reactive T cells and thereby a method to improve tumor immunotherapy in patients with cancer.

Robust B Cell Immunity but Impaired T Cell Proliferation in the Absence of CD134 (OX40)

CD134 (OX40) is a member of the TNF receptor family that is expressed on activated T lymphocytes. T cells from mice that lack expression of CD134 made strong responses to a range of challenges, but they showed impaired proliferation in response to direct stimulation through the TCR with monoclonal anti-CD3ε Ab. CD134-deficient mice controlled infection with Leishmania major, Nippostrongylus brasiliensis, and Theiler’s murine encephalomyelitis virus, and they made overtly normal Ab responses to a variety of antigens. Thus, CD134 is not essential for many T cell responses in vivo, nor is it required for the provision of help to B cells. Nonetheless, a subtle role in the regulation of T cell reactivity is suggested by the effect of CD134 deficiency on in vitro T cell responses.

OX40-deficient mice are defective in Th cell proliferation but are competent in generating B cell and CTL Responses after virus infection

OX40, a member of the TNF receptor superfamily, is expressed on activated T cells and implicated in stimulation of T cells and T-dependent humoral responses. We generated OX40-/- mice and found that the formation of extrafollicular plasma cells, germinal centers, and antibody responses was independent of OX40. After infection with LCMV and influenza virus, OX40-/- mice retain primary and memory cytotoxic T cell responses with normal expansion and decline of specific CTL. In contrast, CD4+ T cell proliferation and the number of IFN-gamma-producing CD4+ T cells were reduced in OX40-/- mice. Moreover, the number of CD4+ T cells infiltrating the lungs of influenza virus-infected OX40-/- mice was reduced. These results define a unique role of OX40 in the generation of optimal CD4+ T cell responses in vivo.

Analysis of 4-1BB Ligand (4-1BBL)-Deficient Mice and of Mice Lacking Both 4-1BBL and CD28 Reveals a Role for 4-1BBL in Skin Allograft Rejection and in the Cytotoxic T Cell Response to Influenza Virus

4-1BB ligand (4-1BBL) is a member of the TNF family expressed on activated APC. 4-1BBL binds to 4-1BB (CD137) on activated CD4 and CD8 T cells and in conjunction with strong signals through the TCR provides a CD28-independent costimulatory signal leading to high level IL-2 production by primary resting T cells. Here we report the immunological characterization of mice lacking 4-1BBL and of mice lacking both 4-1BBL and CD28. 4-1BBL−/− mice mount neutralizing IgM and IgG responses to vesicular stomatitis virus that are indistinguishable from those of wild-type mice. 4-1BBL−/− mice show unimpaired CTL responses to lymphocytic choriomeningitis virus (LCMV) and exhibit normal skin allograft rejection but have a weaker CTL response to influenza virus than wild-type mice. 4-1BBL−/−CD28−/− mice retain the CTL response to LCMV, respond poorly to influenza virus, and exhibit a delay in skin allograft rejection. In agreement with these in vivo results, allogeneic CTL responses of CD28−/− but not CD28+/+ T cells to 4-1BBL-expressing APC are substantially inhibited by soluble 4-1BB receptor as is the in vitro secondary response of CD28+ T cells to influenza virus peptides. TCR-transgenic CD28−/− LCMV glycoprotein-specific T cells are insensitive to the presence of 4-1BBL when a wild-type peptide is used, but the response to a weak agonist peptide is greatly augmented by the presence of 4-1BBL. These results further substantiate the idea that different immune responses vary in their dependence on costimulation and suggest a role for 4-1BBL in augmenting suboptimal CTL responses in vivo.

Role of the Stress Kinase Pathway in Signaling Via the T Cell Costimulatory Receptor 4-1BB

4-1BB is a member of the TNFR superfamily expressed on activated CD4+ and CD8+ T cells. 4-1BB can costimulate IL-2 production by resting primary T cells independently of CD28 ligation. In this study, we report signaling events following 4-1BB receptor aggregation using an Ak-restricted costimulation-dependent T cell hybridoma, C8.A3. Aggregation of 4-1BB on the surface of C8.A3 cells induces TNFR-associated factor 2 recruitment, which in turn recruits and activates apoptosis signal-regulating kinase-1, leading to downstream activation of c-Jun N-terminal/stress-activated protein kinases (JNK/SAPK). 4-1BB ligation also enhances anti-CD3-induced JNK/SAPK activation in primary T cells. Overexpression of a catalytically inactive form of apoptosis signal-regulating kinase-1 in C8.A3 T cells interferes with activation of the SAPK cascade and with IL-2 secretion, consistent with a critical role for JNK/SAPK activation in 4-1BB-dependent IL-2 production. Given the ability of both CD28 and 4-1BB to induce JNK/SAPK activation, we asked whether hyperosmotic shock, another inducer of this cascade, could function to provide a costimulatory signal to T cells. Osmotic shock of resting primary T cells in conjunction with anti-CD3 treatment was found to costimulate IL-2 production by the T cells, consistent with a pivotal role for JNK/SAPK in T cell costimulation.

Cutting Edge: 4-1BB Is a Bona Fide CD8 T Cell Survival Signal

After recognition of Ag/MHC and ligation of a costimulatory molecule, resting T cells will clonally expand and then delete to very low levels. Previously, it was shown that deletion can be prevented by coinjection of cytokines or proinflammatory agents such as adjuvants. Here, we demonstrate that ligation of 4-1BB blocks deletion of superantigen-activated T cells in the absence of adjuvant or additional cytokine treatment. Nearly 10 times as many staphylococcal enterotoxin A-specific T cells were detected in the spleens of mice injected 21 days previously with staphylococcal enterotoxin A and an agonist anti-4-1BB Ab compared with mice given staphylococcal enterotoxin A and a control IgG. Even though both CD4- and CD8-activated T cells expressed 4-1BB, a higher proportion of CD8 T cells were rescued compared CD4 T cells. These data suggest that although 4-1BB provides costimulation, it may also promote long-term T cell survival.