OX40 ligand and CD30 ligand are expressed on adult but not neonatal CD4+CD3- inducer cells: evidence that IL-7 signals regulate CD30 ligand but not OX40 ligand expression

In this report, we have examined the expression of the T cell survival signals, OX40 ligand (OX40L) and CD30 ligand (CD30L) on CD4(+)CD3(-)CD11c(-)B220(-)IL-7Ralpha(+) inducer cells from birth to adulthood in mice. We found that adult but not neonatal inducer cells expressed high levels of OX40L and CD30L, whereas their expression of TNF-related activation-induced cytokine (TRANCE) and receptor activator of NF-kappaB (RANK) was comparable. The failure of neonatal inducer cells to express the ligands that rescue T cells helps to explain why exposure to Ag in neonatal life induces tolerance rather than immunity. The expression of OX40L and CD30L on inducer cells increased gradually in the first few weeks of life achieving essentially normal levels around the time mice were weaned. We found that IL-7 signaling through the common cytokine receptor gamma-chain was critical for the optimal expression of both TNF-related activation-induced cytokine and CD30L but not OX40L. Furthermore, glucocorticoids, which potently suppress T effector function, did not influence the expression of OX40L and CD30L in the presence of IL-7.

Mice deficient in OX40 and CD30 signals lack memory antibody responses because of deficient CD4 T cell memory

Recently, we reported that a CD4(+)CD3(-)CD11c(-) accessory cell provided OX40-dependent survival signals to follicular T cells. These accessory cells express both OX40 ligand and CD30 ligand, and the receptors, OX40 and CD30, are both expressed on Th2-primed CD4 T cells. OX40 and CD30 signals share common signaling pathways, suggesting that CD30 signals might substantially compensate in OX40-deficient mice. In this report we have dissected the signaling roles of CD30 alone and in combination with OX40. CD30-deficient mice showed an impaired capacity to sustain follicular germinal center responses, and recall memory Ab responses were substantially reduced. Deficiencies in OX40 and CD30 signals were additive; secondary Ab responses were ablated in double-deficient mice. Although the initial proliferation of OX40/CD30 double-knockout OTII transgenic T cells was comparable to that of their normal counterparts, they failed to survive in vivo, and this was associated with reduced T cell numbers associated with CD4(+)CD3(-) cells in B follicles. Finally, we show that OX40/CD30 double-knockout OTII transgenic T cells fail to survive compared with normal T cells when cocultured with CD4(+)CD3(-) cells in vitro.

Endothelial cell co-stimulation through OX40 augments and prolongs T cell cytokine synthesis by stabilization of cytokine mRNA

Human endothelial cells (ECs) constitutively express OX40L and co-stimulate memory CD4(+) T cell proliferation that is dependent upon OX40-OX40L interaction. In vivo, OX40 prolongs T cell survival; however, an unanswered question is whether it can also prolong synthesis of proliferation-sustaining cytokines such as IL-2. Here we show that EC co-stimulation results in the secretion of T cell IL-2, IL-3 and IFN-gamma and that in the absence of OX40 signals synthesis largely ceases by 12-18 h, but is prolonged up to 60 h in the presence of OX40 signaling. Blocking OX40-mediated cytokine expression at later times suppresses T cell proliferation and this can be overcome by addition of exogenous IL-2. We find that OX40 signaling has discrete effects on T cell activation as it does not affect expression of IL-10, CD25, CD69 or soluble IL-2R. Also, OX40 does not appear to alter IL-2 transcription, but rather acts to stabilize a subset of cytokine mRNAs, increasing their half-lives by 3-6-fold. We further show that OX40L induces activation of p38 mitogen-activated protein kinase (MAPK) and phosphotidyl-inositol-3-kinase (PI3K) in T cells, and using specific inhibitors, we find that increased mRNA half-life is dependent upon both these pathways but is independent of c-jun-N-terminal kinase (JNK). Thus, EC co-stimulation through OX40 leads to prolonged T cell cytokine synthesis and enhanced proliferation.

Introduction of OX40 ligand into lymphoma cells elicits anti-lymphoma immunity in vivo

OBJECTIVE

OX40, a member of the TNF receptor superfamily, and its ligand (OX40L) play crucial roles in induction and maintenance of integrated T cell immune response. Engagement of OX40L delivers a costimulatory signal to T cells. In this study, we investigated whether inoculation of OX40L-transfected EL4, a murine T cell lymphoma cell line, could induce anti-lymphoma immunity in mice.

MATERIALS AND METHODS

Female C57BL/6 mice were inoculated with 1 x 10(5) cells of parental EL4, OX40L-transfected EL4 (EL4-OX40L), or mock control vector-transfected EL4 (EL4-mock), and then the tumor size, overall survival, CTL activity of spleen cells, and the immunohistochemistry were compared.

RESULTS

While both parental EL4 and EL4-mock grew rapidly, EL4-OX40L was rejected or grew slower than parental EL4 or EL4-mock. Pretreatment of mice with either anti-CD4 or anti-CD8 mAb accelerated the growth of EL4-OX40L, suggesting that both CD4+ and CD8+ T cells were involved in anti-lymphoma immunity. The immunohistochemical study revealed the infiltration of CD8+ T cells into the tumor of EL4-OX40L. In vitro CTL assay demonstrated that spleen cells of mice that had rejected EL4-OX40L had significant cytotoxic activity against parental EL4.

CONCLUSION

The gene transfer of OX40L into lymphoma cells is an eligible and efficient modality to induce anti-lymphoma immunity.

Modulation of TNF Receptor Family Members to Inhibit Autoimmune Disease

Certain members of the TNF-receptor family have shown proinflammatory function during immune activation and can be directly involved with the pathogenic effects observed during an autoimmune episode. The TNF-R family members summarized in this review includes: TNF-RI + II, OX40, and 4-1BB and they are expressed on a variety of leukocytes within the body. Studies within the last decade suggest that all of these proteins or their natural ligands can be targeted with various agents designed to diminish clinical signs of disease in autoimmune models. The data from the preclinical models specifically involving TNF-blockade have led to the development of clinical trials for patients with multiple sclerosis and rheumatoid arthritis. This review will chronicle the preclinical development of agents designed to inhibit OX40 and 4-1BB functions in autoimmunity and discuss relevant preclinical and clinical data associated with TNF-blockade.

CD40 and OX40 ligand are increased on stimulated asthmatic airway smooth muscle

BACKGROUND

Severe, persistent asthma is characterized by airway smooth muscle hyperplasia, inflammatory cell infiltration into the smooth muscle, and increased expression of many cytokines, including IL-4, IL-13, IL-1beta, and TNF-alpha. These cytokines have the potential to alter the expression of surface receptors such as CD40 and OX40 ligand on the airway smooth muscle cell.

OBJECTIVE

To examine whether cytokines alter expression of CD40 and OX40 ligand on airway smooth muscle cells and identify any differences in response between asthmatic and nonasthmatic airway smooth muscle cells.

METHODS

We used flow cytometry and immunohistochemistry to detect CD40 and OX40 ligand on airway smooth muscle cells cultured in the presence of TNF-alpha, IL-1beta, IL-4, or IL-13. Prostaglandin E 2 levels were assessed by ELISA.

RESULTS

TNF-alpha increased expression of both CD40 and OX40 ligand on both asthmatic and nonasthmatic airway smooth muscle cells. The level of expression was significantly greater on the asthmatic cells. IL-1beta alone had no effect, but it attenuated the TNF-induced expression of both CD40 and OX40 ligand. The mechanism of inhibition was COX-dependent for CD40 and was COX-independent but cyclic AMP-dependent for OX40 ligand. IL-4 and IL-13 had no effect.

CONCLUSION

Our study has demonstrated that TNF-alpha and IL-1beta have the potential to modulate differentially the interactions between cells present in the inflamed airways of a patient with asthma and therefore to contribute to the regulation of airway inflammation and remodeling.

Positional identification of TNFSF4, encoding OX40 ligand, as a gene that influences atherosclerosis susceptibility

Ath1 is a quantitative trait locus on mouse chromosome 1 that renders C57BL/6 mice susceptible and C3H/He mice resistant to diet-induced atherosclerosis. The quantitative trait locus region encompasses 11 known genes, including Tnfsf4 (also called Ox40l or Cd134l), which encodes OX40 ligand. Here we report that mice with targeted mutations of Tnfsf4 had significantly (P <or= 0.05) smaller atherosclerotic lesions than did control mice. In addition, mice overexpressing Tnfsf4 had significantly (P <or= 0.05) larger atherosclerotic lesions than did control mice. In two independent human populations, the less common allele of SNP rs3850641 in TNFSF4 was significantly more frequent (P <or= 0.05) in individuals with myocardial infarction than in controls. We therefore conclude that Tnfsf4 underlies Ath1 in mice and that polymorphisms in its human homolog TNFSF4 increase the risk of myocardial infarction in humans.

Co-stimulation: novel methods for preventing viral-induced lung inflammation

Respiratory infections cause significant morbidity and mortality worldwide. Although an immune response is required to eliminate respiratory pathogens, if unchecked, it can damage surrounding tissues and block primary lung function. Based on our knowledge of immune T-cell activation, there are several pathways to which immune intervention could be applied. However, relatively few interventions target only those immune cells that are responding to antigens. OX40 and 4-1BB are members of the tumour necrosis factor receptor family and are expressed on the surface of T cells in several inflammatory conditions. Recently, the inhibition of OX40 has proved beneficial during influenza virus infection. This review highlights the recent advances in the manipulation of such molecules and how they have been applied to inflammatory conditions that are caused by viruses in the lung.

OX40 Signals during Priming on Dendritic Cells Inhibit CD4 T Cell Proliferation: IL-4 Switches off OX40 Signals Enabling Rapid Proliferation of Th2 Effectors

In this study we examined the role and regulation of OX40 signals during CD4 T cell priming on dendritic cells (DCs). Contrary to expectation, OX40-deficient cells proliferated more rapidly than their normal counterparts, particularly when stimulated with peptide in the absence of added cytokines. This proliferative advantage was not apparent for Th2-differentiated cells. When the reasons for this were investigated, we found that the cytokine IL-4 specifically down-regulated expression of OX40 ligand on T, B, and DCs, but not on the CD4(+)CD3(-) cells linked with selection of Th2 cells into the memory compartment. OX40 ligand expression was also down-regulated on rapidly proliferating Th1 effectors. These data are compatible with OX40 signals acting during priming as a check on naive T cell proliferation while T cells integrate additional DC signals. This would serve to limit inappropriate T cell responses. In contrast, OX40 signals from CD4(+)CD3(-) cells located in the outer T zone select proliferating Th2 effectors into the memory T cell pool.

Tumor necrosis factor ligand-receptor superfamily and arthritis

The current studies of apoptosis in rheumatoid arthritis (RA) suggest that the TNF ligand-receptor superfamily (TNFRsF) molecules, downstream pathways (activation of proapoptosis or anti-apoptosis pathway), cell types (lymphocytes and synovial fibroblast), and the mechanism that triggers apoptosis (tolerance induction-related, downmodulation of inflammation-related, or DNA damage-related) all exhibit a capability to determine the induction or prevention of RA. This series of defects at different levels and in different cells have been shown to lead to T cell and synovial hyperproliferation, defective apoptosis, excessive apoptosis, or bone erosion. In this chapter, we summarize the available knowledge of the regulation of TNFRsF and their likely pathogenic roles in RA to help identify candidate target cells and target molecules for delivery of gene constructs to modulate apoptosis to prevent the development of RA in both humans and mice.

Regulation of OX40 gene expression in graft-versus-host disease

OX40 (CD134), a member of the tumor necrosis factor receptor superfamily, is expressed on activated T cells, including CD4(+)CD25(+) T regulatory (Treg) cells. To investigate the kinetics of OX40-OX40L in graft-versus-host disease (GVHD), OX40 mRNA transcript levels were temporally examined in peripheral blood mononuclear cells (PBMCs) from patients undergoing either allogeneic (allo) bone marrow transplantation (alloBMT) or autologous (auto) BMT with the induction of autoGVHD by cyclosporine (CsA) treatment posttransplant. Real-time quantitative PCR analysis revealed that OX40 mRNA expression decreased significantly in PBMCs from patients with either alloGVHD or autoGVHD compared with healthy individuals. No differences were detected between patients developing alloGVHD and those who did not develop this posttransplant complication. On the other hand, a decrease in OX40 mRNA levels correlated directly with the development of autoGVHD. Moreover, the upregulation of OX40 gene expression coincided with the resolution of autoGVHD. Interestingly, expression of OX40 by CD4(+) T lymphocytes after stimulation with autoantigen (Ag) was significantly (>700-fold) increased with a concomitant increase in expression of the Foxp3 regulatory gene. Expression of OX40 was increased (maximum 11-fold) after allo-Ag via mixed-lymphocyte reaction response. CsA suppressed the upregulation of OX40 expression after allo-Ag in a dose-dependent manner. Taken together, these results suggest that the decrease in OX40 expression posttransplant includes the defective reconstitution of Treg cells, and the active inhibition of gene transcription by CsA.

Prevention of diabetes in NOD mice at a late stage by targeting OX40/OX40 ligand interactions

Autoreactive T cells play a major role in the development of insulin-dependent diabetes mellitus, suggesting that costimulatory molecules that regulate T cell responses might be essential for disease progression. In NOD mice, CD28/B7 and CD40/CD40 ligand (L) interactions control the onset of diabetes from 2 to 4 weeks of age, but blocking these molecules has little effect after this time. Hence, it is possible that other ligand/receptor pairs control a later phase of disease. We now show that OX40 is expressed on CD4 and CD8 T cells several weeks prior to islet destruction, which is initiated around weeks 12-14, and that OX40L is present on dendritic cells in both secondary lymphoid organs and the pancreas from 11 to 13 weeks of age. Blocking OX40L at 6, 9, or 15 weeks after birth had little effect on disease; however, inhibiting OX40/OX40L interactions at week 12, or continuous treatment from week 12 onwards, significantly reduced the incidence of diabetes. Histological examination showed that islet destruction was prevented and insulitis reduced by targeting OX40L. These studies show that OX40/OX40L interactions form a late checkpoint in diabetes development and suggest that these molecules are realistic targets for therapeutic intervention.

Anti-tumour therapeutic efficacy of OX40L in murine tumour model

OX40 ligand (OX40L), a member of TNF superfamily, is a co-stimulatory molecule involved in T cell activation. Systemic administration of mOX40L fusion protein significantly inhibited the growth of experimental lung metastasis and subcutaneous (s.c.) established colon (CT26) and breast (4T1) carcinomas. Vaccination with OX40L was significantly enhanced by combination treatment with intra-tumour injection of a disabled infectious single cycle-herpes simplex virus (DISC-HSV) vector encoding murine granulocyte macrophage-colony stimulating factor (mGM-CSF). Tumour rejection in response to OX40L therapy required functional CD4+ and CD8+ T cells and correlated with splenocyte cytotoxic T lymphocytes (CTLs) activity against the AH-1 gp70 peptide of the tumour associated antigen expressed by CT26 cells. These results demonstrate the potential role of the OX40L in cancer immunotherapy.

Molecular transfer of CD40 and OX40 ligands to leukemic human B cells induces expansion of autologous tumor-reactive cytotoxic T lymphocytes

Clinical benefits from monoclonal antibody therapy for B-chronic lymphocytic leukemia (B-CLL) have increased interest in developing additional immunotherapies for the disease. CD40 ligand is an accessory signal for T-cell activation and can overcome T-cell anergy. The OX40-OX40 ligand pathway is involved in the subsequent expansion of memory antigen-specific T cells. We expressed both CD40L and OX40L on B-CLL cells by exploiting the phenomenon of molecular transfer from fibroblasts overexpressing these ligands. We analyzed the effects of the modified B-CLL cells on the number, phenotype, and cytotoxic function of autologous T cells in 7 B-CLL patients. Transfer of CD40L and OX40L was observed in all and was followed by the up-regulation of B7-1 and B7-2. The culture of CD40L/OX40L-expressing B-CLL cells with autologous T cells generated CD4+/CD8+ cytotoxic T-cell lines, which secreted interferon-gamma (IFN-gamma) and granzyme-B/perforin in response to autologous, but not to allogeneic, B-CLL cells or to autologous T-cell blasts. CD40L or OX40L alone was insufficient to expand tumor-reactive T cells. The combination of CD40L and OX40L on B-CLL cells may allow the generation of therapeutic immune responses to B-CLL, either by active immunization with modified tumor cells or by adoptive immunotherapy with tumor-reactive autologous T cells.

4-1BB and OX40 Act Independently to Facilitate Robust CD8 and CD4 Recall Responses

Mice deficient in OX40 or 4-1BB costimulatory pathways show defects in T cell recall responses, with predominant effects on CD4 vs CD8 T cells, respectively. However, OX40L can also stimulate CD8 T cells and 4-1BBL can influence CD4 T cells, raising the possibility of redundancy between the two TNFR family costimulators. To test this possibility, we generated mice deficient in both 4-1BBL and OX40L. In an adoptive transfer model, CD4 T cells expressed 4-1BB and OX40 sequentially in response to immunization, with little or no overlap in the timing of their expression. Under the same conditions, CD8 T cells expressed 4-1BB, but no detectable OX40. Thus, in vivo expression of 4-1BB and OX40 can be temporally and spatially segregated. In the absence of OX40L, there were decreased CD4 T cells late in the primary response and no detectable secondary expansion of adoptively transferred CD4 T cells under conditions in which primary expansion was unaffected. The 4-1BBL had a minor effect on the primary response of CD4 T cells in this model, but showed larger effects on the secondary response, although 4-1BBL(-/-) mice show less impairment in CD4 secondary responses than OX40L(-/-) mice. The 4-1BBL(-/-) and double knockout mice were similarly impaired in the CD8 T cell response, whereas OX40L(-/-) and double knockout mice were similarly impaired in the CD4 T cell response to both protein Ag and influenza virus. Thus, 4-1BB and OX40 act independently and nonredundantly to facilitate robust CD4 and CD8 recall responses.

T cell proliferation by direct cross-talk between OX40 ligand on human mast cells and OX40 on human T cells: comparison of gene expression profiles between human tonsillar and lung-cultured mast cells

Mast cells (MCs) are the primary effector cells in allergic reactions and have also been found to activate T cells and to reside in close physical proximity to T cells. However, the molecular mechanisms involved in the MC-T cell interaction remain unclear. We hypothesized that human tonsillar MCs, which locate in the interfollicular areas, might interact with T cells. Thus, we first established a culture system of human tonsillar MCs and then compared gene expression profiles of tonsillar MCs with that of lung MCs before and after aggregation of FcepsilonRI by using high-density oligonucleotide probe arrays. Here we show that resting tonsillar MCs, when compared with lung MCs, revealed significantly higher expression levels for CC chemokines (CCL3 and 4), which recruit T cells, and for TNFR superfamilies (OX40 ligand and 4-1BB ligand), which induce proliferation of T cells. After aggregation of FcepsilonRI, not only tonsillar MCs but also lung MCs up-regulated the expression of these molecules. We confirmed that T cell proliferation is induced in direct cross-talk by the MC surface molecule OX40 ligand. These results suggest that human MCs may play important roles in adaptive immunity through the T cell responses.

Dendritic cell-mediated T cell polarization

Effective defense against diverse types of micro-organisms that invade our body requires specialized classes of antigen-specific immune responses initiated and maintained by distinct subsets of effector CD4(+) T helper (Th) cells. Excessive or detrimental (e.g., autoimmune) responses by effector T cells are controlled by regulatory T cells. The optimal balance in the development of the different types of effector and regulatory Th cells is orchestrated by dendritic cells (DC). This review discusses the way DC adapt the T cell response to the type of pathogen, focusing on the tools that DC use in this management of the T cell response.

Contribution of CD30/CD153 but not of CD27/CD70, CD134/OX40L, or CD137/4-1BBL to the optimal induction of protective immunity toMycobacterium avium

A panel of monoclonal antibodies specific for CD27 ligand (CD70), CD30 ligand (CD153), CD134 ligand (OX40L), and CD137 ligand (4-1BBL) were screened in vivo for their ability to affect the control of Mycobacterium avium infection in C57Bl/6 mice. Only the blocking of CD153 led to increased mycobacterial burdens. We then used CD30-deficient mice and found an increase in the proliferation of two strains of M. avium in these mice as compared with control animals. The increased mycobacterial growth was associated with decreased T cell expansion and reduced interferon-gamma (IFN-gamma) responses as a result of reduced polarization of the antigen-specific, IFN-gamma-producing T cells. At late times but not early in infection, the lymphoid cuff surrounding granulomas was depleted in the CD30-deficient animals. This report expands our knowledge about tumor necrosis factor superfamily members involved in the immune responses to mycobacterial infection by identifying CD30-CD153 interactions as required for optimal immune control of M. avium infection.

Immunotherapy with OX40L-Fc or anti-CTLA-4 enhances local tissue responses and killing of Leishmania donovani

Enhancing granuloma development and effector function, but without inducing the pathology associated with excess granulomatous inflammation, poses a major challenge for immunotherapeutic intervention against diseases such as visceral leishmaniasis (VL). Here, we demonstrate that a chimeric fusion protein (OX40L-Fc) which stimulates T cells through OX40 and a monoclonal antibody which blocks CTLA-4, an inhibitory receptor on T cells, both enhanced the rate of granuloma maturation, CD4(+) T cell proliferation, and killing of Leishmania. Costimulation-based therapy induced no adverse fibrotic or necrotic reactions, and had no significant effect on the levels of endogenous anti-inflammatory cytokines (IL-10 and TGF-beta). Furthermore, both OX40L-Fc and anti-CTLA4 could be co-administered with conventional anti-leishmanial drugs. Until now, enhancing T cell immunity by the manipulation of costimulatory pathways has only received serious attention for cancer immunotherapy, but our data provide a compelling argument for the evaluation of this approach in human VL and other infectious diseases.

Detection and characterization of OX40 ligand expression in human airway smooth muscle cells: a possible role in asthma?

BACKGROUND

The airway smooth muscle (ASM) cell, originally thought of as a passive structural cell, is now well recognized as an active participant in the pathologic events that occur during persistent asthma. Cell-surface molecules play an important role in the development of an immune response. A number of cell-surface molecules are expressed on ASM cells, and these might contribute to the inflammatory reaction.

OBJECTIVE

The purpose of this study was to determine whether OX40 ligand (OX40L), a molecule known to be involved in T-cell activation, was present on the ASM cell surface.

METHODS

We used real-time RT-PCR to detect mRNA expression and flow cytometry, ELISA, and immunoprecipitation to detect the presence of cell-surface protein on ASM cells isolated from asthmatic and nonasthmatic individuals. ELISAs and Western blotting were used to determine the functional outcomes of engagement of OX40L.

RESULTS

OX40L was present on both asthmatic and nonasthmatic ASM cells. Engagement of OX40L with recombinant OX40:Fc resulted in a significantly greater increase in release of IL-6 from ASM cells of asthmatic patients than from ASM cells of nonasthmatic patients (P<.01). Ligation of OX40L resulted in a rapid translocation of protein kinase C beta2 to the cell membrane.

CONCLUSION

Because the receptor for OX40L, OX40, is expressed on CD4+ T cells within 48 hours of stimulation through the T-cell receptor, elucidation of the cross-talk between OX40 and OX40L could be very important in understanding the interaction of cells present in the inflamed airways of an asthmatic patient.

Adenovirus Vector-Mediated in Vivo Gene Transfer of OX40 Ligand to Tumor Cells Enhances Antitumor Immunity of Tumor-Bearing Hosts

OX40 ligand (OX40L), the ligand for OX40 on activated CD4+ T cells, has adjuvant properties for establishing effective T-cell immunity, a potent effector arm of the immune system against cancer. The hypothesis of this study is that in vivo genetic engineering of tumor cells to express OX40L will stimulate tumor-specific T cells by the OX40L-OX40 engagement, leading to an induction of systemic antitumor immunity. To investigate this hypothesis, s.c. established tumors of three different mouse cancer cells (B16 melanoma, H-2b; Lewis lung carcinoma, H-2b; and Colon-26 colon adenocarcinoma, H-2d) were treated with intratumoral injection of a recombinant adenovirus vector expressing mouse OX40L (AdOX40L). In all tumor models tested, treatment of tumor-bearing mice with AdOX40L induced a significant suppression of tumor growth along with survival advantages in the treated mice. The in vivo AdOX40L modification of tumors evoked tumor-specific cytotoxic T lymphocytes in the treated host correlated with in vivo priming of T helper 1 immune responses in a tumor-specific manner. Consistent with the finding, the antitumor effect provided by intratumoral injection of AdOX40L was completely abrogated in a CD4+ T cell-deficient or CD8+ T cell-deficient condition. In addition, ex vivo AdOX40L-transduced B16 cells also elicited B16-specific cytotoxic T lymphocyte responses, and significantly suppressed the B16 tumor growth in the immunization-challenge experiment. All of these results support the concept that genetic modification of tumor cells with a recombinant OX40L adenovirus vector may be of benefit in cancer immunotherapy protocols.

Essential Role of OX40L on B Cells in Persistent Alloantibody Production Following Repeated Alloimmunizations

OX40/OX40 ligand (OX40L) interactions are implicated in costimulation for both CD4(+) T and B cells in a bidirectional manner. To determine the role of OX40/OX40L interactions in recipient antidonor responses after multiple allogeneic transfusions, we examined alloreactive cytotoxic T lymphocyte (allo-CTL) activity and alloantibody production in repeatedly alloimmunized OX40L-deficient mice. After the fifth alloimmunization, whereas OX40L-deficient mice showed allo-CTL activity with levels comparable to those of wild-type mice, alloantibody production in OX40L-deficient mice was significantly reduced, accompanied by fewer memory B and CD4(+) T cells with reduced function. Furthermore, nu/nu mice that received OX40L-deficient T cells still exhibited impaired alloantibody production with fewer memory CD4(+) T and B cells. In contrast, RAG-2-deficient mice that received both wild-type T cells and OX40L-deficient B cells produced scant alloantibodies with fewer memory B cells, but sufficient memory CD4(+) T cells. Thus, OX40L on B cells, rather than on T cells, is apparently required for adequate and persistent production of alloantibodies after repeated alloimmunizations.

Costimulation of CD8 T Cell Responses by OX40

The persistence of functional CD8 T cell responses is dependent on checkpoints established during priming. Although naive CD8 cells can proliferate with a short period of stimulation, CD4 help, inflammation, and/or high peptide affinity are necessary for the survival of CTL and for effective priming. Using OX40-deficient CD8 cells specific for a defined Ag, and agonist and antagonist OX40 reagents, we show that OX40/OX40 ligand interactions can determine the extent of expansion of CD8 T cells during responses to conventional protein Ag and can provide sufficient signals to confer CTL-mediated protection against tumor growth. OX40 signaling primarily functions to maintain CTL survival during the initial rounds of cell division after Ag encounter. Thus, OX40 is one of the costimulatory molecules that can contribute signals to regulate the accumulation of Ag-reactive CD8 cells during immune responses.