Gene structure and chromosomal localization of the mouse homologue of rat OX40 protein

PKC-ѳ is dispensable for OX40L-induced TCR-independent Treg proliferation but contributes by enabling IL-2 production from effector T-cells

We have previously shown that OX40L/OX40 interaction is critical for TCR-independent selective proliferation of Foxp3⁺ Tregs, but not Foxp3^- effector T-cells (Teff), when CD4⁺ T-cells are co-cultured with GM-CSF derived bone marrow dendritic cells (G-BMDCs). Events downstream of OX40L/OX40 interaction in Tregs responsible for this novel mechanism are not understood. Earlier, OX40L/OX40 interaction has been shown to stimulate CD4⁺ T-cells through the formation of a signalosome involving TRAF2/PKC-Ѳ leading to NF-kB activation. In this study, using CD4⁺ T-cells from WT and OX40^-/- mice we first established that OX40 mediated activation of NF-kB was critical for this Treg proliferation. Although CD4⁺ T-cells from PKC-Ѳ^-/- mice were also defective in G-BMDC induced Treg proliferation ex vivo, this defect could be readily corrected by adding exogenous IL-2 to the co-cultures. Furthermore, by treating WT, OX40^-/-, and PKC-Ѳ^-/- mice with soluble OX40L we established that OX40L/OX40 interaction was required and sufficient to induce Treg proliferation in vivo independent of PKC-Ѳ status. Although PKC-Ѳ is dispensable for TCR-independent Treg proliferation per se, it is essential for optimum IL-2 production by Teff cells. Finally, our findings suggest that OX40L binding to OX40 likely results in recruitment of TRAF1 for downstream signalling.

TNF optimally activatives regulatory T cells by inducing TNF receptor superfamily members TNFR2, 4-1BB and OX40

TNF is a pleiotropic cytokine with intriguing biphasic pro-inflammatory and anti-inflammatory effects. Our previous studies demonstrated that TNF up-regulated FoxP3 expression and activated and expanded CD4+ FoxP3+ regulatory T cells (Tregs) via TNFR2. Furthermore, TNFR2-expressing Tregs exhibited maximal suppressive activity. In this study, we show that TNF, in concert with IL-2, preferentially up-regulated mRNA and surface expression of TNFR2, 4-1BB and OX40 on Tregs. Agonistic antibodies against 4-1BB and OX40 also induced the proliferation of suppressive Tregs. Thus, TNF amplifies its stimulatory effect on Tregs by inducing TNF receptor superfamily (TNFRSF) members. In addition, administration of neutralizing anti-TNF Ab blocked LPS-induced expansion of splenic Tregs and up-regulation of TNFR2, OX40 and 4-1BB receptors on Tregs in vivo, indicating that the expansion of Tregs expressing these co-stimulatory TNFRSF members in response to LPS is mediated by TNF. Altogether, our novel data indicate that TNF preferentially up-regulates TNFR2 on Tregs, and this is amplified by the stimulation of 4-1BB and OX40, resulting in the optimal activation of Tregs and augmented attenuation of excessive inflammatory responses.

Arginine-rich cell-penetrating peptides facilitate delivery of antisense oligomers into murine leukocytes and alter pre-mRNA splicing

Phosphorodiamidate morpholino oligomers (PMO) are synthetic antisense molecules that interfere with translation, pre-mRNA splicing and RNA synthesis. Like other gene-silencing technologies, PMO are poorly taken up by primary leukocytes without the use of physical or chemical delivery techniques. We sought an alternative delivery mechanism of PMO into immune cells that eliminates the need for such manipulations. Here we demonstrate the first use of arginine-rich cell-penetrating peptides (CPPs) to deliver PMO (P-PMO) directly into primary murine leukocytes for inhibition of gene expression and promotion of altered pre-mRNA splicing. We compared the P-PMO delivery efficacy of four arginine-rich CPPs including HIV Tat and penetratin, and one histidine rich CPP, and found that the (RXR)(4) peptide was the most efficacious for PMO delivery and targeted antisense effect. The delivery and antisense effects of P-PMO are time- and dose-dependent and influenced by the activation and maturation states of T cells and dendritic cells, respectively. Targeted expression of several genes using P-PMO is shown including surface signaling proteins (CD45 and OX-40), a cytokine (interleukin-2), and a nuclear transcription factor (Foxp3). Considering the abundance of naturally occurring alternatively spliced gene products involved in immune regulation, P-PMO offer an effective method for modulating gene activity for immunological research and applications beyond traditional antisense approaches.

Characterization and application of two novel monoclonal antibodies against human OX40: costimulation of T cells and expression on tumor as well as normal gland tissues

OX40, a membrane-bound molecule of the tumor-necrosis-factor-receptor superfamily, is a critical costimulatory receptor during the immune response. Here, we newly generated two specific mouse antihuman OX40 monoclonal antibodies (mAbs) (2G2 and 1F7), whose specificities are quite different from the available OX40 mAb (ACT35) by competition assay. It was also found that both mAbs could enhance the proliferation, activation and differentiation of T lymphocytes primed by anti-CD3 mAb. These results evidenced that both were functional antihuman OX40 mAbs. Furthermore, stained by 2G2 and 1F7, FCM and immunohistochemistry detected the constitutive expression of OX40 on tumor cell lines from epithelium, breast cancer and glioma tissues. Meanwhile, the non-tumor tissues (thyroid gland, stomach gland) were also found OX40 expression. These results suggested that OX40 is not only expressed in activated T cells, but also in some tumors as well as normal gland tissues. Such expression pattern indicated that OX40 may be a valuable surface antigen in unveiling its expression and function outside the immune system. Briefly, these novel antibodies may contribute to the evaluation of the mechanism of tumor metastasis and eventually shed light on further study of tumor immunotherapy and autoimmune diseases.

Bone marrow transplantation and approaches to avoid graft-versus-host disease (GVHD)

Haematopoietic stem cell transplantation (HSCT) offers promise for the treatment of haematological and immune disorders, solid tumours, and as a tolerance inducing regimen for organ transplantation. Allogeneic HSCTs engraftment requires immunosuppression and the anti-tumour effects are dependent upon the immune effector cells that are contained within or generated from the donor graft. However, significant toxicities currently limit its efficacy. These problems include: (i) graft-versus-host disease (GVHD) in which donor T cells attack the recipient resulting in multi-organ attack and morbidity, (ii) a profound period of immune deficiency following HSCT, and (iii) donor graft rejection. Currently available methods to prevent or treat GVHD with systemic immunosuppression can lead to impaired immune recovery, increased opportunistic infections, and higher relapse rates. This review will provide an overview of GVHD pathophysiology and discuss the roles of various cells, pathways, and factors in the GVHD generation process and in the preservation of graft-versus-tumour effects. Variables that need to be taken into consideration in attempting to extrapolate preclinical results to the clinical paradigm will be highlighted.

Regulation of CD4 T cell memory by OX40 (CD134)

CD4 memory T cells play a critical role in protection against repeated exposure to infectious agents such as viruses, bacteria, and helminth parasites, yet can also contribute to the aberrant immune responses associated with autoimmune and allergic reactions. Understanding the mechanisms that control effective memory responses has important ramifications for vaccine design and in the management of adverse immune reactions. Recent advances in studies of T cell memory have implicated the tumor-necrosis-factor receptor (TNFR) family member, OX40 (CD134), as a key co-stimulatory molecule involved in the regulation of CD4 memory T cells. In this review we discuss these new developments in the context of past research and current models for the generation, persistence, and re-activation of memory T cells.

OX40 ligation on activated T cells enhances the control of Cryptococcus neoformans and reduces pulmonary eosinophilia

Pulmonary eosinophilia induced in C57BL/6 mice after Cryptococcus neoformans infection is driven by CD4(+) Th2 cells. The immunological mechanisms that protect against eosinophilia are not fully understood. Interaction of OX40 (CD134) and its ligand, OX40L, has been implicated in T cell activation and cell migration. Unlike CD28, OX40 is only expressed on T cells 1-2 days after Ag activation. Manipulation of this pathway would therefore target recently activated T cells, leaving the naive repertoire unaffected. In this study, we show that engagement of OX40 by an OX40L:Ig fusion protein drives IFN-gamma production by CD4(+) T cells and reduces eosinophilia and C. neoformans burden in the lung. Using gene-depleted mice, we show that reduction of eosinophilia and pathogen burden requires IL-12 and/or IFN-gamma. C. neoformans infection itself only partially induces OX40L expression by APCs. Provision of exogenous OX40L reveals a critical role of this pathway in the prevention of C. neoformans-induced eosinophilia.

Structure of the Hodgkin's lymphoma-associated human CD30 gene and the influence of a microsatellite region on its expression in CD30(+) cell lines

The CD30 antigen is a member of the tumor necrosis factor receptor (TNFR) family which is overexpressed on the surface of the tumor cells of Hodgkin's lymphoma, anaplastic large cell lymphoma (ALCL), and embryonal carcinoma of the testis. In this study the entire cd30 gene which is more than 24000 bp long and organized in eight exons was characterized by analyzing cosmid and phage lambda clones from human placental libraries with long-range polymerase chain reaction (PCR) and sequencing. Differences to other genes of the TNFR family were detected in the region encoding the extracellular domain of the cd30 gene. In nearly all other TNFR genes, the coding region of each cysteine-rich repeat is interrupted by one intron, i.e., the 3-4 cysteine-rich repeats of these receptors are encoded by at least 4-5 exons, whereas the six cysteine-rich repeats of the cd30 gene are encoded by two exons, i.e., each of these exons encode three cysteine-rich repeats. In addition, we also found a genetic polymorphism of tetranucleotide ATCC-repeats in the 5' part of the CD30 promoter. This region was amplified by PCR from seven CD30 overexpressing human lymphoid cell lines and five human tissues with an absent or very low CD30 expression. The amplification products showed length differences of more than 550 bp. The number of the ATCC-repeats was higher in CD30(+) cell lines than in normal tissues. Comparison of the individual PCR products in reporter gene assays revealed that the CD30 promoter activity increased with the length of this polymorphic region up to eightfold. The data suggest that the number of ATCC-repeats in the 5' region of the CD30 promoter modulates the regulation of CD30 expression.

Predominant transgene expression in exocrine pancreas directed by the CMV promoter

The enhancer/promoter of the human cytomegalovirus gene encoding the major immediate-early protein (CMVp) is reputed to be one of the strongest and most promiscuous regulatory elements for directing transcription of heterologous genes in vitro. However, transgene expression under the promoter in adult transgenic mice is often more restricted. We selected a CMVp segment from position -350 to +59 to control expression of transgenes for two secretory fusion proteins. Expression was analyzed by immunohistology staining and quantified by Northern blot, Western blot, and ELISA of secretions from explanted tissues. In all six lines of transgenic mice, the highest expression of transgenes at the mRNA and protein level was observed in the exocrine tissue of the pancreas, although the levels of expression varied among the lines. The results indicate not only that CMVp is not a universal promoter in vivo but indeed that it can be relatively specific for the exocrine pancreas, where expression of the gene it controlled was consistently very high.

Reciprocal expression of the TNF family receptor herpes virus entry mediator and its ligand LIGHT on activated T cells: LIGHT down-regulates its own receptor

The TNF receptor (TNFR) family plays a central role in the development of the immune response. Here we describe the reciprocal regulation of the recently identified TNFR superfamily member herpes virus entry mediator (HVEM) (TR2) and its ligand LIGHT (TL4) on T cells following activation and the mechanism of this process. T cell activation resulted in down-regulation of HVEM and up-regulation of LIGHT, which were both more pronounced in CD8(+) than CD4(+) T lymphocytes. The analysis of HVEM and LIGHT mRNA showed an increase in the steady state level of both mRNAs following stimulation. LIGHT, which was present in cytoplasm of resting T cells, was induced both in cytoplasm and at the cell surface. For HVEM, activation resulted in cellular redistribution, with its disappearance from cell surface. HVEM down-regulation did not rely on de novo protein synthesis, in contrast to the partial dependence of LIGHT induction. Matrix metalloproteinase inhibitors did not modify HVEM expression, but did enhance LIGHT accumulation at the cell surface. However, HVEM down-regulation was partially blocked by a neutralizing mAb to LIGHT or an HVEM-Fc fusion protein during activation. As a model, we propose that following stimulation, membrane or secreted LIGHT binds to HVEM and induces receptor down-regulation. Degradation or release of LIGHT by matrix metalloproteinases then contributes to the return to baseline levels for both LIGHT and HVEM. These results reveal a self-regulating ligand/receptor system that contributes to T cell activation through the interaction of T cells with each other and probably with other cells of the immune system.

The OX40 costimulatory receptor determines the development of CD4 memory by regulating primary clonal expansion

The costimulatory receptor OX40 has recently been shown to be involved in primary CD4 responses to several defined Ags. However, to date there has been little information regarding the mechanism of action of OX40, such as whether it regulates T cell numbers, reactivity, or both, and whether it contributes to induction of long-term T cell responses. With an agonist Ab to OX40, and by tracking Ag-specific TCR transgenic T cells in vivo, we show that ligation of OX40 induces clonal expansion and survival of CD4 cells during primary responses, and results in the accumulation of greater numbers of memory cells with time. Significantly, OX40-deficient T cells, from mice generated by gene targeting, secrete IL-2 and proliferate normally during the initial period of activation, but cannot sustain this during the latter phases of the primary response, exhibiting decreased survival over time. Mice lacking OX40 develop only low frequencies of Ag-specific CD4 cells late in primary responses in vivo and generate dramatically lower frequencies of surviving memory cells. These results demonstrate that OX40-OX40L interactions control primary T cell expansion and the ability to retain high numbers of Ag-specific T cells. In this way, OX40 signals promote survival of greater numbers of T cells with time and control the size of the memory T cell pool.

Gene structure and chromosomal assignment of mouse GITR, a member of the tumor necrosis factor/nerve growth factor receptor family

GITR is a type I transmembrane protein that belongs to the tumor necrosis factor/nerve growth factor receptor (TNF/NGFR) family. This receptor is preferentially expressed in activated T lymphocytes and may function as signaling molecule during T-cell development. In the present study, we examined the genomic organization of the entire mouse GITR (mGITR) gene. The gene spans a 2543-bp region and consists of five exons (with a length ranging from 88 bp to 395 bp) and four introns (67 bp to 778 bp). In agreement with GITR expression in activated T cells, consensus elements for transcription factors involved in T-cell development and activation were identified in the 5' flanking region, including a consensus element for NF-kappaB. Two highly significant binding sites for MyoD and one binding site for myogenin were also found, suggesting involvement of GITR in muscle development. The mGITR gene contains 17 transcription initiation sites distributed over a 76-bp region, all used with the same frequency. We localized mGITR to the murine chromosome 4 (E region), where other 4 TNF/NGFR members localize, including m4-1BB and mOX40. These results further indicate that GITR shares several features with OX40, 4-1BB, and CD27, suggesting the existence of a new subfamily of the TNFR family, as also confirmed by the similarity of their cytoplasmic domains.

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.

A Major Linkage Region on Distal Chromosome 4 Confers Susceptibility to Mouse Autoimmune Gastritis

Although much is known about the pathology of human chronic atrophic (type A, autoimmune) gastritis, its cause is poorly understood. Mouse experimental autoimmune gastritis (EAG) is a CD4+ T cell-mediated organ-specific autoimmune disease of the stomach that is induced by neonatal thymectomy of BALB/c mice. It has many features similar to human autoimmune gastritis. To obtain a greater understanding of the genetic components predisposing to autoimmune gastritis, a linkage analysis study was performed on (BALB/cCrSlc × C57BL/6)F2 intercross mice using 126 microsatellite markers covering 95% of the autosomal genome. Two regions with linkage to EAG were identified on distal chromosome 4 and were designated Gasa1 and Gasa2. The Gasa1 gene maps within the same chromosomal segment as the type 1 diabetes and systemic lupus erythematosus susceptibility genes Idd11 and Nba1, respectively. Gasa2 is the more telomeric of the two genes and was mapped within the same chromosomal segment as the type 1 diabetes susceptibility gene Idd9. In addition, there was evidence of quantitative trait locus controlling autoantibody titer within the telomeric segment of chromosome 4. The clustering of genes conferring susceptibility to EAG with those conferring susceptibility to type 1 diabetes is consistent with the coinheritance of gastritis and diabetes within human families. This is the first linkage analysis study of autoimmune gastritis in any organism and as such makes an important and novel contribution to our understanding of the etiology of this disease.

4-1BB and Ox40 are members of a tumor necrosis factor (TNF)-nerve growth factor receptor subfamily that bind TNF receptor-associated factors and activate nuclear factor kappaB

Members of the tumor necrosis factor (TNF)-nerve growth factor (NGF) receptor family have been shown to be important costimulatory molecules for cellular activation. 4-1BB and Ox40 are two recently described members of this protein family which are expressed primarily on activated T cells. To gain insight into the signaling pathways employed by these factors, yeast two-hybrid library screens were performed with the cytoplasmic domains of 4-1BB and Ox40 as baits. TNF receptor-associated factor 2 (TRAF2) was identified as an interacting protein in both screens. The ability of both 4-1BB and Ox40 to interact with TRAF2 was confirmed in mammalian cells by coimmunoprecipitation studies. When the binding of the receptors to other TRAF proteins was investigated, 4-1BB and Ox40 displayed distinct binding patterns. While 4-1BB bound TRAF2 and TRAF1, Ox40 interacted with TRAF3 and TRAF2. Using deletion and alanine scanning analysis, we defined the elements in the cytoplasmic domains of both receptors that mediate these interactions. The 4-1BB receptor was found to have two independent stretches of acidic residues that can mediate association of the TRAF molecules. In contrast, a single TRAF binding domain was identified in the cytoplasmic tail of Ox40. The cytoplasmic domains of both receptors were shown to activate nuclear factor kappaB in a TRAF-dependent manner. Taken together, our results indicate that 4-1BB and Ox40 bind TRAF proteins to initiate a signaling cascade leading to activation of nuclear factor kappaB.

A newly identified member of the tumor necrosis factor receptor superfamily with a wide tissue distribution and involvement in lymphocyte activation

The tumor necrosis factor receptor (TNFR) superfamily consists of approximately 10 characterized members of human proteins. We have identified a new member of the TNFR superfamily, TR2, from a search of an expressed sequence tag data base. cDNA cloning and Northern blot hybridization demonstrated multiple mRNA species, of which a 1.7-kilobase form was most abundant. However, TR2 is encoded by a single gene which, maps to chromosome 1p36.22-36.3, in the same region as several other members of the TNFR superfamily. The most abundant TR2 open reading frame encodes a 283-amino acid single transmembrane protein with a 36-residue signal sequence, two perfect and two imperfect TNFR-like cysteine-rich domains, and a short cytoplasmic tail with some similarity to 4-1BB and CD40. TR2 mRNA is expressed in multiple human tissues and cell lines and shows a constitutive and relatively high expression in peripheral blood T cells, B cells, and monocytes. A TR2-Fc fusion protein inhibited a mixed lymphocyte reaction-mediated proliferation suggesting that the receptor and/or its ligand play a role in T cell stimulation.

Transfer of primed CD4+OX40- T lymphocytes induces increased immunity to experimental Salmonella typhimurium infections in rats

The protective effect of primed CD4 T cells against a lethal dose of Salmonella typhimurium was studied in Lewis rats. Primed CD4 T cells were obtained by inoculating Lewis rats with a non-lethal dose of S. typhimurium. Four weeks after the infection, spleen non-adherent mononuclear cells were isolated. The cells were separated according to their expression of CD4 and the OX40 antigen by FACS. OX40+ and OX40- CD4+ T-cell subpopulations were together with unsorted CD4+ T cells transferred to untreated rats 24 h prior to infection with S. typhimurium. Transfer of either unsorted CD4+ T cells or CD4+ T cells sorted into OX40- or OX40- subpopulations significantly increased animal survival compared to controls. Animals receiving OX40+CD4+ T cells did not differ significantly in survival probability from those receiving unsorted CD+ T cells. However, animals receiving OX40-CD4+ T cells had a significantly better survival compared to animals given unsorted CD4+ T cells. It is concluded that OX40-CD4+ T cells can induce significant protection against S. typhimurium infections in rats. This is most likely due to the fact that the OX40-CD4+ T-cell population contains a significant number of antigen-specific memory T cells that have returned to a resting state.

Molecular, structural, and biological characteristics of the tumor necrosis factor ligand superfamily

The tumor necrosis factor receptor superfamily at present consists of ten different transmembrane (type I) glycoproteins with characteristic limited sequence homology for the cysteine-rich repeats in the extracellular domain. In parallel the tumor necrosis factor ligand super-family has been recognized by discovery of ligands for all members of the receptor superfamily. These molecules are also transmembrane (type II) glycoproteins, with the exception of lymphotoxin-alpha which is the only entirely secreted protein of the tumor necrosis factor-like proteins. Several members of the ligand superfamily, including tumor necrosis factor and CD95L also exist in a biologically active soluble form. The tumor necrosis factor ligand superfamily contains at present ten different proteins. In addition, NGFR p75 binds to a second family of proteins (neurotrophins). These nerve growth factor-like dimeric soluble molecules are basic neurotrophic factors and the five members (NGF, BDNF, NT-3, NT-4, NT-5) are not related to the tumor necrosis factor superfamily ligands. The members of the tumor necrosis factor ligand superfamily (TNF, LT-alpha, LT-beta, CD27L, CD30L, CD40L, CD95L, 4-IBB, OX40L, TRAIL) share common biological activities, but some properties are shared by only some ligands, while others are unique. The diverse biological activities triggered through tumor necrosis factor receptors have been linked to the regulation of cellular activation, including immune responses and inflammatory reactions, but also with the pathology of a series of human diseases.

Human tumor necrosis factor receptor p75/80 (CD120b) gene structure and promoter characterization

Tumor necrosis factor receptor p75 (TNF-R p75) is a 75-kDa type I transmembrane protein expressed predominantly on cells of hematopoietic lineage. TNF-R p75 belongs to the TNF receptor superfamily characterized by cysteine-rich extracellular regions composed of three to six disulfide-linked domains. In the present report we have characterized, for the first time, the complete gene structure for human TNF-R p75, which spans approximately 43 kbp. The gene consists of 10 exons (ranging from 34 base pairs to 2.5 kilobase pairs) and nine introns (343 base pairs to 19 kilobase pairs). Consensus elements for transcription factors involved in T cell development and activation were noted in the 5'-flanking region including T cell factor-1, Ikaros, AP-1, CK-2, interleukin-6 receptor E (IL-6RE), ISRE, GAS, NF-kappaB, and Sp1. The unusual (GATA)n and (GAA)(GGA) repeats found within intron 1 may prove useful for further genome analysis within the 1p36 chromosomal locus. Characterization of the human TNF-R p75 gene structure will permit further assessment of its involvement in normal hematopoietic cell development and function, autoimmune disease, and nonrandom translocations in hematopoietic malignancies.

OX40 is differentially expressed on activated rat and mouse T cells and is the sole receptor for the OX40 ligand

OX40, a member of the tumor necrosis factor (TNF) receptor/nerve growth factor (NGF) receptor superfamily was first identified as a marker of activated rat CD4+ cells with the MRC OX40 monoclonal antibody (mAb). A ligand for OX40 (called OX40 ligand or OX40L) has recently been identified and has sequence similarity to TNF. Mouse OX40L-immunoglobulin fusion protein (OX40L-Ig) binds to activated mouse CD4+ and CD8+ cells (Baum, P. R. et al., EMBO J. 1994. 13: 3992) suggesting that OX40 could have a differential pattern of expression on mouse and rat T cells. This, however, did not rule out the presence of an alternative receptor on CD8+ cells that also binds the OX40L. We have compared the binding of the MRC OX40 mAb with that of OX40L-Ig to activated rat lymph node cells and show that both recognize the same protein, namely OX40 which is expressed on CD4+ and CD4+ CD8 alpha+ cells, but not on CD4-CD8+ cells. We have raised a new mAb (MRC OX86) using recombinant mouse OX40 protein and show by two-color flow cytometry that mouse OX40 is expressed on CD4 and CD8 single-positive cells. In addition, the new MRC OX86 mAb, unlike the MRC OX40 mAb, did not block binding of the OX40L. We conclude that OX40 is differentially expressed on activated mouse and rat T cells and is the sole receptor for the OX40L.