Antibodies to OX-40 (CD134) can identify and eliminate autoreactive T cells: implications for human autoimmune disease

Biologic and small-molecule therapy for treating moderate to severe atopic dermatitis: Mechanistic considerations

Atopic dermatitis (AD) is a complex and heterogeneous skin disease for which achieving complete clinical clearance for most patients has proven challenging through single cytokine inhibition. Current studies integrate biomarkers and evaluate their role in AD, aiming to advance our understanding of the diverse molecular profiles implicated. Although traditionally characterized as a TH2-driven disease, extensive research has recently revealed the involvement of TH1, TH17, and TH22 immune pathways as well as the interplay of pivotal immune molecules, such as OX40, OX40 ligand (OX40L), thymic stromal lymphopoietin, and IL-33. This review explores the mechanistic effects of treatments for AD, focusing on mAbs and Janus kinase inhibitors. It describes how these treatments modulate immune pathways and examines their impact on key inflammatory and barrier biomarkers.

The translational revolution in atopic dermatitis: the paradigm shift from pathogenesis to treatment

Atopic dermatitis (AD) is the most common inflammatory skin disease, and it is considered a complex and heterogeneous condition. Different phenotypes of AD, defined according to the patient age at onset, race, and ethnic background; disease duration; and other disease characteristics, have been recently described, underlying the need for a personalized treatment approach. Recent advancements in understanding AD pathogenesis resulted in a real translational revolution and led to the exponential expansion of the therapeutic pipeline. The study of biomarkers in clinical studies of emerging treatments is helping clarify the role of each cytokine and immune pathway in AD and will allow addressing the unique immune fingerprints of each AD subset. Personalized medicine will be the ultimate goal of this targeted translational research. In this review, we discuss the changes in the concepts of both the pathogenesis of and treatment approach to AD, highlight the scientific rationale behind each targeted treatment and report the most recent clinical efficacy data.

GBR 830, an anti-OX40, improves skin gene signatures and clinical scores in patients with atopic dermatitis

BACKGROUND

GBR 830 is a humanized mAb against OX40, a costimulatory receptor on activated T cells. OX40 inhibition might have a therapeutic role in T cell-mediated diseases, including atopic dermatitis (AD).

OBJECTIVE

This exploratory phase 2a study investigated the safety, efficacy, and tissue effects of GBR 830 in patients with AD.

METHODS

Patients with moderate-to-severe AD (affected body surface area, ≥10%; Eczema Area and Severity Index score, ≥12; and inadequate response to topical treatments) were randomized 3:1 to 10 mg/kg intravenous GBR 830 or placebo on day 1 (baseline) and day 29. Biopsy specimens were collected (n = 40) at days 1, 29, and 71. Primary end points included treatment-emergent adverse events (TEAEs) and changes from baseline in biomarkers (epidermal hyperplasia/cytokines) at days 29 and 71.

RESULTS

GBR 830 was well tolerated, with equal TEAE distribution (GBR 830, 63.0% [29/46]; placebo, 63.0% [10/16]). One serious TEAE in the GBR 830 group was deemed unrelated to study drug. At day 71, the proportion of intent-to-treat subjects achieving 50% or greater improvement in Eczema Area and Severity Index score was greater with GBR 830 (76.9% [20/26]) versus placebo (37.5% [3/8]). GBR 830 induced significant progressive reductions in T_H1 (IFN-γ/CXCL10), T_H2 (IL-31/CCL11/CCL17), and T_H17/T_H22 (IL-23p19/IL-8/S100A12) mRNA expression in lesional skin. Significant progressive reductions until day 71 in the drug group were seen in OX40⁺ T cells and OX40L⁺ dendritic cells (P < .001). Hyperplasia measures (thickness/keratin 16/Ki67) showed greater reductions with GBR 830 (P < .001).

CONCLUSIONS

Two doses of GBR 830 administered 4 weeks apart were well tolerated and induced significant progressive tissue and clinical changes until day 71 (42 days after the last dose), highlighting the potential of OX40 targeting in patients with AD.

TNF superfamily: costimulation and clinical applications

The molecules concerned with costimulation belong either to the immunoglobulin (Ig) or tumor necrosis factor (TNF) superfamily. The tumor necrosis superfamily comprises molecules capable of providing both costimulation and cell death. In this review we briefly summarize certain TNF superfamily receptor-ligand pairs that are endowed with costimulatory properties and their importance in health and disease.

OX40 (CD134) and OX40L

The interaction between OX40 and OX40L plays an important role in antigen-specific T-cell expansion and survival. While OX40 is expressed predominantly on T-lymphocytes early after antigen activation, OX40L is expressed on activated antigen presenting cells and endothelial cells within acute inflammatory environments. We discuss here how ligation of OX40 by OX40L leads to enhanced T-cell survival, along with local inflammatory responses that appear critical for both effective T-cell mediated responses and chronic immune pathologies. We describe how interventions that block or mimic the OX40-OX40L interaction can be applied to treat autoimmune diseases or enhance anti-tumor immune responses. The clinically relevant properties of these agents emphasize the importance of this particular TNFSF-TNFSF in health and disease.

Effects of CD134 monoclonal antibody on hemolysis activities and expression of perforin in peripheral blood mononuclear cells of systemic lupus erythematosus patients

Perforin had been demonstrated to play important roles in the pathology of systemic lupus erythematosus (SLE), which was a potential target of clinical treatment of SLE. CD134 was a member of the tumor necrosis factor receptor family, which had been demonstrated to inhibit expression of perforin mRNA. The aim of the present study was to determine the effects of CD134 monoclonal antibody (MAb) on expression and hemolysis activities of perforin and its mechanisms. Effects of CD134 MAb on hemolysis activities of perforin were measured by rabbit red blood cells. Effects of CD134 MAb on expression of perforin in peripheral blood mononuclear cells (PBMCs) were determined by reverse transcription-polymerase chain reaction (RT-PCR) method and flow cytometry. Then, expression of NF-kappaB P65 was detected by Western blot. The results showed that CD134 MAb could inhibit hemolysis activities and expression of perforin through decreasing expression of NF-kappaB P65. The inhibition effects were positively correlated to the SLE disease activity index (SLEDAI) and 24-hour protein-uria of SLE patients in active state. CD134 MAb is suggested to be a potential treatment for SLE patients.

Effect of antibody to CD134 on perforin-mediated cytolysis in human peripheral blood mononuclear cells

CD134 (OX40) is a new member of the tumor necrosis factor receptor (TNFR) superfamily. The influence of CD134 on perforin-mediated cytolysis remains obscure. In this study, human peripheral blood mononuclear cells (PBMC) were induced with phytohemagglutinin (50 microg/mL), by using anti-CD134 blocking monoclonal antibody (MAb; 1, 5, and 10 microg/mL) for 6, 12, 24, and 48 h. Perforin-mediated cytolysis against human erythrocytes of PBMC was detected. The expression of perforin mRNA and protein was also measured by reverse transcription-polymerase chain reaction and flow cytometry. Our data showed that the perforin-mediated cytolysis of PBMC was downregulated by various concentrations of anti-CD134 blocking MAb for different times and reached a minimum at 24 h at any concentration. Anti-CD134 blocking MAb induced downregulation of perforin-mediated cytolysis of PBMC in a dose-dependent manner in the range of 1-5 microg/mL. Perforin- mediated cytolysis of PBMC reached a plateau when the concentration of anti-CD134 blocking MAb exceeded 5 microg/mL. Anti-CD134 blocking MAb also induced the inhibition of the expression of perforin protein and mRNA in PBMC. In conclusion, anti-CD134 blocking MAb inhibits perforin-mediated cytolysis of PBMC by, at least in some part, decreasing the production of preforin, which may be useful in treating several 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.

TNF-alpha is crucial for the development of autoimmune arthritis in IL-1 receptor antagonist-deficient mice

IL-1 receptor antagonist-deficient (IL-1Ra(-/-)) mice spontaneously develop autoimmune arthritis. We demonstrate here that T cells are required for the induction of arthritis; T cell-deficient IL-1Ra(-/-) mice did not develop arthritis, and transfer of IL-1Ra(-/-) T cells induced arthritis in nu/nu mice. Development of arthritis was also markedly suppressed by TNF-alpha deficiency. We found that TNF-alpha induced OX40 expression on T cells and blocking the interaction between either CD40 and its ligand or OX40 and its ligand suppressed development of arthritis. These findings suggest that IL-1 receptor antagonist deficiency in T cells disrupts homeostasis of the immune system and that TNF-alpha plays an important role in activating T cells through induction of OX40.

CD70 Signaling Is Critical for CD28-Independent CD8+T Cell-Mediated Alloimmune Responses In Vivo

The inability to reproducibly induce robust and durable transplant tolerance using CD28-B7 pathway blockade is in part related to the persistence of alloreactive effector/memory CD8(+) T cells that are less dependent on this pathway for their cellular activation. We studied the role of the novel T cell costimulatory pathway, CD27-CD70, in alloimmunity in the presence and absence of CD28-B7 signaling. CD70 blockade prolonged survival of fully mismatched vascularized cardiac allografts in wild-type murine recipients, and in CD28-deficient mice induced long-term survival while significantly preventing the development of chronic allograft vasculopathy. CD70 blockade had little effect on CD4(+) T cell function but prevented CD8(+) T cell-mediated rejection, inhibited the proliferation and activation of effector CD8(+) T cells, and diminished the expansion of effector and memory CD8(+) T cells in vivo. Thus, the CD27-CD70 pathway is critical for CD28-independent effector/memory CD8(+) alloreactive T cell activation in vivo. These novel findings have important implications for the development of transplantation tolerance-inducing strategies in primates and humans, in which CD8(+) T cell depletion is currently mandatory.

Undefined-antigen vaccines

Our knowledge of the immune system and how it interacts with tumor cells continues to grow. With each advance in basic science comes a new opportunity to develop an effective treatment strategy. Many such opportunities have arisen in the past few decades and this chapter has attempted to describe how these new advances have been combined with a variety of undefined cellular antigen preparations in an attempt to develop effective cancer vaccines. None of the strategies described in this chapter have been sufficiently effective to become part of standard therapy. However, the approaches tested have generally been well-tolerated by patients with advanced cancer and the evidence of immunologic activity and examples of impressive clinical activity in a wide variety of malignancies, suggests that these strategies can be the building blocks upon which new advances are added and effective treatments developed.

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.

CD134 plays a crucial role in the pathogenesis of EAE and is upregulated in the CNS of patients with multiple sclerosis

We investigated the role of the CD134 (also named OX40) molecule in experimental allergic encephalomyelitis (EAE) and multiple sclerosis (MS). We examined the susceptibility of Cd134(-/-) mice to EAE, an autoimmune murine model that is dependent on infiltrating CD4+ T lymphocytes reactive to myelin proteins. EAE induced by myelin oligodendrocyte glycoprotein (MOG) injection in Cd134(-/-) mice showed less severe clinical signs of disease and markedly reduced inflammatory infiltrates within the central nervous system (CNS). Resistance was associated with a strong reduction of pathogenic IFNgamma-producing T cells infiltrating the CNS of Cd134(-/-) mice. Furthermore, analysis of CNS tissue sections from EAE animals and MS patients revealed the presence of CD134+ cells that were localized in active lesions, mainly in perivascular infiltrates. The presence of CD134-expressing T cells in brain tissue of MS patients and EAE affected mice, together with the functional evidence provided by the significant decrease in disease score obtained in Cd134(-/-) mice, indicate that interfering with the CD134 molecule in T cells may be an appropriate target for therapeutic intervention in active MS.

Ligation of OX40 (CD134) regulates graft-versus-host disease (GVHD) and graft rejection in allogeneic bone marrow transplant recipients

OX40 (CD134) is expressed on activated T cells; its ligand, OX40 ligand (OX40L) is expressed on dendritic cells, B cells, and activated endothelial cells. To determine how OX40-OX40L interaction affects graft-versus-host disease (GVHD), we used antagonistic anti-OX40L monoclonal antibody (mAb) or OX40(-/-) donor or OX40L(-/-) recipient mice. Similar degrees of GVHD reduction were observed with each approach. Despite the fact that OX40 is up-regulated on both CD4(+) and CD8(+) T cells isolated during GVHD, the major effects of OX40 ligation were on CD4(+) and not CD8(+) T-cell-mediated alloresponses as assessed in both GVHD and engraftment model systems. GVHD inhibition by blockade of the OX40/OX40L pathway did not require CD28 signaling. Some studies have indicated OX40 is essential for inducing T-helper type 2 (Th2) responses. However, in vivo blockade of OX40-OX40L interactions reduced GVHD mortality induced by either signal transducer and activator of transcription-6(-/-) (Stat-6(-/-)) (Th2-defective) or Stat-4(-/-) (Th1-defective) major histocompatibility complex (MHC)-disparate splenocytes, indicating that the GVHD-ameliorating effects did not require Stat-4 or Stat-6 signaling. Although OX40L has been reported to be expressed on activated T cells, no effects on GVHD were observed when OX40L(-/-) versus OX40L(+/+) T cells were infused in different models. These data provide insights as to the mechanisms responsible for OX40/OX40L regulation of GVHD.

The role of OX40 ligand interactions in the development of the Th2 response to the gastrointestinal nematode parasite Heligmosomoides polygyrus

In these studies, we examined the effects of OX40 ligand (OX40L) deficiency on the development of Th2 cells during the Th2 immune response to the intestinal nematode parasite Heligmosomoides polygyrus. Elevations in IL-4 production and total and Ag-specific serum IgE levels were partially inhibited during both the primary and memory immune responses to H. polygyrus in OX40L(-/-) mice. The host-protective memory response was compromised in OX40L(-/-) mice, as decreased worm expulsion and increased egg production were observed compared with H. polygyrus-inoculated OX40L(+/+) mice. To further examine the nature of the IL-4 defect during priming, adoptively transferred DO11.10 T cells were analyzed in the context of the H. polygyrus response. Although Ag-specific T cell IL-4 production was reduced in the OX40L(-/-) mice following immunization with OVA peptide plus H. polygyrus, Ag-specific T cell expansion, cell cycle progression, CXCR5 expression, and migration were comparable between OX40L(+/+) and OX40L(-/-) mice inoculated with OVA and H. polygyrus. These studies suggest an important role for OX40/OX40L interactions in specifically promoting IL-4 production, as well as associated IgE elevations, in Th2 responses to H. polygyrus. However, OX40L interactions were not required for serum IgG1 elevations, increases in germinal center formation, and Ag-specific Th2 cell expansion and migration to the B cell zone.

Survival in human colorectal cancer correlates with expression of the T-cell costimulatory molecule OX-40 (CD134)

BACKGROUND

The T-cell costimulatory molecule OX-40 (CD134) is expressed on activated CD4(+) ("helper") T cells. Such cells have been detected in human cancers, and engagement of OX-40 improves colon cancer immunity in an animal model.

METHODS

Sections of primary colon cancers, normal margins, mesenteric lymph nodes, and metastases were stained for OX-40 by immunohistochemistry. Cancer registry data were reviewed.

RESULTS

High levels of OX-40 positive tumor-infiltrating lymphocytes were found in 15 of 72 primary tumors. Thirty-one cases had prominent lymphocytic infiltrates expressing OX-40 at the invasive margin of the tumor. Overall, 50% of primary tumors showed high expression of OX-40. Nearly all mesenteric lymph nodes expressed OX-40, whether tumor was present or not. Normal margins of colon did not show high levels of OX-40. High OX-40 expression in the primary tumor correlated with better survival (mean survival high OX-40, 47 months, low OX-40, 35 months, P <0.05), although this correlation was not stage-independent.

CONCLUSIONS

High levels of OX-40 positive lymphocytes are present in half of primary colon cancers, and this expression in primary tumors significantly correlates with better survival. This correlation with survival and our previous preclinical research suggest a basis for an OX-40 immunotherapy trial.

Expression of OX40 in muscles of polymyositis and granulomatous myopathy

OX40 is selectively expressed on activated autoreactive memory T cells and these OX40+ lymphocytes may play a crucial role in autoimmune diseases. To determine whether OX40+ lymphocytes are involved in the pathomechanism of human inflammatory muscle diseases, we immunohistochemically examined the distribution of OX40+ cells in muscles from patients with polymyositis and granulomatous myopathy, and compared with that of cells bearing other activation markers, such as IL-2 receptor (IL-2R) and HLA-DR. In polymyositis, OX40+ mononuclear cells were found predominantly in the perivascular sites and to a lesser degree in the endomysium. Scanty IL-2R+ mononuclear cells were located only in the endomysium and HLA-DR was expressed on half of the mononuclear cells distributed diffusely in the perivascular sites and in the endomysium. Mononuclear cell infiltration in the perivascular sites was greater in the muscles in which OX40+ cells were present in the perivascular sites than in those without OX40+ cells in the perivascular sites (p<0.05). In granulomatous myopathy, OX40+ cells were detected in the centers of the granulomas. In contrast, IL-2R+ cells were present at the periphery of the granulomas and HLA-DR was detected on mononuclear cells throughout the granulomas. OX40+ mononuclear cells with specific distributions in muscles may be involved in the pathomechanism of polymyositis and granulomatous myopathy, and can be a candidate molecule of selective immunotherapy in these diseases.

Engagement of OX40 enhances antigen-specific CD4(+) T cell mobilization/memory development and humoral immunity: comparison of alphaOX-40 with alphaCTLA-4

Increasing the long-term survival of memory T cells after immunization is key to a successful vaccine. In the past, the generation of large numbers of memory T cells in vivo has been difficult because Ag-stimulated T cells are susceptible to activation-induced cell death. Previously, we reported that OX40 engagement resulted in a 60-fold increase in the number of Ag-specific CD4(+) memory T cells that persisted 60 days postimmunization. In this report, we used the D011.10 adoptive transfer model to examine the kinetics of Ag-specific T cell entry into the peripheral blood, the optimal route of administration of Ag and alphaOX40, and the Ag-specific Ab response after immunization with soluble OVA and alphaOX40. Finally, we compared the adjuvant properties of alphaOX40 to those of alphaCTLA-4. Engagement of OX-40 in vivo was most effective when the Ag was administered s.c. Time course studies revealed that it was crucial for alphaOX40 to be delivered within 24-48 h after Ag exposure. Examination of anti-OVA Ab titers revealed a 10-fold increase in mice that received alphaOX40 compared with mice that received OVA alone. Both alphaOX40 and alphaCTLA-4 increased the percentage of OVA-specific CD4(+) T cells early after immunization (day 4), but alphaOX40-treated mice had much higher percentages of OVA-specific memory CD4(+) T cells from days 11 to 29. These studies demonstrate that OX40 engagement early after immunization with soluble Ag enhances long-term T cell and humoral immunity in a manner distinct from that provided by blocking CTLA-4.

Treatment of adjuvant-induced arthritis by oral administration of mycobacterial Hsp65 during disease

OBJECTIVE

Oral administration of antigen prior to disease induction has been shown to induce peripheral tolerance in several experimental autoimmune diseases. However, the clinical benefit of pretreatment with antigens is limited. The aim of this study was to investigate whether adjuvant-induced arthritis (AIA) could be treated by oral administration of mycobacterial heat-shock protein 65 (Hsp65) during ongoing disease.

METHODS

AIA was induced in Lewis rats by immunization with Mycobacterium tuberculosis in Freund's incomplete adjuvant. Oral feeding of Hsp65 in the presence or absence of soybean trypsin inhibitor (SBTI) was started on day 11 after immunization. Arthritis was monitored visually, and joint pathology was examined radiologically.

RESULTS

Oral treatment with Hsp65 during ongoing disease significantly reduced the activity of AIA. However, treatment with Hsp65 was only successful when SBTI was coadministered to prevent breakdown of the Hsp65. The beneficial effect of Hsp65/SBTI treatment during AIA was also represented by a clear reduction of articular destruction, as visualized by radiography. Moreover, feeding Hsp65/SBTI resulted in a lower number of both spleen and mesenteric lymph node (MLN) cells expressing the costimulatory molecule CD80 (B7-1). The number of cells expressing CD86 (B7-2) was not altered. Furthermore, MLN cells from AIA animals treated with Hsp65/SBTI contained a lower number of T cells expressing the activation marker CD134 (Ox-40). In addition, treatment with Hsp65/ SBTI was accompanied by an increased proliferative response of spleen cells to the Hsp65 antigen in vitro. Moreover, Hsp65/SBTI-treated rats showed less Hsp65-specific interferon-gamma and increased production of interleukin-10.

CONCLUSION

Ongoing AIA activity can be reduced by oral administration of Hsp65 only when protein breakdown in the gastrointestinal tract is inhibited.

The initiation of autoimmune diabetes

It is speculated that presentation of self-peptides to autoreactive T cells normally results in T cell tolerance. In autoimmune conditions, breakdown in the tolerization process results in activation of self-reactive T cells and an immune attack on host tissues. Our understanding of the immune cells and signaling pathways that contribute to this breakdown in T cell tolerization mechanisms is beginning to be deciphered. In particular, the elucidation of the mechanisms that contribute to the release of host antigen, the identification of the antigen-presenting cells that present the host peptides to self-reactive T cells and the role of members of the tumor necrosis factor receptor/ligand families that contribute to inappropriate activation of self-reactive T cells is advancing. The accumulating data from these studies will hopefully provide new ideas for combating autoimmunity.

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.

Blocking OX-40/OX-40 Ligand Interaction In Vitro and In Vivo Leads to Decreased T Cell Function and Amelioration of Experimental Allergic Encephalomyelitis

The OX-40R is a member of the TNF receptor family and is expressed primarily on activated CD4+ T cells. When the OX-40R is engaged by the OX-40 ligand (OX-40L), a potent costimulatory signal occurs. We have identified a population of CD11b+ cells, isolated from the central nervous system (CNS) of mice with actively induced experimental allergic encephalomyelitis (EAE), that expresses OX-40L. Moreover, the expression of OX-40L was found to be associated with paralytic episodes of EAE and was reduced or absent at disease recovery. These CD11b+ cells also coexpressed B7 and MHC class II. Therefore, to address the relative contributions of OX-40R/OX-40L and CD28/B7 to the costimulation of myelin-specific T cells, blocking studies were performed using soluble OX-40R and/or soluble CTLA-4. CD11b+ cells isolated from the CNS of mice with actively induced EAE were able to present Ag to proteolipid protein 139–151-specific T cell lines in vitro. The addition of soluble OX-40R:Ig to CD11b+ brain microglia/macrophages inhibited T cell proliferation by 50–70%. The addition of CTLA-4:Ig inhibited T cell proliferation by 20–30%, and the combination inhibited T cell proliferation by 95%. In vivo administration of soluble OX-40R at the onset of actively induced or adoptively transferred EAE reduced ongoing signs of disease, and the mice recovered more quickly from acute disease. The data imply that OX-40L, expressed by CNS-derived APC, acts to provide an important costimulatory signal to EAE effector T cells found within the inflammatory lesions. Furthermore, the data suggest that agents designed to inhibit the OX-40L/OX-40R complex may be useful for treating autoimmune disease.