PD-L2 is a second ligand for PD-1 and inhibits T cell activation

PD-L1/B7H-1 inhibits the effector phase of tumor rejection by T cell receptor (TCR) transgenic CD8+ T cells

Although increased circulating tumor antigen-specific CD8(+) T cells can be achieved by vaccination or adoptive transfer, tumor progression nonetheless often occurs through resistance to effector function. To develop a model for identifying mechanisms of resistance to antigen-specific CTLs, poorly immunogenic B16-F10 melanoma was transduced to express the K(b)-binding peptide SIYRYYGL as a green fluorescent protein fusion protein that should be recognized by high-affinity 2C TCR transgenic T cells. Although B16.SIY cells expressed high levels of antigen and were induced to express K(b) in response to IFN-gamma, they were poorly recognized by primed 2C/RAG2(-/-) T cells. A screen for candidate inhibitory ligands revealed elevated PD-L1/B7H-1 on IFN-gamma-treated B16-F10 cells and also on eight additional mouse tumors and seven human melanoma cell lines. Primed 2C/RAG2(-/-)/PD-1(-/-) T cells showed augmented cytokine production, proliferation, and cytolytic activity against tumor cells compared with wild-type 2C cells. This effect was reproduced with anti-PD-L1 antibody present during the effector phase but not during the priming culture. Adoptive transfer of 2C/RAG2(-/-)/PD-1(-/-) T cells in vivo caused tumor rejection under conditions in which wild-type 2C cells or CTLA-4-deficient 2C cells did not reject. Our results support interfering with PD-L1/PD-1 interactions to augment the effector function of tumor antigen-specific CD8(+) T cells in the tumor microenvironment.

Association of a programmed death 1 gene polymorphism with the development of rheumatoid arthritis, but not systemic lupus erythematosus

OBJECTIVE

The expression of autoimmunity in mice deficient in programmed death 1 (PD-1) suggests that PD-1 is a candidate gene involved in the development of human autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). We therefore tested the potential association between PD-1 and the development of SLE and RA by conducting case-control genetic-association studies.

METHODS

Ninety-eight SLE patients, 84 RA patients, and sex-matched control subjects for each disease group were recruited and genotyped for a single-nucleotide polymorphism, C+872T, in the human PD-1 gene. The significance of the association of the PD-1 gene with SLE or with RA was analyzed by statistical tests for the difference in genotype distribution between disease and control groups.

RESULTS

The human PD-1 gene was found to be significantly associated with disease development in RA patients, but not SLE patients. The risk of RA development appeared to be significantly increased by carriage of the T allele (odds ratio 3.32, P < 0.0001) or the C/T genotype (odds ratio 3.52, P < 0.00005).

CONCLUSION

The PD-1 gene is significantly associated with RA susceptibility, suggesting the possibility that PD-1 may contribute to the pathogenesis of RA.

Microparticulate β-glucan upregulates the expression of B7.1, B7.2, B7-H1, but not B7-DC on cultured murine peritoneal macrophages

Beta-1,3-(D)-glucan from a variety of biological sources has been shown to enhance both humoral and cellular immune responses to a variety of antigens, infectious agents, and tumors. Nevertheless, its mode of action has not been fully defined. We sought to determine whether a 1-2 microm diameter microparticulate form of beta-glucan (MG) from the yeast Saccharomyces cerevisiae could regulate expression of B7 family glycoproteins on resident peritoneal macrophages from BALB/c mice. We discovered that MG uregulated B7.2 mRNA expression and enhanced the surface membrane expression of B7.2 glycoprotein. Although B7.1 mRNA was not upregulated above constitutive levels, MG treatment enhanced B7.1 glycoprotein expression on the macrophages, albeit to a lesser extent than B7.2. At the same time, the gene and cell surface expression of B7-H1, a putative negative regulator of T cell activity, was also upregulated by MG. The expression of B7-DC, another B7 family molecule with negative regulatory activity, was not affected by incubation with MG. This study has demonstrated that a microparticulate form of beta-glucan can enhance B7 co-stimulatory molecule expression on macrophages, thereby enabling these antigen-presenting cells to deliver the second signal to T-lymphocytes that express CD28. In addition, because MG also induces the expression of B7-H1, it may enable macrophages to provide a concomitant downregulatory signal to T-lymphocytes expressing PD-1 or related receptors.

CTLA-4+CD8+ T Cells That Encounter B7-2+ Iris Pigment Epithelial Cells Express Their Own B7-2 to Achieve Global Suppression of T Cell Activation

Pigment epithelial (PE) cells cultured from the eye possess the novel property of suppressing TCR-dependent activation of T cells in vitro. Iris PE (IPE) cells accomplish this suppression by a direct cell contact mechanism in which B7-2 expressed by the PE cells interacts with CTLA-4 on responding T cells. Because CTLA-4 expression is constitutively expressed on a very small proportion of naive splenic T cells and since exposure of splenic T cells to IPE leads to global T cell suppression, we have inquired into the mechanism by which suppression is achieved. Using splenic T cells and IPE from donor mice with disrupted genes for CD80 (B7-1), CD86 (B7-2), CTLA-4, and/or CD28, we report that B7-2(+) IPE in the presence of anti-CD3 supported selectively the activation of CTLA-4(+) CD8(+) T cells that express their own B7-2 and secrete enhanced amounts of active TGFbeta. By contrast, activation of CTLA-4-negative T cells, especially CD4(+) cells, in these cultures was profoundly suppressed. Because global suppression of T cell activation in these cultures was obtained only when both IPE and T cells possessed B7-2 genes and expressed the costimulators as surface molecules, we propose that T cells activated in the presence of parenchymal cells from the eye (an immune privileged site) express B7-2 in a manner that equips them to suppress bystander T cells. Thus, B7-2 expression on T cells participates in their eventual ability to function as regulators in vitro.

Costimulation-based immunotherapy for head and neck cancer

Squamous cell carcinoma of the head and neck (SCCHN) is an important source of cancer-related morbidity and mortality worldwide. Current treatment options are founded on various combinations of surgery, radiation therapy, and chemotherapy, with implementation largely based on patient/disease-specific factors and provider/institutional bias. Despite improvements in surgical techniques and the development of novel chemoradiotherapy strategies for organ preservation, survival has remained relatively unchanged in the past 30 years. Additionally, the impact of these novel treatment approaches on the quality of life of patients remains largely unknown. Improvements in survival and the quality of life for patients with SCCHN will likely require the development of new therapeutic alternatives based on the improved understanding of the molecular pathogenesis of SCCHN and associated changes in the immune response.

Immunology of B7-H1 and its roles in human diseases

B7-H1 was originally identified by homology analysis in comparison with B7-1 and B7-2, two molecules with important immunoregulatory functions. B7-H1, however, was broadly induced in the majority of peripheral tissues as well as hematopoietic cells. Upon binding to an as yet unidentified costimulatory receptor on primed T-cells, B7-H1 costimulates T-cell proliferation and preferentially induces interleukin 10 and interferon gamma. The costimulatory function of B7-H1 may be critical for enhancing maturation and differentiation of T-cells in lymphoid organs. Conversely, by binding to programmed death 1 receptors on activated T-cells and B-cells, B7-H1 may inhibit ongoing T-cell responses in peripheral tissues by inducing apoptosis and arresting cell-cycle progression. Although a positive regulatory role of B7-H1 has been demonstrated in vitro and in various animal models, a negative regulatory role of B7-H1 has also been documented in human diseases, including cancer, rheumatoid arthritis, and human immunodeficiency virus infection. Delineation of the complex interactions between B7-H1 and its receptors as well as its interplay with other ligands is critical for understanding this new immunoregulatory system. Precise manipulation of B7-H1 and its receptors may provide unique opportunities for designing new disease treatments.

Programmed death-1-programmed death-L1 interaction is essential for induction of regulatory cells by intratracheal delivery of alloantigen

BACKGROUND

Programmed death (PD)-1 has been implicated in peripheral tolerance. The authors investigated the roles of PD-1 and its ligands, PD-L1 and PD-L2, in the induction of regulatory cells by intratracheal delivery of alloantigen.

METHODS

CBA (H-2k) mice were pretreated with intratracheal delivery of C57BL/10 (H-2b) splenocytes and administration of monoclonal antibody (mAb) specific for PD-1, PD-L1, or PD-L2. Seven days later, C57BL/10 hearts were transplanted into the pretreated CBA mice. Some naive CBA mice underwent adoptive transfer of splenocytes from the pretreated CBA mice and transplantation of C57BL/10 heart.

RESULTS

Untreated CBA mice rejected C57BL/10 cardiac grafts acutely (median survival time [MST], 7 days). Pretreatment with intratracheal delivery of C57BL/10 splenocytes prolonged graft survival significantly (MST, 65 days). Administration of control immunoglobulin (Ig) G or anti-PD-L2 mAb did not significantly affect the prolongation (MST, 72 and 68 days, respectively). In contrast, anti-PD-1 or anti-PD-L1 mAb abrogated the prolongation (MST, 18 and 17 days, respectively). Adoptive transfer from mice pretreated with intratracheal delivery of alloantigen plus control IgG or anti-PD-L2 mAb prolonged survival of C57BL/10 grafts in secondary CBA recipients (MST, 72 and 56 days, respectively). However, concurrent administration of anti-PD-1 or anti-PD-L1 mAb abrogated prolonged survival after the adoptive transfer (MST, 14 and 20 days, respectively).

CONCLUSIONS

PD-1-PD-L1 interaction was essential for induction of regulatory cells by intratracheal delivery of alloantigen.

Molecular Characterization of Human 4Ig-B7-H3, a Member of the B7 Family with Four Ig-Like Domains

In an effort to characterize molecules with immunoregulatory potential, we raised mAbs to human dendritic cells. We selected an Ab that recognizes a molecule that is induced on monocytes differentiated in vitro toward dendritic cells. Retroviral expression cloning identified this molecule as B7-H3, a member of the B7 family described recently. In contrast to an earlier report, in which B7-H3 was described as a molecule consisting of two Ig-like domains, our cDNA encoded a type I membrane protein with four Ig-like domains, and the molecule identified by us was therefore named 4Ig-B7-H3. mRNA analysis as well as Western blotting experiments performed by us did not reveal evidence for a small B7-H3. B7-H3 is not expressed on peripheral blood lymphocytes, monocytes, or granulocytes. Upon in vitro stimulation, the expression of B7-H3 is induced on T cells, B cells, and NK cells. A number of different approaches were used to investigate the function of human B7-H3. In contrast to an earlier report, our data do not support a costimulatory role of B7-H3 in anti-CD3-mediated activation of the TCR-complex resulting in T cell proliferation and IFN-gamma production.

Absence of programmed death receptor 1 alters thymic development and enhances generation of CD4/CD8 double-negative TCR-transgenic T cells

Programmed death receptor 1 (PD-1) is expressed on thymocytes in addition to activated lymphocyte cells. Its ligation is thought to negatively regulate T cell activation, and PD-1(-/-) mice develop autoimmunity. To study the role of PD-1 on the development and function of a monoclonal CD8(+) T cell population, 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice were generated. Unexpectedly, approximately 30% of peripheral T cells in these mice were CD4/CD8 double negative (DN). Although the DN cells were not activated by Ag-expressing APCs, they functioned normally in response to anti-CD3/anti-CD28. These cells had a naive surface phenotype and lacked expression of NK1.1, B220, and gammadelta TCR; and the majority did not up-regulate CD8alphaalpha expression upon activation, arguing that they are not predominantly diverted gammadelta-lineage cells. The thymus was studied in detail to infer the mechanism of generation of DN peripheral T cells. Total thymus cellularity was reduced in 2C TCR-transgenic/recombination-activating gene 2(-/-)/PD-1(-/-) mice, and a relative increase in DN cells and decrease in double-positive (DP) cells were observed. Increased annexin V(+) cells among the DP population argued for augmented negative selection in PD-1(-/-) mice. In addition, an increased fraction of the DN thymocytes was HSA negative, suggesting that they had undergone positive selection. This possibility was supported by decreased emergence of DN PD-1(-/-) 2C cells in H-2(k) bone marrow chimera recipients. Our results are consistent with a model in which absence of PD-1 leads to greater negative selection of strongly interacting DP cells as well as increased emergence of DN alphabeta peripheral T cells.

The role of ICOS and other costimulatory molecules in allergy and asthma

Activation and differentiation of T cells play a critical role in the pathogenesis of allergies and asthma. Upon encounter with specific antigen, naïve T helper precursor (ThP) cells become activated, an event that is regulated not only by engagement of the T cell receptor (TCR) with peptide presented in the context of MHC class II molecules, but also by a number of costimulatory signals. CD28 engagement by B7-1 and B7-2 on resting ThP cells provides a critical signal for initial cell cycle progression, interleukin-2 production and clonal expansion. However, in recent years, other related members of the immunoglobulin (Ig) family, such as inducible costimulatory molecules (ICOS) and the TNF receptor family members which include OX40, have also been demonstrated to play an important role in providing unique and complementary signals that regulate the outcome of immune responses. These positive costimulatory signals are counterbalanced by signals that dampen down immune responses and include CTLA-4, PD-1 and the recently described Ig superfamily members BTLA and TIM-3. This review discusses the role of these costimulatory signals and their potential involvement in the pathogenesis of asthma and allergic responses.

Blockade of B7-H1 suppresses the development of chronic intestinal inflammation

A newly identified costimulatory molecule, programmed death-1 (PD-1), provides a negative signal that is essential for immune homeostasis. However, it has been suggested that its ligands, B7-H1 (PD-L1) and B7-dendritic cells (B7-DC; PD-L2), could also costimulate T cell proliferation and cytokine secretion. Here we demonstrate the involvement of PD-1/B7-H1 and B7-DC interaction in the development of colitis. We first examined the expression profiles of PD-1 and its ligands in both human inflammatory bowel disease and a murine chronic colitis model induced by adoptive transfer of CD4(+)CD45RB(high) T cells to SCID mice. Second, we assessed the therapeutic potential of neutralizing anti-B7-H1 and/or B7-DC mAbs using this colitis model. We found significantly increased expression of PD-1 on T cells and of B7-H1 on T, B, and macrophage/DCs in inflamed colon from both inflammatory bowel disease patients and colitic mice. Unexpectedly, the administration of anti-B7-H1, but not anti-B7-DC, mAb after transfer of CD4(+)CD45RB(high) T cells suppressed wasting disease with colitis, abrogated leukocyte infiltration, and reduced the production of IFN-gamma, IL-2, and TNF-alpha, but not IL-4 or IL-10, by lamina propria CD4(+) T cells. These data suggest that the interaction of PD-1/B7-H1, but not PD-1/B7-DC, might be involved in intestinal mucosal inflammation and also show a possible role of interaction between B7-H1 and an as yet unidentified receptor for B7-H1 in inducing T cell activation.

Investigation on the effects of soluble programmed death-1 (sPD-1) enhancing anti-tumor immune response

By using semi-quantitative RT-PCR method, it was found that PD-L1 mRNA but not PD-L2 mRNA was expressed in H22 hepatoma cells and both PD-L1 and PD-L2 mRNAs were expressed in tumor tissues of tumor-bearing mice and upregulated as compared with muscle tissues in normal mice and H22 hepatoma cells. PD-L1 and PD-L2 were also expressed on the surface of the activated T cells. The soluble recombinant sPD-1 expressed from the constructed eukaryotic expression vector could enhance the lysis of tumor cells by lymphocytes stimulated specifically with antigen. The expresssion of sPD-1 by local gene therapy on the inoculation site of H22 hepatoma cells could inhibit the growth of tumor. The results of this study indicate that expression of soluble receptor of negative costimulatory molecules could reduce the inhibitory effect on T cells in tumor microenvironment and enhance the cytotoxicity of T cells on tumor cells. This possibly provides a new method of improving efficacy of tumor gene therapy.

Role of costimulatory pathways in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis is an immune-mediated disorder of the central nervous system. T lymphocytes are thought to play a central role in the initiation and potentially in the propagation of this disease. Two signals are required for T-cell activation. The first signal consists of the interaction of the T-cell receptor with antigen presented by the MHC molecule on antigen-presenting cells. The second signal requires engagement of costimulatory receptors on T cells with their ligands on antigen-presenting cells. Several costimulatory pathways have been shown to play an important role in T-lymphocyte activation. Here we will review the current literature on the contribution of the B7-1/2-CD28/CTLA-4, inducible costimulatory molecule-B7h, programmed death pathway 1-programmed death pathway ligand 1/ligand 2, CD40-CD154, OX40-OX40 ligand, and CD137-CD137 ligand pathways to the pathogenesis of multiple sclerosis and their potential roles as therapeutic targets.

Blocking the monocyte chemoattractant protein-1/CCR2 chemokine pathway induces permanent survival of islet allografts through a programmed death-1 ligand-1-dependent mechanism

Islet allografts are subject to rapid rejection through host cellular immune responses involving mononuclear cell recruitment and tissue injury. Interruption of leukocyte recruitment through chemokine receptor targeting is of therapeutic benefit in various experimental models, but little is known about the contribution of chemokine pathways to islet allograft rejection. We found that murine islets produce monocyte chemoattractant protein-1 (MCP-1; CCL2) in vitro and that islet allograft rejection was associated with intragraft expression of MCP-1 and its receptor, CCR2. We therefore investigated whether MCP-1 and CCR2 are required for the rejection of fully MHC-disparate islet allografts. Wild-type mice treated with blocking anti-MCP-1 mAb plus a brief, subtherapeutic course of rapamycin had long-term islet allograft survival, in contrast to the effect of treatment with either mAb or rapamycin alone. CCR2(-/-) mice treated with rapamycin also maintained islet allografts long-term. Both MCP/CCR2- and rapamycin-sensitive signals were required for maximal proliferation of alloreactive T cells, suggesting that MCP-1/CCR2 induce rejection by promoting alloreactive T cell clonal expansion and homing and migration. Prolonged islet allograft survival achieved by blockade of the MCP-1/CCR2 pathway plus rapamycin therapy was accompanied by a mononuclear cell infiltrate expressing the inhibitory receptor, programmed death-1 (PD-1), and its ligand (PD-L1, B7-H1), and prolongation of islet allograft survival was abrogated by anti-PD-L1 mAb therapy. These data show that the blockade of MCP-1 binding to CCR2 in conjunction with subtherapeutic immunosuppression can have profound effects on islet allograft survival and implicate the expression of the PD-1/PD-L1 pathway in the regulation of physiologic responses in vivo.

Altering immune tolerance therapeutically: the power of negative thinking

The etiology of most human autoimmune diseases remains largely unknown. However, investigators have identified several negative regulatory mechanisms acting at the level of innate and/or adaptive immunity. Mutations resulting in a deficiency of some key regulatory molecules are associated with systemic or organ-specific inflammatory disorders, which often have a prominent autoimmune component. Genetic studies have implicated the negative regulator cytotoxic T-lymphocyte antigen 4 (CTLA-4) and other regulatory molecules in human autoimmune diseases. In addition to CTLA-4, key inhibitory molecules include programmed death 1 and B and T lymphocyte attenuator. Transforming growth factor beta1 and interleukin-10 also play major anti-inflammatory and regulatory roles. Tumor cells and infectious agents use negative regulatory pathways to escape immunity. The therapeutic blockage of negative signaling (particularly of CTLA-4) increases immunity against tumor antigens but also induces or aggravates autoimmune diseases. It appears that under normal conditions, the immune system is under strong "negative influences" that prevent autoimmunity and that release of this suppression results in disease. Regulation involves communication between the immune system and nonlymphoid tissues, and the latter can deliver inhibitory or stimulatory signals. Recent studies reveal that the generation of negative signals by selective engagement of inhibitory molecules is feasible and is likely to be of therapeutic benefit in autoimmune diseases and allograft rejection.

Current perspective on the pathogenesis of Graves' disease and ophthalmopathy

Graves' disease (GD) is a very common autoimmune disorder of the thyroid in which stimulatory antibodies bind to the thyrotropin receptor and activate glandular function, resulting in hyperthyroidism. In addition, some patients with GD develop localized manifestations including ophthalmopathy (GO) and dermopathy. Since the cloning of the receptor cDNA, significant progress has been made in understanding the structure-function relationship of the receptor, which has been discussed in a number of earlier reviews. In this paper, we have focused our discussion on studies related to the molecular mechanisms of the disease pathogenesis and the development of animal models for GD. It has become apparent that multiple factors contribute to the etiology of GD, including host genetic as well as environmental factors. Studies in experimental animals indicate that GD is a slowly progressing disease that involves activation and recruitment of thyrotropin receptor-specific T and B cells. This activation eventually results in the production of stimulatory antibodies that can cause hyperthyroidism. Similarly, significant new insights have been gained in our understanding of GO that occurs in a subset of patients with GD. As in GD, both environmental and genetic factors play important roles in the development of GO. Although a number of putative ocular autoantigens have been identified, their role in the pathogenesis of GO awaits confirmation. Extensive analyses of orbital tissues obtained from patients with GO have provided a clearer understanding of the roles of T and B cells, cytokines and chemokines, and various ocular tissues including ocular muscles and fibroblasts. Equally impressive is the progress made in understanding why connective tissues of the orbit and the skin in GO are singled out for activation and undergo extensive remodeling. Results to date indicate that fibroblasts can act as sentinel cells and initiate lymphocyte recruitment and tissue remodeling. Moreover, these fibroblasts can be readily activated by Ig in the sera of patients with GD, suggesting a central role for them in the pathogenesis. Collectively, recent studies have led to a better understanding of the pathogenesis of GD and GO and have opened up potential new avenues for developing novel treatments for GD and GO.

Regulation of immune and autoimmune responses by ICOS

Proper T cell activation and function are regulated by the innate immune system, importantly through positive and negative costimulatory molecules in the B7 superfamily. Inducible costimulator (ICOS), the receptor for B7h (also known as B7RP-1), is expressed on T cells after T cell activation. Recently, using ICOS-deficient mice, we have examined the roles of ICOS in immune responses. ICOS is required for humoral immunity. In organ-specific autoimmune responses, however, ICOS has contrast roles in different disease models. On the one hand, ICOS-/- mice exhibited extreme sensitivity to experimental autoimmune encephalomyelitis (EAE); on the other, ICOS gene deletion led to complete resistance to collagen-induced arthritis (CIA) in mice. Our work not only illustrates the complexity of immune regulation by costimulatory molecules, but also suggests novel therapeutic strategies for various autoimmune diseases.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.

Stimulating PD-1-negative signals concurrent with blocking CD154 co-stimulation induces long-term islet allograft survival

BACKGROUND

A balanced network of positive and negative T-cell co-stimulatory signals is important in regulating T-cell activation. Blocking CD28, CD154 (CD40L), or both co-stimulatory molecules has been efficacious in preventing acute allograft rejection in certain but not all transplantation models. In the present study, the authors tested the hypothesis that stimulating programmed death 1 (PD-1)-triggered negative signals concurrent with blocking CD154 co-stimulatory signals would facilitate islet allograft tolerance.

METHODS

The authors used a dimeric PD-L1 immunoglobulin (Ig) fusion protein to stimulate the inhibitory receptor PD-1, and a monoclonal antibody to block CD154. The effects of PD-1 engagement and CD154 blockade on lymphocyte activation were determined by cell proliferation, flow cytometry, and a model of islet transplantation.

RESULTS

PD-L1Ig inhibited the proliferation of both CD4+ and CD8+ T cells stimulated by anti-CD3. The inhibitory effect of PD-L1Ig was enhanced by concurrent blockade of CD154 co-stimulatory signals, as demonstrated by T-cell proliferation and expression of cell surface activation markers. PD-L1Ig and anti-CD154 also synergistically blocked the activation and maturation of antigen-presenting cells. In an islet transplantation model, treatment of recipient C57BL/6 (H-2b) mice with PD-L1Ig and anti-CD154 induced long-term survival of DBA/2 (H-2d) islet allografts, whereas treatment with each reagent alone failed to prevent islet allograft rejection.

CONCLUSIONS

These results suggest that engaging the negative receptor PD-1 exhibits critical immunoregulatory effects in the allograft response, and blocking positive co-stimulatory molecules with active delivery of inhibitory signals may represent a novel therapeutic strategy in transplantation.

Interaction of B7RP-1 with ICOS negatively regulates antigen presentation by B cells

Stimulation of T cells through the T cell receptor is insufficient for optimal T cell activation. A second activation signal is necessary, being usually provided by the costimulatory molecule CD28. Recently, additional costimulatory pathways have been identified, including inducible costimulator (ICOS) and its ligand B7RP-1. We have examined the role of the B7RP-1/ICOS costimulatory pathway on antigen presentation by B cells, using the I-Ak and I-Ek-positive CH27 B cell line and several different T cell lines. We found that CH27 expressed B7RP-1 and PD-L1 whereas the T cell lines expressed ICOS and PD-1. In the presence of HEL, the T cell hybridomas C10 and 3A9 released IL-2, which is indicative of antigen-specific T cell activation by the CH27 cells. Unexpectedly, blocking antibodies for B7RP-1 and ICOS enhanced the IL-2 response in both T cells. As expected, an increase in the production of IL-2 was seen when blocking antibodies for PD-1 were used. Blocking with antibodies for I-Ak, CD28, B7.1, and B7.2 lead to a decrease in IL-2 production. Additionally we tested a Th1 and a Th2 T cell clone. Blockade of B7RP-1/ICOS lead to an increased IFN-gamma response in Th1 cells (A.E7) and an increased IL-4 response in Th2 cells (D10.G4.1). Intracellular staining also showed an increase in cytokine production when the B7RP-1/ICOS pathway was blocked. In conclusion, the B7RP-1/ICOS pathway is negatively regulating T cell activation by B cells and may play a role similar to that of the PD-L1/PD-1 pathway.

Human muscle cells express a B7-related molecule, B7-H1, with strong negative immune regulatory potential: a novel mechanism of counterbalancing the immune attack in idiopathic inflammatory myopathies

B7-H1 is a novel B7 family protein attributed to costimulatory and immune regulatory functions. Here we report that human myoblasts cultured from control subjects and patients with inflammatory myopathies as well as TE671 muscle rhabdomyosarcoma cells express high levels of B7-H1 after stimulation with the inflammatory cytokine IFN-gamma. Coculture experiments of MHC class I/II-positive myoblasts with CD4 and CD8 T cells in the presence of antigen demonstrated the functional consequences of muscle-related B7-H1 expression: production of inflammatory cytokines, IFN-gamma and IL-2, by CD4 as well CD8 T cells was markedly enhanced in the presence of a neutralizing anti-B7-H1 antibody. This observation was paralleled by an augmented expression of the T cell activation markers CD25, ICOS, and CD69, thus showing B7-H1-mediated inhibition of T cell activation. Further, we investigated 23 muscle biopsy specimens from patients with polymyositis (PM), inclusion body myositis (IBM), dermatomyositis (DM), and nonmyopathic controls for B7-H1 expression by immunohistochemistry: B7-H1 was expressed in PM, IBM, and DM specimens but not in noninflammatory and nonmyopathic controls. Staining was predominantly localized to areas of strong inflammation and to muscle cells as well as mononuclear cells. These data highlight the immune regulatory properties of muscle cells and suggest that B7-H1 expression represents an inhibitory mechanism induced upon inflammatory stimuli and aimed at protecting muscle fibers from immune aggression.

Molecular diagnosis of primary mediastinal B cell lymphoma identifies a clinically favorable subgroup of diffuse large B cell lymphoma related to Hodgkin lymphoma

Using current diagnostic criteria, primary mediastinal B cell lymphoma (PMBL) cannot be distinguished from other types of diffuse large B cell lymphoma (DLBCL) reliably. We used gene expression profiling to develop a more precise molecular diagnosis of PMBL. PMBL patients were considerably younger than other DLBCL patients, and their lymphomas frequently involved other thoracic structures but not extrathoracic sites typical of other DLBCLs. PMBL patients had a relatively favorable clinical outcome, with a 5-yr survival rate of 64% compared with 46% for other DLBCL patients. Gene expression profiling strongly supported a relationship between PMBL and Hodgkin lymphoma: over one third of the genes that were more highly expressed in PMBL than in other DLBCLs were also characteristically expressed in Hodgkin lymphoma cells. PDL2, which encodes a regulator of T cell activation, was the gene that best discriminated PMBL from other DLBCLs and was also highly expressed in Hodgkin lymphoma cells. The genomic loci for PDL2 and several neighboring genes were amplified in over half of the PMBLs and in Hodgkin lymphoma cell lines. The molecular diagnosis of PMBL should significantly aid in the development of therapies tailored to this clinically and pathogenetically distinctive subgroup of DLBCL.

B7x: a widely expressed B7 family member that inhibits T cell activation

B7 family proteins provide costimulatory signals that regulate T cell responses. Here we report the third set of B7 family-related T cell inhibitory molecules with the identification of a homolog of the B7 family, B7x. It is expressed in immune cells, nonlymphoid tissues, and some tumor cell lines. B7x inhibits cell-cycle progression, proliferation, and cytokine production of both CD4+ and CD8+ T cells. B7x binds a receptor that is expressed on activated, but not resting T cells that is distinct from known CD28 family members. Its receptor may be a recently identified inhibitory molecule, B and T lymphocyte attenuator. These studies identify a costimulatory pathway that may have a unique function in downregulation of tissue-specific autoimmunity and antitumor responses.

Finding genes for SLE: complex interactions and complex populations

Many years of work, multiplex family collection and endless genotyping finally give fruit. The original aim cannot be lost. The aim is not only to identify mutations involved in susceptibility for systemic lupus erythematosus (SLE) but to elucidate the disease pathogenesis as well. After genetics comes the biology. We review our recent findings on the genetics of lupus, provide possible mechanisms for disease susceptibility and present some facts on the problematic of identifying susceptibility mutations for complex diseases in complex human populations.

The B7 family member B7-H3 preferentially down-regulates T helper type 1-mediated immune responses

We investigated the in vivo function of the B7 family member B7-H3 (also known as B7RP-2) by gene targeting. B7-H3 inhibited T cell proliferation mediated by antibody to T cell receptor or allogeneic antigen-presenting cells. B7-H3-deficient mice developed more severe airway inflammation than did wild-type mice in conditions in which T helper cells differentiated toward type 1 (T(H)1) rather than type 2 (T(H)2). B7-H3 expression was consistently enhanced by interferon-gamma but suppressed by interleukin 4 in dendritic cells. B7-H3-deficient mice developed experimental autoimmune encephalomyelitis several days earlier than their wild-type littermates, and accumulated higher concentrations of autoantibodies to DNA. Thus, B7-H3 is a negative regulator that preferentially affects T(H)1 responses.

Mouse B7-H3 induces antitumor immunity

Members of the B7 family costimulate the proliferation of lymphocytes during the initiation and maintenance of antigen-specific humoral and cell-mediated immune responses. While B7-1 and -2 are restricted to lymphoid tissues, and activate naïve T cells, recently identified members including B7-H2 and -H3 are widely expressed on nonlymphoid tissues, and regulate effector lymphocytes in the periphery. B7-H3 has properties that suggested it may display antitumor activity, including the ability to stimulate Th1 and cytotoxic T-cell responses. Here, we test this notion by determining whether intratumoral injection of an expression plasmid encoding a newly described mouse homologue of B7-H3 is able to eradicate EL-4 lymphomas. Intratumoral injection of a mouse B7-H3 pcDNA3 expression plasmid led to complete regression of 50% tumors, or otherwise significantly slowed tumor growth. Mice whose tumors completely regressed resisted a challenge with parental tumor cells, indicating systemic immunity had been generated. B7-H3-mediated antitumor immunity was mediated by CD8(+) T and NK cells, with no apparent contribution from CD4(+) T cells. In summary, the results indicate that B7-H3 interactions may play a role in regulating cell-mediated immune responses against cancer, and that B7-H3 is a potential therapeutic tool.

Does the immune system see tumors as foreign or self?

Given the vast number of genetic and epigenetic changes associated with carcinogenesis, it is clear that tumors express many neoantigens. A central question in cancer immunology is whether recognition of tumor antigens by the immune system leads to activation (i.e., surveillance) or tolerance. Paradoxically, while strong evidence exists that specific immune surveillance systems operate at early stages of tumorigenesis, established tumors primarily induce immune tolerance. A unifying hypothesis posits that the fundamental processes of cancer progression, namely tissue invasion and metastasis, are inherently proinflammatory and thus activating for innate and adaptive antitumor immunity. To elude immune surveillance, tumors must develop mechanisms that block the elaboration and sensing of proinflammatory danger signals, thereby shifting the balance from activation to tolerance induction. Elucidation of these mechanisms provides new strategies for cancer immunotherapy.

Impaired germinal center formation and recall T-cell-dependent immune responses in mice lacking the costimulatory ligand B7-H2

B7-H2, which is expressed constitutively on B cells and binds the inducible costimulator (ICOS) on antigen-activated T cells, is a member of the B7 family of costimulatory ligands. We have inactivated B7-H2 in the mouse. B7-H2-/- mice generate normal populations of B and T cells in their various lymphoid organs but have lower basal levels of heavy chain class-switched antibodies in their sera. These mice are able to mount normal immune responses to both type I and type II T-cell-independent antigens. However, their pattern of responses to a T-cell-dependent antigen is altered, with greatly reduced production of antigen-specific heavy chain class-switched antibodies, the levels of which could not be elevated even with repeated immunizations. This suggests a critical role for B7-H2 in the recall phases of the immune response. Germinal center formation is also impaired in the mutant mice. While B cells from the mutant mice could response normally to anti-IgM, anti-CD40, and lipopolysaccharide stimulation, the production of T-helper-type II cytokines such as interleukin-4 (IL-4) and IL-10 by primed CD4+ T cells from mutant mice were reduced. This indicated that the defects in humoral responses and germinal center formation in B7-H2-deficient mice are due to the lack of T-cell-mediated help to the B cells. Hence, B7-H2 on B cells is important for recruiting T-cell help via its interaction with ICOS and plays a critical role in costimulating humoral immune responses.

Informatics and the immune system: the expanding IL-1 and B7 protein families

Remarkable advances in our knowledge of the genome sequence have led to the creation of databases that afford the opportunity to discover genes based on the presence of specific sequence motifs. The application of this strategy to the identification of proteins in families of immunological interest has had a visible impact on the sizes of the interleukin 1/18 and the CD28/B7 costimulatory molecule families, among many others. This accelerated pace of discovery presents a new challenge to match the rate of discovery with biological understanding of the functions of these extended protein families.

Blockade of programmed death-1 engagement accelerates graft-versus-host disease lethality by an IFN-gamma-dependent mechanism

Acute graft-vs-host disease (GVHD) is influenced by pathways that can enhance or reduce lethality by providing positive or negative signals to donor T cells. To date, the only reported pathway to inhibit GVHD is the CTLA-4:B7 pathway. Because absence of the programmed death-1 (PD-1) pathway has been implicated in a predisposition to autoimmunity and hence a lack of negative signals, the effect of PD-1 pathway blockade on GVHD was explored using several distinct approaches. In each, GVHD lethality was markedly accelerated. Coblockade of CTLA-4 and PD-1 was additive in augmenting GVHD, indicating that these pathways are not fully redundant. Although neither perforin nor Fas ligand expression was required for GVHD enhancement, donor IFN-gamma production was required for optimal GVHD acceleration in the absence of PD-1 ligation. These data indicate that PD-1 ligation down-regulates GVHD through modulation of IFN-gamma production and suggest a novel therapeutic target for inhibiting GVHD lethality.

Differential binding properties of B7-H1 and B7-DC to programmed death-1

Programmed death-1 (PD-1) is a negative regulatory receptor expressed on activated T and B cells. Two ligands for PD-1, B7-H1 (PD-L1) and B7-DC (PD-L2), have been identified, but their binding properties have not been characterized yet. In this study, we generated soluble Ig fusion proteins of these molecules and examined the kinetics and relative affinities of the interactions between B7-H1 or B7-DC and PD-1 by flow cytometry and surface plasmon resonance. The interaction of B7-DC/PD-1 exhibited a 2-6-fold higher affinity and had different association/dissociation kinetics compared with the interaction of B7-H1/PD-1. Our results suggest that the differential binding properties of B7-H1 and B7-DC may be responsible for differential contributions of these two PD-1 ligands to immune responses.

Costimulation through the inducible costimulator ligand is essential for both T helper and B cell functions in T cell-dependent B cell responses

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell-dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl(-/-) and Icos(-/-) mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl(-/-) mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell-dependent IgG1 production.

Cooperative B7-1/2 (CD80/CD86) and B7-DC costimulation of CD4+ T cells independent of the PD-1 receptor

B7-DC is a recently discovered member of the B7 family that binds to PD-1 and is selectively expressed by dendritic cells (DCs). It has been shown to either costimulate or inhibit T cell responses. To assess the role of B7-DC in DC-T cell interactions, DCs from B7-DC knockout (KO) mice were generated and compared with DCs from wild-type (WT) and B7-1/B7-2 double KO mice. B7-1/B7-2-deficient DCs, while strongly diminished in their ability to stimulate naive CD4+ T cells, nonetheless retain partial activity. DCs from B7-DC KO mice are diminished in their ability to activate CD4+ T cells, demonstrating that DC-expressed B7-DC serves a predominantly stimulatory rather than inhibitory function in the initiation of T cell responses. B7-DC costimulates expression of CD40L with faster kinetics than B7-1 and displays potent synergy with B7-1 and B7-2 for T cell proliferation and cytokine production, indicating that these B7 family members work in concert to stimulate T cells. Finally, costimulation with B7-DC alone or in conjunction with B7-1 is PD-1 independent, indicating that B7-DC costimulates T cells via a second receptor.

Critical role of the programmed death-1 (PD-1) pathway in regulation of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is mediated by autoantigen-specific T cells dependent on critical costimulatory signals for their full activation and regulation. We report that the programmed death-1 (PD-1) costimulatory pathway plays a critical role in regulating peripheral tolerance in murine EAE and appears to be a major contributor to the resistance of disease induction in CD28-deficient mice. After immunization with myelin oligodendrocyte glycoprotein (MOG) there was a progressive increase in expression of PD-1 and its ligand PD-L1 but not PD-L2 within the central nervous system (CNS) of mice with EAE, peaking after 3 wk. In both wild-type (WT) and CD28-deficient mice, PD-1 blockade resulted in accelerated and more severe disease with increased CNS lymphocyte infiltration. Worsening of disease after PD-1 blockade was associated with a heightened autoimmune response to MOG, manifested by increased frequency of interferon gamma-producing T cells, increased delayed-type hypersensitivity responses, and higher serum levels of anti-MOG antibody. In vivo blockade of PD-1 resulted in increased antigen-specific T cell expansion, activation, and cytokine production. Interestingly, PD-L2 but not PD-L1 blockade in WT animals also resulted in disease augmentation. Our data are the first demonstration that the PD-1 pathway plays a critical role in regulating EAE.

PD-1 inhibits antiviral immunity at the effector phase in the liver

Unlike naive T cells, effector T cells can be activated by either T cell receptor signal or costimulatory signal alone and therefore the absence of costimulatory molecules on tissue cells cannot explain the tolerance mechanism at the effector phase. Here we report that PD-L1, the ligand for the immunoinhibitory receptor PD-1, was expressed on vascular endothelium in peripheral tissues. Liver nonparenchymal cells including sinusoidal endothelial cells and Kupffer cells constitutively expressed PD-L1 and inhibited proliferation and cell division of activated T cells expressing PD-1. The absence of PD-1 induced proliferation of effector T cells in the adenovirus-infected liver and resulted in rapid clearance of the virus. These results indicate that PD-1 plays an important role in T cell tolerance at the effector phase and the blockade of the PD-1 pathway can augment antiviral immunity.

The programmed death-1 (PD-1) pathway regulates autoimmune diabetes in nonobese diabetic (NOD) mice

Programmed death-1 (PD-1) receptor, an inhibitory costimulatory molecule found on activated T cells, has been demonstrated to play a role in the regulation of immune responses and peripheral tolerance. We investigated the role of this pathway in the development of autoimmune diabetes. PD-1 or PD-L1 but not PD-L2 blockade rapidly precipitated diabetes in prediabetic female nonobese diabetic (NOD) mice regardless of age (from 1 to 10-wk-old), although it was most pronounced in the older mice. By contrast, cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) blockade induced disease only in neonates. Male NOD mice also developed diabetes after PD-1-PD-L1 pathway blockade, but NOR mice, congenic to NOD but resistant to the development of diabetes, did not. Insulitis scores were significantly higher and frequency of interferon gamma-producing GAD-reactive splenocytes was increased after PD-1-PD-L1 pathway blockade compared with controls. Interestingly, PD-L1 but not PD-L2 was found to be expressed on inflamed islets of NOD mice. These data demonstrate a central role for PD-1-PD-L1 interaction in the regulation of induction and progression of autoimmune diabetes in the NOD mouse and provide the rationale to develop new therapies to target this costimulatory pathway in this disease.

BTLA is a lymphocyte inhibitory receptor with similarities to CTLA-4 and PD-1

During activation, T cells express receptors for receiving positive and negative costimulatory signals. Here we identify the B and T lymphocyte attenuator (BTLA), an immunoglobulin domain-containing glycoprotein with two immunoreceptor tyrosine-based inhibitory motifs. BTLA is not expressed by naive T cells, but it is induced during activation and remains expressed on T helper type 1 (T(H)1) but not T(H)2 cells. Crosslinking BTLA with antigen receptors induces its tyrosine phosphorylation and association with the Src homology domain 2 (SH2)-containing protein tyrosine phosphatases SHP-1 and SHP-2, and attenuates production of interleukin 2 (IL-2). BTLA-deficient T cells show increased proliferation, and BTLA-deficient mice have increased specific antibody responses and enhanced sensitivity to experimental autoimmune encephalomyelitis. B7x, a peripheral homolog of B7, is a ligand of BTLA. Thus, BTLA is a third inhibitory receptor on T lymphocytes with similarities to cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and programmed death 1 (PD-1).

Some interfaces of dendritic cell biology

The field of dendritic cell (DC) biology is robust, with several new approaches to analyze their role in vivo and many newly recognized functions in the control of immunity and tolerance. There also is no shortage of mysteries and challenges. To introduce this volume, I would like to summarize four interfaces of DC research with other lines of investigation and highlight some current issues. One interface is with hematopoiesis. DCs constitute a distinct lineage of white blood cell development with some unique features, such as their origin from both lymphoid and myeloid progenitors, the existence of several distinct subsets, and an important final stage of differentiation termed "maturation," which occurs in response to inflammation and infection, and is pivotal for determining the subsequent immune response. A second interface is with lymphocyte biology. DCs are now known to influence many different classes of lymphocytes (B, NK, NKT) and many types of T cell responses (Th1/Th2, regulatory T cells, peripheral T cell deletion), not just the initial priming or induction of T cell-mediated immunity, which was the first function to be uncovered. DCs are sentinels, controlling many of the afferent or inductive limbs of immune function, alerting the immune system and controlling its early decisions. A third interface is with cell biology. This is a critical discipline to understand at the subcellular and molecular levels the distinct capacities of DCs to handle antigens, to move about the body in a directed way, to bind and activate lymphocytes, and to exert many quality controls on the type of responses, for both tolerance and immunity. A fourth interface is with medicine. Here DCs are providing new approaches to disease pathogenesis and therapy. This interface is perhaps the most demanding, because it requires research with humans. Human research currently is being slowed by the need to deal with many challenges in the design of such studies, and the need to excite, attract and support the young scientists who are essential to move human investigation forward. Nonetheless, DCs are providing new opportunities to study patients and the many clinical conditions that involve the immune system.

B7DC/PDL2 promotes tumor immunity by a PD-1-independent mechanism

B7H1 (PDL1) and B7DC (PDL2) are two new members of the B7 family that can interact with PD-1, a putative negative regulator for immune function. Recent studies have provided evidence for inhibitory functions of both members via PD-1. Meanwhile, compelling evidence exists for costimulatory function of both members. Here we demonstrate that expression of B7DC on the tumor cells promotes CD8 T cell-mediated rejection of tumor cells, at both the induction and effector phase of antitumor immunity. Moreover, B7DC binds to PD-1(-/-) cells and enhances T cell killing in a PD-1-independent mechanism. Our results demonstrate a novel pathway for B7DC to promote tumor immunity and may reconcile the apparently contradictory findings on the function of B7DC.

B7-H4, a molecule of the B7 family, negatively regulates T cell immunity

We identify a B7 family molecule, B7-H4, by protein sequence analysis and comparative molecular modeling. While B7-H4 mRNA is widely distributed in mouse and human peripheral tissues, cell surface expression of B7-H4 protein is limited and shows an inducible pattern on hematopoietic cells. Putative receptor of B7-H4 can be upregulated on activated T cells. By arresting cell cycle, B7-H4 ligation of T cells has a profound inhibitory effect on the growth, cytokine secretion, and development of cytotoxicity. Administration of B7-H4Ig into mice impairs antigen-specific T cell responses whereas blockade of endogenous B7-H4 by specific monoclonal antibody promotes T cell responses. B7-H4 thus may participate in negative regulation of cell-mediated immunity in peripheral tissues.

B7S1, a novel B7 family member that negatively regulates T cell activation

T cell activation by antigen-presenting cells (APC) is regulated by positive and negative costimulatory molecules in the B7 family. Here we describe a novel addition in this family, designated as B7S1, which is uniquely anchored to the cell membrane via a GPI linkage. B7S1 is expressed on professional APC and widely distributed in nonlymphoid tissues. A soluble B7S1-Ig fusion protein binds to activated but not naive T cells. B7S1-Ig inhibits T cell activation and IL-2 production. A monoclonal antibody that blocks binding of B7S1 to its receptor enhances T cell proliferation in vitro and exacerbates experimental autoimmune encephalomyelitis in vivo. This study identifies a novel negative regulator of T cell activation and further reveals complex costimulatory regulation of immune responses.

Gene array and protein expression profiles suggest post-transcriptional regulation during CD8+ T cell differentiation

Peripheral CD8(+) T cells circulate in a quiescent naive state until they are primed by specific antigen and differentiate into effector cells. In the effector state, CD8(+) T cells acquire cytolytic activity and produce increased levels of cytokines such as interferon-gamma. They also exhibit increased T cell receptor sensitivity, decreased CD28 dependence, and become inhibitable by CTLA-4 and other negative regulatory pathways. We hypothesized that one mechanism by which these two states are regulated is via differential expression of specific genes. To this end, basal gene expression profiles of naive and effector 2C TCR transgenic x RAG2(-/-) CD8(+) T cells were analyzed using Affymetrix arrays representing 11,000 genes. Of the 177 differentially expressed known genes, 68 were expressed at higher levels in effector cells, but 109 were more abundant in naive cells, supporting the notion that the naive state is not passive. Expression of genes related to metabolism, actin cytoskeletal dynamics, and effector function increased with priming, whereas expression of putative anti-proliferative genes decreased. Semiquantitative reverse transcription-PCR was utilized as a secondary validation for selected transcripts, and Western blot analysis was used to examine protein expression for molecules of interest. Surprisingly, for 24 genes examined, 12 showed discordant protein versus mRNA expression. In summary, our study indicates that: 1) not only does the expression of some genes in naive CD8(+) T cells become up-regulated upon priming, but the expression of other genes is down-regulated as well and 2) the complexities of T cell differentiation include regulation at the post-transcriptional level.

Molecular modeling and functional mapping of B7-H1 and B7-DC uncouple costimulatory function from PD-1 interaction

B7-H1 and B7-DC are ligands for PD-1, a receptor implicated in negative regulation of T and B cell functions. These ligands, however, also costimulate T cell responses. It remains elusive whether or not costimulation is mediated through PD-1. By comparative molecular modeling and site-directed mutagenesis, we found that nonconserved residues between these ligands on the A'GFCC'C" face mediate interaction with PD-1. This indicates significant structural heterogeneity of the interactions between PD-1 and its ligands. Importantly, ligand mutants with abolished PD-1 binding capacity could still costimulate proliferation and cytokine production of T cells from normal and PD-1-deficient mice. Our results reveal unique binding characteristics of B7-H1 and B7-DC and provide direct evidence for an independent costimulatory receptor other than PD-1.

Negative T cell costimulation and islet tolerance

Activation of self-reactive T cells that specifically destroy the pancreatic beta-cells is one of the hallmarks in the development of type 1 diabetes. Thus, for prevention and treatment of this autoimmune disease, approaches to induce and maintain T cell tolerance toward the beta-cells, especially in islet transplantation, have been actively pursued. Noticeably, many of the recent protocols for inducing transplant tolerance involve blockade of positive T cell costimulation extrinsically. Though highly effective in prolonging graft survival, these strategies alone might not be universally sufficient to achieve true tolerance. As the mystery of the suppressive and regulatory T cells unfolds, it is becoming appreciated that exploiting the intrinsic molecular and cellular mechanisms that turn off an immune response would perhaps facilitate the current protocols in establishing T cell tolerance. In this perspective, here we summarize the recent findings on the negative costimulation pathways, in particular, the newly identified PD-1 : PD-L interactions. On the basis of these observations, we propose a new principle of curtailing pathogenic T cell response in which blockade of positive T cell costimulation is reinforced by concurrent engagement of the negative costimulation machinery. Such a strategy may hold greater hope for therapeutic intervention of transplant rejection and autoimmune diseases.

B7-H1 pathway and its role in the evasion of tumor immunity

B7-H1 is a recently identified member of the B7 family molecules. Upon ligation to its receptors on T cells it regulates activation and differentiation of T cells. B7-H1 preferentially costimulates IL-10 production in resting T cells and further induces the apoptosis of activated T cells. PD-1 is a receptor of B7-H1 and is shown to mediate the inhibition of activated T cell response, presumably by inhibiting cell cycle progression. The expression of B7-H1 protein is limited to macrophage lineage of cells in normal tissues, although its mRNA transcription is found in a broad range of tissues. In contrast, B7-H1 is abundant in various human cancers. The tumor-associated B7-H1 increases apoptosis of antigen specific T cells, leading to growth of immunogenic tumor growth in vivo. Current data suggest that B7-H1 regulates the organ-specific tolerance in normal tissue and may contribute to immune evasion by cancers. Selective manipulation of B7-H1 pathway thus aids in the design of new regimens in the treatment of human autoimmune disease and the control of malignant cancers.

B7 family molecules are favorably positioned at the human maternal-fetal interface

The human placenta utilizes both active and passive mechanisms to evade rejection by the maternal immune system. We investigated the pattern of expression of the B7 family of immunomodulatory molecules B7-H1 (PD-L1), B7-2 (CD86), and B7-1 (CD80) at the term maternal-fetal interface. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses showed that B7-H1 mRNA is abundant in term placenta and that cytotrophoblasts are sources of this message. Immunohistochemistry demonstrated that B7-H1 is constitutively expressed by the syncytiotrophoblast and by extravillous cytotrophoblasts, both of which are juxtaposed to maternal blood and tissue. By contrast, placental stromal cells, including macrophages, lacked the protein. Expression of B7-H1 protein was low in first-trimester placenta compared to second- and third-trimester tissue (P < 0.05) and was enhanced in cultured cytotrophoblasts by treatment with either interferon-gamma or epidermal growth factor (P < 0.05), suggesting that one or both of these mediators regulates B7-H1 expression in the placenta. RT-PCR and immunofluorescence analysis of term placental tissue revealed different patterns of expression of the immunostimulatory protein, B7-2. In contrast to B7-H1, B7-2 mRNA and protein were absent in cytotrophoblast cells but present in maternal macrophages and some fetal macrophages. The B7-1 mRNA and protein were absent at the maternal-fetal interface. These studies document expression of the B7 family proteins at the maternal-fetal interface and demonstrate that B7-H1 is positioned such that it could facilitate protection of fetal cells against activated maternal leukocytes. Conversely, B7-2 was absent on trophoblasts and was appropriately localized to fetal and maternal macrophages, which may participate in antigen presentation.

PD-L1 and PD-L2 are differentially regulated by Th1 and Th2 cells

PD-L1 and PD-L2 are ligands for PD-1, a costimulatory molecule that plays an inhibitory role in regulating T cell activation in the periphery. We find that PD-L1 is highly expressed on inflammatory macrophages as compared with resident peritoneal macrophages but can be induced on resident macrophages by classical activation stimuli such as lipopolysaccharide, IFN-gamma, and polyinosinic-polycytidylic acid. Further up-regulation of PD-L1 on inflammatory macrophages can also be induced by subsequent exposure to lipopolysaccharide and IFN-gamma. In contrast, PD-L2 is not expressed on inflammatory macrophages but can be induced by alternative activation via IL-4. Although PD-L1 is highly inducible on a variety of antigen-presenting cell lines as well as resident macrophages, PD-L2 is most significantly inducible only on inflammatory macrophages. PD-L1 up-regulation depends on TLR4 and STAT1, whereas PD-L2 expression depends on IL-4R alpha and STAT6. Consistent with these results, T helper 1T helper 2 (Th1/Th2) cells also differentially up-regulate PD-L1 and PD-L2 expression on inflammatory macrophages. Hence, Th1 cells as well as microbial products can enhance PD-L1 expression on many different macrophage populations, whereas Th2 cells instruct only inflammatory macrophages to up-regulate PD-L2. These results suggest that PD-L1 and PD-L2 might have different functions in regulating type 1 and type 2 responses.

T cell costimulatory pathways: blockade for autoimmunity

The activation of T lymphocytes is thought to require at least two signals, one delivered by the T cell receptor (TCR) complex after antigen recognition, and one provided on engagement of costimulatory receptors. The B7-1/B7-2-CD28/cytotoxic T lymphocyte antigen-4 (CTLA-4) and CD154-CD40 pathways have been shown to be crucial in regulating T cell activation and tolerance. Novel members of the B7-CD28 superfamily have recently been discovered and they seem to be particularly important for regulating the responses of previously activated T cells. Superimposition of inhibitory signals like those delivered by CTLA-4 and programmed death (PD)-1-PD-1-ligand (PD-L1) pathway leads to a complex network of positive and negative costimulatory signals, the integration of which modulates immune responses. Furthermore, expression of several B7 homologues on cells other than professional antigen-presenting cells (APCs), indicate new mechanisms for regulating T cell responses in peripheral tissues. This review focuses on our current understanding of the members of the B7-CD28 superfamily and discusses their therapeutic potential.