Programmed death-1 ligand 1 interacts specifically with the B7-1 costimulatory molecule to inhibit T cell responses

The common gamma-chain cytokines IL-2, IL-7, IL-15, and IL-21 induce the expression of programmed death-1 and its ligands

The programmed death (PD)-1 molecule and its ligands (PD-L1 and PD-L2), negative regulatory members of the B7 family, play an important role in peripheral tolerance. Previous studies have demonstrated that PD-1 is up-regulated on T cells following TCR-mediated activation; however, little is known regarding PD-1 and Ag-independent, cytokine-induced T cell activation. The common gamma-chain (gamma c) cytokines IL-2, IL-7, IL-15, and IL-21, which play an important role in peripheral T cell expansion and survival, were found to up-regulate PD-1 and, with the exception of IL-21, PD-L1 on purified T cells in vitro. This effect was most prominent on memory T cells. Furthermore, these cytokines induced, indirectly, the expression of PD-L1 and PD-L2 on monocytes/macrophages in PBMC. The in vivo correlate of these observations was confirmed on PBMC isolated from HIV-infected individuals receiving IL-2 immunotherapy. Exposure of gamma c cytokine pretreated T cells to PD-1 ligand-IgG had no effect on STAT5 activation, T cell proliferation, or survival driven by gamma c cytokines. However, PD-1 ligand-IgG dramatically inhibited anti-CD3/CD28-driven proliferation and Lck activation. Furthermore, following restimulation with anti-CD3/CD28, cytokine secretion by both gamma c cytokine and anti-CD3/CD28 pretreated T cells was suppressed. These data suggest that gamma c cytokine-induced PD-1 does not interfere with cytokine-driven peripheral T cell expansion/survival, but may act to suppress certain effector functions of cytokine-stimulated cells upon TCR engagement, thereby minimizing immune-mediated damage to the host.

Coregulation of CD8+ T cell exhaustion by multiple inhibitory receptors during chronic viral infection

T cell exhaustion often occurs during chronic infections and prevents optimal viral control. The molecular pathways involved in T cell exhaustion, however, remain poorly understood. We demonstrate that exhausted CD8⁺ T cells are subject to complex layers of negative regulation due to co-expression of multiple inhibitory receptors. Exhausted CD8⁺ T cells expressed up to 7 inhibitory receptors. Co-expression of multiple distinct inhibitory receptors correlated with greater T cell exhaustion and more severe infection. Regulation of T cell exhaustion by diverse inhibitory pathways was non-redundant since blockade of PD-1 and LAG-3 simultaneously in vivo synergistically improved T cell responses and reduced viral load. Thus, CD8⁺ T cell responses during chronic viral infections are regulated by complex patterns of co-expressed inhibitory receptors.

PD-L1/PD-1 signal deficiency promotes allogeneic immune responses and accelerates heart allograft rejection

BACKGROUND

PD-L1, a ligand for programmed death 1 (PD-1), delivers a negative costimulatory signal to T cells and plays a critical role in the regulation of peripheral tolerance.

METHODS

We used PD-L1(-/-) mice to evaluate the role of the PD-L1 signal on allogeneic immune responses in vivo and the underlying mechanisms. Heart transplantation was performed from PD-L1(-/-) donors or recipients in major histocompatibility complex fully mismatched mouse combinations. The immunologic function of allograft recipients was evaluated ex vivo by enzyme-linked immunospot, mixed lymphocytes reaction, cytotoxic T lymphocyte, and flow cytometry.

RESULTS

Our results demonstrated that PD-L1(-/-) T cells proliferated vigorously under alloantigen stimulation, and also that the antigen-presenting cells (APCs) from PD-L1(-/-) mice exhibited a stronger allostimulatory activity compared with that in wild-type mice. Heart allografts were rejected at an accelerated rate in both PD-L1(-/-) donors and recipients. This was associated with significantly augmented donor specific T-cell proliferation and antidonor cytotoxic T lymphocyte activities, and enhanced Th1- or Th2-type immune responses of heart allograft recipients.

CONCLUSIONS

Absence of PD-L1 input triggers a stimulatory signal to effector T cells and APCs, accelerating heart allograft rejection. Engagement of the PD-L1 signal on T cells or APCs may be necessary to induce transplant tolerance.

Tumor dormancy and immunoescape

The role of the immune system in tumor dormancy is now well established. In several experimental models it is possible to induce tumor dormancy in immunocompetent hosts by prior immunization against tumor cells. Equilibrium between immune response and tumor cells leads to long-term tumor dormancy. This equilibrium is also observed early in tumor development and adaptive immunity may help contain tumor outgrowth. However, after variable times, tumor dormancy ends and the disease progresses. As the immune response remains active the tumor cells presumably escape dormancy by becoming resistant. Due to the extreme difficulty of isolating dormant tumor cells from patients, such mechanisms are poorly understood. However, experimental models have shown that dormant tumor cells may overexpress B7-H1 and B7.1, and inhibit CTL-mediated lysis. These cells resist apoptosis by methylating SOCS1, and by paracrine production of cytokines. The presence of immunoescape mechanisms in tumor cells from relapsing patients also suggests that the immune equilibrium which maintained dormancy has broken down. Identification of such mechanisms would offer new leads to favor the immune balance, and thus to clear minimal residual disease from patients.

High tumor-infiltrating macrophage density predicts poor prognosis in patients with primary hepatocellular carcinoma after resection

Macrophages constitute a major component of the leukocyte infiltrate of tumors and perform distinct roles in different tumor microenvironments. This study attempted to investigate the prognostic values of tumor-infiltrating macrophages in patients with hepatocellular carcinoma after resection, paying particular attention to their tissue microlocalization. The CD68(+) macrophages were assessed by immunohistochemistry in tissues from 137 patients with hepatocellular carcinoma. Prognostic value of intratumoral, marginal, and peritumoral macrophage densities was evaluated by Kaplan-Meier analysis and Cox regression. Both intratumoral and marginal macrophage densities were associated inversely with overall survival (P = .034 and .004, respectively) and disease-free survival (P = .006 and .008, respectively). In contrast, peritumoral macrophage density was associated with neither overall survival nor disease-free survival. Intratumoral macrophage density emerged as an independent prognosticator of overall survival (hazard ratio = 1.721, P = .049) and disease-free survival (hazard ratio = 2.165, P = .007). Marginal macrophage density, but not intratumoral macrophage density, was associated with vascular invasion, tumor multiplicity, and fibrous capsule formation. Our results demonstrate that high macrophage infiltration predicts poor prognosis in patients with hepatocellular carcinoma. These results, together with our previous report showing the distinct activation patterns of macrophages in different areas of tumor tissue, implies that macrophages in those areas may use different strategies to promote the tumor progression.

Intrafollicular location of marginal zone/CD1d(hi) B cells is associated with autoimmune pathology in a mouse model of lupus

Marginal zone (MZ) B cells contain a large number of autoreactive clones and the expansion of this compartment has been associated with autoimmunity. MZ B cells also efficiently transport blood-borne antigen to the follicles where they activate T cells and differentiate into plasma cells. Using the B6.NZM2410.Sle1.Sle2.Sle3 (B6.TC) model of lupus, we show that the IgM+ CD1d(hi)/MZ B-cell compartment is expanded, and a large number of them reside inside the follicles. Contrary to the peripheral B-cell subset distribution and their activation status, the intrafollicular location of B6.TC IgM+ CD1d(hi)/MZ B cells depends on both bone marrow- and stromal-derived factors. Among the factors responsible for this intrafollicular location, we have identified an increased response to CXCL13 by B6.TC MZ B cells and a decreased expression of VCAM-1 on stromal cells in the B6.TC MZ. However, the reduced number of MZ macrophages observed in B6.TC MZs was independent of the IgM+ CD1d(hi)/B-cell location. B7-2 but not B7-1 deficiency restored IgM+ CD1d(hi)/MZ B-cell follicular exclusion in B6.TC mice, and it correlated with tolerance to dsDNA and a significant reduction of autoimmune pathology. These results suggest that follicular exclusion of IgM+ CD1d(hi)/MZ B cells is an important B-cell tolerance mechanism, and that B7-2 signaling is involved in breaching this tolerance checkpoint.

B7-H1 on hepatocytes facilitates priming of specific CD8 T cells but limits the specific recall of primed responses

BACKGROUND & AIMS

The requirement for costimulation of CD8 T-cell priming and restimulation by nonprofessional antigen-presenting cells is unresolved. Here, we investigated whether B7-H1 (CD274, PD-L1) on hepatocytes (HC) is involved in the specific activation of naive CD8 T cells or activated CD8 T blasts.

METHODS

Naive or activated CD8 T cells from transgenic OT-I mice were primed/restimulated by peptide-pulsed HC, and their proliferation and effector response were determined. We used blocking monoclonal antibodies against B7-H1 and HC from B7-H1-deficient mice to assign a costimulatory or coinhibitory role to B7-H1 for CD8 T-cell priming/restimulation.

RESULTS

Blockade of B7-H1 on HC down modulated interferon (IFN)-gamma production and proliferation of HC-primed CD8 T cells, indicating a costimulatory role for B7-H1 in priming CD8 T cells. In contrast, the PD-1/B7-H1 interaction inhibited proliferation and interferon-gamma release of effector/memory CD8 T blasts specifically restimulated by peptide-pulsed HC.

CONCLUSIONS

B7-H1 differentially modulates the different stages of the specific CD8 T-cell response triggered by HC, and, whereas it costimulates priming and cytokine responses of naive CD8 T cells, it coinhibits their specific local recall of effector cytokine responses. The interaction of CD8 T cells with B7-H1(+) HC can thus fine-tune proliferative and effector responses of specific CD8 T cells reacting locally to nonprofessional antigen-presenting cells infected with hepatotropic agents.

Interactions of T cells with fibroblast-like synoviocytes: role of the B7 family costimulatory ligand B7-H3

Fibroblast-like synoviocytes (FLS) and T cells can activate each other in vitro, and in vivo interactions between these cells may be important in rheumatoid arthritis (RA), yet FLS lack significant expression of CD28 ligands. We sought to identify molecules homologous to CD28 ligands that are strongly expressed by FLS, and documented strong B7-H3 expression on FLS and by fibroblasts of other tissues, which was unaffected by a variety of cytokines. Western blot analysis of FLS lysates showed predominant expression of the larger, four Ig-like domain isoform of B7-H3. Immunohistological sections of RA synovial tissue showed strong staining for B7-H3 on FLS. Cells expressing B7-H3 were distinct from but in close proximity to cells that expressed CD45, CD20, and CD3. Confocal microscopy of FLS and T cell cocultures showed localization of B7-H3 in the region of the T cell-FLS contact point, but distinct from the localization of T cell CD11a/CD18 (LFA-1) and FLS CD54 (ICAM-1). Reduction of B7-H3 expression on FLS by RNA interference affected interactions of FLS with resting T cells or cytokine-activated T cells. Resting T cells showed increased production of TNF-alpha, IFN-gamma, and IL-2, whereas cytokine-activated T cells showed reduced cytokine production relative to control. However, cytokine production by T cells activated through their TCR was not notably altered by knock down of B7-H3. These observations suggest that B7-H3 may be important for the interactions between FLS and T cells in RA, as well as other diseases, and the outcome of such interactions depends on the activation state of the T cell.

Programmed death (PD)-1:PD-ligand 1/PD-ligand 2 pathway inhibits T cell effector functions during human tuberculosis

Protective immunity against Mycobacterium tuberculosis requires the generation of cell-mediated immunity. We investigated the expression and role of programmed death 1 (PD-1) and its ligands, molecules known to modulate T cell activation, in the regulation of IFN-gamma production and lytic degranulation during human tuberculosis. We demonstrated that specific Ag-stimulation increased CD3+PD-1+ lymphocytes in peripheral blood and pleural fluid from tuberculosis patients in direct correlation with IFN-gamma production from these individuals. Moreover, M. tuberculosis-induced IFN-gamma participated in the up-regulation of PD-1 expression. Blockage of PD-1 or PD-1 and its ligands (PD-Ls: PD-L1, PD-L2) enhanced the specific degranulation of CD8+ T cells and the percentage of specific IFN-gamma-producing lymphocytes against the pathogen, demonstrating that the PD-1:PD-Ls pathway inhibits T cell effector functions during active M. tuberculosis infection. Furthermore, the simultaneous blockage of the inhibitory receptor PD-1 together with the activation of the costimulatory protein signaling lymphocytic activation molecule led to the promotion of protective IFN-gamma responses to M. tuberculosis, even in patients with weak cell-mediated immunity against the bacteria. Together, we demonstrated that PD-1 interferes with T cell effector functions against M. tuberculosis, suggesting that PD-1 has a key regulatory role during the immune response of the host to the pathogen.

PD-1/PD-L1, PD-1/PD-L2, and other co-inhibitory signaling pathways in transplantation

Transplantation of cells, tissues and vascularized solid organs is a successful therapeutic intervention for many end-stage chronic diseases. The combination of co-stimulatory blockade with the delivery of negative signals to T cells through co-inhibitory receptors would provide a robust approach to modulating T-cell receptor signaling and improving alloantigen-specific control of transplant rejection. This approach based on fundamental knowledge of APC/T-cell interactions may complement conventional therapies in the near future to reinforce long-term allograft survival, and permit minimal immunosuppression. The focus of this review was primarily on two major co-inhibitory signaling pathways, namely PD-1/PD-L1/PD-L2 and BTLA/CD160/HVEM/LIGHT that have been thoroughly characterized in murine models of transplantation using genetically modified mice, specific monoclonal antibodies and fusion proteins.

Active systemic lupus erythematosus is associated with failure of antigen-presenting cells to express programmed death ligand-1

OBJECTIVE

Antigen-presenting cells (APC) play critical roles in establishing and maintaining peripheral tolerance. This is accomplished in part via expression of negative co-stimulatory molecules such as programmed death ligand-1 (PD-L1) on tolerogenic APC, such as immature myeloid dendritic cells (mDC). Several studies have strongly linked dysfunction of APC, including mDC, to the pathogenesis of SLE. The objective of this study was to determine whether APC expressed PD-L1 protein at normal levels during active lupus.

METHODS

Peripheral blood mononuclear cells (PBMC) were isolated from 19 paediatric patients with SLE and from 17 healthy age-matched controls. PBMC from both cohorts were cultured in the absence of exogenously added stimuli, and leucocyte PD-L1 expression was measured by flow cytometry.

RESULTS

Immature mDC and monocytes (Mo) from healthy children expressed little PD-L1 at initial isolation, but spontaneously up-regulated PD-L1 by 24 h. In contrast, both mDC and Mo from patients with active SLE failed to up-regulate PD-L1 over a 5 day time course, expressing this protein only during disease remissions.

CONCLUSIONS

These data are the first to link active lupus with reversibly decreased PD-L1 expression on professional APC, suggesting a novel mechanism for loss of peripheral tolerance in SLE.

PDL-1 upregulation on monocytes and T cells by HIV via type I interferon: restricted expression of type I interferon receptor by CCR5-expressing leukocytes

The programmed death (PD)-1 interacts with its ligand (PDL-1) delivering a negative signal to T cells. During human immunodeficiency virus (HIV)-1 infection PD-1 and PDL-1 expressions are increased. Here we show that monocytes and CCR5(+) T cells of HIV-uninfected donors upregulated PDL-1 upon in vitro exposure to HIV. HIV-induced PDL-1 required interferon (IFN)-alpha, but not IFN-gamma, production. Inhibition of endocytosis, required for HIV-induced IFN-alpha production, prevented PDL-1 upregulation. IFN-alpha-inducing Toll-like receptor (TLR) agonists increased PDL-1 on monocytes and CCR5(+) T cells. CD80 and CD86 were also increased on monocytes and CCR5(+) T cells after HIV exposure, but only CD80 was IFN-alpha-dependent. IFN-alpha-receptor subunit 2 (IFNAR2), was expressed only by CCR5(+) T cells and monocytes, explaining why these leukocytes responded to HIV-induced IFN-alpha. Finally, T cell proliferation was improved by PDL-1 blockade in HIV-treated PBMC. In the setting of HIV infection, IFN-alpha may negatively affect T cell responses by inducing PDL-1.

Crystal structure of the complex between programmed death-1 (PD-1) and its ligand PD-L2

Programmed death-1 (PD-1) is a member of the CD28/B7 superfamily that delivers negative signals upon interaction with its two ligands, PD-L1 or PD-L2. The high-resolution crystal structure of the complex formed by the complete ectodomains of murine PD-1 and PD-L2 revealed a 1:1 receptor:ligand stoichiometry and displayed a binding interface and overall molecular organization distinct from that observed in the CTLA-4/B7 inhibitory complexes. Furthermore, our structure also provides insights into the association between PD-1 and PD-L1 and highlights differences in the interfaces formed by the two PD-1 ligands (PD-Ls) Mutagenesis studies confirmed the details of the proposed PD-1/PD-L binding interfaces and allowed for the design of a mutant PD-1 receptor with enhanced affinity. These studies define spatial and organizational constraints that control the localization and signaling of PD-1/PD-L complexes within the immunological synapse and provide a basis for manipulating the PD-1 pathways for immunotherapy.

Programmed death 1 ligand signaling regulates the generation of adaptive Foxp3+CD4+ regulatory T cells

Although mature dendritic cells (DCs) are potent initiators of adaptive immune response, immature steady-state DCs contribute to immune tolerance. In this study, we show that ex vivo splenic DCs are capable of inducing conversion of naïve CD4(+) T cells to adaptive Foxp3(+)CD4(+) regulatory T cells (aTreg) in the presence of TGF-beta. In particular, when compared with splenic CD8alpha(-) DCs, the CD8alpha(+) DC subset were superior in inducing higher frequencies of conversion. This was not attributable to the difference in basal level of costimulation, because deficiency of CD40 or CD80/86 signaling did not diminish the differential induction of Foxp3. Conversion was regulated by DC maturation status. Further insights into the molecular mechanisms of conversion were gained by analyzing the contribution of several costimulatory and coinhibitory receptors. Costimulatory signals through GITR suppressed conversion, whereas coinhibitory signaling via programmed death 1 ligand (PD-L1) but not PD-L2 was required for conversion. Ex vivo PD-L1(-/-) DCs failed to support Foxp3 induction in the presence of TGF-beta. In vivo blocking PD-L1 signaling abolished conversion in a tumor-induced aTreg conversion model. Collectively, this study highlights the cellular and molecular parameters that might be exploited to control the de novo generation of aTregs and peripheral tolerance.

Protective role of programmed death 1 ligand 1 (PD-L1)in nonobese diabetic mice: the paradox in transgenic models

OBJECTIVE

Coinhibitory signals mediated via programmed death 1 (PD-1) receptor play a critical role in downregulating immune responses and in maintaining peripheral tolerance. Programmed death 1 ligand 1 (PD-L1), the interacting ligand for PD-1, widely expressed in many cell types, acts as a tissue-specific negative regulator of pathogenic T-cell responses. We investigated the protective potential of PD-L1 on autoimmune diabetes by transgenically overexpressing PD-L1 in pancreatic beta-cells in nonobese diabetic (NOD) mice.

RESEARCH DESIGN AND METHODS

We established an insulin promoter-driven murine PD-L1 transgenic NOD mouse model to directly evaluate the protective effect of an organ-specific PD-L1 transgene against autoimmune diabetes. Transgene expression, insulitis, and diabetic incidence were characterized in these transgenic NOD mice. Lymphocyte development, Th1 cells, and regulatory T-cells were analyzed in these transgenic mice; and T-cell proliferation, adoptive transfer, and islet transplantation were performed to evaluate the PD-L1 transgene-mediated immune-protective mechanisms.

RESULTS

The severity of insulitis in these transgenic mice is significantly decreased, disease onset is delayed, and the incidence of diabetes is markedly decreased compared with littermate controls. NOD/SCID mice that received lymphocytes from transgenic mice became diabetic at a slower rate than mice receiving control lymphocytes. Moreover, lymphocytes collected from recipients transferred by lymphocytes from transgenic mice revealed less proliferative potential than lymphocytes obtained from control recipients. Transgenic islets transplanted in diabetic recipients survived moderately longer than control islets.

CONCLUSIONS

Our results demonstrate the protective potential of transgenic PD-L1 in autoimmune diabetes and illustrate its role in downregulating diabetogenic T-cells in NOD mice.

Interaction of human PD-L1 and B7-1

Numerous studies have pointed to the role of programmed death-1 ligand 1 (PD-L1) in regulating tolerance, chronic infection, and tumor immunity. Recently, we have identified murine B7-1 as a new binding partner for murine PD-L1. Human and mouse B7-1 share only 46% identity, leading us to question whether human B7-1 and PD-L1 can participate in a similar interaction. Here we show that human B7-1 can interact with human PD-L1 with affinity greater than that of B7-1 with CD28, but less than that of B7-1 with CTLA-4 or of PD-L1 with PD-1. We characterize a series of anti-human PD-L1 monoclonal antibodies and identify antibodies that can block interactions of PD-L1 with B7-1, PD-1, or both. Since PD-L1 and CD28 on T cells may compete for B7-1 as a binding partner and CD8 T cells may express high or low levels of CD28, we examined when PD-L1 and CD28 are co-expressed on CD8 T cells. We compared the time-course and extent of PD-L1 induction on CD8 CD28high versus CD28low T cells following stimulation with anti-CD3. We show that PD-L1 is induced to a higher level on CD28high T cells than on CD28low T cells upon activation. These results suggest that PD-L1 may play an important and undervalued role on human T cells.

Preferential costimulation by CD80 results in IL-10-dependent TGF-beta1(+) -adaptive regulatory T cell generation

Costimulatory ligands CD80 and CD86 have different binding preferences and affinities to their receptors, CD28 and CTLA-4. Earlier, we demonstrated that CD80 binds to CTLA-4 with higher affinity and has a role in suppressing T cell response. The current study demonstrates that not only did blockade of CD86 upon Ag presentation by bone marrow-derived dendritic cells (DC) to OVA-specific T cells result in induction of hyporesponsive T cells but also that these T cells could suppress the proliferative response of effector T cells. These T cells showed TGF-beta1 on their surface and secreted TGF-beta1 and IL-10 upon restimulation. Although blockade of CTLA-4 and neutralization of IL-10 profoundly inhibited the induction of these TGF-beta1(+) T cells, their ability to suppress the effector T cell proliferation was abrogated by neutralization of TGF-beta1 alone. Induction of TGF-beta1(+) and IL-10(+) T cells was found to be independent of natural CD4(+)CD25(+) regulatory T cells, demonstrating that preferential ligation of CTLA-4 by CD80 induced IL-10 production by effector T cells, which in turn promoted the secretion of TGF-beta1. Treatment of prediabetic NOD mice with islet beta cell Ag-pulsed CD86(-/-) DCs, but not CD80(-/-) DCs, resulted in the induction of TGF-beta1- and IL-10-producing cells, significant suppression of insulitis, and delay of the onset of hyperglycemia. These observations demonstrate not only that CD80 preferentially binds to CTLA-4 but also that interaction during Ag presentation can result in IL-10-dependent TGF-beta1(+) regulatory T cell induction, reinstating the potential of approaches to preferentially engage CTLA-4 through CD80 during self-Ag presentation in suppressing autoimmunity.

Inhibitory B7-family molecules in the tumour microenvironment

The B7 family consists of activating and inhibitory co-stimulatory molecules that positively and negatively regulate immune responses. Recent studies have shown that human and rodent cancer cells, and stromal cells and immune cells in the cancer microenvironment upregulate expression of inhibitory B7 molecules and that these contribute to tumour immune evasion. In this Review, we focus on the roles of these B7 molecules in the dynamic interactions between tumours and the host immune system, including their expression, regulation and function in the tumour microenvironment. We also discuss novel therapeutic strategies that target these inhibitory B7 molecules and their signalling pathways to treat human cancer.

Dormant tumor cells as a therapeutic target?

Tumor dormancy is characterised by the persistence of residual tumor cells for long periods. Recurrence from minimal residual disease is a major cause of cancer death. Thus, understanding how cancer cells become and remain dormant, may lead to new strategies to prevent relapse. Evidence has emerged that a balance exists between host and dormant tumor cells. Cross-talk between tumor cells and their micro-environment, angiogenesis, and anti-tumor immune response participate in the control of dormant tumor cells. Tumor cells have several mechanisms of maintaining equilibrium, and immune escape, including expression of immuno-regulatory molecules (e.g., increased expression of B7.1 and B7-H1); epigenetic modifications (e.g., silencing of the SOCS1 gene, de-regulating the JAK/STAT pathway); and autocrine loops. These new findings offer new opportunities to design specific treatments, to modify the balance in favor of the host immune response.

CTLA-4 disrupts ZAP70 microcluster formation with reduced T cell/APC dwell times and calcium mobilization

CTLA-4 is a co-receptor that modulates the threshold of T cell activation and autoimmunity. We previously showed that CTLA-4 reverses the TCR-mediated stop signal needed for T cell/APC interactions [Schneider et al., Science 2006, 313: 1972]. In this study, using a different T cell system, we show that CTLA-4 expression changed the behavior of T8.1 T cells by reducing the contact time between T cell and APC, preventing re-inforced contacts, and reducing the contact area at the immunological synapse. This led to a major reduction in Ca(2+) influx/mobilization and interleukin-2 production. Further, anti-CD3/CTLA-4 increased T cell motility on antibody-coated glass slides, concurrent with an abrogation of ZAP70 microcluster formation. Our findings further support a role for CTLA-4 in limiting the interaction between T cell and APC that is needed for optimal activation.

The PD-1/PD-L1 complex resembles the antigen-binding Fv domains of antibodies and T cell receptors

Signaling through the programmed death 1 (PD-1) inhibitory receptor upon binding its ligand, PD-L1, suppresses immune responses against autoantigens and tumors and plays an important role in the maintenance of peripheral immune tolerance. Release from PD-1 inhibitory signaling revives "exhausted" virus-specific T cells in chronic viral infections. Here we present the crystal structure of murine PD-1 in complex with human PD-L1. PD-1 and PD-L1 interact through the conserved front and side of their Ig variable (IgV) domains, as do the IgV domains of antibodies and T cell receptors. This places the loops at the ends of the IgV domains on the same side of the PD-1/PD-L1 complex, forming a surface that is similar to the antigen-binding surface of antibodies and T cell receptors. Mapping conserved residues allowed the identification of residues that are important in forming the PD-1/PD-L1 interface. Based on the structure, we show that some reported loss-of-binding mutations involve the PD-1/PD-L1 interaction but that others compromise protein folding. The PD-1/PD-L1 interaction described here may be blocked by antibodies or by designed small-molecule drugs to lower inhibitory signaling that results in a stronger immune response. The immune receptor-like loops offer a new surface for further study and potentially the design of molecules that would affect PD-1/PD-L1 complex formation and thereby modulate the immune response.

The PD-1/PD-L costimulatory pathway critically affects host resistance to the pathogenic fungus Histoplasma capsulatum

The PD-1 costimulatory receptor inhibits T cell receptor signaling upon interacting with its ligands PD-L1 and PD-L2. The PD-1/PD-L pathway is critical in maintaining self-tolerance. In this study, we examined the role of PD-1 in a mouse model of acute infection with Histoplasma capsulatum, a major human pathogenic fungus. In a lethal model of histoplasmosis, all PD-1-deficient mice survived infection, whereas the wild-type mice died with disseminated disease. PD-L expression on macrophages and splenocytes was up-regulated during infection, and macrophages from infected mice inhibited in vitro T cell activation. Of interest, antibody blocking of PD-1 significantly increased survival of lethally infected wild-type mice. Thus, our studies extend the role of the PD-1/PD-L pathway in regulating antimicrobial immunity to fungal pathogens. The results show that the PD-1/PD-L pathway has a key role in the regulation of antifungal immunity, and suggest that manipulation of this pathway represents a strategy of immunotherapy for histoplasmosis.

B7-H1 is a ubiquitous antiapoptotic receptor on cancer cells

B7-H1 is an immunoglobulin-like immune suppressive molecule broadly detectable on the majority of human and rodent cancers, and its functions have been attributed to delivering an inhibitory signal to its counter-receptor programmed death-1 (PD-1) on T cells. Here we report that B7-H1 on cancer cells receives a signal from PD-1 to rapidly induce resistance against T cell-mediated killing because crippling signaling capacity of B7-H1 but not PD-1 ablates this resistance. Importantly, loss of B7-H1 signaling is accompanied by increased susceptibility to immune-mediated tumoricidal activity. In addition to resistance against T-cell destruction, B7-H1+ cancer cells also become refractory to apoptosis induced by Fas ligation or the protein kinase inhibitor Staurosporine. Our study reveals a new mechanism by which cancer cells use a receptor on immune cells as a ligand to induce resistance to therapy.

Taming cancer by inducing immunity via dendritic cells

Immunotherapy seeks to mobilize a patient's immune system for therapeutic benefit. It can be passive, i.e. transfer of immune effector cells (T cells) or proteins (antibodies), or active, i.e. vaccination. In cancer, passive immunotherapy can lead to some objective clinical responses, thus demonstrating that the immune system can reject tumors. However, passive immunotherapy is not expected to yield long-lived memory T cells that might control tumor outgrowth. Active immunotherapy with dendritic cell (DC)-based vaccines has the potential to induce both tumor-specific effector and memory T cells. Early clinical trials testing vaccination with ex vivo-generated DCs pulsed with tumor antigens provide a proof-of-principle that therapeutic immunity can be elicited. Yet, there is a need to improve their efficacy. The next generation of DC vaccines is expected to generate large numbers of high-avidity effector CD8(+) T cells and to overcome regulatory T cells. Therapeutic vaccination protocols will combine improved ex vivo DC vaccines with therapies that offset the suppressive environment established by tumors.

PD-1 and its ligands in tolerance and immunity

Programmed death 1 (PD-1) and its ligands, PD-L1 and PD-L2, deliver inhibitory signals that regulate the balance between T cell activation, tolerance, and immunopathology. Immune responses to foreign and self-antigens require specific and balanced responses to clear pathogens and tumors and yet maintain tolerance. Induction and maintenance of T cell tolerance requires PD-1, and its ligand PD-L1 on nonhematopoietic cells can limit effector T cell responses and protect tissues from immune-mediated tissue damage. The PD-1:PD-L pathway also has been usurped by microorganisms and tumors to attenuate antimicrobial or tumor immunity and facilitate chronic infection and tumor survival. The identification of B7-1 as an additional binding partner for PD-L1, together with the discovery of an inhibitory bidirectional interaction between PD-L1 and B7-1, reveals new ways the B7:CD28 family regulates T cell activation and tolerance. In this review, we discuss current understanding of the immunoregulatory functions of PD-1 and its ligands and their therapeutic potential.

CD86 has sustained costimulatory effects on CD8 T cells

CD80 and CD86 both costimulate T cell activation. Their individual effects in vivo are difficult to study as they are coordinately up-regulated on APCs. We have studied mice expressing rat insulin promoter (RIP)-CD80 and RIP-CD86 on the NOD and NOD.scid genetic background to generate in vivo models, using diabetes as a readout for cytotoxic T cell activation. Accelerated spontaneous diabetes onset was observed in NOD-RIP-CD80 mice and the transfer of diabetes from 6-wk-old NOD mice to NOD.scid-RIP-CD80 mice was greater compared with NOD-RIP-CD86 and NOD.scid-RIP-CD86 mice, respectively. However, the secondary in vivo response was maintained if T cells were activated through CD86 costimulation compared with CD80. This was demonstrated by greater ability to cause recurrent diabetes in NOD-RIP-CD86 diabetic mice transplanted with 6-wk-old NOD islets and adoptively transferred diabetes from diabetic NOD-RIP-CD86 mice to NOD.scid mice. In vitro, CD80 costimulation enhanced cytotoxicity, proliferation, and cytokine secretion in activated CD8 T cells compared with CD86 costimulation. We demonstrated increased CTLA-4 and programmed death-1 inhibitory molecule expression following costimulation by both CD80 and CD86 (CD80 > CD86). Furthermore, T cells stimulated by CD80 were more susceptible to inhibition by CD4(+)CD25(+) T cells. Overall, while CD86 does not stimulate an initial response as strongly as CD80, there is greater sustained activity that is seen even in the absence of continued costimulation. These functions have implications for the engineered use of costimulatory molecules in altering immune responses in a therapeutic setting.

T cell-encoded CD80 and 4-1BBL induce auto- and transcostimulation, resulting in potent tumor rejection

To reject tumors, T cells must overcome poor tumor immunogenicity and an adverse tumor microenvironment. Providing agonistic costimulatory signals to tumor-infiltrating T cells to augment T cell function remains a challenge for the implementation of safe and effective immunotherapy. We hypothesized that T cells overexpressing selected costimulatory ligands could serve as cellular vehicles mediating powerful, yet constrained, anatomically targeted costimulation. Here, we show that primary human T cells expressing CD80 and 4-1BB ligand (4-1BBL) vigorously respond to tumor cells lacking costimulatory ligands and provoke potent rejection of large, systemic tumors in immunodeficient mice. In addition to showing costimulation of bystander T cells (transcostimulation), we show the effect of CD80 and 4-1BBL binding to their respective receptors in the immunological synapse of isolated single cells (autocostimulation). This new strategy of endowing T cells with constitutively expressed costimulatory ligands could be extended to other ligand-receptor pairs and used to enhance any targeted adoptive transfer therapy.

Programmed death-1 concentration at the immunological synapse is determined by ligand affinity and availability

Despite the importance of programmed death-1 (PD-1) for T cell inhibition, little is known about its intracellular trafficking or requirements for localization to the immunological synapse. Here, we show that in activated T cells, PD-1 is present at the plasma membrane, near the Golgi and in the trans-Golgi network. Unlike CD28 and CTLA-4, PD-1 accumulation at the synapse is extensive only when T cells interact with dendritic cells (DCs) expressing high B7-DC levels. However, B7-H1 is also critically important, especially when the DCs have little B7-DC. Despite this preference, B7-H1(-/-) DCs elicit greater cytokine secretion than B7-DC(-/-) DCs during T cell restimulation, possibly because they also express less B7-DC. PD-1 and CD28 have similar kinetics of synaptic accumulation, suggesting that the process involves T cell receptor-triggered cytoskeletal reorganization followed by ligand binding.

The sound of silence: modulating anergy in T lymphocytes

Understanding the intercellular and intracellular mechanisms that maintain anergy and prevent the induction of full effector function is one avenue that may allow us to manipulate immune responses. Recent studies of T cell receptor (TCR)-proximal signaling events in different models of T cell unresponsiveness have suggested that biochemically distinct forms of anergy may exist in vivo. T cell responsiveness can be altered through the control of the intracellular pool of key second messengers, such as diacylglycerol (DAG) or the lipid modification of signaling molecules, such as the Linker for activated T cells (LAT). Studies on the molecule programmed death-1 (PD-1) and its ligands have revealed that tissue-resident signals are essential in the maintenance of T cell unresponsiveness. Thus, the emerging view is that T cell anergy is a dynamic state whose establishment and maintenance can be influenced by numerous different signaling pathways.

Uncovering the immune tumor microenvironment in non-small cell lung cancer to understand response rates to checkpoint blockade and radiation

The study of immunology has led to breakthroughs in treating non-small cell lung cancer (NSCLC). The recent approval of an anti-PD1 checkpoint drug for NSCLC has generated much interest in novel combination therapies that might provide further benefit for patients. However, a better understanding of which combinations may (or may not) work in NSCLC requires understanding the lung immune microenvironment under homeostatic conditions and the changes in that microenvironment in the setting of cancer progression and with radiotherapy. This review provides background information on immune cells found in the lung and the prognostic significance of these cell types in lung cancer. It also addresses current clinical directions for the combination of checkpoint inhibitors with radiation for NSCLC.