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

Receptors and ligands implicated in human T cell costimulatory processes

It is well established that full activation of T cells that recognize antigens requires additional signals. These second signals are generated by the interaction of costimulatory ligands expressed on antigen presenting cells with their receptors on T cells. In addition, T cell activation processes are negatively regulated by inhibitory costimulatory pathways. Interaction of members of the B7 and the TNF superfamilies with members of the CD28 and TNF-R-superfamilies plays major roles in costimulatory processes. However, a large number of molecules that do not belong to these families have been reported to be involved in the generation of T cell costimulatory signals. In addition to well-defined costimulatory pathways, where both receptors and ligands are known, there are many T cell surface molecules that have been described to generate a second signal under certain experimental conditions, f.i. when ligated with antibodies. Furthermore there are several ligands that have been shown to positively or negatively modulate T cell activation by interacting with as of yet unknown T cell receptors. Here we give a comprehensive overview of molecules that have been implicated in human T cell activation processes and propose criteria that define genuine T cell costimulatory pathways.

Programmed Death-1 antibody blocks therapeutic effects of T-regulatory cells in cockroach antigen-induced allergic asthma

We recently reported that the adoptive transfer of T-regulatory cells (Tregs) isolated from lung and spleen tissue of green fluorescent protein-transgenic mice reversed airway hyperresponsiveness and airway inflammation. Because Programmed Death-1 (PD-1) is a pivotal receptor regulating effector T-cell activation by Tregs, we evaluated whether PD-1 is involved in the therapeutic effect of naturally occurring Tregs (NTregs) and inducible Tregs (iTregs) in cockroach (CRA)-sensitized and challenged mice. The CD4(+)CD25(+) NTregs and CD4(+)CD25(-) iTregs isolated from the lungs and spleens of BALB/c mice were adoptively transferred into CRA-sensitized and CRA-challenged mice with and without anti-PD-1 antibody (100 μg/mice). The CD4(+)CD25(+) T cells in the lung were phenotyped after adoptive transfer. Concentrations of IL-4, IL-5, IL-10, IFN-γ, and IL-13 in bronchoalveolar lavage fluid (BALF) were measured using ELISA. The NTregs and iTregs from either lung or spleen tissue reversed airway hyperresponsiveness for at least 4 wk. However, the therapeutic effect was blocked by administering the anti-PD-1 antibody. The administration of Tregs-recipient mice with anti-PD-1 antibody significantly decreased cytotoxic T-lymphocyte antigen-4 expression, with low concentrations of Forkhead-winged transcriptional factor box 3 (Foxp3) mRNA transcripts in lung CD4(+)CD25(+) T cells. These mice had substantially higher concentrations of BALF IL-4, IL-5, and IL-13, but significantly decreased levels of BALF IL-10. Adoptive therapy recipients without the anti-PD-1 antibody exhibited high levels of CTLA-4 expression and Foxp3 transcripts in lung CD4(+)CD25(+) T cells, with a significant decrease in BALF IL-4, IL-5, and IL-13 concentrations and a substantial increase in BALF IL-10 concentrations. These data suggest that the reversal of airway hyperresponsiveness and airway inflammation by Tregs is mediated in part by PD-1, because other costimulatory molecules (e.g., inducible costimulatory molecule [ICOS] or CTLA-4) have been shown to play a role in Treg-mediated suppression.

Kupffer cell suppression of CD8+ T cells in human hepatocellular carcinoma is mediated by B7-H1/programmed death-1 interactions

B7-H1 is a recently identified B7 family member that, along with one of its receptors, programmed death-1 (PD-1), has been involved in multiple immunopathologic scenarios. However, the nature of B7-H1 and PD-1 in human hepatocellular carcinoma (HCC) remains poorly defined. We investigated the expression and functional relevance of this pathway in patients with HCC. We showed that B7-H1 expression on Kupffer cells (KC) was increased in tumor tissues compared with surrounding nontumor liver tissues in patients with HCC and this correlated with poorer survival. Coculture of HCC cells with monocytes showed that tumor-associated interleukin-10 contributed to the induction of B7-H1 in the HCC environment. We further observed that the levels of PD-1(+)CD8(+) T cells were higher in tumor tissues than in nontumor tissues. B7-H1(+) KCs and PD-1(+) T cells were colocalized in the HCC stroma. PD-1(+)CD8(+) T cells had decreased proliferative ability and effector function as shown by reduced granule and cytokine expression compared with PD-1(-) T cells. Importantly, blocking KC B7-H1 interaction with PD-1(+)CD8(+) cells using neutralizing antibodies recovered effector T-cell function. Our data indicate that the B7-H1/PD-1 axis contributes to immune suppression in human HCC, with blockade of this pathway carrying important therapeutic implications.

Interactions between PD-1 and PD-L1 promote tolerance by blocking the TCR-induced stop signal

Programmed death 1 (PD-1) is an inhibitory molecule expressed on activated T cells; however, the biological context in which PD-1 controls T cell tolerance remains unclear. Using two-photon laser-scanning microscopy, we show here that unlike naive or activated islet antigen-specific T cells, tolerized islet antigen-specific T cells moved freely and did not swarm around antigen-bearing dendritic cells (DCs) in pancreatic lymph nodes. Inhibition of T cell antigen receptor (TCR)-driven stop signals depended on continued interactions between PD-1 and its ligand, PD-L1, as antibody blockade of PD-1 or PD-L1 resulted in lower T cell motility, enhanced T cell-DC contacts and caused autoimmune diabetes. Blockade of the immunomodulatory receptor CTLA-4 did not alter T cell motility or abrogate tolerance. Thus, PD-1-PD-L1 interactions maintain peripheral tolerance by mechanisms fundamentally distinct from those of CTLA-4.

B7-H3 is a potent inhibitor of human T-cell activation: No evidence for B7-H3 and TREML2 interaction

B7-H3 belongs to the B7 superfamily, a group of molecules that costimulate or down-modulate T-cell responses. Although it was shown that B7-H3 could inhibit T-cell responses, several studies - most of them performed in murine systems - found B7-H3 to act in a costimulatory manner. In this study, we have specifically addressed a potential functional dualism of human B7-H3 by assessing the effect of this molecule under varying experimental conditions as well as on different T-cell subsets. We show that B7-H3 does not costimulate human T cells. In the presence of strong activating signals, B7-H3 potently and consistently down-modulated human T-cell responses. This inhibitory effect was evident when analysing proliferation and cytokine production and affected naïve as well as pre-activated T cells. Furthermore, we demonstrate that B7-H3-T-cell interaction is characterised by an early suppression of IL-2 and that T-cell inhibition can be reverted by exogenous IL-2. Since the triggering receptor expressed on myeloid cells like transcript 2 (TREML2/TLT-2) has been recently described as costimulatory receptor of murine B7-H3 we have extensively analysed interaction of human B7-H3 with TREML2/TLT-2. In these experiments we found no evidence for such an interaction. Furthermore, our data do not point to a role for murine TREML2 as a receptor for murine B7-H3.

PD-1 signaling in primary T cells

SUMMARY

Programmed death-1 (PD-1) is a cell surface molecule that regulates the adaptive immune response. Engagement of PD-1 by its ligands PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. While a great deal is known concerning the biologic roles PD-1 plays in regulating the primary immune response and in T-cell exhaustion, comparatively little is known regarding how PD-1 ligation alters signaling pathways. PD-1 ligation is known to inhibit membrane-proximal T-cell signaling events, while ligation of the related inhibitory molecule cytotoxic T-lymphocyte antigen-4 appears to target more downstream signaling pathways. A major obstacle to an in-depth understanding of PD-1 signaling is the lack of physiologic models in which to study signal transduction. This review focuses on: (i) signaling pathways altered by PD-1 ligation, (ii) factors recruited upon PD-1 phosphorylation, and (iii) exploring the hypothesis that PD-1 ligation induces distinct signals during various stages of immune-cell differentiation. Lastly, we describe models to dissect the function of the PD-1 cytoplasmic tail using primary cells in the absence of agonist antibodies.

Sequence, structure, function, immunity: structural genomics of costimulation

SUMMARY

Costimulatory receptors and ligands trigger the signaling pathways that are responsible for modulating the strength, course, and duration of an immune response. High-resolution structures have provided invaluable mechanistic insights by defining the chemical and physical features underlying costimulatory receptor:ligand specificity, affinity, oligomeric state, and valency. Furthermore, these structures revealed general architectural features that are important for the integration of these interactions and their associated signaling pathways into overall cellular physiology. Recent technological advances in structural biology promise unprecedented opportunities for furthering our understanding of the structural features and mechanisms that govern costimulation. In this review, we highlight unique insights that have been revealed by structures of costimulatory molecules from the immunoglobulin and tumor necrosis factor superfamilies and describe a vision for future structural and mechanistic analysis of costimulation. This vision includes simple strategies for the selection of candidate molecules for structure determination and highlights the critical role of structure in the design of mutant costimulatory molecules for the generation of in vivo structure-function correlations in a mammalian model system. This integrated 'atoms-to-animals' paradigm provides a comprehensive approach for defining atomic and molecular mechanisms.

Blockade of CTLA-4 on both effector and regulatory T cell compartments contributes to the antitumor activity of anti-CTLA-4 antibodies

Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is a critical negative regulator of immune responses. Uniquely among known inhibitory receptors, its genetic ablation results in a fulminating and fatal lymphoproliferative disorder. This central regulatory role led to the development of antibodies designed to block CTLA-4 activity in vivo, aiming to enhance immune responses against cancer. Despite their preclinical efficacy and promising clinical activity against late stage metastatic melanoma, the critical cellular targets for their activity remains unclear. In particular, debate has focused on whether the effector T cell (T(eff)) or regulatory T cell (T reg cell) compartment is the primary target of antibody-mediated blockade. We developed a mouse expressing human instead of mouse CTLA-4, allowing us to evaluate the independent contributions of CTLA-4 blockade of each T cell compartment during cancer immunotherapy in an in vivo model of mouse melanoma. The data show that although blockade on effector cells significantly improves tumor protection, unicompartmental blockade on regulatory cells completely fails to enhance antitumor responses. However, concomitant blockade of both compartments leads to a synergistic effect and maximal antitumor activity. We conclude that the combination of direct enhancement of T(eff) cell function and concomitant inhibition of T reg cell activity through blockade of CTLA-4 on both cell types is essential for mediating the full therapeutic effects of anti-CTLA-4 antibodies during cancer immunotherapy.

Dominant human CD8 T cell clonotypes persist simultaneously as memory and effector cells in memory phase

The adaptive immune system plays a critical role in protection at the time of secondary infection. It does so through the rapid and robust reactivation of memory T cells which are maintained long-term, in a phenotypically heterogeneous state, following their primary encounter with Ag. Although most HLA-A*0201/influenza matrix protein(58-66)-specific CD8 T cells from healthy donors display characteristics typical of memory T cells, through our extensive phenotypic analysis we have further shown that up to 20% of these cells express neither the IL-7 receptor CD127 nor the costimulatory molecule CD28. In contrast to the majority of CD28(pos) cells, granzyme B and perforin were frequently expressed by the CD28(neg) cells, suggesting that they are effector cells. Indeed, these cells were able to kill target cells, in an Ag-specific manner, directly ex vivo. Thus, our findings demonstrate the remarkable long-term persistence in healthy humans of not only influenza-specific memory cells, but also of effector T cells. We further observed that granzyme B expression in influenza-specific CD8 T cells paralleled levels in the total CD8 T cell population, suggestive of Ag-nonspecific bystander activation. Sequencing of TCR alpha- and beta-chains showed that the TCR repertoire specific for this epitope was dominated by one, or a few, T cell clonotype per healthy donor. Moreover, our sequencing analysis revealed, for the first time in humans, that identical clonotypes can coexist as both memory and effector T cells, thereby supporting the principle of multipotent clonotypic differentiation.

Target-dependent B7-H1 regulation contributes to clearance of central nervous system infection and dampens morbidity

The neurotropic coronavirus JHM strain of mouse hepatitis virus persists in oligodendroglia despite the presence of virus-specific CD8 T cells. Expression of programmed death 1 (PD-1) and B7-H1 were studied during acute and persistent infection to examine whether this negative regulatory mechanism contributes to CNS viral persistence. The majority of CNS-infiltrating CD8 T cells expressed PD-1, with the highest levels on virus-specific CD8 T cells. Moreover, despite control of infectious virus, CD8 T cells within the CNS of persistently infected mice maintained high PD-1 expression. Analysis of virus-susceptible target cells in vivo revealed that B7-H1 expression was regulated in a cell type-dependent manner. Oligodendroglia and microglia up-regulated B7-H1 following infection; however, although B7-H1 expression on oligodendroglia was prominent and sustained, it was significantly reduced and transient on microglia. Infection of mice deficient in the IFN-gamma or IFN-alpha/beta receptor demonstrated that B7-H1 expression on oligodendroglia is predominantly regulated by IFN-gamma. Ab blockade of B7-H1 on oligodendroglia in vitro enhanced IFN-gamma secretion by virus-specific CD8 T cells. More efficient virus control within the CNS of B7-H1-deficient mice confirmed inhibition of CD8 T cell function in vivo. Nevertheless, the absence of B7-H1 significantly increased morbidity without altering demyelination. These data are the first to demonstrate glia cell type-dependent B7-H1 regulation in vivo, resulting in adverse effects on antiviral CD8 T cell function. However, the beneficial role of PD-1:B7-H1 interactions in limiting morbidity highlights the need to evaluate tissue-specific intervention strategies.

PD-1 and CTLA-4 inhibitory cosignaling pathways in HIV infection and the potential for therapeutic intervention

The balance between proinflammatory mechanisms and the dampening of excessive immune activation is critical for successful clearance of a pathogen without harm to the host. In particular, molecules of the B7:CD28 family play a critical role in regulating T cell activation and peripheral tolerance. Chronic pathogens like HIV, which is characterized by ongoing viral replication despite detectable virus-specific T cell responses, and cancer cells have exploited these pathways to attenuate Ag-specific T cell immunity. This review summarizes evidence that molecules of the B7:CD28 family, PD-1, CTLA-4, and their ligands, play an active and reversible role in virus-specific T cell exhaustion associated with HIV infection in humans and in the SIV model in macaques. We discuss the potential for immunotherapeutic interventions based on manipulation of these inhibitory networks, the promising data obtained with blockade of the PD-1 pathway in animal models, and the challenges to such therapies.

Immune therapy for cancer

Over the past decade, immune therapy has become a standard treatment for a variety of cancers. Monoclonal antibodies, immune adjuvants, and vaccines against oncogenic viruses are now well-established cancer therapies. Immune modulation is a principal element of supportive care for many high-dose chemotherapy regimens. In addition, immune activation is now appreciated as central to the therapeutic mechanism of bone marrow transplantation for hematologic malignancies. Advances in our understanding of the molecular interactions between tumors and the immune system have led to many novel investigational therapies and continue to inform efforts for devising more potent therapeutics. Novel approaches to immune-based cancer treatment strive to augment antitumor immune responses by expanding tumor-reactive T cells, providing exogenous immune-activating stimuli, and antagonizing regulatory pathways that induce immune tolerance. The future of immune therapy for cancer is likely to combine many of these approaches to generate more effective treatments.

Therapeutic vaccines in solid tumours: can they be harmful?

Rosenberg and coworkers caused an uproar in the immunotherapy community when they stated in a position paper that despite great advances in the field of tumour immunology, optimism about the clinical application of therapeutic vaccines lacked justification. They argued that such optimism was based more on surrogate end-points, which lacked robustness, than on proof of anti-tumour effects [Rosenberg SA, Yang JC, Restifo NP. Cancer immunotherapy: moving beyond current vaccines. Nat Med 2004; 10:909-15]. They pointed out that cancer vaccine trials in 440 patients, conducted at the NCI Surgery Branch, had an overall objective response rate of only 2.6%. This was comparable to the 4.0% response rate reported in 40 studies that involved 756 patients. They concluded that surrogate end-points with such low response rates lack robustness. The situation may be even more problematic. Results of a number of larger trials in the past few years have indicated that vaccination therapy can also have a detrimental effect and be associated with worse outcome. These findings need to be looked at seriously and should lead to a critical appraisal of how well we understand these outcomes, how vaccine trials should be designed, monitored and conducted and which opportunities should be considered to be implemented in vaccine trial designs to improve the rationale and chances for a positive outcome.

B7 costimulation is critical for host control of chronic Mycobacterium tuberculosis infection

Although much is understood regarding the role of B7/CD28 family of costimulatory molecules in regulating host resistance in the context of several pathogens, analogous information with Mycobacterium tuberculosis is lacking. To address the requirements of B7-mediated costimulation in host resistance against tuberculosis, mice deficient in both B7.1 and B7.2 (B7DKO) were aerosol infected with M. tuberculosis Erdman and disease progression was monitored. We report herein that B7DKO mice are initially able to contain the bacterial load in the lung, but exhibit enhanced susceptibility during chronic infection. Despite the early control of bacterial replication, B7DKO mice essentially start off with compromised Th1 immunity and slower granulomatous response in the lung, characterized by markedly reduced lymphocytic infiltration. As the infection progresses from acute phase to the chronic phase, the nascent granulomas in the B7DKO lungs never fully achieve the architecture of granulomas developing in wild-type mice. Instead, lesions spread progressively to involve much of the lung in the B7DKO mice, ultimately leading to necrosis. Thus, early control of M. tuberculosis growth in the lung can occur in the absence of B7 costimulation and is less dependent on Th1 immunity and formation of a granulomatous structure. However, B7 costimulation is critical for long-term containment of infection within lung granulomas. These findings suggest that the use of costimulation-based immunomodulators may have significant repercussions on the induction of host protective immunity against tuberculosis.

Role of the immune modulator programmed cell death-1 during development and apoptosis of mouse retinal ganglion cells

PURPOSE

Mammalian programmed cell death (PD)-1 is a membrane-associated receptor regulating the balance between T-cell activation, tolerance, and immunopathology; however, its role in neurons has not yet been defined. The hypothesis that PD-1 signaling actively promotes retinal ganglion cell (RGC) death within the developing mouse retina was investigated.

METHODS

Mature retinal cell types expressing PD-1 were identified by immunofluorescence staining of vertical retina sections; developmental expression was localized by immunostaining and quantified by Western blot analysis. PD-1 involvement in developmental RGC survival was assessed in vitro using retinal explants and in vivo using PD-1 knockout mice. PD-1 ligand gene expression was detected by RT-PCR.

RESULTS

PD-1 is expressed in most adult RGCs and undergoes dynamic upregulation during the early postnatal window of retinal cell maturation and physiological programmed cell death (PCD). In vitro blockade of PD-1 signaling during this time selectively increases the survival of RGCs. Furthermore, PD-1-deficient mice show a selective increase in RGC number in the neonatal retina at the peak of developmental RGC death. Lastly, gene expression of the immune PD-1 ligand genes Pdcd1lg1 and Pdcd1lg2 was found throughout postnatal retina maturation.

CONCLUSIONS

These findings collectively support a novel role for a PD-1-mediated signaling pathway in developmental PCD during postnatal RGC maturation.

Negative regulators of T-cell activation: potential targets for therapeutic intervention in cancer, autoimmune disease, and persistent infections

The generation of productive adaptive immune responses depends on the antigen-specific activation of T and B cells. The outcome of T-cell receptor engagement is influenced by signals from both positive and negative regulatory molecules that can either activate or inhibit T-cell function. CD28 and cytotoxic T-lymphocyte antigen-4 are the prototypical members of an immunoglobulin domain-containing protein family that play important roles in the control of T-cell responses against infection, cancer, and in autoimmune disease. Although the precise molecular details of their functions are still under active investigation, tumors and chronic pathogens seem to have exploited these pathways to achieve immune evasion. Furthermore, malfunction of the inhibitory arm of the immune response appears responsible for the development of multiple autoimmune pathologies. As a result, the negative regulators of T-cell activation have become attractive targets for therapeutic intervention in cancer, chronic infection, and autoimmune disease. The application of findings from basic research has provided insight into the manipulation of these pathways in the clinic and offers promising strategies for the treatment of disease.

CD28/B7-mediated co-stimulation is critical for early control of murine cytomegalovirus infection

Control of acute murine cytomegalovirus (MCMV) infection is dependent upon both innate and adaptive immune responses, relying primarily upon natural killer (NK) and T-cell responses for control. Although CD28/B7 plays a clear role in T-cell responses in many antigen systems including some viral infections, the importance of co-stimulation during MCMV infection is unconfirmed. In addition, recent data suggest that CD28/B7 co-stimulation might also be important to Ly49H+ NK-cell expansion. We therefore hypothesized that CD28/B7 co-stimulation is critical to viral control after MCMV infection, and further that CD28/B7 co-stimulation plays a role in MCMV-specific T- and NK-cell responses. To test these hypotheses, we utilized C57BL/6 mice lacking the co-stimulatory molecules B7-1 and B7-2 or CD28. After primary infection with MCMV, viral titers are significantly elevated in mice lacking CD28 or B7 compared with wild-type mice. Impaired viral control is associated with significant defects in peripheral T-cell responses to MCMV, which appear to be dependent upon CD28/B7 co-stimulation. Abnormal hepatic T-cell responses in CD28(-/-) mice are preceded by impaired MCMV-specific Ly49H+ NK-cell responses. Cytokine evaluations confirm that CD28/B7 co-stimulation is not required for non-specific antiviral responses. We conclude that CD28-mediated co-stimulation is critical for early viral control during acute MCMV infection.

PD-1/PD-L blockade prevents anergy induction and enhances the anti-tumor activities of glycolipid-activated invariant NKT cells

Invariant NKT (iNKT) cells recognize glycolipid Ags, such as the marine sponge-derived glycosphingolipid alpha-galactosylceramide (alphaGalCer) presented by the CD1d protein. In vivo activation of iNKT cells with alphaGalCer results in robust cytokine production, followed by the acquisition of an anergic phenotype. Here we have investigated mechanisms responsible for the establishment of alphaGalCer-induced iNKT cell anergy. We found that alphaGalCer-activated iNKT cells rapidly up-regulated expression of the inhibitory costimulatory receptor programmed death (PD)-1 at their cell surface, and this increased expression was retained for at least one month. Blockade of the interaction between PD-1 and its ligands, PD-L1 and PD-L2, at the time of alphaGalCer treatment prevented the induction iNKT cell anergy, but was unable to reverse established iNKT cell anergy. Consistently, injection of alphaGalCer into PD-1-deficient mice failed to induce iNKT cell anergy. However, blockade of the PD-1/PD-L pathway failed to prevent bacterial- or sulfatide-induced iNKT cell anergy, suggesting additional mechanisms of iNKT cell tolerance. Finally, we showed that blockade of PD-1/PD-L interactions enhanced the antimetastatic activities of alphaGalCer. Collectively, our findings reveal a critical role for the PD-1/PD-L costimulatory pathway in the alphaGalCer-mediated induction of iNKT cell anergy that can be targeted for the development of immunotherapies.

The immune response to tumors

The immune response to tumors is complex. Cells of the immune system can inhibit tumor growth and progression through the recognition and rejection of malignant cells, a process referred to as immunoediting. Yet, immune responses can also promote tumor cell growth, survival, and angiogenesis through the induction of oncogenic inflammation. Immunodeficiency can predispose to the development of spontaneous and virally induced cancer, and established tumors often generate immunosuppressive microenvironments that can block productive antitumor immunity, serving as a substantial barrier to effective immune therapy. Through a deeper understanding of the complicated relationship between tumors and the immune system, tumor immunology strives to harness the immune system to generate protective antitumor responses in patients.

CD28 co-stimulation down regulates Th17 development

Th17 cells are implicated in host defence and autoimmune diseases. CD28/B7 co-stimulation is involved in the induction and progression of autoimmune diseases, but its role in controlling murine Th17 cell fate remains to be clarified. We here report that soluble anti-CD28 mAb suppressed the differentiation of anti-CD3-stimulated naïve CD4⁺ T cells into IL-17-producing cells. CD28 co-stimulation reduced the frequency of proliferating cells that produce IL-17. We provide evidence for an IL-2 and IFN-γ-dependent mechanism of CD28-mediated IL-17 suppression. CD28 blockade of Th17 development was correlated with a decrease rather than an increase in the percentage of Foxp3⁺ T cells. In APC/T cell co-cultures, mature dendritic cells (DC) were less efficient than immature DC in their ability to support Th17 cell differentiation, while CTLA4-Ig, an agent blocking CD28/B7 and CTLA4/B7 interactions, facilitated both murine and human Th17 differentiation. This study identifies the importance of B7 co-stimulatory molecules in the negative regulation of Th17 development. These unexpected results caution targeting the CD28/B7 pathways in the treatment of human autoimmune diseases.

Intestinal tolerance is converted to autoimmune enteritis upon PD-1 ligand blockade

The B7 family member programmed death-1 ligand (PD-L1) has been shown to play an inhibitory role in the regulation of T cell responses in several organs. However, the role of PD-L1 in regulating tolerance to self-Ags of the small intestine has not been previously addressed. In this study, we investigated the role of PD-L1 in CD8(+) T cell tolerance to an intestinal epithelium-specific Ag using the iFABP-tOVA transgenic mouse model, in which OVA is expressed as a self-Ag throughout the small intestine. Using adoptive transfer of naive OVA-specific CD8(+) T cells, we show that loss of PD-1:PD-L1 signaling, by either Ab-mediated PD-L1 blockade or transfer of PD-1(-/-) T cells, leads to considerable expansion of OVA-specific CD8(+) T cells and their differentiation into effector cells capable of producing proinflammatory cytokines. A fatal CD8(+) T cell-mediated inflammatory response develops rapidly against the small bowel causing destruction of the epithelial barrier, severe blunting of intestinal villi, and recruitment and activation of myeloid cells. This response is highly specific because immune destruction selectively targets the small intestine but not other organs. Collectively, these results indicate that loss of the PD-1:PD-L1 inhibitory pathway breaks CD8(+) T cell tolerance to intestinal self-Ag, thus leading to severe enteric autoimmunity.

The diversity of costimulatory and inhibitory receptor pathways and the regulation of antiviral T cell responses

T cell responses are regulated by integrating positive and negative signals from costimulatory and inhibitory receptors. While the function of specific T cell costimulatory molecules during infections has been appreciated for some time, recent observations have now revealed a crucial role for inhibitory receptors in regulating T cell responses to pathogens, especially during chronic infections. A key emerging principle is that there is considerable diversity in the number and type of inhibitory receptors that can be expressed by T cells during both acute and chronic infections. These distinct inhibitory pathways appear to cooperate in regulating T cell function, could have distinct mechanisms of action, and are likely to provide novel therapeutic targets during persisting infections and other diseases.

B7-1 induces immunosuppression when expressed in cultured neonatal mice keratinocytes

BACKGROUND

Chimeric (allo-auto or even xeno-auto) cultured keratinocyte grafting did not exhibit obvious acute rejection or chronic rejection. Although cultured murine keratinocytes were recognized by allogenic CD8+ T cells, they were not rejected. The precise mechanisms underlying this process were unclear. To analyze how keratinocytes attenuated the immune response, we investigated the effect of culturing on neonatal murine keratinocytes and their immunomodulatory properties.

METHODS

Keratinocytes isolated and purified from BALB/c and C57BL/6 neonatal mice were cultured for 7 days. The expression of B7-1, B7-2, B7-H1 and MHC-I was examined by semi-quantitative RT polymerase chain reaction (PCR), fluorescence microscopy and flow cytometry. Cytotoxicity and mixed lymphocyte response (MLR) assays were performed to determine the effects of keratinocytes on cytotoxic T-lymphocyte (CTL) mediated cell lysis and lymphocyte proliferation.

RESULTS

B7-1 was highly expressed in cultured, proliferating murine keratinocytes while no expression of B7-2 and B7-H1 was found. Keratinocytes that expressed B7-1 decreased CTL-mediated cell lysis by an interaction between B7-1 and CTLA-4. In addition, autologous keratinocytes but not allogeneic keratinocytes significantly suppressed auto-specific lymphocyte proliferation in a dose-dependent manner. The modulation was dependent on B7-1 expression and its interaction with CTLA-4.

CONCLUSIONS

Cultured murine keratinocytes expressed B7-1, but not B7-2 or B7-H1. The keratinocytes attenuated CTL-mediated lysis and suppressed lymphocyte proliferation via an interaction with B7-1 and CTLA-4. Therefore, separate expression of B7-1 induced immunosuppression. Non-professional APCs (antigen presenting cells) which separately express B7-1 may possess an ability to induce immunotolerance and thus act as a regulatory APC.

Cytokine treatment of macrophage suppression of T cell activation

High Mphi:T cell ratios suppress the immune response to the retroviral superantigen Mls by IFNgamma-triggered production of the arg- and trp-consuming enzymes iNOS and IDO. Attempts to reverse suppression by treatment with pro-inflammatory cytokines revealed that IL-6 improved the T cell response to Mls and the pro-hematopoietic cyokines IL-3 and GM-CSF increased suppression. GM-CSF treatment increased Mphi expression of CD80, a ligand for the immune suppressive B7H1 and CTLA-4 receptors. These results illustrate potential strategies for reversing the suppression of cell-mediated immunity characteristic of the high Mphi:T cell ratios found in many tumors.

Cross-linking of CD80 on CD4+ T cells activates a calcium-dependent signaling pathway

CD80 expressed on the surface of APCs provides a positive costimulatory signal to naive CD4+ T cells via CD28 during activation. However, CD80 is also expressed on the surface of activated CD4+ T cells, and cross-linking CD80 on the surface of CD4+ T cells activated in the presence of Th1-promoting cytokines induces a direct up-regulation of T-bet, IFN-gamma, and Bcl(XL) expression in primary CD4+ T cells. The present data show that naive CD4+ T cells activated in Th1-promoting conditions in the presence of anti-CD80 mAb increase the level of IFN-gamma produced by increasing the rate of IFN-gamma mRNA transcription, which is supported by an increase in the level of T-bet phosphorylation and T-bet binding to the third intronic enhancer in the IFN-gamma locus. Furthermore, anti-CD80 mAb-induced increase in IFN-gamma expression and T-bet phosphorylation is dependent upon the activation of a Ca2+-dependent pathway as shown by anti-CD80 mAb-induced intracellular Ca2+ flux following CD80 cross-linking. These findings indicate a novel regulatory role for CD80-mediated intracellular signals in CD4+ T cells and have important implications for disease therapies using anti-costimulatory mAbs as use of an intact CD80 mAb may lead to CD80 cross-linking on activated T cells and enhanced proinflammatory cytokine production.

Cancer immunotherapy: co-stimulatory agonists and co-inhibitory antagonists

The generation and maintenance of immune responses are controlled by both co-stimulatory and co-inhibitory signalling through T cell co-receptors, many of which belong to the immunoglobulin-like superfamily or the tumour necrosis factor receptor superfamily. Agonistic or antagonistic monoclonal antibodies targeting these co-receptors have the potential to enhance immunity. Furthermore, their activity on the immunosuppressive regulatory T cell populations which are prevalent within many tumours provides an additional rationale for their use as anti-cancer therapies. This review summarizes the interactions between cancer and the immune system, highlighting the ways in which these new classes of immunostimulatory antibodies might enhance anti-tumour immunity and summarizing early clinical experience with their use.

B7-H1 (PD-L1) on T cells is required for T-cell-mediated conditioning of dendritic cell maturation

Studies have shown that T-cell-dendritic cell (DC) interaction is required for efficient DC maturation. However, the identities of the molecules that mediate the interaction in vivo are largely unknown. Here, we show that maturation of DCs as well as CD8 T-cell responses were impaired in B7-H1-deficient (B7-H1(-/-)) mice to influenza virus infection. Both defects were restored by transferring B7-H1-expressing naïve T cells into B7-H1(-/-) mice. Similarly, transferring DCs from wild-type mice or from RAG1(-/-) mice that had been injected with B7-H1-expressing naïve T cells also restored CD8 T-cell responses in B7-H1(-/-) mice. These results demonstrate that B7-H1 on naïve T cells is required to condition immature DCs to undergo efficient maturation when they encounter microbial infection. In return, the mature DCs stimulate a robust T-cell response against the infecting pathogen.

Combined TLR2 and TLR4 ligation in the context of bacterial or helminth extracts in human monocyte derived dendritic cells: molecular correlates for Th1/Th2 polarization

BACKGROUND

Recognition of pathogens by dendritic cells (DCs) through interaction with pattern recognition receptors, including Toll like receptors (TLR), is crucial for the initiation of appropriate polarized T helper (Th) cell responses. Yet, the characteristics and differences in molecular profiles of DCs with different T cell polarizing capacities are still poorly defined. To address this issue, the molecular profile of human monocyte derived DCs was characterized after exposure to TLR4 ligand LPS in combination with the Th1 promoting bacterial extracts from Listeria monocytogenes and Escherichia coli or the Th2 promoting helminth derived phospholipids from Schistosoma mansoni and Ascaris lumbricoides, all with TLR2 activating capacity.

RESULTS

With regard to the signalling pathways activated upon exposure to LPS and the TLR2 activating compounds, we find that the ratio of activated Mitogen Activated Protein Kinases (MAPK) p-ERK/p-p38 is lower in DCs stimulated with the bacterial products compared to DCs stimulated with the helminth products, which correlates with the Th1 and Th2 polarizing capacity of these compounds. Furthermore, analysis of the mRNA expression levels of a set of 25 carefully selected genes potentially involved in modulation of T cell polarization revealed that the mRNA expression of notch ligand delta-4 and transcription factor c-fos are differentially regulated and show a strong correlation with Th1 and Th2 polarization, respectively.

CONCLUSION

This study shows that combined TLR2 and TLR4 activation in the context of different antigen sources can induce very distinct molecular profiles in DCs and suggests that the Th1/Th2 polarizing capacity of compounds can be predicted with the molecular signature they induce in DCs.

CTLA-4 on alloreactive CD4 T cells interacts with recipient CD80/86 to promote tolerance

Although the inhibitory receptor CTLA-4 (CD152) has been implicated in peripheral CD4 T-cell tolerance, its mechanism of action remains poorly defined. We analyzed mechanisms of CD4 cell tolerance in a model of tolerance induction involving establishment of mixed hematopoietic chimerism in recipients of fully MHC-mismatched allogeneic bone marrow cells with anti-CD154 mAb. Animals lacking CD80 and CD86 failed to achieve chimerism. We detected no T cell-intrinsic requirement for CD28 for chimerism induction. However, a CD4 T cell-intrinsic signal through CTLA-4 was shown to be essential within the first 48 hours of exposure to alloantigen for the establishment of tolerance and mixed chimerism. This signal must be provided by a recipient CD80/86(+) non-T-cell population. Donor CD80/86 expression was insufficient to achieve tolerance. Together, our findings demonstrate a surprising role for interactions of CTLA-4 expressed by alloreactive peripheral CD4 T cells with CD80/86 on recipient antigen-presenting cells (APCs) in the induction of early tolerance, suggesting a 3-cell tolerance model involving directly alloreactive CD4 cells, donor antigen-expressing bone marrow cells, and recipient antigen-presenting cells. This tolerance is independent of regulatory T cells and culminates in the deletion of directly alloreactive CD4 T cells.

Fas expression on antigen-specific T cells has costimulatory, helper, and down-regulatory functions in vivo for cytotoxic T cell responses but not for T cell-dependent B cell responses

Fas-mediated apoptosis is an important contributor to contraction of Ag-driven T cell responses acting only on activated Ag-specific T cells. The effects of targeted Fas deletion on selected cell populations are well described however little is known regarding the consequences of Fas deletion on only activated Ag-specific T cells. We addressed this question using the parent-into-F(1) (P-->F(1)) model of acute or chronic (lupus-like) graft-vs-host disease (GVHD) as a model of either a CTL-mediated or T-dependent B cell-mediated response, respectively. By transferring Fas-deficient lpr donor T cells into Fas-intact F(1) hosts, the in vivo role of Ag-specific T cell Fas can be determined. Our results demonstrate a novel dichotomy of Ag-specific T cell Fas function in that: 1) Fas expression on Ag-activated T cells has costimulatory, helper, and down-regulatory roles in vivo and 2) these roles were observed only in a CTL response (acute GVHD) and not in a T-dependent B cell response (chronic GVHD). Specifically, CD4 T cell Fas expression is important for optimal CD4 initial expansion and absolutely required for help for CD8 effector CTL. Donor CD8 T cell Fas expression played an important but not exclusive role in apoptosis and down-regulation. By contrast, CD4 Fas expression played no detectable role in modulating chronic GVHD induction or disease expression. These results demonstrate a novel role for Ag-specific T cell Fas expression in in vivo CTL responses and support a review of the paradigm by which Fas deficiency accelerates lupus in MRL/lpr lupus-prone mice.

T-cell costimulation and coinhibition in atherosclerosis

Evidence from many human and rodent studies has established that T lymphocytes enhance inflammation in atherosclerotic plaques and contribute to lesion progression and remodeling. Recent work also indicates that regulatory T cells are important in limiting proatherogenic T-cell responses. Given the important role of T cells in atherosclerosis, there is a need to fully understand how proatherogenic T cells are activated and regulated. Antigen-dependent activation of naïve T cells, leading to clonal expansion and effector T-cell differentiation, and effector and memory T cells, is enhanced by signals provided by costimulatory molecules expressed by antigen presenting cells, which bind to receptors on the T cells. In addition, T-cell responses to antigen are negatively regulated by coinhibitory molecules expressed by antigen-presenting cells, which bind to receptors on T cells. Two major families of costimulatory molecules include the B7 and the tumor necrosis factor (TNF) families. These molecules bind to receptors on T cells belonging to the CD28 or TNF receptor families, respectively. The best-defined coinhibitors and their receptors belong to the B7 and CD28 families. Recent work has begun to define how these T-cell costimulatory and coinhibitory pathways influence atherosclerosis, largely in mouse models of the disease. Profound effects are attributable to molecules in both the B7/CD28 (B7-1/2, ICOS, and PDL-1/2) and the TNF/TNF receptor (CD40, OX40, and CD137) families. One emerging theme is that both pathogenic effector T-cell responses and regulatory T cells are influenced by overlapping sets of costimulators and coinhibitors. These complexities must be considered as immunotherapeutic approaches for atherosclerotic disease are developed.

PD-1 ligands expressed on myeloid-derived APC in the CNS regulate T-cell responses in EAE

Disease progression in experimental autoimmune encephalomyelitis (EAE) is regulated by programmed death receptor 1 (PD-1) and its ligands, B7-H1 (programmed death ligand 1 (PD-L1)) and B7-DC (PD-L2). B7-H1 and B7-DC have negative regulatory effects upon binding PD-1 on activated T cells and B7-H1 deficiency increases severity of both diabetes and EAE. However, the role of PD-L expression on different APC in the CNS in regulating local T-cell function during relapsing EAE has not been examined. Our data show that the majority of CNS CD4+ T cells isolated during acute EAE are PD-1+, and T cells specific for relapse-associated epitopes express PD-1 upon antigen stimulation in the CNS. B7-H1 and B7-DC are differentially expressed on discrete APC populations in the inflamed CNS. B7-H1 and PD-1 have mainly inhibitory functions on CNS T cells. B7-H1 negatively regulates the stimulation of activated PD-1+ T(H) cells, in co-cultures with microglia and different CNS-infiltrating APC presenting endogenously processed peptides. The preponderance of IFN-gamma+ versus IL-17+ T cells in the CNS of B7-H1(-/-) mice suggests that B7-H1 more selectively suppresses T(H)-1 than T(H)-17 responses in vivo. In contrast, blockade of B7-DC has less pronounced regulatory effects. Overall, the results demonstrate that B7-H1 expressed by CNS myeloid APC negatively regulates T-cell activation during acute relapsing EAE.

IL-10 and PD-L1 operate through distinct pathways to suppress T-cell activity during persistent viral infection

Suppression of T-cell responses by host-derived regulatory factors is a key event leading to viral persistence. Antibody blockade of either IL-10 or programmed death-ligand 1 (PD-L1) during viral persistence enhances T-cell function and reduces viral titers. Because blockade of these immunoregulatory networks represents a powerful approach to establish immune control during persistent infection, it is important to determine whether these immunoinhibitory factors act independently or jointly and if combined blockade of these factors further enhances T-cell immunity and viral clearance. Herein, we demonstrate that the IL-10 and PD-L1 immunosuppressive pathways are mechanistically distinct. As a result, simultaneous blockade of IL-10 and PD-L1 was significantly more effective in restoring antiviral T-cell responses than blockade of either alone, and led to substantially enhanced control of an established persistent viral infection. Thus, combinatorial blockade of multiple immune-regulatory molecules may ultimately restore the T-cell responses required to tip the balance from viral persistence to immune-mediated control or elimination of persistent infection.

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.