Signaling lymphocyte activation molecule (SLAM), a glycoprotein expressed on activated lymphocytes and antigen-presenting cells, has been shown to be a coregulator of antigen-driven T cell responses and is one of the two receptors for measles virus. Here we show that T cell receptor-induced interleukin (IL)-4 secretion by SLAM(-/-) CD4(+) cells is down-regulated, whereas interferon gamma production by CD4(+) T cells is only slightly up-regulated. Although SLAM controls production of IL-12, tumor necrosis factor, and nitric oxide in response to lipopolysaccharide (LPS) by macrophages, SLAM does not regulate phagocytosis and responses to peptidoglycan or CpG. Thus, SLAM acts as a coreceptor that regulates signals transduced by the major LPS receptor Toll-like receptor 4 on the surface of mouse macrophages. A defective macrophage function resulted in an inability of SLAM(-/-) C57Bl/6 mice to remove the parasite Leishmania major. We conclude that the coreceptor SLAM plays a central role at the interface of acquired and innate immune responses.
Helper T cell differentiation involves silencing as well as activation of gene expression. We have identified a conserved silencer of the gene encoding interleukin 4 (Il4) marked by DNase I hypersensitivity (HS IV) and permissive chromatin structure in all helper T cells. Deletion of HS IV increased Il4 and Il13 transcription by naive T cells and led to T helper type 2 skewing in vitro. HS IV controlled Il4 silencing during T helper type 1 differentiation, as HS IV-deficient T helper type 1 cells that expressed interferon-gamma also produced abundant interleukin 4 in vitro and in vivo. Despite mounting a vigorous interferon-gamma response, HS IV-deficient mice were more susceptible to Leishmania major infection than were wild-type littermate control mice, showing a critical function for Il4 silencing in T helper type 1-mediated immunity.
Kidney podocytes and their slit diaphragms form the final barrier to urinary protein loss. This explains why podocyte injury is typically associated with nephrotic syndrome. The present study uncovered an unanticipated novel role for costimulatory molecule B7-1 in podocytes as an inducible modifier of glomerular permselectivity. B7-1 in podocytes was found in genetic, drug-induced, immune-mediated, and bacterial toxin-induced experimental kidney diseases with nephrotic syndrome. The clinical significance of our results is underscored by the observation that podocyte expression of B7-1 correlated with the severity of human lupus nephritis. In vivo, exposure to low-dose LPS rapidly upregulates B7-1 in podocytes of WT and SCID mice, leading to nephrotic-range proteinuria. Mice lacking B7-1 are protected from LPS-induced nephrotic syndrome, suggesting a link between podocyte B7-1 expression and proteinuria. LPS signaling through toll-like receptor-4 reorganized the podocyte actin cytoskeleton in vitro, and activation of B7-1 in cultured podocytes led to reorganization of vital slit diaphragm proteins. In summary, upregulation of B7-1 in podocytes may contribute to the pathogenesis of proteinuria by disrupting the glomerular filter and provides a novel molecular target to tackle proteinuric kidney diseases. Our findings suggest a novel function for B7-1 in danger signaling by nonimmune cells.
Donor-specific transfusion (DST) can synergize with T cell co-stimulatory blockade in inducing tolerance in several transplant models, but the mechanism of action of DST is poorly characterized. This study used genetically altered mice in an established model of cardiac transplantation to study the role of MHC and co-stimulatory molecule expression on DST cells in mediating the immunomodulatory effects of DST. In addition, to examine the role of indirect antigen presentation in the effect of DST, experiments used recipient mice that do not express MHC class II molecules on peripheral antigen-presenting cells, but do have functional CD4(+) T cells (II(-)4(+)). As previously reported, treatment with DST from wild-type donors in combination with CD154 blockade induced tolerance in wild-type recipients of cardiac allografts. Tolerance in this model is also induced despite the absence of MHC class I and II, CD40, or B7 molecules on transfused cells. In contrast, eliminating the indirect pathway using II(-)4(+) recipients blocked the induction of long-term cardiac allograft survival by DST. These results indicate that the indirect antigen recognition pathway mediates the immunomodulatory effect of DST in inducing transplantation tolerance in vivo.
Both positive and negative regulatory roles have been suggested for the B7 family member PD-L1(B7-H1). PD-L1 is expressed on antigen-presenting cells (APCs), activated T cells, and a variety of tissues, but the functional significance of PD-L1 on each cell type is not yet clear. To dissect the functions of PD-L1 in vivo, we generated PD-L1-deficient (PD-L1(-/-)) mice. CD4(+) and CD8(+) T cell responses were markedly enhanced in PD-L1(-/-) mice compared with wild-type mice in vitro and in vivo. PD-L1(-/-) dendritic cells stimulated greater wild-type CD4(+) T cell responses than wild-type dendritic cells, and PD-L1(-/-) CD4(+) T cells produced more cytokines than wild-type CD4(+) T cells in vitro, demonstrating an inhibitory role for PD-L1 on APCs and T cells. In vivo CD8(+) T cell responses also were significantly enhanced, indicating that PD-L1 has a role in limiting the expansion or survival of CD8(+) T cells. Studies using the myelin oligodendrocyte model of experimental autoimmune encephalomyelitis showed that PD-L1 on T cells and in host tissues limits responses of self-reactive CD4(+) T cells in vivo. PD-L1 deficiency converted the 129S4/SvJae strain from a resistant to experimental autoimmune encephalomyelitis-susceptible strain. Transfer of encephalitogenic T cells from wild-type mice into PD-L1(-/-) recipients led to exacerbated disease. Disease was even more severe in PD-L1(-/-) recipients of PD-L1(-/-) T cells. These results demonstrate that PD-L1 on T cells, APCs, and host tissue inhibits naïve and effector T cell responses and plays a critical role in T cell tolerance.
In these studies, we examined the effects of OX40 ligand (OX40L) deficiency on the development of Th2 cells during the Th2 immune response to the intestinal nematode parasite Heligmosomoides polygyrus. Elevations in IL-4 production and total and Ag-specific serum IgE levels were partially inhibited during both the primary and memory immune responses to H. polygyrus in OX40L(-/-) mice. The host-protective memory response was compromised in OX40L(-/-) mice, as decreased worm expulsion and increased egg production were observed compared with H. polygyrus-inoculated OX40L(+/+) mice. To further examine the nature of the IL-4 defect during priming, adoptively transferred DO11.10 T cells were analyzed in the context of the H. polygyrus response. Although Ag-specific T cell IL-4 production was reduced in the OX40L(-/-) mice following immunization with OVA peptide plus H. polygyrus, Ag-specific T cell expansion, cell cycle progression, CXCR5 expression, and migration were comparable between OX40L(+/+) and OX40L(-/-) mice inoculated with OVA and H. polygyrus. These studies suggest an important role for OX40/OX40L interactions in specifically promoting IL-4 production, as well as associated IgE elevations, in Th2 responses to H. polygyrus. However, OX40L interactions were not required for serum IgG1 elevations, increases in germinal center formation, and Ag-specific Th2 cell expansion and migration to the B cell zone.
Interactions between CD8+ T cells and endothelial cells are important in both protective and pathologic immune responses. Endothelial cells regulate the recruitment of CD8+ T cells into tissues, and the activation of CD8+ T cells by antigen presentation and costimulatory signals. PD-L1 and PD-L2 are recently described B7-family molecules which bind to PD-1 on activated lymphocytes and down-regulate T cell activation. We found that PD-L1 is expressed on interferon-gamma stimulated cultured human and mouse endothelial cells, while PD-L2 was found on stimulated human but not mouse endothelial cells. Expression was further up-regulated by TNF-alpha. Antibody blockade of endothelial cell PD-L1 and PD-L2 enhanced endothelial cell costimulation of PHA-activated human CD8+ T cells. Antibody blockade of mouse endothelial cell PD-L1 enhanced both IFN-gamma secretion and cytolytic activity of CD8+ T cells in response to endothelial cell antigen presentation. These results show that IFN-gamma activated endothelial cells can inhibit T cell activation via expression of the immunoinhibitory PD-L1 and PD-L2 molecules. Endothelial expression of PD-ligands would allow activation and extravasation of T cells without excessive vessel damage. Our findings highlight a potentially important pathway by which endothelial cells down-regulate CD8+ T cell-mediated immune responses.
When antigen-presenting cells (APCs) encounter inflammatory stimuli, they up-regulate their expression of B7. A small amount of B7 is also constitutively expressed on resting APCs, but its function is unclear. Here we show that initiation of T cell responses requires the expression of B7 on immunizing APCs, but the responses are much greater in the absence of basal B7 expression. Transfer of antigen-specific CD4+CD25+ cells reverses the increased responsiveness, and tolerance to a self-protein is broken by immunization in the absence of basal B7, thereby inducing autoimmunity. Similar loss of self-tolerance is seen on depletion of CD25+ cells. Thus, constitutively expressed B7 costimulators function to suppress T cell activation and maintain self-tolerance, principally by sustaining a population of regulatory T cells.
OX40 (CD134) is expressed on activated T cells; its ligand, OX40 ligand (OX40L) is expressed on dendritic cells, B cells, and activated endothelial cells. To determine how OX40-OX40L interaction affects graft-versus-host disease (GVHD), we used antagonistic anti-OX40L monoclonal antibody (mAb) or OX40(-/-) donor or OX40L(-/-) recipient mice. Similar degrees of GVHD reduction were observed with each approach. Despite the fact that OX40 is up-regulated on both CD4(+) and CD8(+) T cells isolated during GVHD, the major effects of OX40 ligation were on CD4(+) and not CD8(+) T-cell-mediated alloresponses as assessed in both GVHD and engraftment model systems. GVHD inhibition by blockade of the OX40/OX40L pathway did not require CD28 signaling. Some studies have indicated OX40 is essential for inducing T-helper type 2 (Th2) responses. However, in vivo blockade of OX40-OX40L interactions reduced GVHD mortality induced by either signal transducer and activator of transcription-6(-/-) (Stat-6(-/-)) (Th2-defective) or Stat-4(-/-) (Th1-defective) major histocompatibility complex (MHC)-disparate splenocytes, indicating that the GVHD-ameliorating effects did not require Stat-4 or Stat-6 signaling. Although OX40L has been reported to be expressed on activated T cells, no effects on GVHD were observed when OX40L(-/-) versus OX40L(+/+) T cells were infused in different models. These data provide insights as to the mechanisms responsible for OX40/OX40L regulation of GVHD.
Adoptive transfer experiments using C57BL/6 mice lacking B7-1 and B7-2 as recipients of wt (wt) encephalitogenic T cells demonstrate a key role for B7 costimulation during the effector phase of experimental autoimmune encephalomyelitis (EAE). Following transfer of encephalitogenic T cells, B7-1/B7-2-deficient (-/-) recipients develop a transient and mild disease as compared to wt recipients. To understand the mechanism by which B7-1/B7-2 may influence the effector phase of EAE, we analyzed T cells, pro-inflammatory cytokines and chemokines within the CNS of wt and B7-1/B7-2-/- recipients at different times after adoptive transfer of activated myelin specific T cells. There was a marked decline in T cells and inflammatory mediators in the CNS of B7-1/B7-2-/- recipients by day 30 post transfer. B7-1/B7-2-/- mice developed more TUNEL+ apoptotic cells in the parenchyma and greater ratios of TUNEL+ cells/parenchymal foci than wt mice resulting in virtual disappearance of parenchymal foci. Therefore, without B7-1 and B7-2 costimulation in the target organ, there is increased T cell apoptosis and attenuation of inflammation. These results indicate that B7-1 and B7-2 provide critical costimulatory signals for sustaining survival of pathogenic T cells within the central nervous system parenchyma during the effector phase of EAE and suggest novel treatment approaches in the effector phase of autoimmune diseases.
Newer members of the B7-CD28 superfamily include the receptor PD-1 and its two ligands, PD-L1 and PD-L2. Here, we characterize the expression of PD-1, PD-L1, and PD-L2 in tissues of naive miceand in target organs from two models of autoimmunity, the pancreas from non-obese diabetic (NOD) mice and brain from mice with experimental autoimmune encephalomyelitis (EAE). In naive mice, proteiexpression of PD-1, PD-L1, and PD-L2 was detected in the thymus, while PD-1 and PD-L1 were detected in the spleen. PD-L1, but not PD-L2, was also detected at low levels on cardiac endothelium, pancreatic islets, and syncyciotrophoblasts in the placenta. In pre-diabetic NOD mice, PD-1 and PD-L1 were expressed on infiltrating cells in the pancreatic islets. Furthermore, PD-L1 was markedly up-regulated on islet cells. In brains from mice with EAE, PD-1, PD-L1, and PD-L2 were expressed on infiltrating inflammatory cells, and PD-L1 was up-regulated on endothelium within EAE brain. The distinct expression patterns of PD-L1 and PD-L2 led us to compare their transcriptional regulation in STAT4(-/-), STAT6(-/-), or NF-kappaB p50(-/-)p65(+/-) dendritic cells (DC).PD-L2, but not PD-L1, expression was dramatically reduced in p50(-/-)p65(+/-) DC. Thus, PD-L1 and PD-L2 exhibit distinct expression patterns and are differentially regulated on the transcriptional level.
Although costimulation plays an important role in activating naive T cells, its role in negative selection is controversial. By following thymocyte deletion induced by endogenous superantigens in mice lacking B7-1 and/or B7-2, we have identified a role for both B7-1 and B7-2 in negative selection. Studies using CD28-deficient and CD28/CTLA-4-double-deficient mice have revealed that either CD28 or another as yet undefined coreceptor can mediate these B7-dependent signals that promote negative selection. Finally, CTLA-4 delivers signals that inhibit selection, suggesting that CTLA-4 and CD28 have opposing functions in thymic development. Combined, the data demonstrate that B7-1/B7-2-dependent signals help shape the T cell repertoire.
Inducible costimulatory molecule (ICOS) plays a pivotal role in T cell activation and Th1/Th2 differentiation. ICOS blockade has disparate effects on immune responses depending on the timing of blockade. Its role in transplantation immunity, however, remains incompletely defined. We used a vascularized mouse cardiac allograft model to explore the role of ICOS signaling at different time points after transplantation, targeting immune initiation (early blockade) or the immune effector phase (delayed blockade). In major histocompatibility-mismatched recipients, ICOS blockade prolonged allograft survival using both protocols but did so more effectively in the delayed-treatment group. By contrast, in minor histocompatibility-mismatched recipients, early blockade accelerated rejection and delayed blockade prolonged graft survival. Alloreactive CD4+ T cell expansion and alloantibody production were suppressed in both treatment groups, whereas only delayed blockade resulted in suppression of effector CD8+ T cell generation. After delayed ICOS blockade, there was a diminished frequency of allospecific IL-10-producing cells and an increased frequency of both IFN-gamma- and IL-4-producing cells. The beneficial effects of ICOS blockade in regulating allograft rejection were seen in the absence of CD28 costimulation but required CD8+ cells, cytotoxic T lymphocyte antigen-4, and an intact signal transducer and activator of transcription-6 pathway. These data define the complex functions of the ICOS-B7h pathway in regulating alloimmune responses in vivo.
Calcineurin links calcium signaling to transcriptional responses in the immune, nervous and cardiovascular systems. To determine the function of the calcipressins, a family of putative calcineurin inhibitors, we assessed the calcineurin-dependent process of T cell activation in mice engineered to lack the gene encoding calcipressin 1 (Csp1). Csp1 regulated calcineurin in vivo, and genes triggered in an immune response had unique transactivation thresholds for T cell receptor stimulation. In the absence of Csp1, the apparent transactivation thresholds for all these genes were shifted because of enhanced calcineurin activity. This unbridled calcineurin activity drove Fas ligand expression, which normally requires high T cell receptor stimulation and results in the premature death of T helper type 1 cells. Thus, calcipressins modulate the pattern of calcineurin-dependent transcription, and may influence calcineurin activity beyond calcium to integrate a broad array of signals into the cellular response.
Asthma is caused by T-helper cell 2 (Th2)-driven immune responses, but the immunological mechanisms that protect against asthma development are poorly understood. T-cell tolerance, induced by respiratory exposure to allergen, can inhibit the development of airway hyperreactivity (AHR), a cardinal feature of asthma, and we show here that regulatory T (T(R)) cells can mediate this protective effect. Mature pulmonary dendritic cells in the bronchial lymph nodes of mice exposed to respiratory allergen induced the development of T(R) cells, in a process that required T-cell costimulation via the inducible costimulator (ICOS-ICOS-ligand pathway. The T(R) cells produced IL-10, and had potent inhibitory activity; when adoptively transferred into sensitized mice, T(R) cells blocked the development of AHR. Both the development and the inhibitory function of regulatory cells were dependent on the presence of IL-10 and on ICOS-ICOS-ligand interactions. These studies demonstrate that T(R) cells and the ICOS-ICOS-ligand signaling pathway are critically involved in respiratory tolerance and in downregulating pulmonary inflammation in asthma.
The B7-1/B7-2-CD28/CTLA-4 pathway is crucial in regulating T-cell activation and tolerance. New B7 and CD28 molecules have recently been discovered and new pathways have been delineated that seem to be important for regulating the responses of previously activated T cells. Several B7 homologues are expressed on cells other than professional antigen-presenting cells, indicating new mechanisms for regulating T-cell responses in peripheral tissues. Some B7 homologues have unknown receptors, indicating that other immunoregulatory pathways remain to be described. Here, we summarize our current understanding of the new members of the B7 and CD28 families, and discuss their therapeutic potential.
Engagement of CTLA-4 is critical for inhibiting T cell immune responses. Recent studies have shown that CTLA-4 plays a key role in regulating peripheral T cell tolerance. It has been suggested that one mechanism by which CTLA-4 performs this function is by regulating cell cycle progression. Here, we investigate in depth the role of CTLA-4 in regulating cell cycle progression in naive T cells by comparing the immune responses in the absence or presence of CTLA-4. In the absence of CLTA-4, T cells exhibit marked increases in T cell proliferation, IL-2 mRNA and protein secretion, and cells cycling in the S and G2-M phase. Analyses of cyclins, cyclin-dependent kinases, and cell cycle inhibitors involved in the transition from the G1 to S phase reveal that cell cycle progression is prolonged in the absence of CTLA-4. This is due to the early exit from the G1 phase, entry into the S phase, and prolonged S phase period. Re-expression of the cell cycle inhibitor p27(kip1) is delayed in the absence of CTLA-4. These studies demonstrate that the B7 : CTLA-4 pathway exerts its major effects on T cell immune responses via regulation of the cell cycle.
The CD28 family member inducible costimulator protein (ICOS) has an important role in T cell differentiation and Ig class switching. To investigate the role of ICOS in vivo, ICOS-/- mice were infected s.c. with Leishmania mexicana. While wild-type mice developed large, cutaneous lesions, the growth of lesions and tissue histopathology was significantly delayed in ICOS-/- mice. ICOS-/- mice exhibited marked decreases in both Th1 and Th2 cytokine production and profound defects in L. mexicana-specific Ig isotype class switching to IgG1 and IgG2a and reduced total IgE levels. Our findings indicate that ICOS is a key regulator of both Th1 and Th2 responses and has a role in controlling cutaneous L. mexicana infection.
B7-1/B7-2 interactions are required for many Th2-cell mediated primary immune responses including the response that follows infection with the intestinal nematode parasite, Heligmosomoides polygyrus. However, few studies have examined the role of B7-1/B7-2/CD28 interactions in the development of a Th2 memory immune response. We examined the development of the memory Th2 response to H. polygyrus in BALB/c mice deficient in both B7-1 and B7-2 (B7-1/B7-2(-/-)) and in BALB/c mice deficient in CD28 (CD28(-/-)). Following primary inoculation with H. polygyrus, adult worms in the gut were cleared with an anti-helminthic drug and mice were subsequently challenge-inoculated with H. polygyrus larvae. The memory Th2 response is readily distinguished by its inhibitory effect on adult worm maturation, resulting in marked reductions in adult worm egg production that are not observed during the primary immune response. Following H. polygyrus challenge inoculation, comparable decreases in egg production and similar increases in mesenteric lymph node cell IL-4 production were observed in B7-1/B7-2(-/-) and B7-1/B7-2(+/+) mice. However, elevations in total serum IgG1 and IgE were reduced, while increases in serum Ag-specific IgG1 and IgE and germinal center formation were blocked in H. polygyrus-challenged B7-1/B7-2(-/-) mice. In contrast, in H. polygyrus-challenged CD28(-/-) mice, marked elevations in Ag-specific IgG1 and IgE and increased germinal center formation were observed. The results of these studies demonstrate that effector Th2 memory cells that produce IL-4 and mediate host defense can develop when B7-1/B7-2 interactions, and associated effector Th2 cell development, are blocked during priming. However, humoral immunity is impaired and differentially affected in B7-1/B7-2(-/-) mice and CD28(-/-) mice following H. polygyrus challenge.
The past year has seen significant advances in our understanding of critical roles of negative immunoregulatory signals delivered through the B7-CD28 superfamily in regulating T cell activation and tolerance. Structural data on CTLA-4 have provided novel insights into the inhibitory functions of CTLA-4. Initial characterization of the PD-1-PD-1-ligand pathway has revealed that this pathway can downregulate TCR- and CD28-mediated signals. Recent studies indicate that ICOS exerts distinct effects at different phases of an immune response: ICOS can inhibit as well as stimulate T cell responses.