Dendritic cells (DCs) competent to express the regulatory enzyme IDO in mice are a small but distinctive subset of DCs. Previously, we reported that a high-dose systemic CpG treatment to ligate TLR9 in vivo induced functional IDO exclusively in splenic CD19(+) DCs, which stimulated resting Foxp3-lineage regulatory T cells (Tregs) to rapidly acquire potent suppressor activity. In this paper, we show that IDO was induced in spleen and peripheral lymph nodes after CpG treatment in a dose-dependent manner. Induced IDO suppressed local T cell responses to exogenous Ags and inhibited proinflammatory cytokine expression in response to TLR9 ligation. IDO induction did not occur in T cell-deficient mice or in mice with defective B7 or programmed death (PD)-1 costimulatory pathways. Consistent with these findings, CTLA4 or PD-1/PD-ligand costimulatory blockade abrogated IDO induction and prevented Treg activation via IDO following high-dose CpG treatment. Consequently, CD4(+)CD25(+) T cells uniformly expressed IL-17 shortly after TLR9 ligation. These data support the hypothesis that constitutive interactions from activated T cells or Tregs and IDO-competent DCs via concomitant CTLA4→B7 and PD-1→PD-ligand signals maintain the default potential to regulate T cell responsiveness via IDO. Acute disruption of these nonredundant interactions abrogated regulation via IDO, providing novel perspectives on the proinflammatory effects of costimulatory blockade therapies. Moreover, interactions between IDO-competent DCs and activated T cells in lymphoid tissues may attenuate proinflammatory responses to adjuvants such as TLR ligands.
The mode locking of the mixed sesquioxide single crystal Yb:LuScO(3) is demonstrated. This crystal is locally disordered and has the broadest emission spectrum of all sesquioxides known so far. Pulse durations as short as 111 and 74 fs were obtained using the semiconductor saturable absorber mirror and Kerr-lens mode locking, respectively. The latter regime was reached using a two-section distributed Bragg-reflector tapered diode laser as a pump source.
Interactions of the inhibitory receptor programmed death-1 (PD-1) with its ligands, programmed death ligand (PD-L)1 and PD-L2, regulate T-cell activation and tolerance. In this study, we investigated the role of PD-L1 and PD-L2 in regulating invariant natural killer T (iNKT)-cell-mediated airway hyperreactivity (AHR) in a murine model of asthma. We found that the severity of AHR and airway inflammation is significantly greater in PD-L2(-/-) mice compared with wild-type mice after either ovalbumin (OVA) sensitization and challenge or administration of alpha-galactosylceramide (alpha-GalCer). iNKT cells from PD-L2(-/-) mice produced significantly more interleukin (IL)-4 than iNKT cells from control mice. Moreover, blockade of PD-L2 interactions of wild-type iNKT cells in vitro with monoclonal antibodies (mAbs) resulted in significantly enhanced levels of IL-4 production. In contrast, PD-L1(-/-) mice showed significantly reduced AHR and enhanced production of interferon-gamma (IFN-gamma) by iNKT cells. iNKT-deficient Jalpha18(-/-) mice reconstituted with iNKT cells from PD-L2(-/-) mice developed high levels of AHR, whereas mice reconstituted with iNKT cells from PD-L1(-/-) mice developed lower levels of AHR compared with control. As PD-L2 is not expressed on iNKT cells but rather is expressed on lung dendritic cells (DCs), in which its expression is upregulated by allergen challenge or IL-4, these findings suggest an important role of PD-L2 on lung DCs in modulating asthma pathogenesis. These studies also indicate that PD-L1 and PD-L2 have important but opposing roles in the regulation of AHR and iNKT-cell-mediated activation.
Regulatory T cells (Tregs) and the PD-1: PD-ligand (PD-L) pathway are both critical to terminating immune responses. Elimination of either can result in the breakdown of tolerance and the development of autoimmunity. The PD-1: PD-L pathway can thwart self-reactive T cells and protect against autoimmunity in many ways. In this review, we highlight how PD-1 and its ligands defend against potentially pathogenic self-reactive effector T cells by simultaneously harnessing two mechanisms of peripheral tolerance: (i) the promotion of Treg development and function and (ii) the direct inhibition of potentially pathogenic self-reactive T cells that have escaped into the periphery. Treg cells induced by the PD-1 pathway may also assist in maintaining immune homeostasis, keeping the threshold for T-cell activation high enough to safeguard against autoimmunity. PD-L1 expression on non-hematopoietic cells as well as hematopoietic cells endows PD-L1 with the capacity to promote Treg development and enhance Treg function in lymphoid organs and tissues that are targets of autoimmune attack. At sites where transforming growth factor-beta is present (e.g. sites of immune privilege or inflammation), PD-L1 may promote the de novo generation of Tregs. When considering the consequences of uncontrolled immunity, it would be therapeutically advantageous to manipulate Treg development and sustain Treg function. Thus, this review also discusses how the PD-1 pathway regulates a number of autoimmune diseases and the therapeutic potential of PD-1: PD-L modulation.
Memory B and plasma cells (PCs) are generated in the germinal center (GC). Because follicular helper T cells (T(FH) cells) have high expression of the immunoinhibitory receptor PD-1, we investigated the role of PD-1 signaling in the humoral response. We found that the PD-1 ligands PD-L1 and PD-L2 were upregulated on GC B cells. Mice deficient in PD-L2 (Pdcd1lg2(-/-)), PD-L1 and PD-L2 (Cd274(-/-)Pdcd1lg2(-/-)) or PD-1 (Pdcd1(-/-)) had fewer long-lived PCs. The mechanism involved more GC cell death and less T(FH) cell cytokine production in the absence of PD-1; the effect was selective, as remaining PCs had greater affinity for antigen. PD-1 expression on T cells and PD-L2 expression on B cells controlled T(FH) cell and PC numbers. Thus, PD-1 regulates selection and survival in the GC, affecting the quantity and quality of long-lived PCs.
The inhibitory receptor programmed death 1 (PD-1) is upregulated on antigen-specific CD8+ T cells during persistent viral infections. Interaction with PD-1 ligand 1 (PD-L1) contributes to functional exhaustion of responding T cells and may limit immunopathology during infection. PD-L1 is expressed on both hematopoietic and nonhematopoietic cells in tissues. However, the exact roles of PD-L1 on hematopoietic versus nonhematopoietic cells in modulating immune responses are unclear. Here we used bone marrow chimeric mice to examine the effects of PD-L1 deficiency in hematopoietic or nonhematopoietic cells during lymphocytic choriomeningitis virus clone 13 (LCMV CL-13) infection. We found that PD-L1 expression on hematopoietic cells inhibited CD8+ T cell numbers and function after LCMV CL-13 infection. In contrast, PD-L1 expression on nonhematopoietic cells limited viral clearance and immunopathology in infected tissues. Together, these data demonstrate that there are distinct roles for PD-L1 on hematopoietic and nonhematopoietic cells in regulating CD8+ T cell responses and viral clearance during chronic viral infection.
Tumor-induced immune defects can weaken host immune response and permit tumor cell growth. In a systemic model of murine acute myeloid leukemia (AML), tumor progression resulted in increased regulatory T cells (Treg) and elevation of program death-1 (PD-1) expression on CD8(+) cytotoxic T cells (CTLs) at the tumor site. PD-1 knockout mice were more resistant to AML despite the presence of similar percentage of Tregs compared with wild type. In vitro, intact Treg suppression of CD8(+) T-cell responses was dependent on PD-1 expression by T cells and Tregs and PD-L1 expression by antigen-presenting cells. In vivo, the function of adoptively transferred AML-reactive CTLs was reduced by AML-associated Tregs. Anti-PD-L1 monoclonal antibody treatment increased the proliferation and function of CTLs at tumor sites, reduced AML tumor burden, and resulted in long-term survivors. Treg depletion followed by PD-1/PD-L1 blockade showed superior efficacy for eradication of established AML. These data demonstrated that interaction between PD-1 and PD-L1 can facilitate Treg-induced suppression of T-effector cells and dampen the antitumor immune response. PD-1/PD-L1 blockade coupled with Treg depletion represents an important new approach that can be readily translated into the clinic to improve the therapeutic efficacy of adoptive AML-reactive CTLs in advanced AML disease.
PURPOSE. Given that dry eye disease (DED) is associated with T cell-mediated inflammation of the ocular surface and that PD-L1 is an important negative or inhibitory regulator of immune responses constitutively expressed at high levels by corneal epithelial cells, the authors studied the expression and function of PD-L1 in DED. METHODS. Dry eye was induced in untreated wild-type mice, PD-L1(-/-) mice, and wild-type mice treated with anti-PD-L1 antibody by exposing these mice to a desiccating environment in the controlled environment chamber modified with subcutaneous administration of scopolamine. Real-time PCR was used to quantify the expression of chemokine gene transcript levels of multiple CC and CXC chemokine ligands and receptors. Epifluorescence microscopy was used to evaluate corneal infiltration of CD3(+) T cells after immunohistochemical staining. RESULTS. The increased expression of specific chemokine ligands and receptors in PD-L1(-/-) corneas of normal mice is associated with significant increases in T-cell homing into these corneas. Similar, and more enhanced, increases in T-cell infiltration were observed in PD-L1(-/-) DED mice or DED mice treated with anti-PD-L1 antibody compared with controls. In addition, the authors found significantly decreased expression of PD-L1 by corneal epithelial cells in DED and significantly increased corneal fluorescein staining score with PD-L1 functional blockade using anti-PD-L1 antibody. CONCLUSIONS. Downregulation of corneal epithelial PD-L1 amplifies dry eye-associated corneal inflammation and epitheliopathy by increasing the expression of chemokine ligands and receptors that promote T-cell homing to the ocular surface.
While the role of PD-1 in inhibiting immunity during chronic infections is well established, its functions during acute infections are much less clear. The PD-1 pathway can dampen CD8 T cell responses during some acute infections and restrain responses by 'helpless' CD8 memory T cells. An emerging role for PD-1 in innate immunity has been revealed by recent studies showing that PD-1 can limit function of DC and macrophages as well as T cell independent B cell responses. Thus, PD-1 can influence adaptive immune responses during acute infections, though precisely how this regulation occurs is only just beginning to be appreciated.
The inducible costimulatory molecule ICOS has been suggested to be important in the development of interleukin 17 (IL-17)-producing helper T cells (T(H)-17 cells) and of follicular helper T cells (T(FH) cells). Here we show that ICOS-deficient mice had no defect in T(H)-17 differentiation but had fewer T(H)-17 cells after IL-23 stimulation and fewer T(FH) cells. We also show that T(FH) cells produced IL-17 and that T(FH) cells in ICOS-deficient mice were defective in IL-17 production. Both T(H)-17 and T(FH) cells had higher expression of the transcription factor c-Maf. Genetic loss of c-Maf resulted in a defect in IL-21 production and fewer T(H)-17 and T(FH) cells. Thus our data suggest that ICOS-induced c-Maf regulates IL-21 production that in turn regulates the expansion of T(H)-17 and T(FH) cells.
The CTLA-4 pathway is recognized as a major immune inhibitory axis and is a key therapeutic target for augmenting antitumor immunity or curbing autoimmunity. CTLA-4-deficient mice provide the archetypal example of dysregulated immune homeostasis, developing lethal lymphoproliferation with multiorgan inflammation. In this study, we show that surprisingly these mice have an enlarged population of Foxp3(+) regulatory T cells (Treg). The increase in Treg is associated with normal thymic output but enhanced proliferation of Foxp3(+) cells in the periphery. We confirmed the effect of CTLA-4 deficiency on the Treg population using OVA-specific Treg which develop normally in the absence of CTLA-4, but show increased proliferation in response to peripheral self-Ag. Functional analysis revealed that Ag-specific Treg lacking CTLA-4 were unable to regulate disease in an adoptive transfer model of diabetes. Collectively, these data suggest that the proliferation of Treg in the periphery is tuned by CTLA-4 signals and that Treg expression of CTLA-4 is required for regulation of pancreas autoimmunity.
Costimulatory molecules, such as B7-1/2 and PD-L1/2 play an important role in the function of APC. The regulation of the surface levels of costimulatory molecules is one mechanism by which APC maintain the balance between tolerance and immunity. We examined the contributions of B7-1/2 and PD-L1/2 to the function of IL-10-treated, immunosuppressive DC as well as therapeutic exosomes derived from these DC. IL-10 treatment of DC significantly downregulated surface expression of MHC II, B7-1, B7-2, and decreased levels of MHC I and PD-L2. IL-10 treatment of DC resulted in a modified costimulatory profile of DC-secreted exosomes with a reduction in B7-1, PD-L1 and PD-L2. We further demonstrate that absence of B7-1 or B7-2 on donor DC results in a loss of ability of IL-10-treated DC and their exosomes to suppress the delayed-type hypersensitivity response, whereas IL-10-treated DC deficient in PD-L1/2 as well as their secreted exosomes retained the ability to suppress delayed-type hypersensitivity responses. We conclude that B7-1 and B7-2, but not PD-L1 and PD-L2, on IL-10-treated DC and DC-derived exosomes play a critical role in immunosuppressive functions of both DC and exosomes.
PURPOSE: Programmed death-1 (PD-1) ligation downregulates active lymphocyte responses. The authors tested whether PD-1 or its ligands are expressed in the posterior segment during active intraocular inflammation.
METHODS: Experimental autoimmune uveitis (EAU) was induced using interphotoreceptor retinoid-binding protein (IRBP 161-180). Ocular inflammation was evaluated by histology and expression of PD-1 ligand tested by immunohistochemistry. PD-1 expression was evaluated by immunohistochemistry, RT-PCR, and Western immunoblotting.
RESULTS: Using immunohistochemistry, PD-1, but not its ligands, was constitutively expressed in retinal neurons of naive mouse retina. Both PD-1 and its ligands were observed, as expected, in sites of active inflammation.
CONCLUSIONS: PD-1 and its ligands were expressed in sites of active inflammation, in accordance with many other models of inflammatory disease. Surprisingly, PD-1, not previously described outside the immune system, was constitutively expressed in retinal neurons, raising the possibility that PD-1 signaling may be important for neuronal function in the absence of an inflammatory insult.
IL-27 has recently been identified as a differentiation factor for the generation of IL-10-producing regulatory type 1 (Tr1) T cells. However, how IL-27 induces the expansion of Tr1 cells has not been elucidated. In this study we demonstrate that IL-27 drives the expansion and differentiation of IL-10-producing murine Tr1 cells by inducing three key elements: the transcription factor c-Maf, the cytokine IL-21, and the costimulatory receptor ICOS. IL-27-driven c-Maf expression transactivates IL-21 production, which acts as an autocrine growth factor for the expansion and/or maintenance of IL-27-induced Tr1 cells. ICOS further promotes IL-27-driven Tr1 cells. Each of those elements is essential, because loss of c-Maf, IL-21-signaling, or ICOS decreases the frequency of IL-27-induced differentiation of IL-10-producing Tr1 cells.
It is generally acknowledged that cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4/CD152) plays a pivotal role in the regulation of T-cell activation and the establishment of self-tolerance in the periphery. CTLA-4-deficient (CTLA-4KO) mice develop a lymphoproliferative disorder and die within 4 weeks of birth, suggesting a role for CTLA-4 in T-cell homeostasis or the development and activity of T-regulatory (Treg) cells. To study the role of CTLA-4 in the control of experimental autoimmune encephalomyelitis (EAE), we have generated a CTLA-4KO mouse in which >90% of all CD4(+) T cells bear a Vbeta8.2 transgenic T-cell receptor that is specific for myelin basic protein peptide Ac1-9 (ASQKRPSQR). These mice do not develop spontaneous lymphoproliferative disease or EAE and are resistant to disease induction. This correlates with a higher frequency of functional FoxP3(+) Treg cells in the spleen and thymus of CTLA-4KO mice. The absence of CTLA-4-mediated suppression of CD28 signaling resulted in the early expression of FoxP3 on double-positive cells in the thymic cortex. We conclude that CTLA-4 is not essential for the peripheral function of FoxP3(+) Treg cells but plays a pivotal role in their thymic selection.
Idd5.1 regulates T1D susceptibility in nonobese diabetic (NOD) mice and has two notable candidate genes, Ctla4 and Icos. Reduced expression of one of the four CTLA-4 isoforms, ligand-independent CTLA-4 (liCTLA-4), which inhibits in vitro T cell activation and cytokine production similarly to full-length CTLA-4 (flCTLA-4), has been hypothesized to increase type 1 diabetes (T1D) susceptibility. However, further support of this hypothesis is required since the Idd5.1 haplotypes of the diabetes-susceptible NOD and the resistant B10 strains differ throughout Ctla4 and Icos. Using haplotype analysis and the generation of novel Idd5.1-congenic strains that differ at the disease-associated Ctla4 exon 2 single-nucleotide polymorphism, we demonstrate that increased expression of liCTLA-4 correlates with reduced T1D susceptibility. To directly assess the ability of liCTLA-4 to modulate T1D, we generated liCTLA-4-transgenic NOD mice and compared their diabetes susceptibility to nontransgenic littermates. NOD liCTLA-4-transgenic mice were protected from T1D to the same extent as NOD.B10 Idd5.1-congenic mice, demonstrating that increased liCTLA-4 expression alone can account for disease protection. To further investigate the in vivo function of liCTLA-4, specifically whether liCTLA-4 can functionally replace flCTLA-4 in vivo, we expressed the liCTLA-4 transgene in CTLA-4(-/-) B6 mice. CTLA-4(-/-) mice expressing liCTLA-4 accumulated fewer activated effector/memory CD4(+) T cells than CTLA-4(-/-) mice and the transgenic mice were partially rescued from the multiorgan inflammation and early lethality caused by the disruption of Ctla4. These results suggest that liCTLA-4 can partially replace some functions of flCTLA-4 in vivo and that this isoform evolved to reinforce the function of flCTLA-4.
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.
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.