Connecting the mechanisms of T-cell regulation: dendritic cells as the missing link

Pregnancy-induced humoral sensitization overrides T cell tolerance to fetus-matched allografts in mice

Immunological tolerance to semiallogeneic fetuses is necessary to achieving successful first pregnancy and permitting subsequent pregnancies with the same father. Paradoxically, pregnancy is an important cause of sensitization, resulting in the accelerated rejection of offspring-matched allografts. The underlying basis for divergent outcomes following reencounter of the same alloantigens on transplanted organs versus fetuses in postpartum females is incompletely understood. Using a mouse model that allows concurrent tracking of endogenous fetus-specific T and B cell responses in a single recipient, we show that semiallogeneic pregnancies simultaneously induce fetus-specific T cell tolerance and humoral sensitization. Pregnancy-induced antibodies, but not B cells, impeded transplantation tolerance elicited by costimulation blockade to offspring-matched cardiac grafts. Remarkably, in B cell-deficient mice, allogeneic pregnancy enabled the spontaneous acceptance of fetus-matched allografts. The presence of pregnancy-sensitized B cells that cannot secrete antibodies at the time of heart transplantation was sufficient to precipitate rejection and override pregnancy-established T cell tolerance. Thus, while induction of memory B cells and alloantibodies by pregnancies establishes formidable barriers to transplant success for multigravid women, our observations raise the possibility that humoral desensitization will not only improve transplantation outcomes, but also reveal an unexpected propensity of multiparous recipients to achieve tolerance to offspring-matched allografts.

Implications of CD39 in immune-related diseases

Extracellular adenosine triphosphate (eATP) mediates pro-inflammatory responses by recruiting and activating inflammatory cells. CD39 can hydrolyze eATP into adenosine monophosphate (AMP), while CD73 can convert AMP into the immunosuppressive nucleoside adenosine (ADO). CD39 is a rate-limiting enzyme in this cascade, which is regarded as an immunological switch shifting the ATP-mediated pro-inflammatory environment to the ADO- mediated anti-inflammatory status. The CD39 expression can be detected in a wide spectrum of immunocytes, which is under the influence of environmental and genetic factors. It is increasingly suggested that, CD39 participates in some pathophysiological processes, like inflammatory bowel disease (IBD), sepsis, multiple sclerosis (MS), allergic diseases, ischemia-reperfusion (I/R) injury, systemic lupus erythematosus (SLE), diabetes and cancer. Here, we focus on the current understanding of CD39 in immunity, and comprehensively illustrate the diverse CD39 functions within a variety of disorders.

Regulatory T Cells in Pregnancy: It Is Not All About FoxP3

In pregnancy, the semi-allogeneic fetus needs to be tolerated by the mother's immune system. Regulatory T cells (Tregs) play a prominent role in this process. Novel technologies allow for in-depth phenotyping of previously unidentified immune cell subsets, which has resulted in the appreciation of a vast heterogeneity of Treg subsets. Similar to other immunological events, there appears to be great diversity within the Treg population during pregnancy, both at the maternal-fetal interface as in the peripheral blood. Different Treg subsets have distinct phenotypes and various ways of functioning. Furthermore, the frequency of individual Treg subsets varies throughout gestation and is altered in aberrant pregnancies. This suggests that distinct Treg subsets play a role at different time points of gestation and that their role in maintaining healthy pregnancy is crucial, as reflected for instance by their reduced frequency in women with recurrent pregnancy loss. Since pregnancy is essential for the existence of mankind, multiple immune regulatory mechanisms and cell types are likely at play to assure successful pregnancy. Therefore, it is important to understand the complete microenvironment of the decidua, preferably in the context of the whole immune cell repertoire of the pregnant woman. So far, most studies have focused on a single mechanism or cell type, which often is the FoxP3 positive regulatory T cell when studying immune regulation. In this review, we instead focus on the contribution of FoxP3 negative Treg subsets to the decidual microenvironment and their possible role in pregnancy complications. Their phenotype, function, and effect in pregnancy are discussed.

Vitamin D-modulated dendritic cells delay lethal graft-versus-host disease through induction of regulatory T cells

Graft-versus-host disease (GVHD) is the most lethal complication after allogeneic bone marrow transplantation (allo-BMT). Current approaches to prevent GVHD rely on donor lymphocyte/T cell depletion or general immunosuppression, leading to opportunistic infections and cancer relapse. Tolerogenic dendritic cells can induce regulatory T cells (Tregs) with the ability to suppress inflammation and prevent transplant rejection, making them an attractive cellular therapy to control GVHD. Active vitamin D (1α,25-dihydroxyvitamin D₃; 1α,25(OH)₂D₃) promotes the generation of tolerogenic dendritic cells (1,25D₃-DCs). This study aimed to determine the ability of ex vivo generated 1,25D₃-DCs to trigger the expansion of Tregs that are able to control lethal xenogeneic GVHD in humanized NOD/LtSz-Prkdc^scidIL2rγ^tm1Wjl (NSG) mice. We demonstrate that 1,25D₃-DCs express lower levels of HLA-DR and costimulatory molecules, such as CD80 and CD86, and produce higher levels of IL-10 and TNF-α and lower amounts of IL-12, compared to vehicle-treated DCs. Moreover, these cells express increased levels of various co-inhibitory molecules such as PD-L1 and ILT-3 and the glycoprotein CD52 that is known to suppress T cell activation. Consequently, 1,25D₃-DCs are poor stimulators of alloantigen-primed T cells, but foster the generation of antigen-specific suppressive Tregs. When adoptively transferred in humanized NSG mice, these 1,25D₃-DC-induced Tregs delayed GVHD caused by the co-transferred autologous human peripheral blood mononuclear cells (PBMCs). These results indicate that 1,25D₃-DC-induced Tregs can inhibit xenogeneic GVHD and maintain their immunomodulatory function under conditions of inflammation.

Vitamin D controls the capacity of human dendritic cells to induce functional regulatory T cells by regulation of glucose metabolism

Tolerogenic dendritic cells (tolDCs) instruct regulatory T cells (Tregs) to dampen autoimmunity. Active vitamin D₃ (1α,25-dihydroxyvitamin D₃; 1α,25(OH)₂D₃) imprints human monocyte-derived DCs with tolerogenic properties by reprogramming their glucose metabolism. Here we identify the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 4 (PFKFB4) as a critical checkpoint and direct transcriptional target of 1α,25(OH)₂D₃ in determining the tolDC profile. Using tracer metabolomics, we show that PFKFB4 activity is essential for glucose metabolism, especially for glucose oxidation, which is elevated upon 1α,25(OH)₂D₃ exposure. Pharmacological inhibition of PFKFB4 reversed the 1α,25(OH)₂D₃-mediated shift in metabolism, DC profile and function, as determined by expression of inhibitory surface markers and secretion of regulatory cytokines and factors. Moreover, PFKFB4 inhibition in 1α,25(OH)₂D₃-treated DCs blocked their hallmark capacity to induce suppressive Tregs. This work demonstrates that alterations in the bioenergetic metabolism of immune cells are central to the immunomodulatory effects induced by 1α,25(OH)₂D₃.

Control of Treg cell homeostasis and immune equilibrium by Lkb1 in dendritic cells

To balance immunity and tolerance, the endogenous pool of Foxp3⁺ regulatory T (T_reg) cells is tightly controlled, but the underlying mechanisms of this control remain poorly understood. Here we show that the number of T_reg cells is negatively regulated by the kinase Lkb1 in dendritic cells (DCs). Conditional knockout of the Lkb1 gene in DCs leads to excessive T_reg cell expansion in multiple organs and dampens antigen-specific T cell immunity. Lkb1-deficient DCs are capable of enhancing, compared with wild-type DCs, T_reg cell proliferation via cell-cell contact involving the IKK/IKBα-independent activation of the NF-κB/OX40L pathway. Intriguingly, treating wild-type mice with lipopolysaccharide selectively depletes Lkb1 protein in DCs, resulting in T_reg cell expansion and suppressed inflammatory injury upon subsequent challenge. Loss of Lkb1 does not obviously upregulate proinflammatory molecules expression on DCs. We thus identify Lkb1 as a regulatory switch in DCs for controlling T_reg cell homeostasis, immune response and tolerance.

Regulatory T (Treg) cells are important for maintaining immune homeostasis by suppressing immune cell activation, but how the Treg cell pool is maintained is still unclear. Here the authors show that a kinase, Lkb1, operates in dendritic cells (DC) to inhibit Treg cell expansion and immunosuppression via mechanisms involving NF-kB/OX40L signalling.

Induction of Immunological Tolerance as a Therapeutic Procedure

A major goal of immunosuppressive therapies is to harness immune tolerance mechanisms so as to minimize unwanted side effects associated with protracted immunosuppressive therapy. Antibody blockade of lymphocyte coreceptor and costimulatory pathways in mice has demonstrated the principle that both naive and primed immune systems can be reprogrammed toward immunological tolerance. Such tolerance can involve the amplification of activity of regulatory T cells, and is maintained through continuous recruitment of such cells through processes of infectious tolerance. We propose that regulatory T cells create around them microenvironments that are anti-inflammatory and endowed with enhanced protection against destructive damage. This acquired immune privilege involves the decommissioning of cells of the innate as well as adaptive immune systems. Evidence is presented that nutrient sensing by immune cells acting through the mammalian target of rapamycin (mTOR) pathway provides one route by which the immune system can be directed toward noninflammatory and regulatory behavior at the expense of destructive functions. Therapeutic control of immune cells so as to harness metabolic routes favoring dominant regulatory mechanisms has offered a new direction for immunosuppressive therapy, whereby short-term treatment may be sufficient for long-term benefit or even cure.

Efficient Presentation of Multiple Endogenous Epitopes to Both CD4+ and CD8+ Diabetogenic T Cells for Tolerance

Antigen-specific immunotherapy of type 1 diabetes, typically via delivery of a single native β cell antigen, has had little clinical benefit to date. With increasing evidence that diabetogenic T cells react against multiple β cell antigens, including previously unappreciated neo-antigens that can be emulated by mimotopes, a shift from protein- to epitope-based therapy is warranted. To this end, we aimed to achieve efficient co-presentation of multiple major epitopes targeting both CD4⁺ and CD8⁺ diabetogenic T cells. We have compared native epitopes versus mimotopes as well as various targeting signals in an effort to optimize recognition by both types of T cells in vitro. Optimal engagement of all T cells was achieved with segregation of CD8 and CD4 epitopes, the latter containing mimotopes and driven by endosome-targeting signals, after delivery into either dendritic or stromal cells. The CD4⁺ T cell responses elicited by the endogenously delivered epitopes were comparable with high concentrations of soluble peptide and included functional regulatory T cells. This work has important implications for the improvement of antigen-specific therapies using an epitope-based approach to restore tolerance in type 1 diabetes and in a variety of other diseases requiring concomitant targeting of CD4⁺ and CD8⁺ T cells.

Glucose-Regulated Protein 78-Induced Myeloid Antigen-Presenting Cells Maintained Tolerogenic Signature upon LPS Stimulation

The 78-kDa glucose-regulated protein (Grp78) is stress-inducible chaperone that mostly reside in the endoplasmic reticulum. Grp78 has been described to be released at times of cellular stress and as having extracellular properties that are anti-inflammatory or favor the resolution of inflammation. As antigen-presenting cells (APCs) play a critical role in both the priming of adaptive immune responses and the induction of self-tolerance, herein, we investigated the effect of Grp78 on the maturation of murine myeloid APCs (CD11c⁺ cells). Results showed that CD11c⁺ cells could be bound by AF488-labeled Grp78 and that Grp78 treatment induced a tolerogenic phenotype comparable to immature cells. Furthermore, when exposed to lipopolysaccharide, Grp78-treated CD11c⁺ cells (DC_Grp78) did not adopt a mature dendritic cell phenotype. DC_Grp78-primed T cells exhibited reduced proliferation along with a concomitant expansion of CD4⁺CD25⁺FoxP3⁺ cells in pancreaticoduodenal lymph nodes and induction of T cell apoptosis in vitro and ex vivo. The above work suggests that Grp78 is an immunomodulatory molecule that could aid resolution of inflammation. It may thus contribute to induce durable tolerance to be of potential therapeutic benefit in transplanted allogeneic grafts and autoimmune diseases such as type I diabetes.

Immune profile of an atypical EAE model in marmoset monkeys immunized with recombinant human myelin oligodendrocyte glycoprotein in incomplete Freund's adjuvant

Background

Experimental autoimmune encephalomyelitis (EAE) in the common marmoset monkey (Callithrix jacchus) is a relevant preclinical model for translational research into immunopathogenic mechanisms operating in multiple sclerosis (MS). Prior studies showed a core pathogenic role of T and B cells specific for myelin oligodendrocyte glycoprotein (MOG). However, in those studies, the quality of the response against MOG epitopes was strongly biased by bacterial antigens in the complete Freund’s adjuvant (CFA), in which the immunizing recombinant human (rh) MOG protein had been formulated. In response to the need of a more refined EAE model, we have tested whether disease could also be induced with rhMOG in incomplete Freund’s adjuvant (IFA).

Method

Marmosets were immunized with rhMOG emulsified in IFA in the dorsal skin. Monkeys that did not develop neurological deficit were given booster immunizations at 28-day interval with the same antigen preparation. In a second experiment, three marmoset twin pairs were sensitized against MOG peptides in IFA to study a possibility for suppressive activity towards pathogenic T cells directed against the encephalitogenic epitope MOG40-48.

Results

Despite the absence of strong danger signals in the rhMOG/IFA inoculum, all monkeys developed clinically evident EAE symptoms. Moreover, in all monkeys, demyelinated lesions were present in the white matter and in two cases also in the cortical grey matter. Immune profiling at height of the disease showed a dominant T cell response against the overlapping peptides 14–36 and 24–46, but reactivity against the pathogenically most relevant peptide 34–56 was conspicuously absent. In the second experiment, there was an indication for a possible suppressive mechanism.

Conclusions

Immunization of marmoset monkeys with rhMOG in IFA elicits clinical EAE in all animals. Moreover, rhMOG contains pathogenic and regulatory epitopes, but the pathogenic hierarchy of rhMOG epitopes is strongly influenced by the adjuvant in which the protein is formulated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0378-5) contains supplementary material, which is available to authorized users.

A Minimal Regulatory Network of Extrinsic and Intrinsic Factors Recovers Observed Patterns of CD4+ T Cell Differentiation and Plasticity

CD4+ T cells orchestrate the adaptive immune response in vertebrates. While both experimental and modeling work has been conducted to understand the molecular genetic mechanisms involved in CD4+ T cell responses and fate attainment, the dynamic role of intrinsic (produced by CD4+ T lymphocytes) versus extrinsic (produced by other cells) components remains unclear, and the mechanistic and dynamic understanding of the plastic responses of these cells remains incomplete. In this work, we studied a regulatory network for the core transcription factors involved in CD4+ T cell-fate attainment. We first show that this core is not sufficient to recover common CD4+ T phenotypes. We thus postulate a minimal Boolean regulatory network model derived from a larger and more comprehensive network that is based on experimental data. The minimal network integrates transcriptional regulation, signaling pathways and the micro-environment. This network model recovers reported configurations of most of the characterized cell types (Th0, Th1, Th2, Th17, Tfh, Th9, iTreg, and Foxp3-independent T regulatory cells). This transcriptional-signaling regulatory network is robust and recovers mutant configurations that have been reported experimentally. Additionally, this model recovers many of the plasticity patterns documented for different T CD4+ cell types, as summarized in a cell-fate map. We tested the effects of various micro-environments and transient perturbations on such transitions among CD4+ T cell types. Interestingly, most cell-fate transitions were induced by transient activations, with the opposite behavior associated with transient inhibitions. Finally, we used a novel methodology was used to establish that T-bet, TGF-β and suppressors of cytokine signaling proteins are keys to recovering observed CD4+ T cell plastic responses. In conclusion, the observed CD4+ T cell-types and transition patterns emerge from the feedback between the intrinsic or intracellular regulatory core and the micro-environment. We discuss the broader use of this approach for other plastic systems and possible therapeutic interventions.

Extracellular adenosine generation in the regulation of pro-inflammatory responses and pathogen colonization

Adenosine, an immunomodulatory biomolecule, is produced by the ecto-enzymes CD39 (nucleoside triphosphate dephosphorylase) and CD73 (ecto-5'-nucleotidase) by dephosphorylation of extracellular ATP. CD73 is expressed by many cell types during injury, infection and during steady-state conditions. Besides host cells, many bacteria also have CD39-CD73-like machinery, which helps the pathogen subvert the host inflammatory response. The major function for adenosine is anti-inflammatory, and most recent research has focused on adenosine's control of inflammatory mechanisms underlying various autoimmune diseases (e.g., colitis, arthritis). Although adenosine generated through CD73 provides a feedback to control tissue damage mediated by a host immune response, it can also contribute to immunosuppression. Thus, inflammation can be a double-edged sword: it may harm the host but eventually helps by killing the invading pathogen. The role of adenosine in dampening inflammation has been an area of active research, but the relevance of the CD39/CD73-axis and adenosine receptor signaling in host defense against infection has received less attention. Here, we review our recent knowledge regarding CD73 expression during murine Salmonellosis and Helicobacter-induced gastric infection and its role in disease pathogenesis and bacterial persistence. We also explored a possible role for the CD73/adenosine pathway in regulating innate host defense function during infection.

Confusing signals: recent progress in CTLA-4 biology

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Cell extrinsic and cell intrinsic mechanisms of action of CTLA-4 are unclear.

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Data suggest that the extracellular domain of CTLA4 elicits regulatory function.

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The function of CTLA-4 tail may lie in regulating localization rather than signaling.

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Membrane levels of CTLA-4 directly impact access of CD28 to shared ligands.

The mechanism of action of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) remains surprisingly unclear. Regulatory T (Treg) cells can use CTLA-4 to elicit suppression; however, CTLA-4 also operates in conventional T cells, reputedly by triggering inhibitory signals. Recently, interactions mediated via the CTLA-4 cytoplasmic domain have been shown to preferentially affect Treg cells, yet other evidence suggests that the extracellular domain of CTLA-4 is sufficient to elicit suppression. Here, we discuss these paradoxical findings in the context of CTLA-4-mediated ligand regulation. We propose that the function of CTLA-4 cytoplasmic domain is not to transmit inhibitory signals but to precisely control the turnover, cellular location, and membrane delivery of CTLA-4 to facilitate its central function: regulating the access of CD28 to their shared ligands.

Breast milk, microbiota, and intestinal immune homeostasis

Newborns adjust to the extrauterine environment by developing intestinal immune homeostasis. Appropriate initial bacterial colonization is necessary for adequate intestinal immune development. An environmental determinant of adequate colonization is breast milk. Although the full-term infant is developmentally capable of mounting an immune response, the effector immune component requires bacterial stimulation. Breast milk stimulates the proliferation of a well-balanced and diverse microbiota, which initially influences a switch from an intrauterine TH2 predominant to a TH1/TH2 balanced response and with activation of T-regulatory cells by breast milk-stimulated specific organisms (Bifidobacteria, Lactobacillus, and Bacteroides). As an example of its effect, oligosaccharides in breast milk are fermented by colonic bacteria producing an acid milieu for bacterial proliferation. In addition, short-chain fatty acids in breast milk activate receptors on T-reg cells and bacterial genes, which preferentially mediate intestinal tight junction expression and anti-inflammation. Other components of breast milk (defensins, lactoferrin, etc.) inhibit pathogens and further contribute to microbiota composition. The breast milk influence on initial intestinal microbiota also prevents expression of immune-mediated diseases (asthma, inflammatory bowel disease, type 1 diabetes) later in life through a balanced initial immune response, underscoring the necessity of breastfeeding as the first source of nutrition.

Polymeric synthetic nanoparticles for the induction of antigen-specific immunological tolerance

Current treatments to control pathological or unwanted immune responses often use broadly immunosuppressive drugs. New approaches to induce antigen-specific immunological tolerance that control both cellular and humoral immune responses are desirable. Here we describe the use of synthetic, biodegradable nanoparticles carrying either protein or peptide antigens and a tolerogenic immunomodulator, rapamycin, to induce durable and antigen-specific immune tolerance, even in the presence of potent Toll-like receptor agonists. Treatment with tolerogenic nanoparticles results in the inhibition of CD4+ and CD8+ T-cell activation, an increase in regulatory cells, durable B-cell tolerance resistant to multiple immunogenic challenges, and the inhibition of antigen-specific hypersensitivity reactions, relapsing experimental autoimmune encephalomyelitis, and antibody responses against coagulation factor VIII in hemophilia A mice, even in animals previously sensitized to antigen. Only encapsulated rapamycin, not the free form, could induce immunological tolerance. Tolerogenic nanoparticle therapy represents a potential novel approach for the treatment of allergies, autoimmune diseases, and prevention of antidrug antibodies against biologic therapies.

Kidney pericytes: roles in regeneration and fibrosis

Renal pericytes have been neglected for many years, but recently they have become an intensively studied cell population in renal biology and pathophysiology. Pericytes are stromal cells that support vasculature, and a subset of pericytes are mesenchymal stem cells. In kidney, pericytes have been reported to play critical roles in angiogenesis, regulation of renal medullary and cortical blood flow, and serve as progenitors of interstitial myofibroblasts in renal fibrogenesis. They interact with endothelial cells through distinct signaling pathways and their activation and detachment from capillaries after acute or chronic kidney injury may be critical for driving chronic kidney disease progression. By contrast, during kidney homeostasis it is likely that pericytes serve as a local stem cell population that replenishes differentiated interstitial and vascular cells lost during aging. This review describes both the regenerative properties of pericytes as well as involvement in pathophysiologic conditions such as fibrogenesis.

Harnessing FOXP3+ regulatory T cells for transplantation tolerance

Early demonstrations that mice could be tolerized to transplanted tissues with short courses of immunosuppressive therapy and that with regard to tolerance to self, CD4+FOXP3+ regulatory T cells (Tregs) appeared to play a critical role, have catalyzed strategies to harness FOXP3-dependent processes to control rejection in human transplantation. This review seeks to examine the scientific underpinning for this new approach to finesse immunosuppression.

Ribosomal protein S6 mRNA is a biomarker upregulated in multiple sclerosis, downregulated by interferon treatment, and affected by season

Background:

Multiple Sclerosis (MS) is an immune-mediated disease of the central nervous system which responds to therapies targeting circulating immune cells.

Objective:

Our aim was to test if the T-cell activation gene expression pattern (TCAGE) we had previously described from whole blood was replicated in an independent cohort.

Methods:

We used RNA-seq to interrogate the whole blood transcriptomes of 72 individuals (40 healthy controls, 32 untreated MS). A cohort of 862 control individuals from the Brisbane Systems Genetics Study (BSGS) was used to assess heritability and seasonal expression. The effect of interferon beta (IFNB) therapy on expression was evaluated.

Results:

The MS/TCAGE association was replicated and rationalized to a single marker, ribosomal protein S6 (RPS6). Expression of RPS6 was higher in MS than controls ( p<0.0004), and lower in winter than summer ( p<4.6E-06). The seasonal pattern correlated with monthly UV light index ( R=0.82, p<0.002), and was also identified in the BSGS cohort ( p<0.0016). Variation in expression of RPS6 was not strongly heritable. RPS6 expression was reduced by IFNB therapy.

Conclusions:

These data support investigation of RPS6 as a potential therapeutic target and candidate biomarker for measuring clinical response to IFNB and other MS therapies, and of MS disease heterogeneity.

Protection of islet grafts through transforming growth factor-β-induced tolerogenic dendritic cells

In type 1 diabetes, the insulin-producing β-cells are destroyed by the immune system. One way of restoring glucose control is to transplant β-cells from a donor. Although this procedure may restore endogenous insulin production, immunosuppressive treatment is needed to prevent the recipient from rejecting the donor-derived islets. We investigated the possibilities of transient expression of the immunosuppressive cytokine transforming growth factor (TGF)-β within islets to achieve long-term graft tolerance. We found that brief expression of TGF-β prevented rejection of syngeneic islets, that there was reduction of dendritic cell (DC) activation in the graft, and that there was reduced reactivation of T cells in the graft-draining lymph nodes. In vitro exposure of bone marrow-derived DCs to TGF-β reduced expression of costimulatory molecules CD80 and CD86, as well as production of proinflammatory cytokines such as interleukin-12 p70 in DCs, but did not alter levels of major histocompatibility complex classes I and II. Furthermore, the capacity of TGF-β-treated bone marrow-derived DCs to activate both CD4(+) and CD8(+) T cells was reduced. Adding TGF-β-conditioned tolerogenic DCs to the grafted islets led to long-term survival of the graft, demonstrating that TGF-β-induced tolerogenic DCs can provide an effective means to restore immune tolerance in an already established autoimmune disease.

Donor bone marrow-derived dendritic cells prolong corneal allograft survival and promote an intragraft immunoregulatory milieu

Investigations into cell therapies for application in organ transplantation have grown. Here, we describe the ex vivo generation of donor bone marrow-derived dendritic cells (BMDCs) and glucocorticoid-treated BMDCs with potent immunomodulatory properties for application in allogeneic transplantation. BMDCs were treated with dexamethasone (Dexa) to induce an immature, maturation-resistant phenotype. BMDC and Dexa BMDC phenotype, antigen presenting cell function, and immunomodulatory properties were fully characterized. Both populations display significant immunomodulatory properties, including, but not limited to, a significant increase in mRNA expression of programmed death-ligand 1 and indoleamine 2,3-dioxygenase. BMDCs and Dexa BMDCs display a profound impaired capacity to stimulate allogeneic lymphocytes. Moreover, in a fully MHC I/II mismatched rat corneal transplantation model, injection of donor-derived, untreated BMDC or Dexa BMDCs (1 × 10(6) cells, day -7) significantly prolonged corneal allograft survival without the need for additional immunosuppression. Although neovascularization was not reduced and evidence of donor-specific alloantibody response was detected, a significant reduction in allograft cellular infiltration combined with a significant increase in the ratio of intragraft FoxP3-expressing regulatory cells was observed. Our comprehensive analysis demonstrates the novel cellular therapeutic approach and significant effect of donor-derived, untreated BMDCs and Dexa BMDCs in preventing corneal allograft rejection.

IL-22 and IDO1 affect immunity and tolerance to murine and human vaginal candidiasis

The ability to tolerate Candida albicans, a human commensal of the gastrointestinal tract and vagina, implicates that host defense mechanisms of resistance and tolerance cooperate to limit fungal burden and inflammation at the different body sites. We evaluated resistance and tolerance to the fungus in experimental and human vulvovaginal candidiasis (VVC) as well as in recurrent VVC (RVVC). Resistance and tolerance mechanisms were both activated in murine VVC, involving IL-22 and IL-10-producing regulatory T cells, respectively, with a major contribution by the enzyme indoleamine 2,3-dioxygenase 1 (IDO1). IDO1 was responsible for the production of tolerogenic kynurenines, such that replacement therapy with kynurenines restored immunoprotection to VVC. In humans, two functional genetic variants in IL22 and IDO1 genes were found to be associated with heightened resistance to RVVC, and they correlated with increased local expression of IL-22, IDO1 and kynurenines. Thus, IL-22 and IDO1 are crucial in balancing resistance with tolerance to Candida, their deficiencies are risk factors for RVVC, and targeting tolerance via therapeutic kynurenines may benefit patients with RVVC.

Mapping I-A(g7) restricted epitopes in murine G6PC2

G6PC2, also known as islet-specific glucose 6-phosphatase catalytic subunit-related protein (IGRP), is a major target of autoreactive CD8(+) T cells in both diabetic human subjects and the non-obese diabetic (NOD) mouse. However, in contrast to the abundant literature regarding the CD8(+) response to this antigen, much less is known about the potential involvement of IGRP-reactive CD4(+) T cells in diabetogenesis. The single previous study that examined this question in NOD mice was based upon a candidate epitope approach and identified three I-A(g7)-restricted epitopes that each elicited spontaneous responses in these animals. However, given the known inaccuracies of MHC class II epitope prediction algorithms, we hypothesized that additional specificities might also be targeted. To address this issue, we immunized NOD mice with membranes from insect cells overexpressing full-length recombinant mouse IGRP and measured recall responses of purified CD4(+) T cells using a library of overlapping peptides encompassing the entire 355-aa primary sequence. Nine peptides representing 8 epitopes gave recall responses, only 1 of which corresponded to any of the previously reported sequences. In each case proliferation was blocked by a monoclonal antibody to I-A(g7), but not the appropriate isotype control. Consistent with a role in diabetogenesis, proliferative responses to 4 of the 9 peptides (3 epitopes) were also detected in CD4(+) T cells purified from the pancreatic draining lymph nodes of pre-diabetic female animals, but not from peripheral lymph nodes or spleens of the same animals. Intriguingly, one of the newly identified spontaneously reactive epitopes (P8 [IGRP(55-72)]) is highly conserved between mice and man, suggesting that it might also be a target of HLA-DQ8-restricted T cells in diabetic human subjects, an hypothesis that we are currently testing.

Divergent contributions of regulatory T cells to the pathogenesis of chronic hepatitis C

Hepatitis C virus, a small single-stranded RNA virus, is a major cause of chronic liver disease. Resolution of primary hepatitis C virus infections depends upon the vigorous responses of CD4(+) and CD8(+) T cells to multiple viral epitopes. Although such broad CD4(+) and CD8(+) T-cell responses are readily detected early during the course of infection regardless of clinical outcome, they are not maintained in individuals who develop chronic disease. Purportedly, a variety of factors contribute to the diminished T-cell responses observed in chronic, virus-infected patients including the induction of and biological suppression by CD4(+)FoxP3(+) regulatory T cells. Indeed, a wealth of evidence suggests that regulatory T cells play diverse roles in the pathogenesis of chronic hepatitis C, impairing the effector T-cell response and viral clearance early during the course of infection and suppressing liver injury as the disease progresses. The factors that affect the generation and biological response of regulatory T cells in chronic, hepatitis C virus-infected patients is discussed.

Long-term prevention of chronic allograft rejection by regulatory T-cell immunotherapy involves host Foxp3-expressing T cells

Administration of donor-specific regulatory T cells prevents chronic rejection of BM and skin allografts in the mouse. Injected regulatory T cells induce the emergence of host regulatory T cells with similar specificity thus ensuring persistence of tolerance.

Amino acid catabolism: a pivotal regulator of innate and adaptive immunity

Enhanced amino acid catabolism is a common response to inflammation, but the immunologic significance of altered amino acid consumption remains unclear. The finding that tryptophan catabolism helped maintain fetal tolerance during pregnancy provided novel insights into the significance of amino acid metabolism in controlling immunity. Recent advances in identifying molecular pathways that enhance amino acid catabolism and downstream mechanisms that affect immune cells in response to inflammatory cues support the notion that amino acid catabolism regulates innate and adaptive immune cells in pathologic settings. Cells expressing enzymes that degrade amino acids modulate antigen-presenting cell and lymphocyte functions and reveal critical roles for amino acid- and catabolite-sensing pathways in controlling gene expression, functions, and survival of immune cells. Basal amino acid catabolism may contribute to immune homeostasis that prevents autoimmunity, whereas elevated amino acid catalytic activity may reinforce immune suppression to promote tumorigenesis and persistence of some pathogens that cause chronic infections. For these reasons, there is considerable interest in generating novel drugs that inhibit or induce amino acid consumption and target downstream molecular pathways that control immunity. In this review, we summarize recent developments and highlight novel concepts and key outstanding questions in this active research field.

Dendritic cells, regulatory T cells and the pathogenesis of chronic hepatitis C

Hepatitis C virus (HCV) is a small, enveloped RNA virus and a major cause of chronic liver disease. Resolution of primary HCV infections depends upon the vigorous responses of CD4⁺ and CD8⁺ T cells to multiple viral epitopes. Although such broad-based responses are readily detected early during the course of infection regardless of clinical outcome, they are not maintained in individuals who develop chronic disease. Ostensibly, a variety of factors contribute to the diminished T cell responses observed in chronic, HCV-infected patients including impaired dendritic cell function and the induction of CD4⁺FoxP3⁺ regulatory T cells. Overwhelming evidence suggests that the complex interaction of dendritic cells and regulatory T cells plays a critical role in the pathogenesis of chronic hepatitis C.

Polyclonal CD4+Foxp3+ Treg cells induce TGFβ-dependent tolerogenic dendritic cells that suppress the murine lupus-like syndrome

Interplay between Foxp3(+) regulatory T cells (Treg) and dendritic cells (DCs) maintains immunologic tolerance, but the effects of each cell on the other are not well understood. We report that polyclonal CD4(+)Foxp3(+) Treg cells induced ex vivo with transforming growth factor beta (TGFβ) (iTreg) suppress a lupus-like chronic graft-versus-host disease by preventing the expansion of immunogenic DCs and inducing protective DCs that generate additional recipient CD4(+)Foxp3(+) cells. The protective effects of the transferred iTreg cells required both interleukin (IL)-10 and TGFβ, but the tolerogenic effects of the iTreg on DCs, and the immunosuppressive effects of these DCs were exclusively TGFβ-dependent. The iTreg were unable to tolerize Tgfbr2-deficient DCs. These results support the essential role of DCs in 'infectious tolerance' and emphasize the central role of TGFβ in protective iTreg/DC interactions in vivo.

Ethylenecarbodiimide-fixed donor splenocyte infusions differentially target direct and indirect pathways of allorecognition for induction of transplant tolerance

Strategic exposure to donor Ags prior to transplantation can be an effective way for inducting donor-specific tolerance in allogeneic recipients. We have recently shown that pretransplant infusion of donor splenocytes treated with the chemical cross-linker ethylenecarbodiimide (ECDI-SPs) induces indefinite islet allograft survival in a full MHC-mismatched model without the need for any immunosuppression. Mechanisms of allograft protection by this strategy remain elusive. In this study, we show that the infused donor ECDI-SPs differentially target T cells with indirect versus direct allospecificities. To target indirect allospecific T cells, ECDI-SPs induce upregulation of negative, but not positive, costimulatory molecules on recipient splenic CD11c(+) dendritic cells phagocytosing the injected ECDI-SPs. Indirect allospecific T cells activated by such CD11c(+) dendritic cells undergo robust initial proliferation followed by rapid clonal depletion. The remaining T cells are sequestered in the spleen without homing to the graft site or the graft draining lymph node. In contrast, direct allospecific T cells interacting with intact donor ECDI-SPs not yet phagocytosed undergo limited proliferation and are subsequently anergized. Furthermore, CD4(+)CD25(+)Foxp3(+) T cells are induced in lymphoid organs and at the graft site by ECDI-SPs. We conclude that donor ECDI-SP infusions target host allogeneic responses via a multitude of mechanisms, including clonal depletion, anergy, and immunoregulation, which act in a synergistic fashion to induce robust transplant tolerance. This simple form of negative vaccination has significant potential for clinical translation in human transplantation.

Immunity and tolerance to fungi in hematopoietic transplantation: principles and perspectives

Resistance and tolerance are two complementary host defense mechanisms that increase fitness in response to low-virulence fungi. Resistance is meant to reduce pathogen burden during infection through innate and adaptive immune mechanisms, whereas tolerance mitigates the substantial cost of resistance to host fitness through a multitude of anti-inflammatory mechanisms, including immunological tolerance. In experimental fungal infections, both defense mechanisms are activated through the delicate equilibrium between Th1/Th17 cells, which provide antifungal resistance, and regulatory T cells limiting the consequences of the ensuing inflammatory pathology. Indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in the tryptophan catabolism, plays a key role in induction of tolerance against fungi. Both hematopoietic and non-hematopoietic compartments contribute to the resistance/tolerance balance against Aspergillus fumigatus via the involvement of selected innate receptors converging on IDO. Several genetic polymorphisms in pattern recognition receptors influence resistance and tolerance to fungal infections in human hematopoietic transplantation. Thus, tolerance mechanisms may be exploited for novel diagnostics and therapeutics against fungal infections and diseases.

Regulation and function of mTOR signalling in T cell fate decisions

The evolutionarily conserved kinase mTOR (mammalian target of rapamycin) couples cell growth and metabolism to environmental inputs in eukaryotes. T cells depend on mTOR signalling to integrate immune signals and metabolic cues for their proper maintenance and activation. Under steady-state conditions, mTOR is actively controlled by multiple inhibitory mechanisms, and this enforces normal T cell homeostasis. Antigen recognition by naive CD4(+) and CD8(+) T cells triggers mTOR activation, which in turn programmes the differentiation of these cells into functionally distinct lineages. This Review focuses on the signalling mechanisms of mTOR in T cell homeostatic and functional fates, and discusses the therapeutic implications of targeting mTOR in T cells.

Neuroantigen-specific, tolerogenic vaccines: GM-CSF is a fusion partner that facilitates tolerance rather than immunity to dominant self-epitopes of myelin in murine models of experimental autoimmune encephalomyelitis (EAE)

Background

Vaccination strategies that elicit antigen-specific tolerance are needed as therapies for autoimmune disease. This study focused on whether cytokine-neuroantigen (NAg) fusion proteins could inhibit disease in chronic murine models of experimental autoimmune encephalomyelitis (EAE) and thus serve as potential therapeutic modalities for multiple sclerosis.

Results

A fusion protein comprised of murine GM-CSF as the N-terminal domain and the encephalitogenic MOG35-55 peptide as the C-terminal domain was tested as a tolerogenic, therapeutic vaccine (TTV) in the C57BL/6 model of EAE. Administration of GMCSF-MOG before active induction of EAE, or alternatively, at the onset of EAE blocked the development and progression of EAE. Covalent linkage of the GM-CSF and MOG35-55 domains was required for tolerogenic activity. Likewise, a TTV comprised of GM-CSF and PLP139-151 was a tolerogen in the SJL model of EAE.

Conclusion

These data indicated that fusion proteins containing GM-CSF coupled to myelin auto-antigens elicit tolerance rather than immunity.

The battle against immunopathology: infectious tolerance mediated by regulatory T cells

Infectious tolerance is a process whereby one regulatory lymphoid population confers suppressive capacity on another. Diverse immune responses are induced following infection or inflammatory insult that can protect the host, or potentially cause damage if not properly controlled. Thus, the process of infectious tolerance may be critical in vivo for exerting effective immune control and maintaining immune homeostasis by generating specialized regulatory sub-populations with distinct mechanistic capabilities. Foxp3(+) regulatory T cells (T(regs)) are a central mediator of infectious tolerance through their ability to convert conventional T cells into induced regulatory T cells (iT(regs)) directly by secretion of the suppressive cytokines TGF-β, IL-10, or IL-35, or indirectly via dendritic cells. In this review, we will discuss the mechanisms and cell populations that mediate and contribute to infectious tolerance, with a focus on the intestinal environment, where tolerance induction to foreign material is critical.

Common gamma chain cytokines promote rapid in vitro expansion of allo-specific human CD8+ suppressor T cells

Human CD8⁺ regulatory T cells, particularly the CD8⁺CD28⁻ T suppressor cells, have emerged as an important modulator of alloimmunity. Understanding the conditions under which these cells are induced and/or expanded would greatly facilitate their application in future clinical trials. In the current study, we develop a novel strategy that combines common gamma chain (γc) cytokines IL-2, IL-7 and IL-15 and donor antigen presenting cells (APCs) to stimulate full HLA-mismatched allogeneic human CD8⁺ T cells which results in significant expansions of donor-specific CD8⁺CD28⁻ T suppressor cells in vitro. The expanded CD8⁺CD28⁻ T cells exhibit increased expressions of CTLA-4, FoxP3, and CD25, while down-regulate expressions of CD56, CD57, CD127, and perforin. Furthermore, these cells suppress proliferation of CD4⁺ T cells in a contact-dependent and cytokine-independent manner. Interestingly, the specificity of suppression is restricted by the donor HLA class I antigens but promiscuous to HLA class II antigens, providing a potential mechanism for linked suppression. Taken together, our results demonstrate a novel role for common γc cytokines in combination with donor APCs in the expansion of donor-specific CD8⁺CD28⁻ T suppressor cells, and represent a robust strategy for in vitro generation of such cells for adoptive cellular immunotherapy in transplantation.

IDO and regulatory T cell support are critical for cytotoxic T lymphocyte-associated Ag-4 Ig-mediated long-term solid organ allograft survival

Costimulatory blockade of CD28-B7 interaction with CTLA4Ig is a well-established strategy to induce transplantation tolerance. Although previous in vitro studies suggest that CTLA4Ig upregulates expression of the immunoregulatory enzyme IDO in dendritic cells, the relationship of CTLA4Ig and IDO in in vivo organ transplantation remains unclear. In this study, we studied whether concerted immunomodulation in vivo by CTLA4Ig depends on IDO. C57BL/6 recipients receiving a fully MHC-mismatched BALB/c heart graft treated with CTLA4Ig + donor-specific transfusion showed indefinite graft survival (>100 d) without signs of chronic rejection or donor specific Ab formation. Recipients with long-term surviving grafts had significantly higher systemic IDO activity as compared with rejectors, which markedly correlated with intragraft IDO and Foxp3 levels. IDO inhibition with 1-methyl-dl-tryptophan, either at transplant or at postoperative day 50, abrogated CTLA4Ig + DST-induced long-term graft survival. Importantly, IDO1 knockout recipients experienced acute rejection and graft survival comparable to controls. In addition, αCD25 mAb-mediated depletion of regulatory T cells (Tregs) resulted in decreased IDO activity and again prevented CTLA4Ig + DST induced indefinite graft survival. Our results suggest that CTLA4Ig-induced tolerance to murine cardiac allografts is critically dependent on synergistic cross-linked interplay of IDO and Tregs. These results have important implications for the clinical development of this costimulatory blocker.

Immunobiology of transplantation: impact on targets for large and small molecules

Organ transplantation is the preferred method of treatment for many forms of end-stage organ failure. However, immunosuppressive drugs that are used to avoid rejection can result in numerous undesirable effects (infection, malignancy, hypertension, diabetes, and accelerated arteriosclerosis). Moreover, they are not effective at preventing chronic rejection resulting in late graft loss. This review summarizes the fundamental concepts underlying the rejection of solid-organ allografts with the aim of highlighting potential new targets for therapeutics. Future improvement will depend on new therapeutic moieties, including biologics, to target various pathways of both the innate and adaptive arms of immunity. Results from some of the most recent clinical trials in transplantation and emerging new therapies are also discussed.

Physiologic control of IDO competence in splenic dendritic cells

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.

Adenosine modulates Toll-like receptor function: basic mechanisms and translational opportunities

Adenosine is an endogenous purine metabolite whose concentration in human blood plasma rises from nanomolar to micromolar concentrations during the inflammatory process. Leukocytes express seven-transmembrane adenosine receptors whose engagement modulates Toll-like receptor-mediated cytokine responses, in part via modulation of intracellular cyclic adenosine monophosphate. Adenosine analogs are used clinically to treat arrhythmias and apnea of prematurity. Herein, we consider the potential of adenosine analogs as innate immune response modifiers to prevent and/or treat infection.

Comparative study of clinical grade human tolerogenic dendritic cells

BACKGROUND

The use of tolerogenic DCs is a promising therapeutic strategy for transplantation and autoimmune disorders. Immunomodulatory DCs are primarily generated from monocytes (MDDCs) for in vitro experiments following protocols that fail to fulfil the strict regulatory rules of clinically applicable products. Here, we compared the efficacy of three different tolerance-inducing agents, dexamethasone, rapamycin and vitamin D3, on DC biology using GMP (Good Manufacturing Practice) or clinical grade reagents with the aim of defining their use for human cell therapy.

METHODS

Tolerogenic MDDCs were generated by adding tolerogenic agents prior to the induction of maturation using TNF-α, IL-β and PGE2. We evaluated the effects of each agent on viability, efficiency of differentiation, phenotype, cytokine secretion and stability, the stimulatory capacity of tol-DCs and the T-cell profiles induced.

RESULTS

Differences relevant to therapeutic applicability were observed with the cellular products that were obtained. VitD3-induced tol-DCs exhibited a slightly reduced viability and yield compared to Dexa-and Rapa-tol-DCs. Phenotypically, while Dexa-and VitD3-tol-DCs were similar to immature DCs, Rapa-tol-DCs were not distinguishable from mature DCs. In addition, only Dexa-and moderately VitD3-tol-DCs exhibited IL-10 production. Interestingly, in all cases, the cytokine secretion profiles of tol-DCs were not modified by a subsequent TLR stimulation with LPS, indicating that all products had stable phenotypes. Functionally, clearly reduced alloantigen T cell proliferation was induced by tol-DCs obtained using any of these agent. Also, total interferon-gamma (IFN-γ) secretion by T cells stimulated with allogeneic tol-DCs was reduced in all three cases, but only T cells co-cultured with Rapa-tol-DCs showed impaired intracellular IFN-γ production. In addition, Rapa-DCs promoted CD4+ CD127 low/negative CD25high and Foxp3+ T cells.

CONCLUSIONS

Our results demonstrate contrasting influences of different clinical-grade pharmacological agents on human tol-DC generation. This should be taken into account for decisions on the use of a specific agent for the appropriate cellular therapy in the context of a particular disease.

Identification of IDO-positive and IDO-negative human dendritic cells after activation by various proinflammatory stimuli

Dendritic cells (DCs) can induce tolerance or immunity. We identified and characterized an IDO-expressing and an IDO-negative human DC population after stimulation by various proinflammatory stimuli. IDO expression was strongly dependent on the maturation status of the cells (CD83-positive cells only). The two DC subpopulations remained IDO positive and IDO negative, respectively, over a time period of at least 48 h. IDO enzyme activity of human DCs was highest during stimulation by strongly maturation-inducing TLR ligands such as highly purified LPS (TLR4 ligand) or polyriboinosinic-polyribocytidilic acid (TLR3 ligand); factors of the adaptive immune system such as IFN-γ, a mixture of cytokines, and IFN-α had lesser stimulatory capacity for IDO induction and activity. After stimulation with CD40L, IDO-positive DCs expressed significantly increased levels of B7 family molecules such as CD40, CD80, CD86, ICOS ligand, as well as PD-L1 (B7-H1) and PD-L2 (B7-DC) compared with the IDO-negative DC subset. At the same time, the inhibitory receptors Ig-like transcripts 3 and 4 were significantly downregulated on IDO-positive cells. Functionally, IDO-positive DCs produced significantly more IL-1β and IL-15 and less IL-10 and IL-6 than the IDO-negative subset after CD40L stimulation. These results show that IDO expression is associated with a distinctive phenotype and functional capacity in mature DCs. It seems likely that the IDO-positive DC subset possesses a regulatory function and might skew a T cell response toward tolerance.

Biomarkers of transplantation tolerance: more hopeful than helpful?

A major limitation to the translation of tolerogenic therapies to clinical transplantation is a lack of biomarkers that can be used as surrogate measures for predicting the successful induction of immune tolerance which would allow for the safe withdrawal of immunosuppression. We have used three different mouse models of donor specific tolerance to skin grafts together with quantitative RT-PCR to search for potential biomarkers of tolerance using criteria based on the presence or activity of regulatory T cells and antigen presenting cells (APCs) within grafts or lymphoid organs. We find that significant differences in gene expression between tolerated and rejecting grafts are observed primarily within the grafted skin and not systemically or in the draining lymph node. The pattern of gene expression within long-term surviving tolerated grafts appear very similar to syngeneic grafts, with both having low levels of T cell and APC infiltration and a bias toward relative over-expression of “regulatory-associated” genes, while allografts destined for rejection show an overall increase in both “regulatory” and “effector” cell associated transcripts. We also, however, find an increase in a large number of regulatory genes, of both innate and T cell origin, even after grafting syngeneic skin. Taken together, these findings suggest that there may be no tissue biomarkers uniquely able to predict donor antigen specific tolerance per se, but that patterns of gene expression within tolerated grafts may be similar to those found in self tissues recovering from an inflammatory insult.

Development of human CD4+FoxP3+ regulatory T cells in human stem cell factor-, granulocyte-macrophage colony-stimulating factor-, and interleukin-3-expressing NOD-SCID IL2Rγ(null) humanized mice

Human hematolymphoid mice have become valuable tools for the study of human hematopoiesis and uniquely human pathogens in vivo. Recent improvements in xenorecipient strains allow for long-term reconstitution with a human immune system. However, certain hematopoietic lineages, for example, the myeloid lineage, are underrepresented, possibly because of the limited cross-reactivity of murine and human cytokines. Therefore, we created a nonobese diabetic/severe combined immunodeficiency/interleukin-2 receptor-γ-null (NOD-SCID IL2Rγ(null)) mouse strain that expressed human stem cell factor, granulocyte-macrophage colony-stimulating factor, and interleukin-3, termed NSG-SGM3. Transplantation of CD34(+) human hematopoietic stem cells into NSG-SGM3 mice led to robust human hematopoietic reconstitution in blood, spleen, bone marrow, and liver. Human myeloid cell frequencies, specifically, myeloid dendritic cells, were elevated in the bone marrow of humanized NSG-SGM3 mice compared with nontransgenic NSG recipients. Most significant, however, was the increase in the CD4(+)FoxP3(+) regulatory T-cell population in all compartments analyzed. These CD4(+)FoxP3(+) regulatory T cells were functional, as evidenced by their ability to suppress T-cell proliferation. In conclusion, humanized NSG-SGM3 mice might serve as a useful model to study human regulatory T-cell development in vivo, but this unexpected lineage skewing also highlights the importance of adequate spatiotemporal expression of human cytokines for future xenorecipient strain development.

The dynamic processing of CD46 intracellular domains provides a molecular rheostat for T cell activation

Background

Adequate termination of an immune response is as important as the induction of an appropriate response. CD46, a regulator of complement activity, promotes T cell activation and differentiation towards a regulatory Tr1 phenotype. This Tr1 differentiation pathway is defective in patients with MS, asthma and rheumatoid arthritis, underlying its importance in controlling T cell function and the need to understand its regulatory mechanisms. CD46 has two cytoplasmic tails, Cyt1 and Cyt2, derived from alternative splicing, which are co-expressed in all nucleated human cells. The regulation of their expression and precise functions in regulating human T cell activation has not been fully elucidated.

Methodology/Principal Findings

Here, we first report the novel role of CD46 in terminating T cell activation. Second, we demonstrate that its functions as an activator and inhibitor of T cell responses are mediated through the temporal processing of its cytoplasmic tails. Cyt1 processing is required to turn T cell activation on, while processing of Cyt2 switches T cell activation off, as demonstrated by proliferation, CD25 expression and cytokine secretion. Both tails require processing by Presenilin/γSecretase (P/γS) to exert these functions. This was confirmed by expressing wild-type Cyt1 and Cyt2 tails and uncleavable mutant tails in primary T cells. The role of CD46 tails was also demonstrated with T cells expressing CD19 ectodomain-CD46 C-Terminal Fragment (CTF) fusions, which allowed specific triggering of each tail individually.

Conclusions/Significance

We conclude that CD46 acts as a molecular rheostat to control human T cell activation through the regulation of processing of its cytoplasmic tails.