Induction of regulatory T cells and dominant tolerance by dendritic cells incapable of full activation

An Experimental Model of Neuro-Immune Interactions in the Eye: Corneal Sensory Nerves and Resident Dendritic Cells

The cornea is an avascular connective tissue that is crucial, not only as the primary barrier of the eye but also as a proper transparent refractive structure. Corneal transparency is necessary for vision and is the result of several factors, including its highly organized structure, the physiology of its few cellular components, the lack of myelinated nerves (although it is extremely innervated), the tightly controlled hydration state, and the absence of blood and lymphatic vessels in healthy conditions, among others. The avascular, immune-privileged tissue of the cornea is an ideal model to study the interactions between its well-characterized and dense sensory nerves (easily accessible for both focal electrophysiological recording and morphological studies) and the low number of resident immune cell types, distinguished from those cells migrating from blood vessels. This paper presents an overview of the corneal structure and innervation, the resident dendritic cell (DC) subpopulations present in the cornea, their distribution in relation to corneal nerves, and their role in ocular inflammatory diseases. A mouse model in which sensory axons are constitutively labeled with tdTomato and DCs with green fluorescent protein (GFP) allows further analysis of the neuro-immune crosstalk under inflammatory and steady-state conditions of the eye.

Non-immunogenic Induced Pluripotent Stem Cells, a Promising Way Forward for Allogenic Transplantations for Neurological Disorders

Neurological disorder is a general term used for diseases affecting the function of the brain and nervous system. Those include a broad range of diseases from developmental disorders (e.g., Autism) over injury related disorders (e.g., stroke and brain tumors) to age related neurodegeneration (e.g., Alzheimer's disease), affecting up to 1 billion people worldwide. For most of those disorders, no curative treatment exists leaving symptomatic treatment as the primary mean of alleviation. Human induced pluripotent stem cells (hiPSC) in combination with animal models have been instrumental to foster our understanding of underlying disease mechanisms in the brain. Of specific interest are patient derived hiPSC which allow for targeted gene editing in the cases of known mutations. Such personalized treatment would include (1) acquisition of primary cells from the patient, (2) reprogramming of those into hiPSC via non-integrative methods, (3) corrective intervention via CRISPR-Cas9 gene editing of mutations, (4) quality control to ensure successful correction and absence of off-target effects, and (5) subsequent transplantation of hiPSC or pre-differentiated precursor cells for cell replacement therapies. This would be the ideal scenario but it is time consuming and expensive. Therefore, it would be of great benefit if transplanted hiPSC could be modulated to become invisible to the recipient's immune system, avoiding graft rejection and allowing for allogenic transplantations. This review will focus on the current status of gene editing to generate non-immunogenic hiPSC and how these cells can be used to treat neurological disorders by using cell replacement therapy. By providing an overview of current limitations and challenges in stem cell replacement therapies and the treatment of neurological disorders, this review outlines how gene editing and non-immunogenic hiPSC can contribute and pave the road for new therapeutic advances. Finally, the combination of using non-immunogenic hiPSC and in vivo animal modeling will highlight the importance of models with translational value for safety efficacy testing; before embarking on human trials.

Particulate-Based Single-Dose Local Immunosuppressive Regimen for Inducing Tolerogenic Dendritic Cells in Xenogeneic Islet Transplantation

Recent studies emphasize on developing immune tolerance by an interim administration of various immunosuppressive drugs. In this study, a robust protocol is reported for local immunomodulation using a single-dose of FK506 microspheres and clodronate liposomes (mFK+CLO) in a xenogeneic model of islet transplantation. Surprisingly, the single-dose treatment with mFK+CLO induce tolerance to the islet xenograft. The recipient mice display tolerogenic dendritic cells (tDCs) with decreased antigen presenting ability and T cell activation capacity. Furthermore, a reduced percentage of CD4⁺ and CD8⁺ T cells and an impaired differentiation of naïve CD4⁺ T cells into interferon-γ producing Th1 and interleukin-17 producing Th17 cells are observed. In addition, the immunosuppressive protocol leads to the generation of Foxp3⁺ regulatory T cells (Tregs) which are required for the long-term graft survival. The enhanced generation of tDCs and Tregs by the single treatment of mFK+CLO cause xenograft tolerance, suggesting a possible clinical strategy which may pave the way towards improving therapeutic outcomes of clinical islet transplantation.

Indirectly Activated Treg Allow Dominant Tolerance to Murine Skin-grafts Across an MHC Class I Mismatch After a Single Donor-specific Transfusion

BACKGROUND

Tolerance induced in stringent animal transplant models using donor-specific transfusions (DST) has previously required additional immunological manipulation. Here, we demonstrate a dominant skin-allograft tolerance model induced by a single DST across an major histocompatibility class I mismatch in an unmanipulated B6 host.

METHODS

C57BL/6 (H-2) (B6) mice were injected intravenously with splenocytes from B6.C.H-2 (H-2k) (bm1) or F1 (B6 × bm1) mice before skin transplantation. Mice were transplanted 7 days postinjection with donor (bm1 or F1) and third-party B10.BR (H-2) skin grafts.

RESULTS

B6 hosts acutely rejected skin grafts from B6.C.H-2 (bm1) and F1 (B6 × bm1) mice. A single transfusion of F1 splenocytes into B6 mice without any additional immune modulation led to permanent acceptance of F1 skin grafts. This graft acceptance was associated with persistence of donor cells long-term in vivo. The more rapid removal of DST bm1 cells than F1 cells was reduced by natural killer-cell depletion. Tolerant grafts survived an in vivo challenge with naive splenocytes. Both CD4CD25 and CD4CD25 T cells from F1 DST treated B6 mice suppressed alloproliferation in vitro. Tolerance was associated with expansion of peripheral Foxp3CD4CD25 regulatory T cells (Treg) and increased forkhead box P3 (Foxp3) expression in tolerant grafts. In tolerant mice, Foxp3 Treg arises from the proliferation of indirectly activated natural Foxp3 Treg (nTreg) and depletion of Foxp3 Treg abrogates skin-graft tolerance.

CONCLUSIONS

This study demonstrates that the persistence of transfused semiallogeneic donor cells mismatched at major histocompatibility class I can enhance tolerance to subsequent skin allografts through indirectly expanded nTreg leading to dominant tolerance without additional immunological manipulation.

Human Dendritic Cells: Their Heterogeneity and Clinical Application Potential in Cancer Immunotherapy

Dendritic cells (DC) are professional antigen presenting cells, uniquely able to induce naïve T cell activation and effector differentiation. They are, likewise, involved in the induction and maintenance of immune tolerance in homeostatic conditions. Their phenotypic and functional heterogeneity points to their great plasticity and ability to modulate, according to their microenvironment, the acquired immune response and, at the same time, makes their precise classification complex and frequently subject to reviews and improvement. This review will present general aspects of the DC physiology and classification and will address their potential and actual uses in the management of human disease, more specifically cancer, as therapeutic and monitoring tools. New combination treatments with the participation of DC will be also discussed.

Method of Generating Tolerogenic Maturation-Resistant Dendritic Cells and Testing for Their Immune-Regulatory Functions In Vivo in the Context of Transplantation

During that past two decades, advances in techniques for generating in vitro immune-suppressive dendritic cells (DCs) have heralded the use of these pro-tolerogenic DCs as therapeutics against transplant rejection and autoimmune diseases. In transplantation, previous dogma assumed that systemically administered therapeutic DCs bearing donor antigens (Ags) control the anti-donor response by directly interacting with anti-donor T cells in vivo. However, recent evidence indicates that the exogenously-administered therapeutic DCs instead function as Ag-transporting cells that transfer donor Ags to recipient's Ag-presenting cells (APCs) for presentation to T cells. In secondary lymphoid organs, presentation of acquired donor Ags by recipient's quiescent DCs triggers deficient activation and eventual apoptosis of donor-specific effector T cells, leading to a relative increase in the percentage of donor-specific regulatory T cells. This chapter describes the methodology to generate in vitro immune-suppressive DCs that are resistant to maturation, and to assess in vivo both their survival and their ability to regulate donor-specific T cells in a mouse model.

Tolerance through Education: How Tolerogenic Dendritic Cells Shape Immunity

Dendritic cells (DCs) are central players in the initiation and control of responses, regulating the balance between tolerance and immunity. Tolerogenic DCs are essential in the maintenance of central and peripheral tolerance by induction of clonal T cell deletion and T cell anergy, inhibition of memory and effector T cell responses, and generation and activation of regulatory T cells. Therefore, tolerogenic DCs are promising candidates for specific cellular therapy of allergic and autoimmune diseases and for treatment of transplant rejection. Studies performed in rodents have demonstrated the efficacy and feasibility of tolerogenic DCs for tolerance induction in various inflammatory diseases. In the last years, numerous protocols for the generation of human monocyte-derived tolerogenic DCs have been established and some first phase I trials have been conducted in patients suffering from autoimmune disorders, demonstrating the safety and efficiency of this cell-based immunotherapy. This review gives an overview about methods and protocols for the generation of human tolerogenic DCs and their mechanisms of tolerance induction with the focus on interleukin-10-modulated DCs. In addition, we will discuss the prerequisites for optimal clinical grade tolerogenic DC subsets and results of clinical trials with tolerogenic DCs in autoimmune diseases.

Precise immune tolerance for hPSC derivatives in clinical application

Human pluripotent stem cells (hPSCs) promise a foreseeing future for regeneration medicine and cell replacement therapy with their abilities to produce almost any types of somatic cells of the body. The complicated immunogenicity of hPSC derivatives and context dependent responses in variable transplantations greatly hurdle the practical application of hPSCs in clinic. Especially for applications of hPSCs, induction of immune tolerance at the same time increases the risks of tumorigenesis. Over the past few years, thanks to the progress in immunology and practices in organ transplantation, endeavors on exploring strategies to induce long term protection of allogeneic transplants have shed light on overcoming this barrier. Novel genetic engineering techniques also allow to precisely cradle the immune response of transplantation. Here we reviewed the current understanding on immunogenicity, and efforts have been attempted on inducing immune tolerance for hPSC derivatives, with extra focus on modifying the graft cells. We also glimpse on employing cutting-edge genome editing technologies for this purpose, which will potentially endow hPSC derivatives with the nature of wide spectrum drugs for therapy.

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.

Harnessing the properties of dendritic cells in the pursuit of immunological tolerance

The acquisition of self-perpetuating, immunological tolerance specific for graft alloantigens has long been described as the "holy grail" of clinical transplantation. By removing the need for life-long immunosuppression following engraftment, the adverse consequences of immunosuppressive regimens, including chronic infections and malignancy, may be avoided. Furthermore, autoimmune diseases and allergy are, by definition, driven by aberrant immunological responses to ordinarily innocuous antigens. The re-establishment of permanent tolerance towards instigating antigens may, therefore, provide a cure to these common diseases. Whilst various cell types exhibiting a tolerogenic phenotype have been proposed for such a task, tolerogenic dendritic cells (tol-DCs) are exquisitely adapted for antigen presentation and interact with many facets of the immune system: as such, they are attractive candidates for use in strategies for immune intervention. We review here our current understanding of tol-DC mediated induction and maintenance of immunological tolerance. Additionally, we discuss recent in vitro findings from animal models and clinical trials of tol-DC immunotherapy in the setting of transplantation, autoimmunity and allergy which highlight their promising therapeutic potential, and speculate how tol-DC therapy may be developed in the future.

Anti-CD3 treatment up-regulates programmed cell death protein-1 expression on activated effector T cells and severely impairs their inflammatory capacity

T cells play a key role in the pathogenesis of type 1 diabetes, and targeting the CD3 component of the T‐cell receptor complex provides one therapeutic approach. Anti‐CD3 treatment can reverse overt disease in spontaneously diabetic non‐obese diabetic mice, an effect proposed to, at least in part, be caused by a selective depletion of pathogenic cells. We have used a transfer model to further investigate the effects of anti‐CD3 treatment on green fluorescent protein (GFP)⁺ islet‐specific effector T cells in vivo. The GFP expression allowed us to isolate the known effectors at different time‐points during treatment to assess cell presence in various organs as well as gene expression and cytokine production. We find, in this model, that anti‐CD3 treatment does not preferentially deplete the transferred effector cells, but instead inhibits their metabolic function and their production of interferon‐γ. Programmed cell death protein 1 (PD‐1) expression was up‐regulated on the effector cells from anti‐CD3‐treated mice, and diabetes induced through anti‐PD‐L1 antibody could only be reversed with anti‐CD3 antibody if the anti‐CD3 treatment lasted beyond the point when the anti‐PD‐L1 antibody was washed out of the system. This suggests that PD‐1/PD‐L1 interaction plays an important role in the anti‐CD3 antibody mediated protection. Our data demonstrate an additional mechanism by which anti‐CD3 therapy can reverse diabetogenesis.

Is the skin a sanctuary for breast cancer cells during treatment with anti-HER2 antibodies?

The occurrence of skin metastases is a common event in patients affected by advanced breast cancer, usually associated with systemic disease progression. Here we describe 2 cases of diffuse cutaneous metastases from HER2-overexpressing breast cancer occurring despite a dramatic response in liver and bone, respectively, during treatment with anti-HER2 antibodies Trastuzumab and Pertuzumab. We discuss the reasons for this discrepancy and suggest a possible implication of impaired immune response in the skin. Future research should provide strategies to overcome the induction of immune privilege in the skin in order to avoid discontinuation of effective treatments.

Metabolism Is Central to Tolerogenic Dendritic Cell Function

Immunological tolerance is a fundamental tenant of immune homeostasis and overall health. Self-tolerance is a critical component of the immune system that allows for the recognition of self, resulting in hyporeactivity instead of immunogenicity. Dendritic cells are central to the establishment of dominant immune tolerance through the secretion of immunosuppressive cytokines and regulatory polarization of T cells. Cellular metabolism holds the key to determining DC immunogenic or tolerogenic cell fate. Recent studies have demonstrated that dendritic cell maturation leads to a shift toward a glycolytic metabolic state and preferred use of glucose as a carbon source. In contrast, tolerogenic dendritic cells favor oxidative phosphorylation and fatty acid oxidation. This dichotomous metabolic reprogramming of dendritic cells drives differential cellular function and plays a role in pathologies, such as autoimmune disease. Pharmacological alterations in metabolism have promising therapeutic potential.

Dendritic cells and pluripotency: unlikely allies in the pursuit of immunotherapy

As the fulcrum on which the balance between the opposing forces of tolerance and immunity has been shown to pivot, dendritic cells (DC) hold significant promise for immune intervention in a variety of disease states. Here we discuss how the directed differentiation of human pluripotent stem cells may address many of the current obstacles to the use of monocyte-derived DC in immunotherapy, providing a novel source of previously inaccessible DC subsets and opportunities for their scale-up, quality control and genetic modification. Indeed, given that it is the immunological legacy DC leave behind that is of therapeutic value, rather than their persistence per se, we propose that immunotherapy should serve as an early target for the clinical application of pluripotent stem cells.

Understanding Stem Cell Immunogenicity in Therapeutic Applications

Stem cells and their differentiated progeny offer great hope for treating disease by providing an unlimited source of cells for repairing or replacing damaged tissue. Initial studies suggested that, unlike 'normal' transplants, specific characteristics of stem cells enabled them to avoid immune attack. However, recent findings have revealed that the immunogenicity of stem cells may have been underestimated. Here, we review the current understanding of the mechanisms of immune recognition associated with stem cell immunogenicity, and discuss the relevance of reprogramming and differentiation strategies used to generate cells or tissue from stem cells for implantation in eliciting an immune response. We examine the effectiveness of current strategies for minimising immune attack in light of our experience in the transplantation field and, in this context, outline important challenges moving forward.

The paradoxical roles of C1q and C3 in autoimmunity

In this review we will focus on the links between complement and autoimmune diseases and will highlight how animal models have provided insights into the manner by which C1q and C3 act to modulate both adaptive and innate immune responses. In particular we will highlight how C1q may not only act as initiator of the classical complement pathway, but can also mediate multiple immune responses in a complement activation independent manner.

Intranasal peptide-induced tolerance and linked suppression: consequences of complement deficiency

A role for complement, particularly the classical pathway, in the regulation of immune responses is well documented. Deficiencies in C1q or C4 predispose to autoimmunity, while deficiency in C3 affects the suppression of contact sensitization and generation of oral tolerance. Complement components including C3 have been shown to be required for both B-cell and T-cell priming. The mechanisms whereby complement can mediate these diverse regulatory effects are poorly understood. Our previous work, using the mouse minor histocompatibility (HY) model of skin graft rejection, showed that both C1q and C3 were required for the induction of tolerance following intranasal peptide administration. By comparing tolerance induction in wild-type C57BL/6 and C1q-, C3-, C4- and C5-deficient C57BL/6 female mice, we show here that the classical pathway components including C3 are required for tolerance induction, whereas C5 plays no role. C3-deficient mice failed to generate a functional regulatory T (Treg) -dendritic cell (DC) tolerogenic loop required for tolerance induction. This was related to the inability of C3-deficient DC to up-regulate the arginine-consuming enzyme, inducible nitric oxide synthase (Nos-2), in the presence of antigen-specific Treg cells and peptide, leading to reduced Treg cell generation. Our findings demonstrate that the classical pathway and C3 play a critical role in the peptide-mediated induction of tolerance to HY by modulating DC function.

Effect of CD40 silenced dendritic cells by RNA interference on mice skin allograft rejection

Aim: Tolerogenic dendritic cells (DCs) play a critical role in inducing and maintaining tolerance. CD40 is a member of tumor necrosis factor receptor super family and is a potent T-cell costimulatory molecule. Therefore, in this study we evaluated the effect of CD40 silenced DCs by RNA interference on mice skin allograft rejection. Materials & methods: Skin transplantation was performed from C57BL/6 to BALB/c mouse. Skin allograft recipients were assigned to four groups (n = 5). CD40 downregulated DCs were injected to the BALB/c mice intravenously 7 days before transplantation. Then, graft survival time, Treg generation, CD4+ and CD8+ T cells infiltration and cytokine levels in serum of this group were compared with those of untreated and cyclosporine groups. Results: In comparison with untreated group, BALB/c mice injected with CD40 siRNA transfected DCs showed an increased graft survival time, Treg cells, IL-4 and IL-10 cytokine levels as well as decreased number of intragraft CD4+ and CD8+ T cells. IFN-γ and IL-12 secretion were diminished, too. Conclusion: Taken together, these data demonstrate that downregulation of CD40 in DCs can expand Treg cells and increase skin allograft survival.

Modulation of dendritic cell immunobiology via inhibition of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase

The maturation status of dendritic cells determines whether interacting T cells are activated or if they become tolerant. Previously we could induce T cell tolerance by applying a 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitor (HMGCRI) atorvastatin, which also modulates MHC class II expression and has therapeutic potential in autoimmune disease. Here, we aimed at elucidating the impact of this therapeutic strategy on T cell differentiation as a consequence of alterations in dendritic cell function. We investigated the effect of HMGCRI during differentiation of peripheral human monocytes and murine bone marrow precursors to immature DC in vitro and assessed their phenotype. To examine the stimulatory and tolerogenic capacity of these modulated immature dendritic cells, we measured proliferation and suppressive function of CD4+ T cells after stimulation with the modulated immature dendritic cells. We found that an HMGCRI, atorvastatin, prevents dendrite formation during the generation of immature dendritic cells. The modulated immature dendritic cells had a diminished capacity to take up and present antigen as well as to induce an immune response. Of note, the consequence was an increased capacity to differentiate naïve T cells towards a suppressor phenotype that is less sensitive to proinflammatory stimuli and can effectively inhibit the proliferation of T effector cells in vitro. Thus, manipulation of antigen-presenting cells by HMGCRI contributes to an attenuated immune response as shown by promotion of T cells with suppressive capacities.

Regulatory dendritic cells for immunotherapy in immunologic diseases

We recognize well the abilities of dendritic cells to activate effector T cell (Teff cell) responses to an array of antigens and think of these cells in this context as pre-eminent antigen-presenting cells, but dendritic cells are also critical to the induction of immunologic tolerance. Herein, we review our knowledge on the different kinds of tolerogenic or regulatory dendritic cells that are present or can be induced in experimental settings and humans, how they operate, and the diseases in which they are effective, from allergic to autoimmune diseases and transplant tolerance. The primary conclusions that arise from these cumulative studies clearly indicate that the agent(s) used to induce the tolerogenic phenotype and the status of the dendritic cell at the time of induction influence not only the phenotype of the dendritic cell, but also that of the regulatory T cell responses that they in turn mobilize. For example, while many, if not most, types of induced regulatory dendritic cells lead CD4⁺ naïve or Teff cells to adopt a CD25⁺Foxp3⁺ Treg phenotype, exposure of Langerhans cells or dermal dendritic cells to vitamin D leads in one case to the downstream induction of CD25⁺Foxp3⁺ regulatory T cell responses, while in the other to Foxp3⁻ type 1 regulatory T cells (Tr1) responses. Similarly, exposure of human immature versus semi-mature dendritic cells to IL-10 leads to distinct regulatory T cell outcomes. Thus, it should be possible to shape our dendritic cell immunotherapy approaches for selective induction of different types of T cell tolerance or to simultaneously induce multiple types of regulatory T cell responses. This may prove to be an important option as we target diseases in different anatomic compartments or with divergent pathologies in the clinic. Finally, we provide an overview of the use and potential use of these cells clinically, highlighting their potential as tools in an array of settings.

Regulatory T-cell therapy in transplantation: moving to the clinic

Regulatory T cells (Tregs) are essential to transplantation tolerance and their therapeutic efficacy is well documented in animal models. Moreover, human Tregs can be identified, isolated, and expanded in short-term ex vivo cultures so that a therapeutic product can be manufactured at relevant doses. Treg therapy is being planned at multiple transplant centers around the world. In this article, we review topics critical to effective implementation of Treg therapy in transplantation. We will address issues such as Treg dose, antigen specificity, and adjunct therapies required for transplant tolerance induction. We will summarize technical advances in Treg manufacturing and provide guidelines for identity and purity assurance of Treg products. Clinical trial designs and Treg manufacturing plans that incorporate the most up-to-date scientific understanding in Treg biology will be essential for harnessing the tolerogenic potential of Treg therapy in transplantation.

Prolongation of rat renal allograft survival by CD4⁺CD25⁻ T cells induced by recipient dendritic cells transfected with IKK2dn

Previous studies have demonstrated that recipient-derived immature dendritic cells transfected by recombinant adenovirus-mediated IKK2dn (Adv‑IKK2dn) and loaded with donor splenocyte lysate generate CD4+CD25- T cells (Adv-IKK2dn-CD4+CD25- T cells). These cells may inhibit T cell responses in vitro. In the present study, Lewis (LW) rats were administered with an intravenous injection of naive CD4+ T cells, empty adenovirus (Adv-0)-dendritic cell-generated CD4+CD25- T cells (Adv-0-CD4+CD25- T cells), Adv-IKK2dn-CD4+CD25- T cells or an equal volume of normal saline, seven days prior to transplantation. The potency and the mechanism of action of Adv-IKK2dn-CD4+CD25- T cells was analyzed, as well as an investigation of their tolerogenic properties in vivo. Administration of Adv-IKK2dn-CD4+CD25- T cells in vivo to LW rats was observed to markedly prolong the survival of a kidney allograft from Brown Norway rats. Furthermore, the Adv-IKK2dn-CD4+CD25- T cell-treated group exhibited significantly reduced levels of interleukin (Il)-2 and interferon-γ production and increased Il-10 and transforming growth factor-β (TGF-β) secretion. The serum creatinine levels remained at low levels in the Adv-IKK2dn-CD4+CD25- T cell-treated group. Their ability to induce allogeneic T cell proliferation was markedly reduced compared with the other groups. These observations indicated that Adv-IKK2dn-CD4+CD25- T cells induce prolongation of kidney allograft survival in vivo, which is hypothesized to be due to the high expression levels of Il-10 and TGF-β.

Adoptive transfusion of tolerance dendritic cells prolongs the survival of skin allografts in mice: a systematic review

OBJECTIVE

We aim to systematically review adoptive transfusion of tolerogenic dendritic cells (Tol-DCs) induced by different ways to affect skin allograft survival in mice.

METHODS

We searched PubMed and EMbase for relevant studies and evaluated the quality of included ones. Taking skin allograft survival time as endpoint outcome, we displayed outcomes of each group using one forest map and dissected possible mechanisms underlying survival prolongation.

RESULTS

We included 21 studies, which reported four methods of inducing Tol-DCs with different extents of average allograft survival prolongation: skin allograft survival time was prolonged (the drug intervention group, 63.08 ± 42.92 days, 4.6 folds to control; the cytokine induction group: 26.17 ± 16.20 days, 1.8 folds; the gene modification group: 14.65 ± 17.89 days, 1.5 folds; other derivation group 9.63 ± 24.38 days, 0.5 fold). Possible mechanisms underlying survival prolongation included induction of donor-specific T cell hyporesponsiveness, reduction of cytotoxicity against allografts, Th0 skewing to Th2, and generation or expansion of Treg. Infusion of Tol-DCs in combination with immunosuppressive agents or costimulatory blockade contributed to longer prolongation. Compared to MiHA mismatch, MHCI/II mismatch was a much more important factor to cause skin allograft rejection.

CONCLUSION

For MHC or MiHA mismatched, allogeneic skin transplants inbred recipients, adoptive transfusion of Tol-DCs induced by 4 methods prolong skin allograft survival to different extents. Drug intervened Tol-DCs works best. Immunosuppressive agents and/or co-stimulatory blockade contribute to better outcomes. Yet more rigorous studies with larger sample size are needed and more attention to mechanisms should be paid.

Pluripotent stem cells: immune to the immune system?

Human embryonic stem cells (hESCs), initially thought to be immune privileged cells, are now known to be susceptible to immune recognition. Human induced pluripotent stem cells (iPSCs) have been proposed as a potential source of autologous stem cells for therapy, but even these autologous stem cells may be targets of immune rejection. With clinical trials on the horizon, it is imperative that the immunogenicity of hESCs and iPSCs be definitively understood.

Induction of alloimmune tolerance in heart transplantation through gene silencing of TLR adaptors

Toll-like receptors (TLRs) activate biochemical pathways that evoke activation of innate immunity, which leads to dendritic cell (DC) maturation and initiation of adaptive immune responses that provoke allograft rejection. We aimed to prolong allograft survival by selectively inhibiting expression of the common adaptors of TLR signaling, namely MyD88 and TRIF, using siRNA. In vitro we demonstrated that blocking expression of MyD88 and TRIF led to reduced DC maturation. In vivo treatment of recipients with MyD88 and TRIF siRNA significantly prolonged allograft survival in the BALB/c > C57BL6 cardiac transplant model. Moreover, the combination of MyD88 and TRIF siRNA along with a low dose of rapamycin further extended the allograft survival (88.8 ± 7.1 days). Tissue histopathology demonstrated an overall reduction in lymphocyte interstitium infiltration, vascular obstruction and hemorrhage in mice treated with MyD88 and TRIF siRNA vector plus rapamycin. Furthermore, treatment was associated with an increase in the numbers of CD4(+) CD25(+) FoxP3(+) regulatory T cells and Th2 deviation. To our knowledge, this study is the first demonstration of prolonging the survival of allogeneic heart grafts through gene silencing of TLR signaling adaptors, highlighting the therapeutic potential of siRNA in clinical transplantation.

Memory T-cell-specific therapeutics attenuate allograft rejection via mediation of alloreactivity in memory cells

Many means in inbred rodent models promoted long-term graft survival or donor-specific tolerance, but less so in nonhuman primates, outbred rodents or human patients. A diverse repertoire of memory T cells, derived from heterologous immunity or prior to exposure to alloantigen, has been believed to be an important part of this barrier. Memory T cells have a unique capacity to generate effector functions quickly upon re-exposure to antigen, and this capacity is achieved by reduced activation thresholds, and expressed high level trafficking and adhesion molecules, which is likely responsible for their exhibiting differential susceptibility to immune therapeutics compared with naïve T cells. This review outlines recent progress on characteristics of memory T cells and focuses on these potential therapies targeting memory T cells which are likely to ameliorate allograft rejection by inducing transplant tolerance.

Rapamycin conditioning of dendritic cells differentiated from human ES cells promotes a tolerogenic phenotype

While human embryonic stem cells (hESCs) may one day facilitate the treatment of degenerative diseases requiring cell replacement therapy, the success of regenerative medicine is predicated on overcoming the rejection of replacement tissues. Given the role played by dendritic cells (DCs) in the establishment of immunological tolerance, we have proposed that DC, rendered tolerogenic during their differentiation from hESC, might predispose recipients to accept replacement tissues. As a first step towards this goal, we demonstrate that DC differentiated from H1 hESCs (H1-DCs) are particularly responsive to the immunosuppressive agent rapamycin compared to monocyte-derived DC (moDC). While rapamycin had only modest impact on the phenotype and function of moDC, H1-DC failed to upregulate CD40 upon maturation and displayed reduced immunostimulatory capacity. Furthermore, coculture of naïve allogeneic T cells with rapamycin-treated H1-DC promoted an increased appearance of CD25^hi Foxp3⁺ regulatory T cells, compared to moDC. Our findings suggest that conditioning of hESC-derived DC with rapamycin favours a tolerogenic phenotype.

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.

HLA-G level on monocytoid dendritic cells correlates with regulatory T-cell Foxp3 expression in liver transplant tolerance

BACKGROUND

Human leukocyte antigen (HLA)-G is a nonclassical HLA class I molecule expressed as membrane-bound and soluble isoforms. Interaction of HLA-G with its receptor, immunoglobulin-like transcript 4 on dendritic cells (DCs) down-regulates their T-cell stimulatory ability.

METHODS

We examined expression of HLA-G, immunoglobulin-like transcript 4, other immune regulatory molecules (inducible costimulator ligand and glucocorticoid-induced tumor necrosis factor-related receptor ligand), and the activation marker CMRF44 on circulating monocytoid dendritic cell (mDC) and plasmacytoid dendritic cell by monoclonal antibody staining and flow cytometry. Three groups of stable liver transplant recipients: operationally tolerant (TOL), prospective immunosuppressive drug weaning, and maintenance immunosuppression (MI) were studied, together with healthy controls (HC). Serum HLA-G levels were measured by enzyme-linked immunosorbent assay.

RESULTS

In TOL patients, monocytoid dendritic cell (mDC) but not plasmacytoid dendritic cell expressed higher HLA-G than in MI patients or HC. In TOL patients, the incidence of CD4(+)CD25(hi)CD127(-) regulatory T cells (Treg) and the intensity of Treg forkhead box p3 (Foxp3) expression were significantly higher than in the MI group. HLA-G expression on circulating mDC correlated significantly with that of Foxp3 in the TOL group. There was no correlation between immunosuppressive drug (tacrolimus) dose or trough level and HLA-G expression or Treg frequency or Foxp3 expression. The incidence of patients with circulating HLA-G levels more than 100 ng/mL was highest in the TOL group, although statistical significance was not achieved.

CONCLUSIONS

Higher HLA-G expression on circulating mDC in TOL recipients compared with MI or HC, suggests a possible role of HLA-G in immune regulation possibly mediated by enhanced host Treg Foxp3 expression.

Molecular mechanisms of induction of antigen-specific allograft tolerance by intranasal peptide administration

We have previously shown that intranasal (i.n.) administration of a single MHC class II-restricted HY peptide to female mice induces tolerance to up to five additional epitopes expressed on test male grafts, a phenomenon known as linked suppression. In this study, we investigated the molecular mechanisms involved both in the induction phase following peptide administration and during linked suppression after grafting. We report that following initial i.n. administration, peptide is widely disseminated and is presented by functionally immature dendritic cells. These fail to cause optimal stimulation of the responding HY-specific CD4(+) T cells that express genes characteristic of regulatory T cells. Following i.n. peptide plus LPS administration, causing immunization, HY-specific CD4(+) T cells express genes characteristic of activated T cells. We further find that following male skin grafting, HY-specific CD8(+) T cells from peptide-treated tolerant mice display both quantitative and qualitative differences compared with similar cells from untreated mice that reject their grafts. In tolerant mice there are fewer HY-specific CD8(+) cells and they express several genes characteristic of exhausted T cells. Furthermore, associated with specific chemokine receptor and integrin expression, HY-specific CD8(+) T cells show more limited migration from the graft draining lymph node into other tissues.

Future therapeutics for the induction of peripheral immune tolerance in autoimmune disease and organ transplantation

Rodent models of transplantation and autoimmune disease have demonstrated that it is possible to induce lifelong and specific immunological tolerance to both self and graft antigens in the absence of any continued immunosuppression. If this situation could be achieved clinically, it would avoid many of the longer-term complications of immunosuppression, such as the increased risk of infection, cancer and other side effects, such as nephrotoxicity. In this review, we shall consider the interplay between regulatory T cells, dendritic cells and the tissue itself, and the resulting local protective mechanisms that are coordinated to maintain the tolerant state and an acquired local immune privilege. The current status of attempts to apply tolerogenic approaches to the clinical treatment of autoimmune diseases and to induce either tolerance to organ grafts or sufficient immune regulation so that conventional immunosuppression can be minimized will also be considered.

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

A variety of different molecular mechanisms have been proposed to explain the suppressive action of regulatory T cells, including the production of anti-inflammatory cytokines, negative costimulatory ligands, indoleamine 2,3-dioxygenase-mediated tryptophan catabolism, CD73-mediated adenosine generation, and downregulation of antigen-presenting cells. Until now it has been unclear how important each of these different mechanisms might be and how they are coordinated. In this review, we examine the hypothesis that it is the interaction between regulatory T cells and dendritic cells that creates a local microenvironment depleted of essential amino acids and rich in adenosine that leads to the amplification of a range of different tolerogenic signals. These signals are all eventually integrated by mammalian target of rapamycin inhibition, which enables the induction of new forkhead box protein 3-expressing Tregs. If correct, this provides a molecular explanation for the in vivo phenomena of linked suppression and infectious tolerance.

Immunotherapy with myeloid cells for tolerance induction

PURPOSE OF REVIEW

Understanding the interplay between myeloid dendritic cells and T cells under tolerogenic conditions, and whether their interactions induce the development of antigen-specific regulatory T cells (Tregs) is critical to uncover the mechanisms involved in the induction of indefinite allograft survival.

RECENT FINDINGS

Myeloid dendritic cell-T-cell interactions are seminal events that determine the outcome of the immune response, and multiple in-vitro protocols suggest the generation of tolerogenic myeloid dendritic cells that modulate T-cell responses, and determine the outcome of the immune response to an allograft following adoptive transfer. We believe that identifying specific conditions that lead to the generation of tolerogenic myeloid dendritic cells and Tregs are critical for the manipulation of the immune response towards the development of transplantation tolerance.

SUMMARY

We summarize recent findings regarding specific culture conditions that generate tolerogenic myeloid dendritic cells that induce T-cell hyporesponsiveness and Treg development, which represents a novel immunotherapeutic approach to promote the induction of indefinite graft survival prolongation. The interpretations presented here illustrate that different mechanisms govern the generation of tolerogenic myeloid dendritic cells, and we discuss the concomitant therapeutic implications.

Mechanisms of tolerance and allergic sensitization in the airways and the lungs

The respiratory mucosa is constantly exposed to non-infectious substances that have the potential of triggering inflammation. While many particles are excluded, soluble molecules can reach the epithelium surface, where they can be uptaken by dendritic cells and stimulate an adaptive immune response. Most mucosal responses result in tolerance to subsequent antigen encounters, which is mediated by Foxp3(+) regulatory T cells. Genetic and environmental factors, added to the ability of certain allergens to induce innate responses, can predispose to allergic sensitization. In this review we discuss recent advances in the understanding of the mechanisms of tolerance and allergic sensitization to airborne allergens.

Approaches for immunological tolerance induction to stem cell-derived cell replacement therapies

The shortage of donors for organ transplantation and also to treat degenerative diseases has led to the development of the new field of regenerative medicine. One aim of this field, in addition to in vivo induction of endogenous tissue regeneration, is to utilize stem cells as a supplementary source of cells to repair or replace tissues or organs that have ceased to function owing to ageing or autoimmunity. Embryonic stem cells hold promise in this respect because of their developmental capacity to generate all tissues within the body. More recently, the discovery of induced pluripotent stem cells, somatic cells reprogrammed to a primitive embryonic-like state by the introduction of pluripotency factors, may also act as an important cell source for cell replacement therapy. However, before cell replacement therapy can become a reality, one must consider how to overcome the potential transplant rejection of stem cell-derived products. There are several potential ways to circumvent the hurdles presented by the immune system in this setting, not least the induction of immunological tolerance in the host. In this review, we consider this and other approaches for engendering acceptance of stem cell-derived tissues.

Tolerance: an overview and perspectives

Self tolerance is dependent on mechanisms that operate on T cells and B cells from the earliest stages, that is, from when they first express anti-self-receptors in the primary lymphoid organs of the thymus and bone marrow, all the way through to when they engage with self antigens in the peripheral immune system and within tissues themselves. This continuum of checkpoints and fail-safes ensures that the risk of developing harmful autoimmune diseases remains very small. Certain tissues have a degree of privilege that allows them to mute the immune response against them by mechanisms that are also well represented in cancers. An understanding of the underlying mechanisms of self tolerance is hoped to spawn a new range of therapeutics designed to both reprogram the immune system to avoid long-term intense immunosuppression, and to override the immune system to achieve more effective immunity against cancers and persistent viral infections.

Tolerogenicity is not an absolute property of a dendritic cell

Pharmacological modulation is known to temper the immune capacity of DC, enhancing the notion that modulated Ag-bearing DC might be used therapeutically to induce tolerance. We have investigated phenotypic features shared by such DC, and queried their potential to tolerize in different settings. Immature, IL-10, TGF-beta and 1alpha,25-dihydroxyvitamin D(3)-modulated BMDC all induced tolerance to male skin in female TCR transgenic A1.RAG mice, and the modulated DC also tolerized after exposure to the TLR4-ligand LPS. Transcript profiling revealed that this was achieved despite retaining much of the normal LPS-maturation response. No shared tolerance-associated transcripts could be identified. Equivalent BMDC could not tolerize in Marilyn TCR-transgenic mice. Simultaneous presentation of both A1.RAG and Marilyn peptide-Ag (Dby-H2E(k) and Dby-H2A(b)) on immature (C57BL/6JxCBA/Ca) F1 BMDC also only achieved tolerance in A1.RAG mice. Both strains registered Ag, but Foxp3(+) Treg were only induced in A1.RAG mice. In contrast, Marilyn T cells showed greater proliferation and an inflammatory bias, in response to Ag presented by immature F1 BMDC in vitro. In summary, while pharmacological agents can skew DC to reinforce their immature tolerogenic phenotype, the outcome of presentation is ultimately an integrated response including T-cell-intrinsic components that can over-ride for immune activation.

Immunohistochemical study of Langerhans cells in cutaneous lesions of the Jorge Lobo's disease

Jorge Lobo's disease is a chronic infection caused by the fungus Lacazia loboi endemic in South America. The infection is characterized by the appearance of parakeloidal, ulcerated or verrucous nodular or plaque-like cutaneous lesions. The histopathological aspect is characterized by poorly organized granulomas with histiocytes and multinucleated giant cells. Little is known about local immune response in lobomycosis skin lesions. Thirty-three skin biopsies from patients with Jorge Lobo's disease were selected from Ambulatory of Dermatology, UFPA. The control group was constituted by ten biopsies from normal skin. Langerhans cells were identified by immunohistochemistry using anti-CD1a antibody (Serotec). The number of positive cells was statistically analyzed. Langerhans cells were visualized along the epidermis in biopsies from Jorge Lobo's disease and the morphology and the number of Langerhans cells did not differ from normal skin (p>0.05). In Jorge Lobo's disease, this cell population probably presents some escape mechanism of the local immune system to evade the antigen presentation by those cells.

Tolerogenic dendritic cells: all present and correct?

Although well-recognized for their sentinel role and, when activated, their immunostimulatory function, bone marrow-derived dendritic cells (DC) possess inherent tolerogenic (tol) ability. Under quiescent conditions, these cells maintain central and peripheral self tolerance. When appropriately conditioned, in vitro or in vivo, they inhibit innate and adaptive immunity to foreign antigens, including memory T-cell responses. This suppressive function is mediated by various mechanisms, including the expansion and induction of antigen-specific regulatory T cells. Extensive experience in rodent models and recent work in nonhuman primates, indicate the potential of pharmacologically-modified, tol DC (tolDC) to regulate alloimmunity in vivo and to promote lasting, alloantigen-specific T-cell unresponsiveness and transplant survival. While there are many questions yet to be addressed concerning the functional biology of tolDC in humans, these cells offer considerable potential as natural, safe and antigen-specific regulators for long-term control of the outcome of organ and hematopoietic cell transplantation. This minireview surveys recent findings that enhance understanding of the functional biology and therapeutic application of tolDC, with special reference to transplantation.

CCR5 blockade in combination with rapamycin prolongs cardiac allograft survival in mice

Both chemokine receptor 5 (CCR5) blockade and rapamycin (rapa) are effective in modulating transplant immunity and led to prolonged allograft survival, yet a great many grafts were ultimately lost to acute rejection. In this study we examined the inhibition of CCR5 in combination with the treatment with rapa in cardiac transplantation. Fully major histocompatibility complex-mismatched murine cardiac allograft models were randomized to five groups. They were administered with anti-CCR5 antibody or control antibody and rapa or phosphate-buffered saline (PBS), respectively. An additional group was treated with anti-CCR5 antibody, rapa and anti-CD25 antibody. Allograft rejection was investigated by flow cytometric analyses and enzyme-linked immunospot assay. Allografts treated with anti-CCR5 antibody plus rapa showed significantly prolonged survival (83 +/- 3 days, P < 0.001) compared with control antibody plus PBS-treated allografts (6 +/- 1 days). Treatment with anti-CCR5 monoclonal antibody (mAb) plus rapa inhibited significantly the progression of chronic rejction. Further analysis of donor hearts in the anti-CCR5 antibody plus rapa-treated group demonstrated increased infiltration of CD4(+)CD25(+)forkhead box P3(+) regulatory T cells, and depletion of CD25(+) cells resulted in acute rejection of allografts in 18 +/- 1 day. CCR5 blockade in combination with rapa is effective in preventing acute and chronic rejection in a robust murine model. This effect is mediated by CD25(+) T cell recruitment and control of T lymphocyte proliferation.

Donor reactive regulatory T cells

PURPOSE OF REVIEW

Donor reactive regulatory T cells (Treg) play an important role in tolerance induction and maintenance in experimental transplant models. In this review we focus on the formation of the donor reactive Treg pool and explore the potential of these cells for therapeutic application in clinical transplantation.

RECENT FINDINGS

Donor reactive Treg can arise by both conversion of alloreactive nonregulatory cells and expansion of naturally occurring Treg (nTreg) cross-reactive with donor alloantigen but the quantitative contribution of each of these pathways is at present unclear. However, the fact that donor reactive Treg can be driven both in vivo and ex vivo by alloantigen challenge of nonregulatory precursors is encouraging as it demonstrates that the functional potential of these cells for use in clinical transplantation will not be limited by fortuitous cross-reactivity between nTreg and donor alloantigens. Treg can be generated in vivo by transplantation or alloantigen challenge in combination with Treg-permissive immunosuppression, or ex vivo by phenotypic selection or by polyclonal or antigen-specific stimulation. A number of ex-vivo protocols exist for the enrichment of Treg in the laboratory and in many cases these cells have demonstrable function both in vitro and in relevant graft-versus-host disease (GVHD) or organ transplant models. The challenge now is to understand the clinical opportunities and limitations that these populations present.

SUMMARY

Combined with appropriate immunosuppression, Treg generated/expanded in vivo or ex vivo may hold the final key to operational tolerance in clinical setting.