Dendritic cells as regulators of immune reactivity: implications for transplantation

Regulatory dendritic cells for human organ transplantation

Current immunosuppressive (IS) regimens used to prevent organ allograft rejection have well-recognized side effects, that include enhanced risk of infection and certain types of cancer, metabolic disorders, cardiovascular disease, renal complications and failure to control chronic allograft rejection. The life-long dependency of patients on these IS agents reflects their inability to induce donor-specific tolerance. Extensive studies in rodent and non-human primate models have demonstrated the ability of adoptively-transferred regulatory immune cells (either regulatory myeloid cells or regulatory T cells) to promote transplant tolerance. Consequently, there is considerable interest in the potential of regulatory immune cell therapy to allow safe minimization/complete withdrawal of immunosuppression and the promotion of organ transplant tolerance in the clinic. Here, we review the properties of regulatory dendritic cells (DCreg) with a focus on the approaches being taken to generate human DCreg for clinical testing. We also document the early phase clinical trials that are underway to assess DCreg therapy in clinical organ transplantation as well as in autoimmune disorders.

Laser scanning in vivo confocal microscopic characterization of equine immune-mediated keratitis

OBJECTIVE

To describe the corneal in vivo confocal microscopy (IVCM) findings in horses with putative immune-mediated keratitis (IMMK).

ANIMALS

Sixty five horses with IMMK.

PROCEDURES

Horses diagnosed with IMMK were examined with a modified Heidelberg Retina Tomograph II and Rostock Cornea Module. The findings from the IVCM examinations were correlated with clinical details from ophthalmic examination and diagnostic test results.

RESULTS

Eighty eyes from 65 horses were examined. Clinical IMMK lesions were categorized as epithelial (n = 17 eyes), superficial stromal (n = 38), midstromal (n = 18), and endothelial (n = 7). Epithelial, superficial stromal, and midstromal lesions were characterized with IVCM by variable corneal leukocyte infiltrates and vascularization of the approximate corneal anatomic region that was clinically affected as determined by biomicroscopy. In addition, all horses displayed a dense network of dendritic cells in the epithelial basement membrane and immediate subepithelial stroma. Less consistent IVCM findings included epithelial disorganization, corneal edema, mineral deposition, stromal fibrosis, and epithelial pigment granules. Endothelial IMMK was distinct from the other forms of IMMK and characterized with IVCM by stromal edema, endothelium disorganization, endothelial cell loss, and multifocal accumulations of highly reflective material within the endothelium.

CONCLUSIONS AND CLINICAL RELEVANCE

The distinguishing feature of epithelial and stromal forms of IMMK is a dense accumulation of dendritic cells in the epithelial basement membrane and immediate subepithelial stroma. Cellular changes in endothelial IMMK were largely confined to the endothelium and distinct from the other forms of IMMK evaluated.

Co-Stimulation-Impaired Bone Marrow-Derived Dendritic Cells Prevent Dextran Sodium Sulfate-Induced Colitis in Mice

Dendritic cells (DC) are important in the onset and severity of inflammatory bowel disease (IBD). Tolerogenic DC induce T-cells to become therapeutic Foxp3+ regulatory T-cells (Tregs). We therefore asked if experimental IBD could be prevented by administration of bone marrow-derived DC generated under conventional GM-CSF/IL-4 conditions but in the presence of a mixture of antisense DNA oligonucleotides targeting the primary transcripts of CD40, CD80, and CD86. These cell products (which we call AS-ODN BM-DC) have demonstrated tolerogenic activity in preventing type 1 diabetes and preserving beta cell mass in new-onset type 1 diabetes in the NOD mouse strain, in earlier studies. In addition to measuring efficacy in prevention of experimental IBD, we also sought to identify possible mechanism(s) of action. Weight, behavior, stool frequency, and character were observed daily for 7–10 days in experimental colitis in mice exposed to dextran sodium sulfate (DSS) following injection of the AS-ODN BM-DC. After euthanasia, the colons were processed for histology while spleen and mesenteric lymph nodes (MLNs) were made into single cells to measure Foxp3+ Treg as well as IL-10+ regulatory B-cell (Breg) population frequency by flow cytometry. AS-ODN BM-DC prevented DSS-induced colitis development. Recipients of these cells exhibited significant increases in Foxp3+ Treg and IL-10+ Breg in MLN and spleen. Histological examination of colon sections of colitis-free mice remained largely architecturally physiologic and mostly free of leukocyte infiltration when compared with DSS-treated animals. Although DSS colitis is mainly an innate immunity-driven condition, our study adds to the growing body of evidence showing that Foxp3+ Treg and IL-10 Bregs can suppress a mainly innate-driven inflammation. The already-established safety of human DC generated from monocytic progenitors in the presence of the mixture of antisense DNA targeting the primary transcripts of CD40, CD80, and CD86 in humans offers the potential to adapt them for clinical IBD therapy.

The immunologic considerations in human head transplantation

The idea of head transplantation appears at first as unrealistic, unethical, and futile. Here we discuss immunological considerations in human head transplantation. In a separate accompanying article we discuss surgical, ethical, and psychosocial issues concerned in body-to-head transplantation (BHT) [1]. The success of such an unusual allograft, where the donor and the recipient can reject each other, depends on prevention of complex immunologic reactions, especially rejection of the head by the body (graft-vs-host) or probably less likely, the possibility of the head rejecting the total body allograft (host-vs-graft). The technical and immunologic difficulties are enormous, especially since rapid nerve and cord connections and regeneration have not yet been possible to achieve. In this article we begin by briefly reviewing neuro-immunologic issues that may favor BHT such as the blood brain barrier (BBB) and point out its shortcomings. And we touch on the cellular and humoral elements in the brain proper that differ in some respects from those in other organs and in the periphery. Based on recent successes in vascular composite allografts (VCAs), we will elaborate on potential specific advantages and difficulties in BHT of various available immunosuppressive medications already utilized in VCAs. The risk/benefit ratio of these drugs will be emphasized in relation to direct brain toxicity such as seizure disorders, interference, or promotion of nerve regeneration, and potentiation of cerebral viral infections. The final portion of this article will focus on pre-transplant immunologic manipulation of the deceased donor body along with pretreatment of the recipient.

Zinc finger protein A20 inhibits maturation of dendritic cells resident in rat liver allograft

BACKGROUND

In organ transplant field, although viewed traditionally as instigators of organ allograft rejection, donor-derived interstitial dendritic cells (DCs), including those resident in liver, or host DCs have also been implicated in transplant tolerance in experimental models. This functional dichotomy of DC is governed by various factors, the most important of which appears to be their stage of maturation. This study was designed to examine the effect of zinc finger protein A20 on maturation of DCs resident in rat liver allograft.

MATERIALS AND METHODS

Allogeneic (Dark Agouti [DA] rat to Lewis rat) liver transplantation was performed. Adenovirus carrying the full length of A20 was introduced into liver allografts by ex vivo perfusion via the portal vein during preservation (group A20), physiological saline (group PS), and empty Ad vector rAdEasy (group rAdEasy) that served as controls. Acute liver allograft rejection was assessed, and DCs resident in liver allografts were isolated on day 7 after transplantation. Nuclear factor kappa B (NF-κB)-binding activities, surface expression of costimulatory molecules (CD40, CD80, and CD86), expression of interleukin (IL) 12 messenger RNA (mRNA), and allocostimulatory capacity of DCs were measured with electrophoretic mobility shift assay, flow cytometry, reverse transcription-polymerase chain reaction, and mixed lymphocyte reaction (MLR), respectively.

RESULTS

Ex vivo transfer of A20 adenovirus by portal vein infusion resulted in overexpression of A20 protein in liver allograft after transplantation. On day 7 after transplantation, histologic examination revealed a mild rejection in group A20 but a more severe rejection in group PS and group rAdEasy. DCs from group A20 liver allografts exhibited features of immature DC with detectable but very low level of NF-κB activity, IL-12 mRNA expression, and surface expression of costimulatory molecules (CD40, CD80, and CD86), whereas DCs from group rAdEasy and group PS liver allograft displayed features of mature DC with high level of NF-κB activity, IL-12 mRNA expression, and surface expression of costimulatory molecules (CD40, CD80, and CD86). DCs from group PS and group rAdEasy liver allograft were potent inducers of DNA synthesis and interferon γ production in MLR, and DCs from group A20 liver allografts induced only minimal levels of cell proliferation and interferon γ production in MLR.

CONCLUSIONS

These data suggest that A20 overexpression could effectively inhibit maturation of DCs resident in liver allograft and consequently suppress acute liver allograft rejection.

Stem cell transplantation in living donor renal transplantation for minimization of immunosuppression

BACKGROUND

We present our experience of living-donor renal transplantation (LDRT) using pretransplant stem cell transplantation (SCT) where we have successfully achieved minimization of immunosuppression.

METHODS

Nine hundred sixteen patients underwent LDRT between 2007 and 2011: 606 under tolerance induction protocol (TIP) and 310 with our usual triple immunosuppression of calcineurin inhibitors (CNI), mycofenolate sodium (MMF), and prednisone (controls). The test group (TIP) was stratified into group 1, 1 haplomatch or greater (n=392), group 3, less than 1 haplomatch (n=214); controls were similarly stratified to group 2, 1 haplomatch or greater (n=179) and group 4, less than 1 haplomatch (n=131). The TIP consisted of donor-specific transfusion, adipose tissue-derived mesenchymal and hematopoietic stem cell transplantation, and nonmyeloablative conditioning with total lymphoid irradiation, cyclophosphamide, and rabbit-antithymocyte globulin. Posttransplant IS consisted of prednisone, CNI, or MMF, all in low doses.

RESULTS

Four-year patient survival was 93.5%, 90.7%, 88.7%, and 82.7% in groups 1 through 4, respectively, and death-censored 4-year allograft survival was 94.8%, 95.4%, 94.5%, and 74.6%, respectively. Mean serum creatinine (mg/dL) for groups 1 through 4, respectively, at 4 years was 1.26, 1.57, 1.29, and 2.1. The number of rejection episodes was highest in group 4 and lowest in group 1. Minimization of IS was successfully achieved in 82.9% patients in group 1 and in 61.7% patients in group 3, whereas no minimization in groups 2 and 4. Stem cell transplantations were safe.

CONCLUSION

Stem cell transplantation is effective in IS minimization in LDRT resulting in good graft function and patient and graft survival at 4 years.

Differentiation and characterization of dendritic cells from human embryonic stem cells

Human embryonic stem cells (hESCs) offer great hope in regenerative medicine. Their ability to give rise to almost any type of cell present in the adult body makes them an invaluable tool in finding cures for a variety of diseases. While considerable protocols have been devised to efficiently differentiate hESCs into various cells types including cells of hematopoietic origin, this protocol will focus on the derivation of dendritic cells (DC), a potent antigen-presenting cell. DCs are a highly important arm of the immune system, as they represent one of the few cells that bridge the innate and adaptive systems, leading to effective pathogen clearance. The study of DCs has led to potential applications in diverse fields, such as vaccine development, tumor immunology, and transplantation. In this protocol, we describe two different methods of differentiating hESCs into DCs. The first method uses OP9 bone marrow stromal supporting cells as a coculture system, while the second method utilizes the formation of embryoid body (EB, cellular aggregate) as an approach. To assure the successful outcome and subsequent assessment of the differentiated DCs, supporting protocols have been included in this chapter.

Effects of TLR agonists on the hypoxia-regulated transcription factor HIF-1alpha and dendritic cell maturation under normoxic conditions

Dendritic cells (DC) are professional antigen presenting cells that represent an important link between innate and adaptive immunity. Danger signals such as toll-like receptor (TLR) agonists induce maturation of DC leading to a T-cell mediated adaptive immune response. In this study, we show that exogenous as well as endogenous inflammatory stimuli for TLR4 and TLR2 induce the expression of HIF-1alpha in human monocyte-derived DC under normoxic conditions. On the functional level, inhibition of HIF-1alpha using chetomin (CTM), YC-1 and digoxin lead to no consistent effect on MoDC maturation, or cytokine secretion despite having the common effect of blocking HIF-1alpha stabilization or activity through different mechanisms. Stabilization of HIF-1alpha protein by hypoxia or CoCl(2) did not result in maturation of human DC. In addition, we could show that TLR stimulation resulted in an increase of HIF-1alpha controlled VEGF secretion. These results show that stimulation of human MoDC with exogenous as well as endogenous TLR agonists induces the expression of HIF-1alpha in a time-dependent manner. Hypoxia alone does not induce maturation of DC, but is able to augment maturation after TLR ligation. Current evidence suggests that different target genes may be affected by HIF-1alpha under normoxic conditions with physiological roles that differ from those induced by hypoxia.

Stratification of Antigen-presenting Cells within the Normal Cornea

The composition and location of professional antigen presenting cells (APC) varies in different mucosal surfaces. The cornea, long considered an immune-privileged tissue devoid of APCs, is now known to host a heterogeneous network of bone marrow-derived cells. Here, we utilized transgenic mice that express enhanced green fluorescent protein (EGFP) from the CD11c promoter (pCD11c) in conjunction with immunohistochemical staining to demonstrate an interesting stratification of APCs within non-inflamed murine corneas. pCD11c⁺ dendritic cells (DCs) reside in the basal epithelium, seemingly embedded in the basement membrane. Most DCs express MHC class II on at least some dendrites, which extend up to 50 μm in length and traverse up 20 μm tangentially towards the apical surface of the epithelium. The DC density diminishes from peripheral to central cornea. Beneath the DCs and adjacent to the stromal side of the basement membrane reside pCD11c⁻CD11b⁺ putative macrophages that express low levels of MHC class II. Finally, MHC class II^-pCD11c⁻CD11b⁺ cells form a network throughout the remainder of the stroma. This highly reproducible stratification of bone marrow-derived cells is suggestive of a progression from an APC function at the exposed corneal surface to an innate immune barrier function deeper in the stroma.

Biomarkers to discern transplantation tolerance after allogeneic hematopoietic cell transplantation

Although it is commonly accepted that allogeneic hematopoietic cell transplant (HCT) recipients develop transplantation tolerance and can quickly discontinue all immunosuppressive drugs, existing data does not support this concept. Most patients will require a prolonged duration of immunosuppression, lasting commonly several years. This has even greater importance, as the majority of transplants are now performed utilizing peripheral blood mobilized stem cells, which are associated with an increased risk of chronic graft-versus-host disease (cGVHD) and prolonged duration of immunosuppression. Despite these challenges, the approach to liberation from immunosuppression after HCT is empiric, and biomarkers of operational tolerance after HCT are lacking. Conversely, investigators in solid organ allografting have begun to examine tolerance associated gene expression in renal and hepatic allograft recipients. Significant challenges in the design and interpretation of these studies potentially limit comparisons. However, a relatively unified model is beginning to emerge, which largely recapitulates previously established mechanisms of immune tolerance. This evidence supports a state of immune quiescence with reduced expression of costimulation and immune response genes, and upregulation of cell cycle control genes. Data indirectly supports the importance of tumor growth factor (TGF)-beta, supports the role of CD4(+)CD25(+) regulatory T cells, and offers new insights into the role of natural killer (NK) cells. Distinct in hepatic allograft tolerance, emerging evidence highlights the importance of gammadeltaT cells, and selection of the Vgammadelta1+ subtype among the gammadeltaT cell population. The deficiencies in the current understanding of transplantation tolerance after HCT, as well as the inadequacies evident in the current empiric approach to immunosuppressive medication (IS) management after HCT make clear the rationale for investigation aimed at elucidating tolerance associated biomarkers after HCT.

Induction of allogeneic mixed chimerism by immature dendritic cells and bone marrow transplantation leads to prolonged tolerance to major histocompatibility complex disparate allografts

Mixed chimerism has been shown to lead to prolonged major histocompatibility complex (MHC) disparate allograft survival and immune-specific tolerance; however, traditional conditioning regimes often involve myeloablation, which may pose a significant safety risk. In this study we examined the use of donor C57BL/6 (H-2(b)) immature dendritic cells (imDCs) to tolerize the BALB/c (H-2(d)) recipient to bone marrow transplantation (BMT), allowing the induction of mixed chimerism without immunosuppression or myeloablation. We showed that successful mismatched bone marrow engraftment can be achieved using imDCs given up to 3 days prior to BMT and that mixed chimerism can be established and detected in excess of 100 days post-BMT without evidence of graft-versus-host disease. Furthermore, we showed that imDCs can suppress lymphocyte proliferation in response to mismatched MHC stimulation, leading to increased expression of interleukin (IL)-4 and IL-10 and decreased expression of IL-2 and interferon-gamma (IFN-gamma). The induction of stable chimeras through pre-conditioning of mice with donor imDCs followed by BMT led to tolerance, allowing the long-term survival (> 110 days) of mismatched cardiac allografts and the prolonged survival of mismatched skin allografts without the need for immunosuppression or myeloablation. Transplantation with third-party C3H allografts were rapidly rejected in this model, suggesting that immune-specific tolerance was achieved. The induction of immune-specific tolerance without the need for immunosuppression or myeloablation represents a significant advance in transplant immunology and may provide clinicians with a plausible alternative in combating organ rejection following transplantation.

A pcDNA-Ehcpadh vaccine against Entamoeba histolytica elicits a protective Th1-like response in hamster liver

DNA vaccines are promising tools to fight parasitic diseases, including amoebiasis caused by the protozoan Entamoeba histolytica. Here we studied the immunogenicity and protective efficacy of a DNA vaccine against this parasite composed by the EhCPADH surface complex encoding genes (Ehcp112 and Ehadh112). EhCPADH is formed by an adhesin (EhADH112) and a cysteine proteinase (EhCP112), both involved in the parasite virulence. We evaluated transcription, protein expression, immunological response and protection against hepatic amoebiasis in hamsters intradermally and intramuscularly immunized with a mixture of pcDNA-Ehadh112 and pcDNA-Ehcp112 plasmids. RT-PCR and immunohistochemical assays showed that both antigens were differentially expressed in spleen and liver of immunized animals. No significant antibody immune response was induced by either route. However, intradermally inoculated hamsters presented a robust Th1-like immune response, characterized by high levels of INF-gamma and TNF-alpha cytokines, detected in the liver of animals challenged with virulent trophozoites. Animals showed significant protection against amoebiasis manifested by a higher survival rate and a significant prevention of liver abscess formation. We conclude that a refinement of this DNA vaccine could be a good choice to control hepatic amoebiasis.

New strategies in immune tolerance induction

Induction of tolerance in clinical organ transplantation that will obviate the use of chronic immunosuppression and preserve host immune response to other antigens remains the goal of transplant research. The thymus plays a critical role in the ability of the immune system to discriminate between self- and nonself-antigens or harmful and harmless alloantigens. We now know that multiple factors determine how the immune system responds to a self-antigen or foreign antigen. These determinants include developmental stage of the host, stage of T-cell maturity, site of antigen encounter, type and maturity of antigen-presenting cells, and presence and type of costimulatory molecules. Our understanding of the mechanisms of T-cell interactions with peptide/ major histocompatibility complex in peripheral lymphoid organs has led to experiments that translate into peripheral T-cell tolerance. The induction of high-avidity peripheral alloreactive T cells in the early phase of organ transplantation makes it difficult to achieve long-term alloantigen-specific tolerance without the use of transient perioperative immunosuppression. Therefore, protocols that induce robust tolerance in rodent and nonhuman primate models involve the use of donor antigen combined with a short course of perioperative immunosuppression. These studies suggest that the underlying mechanisms of peripheral tolerance include deletion, anergy, immune deviation, and regulatory T cells. This review focuses on recent advances in tolerance induction in experimental animal models and discusses their relevance to the development of protocols for the induction and maintenance of clinical transplant tolerance.

The complement inhibitor low molecular weight dextran sulfate prevents TLR4-induced phenotypic and functional maturation of human dendritic cells

Low molecular weight dextran sulfate (DXS) has been reported to inhibit the classical, alternative pathway as well as the mannan-binding lectin pathway of the complement system. Furthermore, it acts as an endothelial cell protectant inhibiting complement-mediated endothelial cell damage. Endothelial cells are covered with a layer of heparan sulfate (HS), which is rapidly released under conditions of inflammation and tissue injury. Soluble HS induces maturation of dendritic cells (DC) via TLR4. In this study, we show the inhibitory effect of DXS on human DC maturation. DXS significantly prevents phenotypic maturation of monocyte-derived DC and peripheral myeloid DC by inhibiting the up-regulation of CD40, CD80, CD83, CD86, ICAM-1, and HLA-DR and down-regulates DC-SIGN in response to HS or exogenous TLR ligands. DXS also inhibits the functional maturation of DC as demonstrated by reduced T cell proliferation, and strongly impairs secretion of the proinflammatory mediators IL-1beta, IL-6, IL-12p70, and TNF-alpha. Exposure to DXS leads to a reduced production of the complement component C1q and a decreased phagocytic activity, whereas C3 secretion is increased. Moreover, DXS was found to inhibit phosphorylation of IkappaB-alpha and activation of NF-kappaB. These findings suggest that DXS prevents TLR-induced maturation of human DC and may therefore be a useful reagent to impede the link between innate and adaptive immunity.

CTLA4-Ig-modified dendritic cells inhibit lymphocyte-mediated alloimmune responses and prolong the islet graft survival in mice

The induction of antigen specific tolerance is critical for prevention and treatment of allograft rejection. In this study, we transfected CTLA4-Ig gene into dendritic cells (DCs), and investigated their effect on inhibition of lymphocyte activity in vitro and induction of immune tolerance on pancreatic islet allograft in mice. An IDDM C57BL/6 murine model induced by streptozotocin is as model mouse. The model mice were transplanted of the islet cells isolated from the BALB/c mice to their kidney capsules, and injected of CTLA4-Ig modified DCs (mDCs). The results showed that mDCs could significantly inhibit T lymphocyte proliferation and induce its apoptosis; whereas, unmodified DCs (umDCs) promoted the murine lymphocyte proliferation. Compared with injection of umDCs and IgG1 modified DCs, the injection of mDCs prolonged IDDM mice's allograft survival, and normalized their plasma glucose (PG) levels within 3 days and maintained over 2 weeks. The level of IFN-gamma was lower and the level of IL-4 was higher in mDCs treated recipient mice than that in control mice, it indicated that mDCs led to Th1/Th2 deviation. After 7 days of islet transplantation, HE stain of the renal specimens showed that the islets and kidneys were intact in structure, and islet cells numbers are increased in mDCs treated mice. Our studies suggest that DCs expressing CTLA4-Ig fusion protein can induce the immune tolerance to islet graft and prolong the allograft survival through the inhibition of T cell proliferation in allogeneic mice.

NK026680, a novel compound suppressive of dendritic cell function, ameliorates mortality in acute lethal graft-versus-host reaction in mice

A role for dendritic cells (DCs) has been emphasized in the onset of acute graft-versus-host disease (GVHD). We have made efforts to develop a new strategy for suppression of DC functions with a chemical compound in the treatment of acute GVHD. We here describe the immunological characterization of the new chemical compound NK026680. It was found that NK026680 significantly suppressed (1) expression of CD83, CD86, and major histocompatibility complex (MHC) class I and II antigens on human monocyte-derived DCs, (2) excretion of interleukin-12p40 on activation of monocyte-derived DCs, (3) allogeneic responses of human and mouse T cells and (4) mortality in mice with acute GVHD evoked across MHC class I or II. The beneficial effect of NK026680 administered orally was without any recognizable adverse effects. Early intervention in acute GVHD was required for this effect, indicating that an early event in acute GVHD is a critical target of NK026680. We propose the use of NK026680 as a prophylactic for acute GVHD.

Facilitating physiologic self-regeneration: a step beyond islet cell replacement

Type 1 diabetes (T1D) is an autoimmune disease, the clinical onset of which most frequently presents in children and adolescents who are genetically predisposed. T1D is characterized by specific insulin-producing beta cell destruction. The well-differentiated and specialized islet beta cells seem to physiologically retain the ability to compensate for the cells lost by reproducing themselves, whereas undifferentiated cell sources may help in generating new ones, even while the autoimmune process takes place. Diabetes clinical onset, i.e., establishment of a detectable, chronic hyperglycemia, occurs at a critical stage when autoimmunity, having acted for a while, supersedes the regenerative effort and reduces the number of beta cells below the physiologic threshold at which the produced insulin becomes insufficient for the body's needs. Clinical solutions aimed at avoiding cumbersome daily insulin administrations by the reestablishment of physiologic insulin production, like whole pancreas or pancreatic islet allotransplantation, are limited by the scarcity of pancreas donors and by the toxic effects of the immunosuppressive drugs administered to prevent rejection. However, new accumulating evidence suggests that, once autoimmunity is abrogated, the endocrine pancreas properties may be sufficient to allow the physiological regenerative process to restore endogenous insulin production, even after the disease has become clinically manifest. Knowledge of these properties of the endocrine pancreas suggests the testing of reliable and clinically translatable protocols for obliterating autoimmunity, thus allowing the regeneration of the patient's own endocrine cells. The safe induction of an autoimmunity-free status might become a new promising therapy for T1D.

Plasmacytoid precursor dendritic cells facilitate allogeneic hematopoietic stem cell engraftment

Bone marrow transplantation offers great promise for treating a number of disease states. However, the widespread application of this approach is dependent upon the development of less toxic methods to establish chimerism and avoid graft-versus-host disease (GVHD). CD8⁺/TCR⁻ facilitating cells (FCs) have been shown to enhance engraftment of hematopoietic stem cells (HSCs) in allogeneic recipients without causing GVHD. In the present studies, we have identified the main subpopulation of FCs as plasmacytoid precursor dendritic cells (p-preDCs). FCs and p-preDCs share many phenotypic, morphological, and functional features: both produce IFN-α and TNF-α, both are activated by toll-like receptor (TLR)-9 ligand (CpG ODN) stimulation, and both expand and mature after Flt3 ligand (FL) treatment. FL-mobilized FCs, most of which express a preDC phenotype, significantly enhance engraftment of HSCs and induce donor-specific tolerance to skin allografts. However, p-preDCs alone or p-preDCs from the FC population facilitate HSC engraftment less efficiently than total FCs. Moreover, FCs depleted of preDCs completely fail to facilitate HSC engraftment. These results are the first to define a direct functional role for p-preDCs in HSC engraftment, and also suggest that p-preDCs need to be in a certain state of maturation/activation to be fully functional.

Transplantation tolerance in pediatric recipients: lessons and challenges

Clinical transplantation tolerance has remained an elusive goal in the 50 yr since it was first described in experimental animals. Greater understanding of the molecular mechanisms responsible for allorecognition have allowed for the development of promising immunosuppressive strategies that may bring us closer to reproducible induction of tolerance; consideration of past successes and failures from both clinical and basic science is required to define future challenges facing this field. This article reviews mechanisms of self and transplantation tolerance, translation of basic science research to clinical protocols in animals and human beings, the changing role of immunosuppression, complications following tolerance induction and controversies surrounding the choice of patients for tolerance trials with a focus on issues relevant to pediatric patients. The role of the Immune Tolerance Network is discussed along with realistic goals for tolerance induction in human beings over the next decade.

Prolonged survival of donor-specific rat intestinal allograft by administration of bone-marrow-derived immature dendritic cells

It has been reported that intraportal administration of donor antigens induced donor-specific hyporesponsiveness. We studied here the effects of transplantation of BM-derived immature dendritic cells (imDCs) and mature DCs (mDCs) via portal vein on rat small intestinal allograft survival. This study comprised four treatment groups: 1) untreated controls; 2) FK506 alone; 3) intraportal donor-specific BM-derived imDCs transplantation+FK506; 4) mDCs/Tx+FK506. Allograft survival was minimal in control group (5.2+/-0.8 days) and maximal in imDC+FK506 group (28.4+/-3.0 days). The rats in mDC+FK506 group showed systemic inflammatory reaction due to GVHR, and died within 10 days after transplantation. The in vitro MLR reaction using imDCs was also strongly inhibited both in direct and indirect recognition pathways. The impact of imDCs for the specific induction of transplant tolerance may suggest that immunization with donor-specific imDCs has therapeutic potential in organ transplantation.

Systematic evaluation of the conditions required for the generation of immature rat bone marrow-derived dendritic cells and their phenotypic and functional characterization

This study systematically evaluated the conditions required for generating immature rat bone marrow-derived dendritic cells (BMDCs) and characterized their phenotype. The culture of Wistar rat bone marrow cells for 7 days in an optimal cytokine environment (granulocyte macrophage-colony stimulating factor (GM-CSF), 10 ng/ml; IL-4, 5 ng/ml) resulted in adherent and non-adherent cell populations, but only the adherent population predominantly expressed the rat DC marker OX62. Adherent OX62+ cells were immature, in that they expressed lower levels of CD86 and MHC class II and were more phagocytic than their non-adherent OX62+ counterparts. Adherent BMDCs constitutively produced low levels of IL-12 and nitric oxide (NO), levels of both of which were markedly increased following lipopolysaccharide (LPS) activation. Activation also increased the proportion of OX62+ cells expressing CD40, CD54 and CD86 and their intensity of expression, however, unlike murine BMDCs, it had no effect on CD80 and MHC class II expression. Although the proliferation of allogeneic Lewis splenocytes in response to immature resting and LPS-activated (mature) Wistar BMDCs was of a similar magnitude, levels of IL-12 after 5 days were significantly higher in cultures containing LPS-activated BMDCs and the IFN-gamma/IL-4 cytokine ratio differed markedly (2.35 vs. 6.66, respectively). This study systematically defines conditions for generating immature rat BMDC populations and demonstrates qualitative differences in the phenotype of immune responses induced by resting and LPS-activated BMDC populations.

Antisense Oligonucleotides Down-Regulating Costimulation Confer Diabetes-Preventive Properties to Nonobese Diabetic Mouse Dendritic Cells

Phenotypically "immature" dendritic cells (DCs), defined by low cell surface CD40, CD80, and CD86 can elicit host immune suppression in allotransplantation and autoimmunity. Herein, we report the most direct means of achieving phenotypic immaturity in NOD bone marrow-derived DCs aiming at preventing diabetes in syngeneic recipients. CD40, CD80, and CD86 cell surface molecules were specifically down-regulated by treating NOD DCs ex vivo with a mixture of antisense oligonucleotides targeting the CD40, CD80, and CD86 primary transcripts. The incidence of diabetes was significantly delayed by a single injection of the engineered NOD DCs into syngeneic recipients. Insulitis was absent in diabetes-free recipients and their splenic T cells proliferated in response to alloantigen. Engineered DC promoted an increased prevalence of CD4(+)CD25(+) T cells in NOD recipients at all ages examined and diabetes-free recipients exhibited significantly greater numbers of CD4(+)CD25(+) T cells compared with untreated NOD mice. In NOD-scid recipients, antisense-treated NOD DC promoted an increased prevalence of these putative regulatory T cells. Collectively, these data demonstrate that direct interference of cell surface expression of the major costimulatory molecules at the transcriptional level confers diabetes protection by promoting, in part, the proliferation and/or survival of regulatory T cells. This approach is a useful tool by which DC-mediated activation of regulatory T cells can be studied as well as a potential therapeutic option for type 1 diabetes.

Induction of Tolerance in Quadruple Chimeric Mice

Human cord blood (CB) contains hematopoietic stem cells and progenitors. Because the major limitation to a widespread use of CB for transplantation lies in its limited volume, it is necessary to combine the CB from several donors. In this study, we show that lethally irradiated mice can be reconstituted with the injection of a mixture of T cell-depleted bone marrow cells (BMCs; total, 3 x 10(6)) obtained from three fully allogeneic mouse strains in two different mouse combinations. A higher survival rate was obtained in the triple injection group than in mice injected with BMCs (1 x10(6)) obtained from a single mouse strain. In the mixed chimeric mice, three kinds of donor-type and recipient-type cells were detected in all the hematopoietic organs 1 month after bone marrow transplantation (BMT). Mixed-lymphocyte reaction showed that the tolerance to both recipient-type and donor-type major histocompatibility complex determinants was induced in the chimeric mice. In the peripheral blood (PB) of these mice, only one type of cells from the three different donor strains became dominant in most chimeric mice and reached a stable level about 4 months after BMT. Polymerase chain reaction analyses, however, revealed that the skins from all the donors were accepted even when no cells with their phenotypes could be detected in the PB. These results suggest that both hemato-lymphoid reconstitution and stable tolerance to not only the recipient strain but also all the donor strains can be achieved in chimeric mice, indicating the possibility of mixed CB transplantation in humans.

Clinical transplantation tolerance: The promise and challenges

Organ transplantation is now well established as a preferred option for the treatment of end-stage organ failure. However, there is a severe shortage of donor organs and continued loss of a significant number of organ grafts due to chronic allograft dysfunction. Induction of tolerance of a transplant recipient toward their foreign organ graft, therefore, remains the "Holy Grail" of transplantation immunobiologists. Recently, clinical trials to explore pilot tolerance protocols in humans have been initiated. Defining the ideal strategy(ies) and the role of immunosuppressive drugs, developing tolerance assay(s), and enhancing cooperation between transplant professionals, industry, and the government are some of the challenges to achieving clinical transplantation tolerance. This article reviews the promise and the challenges of achieving clinical transplantation tolerance in human organ transplant recipients.

Prolongation of skin graft survival by modulation of the alloimmune response with alternatively activated dendritic cells1

BACKGROUND

Activation of immature dendritic cells (DC) in the presence of the glucocorticoid hormone dexamethasone (DEX) results in alternatively matured DC that present antigen in the absence of a proper co-stimulatory context. This maturation process is irreversible, making these cells an attractive potential tool for the induction of antigen-specific T-cell tolerance in vivo. The authors explored the possibility of using these DC for the induction of transplantation tolerance in a fully allogeneic setting in mice.

METHODS

Immature dendritic cells (D1, an immature splenic DC line derived from B6 mice) were pretreated with DEX for 24 hr, after which lipopolysaccharide or nothing was added to the culture for another 48 hr. These cells were analyzed for their in vitro and in vivo stimulating or tolerizing capacities.

RESULTS

In line with their phenotype, including decreased interleukin (IL)-12 production, in vitro co-culture of alternatively matured D1 (B6 origin; H-2b) with completely allogeneic T cells of BALB/c origin led to a significant decrease in the alloreactive T-cell response. A single injection of 1 x 10(6) alternatively matured H-2b DC into BALB/c mice induced a different alloimmune response compared with mature DC. The responding T cells showed a lower proliferation rate and a lower interferon-gamma production, whereas a significantly higher proportion of the cells produced IL-10 as measured ex vivo by enzyme-linked immunospot assay. Furthermore, injection with alternatively matured DC, followed by transplantation of fully mismatched skin grafts (C57BL/6), led to a significantly prolonged survival compared with that of mature DC-pretreated mice or untreated mice. The immunomodulatory effect was antigen specific, as third-party reactive alloresponses were not affected.

CONCLUSIONS

The authors' data constitute the first direct demonstration that DC alternatively matured in the presence of glucocorticoid hormones can be exploited for the specific suppression of the alloreactive Th1 response, resulting in a delayed skin graft rejection in a complete major histocompatibility complex-incompatible strain combination.

Interleukin-10 modified dendritic cells induce allo-hyporesponsiveness and prolong small intestine allograft survival

AIM: To investigate whether IL-10-transduced dendritic cells (DCs) could induce tolerogenicity and prolong allograft survival in rat intestinal transplantation.

METHODS: Spleen-derived DCs were prepared and genetically modified by hIL-10 gene. The level of IL-10 expression was quantitated by ELISA. DC function was assessed by MTT in mixed leukocyte reaction. Allogeneic T-cell apoptosis was examined by flow cytometric analysis. Seven days before heterotopic intestinal transplantation, 2 × 10⁶ donor-derived IL-10-DC were injected intravenously, then transplantation was performed between SD donor and Wistar recipient.

RESULTS: Compared with untransduced DC, IL-10-DC could suppress allogeneic mixed leukocyte reaction (MLR). The inhibitory effect was the most striking with the stimulator/effector (S/E) ratio of 1:10. The inhibition rate was 33.25%, 41.19% (P < 0.01) and 22.92% with the S/E ratio of 1:1, 1:10 and 1:50 respectively. At 48 h and 72 h by flow cytometry counting, apoptotic T cells responded to IL-10-DC in MLR were 13.8% and 30.1%, while untransduced group did not undergo significant apoptosis (P < 0.05). IL-10-DC pretreated recipients had a moderate survival prolongation with a mean allograft survival of 19.8 d (P < 0.01), compared with 7.3 ± 2.4 d in control group and 8.3 ± 2.9 d in untransduced DC group. Rejection occurred in the control group within three days. The difference between untreated DC group and control group was not significant.

CONCLUSION: IL-10-DC can induce allogenic T-cell hyporesponsiveness in vitro and apoptosis may be involved in it. IL-10-DC pretreatment can prolong intestinal allograft survival in the recipient.

Gene- and cell-based therapeutics for type I diabetes mellitus

Type 1 diabetes mellitus, an autoimmune disorder is an attractive candidate for gene and cell-based therapy. From the use of gene-engineered immune cells to induce hyporesponsiveness to autoantigens to islet and beta cell surrogate transplants expressing immunoregulatory genes to provide a local pocket of immune privilege, these strategies have demonstrated proof of concept to the point where translational studies can be initiated. Nonetheless, along with the proof of concept, a number of important issues have been raised by the choice of vector and expression system as well as the point of intervention; prophylactic or therapeutic. An assessment of the current state of the science and potential leads to the conclusion that some strategies are ready for safety trials while others require varying degrees of technical and conceptual refinement.

Immature rat myeloid dendritic cells generated in low-dose granulocyte macrophage-colony stimulating factor prolong donor-specific rat cardiac allograft survival

BACKGROUND

Because the differential polarization of T cells in response to antigen presentation is dependent on the maturational state of dendritic cells (DCs), we hypothesized that the adoptive transfer of immature myeloid DCs (iMDCs) would prolong graft survival.

METHODS

To evaluate this hypothesis, we studied the effects of transfer of iMDCs and mature myeloid DCs (mMDCs) on rat cardiac allograft survival.

RESULTS

Whereas iMDCs that do not express costimulatory molecules induce allogeneic T-cell hyporesponsiveness in coculture studies, mMDCs that express high levels of major histocompatibility complex class II costimulatory and maturation molecules induce a robust allostimulatory T-cell response. Adoptive transfer of Wistar Furth iMDCs, unlike mMDCs, 7 days before cardiac transplantation significantly prolonged graft survival. It was important that adoptive transfer of iMDCs combined with 0.5 mL antilymphocyte serum (ALS) transient immunosuppression on day -7 led to donor-specific permanent graft survival in 50% of recipients. In contrast, adoptive transfer of mMDCs combined with ALS led to graft survival similar to that in recipients treated with ALS alone. Stimulation of CD4 T cells isolated from the spleen of unresponsive allograft recipients with donor antigen resulted in donor-specific hyporesponsiveness and production of interleukin (IL)-10 and transforming growth factor-beta but not IL-4 and interferon-gamma. The tolerant T-cell unresponsiveness was reversed by the addition of IL-2.

CONCLUSION

Our data confirming the immunoregulatory effect of immature DCs indicate that induction of transplant tolerance by iMDCs is partly dependent on in vivo generation of regulatory T cells. This finding suggests that immunization with immature donor DCs has therapeutic potential for the induction of transplant tolerance and treatment of autoimmune diseases.

Rapamycin specifically interferes with GM-CSF signaling in human dendritic cells, leading to apoptosis via increased p27KIP1 expression

The longevity of dendritic cells (DCs) is a critical regulatory factor influencing the outcome of immune responses. Recently, we demonstrated that the immunosuppressive drug rapamycin (Rapa) specifically induces apoptosis in DCs but not in other myeloid cell types. The present study unraveled the mechanism used by Rapa to induce apoptosis in human monocyte-derived DCs. Our data demonstrate that granulocyte-macrophage colony-stimulating factor (GM-CSF) preserves DC survival specifically via the phosphatidylinositol-3 lipid kinase/mammalian target of rapamycin (PI3K/mTOR) signaling pathway, which is abrogated by Rapa at the level of mTOR. Disruption of this GM-CSF signaling pathway induced loss of mitochondrial membrane potential, phosphatidyl-serine exposure, and nuclear changes. Apoptosis of these nonproliferating DCs was preceded by an up-regulation of the cell cycle inhibitor p27(KIP1). Overexpression of p27(KIP1) in DCs using adenoviral gene transduction revealed that apoptosis is directly regulated by p27(KIP1). Furthermore, both overexpression of p27(KIP1) and disruption of the GM-CSF/PI3K/mTOR signaling pathway decreased the expression of the antiapoptotic protein mcl-1. This mTOR/p27(KIP1)/mcl-1 survival seems unique for DCs and may provide novel opportunities to influence immune responses by specific interference with the life span of these cells.

Inhibitory feedback loop between tolerogenic dendritic cells and regulatory T cells in transplant tolerance

An active role of T regulatory cells (Treg) and tolerogenic dendritic cells (Tol-DC) is believed important for the induction and maintenance of transplantation tolerance. However, interactions between these cells remain unclear. We induced donor-specific tolerance in a fully MHC-mismatched murine model of cardiac transplantation by simultaneously targeting T cell and DC function using anti-CD45RB mAb and LF 15-0195, a novel analog of the antirejection drug 15-deoxyspergualin, respectively. Increases in splenic Treg and Tol-DC were observed in tolerant recipients as assessed by an increase in CD4(+)CD25(+) T cells and DC with immature phenotype. Both these cell types exerted suppressive effects in MLR. Tol-DC purified from tolerant recipients incubated with naive T cells induced the generation/expansion of CD4(+)CD25(+) Treg. Furthermore, incubation of Treg isolated from tolerant recipients with DC progenitors resulted in the generation of DC with Tol-DC phenotype. Treg and Tol-DC generated in vitro were functional based on their suppressive activity in vitro. These results are consistent with the notion that tolerance induction is associated with a self-maintaining regulatory loop in which Tol-DC induce the generation of Treg from naive T cells and Treg programs the generation of Tol-DC from DC progenitors.

Polybrene and interleukin-4: two opposing factors for retroviral transduction of bone-marrow-derived dendritic cells

BACKGROUND

Gene transfer using retroviral transduction offers the advantage of long-term transgene expression in developing strategies that use dendritic cells (DCs) for immunotherapy. The goal of this study was to infect DCs in an immature state in order to take advantage of their proliferating and tolerogenic potential.

METHODS

Immature DCs were generated from murine bone marrow (BM) using either GM-CSF alone or GM-CSF plus IL-4. The cells were transduced directly with retroviral supernatants or by co-culture with the GP + E-86 retroviral packaging cell line in the presence of two different cationic polymers: polybrene and protamine sulfate. Phenotypic and functional characterization of the transduced cells were then performed.

RESULTS

Our results show a low efficiency of retroviral infection of DCs in the presence of polybrene. This cationic polymer was found to be directly cytotoxic to murine DCs and thus favored the growth of contaminating macrophages. This effect was not observed using protamine sulfate. Furthermore, stimulation by IL-4 early in the culture increased DC differentiation, proliferation and transduction. However, we found that DCs generated in GM-CSF plus IL-4 presented a more mature phenotype with an enhanced allogeneic stimulating activity. Finally, we showed that DCs themselves down-regulated transgene expression in the co-cultured packaging cell line in a promoter-dependent manner.

CONCLUSIONS

We have defined optimal conditions to generate and transduce murine BM-derived DCs. This included: the use of protamine sulfate during exposure to retroviral infectious supernatant and the addition of IL-4 at an early stage of the culture. Nevertheless, this cytokine also induced DC maturation. These findings have potential implications in experimental gene therapy.

Direct and indirect cross-tolerance of alloreactive T cells by dendritic cells retained in the immature stage

BACKGROUND

Rejection of allografts entails the direct and indirect cross-recognition of donor major histocompatibility complex molecules by recipient alloreactive T cells. The ability to manipulate the state of dendritic cell (DC) maturation in vitro has enabled us to induce tolerance specifically targeting the alloreactive T-cell compartment. In this study, the immunoregulatory effect of alloantigen presentation by ex vivo-generated donor and recipient DCs retained in immature stage was investigated.

METHODS

Dendritic cell were generated by culturing monocytes with granulocyte-macrophage colony-stimulating factor and interleukin-4. Ex vivo-generated tolerogenic DCs were characterized by flow cytometry and confocal microscopy. Recipient T-cell responses to donor or recipient DCs loaded with donor-derived apoptotic cells were assessed in a two-step culture system.

RESULTS

Dendritic cells maintained their phagocytic and endocytic activities, and had significantly reduced capacity to prime recipient T cells. Moreover, primary coculture of recipient T cells with donor tolerogenic DCs rendered alloantigen-specific T cells hyporesponsive to a subsequent challenge with donor immunogenic DCs as evidenced by decreased proliferation and cytokine secretion. Importantly, recipient tolerogenic DCs loaded with donor-derived apoptotic cells were able to cross-tolerize recipient T cells. This was revealed by alloantigen-specific T-cell hyporesponsiveness on restimulation with the recipient immunogenic DCs loaded with different tissue-derived apoptotic cells obtained from the same donor.

CONCLUSIONS

Dendritic cells retained in immature stage induce direct and most importantly indirect cross-tolerance of alloantigen-specific T cells. It may be likely that administration of donor and/or recipient DCs could be one means with which to promote tolerance induction in acute and chronic phases of organ transplant.

Regulatory functions of human CD4(+) T cells recognizing allopeptides in the context of self-HLA class II

Pretransplant blood transfusions sharing one human leukocyte antigen DR (HLA-DR) with the recipient have been shown to enhance graft survival, whereas HLA-DR mismatched blood transfusions will lead to immunization of the patient. The involvement of self HLA-DR suggests a role for CD4(+) regulatory T cells recognizing allopeptides in the context of self HLA class II molecules. Specific immunoregulation may be due to recognition of these allopeptides in the DR molecules of autologous T cells or dendritic cells. We tested this hypothesis on the basis of the reactivity of cell line ThoU6 which recognizes a peptide derived from an allo DR3 molecule, in the context of self DPB1*0301, and EL26, a CD4(+) T-cell clone recognizing HLA-A2 peptide in the presence of DRB1*1501. Addition of the line and clone to an assay in which the alloreactive cytotoxic T cell response (in a limiting dilution analysis) of PBLs sharing the restriction element was measured, resulted in a suppression of the anti-donor response but only when the proper peptide was added. These regulatory CD4(+) T cells were cytotoxic for targets presenting the proper peptide in the context of self MHC class II. Furthermore, these cells produced IL-10 after stimulation with the specific MHC/peptide combinations. Despite the similarity in function, EL26 and ThoU6 showed some differences in their phenotypic characteristics. Although both were CD25(+), EL26 expressed surface TGF-beta and CTLA-4, while ThoU6 did not. Similar regulatory T cells may explain the enhanced graft survival after HLA-DR shared blood transfusions either by their interaction with autologous alloreactive T cells or by modulation of autologous dendritic cells presenting the peptide involved.

Reduction in the circulating pDC1/pDC2 ratio and impaired function of ex vivo-generated DC1 in chronic hepatitis B infection

Dendritic cells (DCs) induce and regulate T-cell-mediated immune responses. Circulating precursor (p)DC1 and pDC2 from patients with chronic hepatitis B virus (HBV) infection were quantified by flow cytometry. To assess their function, DC1 were cultured from patients and compared to those of healthy volunteers. HBV patients exhibited a significant decrease in the proportion of freshly isolated pDC1 to pDC2. DC1 propagated from patients showed lower expression of costimulatory molecules and impaired allostimulatory capacity in comparison to controls. After exposure to proinflammatory cytokines, expression of costimulatory molecules, secretion of interleukin-12 (IL-12) and allostimulatory properties increased, but capacity for T-cell stimulation and IL-12 production remained inferior to that of control DCs. HBV-DNA was amplified by polymerase chain reaction in DC1 cultured from all patients. Viral particles were visible in DC1 by electron microscopy. These results suggest that intracellular presence of HBV impairs DC1 functional maturation and subsequent deficits in T-lymphocyte activation may contribute to viral persistence.

Microgravity culture condition reduces immunogenicity and improves function of pancreatic islets1

BACKGROUND

The failure of pancreatic islet allotransplants observed in almost all clinical attempts is related to poor initial islet function and allograft rejection. To remedy these problems we cultured islets in microgravity conditions to improve their function and to reduce their immunogenicity.

METHODS

Fresh mouse islets or mouse islets cultured in stationary dishes or microgravity bioreactors were transplanted to streptozotocin-induced diabetic mouse recipients.

RESULTS

Both allogeneic dish- or bioreactor-cultured islets survived more than 100 days compared with fresh allogeneic islets, which were rejected in less than 15 days. Islet titration studies revealed that 250 fresh or dish-cultured, but only 30 to 120 bioreactor-cultured, islets were necessary to produce euglycemia. Furthermore, glucose tolerance tests showed that bioreactor-cultured islets functioned better compared with fresh and dish-cultured islets on day 30 postgrafting. Immunostaining and transmission electron microscopy (TEM) analyses showed the gradual disappearance of dendritic cells in cultured islets compared with fresh islets. TEM revealed that the ultrastructure of islets from bioreactor, but not dish, appeared healthy and closely resembled fresh islets. Interestingly, TEM and scanning electron microscopy showed that only bioreactor-cultured islets developed unique and multiple nutritional channels between arrays of islet cells. TEM with colloidal lanthanum tracer revealed that only bioreactor islet cell cultures were devoid of tight junctional complexes, which may facilitate channel formation.

CONCLUSION

Microgravity condition decreases immunogenicity and significantly improves the function of secretory cells.

Dendritic cells

Dendritic cells (DC) are the most effective or 'professional' of the antigen-presenting cells (APC) that initiate primary immune responses. They are located at surveillance sites where they capture and process antigens. They then initiate and regulate T- and B-cell responses by expressing lymphocyte costimulatory molecules, migrating to lymphoid organs and secreting biologically active molecules. Dendritic cells not only activate lymphocytes to induce the immune response, but they also minimize autoimmune reactions by tolerizing T cells to self-antigens. Recent Phase I and II clinical studies have shown promise in the use of antigen-pulsed autologous DC for vaccination of cancer patients. Dendritic cells also have applications in preventing rejection after transplantation, immunization against viral infections and immunosuppression in autoimmune diseases.

Corticosteroids prevent generation of CD34+-derived dermal dendritic cells but do not inhibit Langerhans cell development

Corticosteroids (CS) have been shown to exert strong inhibitory effects on dendritic cell (DC) differentiation and function. Those studies were mostly performed with monocyte-derived DC, which represents only one subpopulation from the wide variety of DC types. In the present study the effects of the CS dexamethasone and prednisolone were investigated on the differentiation of CD34(+) hemopoietic progenitor cells into 1) Langerhans cells (LC), which differentiate directly into CD1a(+) DC; and 2) dermal/interstitial DC, which differentiate via a CD14(+)CD1a(-) phenotype into CD14(-)CD1a(+) DC. CS present during the entire 11-day culture period, resulting in fully differentiated CD1a(+) DC, increased the percentage of langerin(+) DC within the CD1a(+) population. In line with these data, CS treatment during the first 6 days of differentiation reduced the development of CD14(+) dermal DC precursors and thereby seemed to support the generation of CD1a(+) LC precursors. Addition of CS from day 6 onward specifically blocked the development of CD1a(+) dermal DC by both inhibition of spontaneous and IL-4-induced differentiation of CD14(+) DC precursors into CD1a(+) DC as well as induction of apoptosis in CD14(+) DC precursors. Apoptosis was not found in CD14(+) macrophage precursors derived from the same CD34(+) progenitors. The development and function of LC were not affected by CS, as demonstrated by a normal T cell stimulatory capacity and IL-12 production. These data demonstrate that CS interfere with the normal development of DC from CD34(+) progenitors by specific induction of apoptosis in precursors of dermal/interstitial DC. In view of the different functional capacities of dermal/interstitial DC and Langerhans cells, this might affect the overall cellular immune response.

New approaches to immunosuppression in liver transplantation

With the continued improvements in outcome following liver transplantation, the drawbacks associated with conventional immunosuppression regimens become increasingly apparent. Although up to 70% of patients develop a histological infiltrate of the graft (acute rejection), many of these will resolve spontaneously, and chronic rejection is rare. If a robust form of allograft acceptance or tolerance can be established, then immunosuppression can be withdrawn along with all the accompanying risks. The liver is already known to be associated with downregulated immune responses; the mechanism for this is unclear, but may be related to a number of mechanisms known to be involved in peripheral tolerance. There are many strategies being studied for achieving allograft tolerance, including the use of modern immunosuppressants, antibodies that target key molecules in the immune response, and recruitment of leukocytes to allografts. In the interim, it is necessary to look for safe protocols that allow trials of tolerance strategies without putting patients at increased risk.

Dendritic cells and second signal blockade: a step toward allograft tolerance?

The ultimate goal of clinical organ transplantation is to induce immunological tolerance to the allograft. Dendritic cells, the main antigen-presenting cells, are a complex system including numerous subsets, capable of inducing T-cell activation, and thus initiating an immune response, but also of inducing tolerance. In organ transplantation, the problem is even more complex because of the coexistence of dendritic cells from the donor, responsible for direct recognition of foreign antigens by T cells, and dendritic cells from the recipient, responsible for indirect antigen presentation to host T cells. Among the various methods of immunomanipulation aiming at inducing tolerance, blockade of the second signal pathway by monoclonal antibodies (anti-CD28, anti-B7, anti CD40, anti-CD40L, CTLA4-Ig) has yielded promising but incomplete results. Viral transfection of recipient immature dendritic cells encoding for immunosuppressive or apoptotic molecules, such as interleukin 10, transforming growth factor-beta, CTLA4-Ig, or Fas ligand, is also able to induce hyporesponsiveness. Another very promising method consists of associating donor cells (bone marrow cells, CD34+ cells, dendritic cells) and polyclonal or monoclonal antibodies (anti-CD4, anti-CD40L, CTLA4-Ig) to induce microchimerism and partial tolerance. Reflecting on these data would seem to be an interesting direction for future prospects.

Dendritic cells: to where do they lead?

Dendritic cells are a heterogeneous population of bone marrow-derived cells present in most peripheral tissues. These cells are able to capture and present antigens to T cells. Such presentation can lead to two opposite outcomes: potent activation (immunogenicity) or inhibition (tolerance) of the immune response. The fine regulation of these two distinct functions is not completely understood to date. In this review, we discuss three potential variables that may influence dendritic cell function: the origin of dendritic cells, their maturation state, and their capture properties. Each hypothesis is illustrated with examples in the field of transplantation. Lastly, the criteria necessary for proposing tolerogenic dendritic cells to promote engraftment and long-term allograft survival are discussed.

Dendritic cells: immune regulators in health and disease

Dendritic cells (DCs) are bone marrow-derived cells of both lymphoid and myeloid stem cell origin that populate all lymphoid organs including the thymus, spleen, and lymph nodes, as well as nearly all nonlymphoid tissues and organs. Although DCs are a moderately diverse set of cells, they all have potent antigen-presenting capacity for stimulating naive, memory, and effector T cells. DCs are members of the innate immune system in that they can respond to dangers in the host environment by immediately generating protective cytokines. Most important, immature DCs respond to danger signals in the microenvironment by maturing, i.e., differentiating, and acquiring the capacity to direct the development of primary immune responses appropriate to the type of danger perceived. The powerful adjuvant activity that DCs possess in stimulating specific CD4 and CD8 T cell responses has made them targets in vaccine development strategies for the prevention and treatment of infections, allograft reactions, allergic and autoimmune diseases, and cancer. This review addresses the origins and migration of DCs to their sites of activity, their basic biology as antigen-presenting cells, their roles in important human diseases and, finally, selected strategies being pursued to harness their potent antigen-stimulating activity.

Rare-event analysis of circulating human dendritic cell subsets and their presumptive mouse counterparts

BACKGROUND

Considerable interest has focused recently on murine CD8alpha- and CD8alpha+ dendritic cell (DC) subsets, because of their roles in initiating and regulating immune responses. Attention has also centered on their presumed human counterparts, DC1 and DC2, respectively, and their precursors. Identification and quantification of these subsets in the blood may be crucial to understanding and monitoring of their immunologic significance, particularly in humans, where blood may be the only tissue readily or routinely available.

METHODS

Leukocytes were isolated from anticoagulated human or mouse (C57BL/10J) blood using conventional procedures. Four-color, rare-event, flow cytometric analysis was used to identify DC1 precursors (pDC1; lineage [lin]- CD4+ CD11c+ HLA-DR+) or DC2 precursors (pDC2; lin- CD4+ CD11c- CD123(hi) [IL-3Ralpha(hi)] HLA-DR+) in normal humans. In mice, CD8alpha+ (CD11b(lo), CD11c+) and CD8alpha- (CD11b(hi), CD11c+) DC subsets were identified both in normal animals and after administration of the potent DC growth factor, fms-like tyrosine kinase 3 ligand (Flt3L).

RESULTS

All human subjects examined had discrete populations of pDC1 and pDC2 comprising approximately 0.6% and 0.1% of blood mononuclear cells. CD8alpha- and CD8alpha+ DC constituted approximately 0.75% and 0.2%, respectively, of blood mononuclear cells in normal mice, and 12% and 0.5%, respectively, in Flt3L-treated animals. Flt3L administration substantially increased the absolute numbers of circulating CD11c+ DC by approximately 200-fold.

CONCLUSIONS

In addition to pDC1 and CD8alpha- DC, pDC2 and CD8alpha+ DC can be identified in normal human or mouse blood, respectively. Monitoring and isolation or characterization of these cells may provide novel insights into their functional significance in transplantation and other clinical conditions.

Activation of human dendritic cells by porcine aortic endothelial cells: transactivation of naïve T cells through costimulation and cytokine generation

BACKGROUND

Dendritic cells (DC) are the most potent antigen-presenting cells in the immune system. To define the role of human DC in human anti-porcine immune responses, we defined the interaction of human DC with porcine aortic endothelial cells (PAEC).

METHODS

To determine the immune responses, both monocyte-derived and peripheral blood DC were cultured with porcine and human endothelial cells. We analyzed the role of CD11a, CD11b, and CD54 in a cell-to-cell adhesion assay using antibodies against these molecules. The expression pattern of costimulatory molecules (CD40, CD80, CD86), adhesion molecules (CD54), and intracellular cytokines (interleukin-12p70 and tumor necrosis factor [TNF]-alpha) in DC after interaction with endothelial cells was determined by immunofluorescence.

RESULTS

Human DC significantly adhered to PAEC (38-40%), and this adhesion was augmented (>50%) upon treatment with either recombinant swine interferon-gamma or recombinant human TNF-alpha. Addition of human DC to PAEC was blocked by pretreatment of DC with antibodies specific to human leukocyte function-associated antigen-1 or CD54. Adhesion of DC to PAEC also resulted in the activation of DC, which was manifested by up-regulation of costimulatory molecules (CD40, CD80, CD86), adhesion molecules (CD54), and HLA-DR. PAEC-activated human DC provided proliferative signals to the naïve autologous CD4+ T cells and synthesized interleukin-12p70 and TNF-alpha. However, activated DCs failed to lyse PAEC in such interaction.

CONCLUSION

Human DC effectively adhered to PAEC and were activated by xenoantigen, resulting in highly efficient antigen presentation and proliferation of CD4+ T cells. Further, this interaction of human DC to PAEC is regulated by the participation of costimulatory and adherence molecules and cytokines.

Costimulation blockade promotes the apoptotic death of graft-infiltrating T cells and prolongs survival of hepatic allografts from FLT3L-treated donors

BACKGROUND

Mouse liver grafts are accepted across major histocompatibility complex (MHC) barriers and induce donor-specific tolerance without immunosuppressive therapy. By contrast, hepatic allografts from donors treated with the hematopoietic growth factor fms-like tyrosine kinase 3 ligand (FL), which dramatically increases hepatic interstitial dendritic cells (DC), are rejected acutely (median survival time 5 days). This switch from tolerance to rejection is associated with a marked reduction in apoptotic activity of graft-infiltrating T cells. We hypothesized that T-cell costimulation, provided by markedly enhanced numbers of donor antigen presenting cells (APCs), might inhibit apoptosis, promote expansion of T helper 1 cells and play a key role in acute liver rejection.

METHODS

C3H (H2k) recipients of orthotopic liver grafts from FL-treated B10 (H2b) donors were given cytotoxic T-lymphocyte antigen 4: immunoglobulin (CTLA4Ig), a chimeric fusion protein that blocks the B7-CD28 costimulatory pathway, or control human immunoglobulin (200 microg) on the day of transplantation (day 0). Livers and spleens were removed on day 4. Cryostat sections were stained for interleukin (IL)-12 or by terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling (TUNEL). Expression of mRNA encoding interferon (IFN)-gamma and IL-10 was determined by RNase protection assay. Suspensions of graft-infiltrating cells (GICs) and spleen cells were analyzed for apoptotic (TUNEL+) T-cell subsets by flow cytometry. CTL activity of GICs and circulating alloantibody levels were determined by cytotoxicity assays.

RESULTS

Survival of liver grafts from FL donors was markedly prolonged by CTLA4Ig administration. This effect was associated with reductions in IFN-gamma and IL-10 gene transcripts within the GIC population, and with decreases in donor-specific CTL and NK cell activities and circulating anti-donor alloantibody levels. At the same time, there were marked increases in TUNEL+ CD4+ and especially CD8+ cells, both within the grafts and in the spleens of CTLA4Ig-treated mice.

CONCLUSIONS

Signaling via the B7-CD28 pathway appears to play a key role in the switch from tolerance to rejection that is precipitated by markedly enhanced numbers of donor DCs. Inhibition of acute liver allograft rejection by CTLA4Ig, linked to restoration of apoptotic activity of graft-infiltrating T cells, further suggests that deletion of these cells may be critical for promotion of long-term allograft survival.