Liver-derived DEC205+B220+CD19- dendritic cells regulate T cell responses

PARG regulates the proliferation and differentiation of DCs and T cells via PARP/NF‑κB in tumour metastases of colon carcinoma

The present study investigated the effect of poly(ADP-ribose) glycohydrolase (PARG) on the immune response in tumour metastases of colon carcinoma. CT26 cells were transfected with lentivirus PARG-short hairpin RNA (shRNA). A liver metastasis model of colon carcinoma was successfully established by splenic subcapsular inoculation of the various groups of CT26 cells into BALB/c mice. Next, changes in the liver metastases of colon carcinoma nodules and alterations in the survival times were observed in tumour-bearing mice. The numbers of B220⁺DEC205⁺ dendritic cells (B220⁺DEC205⁺DC) and CD11c⁺CD11b⁺ dendritic cells (CD11c⁺CD11b⁺DC) in the spleen and liver were measured by the double-label immunofluorescence assay. The distribution pattern of CD4⁺T cells and CD8⁺T cells in the spleen and liver was investigated by immunofluorescence staining. The expression levels of PARG, PARP and nuclear factor-κB (NF-κB) proteins in spleen transplant tumours and liver metastases of colon carcinoma were detected by western blotting. An ELISA was used to detect the levels of IL-10 and TGF-β in the serum of tumour-bearing mice and from the supernatant of tumour cells. The numbers and grading of metastatic liver nodules in the PARG-silenced group were clearly lower than those in the control group. The survival time of the PARG-silenced group mice was longer than that in the control group. In the PARG-silenced group, the levels of B220⁺DEC205⁺DC in the spleen and liver were lower and the numbers of CD11c⁺CD11b⁺DC in the spleen and liver were more than those in the control group. The ratio of CD4⁺/CD8⁺ in the spleen and liver in the PARG-silenced group was increased compared with that in the control group (P<0.05). The levels of PARG, PARP and NF-κB in spleen transplant tumours and liver metastases of colon carcinoma were lower in the PARG-silenced group than in the control group. In addition, the levels of IL-10 and TGF-β in the serum of tumour-bearing mice and supernatants of tumour cells were both reduced in the PARG-silenced group compared with those in the control group. The present research suggests that the liver metastases of colon carcinoma could be restrained by silencing PARG. Likely, the silencing of PARG could suppress the expression of PARP and NF-κB and subsequently suppress the secretion of IL-10 and TGF-α, finally affecting the proliferation and differentiation of DC and T cells.

Neural Stem Cells

Neural stem cell (NSC) transplantation has provided the basis for the development of potentially powerful new therapeutic cell-based strategies for a broad spectrum of clinical diseases, including stroke, psychiatric illnesses such as fetal alcohol spectrum disorders, and cancer. Here, we discuss pertinent preclinical investigations involving NSCs, including how NSCs can ameliorate these diseases, the current barriers hindering NSC-based treatments, and future directions for NSC research. There are still many translational requirements to overcome before clinical therapeutic applications, such as establishing optimal dosing, route of delivery, and timing regimens and understanding the exact mechanism by which transplanted NSCs lead to enhanced recovery. Such critical lab-to-clinic investigations will be necessary in order to refine NSC-based therapies for debilitating human disorders.

Harnessing neural stem cells for treating psychiatric symptoms associated with fetal alcohol spectrum disorder and epilepsy

Brain insults with progressive neurodegeneration are inherent in pathological symptoms that represent many psychiatric illnesses. Neural network disruptions characterized by impaired neurogenesis have been recognized to precede, accompany, and possibly even exacerbate the evolution and progression of symptoms of psychiatric disorders. Here, we focus on the neurodegeneration and the resulting psychiatric symptoms observed in fetal alcohol spectrum disorder and epilepsy, in an effort to show that these two diseases are candidate targets for stem cell therapy. In particular, we provide preclinical evidence in the transplantation of neural stem cells (NSCs) in both conditions, highlighting the potential of this cell-based treatment for correcting the psychiatric symptoms that plague these two disorders. Additionally, we discuss the challenges of NSC transplantation and offer insights into the mechanisms that may mediate the therapeutic benefits and can be exploited to overcome the hurdles of translating this therapy from the laboratory to the clinic. Our ultimate goal is to advance stem cell therapy for the treatment of psychiatric disorders.

The CD8 T-cell response during tolerance induction in liver transplantation

Both experimental and clinical studies have shown that the liver possesses unique tolerogenic properties. Liver allografts can be spontaneously accepted across complete major histocompatibility mismatch in some animal models. In addition, some liver transplant patients can be successfully withdrawn from immunosuppressive medications, developing ‘operational tolerance'. Multiple mechanisms have been shown to be involved in inducing and maintaining alloimmune tolerance associated with liver transplantation. Here, we focus on CD8 T-cell tolerance in this setting. We first discuss how alloreactive cytotoxic T-cell responses are generated against allografts, before reviewing how the liver parenchyma, donor passenger leucocytes and the host immune system function together to attenuate alloreactive CD8 T-cell responses to promote the long-term survival of liver transplants.

The potential of neural stem cell transplantation for the treatment of fetal alcohol spectrum disorder

Fetal alcohol spectrum disorder (FASD) is caused by intrauterine exposure to alcohol and can cause a full range of abnormalities to brain development, as well as long-term sequelae of cognitive, sensory and motor impairments. The incidence is estimated to be as high as 2% to 5% in children born within the US, however the prevalence is even higher in low socioeconomic populations. Despite the various mechanisms thought to explain the etiology of FASD, molecular targets of ethanol toxicity during development are not completely understood. More recent findings explore the role of GABA-A and GABA-B mechanisms, as well as cell death, cell signaling and gene expression malfunctions. Stem cell based therapies have grown exponentially over the last decade, which have lead to novel clinical interventions across many disciplines. Thus, early detailed understanding of the therapeutic potential of stem cell research has provided promising applications across a wide range of illnesses. Consequently, these potential benefits may ultimately lead to a reduced incidence and severity of this highly preventable and prevalent birth defect. It is recognized that stem cell derivations provide unique difficulties and limitations of therapeutic applications. This review will outline the current knowledge, along with the benefits and challenges of stem cell therapy for FASD.

Targeting of autoantigens to DEC205⁺ dendritic cells in vivo suppresses experimental allergic encephalomyelitis in mice

The dendritic and epithelial cell receptor with a m.w. of 205 kDa (DEC205) is expressed by dendritic cells (DCs) and facilitates Ag presentation. After injection of Ags coupled to Abs specific for DEC205 into mice, Ag presentation occurs by nonactivated DCs, which leads to induction of regulatory T cells (Tregs). To test this system for tolerance induction in experimental allergic encephalomyelitis (EAE), we created single-chain fragment variables (scFv) specific for DEC205 and fused the scFv to the self-Ag myelin oligodendrocyte glycoprotein (MOG; scFv DEC:MOG). An anti-β-galactosidase scFv:MOG fusion protein (scFv GL117:MOG) served as isotype control. After staining of DCs in vitro with purified scFv DEC:MOG, binding to DCs and colocalization with MHC class II was apparent, whereas isotype controls did not bind. We next injected scFv DEC:MOG into mice and observed elevated numbers of highly activated, IL-10-producing CD4⁺CD25⁺Foxp3⁺ Tregs (17% of CD4) in spleens, as compared with isotype controls and uninjected mice (12% of CD4). Furthermore, DCs isolated from scFv DEC:MOG-injected animals produced significantly increased levels of TGF-β. Most importantly, when EAE was induced in scFv DEC:MOG-injected mice, 90% of the mice were protected from EAE, whereas all mice in the isotype controls (scFv GL117:MOG) experienced development of EAE. When applying scFv DEC:MOG to mice that had already experienced EAE symptoms, abrogation of the disease in 90% of the animals was apparent, whereas all animals in the control groups experienced development of severe EAE. Thus, these data indicate that targeting of MOG to "steady-state" DCs in vivo may provide a tool to prevent and to treat EAE by a DC/Treg-driven mechanism.

Hepatic antigen-presenting cells and regulation of liver transplant outcome

In the steady state, hepatic antigen (Ag)-presenting cells (APC) generally dampen systemic inflammatory responses to gut-derived Ags. Our studies focus on the role of specific liver APC populations, both non-parenchymal cells (dendritic cells [DC], Kupffer cells, and hepatic stellate cells [HSC]) and parenchymal cells, in the molecular regulation of tissue damage (ischemia and reperfusion [I/R] injury) and immunity following liver transplantation. We focus on factors that either promote or overwhelm the natural tendency of the liver to suppress inflammatory/immune responses. We are also examining molecular mechanisms that regulate liver DC maturation and function and that determine their role in the control of allogeneic T-cell function and the fate of the transplanted liver. Our studies are also aimed at elucidating mechanisms by which HSC regulate DC and T-cell function. These investigations may provide new targets for therapeutic intervention in liver inflammation.

Role of dendritic cells: a step forward for the hygiene hypothesis

The hygiene hypothesis was proposed more than two decades ago, but its mechanism remains unclear. This review focuses on recent advances in the field, especially on the role played by dendritic cells (DCs) and their modulating effects on various infections and allergic diseases, including allergic asthma. DCs isolated from mice long after the resolution of an infection were reported to have a significant modulating effect on allergen-specific Th2 responses in both in vitro and in vivo systems. These DCs showed DC1-like and/or tolerogenic DC capacity, which allowed for the inhibition of allergic responses by immune deviation (enhancing Th1 response) and immune regulation (through regulatory T-cell and Th2 hyporesponsiveness) mechanisms. These findings represented a significant advance in the elucidation of the mechanisms underlying the hygiene hypothesis. Further investigation on the mechanisms by which DCs are 'educated' by infectious agents and the influence of the type, time, and extent of infections on this 'education' process will help us understand immune regulation in disease settings and in the rational design of preventive/therapeutic approaches to allergy/asthma and infections.

Dendritic cells and regulation of alloimmune responses: relevance to outcome and therapy of organ transplantation

Dendritic cells are uniquely well-equipped for antigen capture, processing and presentation. They are highly-efficient antigen-presenting cells that induce and regulate T-cell reactivity. Due to their inherent tolerogenicity, immature dendritic cells offer considerable potential as candidate cellular vaccines for negative regulation of immune reactivity/promotion of tolerance. Both classic myeloid and, more recently, characterized plasmacytoid dendritic cells, exhibit tolerogenic properties. Manipulation of dendritic cells differentiation/ maturation in the laboratory using cytokines, pharmacologic agents or genetic engineering approaches can render stably immature dendritic cells that promote organ transplant tolerance in rodents. There are also indications from human studies of the ability of dendritic cells to promote T-cell tolerance and induce T-regulatory cells, with potential for therapeutic application in organ transplantation. In addition, recent clinical observations suggest that modulation of dendritic cell function (e.g., by immunosuppressive drugs) affects the outcome of transplantation. The challenge confronting applied dendritic cell biology is the identification of optimal strategies and therapeutic regimens to allow the potential of these powerful immune regulatory cells to be realized in the clinic.

Increased CD4+CD25+Foxp3+ regulatory T cells in tolerance induced by portal venous injection

BACKGROUND

A portal vein injection (PVI) of allogeneic donor antigen is known to prolong the survival of a subsequently transplanted allograft; however, the underlying mechanism remains to be clarified.

METHODS

Irradiated C57BL/6 (B6) splenocytes were injected into BALB/c mice via the portal vein. Seven days after injection, the proportions of CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells were determined in the blood, liver, and spleen. CD4(+) and CD8(+) T cells were isolated from BALB/c mice that received PVI of B6 splenocytes (PVI mice), adoptively transferred into recipient BALB/c mice 1 day before B6 or third-party C3H heart transplantation, and graft survival was compared. B6 or C3H heart allografts were implanted into anti-CD25 monoclonal antibody (mAb)-treated PVI and untreated PVI mice, and graft survivals were compared. The percentages of CD4(+)CD25(+)Foxp3(+) Treg, cytokine profiles, and ratios of apoptosis were determined in anti-CD25 mAb-treated PVI and untreated PVI mice.

RESULTS

PVI of allogeneic cells induced antigen-specific tolerance and increased the percentage of CD4(+)CD25(+)Foxp3(+) Treg. Adoptive transfer of CD4(+) T cells, but not CD8(+) T cells, from PVI mice prolonged B6 heart allograft survival. Depletion of CD4(+)CD25(+) T cells prevented the induction of tolerance and decreased the percentage of CD4(+)CD25(+)Foxp3(+) Treg in the CD3(+) T-cell pool, and thus was associated with decreased production of interleukin (IL)-4 and apoptosis of T cells.

CONCLUSION

Increased CD4(+)CD25(+)Foxp3(+) Treg play an important role in portal vein tolerance induction, at least partly via increasing the production of IL-4 and decreasing apoptosis of T cells.

Immune regulation by non-lymphoid cells in transplantation

Regulatory cells play a crucial role in the induction and maintenance of tolerance by controlling T cell as well as B and natural killer (NK) cell-mediated immunity. In transplantation, CD4+CD25+forkhead box P3+ T regulatory cells are instrumental in the maintenance of immunological tolerance, as are several other T cell subsets such as NK T cells, double negative CD3+ T cells, gammadelta T cells, interleukin-10-producing regulatory type 1 cells, transforming growth factor-beta-producing T helper type 3 cells and CD8+CD28(-) cells. However, not only T cells have immunosuppressive properties, as it is becoming increasingly clear that both T and non-T regulatory cells co-operate and form a network of cellular interactions controlling immune responses. Non-T regulatory cells include tolerogenic dendritic cells, plasmacytoid dendritic cells, mesenchymal stem cells, different types of stem cells, various types of alternatively activated macrophages and myeloid-derived suppressor cells. Here, we review the mechanism of action of these non-lymphoid regulatory cells as they relate to the induction or maintenance of tolerance in organ transplantation.

Plasmacytoid dendritic cells mediate oral tolerance

Oral tolerance prevents oral sensitization to dietary antigens (Ags), including proteins and haptens, and development of delayed-type hypersensitivity (DTH) responses. We showed here that plasmacytoid dendritic cells (pDCs) prevented oral T cell priming and were responsible for systemic tolerance to CD4(+) and CD8(+) T cell-mediated DTH responses induced by Ag feeding. Systemic depletion of pDCs prevented induction of tolerance by antigen feeding. Transfer of oral Ag-loaded liver pDCs to naive recipient mice induced Ag-specific suppression of CD4(+) and CD8(+) T cell responses to protein and hapten, respectively. Liver is a site of oral Ag presentation, and pDCs appeared to induce anergy or deletion of Ag-specific T cells in the liver relatively rapidly via a CD4(+) T cell-independent mechanism. These data demonstrate that oral tolerance relies on Ag presentation by pDC to T cells and suggest that pDC could represent a key therapeutic target for intestinal and systemic inflammatory diseases.

Delivery of rapamycin by PLGA nanoparticles enhances its suppressive activity on dendritic cells

The purpose of this study was to evaluate the effect of rapamycin delivery by poly (D,L-lactic-co-glycolic acid) (PLGA) nanoparticles on the maturation of dendritic cells (DCs). DCs were generated from mouse bone marrow and exposed to particulate and soluble rapamycin without any additional treatment, or with pre- or posttreatment with lipopolysaccharide (LPS). Annexin V-FITC/PI staining was performed on DC cultures to assess the viability of DCs during study. Surface phenotype of DCs was characterized for the expression of maturation markers, that is, MHC class II, CD86, and CD40 by flow cytometry. Cell culture supernatants were analyzed for the production of TGF-beta, IL-12, and IL-10 cytokines using sandwich ELISA method. DCs from Balb/C mice were cocultured with T cells from C57BL/6 mice and allogenic mixed lymphocyte reaction was assessed by [3H]-Thymidine incorporation. Unlike free rapamycin that has shown little if any effect on the expression of maturation markers in immature DCs, PLGA encapsulated rapamycin decreased the expression of all maturation markers under the study, that is, MHC class II, CD86, and CD40, significantly. LPS pre- or posttreated DCs demonstrated decreased expression of MHC class II, CD86, and CD40 in the presence of soluble or encapsulated rapamycin. The cytokine secretion profiles revealed high levels of TGF-beta and very low levels of IL-10 and IL-12 production. Rapamycin in soluble or encapsulated form significantly inhibited mixed lymphocyte reaction in DCs. The inhibitory effect of rapamycin on the maturation of DCs with respect to DC phenotype, cytokine production, and functional effects on the proliferation of T cells was significantly increased by PLGA delivery.

Molecular mechanisms of portal vein tolerance

The liver has been considered as a tolerogenic organ in the sense that favors the induction of peripheral tolerance. The administration of antigens (Ags) via the portal vein causes tolerance, which is termed portal vein tolerance and can explain the occurrence of tolerogenic responses in the liver. Here we discuss the fundamental mechanisms accounting for portal vein tolerance. Antigen-presenting cells (APCs) in the liver, especially dendritic cells and sinusoidal endothelial cells, have limited the ability to produce pro-inflammatory cytokines upon stimulation with endotoxin, an effect that could be due to the continuous exposure to bacterial Ags derived from intestinal microflora. Ag presentation by liver APCs results in T cell tolerance through clonal deletion and selection of regulatory T cells. Thus, APCs with immunosuppressive functions are associated with the achievement of portal vein tolerance via the induction of clonal deletion and generation of regulatory T cells.

Regulatory conversation between antigen presenting cells and regulatory T cells enhance immune suppression

Regulatory T cells (Treg) were originally described by their suppressive function exerted on effector T cells, but recent evidence also reveals interactions with antigen presenting cells (APCs). In general, all major subpopulations of APCs, i.e., dendritic cells (DC), B cells and monocytes/macrophages (Mvarphi), respond to exposure to Treg by down regulation of their antigen presenting function, upregulation of immunosuppressive molecules and secretion of immunosuppressive cytokines. Thus, Treg gain influence on the innate immune system and are able to augment their immunosuppressive capacities by blocking the effective priming of T effector cells by APCs. Conversely, APCs have an important role in nurturing peripheral Treg populations, since it has been shown that immature DC, as well as alternatively activated Mvarphi, are able to induce Treg de novo. These properties are dependent on the expression of surface molecules (CTLA-4, F4/80) and the production of soluble factors such as IL-10 and Indoleamine 2,3-dioxygenase by the APC subpopulations. On the whole, the mutual interaction of Treg and APCs enables Treg to sustain their immunosuppressive functions which, in healthy individuals, may be crucial for the maintenance of peripheral tolerance.

Dendritic cells, the liver, and transplantation

Interstitial liver dendritic cells (DCs) exhibit phenotypic diversity and functional plasticity. They play important roles in both innate and adaptive immunity. Their comparatively low inherent T cell stimulatory ability and the outcome of their interactions with CD4(+) and CD8(+) T cells, as well as with natural killer (NK) T cells and NK cells within the liver, may contribute to regulation of hepatic inflammatory responses and liver allograft outcome. Liver DCs migrate in the steady state and after liver transplantation to secondary lymphoid tissues, where the outcome of their interaction with antigen-specific T cells determines the balance between tolerance and immunity. Systemic and local environmental factors that are modulated by ischemia-reperfusion injury, liver regeneration, microbial infection, and malignancy influence hepatic DC migration, maturation, and function. Current research in DC biology is providing new insights into the role of these important antigen-presenting cells in the complex events that affect liver transplant outcome.

The role of liver-derived regulatory dendritic cells in prevention of type 1 diabetes

Development of type 1 diabetes has been attributed to T-cell-mediated autoimmunity, which is regulated by antigen-presenting cells. To study the role of liver-derived B220(+) regulatory dendritic cells (DCs) in the development of diabetes in non-obese diabetic (NOD) mice, we found that liver 220(+) DCs could easily be propagated from young NOD mice, but that such propagation was extremely difficult from mice older than 11 weeks, when insulitis began. This was not simply an age-related phenomenon, because liver B220(+) DCs were readily propagated from both young and old congenic non-obese diabetic-resistant (NOR) and normal BALB/c mice. It was therefore speculated that the development of diabetes might be associated with a lack of precursors of B220(+) DC in the liver in this animal model. Unfortunately, the specific marker for precursors of liver B220(+) DC has not been identified. An alternative approach to supplement liver B220(+) DCs by intravenous administration significantly inhibited the development of diabetes by inducing T-cell hyporesponsiveness via enhancement of their apoptotic death. Liver B220(+) DCs were capable of effectively presenting antigens but, unlike plasmacytoid DCs, did not express CD11c and were not interferon-alpha producers. These observations may throw new light on the aetiopathology of type 1 diabetes.

Antigen processing and CD24 expression determine antigen presentation by splenic CD4+ and CD8+ dendritic cells

To examine heterogeneity in dendritic cell (DC) antigen presentation function, murine splenic DCs were separated into CD4+ and CD8+ populations and assessed for the ability to process and present particulate antigen to CD4+ and CD8+ T cells. CD4+ and CD8+ DCs both processed exogenous particulate antigen, but CD8+ DCs were much more efficient than CD4+ DCs for both major histocompatibility complex (MHC) class II antigen presentation and MHC class I cross-presentation. While antigen processing efficiency contributed to the superior antigen presentation function of CD8+ DCs, our studies also revealed an important contribution of CD24. CD8+ DCs were also more efficient than CD4+ DCs in inducing naïve T cells to acquire certain effector T-cell functions, for example generation of cytotoxic CD8+ T cells and interferon (IFN)-gamma-producing CD4+ T cells. In summary, CD8+ DCs are particularly potent antigen-presenting cells that express critical costimulators and efficiently process exogenous antigen for presentation by both MHC class I and II molecules.

In vivo expansion of two distinct dendritic cells in mouse livers and its impact on liver immune regulation

Liver transplant tolerance in pigs, rats, and mice has been disclosed for decades, but the underlying mechanisms are not completely understood. Accumulating data indicate that residing dendritic cells (DC) are important in determining direction of immune responses in the liver. However, our knowledge remains very limited due to the difficulties in obtaining sufficient liver DC. Most of the previous studies were dependent on DC propagated in vitro with growth factors and cytokines. In this study, we adopted an approach to transfect genes into the mouse liver by tail vein injection of plasmid DNA. Transfection with plasmid granulocyte-macrophage colony-stimulating factor markedly expanded liver CD11c(+) DC mainly located in portal regions, while liver B220(+) DC were dramatically generated after injection with plasmid interleukin (IL)-3/CD40L largely present in the lobules. Although both were phenotypically mature and strong T-cell stimulators, CD11c(+)DC induced potent T-cell response while B220(+)DC induced T-cell hyporesponsiveness. Administration of CD11c(+)DC accelerated cardiac allograft rejection, while B220(+)DC significantly prolonged graft survival. This hyporesponsiveness is not due to inhibition of DC/T-cell interaction, but rather through an active process of stimulating T-cell apoptosis. Compared to B220(+) DC that expressed messenger RNA of (TLR) 1, 2, 6, 7, and 9, CD11c(+)DC expressed all TLR 1 to 9. TLR 9 ligation stimulated very high IL-12 in CD11c(+) DC, but high IL-10 and no IL-12 in B220(+) DC. In conclusion, through these mechanisms, liver DC may be actively involved in immune regulation in the liver.

CD25+CD4+ Regulatory T Cells Develop in Mice Not Only During Spontaneous Acceptance of Liver Allografts but Also After Acute Allograft Rejection

BACKGROUND

Liver grafts transplanted across a major histocompatibility barrier are accepted spontaneously and induce donor specific tolerance in some species. Here, we investigated whether liver allograft acceptance is characterized by, and depends upon, the presence of donor reactive CD25CD4 regulatory T cells.

METHODS

CD25 and CD25CD4 T cells, isolated from CBA. Ca (H2) recipients of C57BL/10 (B10; H2) liver and heart allografts 10 days after transplantation, were transferred into CBA. Rag1 mice to investigate their influence on skin allograft rejection mediated by CD45RBCD4 effector T Cells.

RESULTS

Fully allogeneic B10 liver allografts were spontaneously accepted by naive CBA.Ca recipient mice, whereas B10 cardiac allografts were acutely rejected (mean survival time=7 days). Strikingly, however, CD25CD4 T cells isolated from both liver and cardiac allograft recipients were able to prevent skin allograft rejection in this adoptive transfer model. Interestingly, CD25CD4 T cells isolated from liver graft recipients also showed suppressive potency upon adoptive transfer. Furthermore, depletion of CD25CD4 T cells in primary liver allograft recipients did not prevent the acceptance of a secondary donor-specific skin graft.

CONCLUSIONS

Our data provide evidence that the presence of CD25CD4 regulatory T cells is not a unique feature of allograft acceptance and is more likely the result of sustained exposure to donor alloantigens in vivo.

Increasing numbers of hepatic dendritic cells promote HMGB1-mediated ischemia-reperfusion injury

Endogenous ligands released from damaged cells, so-called damage-associated molecular pattern molecules (DAMPs), activate innate signaling pathways including the TLRs. We have shown that hepatic, warm ischemia and reperfusion (I/R) injury, generating local, noninfectious DAMPs, promotes inflammation, which is largely TLR4-dependent. Here, we demonstrate that increasing dendritic cell (DC) numbers enhance inflammation and organ injury after hepatic I/R. High-mobility group box 1 (HMGB1), a NF released by necrotic cells or secreted by stimulated cells, is one of a number of ligands promoting TLR4 reactivity. Augmentation of DC numbers in the liver with GM-CSF hydrodynamic transfection significantly increased liver damage after I/R when compared with controls. TLR4 engagement on hepatic DC was required for the I/R-induced injury, as augmentation of DC numbers in TLR4 mutant (C3H/HeJ) mice did not worsen hepatic damage. It is interesting that TLR4 expression was increased in hepatic DC following HMGB1 stimulation in vitro, suggesting a mechanism for the increased liver injury following I/R. It thus appears that functional TLR4 on DC is required for I/R-induced injury. Furthermore, HMGB1 may direct the inflammatory responses mediated by DC, at least in part, by enhancing TLR4 expression and reactivity to it and other DAMPs.

Regulatory dendritic cell therapy in organ transplantation

Dendritic cells (DCs) are uniquely well equipped antigen (Ag)-presenting cells. Their classic function was thought to be that of potent initiators of innate and adaptive immunity to infectious organisms and other Ags (including transplanted organs). Evidence has emerged, however, that DCs have a central and crucial role in determining the fate of immune responses toward either immunity or tolerance. This dichotomous function of DCs, coupled with their remarkable plasticity, renders them attractive therapeutic targets for immune modulation. In transplantation, much recent work has focused on the ability of DCs to silence immune reactivity in an Ag-specific manner in the hope of preventing rejection and diminishing reliance on potentially harmful immunosuppressive agents. Experimental strategies have included in vivo targeting of DCs, as well as ex vivo generation of regulatory (or tolerogenic) DCs with subsequent reinfusion (i.e. cell therapy). Different approaches to 'program' DC toward tolerogenic properties include genetic (transgene insertion), biologic (differential culture conditions, anti-inflammatory cytokine exposure) and pharmacologic manipulation. Recent data suggest a promising role for pharmacologic treatment as a means of generating potent regulatory DCs and have further stimulated speculation regarding their potential clinical application. Herein, we discuss evidence that the potential of regulatory DC therapy is considerable and that there are compelling reasons to evaluate it in the setting of organ transplantation in the near future.

Role for dendritic cells in immunoregulation during experimental vaginal candidiasis

Vulvovaginal candidiasis (VVC) caused by the commensal organism Candida albicans remains a significant problem among women of childbearing age, with protection against and susceptibility to infection still poorly understood. While cell-mediated immunity by CD4+ Th1-type cells is protective against most forms of mucosal candidiasis, no protective role for adaptive immunity has been identified against VVC. This is postulated to be due to immunoregulation that prohibits a more profound Candida-specific CD4+ T-cell response against infection. The purpose of this study was to examine the role of dendritic cells (DCs) in the induction phase of the immune response as a means to understand the initiation of the immunoregulatory events. Immunostaining of DCs in sectioned murine lymph nodes draining the vagina revealed a profound cellular reorganization with DCs becoming concentrated in the T-cell zone throughout the course of experimental vaginal Candida infection consistent with cell-mediated immune responsiveness. However, analysis of draining lymph node DC subsets revealed a predominance of immunoregulation-associated CD11c+ B220+ plasmacytoid DCs (pDCs) under both uninfected and infected conditions. Staining of vaginal DCs showed the presence of both DEC-205+ and pDCs, with extension of dendrites into the vaginal lumen of infected mice in close contact with Candida. Flow cytometric analysis of draining lymph node DC costimulatory molecules and activation markers from infected mice indicated a lack of upregulation of major histocompatibility complex class II, CD80, CD86, and CD40 during infection, consistent with a tolerizing condition. Together, the results suggest that DCs are involved in the immunoregulatory events manifested during a vaginal Candida infection and potentially through the action of pDCs.

Regulatory dendritic cells modulate immune responses via induction of T-cell apoptotic death

We describe a regulatory lymphoid dendritic cell (LDC) population propagated from mouse liver nonparenchymal cells (NPC) in IL-3 and anti-CD40 monoclonal antibody that are phenotypically mature, and induce T-cell hyporesponsiveness by promoting T-cell apoptotic death, which is partially caspase-dependent, but is unlikely to be mediated by soluble factor(s). In vivo administration of liver LDC significantly prolonged the survival of vascularized cardiac allografts in an alloantigen-specific manner. This is associated with enhanced T-cell death in secondary lymphoid organs.

Immune regulatory activity of liver-derived dendritic cells generated in vivo

Hepatic tolerance is demonstrated by spontaneous acceptance of liver allografts in mice. Hepatic dendritic cells (DC) play a crucial role in determining immunity or tolerance. In this study, we adopted an approach to transfect gene(s) into the mouse liver by tail-vein injection of plasmid-carrying genes. Transfection with GM-CSF expanded liver CD11c+ myeloid DC (LMDC), while liver B220+CD11c- lymphoid DC (LLDC) were expanded after transfection of IL-3 and CD40L. Flow analysis revealed that these liver DC subsets were phenotypically mature following overnight culture. However, in contrast to LMDC, LLDC induced hyporesponsiveness in allogeneic T-cells, with suppressed secretion of both IL-2 and IFN-gamma, and prolonged cardiac allograft survival. This immune regulatory DC population in the liver may play a role in modulating T-cell immunity in the liver.

Dendritic cells: key cells for the induction of regulatory T cells?

Even though dendritic cells (DCs) are well known for their capacity to induce immune responses, recent results show that they are also involved in the induction of tolerance. These two contrary effects of otherwise homologous DCs on a developing immune response maybe explainedby different DC developmental stages, i.e., different subsets of DCs may exist and/or different spatial distribution of DCs in the body might influence their function. However, independently from the subtype(s), it is obvious that the ability of DCs to act in a tolerogenic fashion depends on the maturation status, since immature DCs are prone to induce regulatory T cells and hence promote tolerance, whereas mature DCs stimulate effector T cells, facilitating immunity. The means by which DCs convey tolerance are not entirely clear yet, but secretion of suppressive cytokines such as IL-10 and induction of regulatory lymphocytes are involved. In this review we focus on the interaction between DCs and T cells and highlight some mechanisms in the decision-making process of whether immunity or tolerance is induced.

Induction of antigen-specific regulatory T cells in the liver-draining celiac lymph node following oral antigen administration

Regulatory T cells are induced by oral administration of an antigen, but the physiological requirements and localization of the inductive sites are largely unknown. Using an adoptive transfer system of cells transgenic for ovalbumin T-cell receptor (OVA TCR tg), we found that antigen-specific CD4+ T cells were activated in the liver-draining celiac lymph node (CLN) shortly after ovalbumin feeding, and that a significantly higher proportion of the T cells in the CLN developed into the putative regulatory phenotype [co-expressing CD25 with the glucocortico-induced tumour necrosis factor (TNF) receptor family related gene (GITR), cytotoxic T-lymphocyte antigen (CTLA)-4 and CD103] than in Peyer's patches, the mesenteric and peripheral lymph nodes and the spleen. In addition, a particularly high level of expression of CD103 on the OVA-specific T cells in the CLN may favour homing to the epithelium of the intestine. While equally suppressive, OVA tg T cells isolated from the CLN of OVA-fed DO11.10 mice were less dependent on transforming growth factor (TGF)-beta for suppression than cells isolated from the peripheral and mesenteric lymph nodes, which indicates the involvement of an additional suppressive mechanism. The expression of FoxP3 was not up-regulated in any of the lymph node compartments studied. Our phenotypic and functional findings suggest that the induction of regulatory T cells in the CLN may be relevant in the control of the immune response to dietary antigens.

Liver inflammation and cytokine production, but not acute phase protein synthesis, accompany the adult liver progenitor (oval) cell response to chronic liver injury

Oval cells are facultative liver progenitor cells, which are invoked during chronic liver injury in order to replenish damaged hepatocytes and bile duct cells. Previous studies have observed inflammation and cytokine production in the liver during chronic injury. Further, it has been proposed that inflammatory growth factors may mediate the proliferation of oval cells during disease progression. We have undertaken a detailed examination of inflammation and cytokine production during a time course of liver injury and repair, invoked by feeding mice a choline-deficient, ethionine-supplemented (CDE) diet. We show that immediately following initial liver injury, B220-expressing leucocytes transiently infiltrate the liver. This inflammatory response occurred immediately before oval cell numbers began to expand in the liver, suggesting that the two events may be linked. Two waves of liver cytokine production were observed during the CDE time course. The first occurred shortly following commencement of the diet, suggesting that it may represent a hepatic acute phase response. However, examination of acute phase marker expression in CDE-fed mice did not support this hypothesis. The second wave of cytokine expression correlated with the expansion of oval cell numbers in the liver, suggesting that these factors may mediate oval cell proliferation. No inflammatory signalling was detected following withdrawal of the injury stimulus. In summary, our results document a close correlation between inflammation, cytokine production and the expansion of oval cells in the liver during experimental chronic injury.

Inhibitory oligodeoxynucleotides - therapeutic promise for systemic autoimmune diseases?

Recent studies have shed new light on a possible link between the innate activation of plasmocytoid dendritic cells and marginal zone B cells and the pathogenesis of systemic lupus erythematosus. Animal studies have identified that this response requires the Toll-like receptor 9 (TLR9). Engagement of the TLR9 by various ligands, including non-canonical CpG-motifs, can cause or aggravate pathogenic autoantibody production and cytokine secretion in lupus. Attempts to neutralize this activity either by blocking the acidification of the endosomal compartment with chloroquine and related compounds, or by preventing the interaction between the CpG-DNA sequences and TLR9 using inhibitory oligonucleotides could be a promising therapeutic option for lupus.

IL-10-producing T regulatory type 1 cells and oral tolerance

Oral tolerance is mediated by multiple mechanisms such as anergy and/or active suppression of antigen-specific effector T cells by T regulatory (Tr) cells. Among the CD4(+) Tr cells, T regulatory type 1 cells (Tr1) have been shown to downmodulate immune responses through production of the immunosuppressive cytokines IL-10 and TGF-beta. Human Tr1 cells can be induced to differentiate in vitro by IL-10 1 IFN-alpha or after stimulation by immature dendritic cells (DCs) or DCs rendered tolerogenic by exposure to immunomodulatory compounds. Murine Tr1 cells can be induced to differentiate in vitro by activating naive CD4(+) T cells in the presence of high doses of IL-10. Several protocols for induction of oral tolerance, including oral administration of the antigen with IL-10, have been shown to induce antigen-specific Tr1 cells that suppress undesired immune responses toward self-antigens, allergens, and food antigens. Overall, these data demonstrate that IL-10-producing Tr1 cells play a central role in the induction of oral as well as systemic tolerance.

Differentiation of Tr1 cells by immature dendritic cells requires IL-10 but not CD25+CD4+ Tr cells

Dendritic cells (DCs) are specialized antigen-presenting cells that monitor the antigenic environment and activate naive T cells. The role of DCs is not only to sense danger but also to tolerize the immune system to antigens encountered in the absence of maturation/inflammatory stimuli. Indeed, if a naive T cell encounters its antigen on immature DCs (iDCs), it may differentiate into a T-regulatory (Tr) rather than a T-effector cell. However, little is known about the mechanisms by which iDCs differentiate Tr cells. We developed a standardized and highly reproducible protocol to differentiate Tr cells by repetitive exposure of naive peripheral blood CD4+ T cells to allogeneic iDCs. The resultant Tr cells are phenotypically and functionally identical to type 1 Tr (Tr1) cells because their generation requires production of IL-10 by iDCs, and they suppress T-cell responses through an interleukin-10 (IL-10)– and a transforming growth factor β (TGF-β)–dependent mechanism. In addition, Tr1 cells induced by iDCs do not require the presence of CD4+CD25+ Tr cells for their generation, nor do they express high constitutive levels of CD25 or the transcription factor FoxP3. Thus, iDCs can drive the differentiation of Tr1 cells and can be used to generate large numbers of alloantigen-specific Tr1 cells for clinical use as a cellular therapy to restore peripheral tolerance.

Regulating the immune system: the induction of regulatory T cells in the periphery

The immune system has evolved a variety of mechanisms to achieve and maintain tolerance both centrally and in the periphery. Central tolerance is achieved through negative selection of autoreactive T cells, while peripheral tolerance is achieved primarily via three mechanisms: activation-induced cell death, anergy, and the induction of regulatory T cells. Three forms of these regulatory T cells have been described: those that function via the production of the cytokine IL-10 (T regulatory 1 cells), transforming growth factor beta (Th3 cells), and a population of T cells that suppresses proliferation via a cell-contact-dependent mechanism (CD4+CD25+ TR cells). The present review focuses on the third form of peripheral tolerance - the induction of regulatory T cells. The review will address the induction of the three types of regulatory T cells, the mechanisms by which they suppress T-cell responses in the periphery, the role they play in immune homeostasis, and the potential these cells have as therapeutic agents in immune-mediated disease.

IL-10-Producing B220+CD11c−APC in Mouse Spleen

APC acting at the early stages of an immune response can shape the nature of that response. Such APC will include dendritic cells (DCs) but may also include populations of B cells such as marginal zone B cells in the spleen. In this study, we analyze APC populations in mouse spleen and compare the phenotype and function of B220(+)CD11c(-) populations with those of CD11c(+) spleen DC subsets. Low-density B220(+) cells had morphology similar to DCs and, like DCs, they could stimulate naive T cells, and expressed high levels of MHC and costimulatory molecules. However, the majority of the B220(+) cells appeared to be of B cell lineage as demonstrated by coexpression of CD19 and surface Ig, and by their absence from RAG-2(-/-) mice. The phenotype of these DC-like B cells was consistent with that of B cells in the marginal zone of the spleen. On bacterial stimulation, they preferentially produced IL-10 in contrast to the DCs, which produced IL-12. Conventional B cells did not produce IL-10. The DC-like B cells could be induced to express low levels of the DC marker CD11c with maturational stimuli. A minority of the B220(+)CD11c(-) low-density cells did not express CD19 and surface Ig and may be a DC subset; this population also produced IL-10 on bacterial stimulation. B220(+) APC in mouse spleen that stimulate naive T cells and preferentially produce IL-10 may be involved in activating regulatory immune responses.

DEC-205 expression on migrating dendritic cells in afferent lymph

Previous studies have identified a 210 000-molecular weight molecule expressed at a high level on the surface of dendritic cells (DCs) in afferent lymph of cattle and evident on cells with the morphology of DCs in lymphoid tissues. Expression is either absent from other immune cells or is present at a lower level. The molecular weight and cellular distribution suggested that the molecule, called bovine WC6 antigen (workshop cluster), might be an orthologue of human DEC-205 (CD205). To establish whether this was the case, the open reading frame of bovine DEC-205 was amplified, by polymerase chain reaction, from thymic cDNA (accession no. AY264845). The cDNA sequence of bovine DEC-205 had 86% and 78% nucleic acid identity with human and mouse molecules, respectively. COS-7 cells transfected with a plasmid containing the cattle DEC-205 coding region expressed a molecule that stained with WC6-specific monoclonal antibody, showing that ruminant WC6 is an orthologue of DEC-205. Two-colour flow cytometry of mononuclear cells from afferent lymph draining cattle skin, and from blood, confirmed the high level of expression on large cells in lymph that were uniformly DC-LAMP positive and major histocompatibility complex class II positive. Within this DEC-205+ DC-LAMP+ population were subpopulations of cells that expressed the mannose receptor or SIRPalpha. The observations imply that DCs in afferent lymph are all DEC-205high, but not a uniform population of homogeneous mature DCs.

Induction of tolerogenic DCs: 'you are what you eat'

Dendritic cells (DCs) take up antigens using antigen receptors that can be divided into three major classes: C-type lectins, integrins and Fc receptors. These receptors facilitate effective presentation of MHC-peptide complexes to T cells, resulting in the induction of immune responses. However, we discuss recent evidence that some receptors also cause induction of tolerance. Signaling motifs within the receptors either block maturation of DCs or induce signals that render DCs tolerogenic. These DCs then either induce regulatory T cells or cause deletion of effector T cells, resulting in the induction of tolerance. Antigen receptors expressed by DCs might therefore have an important role in the induction and maintenance of peripheral tolerance.

Mechanistic Insights into Impaired Dendritic Cell Function by Rapamycin: Inhibition of Jak2/Stat4 Signaling Pathway

The suppressive effect of rapamycin on T cells has been extensively studied, but its influence on the function of APC is less clear. The data in this study demonstrated that immunostimulatory activity of B10 (H2(b)) dendritic cells (DC) exposed to rapamycin (rapa-DC) was markedly suppressed as evidenced by the induction of low proliferative responses and specific CTL activity in allogeneic (C3H, H2(k)) T cells. Administration of rapa-DC significantly prolonged survival of B10 cardiac allografts in C3H recipients. Treatment with rapamycin did not affect DC expression of MHC class II and costimulatory molecules or IL-12 production. Rapamycin did not inhibit DC NF-kappaB pathway, however, IL-12 signaling through Janus kinase 2/Stat4 activation was markedly suppressed. Indeed, Stat4(-/-) DC similarly displayed poor allostimulatory activity. The Stat4 downstream product, IFN-gamma, was also inhibited by rapamycin, but DC dysfunction could not solely be attributed to low IFN-gamma production as DC deficient in IFN-gamma still exhibited vigorous allostimulatory activity. Rapamycin did not affect DC IL-12R expression, but markedly suppressed IL-18Ralpha and beta expression, which may in turn down-regulate DC IL-12 autocrine activation.

Prevention of diabetes in NOD mice by administration of dendritic cells deficient in nuclear transcription factor-kappaB activity

Abnormalities of dendritic cells (DCs) have been identified in type 1 diabetic patients and in nonobese diabetic (NOD) mice that are associated with augmented nuclear transcription factor (NF)-kappaB activity. An imbalance that favors development of the immunogenic DCs may predispose to the disease, and restoration of the balance by administration of DCs deficient in NF-kappaB activity may prevent diabetes. DCs propagated from NOD mouse bone marrow and treated with NF-kappaB-specific oligodeoxyribonucleotide (ODN) in vitro (NF-kappaB ODN DC) were assessed for efficacy in prevention of diabetes development in vivo. Gel shift assay with DC nuclear extracts confirmed specific inhibition of NF-kappaB DNA binding by NF-kappaB ODN. The costimulatory molecule expression, interleukin (IL)-12 production, and immunostimulatory capacity in presenting allo- and islet-associated antigens by NF-kappaB ODN DC were significantly suppressed. NF-kappaB ODN renders DCs resistant to lipopolysaccharide stimulation. Administration of 2 x 10(6) NF-kappaB ODN DCs into NOD mice aged 6-7 weeks effectively prevented the onset of diabetes. T-cells from pancreatic lymph nodes of NF-kappaB ODN DC-treated animals exhibited hyporesponsiveness to islet antigens with low production of interferon-gamma and IL-2. These findings provide novel insights into the mechanisms of autoimmune diabetes and may lead to development of novel preventive strategies.

Four-color flow cytometric analysis of peripheral blood donor cell chimerism

Passenger leukocytes have been demonstrated to play significant roles in initiating and also regulating immune reactions after organ transplantation. Reliable techniques to detect donor leukocytes in recipients after organ transplantation are essential to analyze the role, function, and behavior of these leukocytes. In this report we describe a simple, reliable method to detect donor cells with low frequencies using peripheral blood samples. Detection of small numbers of major histocompatibility complex (MHC) mismatched cells was first studied using four-color flow cytometry in artificially created cell mixtures. By selecting the CD45(+) population and simultaneous staining with several leukocyte lineage markers (CD3, CD4, CD8, CD56, and CD19), MHC-mismatched leukocytes were consistently detected in cell suspensions prepared from directly stained whole blood samples with a threshold sensitivity as low as 0.1%-0.2%. When the fresh peripheral blood mononuclear cells were separated by conventional Ficoll gradient purification, similar, but slightly lower levels of donor cells were detected. Blood samples obtained 1-5 months after liver, kidney, and intestine transplants revealed that the kind of organ allograft influenced levels and lineage pattern of the circulating donor cells. This procedure provided a simple and reliable method in determining early chimerism in transplant recipients. However, the detection of MHC-mismatched leukocytes of all lineages was much lower when frozen peripheral blood mononuclear cells were used.

Interleukin-3Ralpha+ myeloid dendritic cells and mast cells develop simultaneously from different bone marrow precursors in cultures with interleukin-3

The distinct developmental routes of dendritic cells and mast cells from murine bone marrow cultures with interleukin-3 are unclear. We found that short-term bone marrow cultures with interleukin-3 after 8-10 d consist of about 10%-30% dendritic cells and 70%-90% mast cell precursors, and only after 4-6 wk do homogeneous populations of mast cells emerge. Phenotypical and functional analysis of interleukin-3/dendritic cells revealed a high similarity with myeloid dendritic cells generated with granulocyte-macrophage colony stimulating factor in the expression of myeloid dendritic cell markers (CD11c+ B220- CD8alpha- CD11b+), major histocompatibility complex II and costimulatory molecules, endocytosis, maturation potential, interleukin-12 production, and T cell priming. Interleukin-3/dendritic cells expressed higher levels of interleukin-3 receptor, however. To dissect the interleukin-3/dendritic cell and mast cell development, we sorted fresh bone marrow cells into six subsets by the antibodies ER-MP12 (CD31) and ER-MP20 (Ly-6C). Both interelukin-3/dendritic cells and granulocyte-macrophage colony stimulating factor/dendritic cells develop from the same bone marrow populations, including the ER-MP12neg, ER-MP20high bone marrow monocytes. In contrast, mast cells only developed from ER-MP12(int+high), ER-MP20neg bone marrow cell subsets, indicating that different precursors exist for interleukin-3/dendritic cells and mast cells. Established mast cell cultures could not be converted to dendritic cells or stimulated to express major histocompatibility complex II molecules in vitro or home to lymph node T cell areas in vivo. In summary, we show that dendritic cells generated from bone marrow precursors with interleukin-3 are clearly myeloid and develop via a different pathway compared to bone marrow mast cells.

Dendritic cells, T cell tolerance and therapy of adverse immune reactions

Dendritic cells (DC) are uniquely able to either induce immune responses or to maintain the state of self tolerance. Recent evidence has shown that the ability of DC to induce tolerance in the steady state is critical to the prevention of the autoimmune response. Likewise, DC have been shown to induce several type of regulatory T cells including Th2, Tr1, Ts and NKT cells, depending on the maturation state of the DC and the local microenvironment. DC have been shown to have therapeutic value in models of allograft rejection and autoimmunity, although no success has been reported in allergy. Several strategies, including the use of specific DC subsets, genetic modification of DC and the use of DC at various maturation stages for the treatment of allograft rejection and autoimmune disease are discussed. The challenge for the future use of DC therapy in human disease is to identify the appropriate DC for the proposed therapy; a task made more daunting by the extreme plasticity of DC that has recently been demonstrated. However, the progress achieved to date suggests that these are not insurmountable obstacles and that DC may become a useful therapeutic tool in transplantation and autoimmune disease.

Induction of CD4+/CD25+ regulatory T cells by targeting of antigens to immature dendritic cells

Coupling of ovalbumin (OVA) to anti-DEC-205 monoclonal antibody (mAb) (alphaDEC) induced the proliferation of OVA-specific T cells in vivo. Expansion was short-lived, caused by dendritic cells (DCs), and rendered T cells anergic thereafter. Phenotypic analysis revealed the induction of CD25+/CTLA-4+ T cells suppressing proliferation and interleukin-2 (IL-2) production of effector CD4+ T cells. The findings were supported by 2 disease models: (1) CD4+ T-cell-mediated hypersensitivity reactions were suppressed by the injection of alphaDEC-OVA and (2) the application of hapten-coupled alphaDEC-205 reduced CD8+ T-cell-mediated allergic reactions. Thus, targeting of antigens to immature DCs through alphaDEC antibodies led to the induction of regulatory T cells, providing the basis for novel strategies to induce regulatory T cells in vivo.

Active suppression of allogeneic proliferative responses by dendritic cells after induction of long-term allograft survival by CTLA4Ig

Costimulatory blockade using cytotoxic T lymphocyte-associated antigen 4 immunoglobulin (CTLA4Ig) efficiently down-regulates immune responses in animal models and is currently used in autoimmune and transplantation clinical trials, but the precise cellular and molecular mechanisms involved remain unclear. Rats that received allogeneic heart transplants and were treated with adenoviruses coding for CTLA4Ig show long-term allograft survival. The immune mechanisms regulating induction of long-term allograft acceptance were analyzed in splenocytes using mixed leukocyte reactions (MLRs). MLRs of splenocytes but not purified T cells from CTLA4Ig-treated rats showed higher than 75% inhibition compared with controls. Splenocytes from CTLA4Ig-treated rats inhibited proliferation of naive and allogeneically primed splenocytes or T cells. MLR suppression was dependent on soluble secreted product(s). Production of soluble inhibitory product(s) was triggered by a donor antigen-specific stimulation and inhibited proliferation in an antigen-nonspecific manner. CTLA4Ig levels in the culture supernatant were undetectable and neither interleukin-10 (IL-10), transforming growth factor beta 1 (TGF beta 1), IL-4, nor IL-13 were responsible for suppression of MLRs. Inhibition of nitric oxide (NO) production or addition of IL-2 could not restore proliferation independently, but the combined treatment synergistically induced proliferation comparable with controls. Stimulation of APCs using tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE) or CD40L and addition of IL-2 normalized MLRs of CTLA4Ig-treated splenocytes. Finally, dendritic cells (DCs), but not T cells, from CTLA4Ig-treated rats inhibited naive MLRs. Altogether, these results provide evidence that after in vivo CTLA4Ig treatment, splenocytes, and in particular DCs, can inhibit alloantigen-induced proliferative responses through secretion of inhibitory products, thus promoting alloantigen-specific tolerance in vivo.

Two developmentally distinct populations of dendritic cells inhabit the adult mouse thymus: demonstration by differential importation of hematogenous precursors under steady state conditions

Although a variety of lymphoid and myeloid precursors can generate thymic dendritic cells (DCs) under defined experimental conditions, the developmental origin(s) of DCs in the steady state thymus is unknown. Having previously used selective combinations of normal, parabiotic, and radioablated mice to demonstrate that blood-borne prothymocytes are imported in a gated and competitive manner, we used a similar approach in this study to investigate the importation of the hematogenous precursors of thymic DCs. The results indicate that two developmentally distinct populations of DC precursors normally enter the adult mouse thymus. The first population is indistinguishable from prothymocytes according to the following criteria: 1) inefficient (<20%) exchange between parabiotic partners; 2) gated importation by the thymus; 3) competitive antagonism for intrathymic niches; 4) temporally linked generation of thymocytes and CD8alpha(high) DCs; and 5) absence from prothymocyte-poor blood samples. The second population differs diametrically from prothymocytes in each of these properties, and appears to enter the thymus in at least a partially differentiated state. The resulting population of DCs has a CD8alpha(-/low) phenotype, and constitutes approximately 50% of total thymic DCs. The presence of two discrete populations of DCs in the steady state thymus implies functional heterogeneity consistent with evidence implicating lymphoid DCs in the negative selection of effector thymocytes and myeloid DCs in the positive selection of regulatory thymocytes.

Flt3 ligand-treated neonatal mice have increased innate immunity against intracellular pathogens and efficiently control virus infections

Flt-3 ligand (FL), a hematopoetic growth factor, increases the number of dendritic cells (DCs), B cells, and natural killer cells in adult mice but the effect in neonates was unknown. We show that FL treatment of newborn mice induced a >100-fold increase in the innate resistance against infection with herpes simplex virus type 1 and Listeria monocytogenes. This resistance required interferon (IFN)-alpha/beta for viral and interleukin (IL)-12 for bacterial infections. Long-term survival after viral but not bacterial infection was increased approximately 100-fold by FL treatment. After treatment, CD11c(+)/major histocompatibility complex type II(+) and CD11c(+)/B220(+) DC lineage cells were the only cell populations increased in the spleen, liver, peritoneum, and skin. DC induction was independent of IFNs, IL-2, -4, -7, -9, -15, and mature T and B cells. The data suggest that FL increases the number of DCs in neonates and possibly in other immune-compromised individuals, which in turn improves IFN-alpha/beta- and IL-12-associated immune responses.