Donor pretreatment with Flt-3 ligand augments antidonor cytotoxic T lymphocyte, natural killer, and lymphokine-activated killer cell activities within liver allografts and alters the pattern of intragraft apoptotic activity

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 In addition, some liver transplant patients can be successfully withdrawn from immunosuppressive medications, developing ‘operational tolerance'.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.

Stromal-Derived Factor-1α and Interleukin-7 Treatment Improves Homeostatic Proliferation of Naïve CD4(+) T Cells after Allogeneic Stem Cell Transplantation

Graft-versus-host disease (GVHD) impairs immune reconstitution after allogeneic stem cell transplantation (allo-SCT) and effective therapies aimed at restoring T cell counts in GVHD patients have yet to be developed. During GVHD, CD4(+) T cell reconstitution is particularly affected and current models hold that GVHD insult to the peripheral lymphoid niche is responsible for this effect. Here, we show that naïve CD4(+) T cell homeostatic proliferation (HP) is lost during GVHD because of low systemic IL-7 and impaired dendritic cell (DC) regeneration. We assessed factors involved in DC differentiation and found that although fms-like tyrosine kinase 3 ligand (Flt3-L) levels were normal, stromal-derived factor-1α (SDF-1α) was diminished in the blood of GVHD mice. Unlike Flt3-L treatment, the administration of SDF-1α specifically increased CD8α(+) DC numbers and did not worsen GVHD. Importantly, CD4(+) T cell HP was enhanced only when IL-7 and SDF-1α or Flt3L were coadministered, confirming the crucial role of DCs and IL-7 in restoring CD4(+) T cell regeneration during GVHD. Altogether, our results indicate that CD8α(+) DCs are part of the peripheral niche that controls CD4(+) T cell HP and that their depletion, combined with low systemic IL-7, explains how GVHD constrains naïve CD4(+) T cell reconstitution after allo-SCT.

DAP12 deficiency in liver allografts results in enhanced donor DC migration, augmented effector T cell responses and abrogation of transplant tolerance

Liver interstitial dendritic cells (DC) have been implicated in immune regulation and tolerance induction. We found that the transmembrane immuno-adaptor DNAX-activating protein of 12 kDa (DAP12) negatively regulated conventional liver myeloid (m) DC maturation and their in vivo migratory and T cell allostimulatory ability. Livers were transplanted from C57BL/6(H2(b) ) (B6) WT or DAP12(-/-) mice into WT C3H (H2(k) ) recipients. Donor mDC (H2-K(b+) CD11c(+) ) were quantified in spleens by flow cytometry. Anti-donor T cell reactivity was evaluated by ex vivo carboxyfluorescein diacetate succinimidyl ester-mixed leukocyte reaction and delayed-type hypersensitivity responses, while T effector and regulatory T cells were determined by flow analysis. A threefold to fourfold increase in donor-derived DC was detected in spleens of DAP12(-/-) liver recipients compared with those given WT grafts. Moreover, pro-inflammatory cytokine gene expression in the graft, interferon gamma (IFNγ) production by graft-infiltrating CD8(+) T cells and systemic levels of IFNγ were all elevated significantly in DAP12(-/-) liver recipients. DAP12(-/-) grafts also exhibited reduced incidences of CD4(+) Foxp3(+) cells and enhanced CD8(+) T cell IFNγ secretion in response to donor antigen challenge. Unlike WT grafts, DAP12(-/-) livers failed to induce tolerance and were rejected acutely. Thus, DAP12 expression in liver grafts regulates donor mDC migration to host lymphoid tissue, alloreactive T cell responses and transplant tolerance.

Mechanisms of murine spontaneous liver transplant tolerance

Liver transplant is associated with the induction of peripheral immune tolerance. Liver allografts are accepted spontaneously in most combinations of mismatch in major histocompatibility complex, without any requirements for immunosuppression. Liver nonparenchymal cells (especially dendritic cells and Kupffer cells), costimulatory pathways, and activated T-cell apoptosis may contribute to the induction of liver tolerance. Therefore, liver tolerance is an active process that includes T-cell activation, proliferation, infiltration of the allograft, and T-cell apoptosis. Liver dendritic cells may modulate the amount of alloreactive T cells in liver graft recipients by expressing the coinhibitory molecule programmed death-ligand 1 and the immunosuppressive enzyme indoleamine 2,3-dioxygenase. Liver dendritic cells also may induce activated T-cell apoptosis and Foxp3+ regulatory T cells. Future studies may clarify the precise function of liver nonparenchymal cells, the interactions between programmed death-ligand 1 and other costimulatory signals, and the contribution of the liver microenvironment to the induction and expansion of Foxp 3 regulatory T cells.

Differential migration of passenger leukocytes and rapid deletion of naive alloreactive CD8 T cells after mouse liver transplantation

Donor passenger leukocytes (PLs) from transplanted livers migrate to recipient lymphoid tissues, where they are thought to induce the deletion of donor-specific T cells and tolerance. Difficulties in tracking alloreactive T cells and PLs in rats and in performing this complex surgery in mice have limited progress in identifying the contribution of PL subsets and sites and the kinetics of T cell deletion. Here we developed a mouse liver transplant model in which PLs, recipient cells, and a reporter population of transgenic CD8 T cells specific for the graft could be easily distinguished and quantified in allografts and recipient organs by flow cytometry. All PL subsets circulated rapidly via the blood as soon as 1.5 hours after transplantation. By 24 hours, PLs were distributed differently in the lymph nodes and spleen, whereas donor natural killer and natural killer T cells remained in the liver and blood. Reporter T cells were activated in both liver and lymphoid tissues, but their numbers dramatically decreased within the first 48 hours. These results provide the first unequivocal demonstration of the differential recirculation of liver PL subsets after transplantation, and show that alloreactive CD8 T cells are deleted more rapidly than initially reported. This model will be useful for dissecting early events leading to the spontaneous acceptance of liver transplants.

Natural killer cells in asthma

The worldwide prevalence, morbidity, and mortality of asthma have dramatically increased over the last few decades and there is a clear need to identify new effective therapeutic and prophylactic strategies. Despite high numbers of NK cells in the lung and their ability to generate a variety of immunomodulatory mediators, the potential of NK cells as therapeutic targets in allergic airway disease has been largely overlooked. The fact that IgE, acting through FcγRIII, can activate NK cells resulting in cytokine/chemokine production implies that NK cells may contribute to IgE-mediated allergic responses. Indeed, current evidence suggests that NK cells can promote allergic airway responses during sensitization and ongoing inflammation. In animal models, increased NK cells are observed in the lung following antigen challenge and depletion of the cells before immunization inhibits allergic airway inflammation. Moreover, in asthmatics, NK cell phenotype is altered and may contribute to the promotion of a pro-inflammatory Th2-type environment. Conversely, driving NK cells toward an IFN-γ-secreting phenotype can reduce features of the allergic airway response in animal models. However, we have limited knowledge of the signals that drive the development of distinct subsets and functional phenotypes of NK cells in the lung and thus the role and therapeutic potential of NK cells in the allergic airway remains unclear. Here we review the potentially diverse role of NK cells in allergic airway disease, identify gaps in current knowledge, and discuss the potential of modulating NK cell function as a treatment strategy in asthma.

Roles of dendritic cells in murine hepatic warm and liver transplantation-induced cold ischemia/reperfusion injury

Dendritic cells (DCs) induce and regulate both innate and adaptive immune responses; however, their in vivo functional importance in hepatic ischemia/reperfusion (IR) injury is perplexing. We hypothesized that liver-resident DC and locally recruited blood-borne DC might have distinctive roles in hepatic IR injury. We tested this hypothesis by using DC-deficient, fms-like tyrosine kinase 3 ligand (Flt3L) knockout (KO) mice in hepatic warm (70% partial clamping for 60 minutes) and cold IR injury (liver transplant [LTx] with 24-hour cold storage). Flt3L KO liver and lymphoid organs contained virtually no CD11c+ F4/80- DC. Hepatic warm IR injury was significantly lower in Flt3L KO than in wildtype (WT) mice with lower alanine aminotransferase (ALT) levels, reduced hepatic necrosis, and lower neutrophil infiltration. Hepatic messenger RNA (mRNA) and protein levels for inflammatory cytokines (tumor necrosis factor alpha [TNFα], interleukin [IL]-6) and chemokines (CCL2, CXCL2) were also significantly lower in Flt3L KO than in WT mice, indicating that lack of both liver-resident and blood-borne DC ameliorated hepatic warm IR injury. Adoptive transfer of splenic or hepatic WT DC into Flt3L KO or WT mice increased hepatic warm IR injury, suggesting injurious roles of DC infusion. When Flt3L KO liver was transplanted into WT mice, ALT levels were significantly higher than in WT to WT LTx, with enhanced hepatic necrosis and neutrophil infiltration, indicating a protective role of liver-resident DC.

CONCLUSION

Using both warm and cold hepatic IR models, this study suggests differential roles of liver-resident versus blood-borne DC, and points to the importance of the local microenvironment in determining DC function during hepatic IR injury.

Distinct response of liver myeloid dendritic cells to endotoxin is mediated by IL-27

BACKGROUND/AIMS

The liver lies downstream of the gut, and is constantly exposed to bacteria. Liver dendritic cells (DC) are known to possess properties of tolerance, and respond to LPS differently when compared to conventional DC, but the underlying mechanisms are not completely understood. The aim of this study was to evaluate liver DC response to LPS stimulation.

METHODS

Liver or spleen derived DC were isoloated from mice treated with plasmid-GM-CSF hydrodynamic injection. The surface molecules and TLR4 expression on DC and cytokine productions of LPS stimulated DC were determinded by FACS analysis, ELISA and qPCR. The ability of DC to elicit T cell responses and differentiation were examined by MLR/CTL assay and qPCR for molecular markers related to Th1/Th2/Treg.

RESULTS

In this study, we demonstrated that the threshold of LPS stimulation for liver DC was markedly higher than spleen DC, even though the expression of TLR4 on both DCs was comparable. In contrast to spleen DC that produced high levels of IL-12 and induced Th1 response upon LPS stimulation, LPS-liver DC preferentially produced IL-10 and IL-27, instead of IL-12. In addition, liver DC induced T cell hyporesponsiveness, associated with selective expansion of CD4(+)Foxp3(+)T regulatory cells. Addition of exogenous IL-12 only slightly enhanced liver DC-induced T cell response. Interestingly, abrogation of IL-27 ligation by using IL-27R(-/-) T cells synergistically augmented the effect of IL-12, suggesting that IL-27 produced by liver DC plays a crucial role in induction of T cell hyporesponsiveness.

CONCLUSIONS

Liver DC respond distinctly to LPS stimulation by secreting IL-27 which synergizes with silencing of bioactive IL-12 activity leading to profound T cell inhibition.

Murine liver plasmacytoid dendritic cells become potent immunostimulatory cells after Flt-3 ligand expansion

The liver has unique immunological properties. Although dendritic cells (DCs) are central mediators of immune regulation, little is known about liver DCs. Plasmacytoid DCs (pDCs) are a recently identified subtype of murine liver DC. We sought to define the function of freshly isolated murine liver pDCs. We found that normal liver pDCs were weak in stimulating T cells, yet they possessed a proinflammatory cytokine profile with high tumor necrosis factor-alpha and low IL-10 secretion. To facilitate the investigation of murine liver pDCs, we expanded them in vivo with fms-like tyrosine kinase 3 ligand (Flt3L). After Toll-like receptor-9 ligation, expanded liver pDCs secreted high levels of IFN-alpha and were able to stimulate NK cells, NKT cells, and antigen-specific CD8+ T cells in vitro. In addition, Flt3L expansion alone generated pDCs capable of activating antigen-specific CD8+ T cells in vivo.

CONCLUSION

Unstimulated liver pDCs exist in a latent state with the potential to become potent activators of the innate and adaptive immune systems through their interactions with other immune effectors. Our findings have implications for understanding the role of the liver in tolerance and immunity.

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.

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.

CTLA4 engagement is required for induction of murine liver transplant spontaneous tolerance

Liver transplantation in mice is accepted spontaneously in all strain combinations. The mechanisms remain largely undefined. We hypothesize that signaling via the B7-CTLA4 receptor pathway is required for induction of liver transplant tolerance. Liver transplantation was performed from B10 (H2(b)) to C3H (H2(k)) mice. The recipients received anti-mouse CTLA4 mAb 0.25 mg i.p. every other day post-operatively. Liver grafts in anti-CTLA4 mAb treated recipients were acutely rejected. The allo-specific proliferative responses, anti-donor CTL and NK cell activities of GIC and SC and the serum levels of IFN-gamma and IL-2 from anti-CTLA4 mAb treated recipients were elevated significantly in comparison to the control mice. The frequency of IFN-gamma and IL-2 producing cells were markedly increased also in the anti-CTLA4 treated recipients. The immunohistology of liver grafts from anti-CTLA4 mAb treated mice showed extensively increased lymphocyte infiltration in the portal and general parenchymal areas, and expanded T-cell area in the spleen, with a reduction in the frequency of apoptotic cells observed by TUNEL staining compared with control mice. Thus CTLA4 signaling is critical for murine liver transplant tolerance induction. CTLA4 blockade promotes donor specific T-cell activation, cytotoxicity and Th1 polarization; protects alloreactive T cells from apoptotic death and induces liver allograft acute rejection.

Prolongation of liver allograft survival by dendritic cells modified with NF-κB decoy oligodeoxynucleotides

AIM: To induce the tolerance of rat liver allograft by dendritic cells (DCs) modified with NF-κB decoy oligodeoxynucleotides (ODNs).

METHODS: Bone marrow (BM)-derived DCs from SD rats were propagated in the presence of GM-CSF or GM-CSF + IL-4 to obtain immature DCs or mature DCs. GM-CSF+IL-4-propagated DCs were treated with double-strand NF-κB decoy ODNs containing two NF-κB binding sites or scrambled ODNs to ascertain whether NF-κB decoy ODNs might prevent DC maturation. GM-CSF-propagated DCs, GM-CSF + NF-κB decoy ODNs or scrambled ODNs-propagated DCs were treated with LPS for 18 h to determine whether NF-κB decoy ODNs could prevent LPS-induced IL-12 production in DCs. NF-κB binding activities, costimulatory molecule (CD40, CD80, CD86) surface expression, IL-12 protein expression and allostimulatory capacity of DCs were measured with electrophoretic mobility shift assay (EMSA), flow cytometry, Western blotting, and mixed lymphocyte reaction (MLR), respectively. GM-CSF-propagated DCs, GM-CSF + IL-4 -propagated DCs, and GM-CSF + NF-κB decoy ODNs or scrambled ODNs-propagated DCs were injected intravenously into recipient LEW rats 7 d prior to liver transplantation and immediately after liver transplantation. Histological grading of liver graft rejection was determined 7 d after liver transplantation. Expression of IL-2, IL-4 and IFN-γ mRNA in liver graft and in recipient spleen was analyzed by semiquantitative RT-PCR. Apoptosis of liver allograft-infiltrating cells was measured with TUNEL staining.

RESULTS: GM-CSF-propagated DCs, GM-CSF+NF-κB decoy ODNs-propagated DCs and GM-CSF+ scrambled ODNs-propagated DCs exhibited features of immature DCs, with similar low level of costimulatory molecule(CD40, CD80, CD86) surface expression, absence of NF-κB activation, and few allocostimulatory activities. GM-CSF + IL-4-propagated DCs displayed features of mature DCs, with high levels of costimulatory molecule (CD40, CD80, CD86) surface expression, marked NF-κB activation, and significant allocostimulatory activity. NF-κB decoy ODNs completely abrogated IL-4-induced DC maturation and allocostimulatory activity as well as LPS-induced NF-κB activation and IL-12 protein expression in DCs. GM-CSF + NF-κB decoy ODNs-propagated DCs promoted apoptosis of liver allograft-infiltrating cells within portal areas, and significantly decreased the expression of IL-2 and IFN-γ mRNA but markedly elevated IL-4 mRNA expression both in liver allograft and in recipient spleen, and consequently suppressed liver allograft rejection, and promoted liver allograft survival.

CONCLUSION: NF-κB decoy ODNs-modified DCs can prolong liver allograft survival by promoting apoptosis of graft-infiltrating cells within portal areas as well as down-regulating IL-2 and IFN-γ mRNA and up-regulating IL-4 mRNA expression both in liver graft and in recipient spleen.

Increased mononuclear cell activation and apoptosis early after human liver transplantation is associated with a reduced frequency of acute rejection

Experimental models of orthotopic liver transplantation (OLT) have shown that the very early events post-OLT are critical in distinguishing immunogenic and tolerogenic reactions. In rodents, increased leukocyte apoptosis and cytokine expression have been demonstrated in tolerogenic strain combinations. Information from human OLT recipients is less abundant. The aim of this study was to determine the amount of early leukocyte activation and apoptosis following human OLT, and to correlate this with subsequent rejection status. Peripheral blood mononuclear cells (PBMC) were isolated from 76 patients undergoing OLT - on the day prior, 5 hrs after reperfusion (day 0), and 18-24 hrs post-OLT (day 1). The mean level of apoptotic PBMCs on post OLT day 1 was higher than healthy recipients (0.9% +/- 0.2 vs. 0.2% +/- 0.1, p=0.013). Apoptosis was greater in nonrejecting (NR) (1.1% +/- 0.3) compared with acutely-rejecting (R) (0.3% +/- 0.1, p=0.021) patients. On day 1, PBMC from NR patients had increased expression of IFN-gamma (p=0.006), IL-10 (p=0.016), and CD40 ligand (p=0.02) compared with R. Donor cell chimerism on day 1 did not differ between the groups indicating that this was unlikely to account for increased PBMC apoptosis in the NR group. Interestingly, the level of chimerism on day 0 was significantly higher in NR (3.8% +/- 0.6) compared with R (1.2% +/- 0.4, p=0.004) patients and there was a close correlation between chimerism on day 0 and cytokine expression on day 1. These results imply that similar mechanisms are occurring in the human liver to promote graft acceptance as in the experimental models of liver transplantation and suggest that strategies that promote liver transplant acceptance in rodents might be applicable to humans.

Human hepatic lymph nodes contain normal numbers of mature myeloid dendritic cells but few plasmacytoid dendritic cells

We investigated whether the hyporesponsiveness of the adaptive arm of the liver immune system is related to the composition of the dendritic cell (DC) population in hepatic lymph nodes. Noninflamed human hepatic lymph nodes (LN) were obtained from multiorgan donors, inflamed hepatic LN from liver transplant recipients with autoimmune cholestatic liver diseases, and inguinal LN from kidney transplant recipients. Quantitative immunohistochemistry showed that all three types of LN contained comparable numbers of mature and immature myeloid DC, but that noninflamed and inflamed hepatic LN contained significantly fewer plasmacytoid DC as compared to inguinal LN. Likewise, DC-enriched cell preparations from hepatic LN contained relatively few plasmacytoid DC. The difference in numbers of plasmacytoid DC was confirmed in comparisons of hepatic LN and ileacal LN from the same organ-donors. Myeloid DC from hepatic LN showed similar expressions of HLA-DR, CD83, and CD86, and higher expression of CD80 compared to myeloid DC from inguinal LN. In conclusion, hepatic LN contain similar numbers of myeloid DC as muscle/skin lymph-draining LN, with no signs of immaturity, but relatively few plasmacytoid DC.

Murine Flt3 ligand expands distinct dendritic cells with both tolerogenic and immunogenic properties

Human Flt3 ligand can expand dendritic cells (DC) and enhance immunogenicity in mice. However, little is known about the effects of murine Flt3 ligand (mFlt3L) on mouse DC development and function. We constructed a vector to transiently overexpress mFlt3L in mice. After a single treatment, up to 44% of splenocytes became CD11c(+) and the total number of DC increased 100-fold. DC expansion effects lasted for >35 days. mFlt3L DC were both phenotypically and functionally distinct. They had increased expression of MHC and costimulatory molecules and expressed elevated levels of B220 and DEC205 but had minimal CD4 staining. mFlt3L DC also had a markedly altered cytokine profile, including lowered secretion of IL-6, IL-10, IFN-gamma, and TNF-alpha, but had a slightly increased capacity to stimulate T cells in vitro. However, in a variety of in vivo models, DC expanded by mFlt3L induced tolerogenic effects on T cells. Adoptive transfer of Ag-pulsed mFlt3L splenic DC to naive mice actually caused faster rates of tumor growth and induced minimal CTL compared with control DC. mFlt3L also failed to protect against tumors in which human Flt3 ligand was protective, but depletion of CD4(+) T cells restored tumor protection. Our findings 1) demonstrate that mFlt3L has distinct effects on DC development, 2) suggest an important role for mFlt3L in generating DC that have tolerogenic effects on T cells, and 3) may have application in immunotherapy in generating massive numbers of DC for an extended duration.

Flt3 ligand augmentation of T cell mitogenesis and expansion of type 1 effector/memory T cells

Herein we report mechanisms whereby Flt3 ligand (FL) augments steady state T cell activity in addition to the expansion of dendritic cells (DCs). We demonstrate that in vivo administration of FL increases the frequency and absolute number of effector/memory T cells and preferentially expands T cells that express a type-1 cytokine phenotype. In addition, FL enhances T cell proliferative responses to Concanavalin A that directly correlated with increased frequencies in effector/memory T cells and expansion of lymphoid-derived (type 1) DCs (DC1s). Together, these data demonstrate that mechanisms of FL-induced T cell regulation include not only the expansion of DC subsets, but also the preferential expansion of type 1 -effector/memory T cell populations, and suggest multiple mechanisms of action for FL as a vaccine adjuvant and as a therapeutic modality.

Flt3 ligand therapy for recipients of allogeneic bone marrow transplants expands host CD8 alpha(+) dendritic cells and reduces experimental acute graft-versus-host disease

Recent evidence suggests that dendritic cells (DCs) can regulate and amplify immune responses. Flt3 ligand (FL)-derived DC function was tested as a stimulator of allogeneic lymphocytes in vitro and in vivo. Treatment of mice with FL dramatically expanded DC number, but DCs isolated from FL-treated mice (FL DCs) were poor stimulators of allogeneic T-cell responses in vitro. Further activation of FL DCs did not restore their stimulatory ability, and FL DCs did not suppress the stimulation of the allogeneic T cells by normal DCs. FL treatment significantly increased the CD8 alpha(+) DC subset, which appeared to be the reason for their poor stimulatory capacity. These observations were confirmed in vivo using a mouse model of acute graft-versus-host disease (GVHD) wherein host DCs play a critical role. FL treatment of recipients before allogeneic bone marrow transplantation dramatically suppressed donor T-cell responses to host antigens, thereby reducing GVHD mortality (P <.01). These data represent a novel strategy that alters host DCs and reduces acute GVHD.

Spontaneous acceptance of liver transplants in rodents: evidence that liver leucocytes induce recipient T-cell death by neglect

In many animal models transplanted livers are not rejected, even when there is a complete MHC mismatch between the donor and recipient and the recipient is not immunosuppressed. This distinguishes liver transplants from other organs, such as kidneys and hearts, which are rapidly rejected in mismatched individuals. Acceptance of transplanted livers in a rat model is not due to the absence of an immune response to the liver and there is a rapid, abortive response that is ultimately exhausted. Donor leucocytes transferred with the liver appear to be responsible for both liver acceptance and the abortive activation of the recipient's T cells. The immune mechanism of liver transplant acceptance appears to be due to 'death by neglect' in which T cells are activated to express IL-2 and IFN-gamma mRNA in the recipient lymphoid tissues, but not at adequate levels within the graft. Subsequently the activated T cells die leading to specific clonal deletion of liver donor-reactive T cells. These findings have important implications for liver transplant patients as immunosuppressive drugs that are given to prevent rejection can also interfere with this form of tolerance. In addition, it might be possible to modify the immunosuppressive drug treatment of transplant patients to promote the process of death by neglect of recipient alloreactive T cells.

Liver-derived dendritic cells induce donor-specific hyporesponsiveness: use of sponge implant as a cell transplant model

Spontaneously accepted mouse liver allografts are capable of protecting subsequently transplanted donor organs from rejection; however, the underlying mechanisms are unclear. Dendritic cells (DC) residing in liver grafts are likely important in tolerance induction. DC propagated from mouse liver with GM-CSF are phenotypically and functionally immature. They are poor allostimulators in MLR and prolong survival of pancreatic islet allografts. It has been problematic to perform mechanistic studies in an islet transplant model because of difficulties in obtaining sufficient graft infiltrating cells. In this study, we used a sponge allograft model [i.e.. a subcutaneously implanted sponge matrix loaded with B10 (H2b) spleen cells]. To investigate the influence of administration of donor (B10) liver-derived DC on alloimmune reactivity of C3H (H2k) hosts, sponge graft infiltrating cells (SGIC) and recipient spleen cells were isolated, and their immunophenotype and donor-specific cytotoxic T lymphocyte (CTL) activity were examined. The results illustrate that donor-specific CTL activity of T cells are lower in recipients that had received systemic treatment with liver-derived immature DC, associated with a decrease in CD8+ cell population and an increase in Gr-1+ cells in SGIC, compared with recipients treated with mature bone marrow (BM)-derived DC. Interestingly, administration of liver DC directly into the sponge did not inhibit T cell responses. These data suggest that systemic administration of donor liver DC induces donor-specific hyporesponsiveness, probably not by direct inhibition of graft infiltrating T cells. The increased Gr-1+ cells may play immune regulatory roles in induction of host donor-specific hyporesponsiveness.

Flt3 ligand: a novel cytokine prevents allergic asthma in a mouse model

Flt-3 ligand (FL), a recently described growth factor affecting early hematopoietic progenitor cells, can also support the expansion of dendritic cells secreting IL-12. Since type 2 T cells predominate in asthma and IL-12 prevents the differentiation of naive T lymphocytes to a type 2 phenotype, we hypothesized that FL could prevent the development of asthma-like conditions in the ovalbumin mouse model. We found that co-administration of FL during ovalbumin sensitization abrogated late allergic responses, but had no effect on early allergic responses. Airway hyperresponsiveness to methacholine was also blocked by FL treatment. Analysis of bronchoalveolar lavage (BAL) fluid demonstrated a significant reduction in eosinophils, with concomitant decreases in IL-5 and increases in IFN-gamma levels. However, there was no change in BAL fluid IL-4 and serum IgE levels. These data suggest that FL treatment prevents ovalbumin-induced asthma in the mouse and may provide a useful adjuvant in the treatment of human asthma.

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.

Investigation of human spleen dendritic cell phenotype and distribution reveals evidence of in vivo activation in a subset of organ donors

Although the mouse spleen dendritic cell (DC) is perhaps the most intensively studied DC type, little has been published concerning its human equivalent. In this report, rare event flow cytometry and in situ immunofluorescence were used to study the surface phenotype and distribution of HLA-DR(+) CD3(-)14(-)16(-)19(-) human spleen DC. Spleens from organ donors with different clinical histories were used. Most (81% +/- 9%; n = 14) spleen DCs expressed high levels of the integrin CD11c. CD11c(+) DCs were distributed in 3 distinct regions-the peri-arteriolar T-cell zones, the B-cell zones, and the marginal zone, where they formed a ring of cells surrounding the white pulp, just inside a ring of CD14(+) red pulp macrophages, apparently more regularly organized than the previously described marginating DC population in the mouse spleen. The T-cell zones contained CD86(+) DCs, among which a subpopulation expressed CD83. These mature/activated CD86(+) DCs represented a minority (12% +/- 8%) of total spleen DCs in most organ donors: most spleen DCs are immature. In 3 of 18 (17%) donors, however, most (54%-81%) of spleen DCs were CD86(+), suggesting that in vivo DC activation had occurred. In one donor, a radical shift in DC distribution from the marginal zone to the T-cell zones was also observed. This activation of spleen DCs in vivo was reminiscent of the effects of experimental microbial product injection in mice, and it seemed to correlate with bacterial infection or multiple trauma. (Blood. 2001;97:3470-3477)

Il-12 antagonism enhances apoptotic death of T cells within hepatic allografts from Flt3 ligand-treated donors and promotes graft acceptance

Mouse livers are accepted across MHC barriers and induce donor-specific tolerance without immunosuppressive therapy. By contrast, livers from donors treated with Flt3 ligand, which dramatically increases hepatic interstitial dendritic cells, 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 cells. We hypothesized that IL-12 production by enhanced numbers of donor APC might inhibit apoptosis, promote expansion of Th1 cells, and play a key role in liver rejection. Therefore, C3H (H2(k)) recipients of liver grafts from Flt3 ligand-treated B10 donors were given neutralizing anti-IL-12 mAb (200 or 500 microg) on days 0 and 2 after transplant. Graft survival was markedly prolonged at the higher mAb dose, with 50% of grafts surviving >100 days. This effect was associated with reductions in IFN-gamma gene transcripts within the graft-infiltrating cell population and with reductions in circulating IFN-gamma and IL-10 levels, donor-specific CTL and NK cell activities, and circulating alloantibody levels. At the same time, there were marked increases in apoptotic (TUNEL(+)) CD4(+) and especially CD8(+) cells, both within the grafts and in spleens of anti-IL-12 mAb-treated mice. In vitro, exogenous IL-12 inhibited apoptotic death induced in naive allogeneic T cells by liver nonparenchymal cells. These findings suggest that suppression of rejection by IL-12 antagonism, linked to restoration of apoptotic activity within the peripheral alloreactive T cell population, is important for liver allograft survival and tolerance induction.

Immune activation is required for the induction of liver allograft tolerance: implications for immunosuppressive therapy

Liver transplants in many animal models are unusual because often they are not rejected even when transplanted across complete major histocompatibility complex barriers without immunosuppression. Their paradoxical behavior is even more obvious when the immune mechanism of acceptance is examined. Instead of acceptance resulting from a lack of immune response to the graft, the opposite occurs, and there is an unusual extensive increase in immune activation in acceptance compared with rejection. This abnormal extensive immune activation is driven by donor leukocytes transferred with the liver and results in death of the recipient cells that would normally reject the transplant. Some forms of immunosuppression inhibit this activation-associated liver transplant tolerance. The significance of these findings and possible means to design future treatment protocols for clinical transplantation that optimize management of liver transplant recipients are discussed.

Preferential induction of Th1 responses by functionally mature hepatic (CD8alpha- and CD8alpha+) dendritic cells: association with conversion from liver transplant tolerance to acute rejection

BACKGROUND

Liver grafts are accepted across major histocompatibility barriers in mice without immunosuppressive therapy. Potentially tolerogenic immature donor dendritic cells (DC) may play a key role in this phenomenon, but recovery of purified DC from normal livers for functional analysis is inherently difficult. Administration of in vitro propagated immature donor DC to recipients of different types of allograft can prolong transplant survival. By contrast, marked increases in donor liver DC as the result of Flt3 ligand (FL) administration and the resulting augmentation of allostimulatory activity within host lymphoid tissue, is associated with acute graft rejection. Here, we compared the capacity of in vitro generated normal liver immature DC and FL-treated donor liver DC to induce alloimmune CD4+ T helper (Th) 1/Th2 and CD8+ T cytotoxic (Tc) 1/Tc2 responses, in vitro and in vivo.

METHODS

B10 (H2b, IAb) immature liver DC were propagated from normal hepatic nonparenchymal cells in granulocyte macrophage-colony stimulating factor (GM-CSF) for 6-8 days. Freshly isolated DC from livers of FL-treated mice (FL-liver DC) were cultured overnight (o/n) in GM-CSF, and both myeloid (CD11c+ CD8alpha-) and lymphoid DC (CD11c+ CD8alpha+) flow-sorted for functional analysis. Proliferative activity and production of interferon (IFN)-gamma, interleukin (IL)-4, and IL-10 by naive C3H (H2k, IEk) T cells in response to DC stimulation was assessed by [3H]thymidine incorporation, and by multicolor flow cytometric analysis, respectively, after 3-day mixed leukocyte reactions. To investigate their in vivo trafficking, B10 DC were injected subcutaneously into normal C3H mice. Sections of lymphoid tissue were immunostained for donor MHC class II+ (IAb+) cells, and for IFN-gamma, IL-4, and IL-10 production. Donor cells and clusters of specific cytokine-secreting cells were enumerated.

RESULTS

Both in vitro propagated normal liver-derived DC, and freshly isolated bulk FL-liver DC showed an immature phenotype (MHC class II(lo), CD40-, CD80-, and CD86-) and were weak stimulators of naive allogeneic T cells. After o/n incubation in GM-CSF, both CD8alpha- and CD8alpha+ FL-liver DC exhibited marked up-regulation of surface MHC class II and costimulatory molecules, and acquired potent stimulatory activity for Th1 (mainly) and Th2 cells. Both in vitro propagated immature DC and o/n-cultured mature FL-liver DC homed in vivo to host lymphoid tissues, but with different kinetics. Whereas the mature allogeneic FL-liver DC induced IFN-gamma+ clusters in splenic T-cell areas within 2 days, the IFN-gamma response to immature DC was much slower and weaker.

CONCLUSIONS

FL-treated donor livers that are rejected acutely contain markedly enhanced numbers of myeloid (CD8alpha-) and lymphoid (CD8alpha+) DC, many of which are capable of maturing rapidly into strong inducers of Th1 and Tcl responses. Substantial differences in quantity, and both the phenotypic and functional characteristics of the DC constituency of donor livers, may contribute significantly toward the distinct outcomes of liver transplant tolerance and rejection.

Flt3 ligand (FL) and its influence on immune reactivity

Flt3 (fms-like tyrosine kinase 3) ligand (FL) is a potent hematopoietic cytokine that affects the growth and differentiation of progenitor and stem cells both in vivo and in vitro. Its capacity to augment strikingly the numbers of dendritic cells (rare antigen-presenting cells that induce and regulate immune responses) in mice and humans has stimulated considerable interest in its value as an investigational tool and therapeutic agent. In this review, we survey the hematopoietic properties and immunobiology of FL, and examine its therapeutic potential.

Trafficking of APC from liver allografts of Flt3L-treated donors: augmentation of potent allostimulatory cells in recipient lymphoid tissue is associated with a switch from tolerance to rejection

Livers transplanted across major histocompatibility complex (MHC) barriers in mice are normally accepted without recipient immune suppression, and induce a state of functional tolerance. However, markedly increasing functional dendritic cells (DC) in the 'passenger leucocyte' population by donor pretreatment with the hematopoietic growth factor Flt3-ligand (Flt3L; 10 microg/day for 10 days) results in acute allograft rejection. In this study, molecular, immunohistochemical and flow cytometric analysis of donor cell traffick into recipient lymphoid tissue 24 h after liver transplantation (C57BL/10 [H2b]-->C3H [H2k]) was performed. In addition, the capacity of donor-derived cells in these tissues to stimulate host T cell proliferation was examined. Reverse transcriptase polymerase chain reaction analysis revealed increases in donor genomic DNA in both thymi and spleens of mice given livers from Flt3L-treated donors compared to controls. Donor MHC class II+ (IAb+) cells in spleens were strikingly elevated (10-fold) in the former group. Two-colour flow cytometry revealed a similar increase in donor-derived H-2Kb+/I-Ab+ cells, and in the incidence of donor leucocytes expressing CD40, CD80, and CD86. CD11c+ DC comprised approximately 40% of the I-Ab+ cells in spleens of mice given livers from Flt3L-treated donors. These changes were associated with the presence, in spleens, of potent allostimulatory activity for naive recipient strain T cells, that was not observed in normal liver recipients. Elicitation of allograft rejection, associated with enhanced trafficking of stimulatory donor antigen-presenting cells (APC), in particular DC, suggests that normal liver graft survival and tolerance induction may be linked to failure/counter-regulation of APC-driven stimulation of effective anti-donor T cell responses.