Dendritic cells and prospects for transplantation tolerance

The Role of Kynurenines Produced by Indolamine-2,3-Dioxygenase 1 in Sepsis

BACKGROUND

The enzyme indolamine-2,3-dioxygenase 1 (IDO1) is the rate-limiting enzyme of the kynurenine (KYN) pathway and metabolizes the essential amino acid tryptophan to KYNs. The depletion of tryptophan and the generation of KYNs were shown to be involved in the global downregulation of the immune system during the later stages of sepsis, also referred to as sepsis-associated immunosuppression.

SUMMARY

The generation of KYNs by IDO1 leads to a depletion of effector T cells, including increased rate of apoptosis, decreased ability of T-cell proliferation and activation, and the generation of FoxP3+ regulatory T cells. Furthermore, KYN was shown a potent vasorelaxant during inflammation-induced hypotension. Experimental studies in murine sepsis models and in humans show promising data for using the activation of IDO1 both as a prognostic marker and potential drug target in sepsis.

Two polysaccharides from Porphyra modulate immune homeostasis by NF-κB-dependent immunocyte differentiation

Porphyra polysaccharides possess multiple pharmacological activities, such as immunoregulatory, anti-tumor and anti-inflammatory effects, but the specific underlying mechanisms are not fully understood.

Prolongation of kidney allograft survival regulated by indoleamine 2, 3-dioxygenase in immature dendritic cells generated from recipient type bone marrow progenitors

Immature dendritic cells (iDCs) are bone marrow-derived professional antigen-presenting cells, exhibit very low levels of the co-stimulatory molecules CD80 (B7-1), CD86 (B7-2), and CD40 and major histocompatibility complex (MHC) class II and play a critical role in triggering antigen-specific immunotolerance. The enzyme indoleamine 2, 3-dioxygenase (IDO) is a cytosolic tryptophan catabolism rate-limiting step enzyme. IDO secreted by DCs shows an association with the suppression of T-cell responses and promotion of tolerance. In this study, BN rat recipients were pre-injected with donor renal alloantigen-treated recipient iDCs before kidney transplantation. The renal allograft exhibited a lighter renal rejection response, prolonged graft survival time, and an increasing content of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Additionally, up-regulated secretion of Th2 cytokines were found in recipient sera post-transplantation. Transfection of si-IDO1 RNA into renal-antigen-treated recipient iDCs reversed these changes, which suggested that IDO channel signaling may be involved in iDC-induced allograft immunotolerance. These results suggested that iDC-induced and IDO-mediated allograft immunotolerance might be a potentially feasible tactic to prolong allograft survival, in addition to immunosuppressive drugs.

Immune tolerance of skin allograft transplantation induced by immature dendritic cells of a third party carrying donor antigens in mice

BACKGROUND

Dendritic cells (DCs) are the most powerful antigen-presenting cells in the body. Immature DCs (imDCs) can induce transplantation tolerance. In this study, using a mouse model of skin transplantation. We explored the antigen uptake by imDCs, changes in phenotype and function after antigen loading, as well as survival of skin grafts.

METHODS

Mononuclear cells from C57BL/6 mice mixed with a tritiated leucine ([(3)H]Leu) antigen supernate were incubated with Kunming mice imDC and mature DCs. We recorded the expressions of surface molecules that were detected using flow cytometry, mixed lymphocyte reactions, mean survival times, and postoperative morphological changes in skin grafts.

RESULTS

After the addition of allogeneic antigen supernate, the counts per minute of imDCs were significantly higher than those of mature DCs. The expression rates of I(A)/I(E) and CD80 significantly increased on the cell surface of imDCs. The counts per minute of imDCs in mixed lymphocyte reactions in the presence of allogeneic antigens was significantly higher than those of controls. Comparing mean survival times with controls, skin grafts were significantly longer in the imDCs groups from donors or from a third party carrying donor antigens.

CONCLUSIONS

ImDCs display a strong antigen uptake, gradually maturing in terms of phenotype and function after loading. Complementary application of cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin blocks the immune response of imDCs. Both imDCs from the third party carrying donor antigens and those from the donor strain can establish antigen-specific immune tolerance to allogeneic skin grafts.

Hepatic stellate cells promote immunotolerance following orthotopic liver transplantation in rats via induction of T cell apoptosis and regulation of Th2/Th3-like cell cytokine production

Hepatic stellate cells (HSCs) have been demonstrated to have immunoinhibitory activity. The aim of this study was to investigate the role of HSCs in the development of immunotolerance following liver transplantation. A rat liver transplantation tolerance model [donor Lewis into recipient Dark Agouti (DA)] and rejection (donor DA into recipient Lewis) was established. On the 7th day following transplantation, the HSCs and T cells were isolated from the rats of either the tolerance or rejection group and cultured together. The apoptosis rate of the T cells was determined 24 h later by flow cytometry following staining with anti-CD3 mAb and Annexin V-FITC/PI. Additionally, the FasL expression of HSCs was determined by flow cytometry following staining with anti-FasL mAb. The protein levels of IL-2, TNF-α, TGF-β and IL-10 in the supernatant collected from mixed lymphocyte reaction cultures of HSCs and T cells for 5 days were measured using ELISA assays. HSCs isolated from the tolerance group had a higher T-cell apoptosis induction activity compared with those of the rejection group. The activity of the HSCs was partially reversed by FasL blocking mAb. Accordingly, the FasL expression level of HSCs in the tolerance group was revealed to be higher than that of the rejection group. Moreover, HSCs stimulated IL-10 and TGF-β production in the tolerance group. This study suggests that HSCs are involved in liver transplantation immune tolerance via the induction of T-cell apoptosis partially mediated by the Fas/FasL pathway and the activation of Th2/Th3-like cell cytokine production.

The Combination of Immunosuppressive Drugs with 8-Methoxypsoralen and Ultraviolet a Light Modulates the Myeloid-Derived Dendritic Cell Function

The functional properties of myeloid dendritic cells (DCs) differ, depending on microenvironmental factors as well as on their stage of maturation. The main approaches for the selective enhancement of the tolerogenic properties of DCs include the induction of a pharmacological arrest of the DCs maturation and the genetical engineering of DCs expressing immunosuppressive molecules. Several immunosuppressive/anti-inflammatory agents have been discovered that potentially inhibit DC maturation and immunogenicity. Photopheresis (ECP) is an immunomodulatory therapy in which leucocytes are exposed to 8-methoxypsoralen (8-MOP) and ultraviolet (UV) A radiation (PUVA). The combination of ECP with immunosuppressive agents has demonstrated efficacy in the management of transplanted patients by reducing either the incidence of organ rejection or the pharmacological toxicity. In particular, we have observed in hepatitis C virus (HCV)-positive patients that the same combination has reduced the immunosuppressive burden and improved sustainability and efficacy of pre-emptive antiviral therapy after liver transplantation. Therefore, in our work we investigated the in vitro effects of PUVA, combined with immunosuppressive drugs (IDs), on both in vitro human DC generation and maturation, in order to contribute to understanding the immunological mechanisms underlying this pharmacological combination. Monocyte PUVA-treatment was performed by using an in vitro experimental protocol that we previously described. PUVA-treated or -untreated highly purified CD14+ cells were incubated with the association of the immunosuppressive drugs, used in the management of liver transplantation, at two different concentrations, in the presence of IL-4 and GM-CSF. The treatment with IDs at the highest concentration (corresponding to that used in clinical practice), alone or in association with PUVA, induced an immunosuppressive effect, by impairing both DC generation and maturation. Neither immunosuppressive drugs at the lowest concentration nor their combination with PUVA affected myeloid DC generation, but modified DC functions, strengthening the induction of a tolerogenic pattern. As this ID concentration was arbitrarily chosen, further experiments could highlight whether lower concentrations than those used in clinical practice would elicit the same effect on DCs and potentially improve their functional properties. This work describes an original experimental approach exploring the in vitro mechanism of action of the combined procedure of PUVA with immunosuppressive drugs, used in liver transplantation, on DCs generation and function. Our results contribute to the knowledge of the mechanisms of action of this combined procedure on DCs, suggesting useful therapeutic implications for the in vivo therapy.

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

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

New strategies in immune tolerance induction

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

Human embryonic stem cell (hES) derived dendritic cells are functionally normal and are susceptible to HIV-1 infection

Background

Human embryonic stem (hES) cells hold considerable promise for cell replacement and gene therapies. Their remarkable properties of pluripotency, self-renewal, and tractability for genetic modification potentially allows for the production of sizeable quantities of therapeutic cells of the hematopoietic lineage. Dendritic cells (DC) arise from CD34+ hematopoietic progenitor cells (HPCs) and are important in many innate and adaptive immune functions. With respect to HIV-1 infection, DCs play an important role in the efficient capture and transfer of the virus to susceptible cells. With an aim of generating DCs from a renewable source for HIV-1 studies, here we evaluated the capacity of hES cell derived CD34+ cells to give rise to DCs which can support HIV-1 infection.

Results

Undifferentiated hES cells were cultured on S17 mouse bone marrow stromal cell layers to derive CD34+ HPCs which were subsequently grown in specific cytokine differentiation media to promote the development of DCs. The hES derived DCs (hES-DC) were subjected to phenotypic and functional analyses and compared with DCs derived from fetal liver CD34+ HPC (FL-DC). The mature hES-DCs displayed typical DC morphology consisting of veiled stellate cells. The hES-DCs also displayed characteristic phenotypic surface markers CD1a, HLA-DR, B7.1, B7.2, and DC-SIGN. The hES-DCs were found to be capable of antigen uptake and stimulating naïve allogeneic CD4+ T cells in a mixed leukocyte reaction assay. Furthermore, the hES-DCs supported productive HIV-1 viral infection akin to standard DCs.

Conclusion

Phenotypically normal and functionally competent DCs that support HIV-1 infection can be derived from hES cells. hES-DCs can now be exploited in applied immunology and HIV-1 infection studies. Using gene therapy approaches, it is now possible to generate HIV-1 resistant DCs from anti-HIV gene transduced hES-CD34+ hematopoietic progenitor cells.

Immune modulation and tolerance induction by RelB-silenced dendritic cells through RNA interference

Dendritic cells (DC), the most potent APCs, can initiate the immune response or help induce immune tolerance, depending upon their level of maturation. DC maturation is associated with activation of the NF-kappaB pathway, and the primary NF-kappaB protein involved in DC maturation is RelB, which coordinates RelA/p50-mediated DC differentiation. In this study, we show that silencing RelB using small interfering RNA results in arrest of DC maturation with reduced expression of the MHC class II, CD80, and CD86. Functionally, RelB-silenced DC inhibited MLR, and inhibitory effects on alloreactive immune responses were in an Ag-specific fashion. RelB-silenced DC also displayed strong in vivo immune regulation. An inhibited Ag-specific response was seen after immunization with keyhole limpet hemocyanin-pulsed and RelB-silenced DC, due to the expansion of T regulatory cells. Administration of donor-derived RelB-silenced DC significantly prevented allograft rejection in murine heart transplantation. This study demonstrates for the first time that transplant tolerance can be induced by means of RNA interference using in vitro-generated tolerogenic DC.

Allosuppressive Donor CD4+CD25+ Regulatory T Cells Detach from the Graft and Circulate in Recipients after Liver Transplantation

Organ transplantation (Tx) results in a transfer of donor leukocytes from the graft to the recipient, which can lead to chimerism and may promote tolerance. It remains unclear whether this tolerance involves donor-derived regulatory T cells (Tregs). In this study, we examined the presence and allosuppressive activity of CD4+CD25+Foxp3+ Tregs in perfusates of human liver grafts and monitored the cells presence in the circulation of recipients after liver Tx. Vascular perfusions of 22 liver grafts were performed with University of Wisconsin preservation and albumin solutions. Flow cytometric analysis revealed that perfusate T cells had high LFA-1 integrin expression and had a reversed CD4 to CD8 ratio compared with control blood of healthy individuals. These findings indicate that perfusate cells are of liver origin and not derived from residual donor blood. Further characterization of perfusate mononuclear cells showed an increased proportion of CD4+CD25+CTLA4+ T cells compared with healthy control blood. Increased percentages of Foxp3+ cells, which were negative for CD127, confirmed the enrichment of Tregs in perfusates. In MLR, CD4+CD25+ T cells from perfusates suppressed proliferation and IFN-gamma production of donor and recipient T cells. In vivo within the first weeks after Tx, up to 5% of CD4+CD25+CTLA4+ T cells in recipient blood were derived from the donor liver. In conclusion, a substantial number of donor Tregs detach from the liver graft during perfusion and continue to migrate into the recipient after Tx. These donor Tregs suppress the direct pathway alloresponses and may in vivo contribute to chimerism-associated tolerance early after liver Tx.

Genetic modification of dendritic cells through the directed differentiation of embryonic stem cells

Recent years have witnessed a progressive acceptance of the dual role played by dendritic cells (DC) in the initiation of immune responses and their specific attenuation through the induction of immunological tolerance. Nevertheless, as terminally differentiated cells of the myeloid lineage, DC share with macrophages an inherent resistance to genetic modification, greatly restricting strategies available for studying their physiology and function. Consequently, little is known of the molecular interactions provided by DC that underlie the critical decision between tolerance and immunity. Embryonic stem (ES) cells are, by contrast, relatively amenable to genetic modification. Furthermore, their propensity for self-renewal, one of the cardinal features of a stem cell, permits cloning at the single cell level and the rational design of ES cell lines, uniformly expressing a desired, mutant phenotype. Here, we describe how another defining property of ES cells, their demonstrable pluripotency, may be harnessed for their directed differentiation along the DC pathway, enabling the generation of limitless numbers of DC faithfully expressing candidate genes of interest. The protocols we outline in this chapter may, therefore, offer new opportunities for dissecting the biology of DC and the molecular basis of their unique properties.

Dopaminergic neuroprotection by neurturin-expressing c17.2 neural stem cells in a rat model of Parkinson's disease

Genetically engineered neural stem cell (NSC) lines are promising vectors for the treatment of regenerative diseases, especially Parkinson's disease (PD). Neurturin (NTN), a member of the glial cell line-derived neurotrophic factor-family, has been demonstrated to act specifically on mesencephalic dopaminergic neurons, suggesting its therapeutic potential for PD. Here, we have generated a NTN-secreting c17.2 NSC line and investigated the protective effect of NTN-c17.2 on PD rat models. These NTN-releasing NSCs engrafted and integrated in the host striatum with good success, gave rise to neurons, astrocytes and oligodendrocytes, and maintained stable, high-level NTN expression. In addition, inverse transfer of NTN protein into the substantia nigra (SN) was able to protect dopaminergic neurons from 6-OHDA toxicity. Observation of rotational behavior showed that the NTN group performed significantly better than the Mock group, and the protective effect of NTN lasted for at least 4 months. HPLC tests indicated that the contents of neurotransmitters (e.g. dopamine) in the corpus striatum area of the NTN-c17.2 group and the Mock-c17.2 group were significantly higher than in the PBS group, but there was no significant difference between expression in the NTN-c17.2 and Mock-c17.2 groups. Taken together, our results suggest that transplantation of NTN-secreting NSCs exerted protective on PD rat models.

Induction of regulatory T cells from mature T cells by allogeneic thymic epithelial cells in vitro

The ability of thymic epithelial cells (TEC) to re-educate mature T cells to be regulatory T cells has not been addressed. In the present study, this issue was directly investigated by co-culturing of mature T cells and allo-TECs. B6 macrophage cell line 1C21-cultured BALB/c splenocytes responded to B6 antigens in vitro. However, BALB/c splenocytes precultured with B6-derived TECs 1-4C18 or 1C6 did not proliferate to B6 antigens, but responded to rat antigens. Exogenous interleukin-2 (IL-2) failed to revise the unresponsiveness of these T cells. Allo-TEC-cultured T cells predominantly expressed Th2 cytokines (IL-4 and IL-10). B6 TEC-cultured BALB/c splenocytes markedly inhibited the immune responses of naïve BALB/c splenocytes to B6 antigens, but not to rat or the third-party mouse antigens. BALB/c nude mice that received naïve syngeneic splenocytes rejected B6 or rat skin grafts by 17 days postskin grafting; however, co-injection of B6 TEC-cultured BALB/c splenocytes significantly delayed B6 skin graft rejection (P < 0.01), with the unchanged rejection of rat skin grafts. These studies demonstrate that allo-TECs are able to 'educate' mature T cells to be regulatory cells, and suggest that regulatory cells derived from mature T cells by TECs may play an important role in T cell tolerance to allo- and auto-antigens.

Immunomodulatory strategies for celiac disease

Celiac disease (CD) is the most common food-sensitive enteropathy in humans and is caused by the lack of immune tolerance (oral tolerance) to gluten. The identification of gluten-specific T cells in the lamina propria of celiacs and the strong association with HLA-DQ2 and -DQ8 genes support a central role of CD4(+) T cells in CD pathogenesis. Studies focused on the modulation of autoimmunity in different experimental models highlighted possible immune therapeutic protocols useful also for the management of CD. On the basis of these observations, a series of strategies have been designed: some of them are based on the identification of immunogenic epitopes and their suppression via enzymatic treatment or by using peptide analogues; others rely on the delivery of unmodified antigen through the nasal route or coadministered with downregulatory cytokines. studies are generally early stage but encouraging in paving a way for an alternative treatment for celiac disease.

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.

Recipient immature dendritic cells do not prolong allograft survival

Experimental studies on allogeneic transplantation have shown that recipient dendritic cells (DC) play a role in peripheral tolerance as well as in rejection of allografts. It is not known whether DCs exert their tolerogenic function in the graft or in recipient lymphoid tissue. To answer this question we created a chimeric heart model deprived of its own DCs and repopulated with recipient DCs. The rationale for this model was to observe whether recipient mature and immature DCs located in the graft attenuate recruitment and stimulation of recipient lymphocytes, subsequently prolonging graft survival. Vascularized bone marrow transplants from the prospective recipient to the lethally irradiated heart donor, which function for a period of 14 days, were used to replace donor DCs with prospective recipient either mature or immature DCs. Replacement of the donor heart with either of these cells did not prolong graft survival. The intragraft microchimerism did not mitigate the allogeneic rejection reaction.

New trends in immunosuppression

Immunosuppressive agents are the mainstay treatment for patients that have received organ grafts and are becoming increasingly important in the treatment of autoimmune diseases. There are, however, many problems with both the concept and reality of long-term immunosuppression as a therapeutic modality, both in terms of the nonspecific toxicity of the drugs that are currently available and the increased risk of infections and tumours arising from global suppression of the immune system. This special issue of International Immunopharmacology includes papers submitted at the 6th International Conference on New Trends in Immunosuppression that was held in Salzburg during February 2004 that show some of the recent advances, particularly in the field of transplantation tolerance, and demonstrate the complexity of issues limiting our application of experimental developments into effective clinical strategies.

Stem cell transplantation as a therapeutic approach to organ failure

BACKGROUND

Stem cell transplantation is one of the next great frontiers for surgery. Stem cells, which are undifferentiated and self-renewing, have shown the ability to differentiate into cardiomyocytes, as well as many other cell types for potential therapeutic use by surgeons.

MATERIALS AND METHODS

As a result, stem cells have the potential to undo irreversible cellular damage, something traditional therapies could not cure. However, numerous issues must be resolved to permit safe and effective clinical application of stem cell therapy. These include the interpretation of cellular labeling, the origin of replicating myocytes, the homing mechanism of stem cells, and the differentiation process.

RESULTS

Successful translational research will depend on precise delivery of these cells in real time to the area of interest, e.g., the spinal cord, liver, or heart. Surgeons will be better able to excise and replace/regrow, rather than excise alone. As such, a basic understanding of stem cell biology will benefit the surgeon scientist and clinical surgeon.

CONCLUSIONS

The review: 1) discusses myocardial regeneration; 2) defines and categorizes stem cells; 3) presents evidence of stem cell transdifferentiation into cardiomyocytes; and, 4) delineates the therapeutic potential of stem cells in the treatment of ischemic heart disease.

Generation of antigen-specific, Foxp3-expressing CD4+ regulatory T cells by inhibition of APC proteosome function

We tested the hypothesis that immature APC, whose NF-kappaB-signaling pathway and thus maturation was blocked by the proteosome inhibitor benzyloxycarbonyl-isoleucyl-glutamyl(O-tert-butyl)-alanyl-leucinal (PSI), could be a source of Ag-specific regulatory T (Treg) cells. DO11.10 CD4(+) T cells that were incubated with Ag- and PSI-pulsed APC proliferated poorly, produced less IL-2, IFN-gamma, and IL-10 in secondary cultures, and inhibited the response of both naive and memory CD4(+) T cells stimulated by Ag-pulsed APC. The generation of PSI-APC Treg cells required IL-10 production by APC. PSI-APC Treg cell inhibition required cell-cell contact but not IL-10 or TGF-beta. Addition of IL-2 did not reverse, but Ab to CTLA-4 did reverse partially the inhibitory effect. Depletion of CD25(+) T cells before initial culture with PSI-APC did not affect Treg generation. PSI-APC Treg cells expressed high levels of Foxp3, inhibited proliferation of naive DO11.10 T cells in vivo, and abrogated colitis driven by a memory Th1 response to bacterial-associated Ag. We conclude that NF-kappaB-blocked, immature APC are able to induce the differentiation of Treg cells that can function in vitro and in vivo in an Ag-specific manner.

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

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

Embryonic stem cells: a novel source of dendritic cells for clinical applications

As arbitrators of the immune response, dendritic cells (DC) are uniquely placed to negotiate the balance between the opposing forces of tolerance and immunity, making them attractive candidates for clinical applications. Accordingly, DC have been used successfully in the treatment of cancer, enhancing immune responses to tumour-associated antigens (TAA) in experimental animal models and phase I clinical trials. A novel source of DC that has recently been described is the embryonic stem (ES) cell whose differentiation in vitro may be directed along multiple lineage pathways. Such pluripotency offers unparalleled opportunities for the treatment of chronic and degenerative disease states by the replacement of affected tissues, a vision which has inspired the emerging field of regenerative medicine. By sharing the genotype of therapeutic cell types, such as cardiomyocytes and dopaminergic neurons derived from the same ES cell line, so-called esDC may offer prospects for reprogramming the immune system to tolerate the grafted tissues. Here, we describe how the unique properties of esDC and the ES cells from which they derive, make them eminently suited to clinical applications, overcoming many of the issues that currently limit the effectiveness of DC-based immune intervention.

Comparison of two different protocols for the induction of maturation of human dendritic cells in vitro

BACKGROUND

Dendritic cells (DC) have been used for immunotherapy of malignant tumors, different kinds of infections, and other clinical conditions. For that purpose, optimal conditions for the generation of functionally mature DC in vitro are required. Two different protocols for the induction of maturation of monocyte-derived DC (MDDC) were compared in this study.

METHODS

MDDC were generated in vitro by cultivating adherent monocytes of healthy volunteers with granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin 4 (IL-4) during 6-days period. The immature DC thus prepared were induced to mature using two protocols. DC were stimulated for 2 days with lipopolysaccharide (LPS), or with a cocktail of proinflammatory mediators (PM) containing IL-1beta, IL-6, tumor necrosis factor alpha (TNFalpha), and prostaglandin E2 (PGE2), respectively. Phenotypic characteristics of MDDC and their endocytic activity were studied by flow cytometry. Allostimulatory activity of these cells was tested in the mixed leukocyte reaction (MLR), whereas the production of cytokines was determined by ELISA kits.

RESULTS

MDDC matured with PM (PM-DC) were predominantly non-adherent cells, while about 30% of LPS-matured DC were adherent cells. In comparison with LPS-DC, PM-DC expressed higher levels of CD86 and CD83, had lower endocytic activity, produced higher levels of IL-10 and lower levels of IL-12, and more strongly stimulated proliferation of allogeneic lymphocytes.

CONCLUSION

The protocol based on the combination of proinflammatory cytokines and PGE2 is better for the induction of maturation of human MDDC in vitro than the protocol using LPS alone.

IDO expression by dendritic cells: tolerance and tryptophan catabolism

Indoleamine 2,3-dioxygenase (IDO) is an enzyme that degrades the essential amino acid tryptophan. The concept that cells expressing IDO can suppress T-cell responses and promote tolerance is a relatively new paradigm in immunology. Considerable evidence now supports this hypothesis, including studies of mammalian pregnancy, tumour resistance, chronic infections and autoimmune diseases. In this review, we summarize key recent developments and propose a unifying model for the role of IDO in tolerance induction.

Hyporesponsiveness in alloreactive T-cells by NF-kappaB inhibitor-treated dendritic cells: resistance to calcineurin inhibition

Dendritic cells (DC) are the most potent antigen-presenting cells (APC) initiating primary T-cell responses. Beyond this immunostimulatory function, certain DC subsets have been shown to induce T-cell tolerance in vitro and in vivo. In this study, immature monocyte-derived DC were activated in the presence of the NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC) and characterized with regard to phenotype, cytokine production and allostimulatory potential. Furthermore, the functional consequences of calcineurin inhibition were studied in T cells exposed to PDTC-modulated DC. We demonstrate that PDTC treatment of DC leads to an arrest in maturation as reflected by down-regulated major histocompatibility complex (MHC) antigens and costimulatory molecules, suppressed immunostimulatory cytokines and an impaired capability to support allogeneic T-cell activation. Allogeneic T cells challenged with PDTC-treated DC are refractory upon restimulation with alloantigens but not to polyclonal stimuli. Interestingly, the successful establishment of alloantigenic hyporesponsiveness is not prevented by concomitant calcineurin inhibition in vitro as well as in T cells from patients under cyclosporine A (CsA)-based immunosuppression ex vivo. These data may have important implications for the design of clinical regimens for the establishment of antidonor hyporeactivity in organ transplantation using in vitro-modulated DC.

Embryonic stem cells and the challenge of transplantation tolerance

Isolated from early blastocysts, embryonic stem (ES) cells capture the brief moment of pluripotency in the developing embryo, as evidenced by their differentiation into many somatic cell types in vitro. Although these properties might help meet the growing demand for 'spare parts' to replace diseased or worn-out tissues, their use in so-called cell replacement therapy (CRT) poses several challenges, not least of which is the prevention of their subsequent rejection. Here we explore the notion that ES cells might spawn cell types necessary for the treatment of disease while acting as a plentiful source of hematopoietic stem cells (HSCs) or terminally differentiated dendritic cells (DCs) that might facilitate the induction of transplantation tolerance to the replacement tissues.

Functional antigen-presenting leucocytes derived from human embryonic stem cells in vitro

BACKGROUND

Differentiated cells derived from pluripotent human embryonic stem (hES) cells offer the opportunity for new transplantation therapies. However, hES cells and their differentiated progeny express highly polymorphic MHC molecules that serve as major graft rejection antigens to the immune system of allogeneic hosts. To achieve sustained engraftment of donor cells, strategies must be developed to overcome graft rejection without broadly suppressing host immunity. One approach entails induction of donor-specific immune tolerance by establishing chimeric engraftment in hosts with haemopoietic cells derived from an existing hES cell line. We aimed to develop methods to efficiently differentiate hES cells to haemopoietic cells, including immune-modulating leucocytes, a prerequisite of the tolerance induction strategies applying to hES cell-mediated transplantation.

METHODS

We developed a method to generate a broad range of haemopoietic cells from hES-generated embryonic bodies in the absence of murine stromal feeder cells. Embryonic bodies were further cultured in the presence of haemopoietic cytokines. In addition to flow cytometric analyses of haemopoietic cell markers, we analysed the hES cell-derived haemopoietic cells by colony-forming assays (for erythroid and myeloid progenitor cells), cytochemical staining, and mixed leucocyte reactions to determine the functional capacity of the generated antigen-presenting cells.

FINDINGS

12 independent experiments were done. When selected growth factors were added, leucocytes expressing CD45 were generated and released into culture media for 6-7 weeks. Under the condition used, both erythroid and myeloid progenitor cells were generated. About 25% of the generated leucocytes acquired MHC class II and costimulatory molecule expression. These hES-derived, MHC class II+ leucocytes resembled dendritic cells and macrophages, and they functioned as antigen-presenting cells capable of eliciting allogeneic CD4 and CD8 T-cell responses in culture.

INTERPRETATION

The hES cell-derived antigen-presenting cells could be used to regulate alloreactive T cells and induce immune tolerance for improvement of the transplant acceptance of hES-cell derivatives.

Generation of Anergic and Regulatory T Cells following Prolonged Exposure to a Harmless Antigen

Regulatory CD4(+) T cells are known to develop during the induction of donor-specific peripheral tolerance to transplanted tissues; it is proposed that such tolerance is a consequence of persistent, danger-free stimulation by Ag. To test this hypothesis, male RAG-1(-/-) mice were recolonized with small numbers of monospecific CD4(+) T cells specific for the male H-2E(k)-restricted Ag Dby. After 6 wk in the male environment, the monospecific CD4(+) T cells, having recolonized the host, had become anergic to stimulation in vitro and had acquired a regulatory capacity. CD4(+) T cells in these mice expressed higher levels of CTLA-4 and glucocorticoid-induced TNF-related receptor than naive CD4(+) T cells, but only 3% of the recolonizing cells were CD25(+) and did not express significant foxP3 mRNA. In vivo, these tolerant T cells could censor accumulation of, and IFN-gamma production by, naive T cells, with only a slight inhibition of proliferation. This suppressive effect was not reversed by the addition of fresh bone marrow-derived male dendritic cells. These results suggest that persistent exposure to Ag in conditions that fail to evoke proinflammatory stimuli leads to the development of T cells that are both anergic and regulatory.

Antibody-induced transplantation tolerance: the role of dominant regulation

A short-treatment with nondepleting antibodies, such as those targeting CD4 or CD154 (CD40 ligand), allows long-term graft survival without the need for continuous immunosuppression. This state of immune tolerance is maintained by regulatory CD4+ T cells present within both the lymphoid tissue and the tolerated graft. The nature of such regulatory T cells, their relationship to CD4+CD25+ T cells, and their mode of action have all been the subjects of much attention recently. Here, we review recent progress on understanding the nature, specificity, and mechanisms of action of T cells mediating dominant tolerance brought about by antibody therapy.

IL-10-Conditioned Dendritic Cells, Decommissioned for Recruitment of Adaptive Immunity, Elicit Innate Inflammatory Gene Products in Response to Danger Signals

Dendritic cells (DCs) are the professional APCs of the immune system, enabling T cells to perceive and respond appropriately to potentially dangerous microbes, while also being able to maintain T cell tolerance toward self. In part, such tolerance can be determined by IL-10 released from certain types of regulatory T cells. IL-10 has previously been shown to render DCs unable to activate T cells and it has been assumed that this process represents a general block in maturation. Using serial analysis of gene expression, we show that IL-10 pretreatment of murine bone marrow-derived DCs alone causes significant changes in gene expression. Furthermore, these cells retain the ability to respond to Toll-like receptor agonists, but in a manner skewed toward the selective induction of mediators known to enhance local inflammation and innate immunity, among which we highlight a novel CXCR2 ligand, DC inflammatory protein-1. These data suggest that, while the presence of a protolerogenic and purportedly anti-inflammatory agent such as IL-10 precludes DCs from acquiring their potential as initiators of adaptive immunity, their ability to act as initiators of innate immunity in response to Toll-like receptor signaling is enhanced.

Differentiation of human dendritic cells from monocytes in vitro using granulocyte-macrophage colony stimulating factor and low concentration of interleukin-4

Several laboratories have developed culture systems that allow the generation of large numbers of human dendritic cells (DC) from monocytes using granulocyte-macrophage colony stimulating factor (GM-CSF), and interleukin-4 (IL-4). In this work we provided evidence that GM-CSF (100 ng/ml) in combination with a low concentration of IL-4 (5 ng/ml) was efficient in the generation of immature, non-adherent, monocyte-derived DC as the same concentration of GM-CSF, and ten times higher concentration of IL-4 (50 ng/ml). This conclusion was based on the similar phenotype profile of DC, such as the expression of CD1a, CD80, CD86, and HLA-DR, down-regulation of CD14, and the absence of CD83, as well as on their similar allostimulatory activity for T cells. A higher number of cells remained adherent in cultures with lower concentrations of IL-4 than in cultures with higher concentrations of the cytokine. However, most of these adherent cells down-regulated CD14 and stimulated the proliferation of alloreactive T cells. In contrast, adherent cells cultivated with GM-CSF alone were predominantly macrophages, as judged by the expression of CD14 and the inefficiency to stimulate alloreactive T cells. DC generated in the presence of lower concentrations of IL-4 had higher proapoptotic potential for the Jurkat cell line than DC differentiated with higher concentrations of IL-4, suggesting their stronger cytotoxic, anti-tumor effect.

Regulatory T cells and dendritic cells in transplantation tolerance: molecular markers and mechanisms

Transplantation tolerance can be induced in adult rodents using monoclonal antibodies against coreceptor or costimulation molecules on the surface of T cells. There are currently two well-characterized populations of T cells, demonstrating regulatory capacity: the "natural" CD4+CD25+ T cells and the interleukin (IL)-10-producing Tr1 cells. Although both types of regulatory T cells can induce transplantation tolerance under appropriate conditions, it is not clear whether either one plays any role in drug-induced dominant tolerance, primarily due to a lack of clear-cut molecular or functional markers. Similarly, although dendritic cells (DCs) can be pharmacologically manipulated to promote tolerance, the phenotype of such populations remains poorly defined. We have used serial analysis of gene expression (SAGE) with 29 different T-cell and antigen-presenting cell libraries to identify gene-expression signatures associated with immune regulation. We found that independently derived, regulatory Tr1-like clones were highly concordant in their patterns of gene expression but were quite distinct from CD4+CD25+ regulatory T cells from the spleen. DCs that were treated with the tolerance-enhancing agents IL-10 or vitamin D3 expressed a gene signature reflecting a functional specification in common with the most immature DCs derived from embryonic stem cells.

Prevention of CD40-triggered dendritic cell maturation and induction of T-cell hyporeactivity by targeting of Janus kinase 3

Pharmacological targeting of Janus kinase 3 (JAK3) has been employed successfully to control allograft rejection and graft-vs.-host disease (GVHD). Recent evidence suggests that in addition to its involvement in common-gamma chain (cgamma) signaling of cytokine receptors, JAK3 is also engaged in the CD40 signaling pathway of peripheral blood monocytes. In this study, we assessed the consequences of JAK3 inhibition during CD40-induced maturation of myeloid dendritic cells (DCs), and tested the impact thereof on the induction of T-cell alloreactivity. Dendritic cells triggering through CD40 induced JAK3 activity, the expression of costimulatory molecules, production of IL-12, and potent allogeneic stimulatory capacity. In contrast, JAK3 inhibition with the rationally designed JAK3 inhibitor WHI-P-154 prevented these effects arresting the DCs at an immature level. Interestingly, DCs exposed to the JAK3-inhibitor during CD40-ligation induced a state of hyporeactivity in alloreactive T cells that was reversible upon exogenous IL-2 supplementation to secondary cultures. These results suggest that immunosuppressive therapies targeting the tyrosine kinase JAK3 may also affect the function of myeloid cells. This property of JAK3 inhibitors therefore represents a further level of interference, which together with the well-established suppression of cgamma signaling could be responsible for their clinical efficacy.

Mobilizing dendritic cells for tolerance by engagement of immune inhibitory receptors for HLA-G

The presence of dendritic cells (DC) in the maternal decidua has pointed to a biologic role of antigen-presenting cell in maternal-fetal interaction. The expression of immune inhibitory receptors on DC opens the intriguing possibility that these types of receptors are directly involved in maturation/activation of DC and modulate their function. We show that the triggering of the murine inhibitory receptor paired immunoglobulin-like receptor-B by cross-linking or by human leukocyte antigen (HLA)-G tetramer resulted in the modulation of DC function and prolongation of allogeneic graft survival. In addition, we found that the engagement of human inhibitory receptor ILT4 by its natural ligand, HLA-G, alters maturation of human DC. In this study, we examined the mechanisms for the modulation of antigen-presenting cells by HLA-G. These findings have established an important link between HLA-G and immune inhibitory receptor regulation in vivo and in vitro, thereby placing HLA-G in the inhibitory pathway.

Dominant transplantation tolerance

Long-term allograft survival in the absence of continuous immunosuppression can be induced following a short treatment of nondepleting antibodies, such as those that target CD4 or CD154 (CD40 ligand). It is now established that this may involve dominant tolerance mechanisms that are maintained by CD4+ regulatory T cells present within the lymphoid tissue and the tolerated graft. The phenotype of these cells, their relationship to CD4+CD25+ T cells, and the mechanism of action are still controversial.

Repopulation of donor heart by recipient bone marrow–derived dendritic cells prior to transplantation causes acute rejection by both the allogeneic and syngeneic recipient

Experimental studies on allogeneic transplantation have shown that recipient dendritic cells (DC) play a role in peripheral tolerance as well as in rejection of allografts. It is not known whether DC exert their tolerogenic function in recipient lymphoid tissue, and whether they process shed alloantigen in the graft itself. To answer this question we created a chimeric heart model deprived of its own DC and repopulated by recipient DC. The rationale for this model was to observe whether recipient DC located in the graft attenuate recruitment and stimulation of recipient lymphocytes, subsequently prolonging graft survival. Vascularized bone marrow transplants (VBMTx) from the prospective recipient to the lethally irradiated heart donor, which function for a period of 14 days, were used to replace donor DC with prospective recipient DC. Hearts from chimeric LEW rats (with BN DC) were transplanted to untreated BN rats. Also, hearts from chimeric LEW rats (with BN DC) were returned to untreated LEW rats. Replacement of the donor heart with recipient DC did not prolong graft survival. Rather, it initiated a rejection reaction that was already present in the donor. Recipient DC retained their immunogenic properties also when the graft was returned back to a donor strain animal.

Stable lines of genetically modified dendritic cells from mouse embryonic stem cells

BACKGROUND

The capacity to activate naïve T cells sets dendritic cells (DCs) apart from other antigen-presenting cells, making them attractive targets for immune intervention during deleterious immune responses. The inherent resistance of terminally differentiated DCs to conventional strategies for genetic modification has, however, greatly limited our understanding of the molecular mechanisms underlying their function.

METHODS AND RESULTS

We report the derivation of long-term cultures of untransformed DCs, uniformly expressing a defined mutant phenotype by the directed differentiation of cloned embryonic stem cells, stably transfected with a reporter gene. Introduction of the gene encoding enhanced green fluorescent protein into pluripotent stem cells demonstrated no observable impact on the phenotype, immunogenicity, or capacity for maturation of DCs differentiated from them.

CONCLUSIONS

The production of unlimited numbers of mutant DCs from genetically modified embryonic stem cells paves the way for the systematic elucidation of gene function in this cell type and the rational design of DCs for use in immunotherapy.

[Allogeneic adoptive immunotherapy]

Recognition of the importance of immune cells present in a hematopoietic graft has resulted in a significant change in the perception of allogeneic hematopoietic transplantation. Such a transplant modality is now perceived has a very efficient form of adoptive allogeneic immunotherapy unfortunately associated with significant toxicity.

Functional modulation of dendritic cells to suppress adaptive immune responses

In recent years, dendritic cells (DCs) have entered the center court of immune regulation. Dependent on their ontogeny, state of differentiation, and maturation and thereby a variable expression of membrane-bound and soluble molecules, DCs can induce immunostimulatory as well as immunoregulatory responses. This dual function has made them potential targets in vaccine development in cancer and infections as well as for the prevention and treatment of allograft rejection and autoimmune diseases. The present review is focused on the effect of immune-modulatory factors, such as cytokines and immunosuppressive drugs, and on the survival, differentiation, migration, and maturation of DC human subsets. A better understanding of DC immunobiology may lead to the development of specific therapies to prevent or dampen immune responses.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Tryptophan deprivation sensitizes activated T cells to apoptosis prior to cell division

Cells expressing indoleamine 2,3-dioxygenase (IDO), an enzyme which catabolizes tryptophan, prevent T-cell proliferation in vitro, suppress maternal antifetal immunity during pregnancy and inhibit T-cell-mediated responses to tumour-associated antigens. To examine the mechanistic basis of these phenomena we activated naïve murine T cells in chemically defined tryptophan-free media. Under these conditions T cells expressed CD25 and CD69 and progressed through the first 12 hr of G0/G1 phase but did not express CD71, cyclin D3, cdk4, begin DNA synthesis, or differentiate into cytotoxic effector cells. In addition, activated T cells with their growth arrested by tryptophan deprivation exhibited enhanced tendencies to die via apoptosis when exposed to anti-Fas antibodies. Apoptosis was inhibited by caspase inhibitor and was not observed when T cells originated from Fas-deficient mice. These findings suggest that T cells activated in the absence of free tryptophan entered the cell cycle but cell cycle progression ceased in mid-G1 phase and T cells became susceptible to death via apoptosis, in part though Fas-mediated signalling. Thus, mature antigen-presenting cells expressing IDO and Fas-ligand may induce antigen-specific T-cell tolerance by blocking T-cell cycle progression and by rapid induction of T-cell activation induced cell death in local tissue microenvironments.

Stem cell medicine encounters the immune system

Recent progress in deriving human embryonic stem (hES) cells and defining their capacity to differentiate has inspired hope that they could become a source of replacement cells for damaged or diseased tissues. We review the immunological barriers to transplanting hES cells and consider several potential solutions, including stem-cell banking, modification of the immunogenicity of donor cells and induction of tolerance to the graft. We evaluate the probable efficacy of these approaches with a view to facilitating the use of hES cells in clinical practice.

Role of dendritic cells in graft-versus-host disease

A major barrier to successful allogeneic hematopoietic stem cell transplantation is graft-versus-host disease (GVHD). Until recently, the role of antigen presentation in the development of this disorder was unknown. The experimental finding that recipient antigen-presenting cells (APCs) were required for the development of CD8(+) T cell-dependent GVHD has led to a fundamental reappraisal of our ideas concerning the pathogenesis of this disease. Following transplantation, the origin (donor or recipient), number, lineage, and function of APCs within the recipient are altered significantly. Studies that test the influence of each of these factors upon graft-versus-host responses, including graft-versus-tumor responses, are beginning to emerge and suggest that APCs, such as dendritic cells, constitute a potential target for therapeutic manipulation.

Chemokines, their receptors, and transplant outcome

Organ transplant rejection is mediated largely by circulating peripheral leukocytes induced to infiltrate the graft by various inflammatory stimuli. Of these, chemotactic cytokines called chemokines, expressed by inflamed graft tissues, as well as by early innate-responding leukocytes that infiltrate the graft, are responsible for the recruitment of alloreactive leukocytes. This report discusses the impact of these leukocyte-directing proteins on transplant outcome and novel therapeutic approaches for antirejection therapy based on targeting of chemokines and/or their receptors.

Tacrolimus and cyclosporine differ in their capacity to overcome ongoing allograft rejection as a result of their differential abilities to inhibit interleukin-10 production

BACKGROUND

Accumulated evidence from clinical transplantation has suggested that tacrolimus-based treatment can reverse ongoing allograft rejection in patients treated with cyclosporine (CsA)-based immunosuppression, even when a high dose of antirejection rescue therapy has failed. This evidence prompted us to investigate whether these two compounds, which share an in vitro mechanism, would differ in their abilities to regulate in situ cellular and molecular events during ongoing allograft rejection.

METHODS

The equivalent effective doses of tacrolimus (3.2 mg/kg/day) and CsA (10 mg/kg/day), when administered orally to Lewis rats for 10 days (day 0-9), were predetermined and defined as the ability of the drug to induce a similar survival of Brown Norway rat heart allografts with an equal suppression of intragraft interleukin (IL)-2 mRNA expression. To investigate the ability of each drug to rescue ongoing allograft rejection, Lewis recipients of Brown Norway rat heart grafts were left untreated for the first 5 days after transplantation. Tacrolimus or CsA was then administered at the equivalent effective dose for 10 days (days 5-14). Heart grafts and blood samples, harvested on days 3, 5, 7, and 10, were analyzed by reverse transcriptase-polymerase chain reaction, real-time quantitative polymerase chain reaction, ELISA, and immunohistology.

RESULTS

Ongoing allograft rejection was found to be rescued by tacrolimus but not by CsA at the equivalent dose (median survival time: untreated, 6 days; tacrolimus, 18 days; and CsA, 7 days). A significant suppression of local intragraft IL-10 mRNA expression and serum protein production along with a dramatic down-regulation of functional CD8+ T and NKR-P1a+ natural killer cell local infiltration by means of decreased of cytotoxic factor release, including granzyme B and perforin 1, was found to be associated with tacrolimus but not CsA treatment. However, both drugs inhibited other immune cells (CD4+ T cell, ED2+ macrophage) and cytokines (IL-1beta, IL-2, IL-4, IL-6, IL-12, interferon-gamma, transforming growth factor-beta, and tumor necrosis factor-alpha) at almost the same levels. The inability of CsA to overcome ongoing allograft rejection could be rescued by cotreating recipients with neutralizing anti-IL-10 antibody on day 5 and day 6 after transplantation: anti-IL-10 antibody alone did not show such an effect.

CONCLUSIONS

Inhibition of IL-10 production is a critical factor in the ability of tacrolimus to reverse ongoing allograft rejection.