Rapamycin-treated, alloantigen-pulsed host dendritic cells induce ag-specific T cell regulation and prolong graft survival

Impairment of Circulating Myeloid Dendritic Cells in Immunosuppressed Renal/Pancreas Transplant Recipients

BACKGROUND

Immunosuppressive drugs used after organ transplantation are known to impair lymphocyte function, resulting in an increased incidence of viral associated malignancies. In vitro data indicate that immunosuppressive drugs also target dendritic cells (DCs). Our study aimed to investigate the phenotype and function of circulating myeloid DCs (mDCs) from renal/pancreas transplant recipients receiving immunosuppressive drugs. In addition, we analyzed the potential of patient monocytes to differentiate into mature DCs (MoDCs) in vitro.

METHODS

Phenotype of mDCs was analyzed by fluorescence-activated cell sorter (FACS) analysis. The ability of mDCs to undergo activation was determined using cytokine bead array and FACS analysis. Allostimulatory capacity was determined by mixed leukocyte reaction. MoDCs were generated using a defined cytokine cocktail and analyzed for maturity of phenotype and function. The ability of patient-MoDCs to expand antigen-specific T cells was analyzed by tetramer staining and interferon (IFN)-gamma ELISPOT assay.

RESULTS

We observed a reduced expression of CD54 (P=0.001), CD86 (P=0.032), HLA-DR (P=0.013), and CD38 (P=0.006) on patient mDCs. Upon stimulation, the expression of HLA-ABC, HLA-DR and CD86 was upregulated on patient mDCs to the same level as on control mDCs. MoDCs were equivalent to control-MoDCs regarding phenotype and function. Co-culture of peptide-pulsed patient-MoDCs with T cells resulted in significant expansion of autologous Epstein-Barr virus-specific, IFN-gamma secreting T cells.

CONCLUSIONS

Our data support the notion that immunosuppressive drugs target DCs and induce a maturation defect in circulating mDCs. However, ex vivo stimulated mDCs as well as MoDCs do not show a significant impairment, suggesting that MoDCs from immunosuppressed patients can be used for immunotherapeutic strategies.

Regulatory dendritic cell therapy in organ transplantation

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

Dendritic cells as promoters of transplant tolerance

Dendritic cells (DCs) play a crucial role during the initiation of immune responses against non-self antigens. Following organ transplantation, activated donor- and recipient-derived DCs participate actively in graft rejection by sensitising recipient T cells via the direct or indirect pathways of allorecognition, respectively. There is increasing evidence that immature/semi-mature DCs induce antigen-specific unresponsiveness or tolerance to self antigens, both in central lymphoid tissue and in the periphery, through a variety of mechanisms (deletion, anergy and regulation). In the past few years, DC-based therapy of experimental allograft rejection has focused on ex vivo biological, pharmacological and genetic engineering of DCs to mimic/enhance their natural tolerogenicity. Successful outcomes in rodent models have built the case that DC-based therapy may provide a novel approach to transplant tolerance. Ongoing research into the role that DCs play in the induction of tolerance should allow for its clinical application in the near future.

Rapamycin induces a caspase-independent cell death in human monocytes

The immunosuppressive activity of rapamycin (RAPA) and its efficacy as an anti-rejection agent in organ transplantation have been ascribed principally to its anti-proliferative effects on T cells, while the activity on monocytes is partially unknown. In vitro, RAPA reduced monocyte survival by inducing a caspase-independent cell death. RAPA-induced monocyte cell death (RAPA-CD) was impeded by activation of granulocyte macrophage-colony stimulating factor family receptors or toll-like receptor 4, and by exposure to inflammatory cytokines. In vivo, in patients who received RAPA monotherapy as part of pre-conditioning for islet transplantation, RAPA affected survival of myeloid lineage cells. In the peripheral blood, CD33(+) and CD14(+) cells decreased, whereas lymphocytes appeared unaffected. In the bone marrow, myeloid precursors such as CD15(+) and CD15(+)/CD16(+) were selectively and significantly decreased, but no major cytotoxic effects were observed. The RAPA-CD suggests a dependence of monocytes on mammalian target of RAPA pathways for nutrient usage, and this feature implies that RAPA could be selectively useful as a treatment to reduce monocytes or myeloid cells in conditions where these cells negatively affect patient, suggesting a potential anti-inflammatory action of this drug.

Rapamycin Does Not Induce Anergy but Inhibits Expansion and Differentiation of Alloreactive Human T Cells

BACKGROUND

Studies in mice have shown that rapamycin inhibits cell cycle progression and promotes the development of clonal anergy. We here addressed the question if rapamycin can induce anergy of human T cells and studied the effects of rapamycin on activation, proliferation and expression of cytotoxic effector molecules of alloresponsive T cells in mixed lymphocyte cultures.

METHODS

Peripheral blood mononuclear cells from healthy individuals were labeled with CFSE to monitor subsequent cell divisions. Cells were cocultured with allogeneic irradiated cells in the presence or absence of rapamycin. Flowcytometric analysis was performed after staining for surface CD4, CD8, and CD25 and for intracellular perforin, granzyme B, active caspase-3, and TGF-beta. Bio-Plex cytokine assay was done to measure the secretion of IL-2, IL-4, IL-10, and IFN-gamma.

RESULTS

Addition of rapamycin at a final concentration of 10 ng/ml strongly decreased precursor frequencies of alloreactive CD4+ and CD8+ T cells. However, when these cells were washed and subsequently specifically restimulated in the absence of rapamycin, the proliferative capacity appeared normal. Next to lowering precursor frequencies, rapamycin also inhibited T cell expansion by inducing apoptosis in divided alloreactive CD4+ and CD8+ T cells. Rapamycin did not interfere with the formation of CD25brightCD4+ T cells during allogeneic stimulation and did not inhibit their suppressive function. Furthermore, the drug decreased production of effector molecules perforin and granzyme B by alloreactive T cells and diminished alloreactive cytotoxicity.

CONCLUSION

Our data show that rapamycin strongly inhibits proliferation and effector functions of alloreactive T cells in vitro, but does not induce alloantigen specific nonresponsiveness.

Dendritic cells in allogeneic hematopoietic stem cell transplantation

In allogeneic hematopoietic stem cell transplantation (SCT), dendritic cells (DCs) as the most potent antigen-presenting cells play a central role in the development of acute and chronic graft-vs-host disease (GVHD), in graft-vs-leukemia or -malignancy reactions and in fighting infectious complications. Functional maturity and distribution of DC sub-types (DC1 and DC2) differ between the different stem cell sources used (bone marrow, granulocyte colony-stimulating factor-mobilised peripheral blood and cord blood) resulting in various rates of graft-vs-host disease and graft-vs-leukemia activity. Although DC recovery following stem cell transplantation is prompt, graft-vs-host disease and the use of immunosuppressive drugs result in qualitative and quantitative disturbances in DC homeostasis and have been observed for up to 1 year after transplantation. Complete donor DC chimerism seems to be a pre-requisite for the development of chronic GVHD and for graft-vs-leukemia activity, at least following reduced-intensity transplants, although in the early phase of acute graft-vs-host disease the presence of host antigen-presenting cells is essential. Preliminary data show promising results with DC-based immunotherapy for treatment of viral and fungal infections and of leukemic relapse following allogeneic stem cell transplantation. More information on the mechanisms and interactions between dendritic cells and regulatory T cells is needed for DC vaccination concepts for modulation of graft-vs-host disease.

Dendritic cells: tools and targets for transplant tolerance

Our knowledge of the role of dendritic cells (DC) in the generation and maintenance of T-cell tolerance has expanded rapidly and is now a key area of research in basic and applied DC biology. This minireview highlights recent developments in the field that are leading to new avenues for exploiting DC in the promotion of transplant tolerance.