Potential of tolerogenic dendritic cells for transplantation

Immune regulation by regulatory T cells: implications for transplantation

The induction of antigen-specific T cell tolerance and its maintenance in the periphery are critical for the immune system to prevent autoaggressive immune responses. Our current state of knowledge about the immunoregulatory mechanisms responsible for T cell tolerance in the periphery offers new possibilities for immunomodulation to prevent transplant rejection as well as to diminish autoimmune reaction or chronic allergy. There is growing evidence that dendritic cells, besides their well-known T cell stimulatory functions, also maintain and regulate T cell tolerance in the periphery. This control function is exerted by certain maturation stages and subsets of dendritic cells, and can be further influenced and modulated by immunoregulatory cytokines and drugs. The regulatory functions of dendritic cells include the induction of T cell anergy, of T cells with regulatory properties and of T cells that produce immunosuppressive cytokines such as IL-10 or TGF-beta. Additionally, distinct subsets of resident regulatory T cells generated in the thymus play a central role in maintenance of peripheral tolerance by active suppression of effector T cell populations. These CD4(+)CD25(+) regulatory T cells inhibit a variety of autoimmune and inflammatory diseases and they are also efficient in the suppression of alloantigen responses. This review summarises the current knowledge regarding the immunoregulatory role of dendritic cells and the functional activities of resident regulatory T cells as guardians for peripheral T cell tolerance.

Influence of immunosuppressive drugs on dendritic cells

Immunosuppressive drugs used to control allograft rejection and in efforts to promote transplant tolerance are well recognized for their abilities to inhibit lymphocyte activation and proliferation. In recent years, evidence has accumulated that these diversely acting agents (anti-proliferative drugs, calcineurin inhibitors, rapamycin, deoxyspergualin and glucocorticoids) also affect the development and functional immunobiology of dendritic cells, in vitro and in vivo. Here we review the influence of immunosuppressive drugs on the differentiation and function of these important antigen-presenting cells. We also consider how these effects influence immune reactivity and tolerance induction, implications for furthermore understanding of dendritic cell biology and prospects for improving the outcome of organ transplantation and therapy of other immune-mediated disorders by impacting dendritic cell function.

Dendritic cell subset ratio in peripheral blood correlates with successful withdrawal of immunosuppression in liver transplant patients

Human dendritic cell (DC) subsets appear to play distinct roles in the induction and regulation of immune responses. While monocytoid DC (DC1) induce T-helper (Th) 1-type responses, plasmacytoid DC (DC2) have been reported to selectively induce Th2 responses. In blood, their precursors (p) can be identified as HLA-DR+ lineage- cells that are further characterized as CD11c+ CD123-/lo (IL-3Ralpha-/lo) (pDC1) or as CD11c- CD123hi (pDC2) by rare event, flow cytometric analysis. We compared the incidences of pDC1 and pDC2 in peripheral blood mononuclear cell populations isolated from normal healthy controls and from 3 groups of clinically stable liver transplant patients. Group A had been successfully withdrawn from immunosuppression, whereas group B were undergoing prospective drug weaning and on minimal anti-rejection therapy. In group C, drug withdrawal had either failed or never been attempted and patients were on maintenance immunosuppression. Assessment of DC subsets and the pDC2 : pDC1 ratio showed good intra-and interassay reproducibility. Compared with patients in group C, those in groups A and B demonstrated a significantly higher relative incidence of pDC2 and a lower incidence of pDC1 - similar to those values observed in normal healthy controls. Moreover, the pDC2 : pDC1 ratio was significantly higher in patients undergoing (successful) weaning and in those off immunosuppression compared with patients on maintenance immunosuppression.

Donor-derived hematopoietic cells in organ transplantation: a major step toward allograft tolerance?

Infusion of donor-derived cells can improve organ allograft survival in animal models. Under certain conditions, it can even induce tolerance (i.e., unlimited organ survival without any maintenance immunosuppressive therapy). Use of nonmyeloablative regimens allows engraftment of donor-derived bone marrow cells, induction of mixed chimerism, and tolerance in rodents. High doses of bone marrow cells together with anti-T-cell antibodies can even result in mixed chimerism without cytoablative host conditioning. Cultured donor-derived CD34+ cells or donor-derived immature (or even mature) dendritic cells associated with monoclonal antibodies directed against co-stimulatory molecules might also induce tolerance. Among the numerous experimental protocols leading to tolerance of solid organs in animal models, how can we find our bearings in human transplantation? Numerous problems have yet to be solved: the type and amount of donor-derived cells (including stromal cells) to be used, the timing for infusion of donor cells in keeping with organ transplantation, the route of infusion (should it be intravenous, into the portal vein?), and the conditioning regimen. The first clinical trials would appear to indicate that tolerance induction in humans using donor-derived cells is a relatively safe solution that is both promising and realistic.

Induction of peripheral tolerance to experimental autoimmune myasthenia gravis by acetylcholine receptor-pulsed dendritic cells

Dendritic cells (DC) are usually regarded as antigen-presenting cells involved in T cell activation, but DC also directly and indirectly affect B cell activation, antibody synthesis, and isotype switch. In the present study, bone marrow (BM)-derived DC from healthy rats were pulsed in vitro with acetylcholine receptor (AChR) and injected subcutaneously into healthy Lewis rats. No clinical signs of the first phase of experimental autoimmune myasthenia gravis (EAMG) were observed during 3 weeks of observation. Upon immunization with AChR and complete Freund's adjuvant, the rats that had received AChR-pulsed DC did not develop clinical EAMG. This tolerance of rats injected with AChR-pulsed DC was associated with reduced expression of B cell-activating factor (BAFF) and by reduced numbers of B cells among splenic mononuclear cells (MNC) compared to rats injected with medium or unpulsed DC. Anti-AChR IgG antibody-secreting cells were decreased, while the ratio of IgG1:IgG2b isotypes was enhanced in rats treated with AChR-pulsed DC compared to control EAMG rats. These results demonstrate that AChR-pulsed DC induce peripheral tolerance to EAMG by possibly inhibiting the expression of BAFF and production of anti-AChR antibodies, providing a possible potential for immunotherapy of antibody-mediated autoimmune diseases.

Synergistic tolerance induced by LF15-0195 and anti-CD45RB monoclonal antibody through suppressive dendritic cells

BACKGROUND

LF 15-0195 (LF), a novel analogue of 15-deoxyspergualin (DSG), inhibits maturation of dendritic cells (DC). Anti-CD45RB is a monoclonal antibody (mAb) that blocks activation of T-helper (Th) 1 cells and generates T-regulatory cells. This study addressed whether these two reagents act synergistically to inducing tolerance, and investigated associated cellular mechanisms.

METHODS

BALB/c recipients were treated by a short course of mAb alone, LF alone, or the combination of both agents. Mice that accepted a C57BL/6 cardiac allograft for more than 100 days were considered tolerant. Splenic DC were purified using positive selection for CD11c. Bone marrow DC were generated by culture with interleukin-4 and granulocyte-macrophage colony-stimulating factor. Surface marker expression was determined by fluorescence-activated cell sorter analysis. DC function was assessed by the ability to stimulate or inhibit T cells in vitro.

RESULTS

Although monotherapy with LF or mAb failed to induce tolerance, combination therapy resulted in long-lasting acceptance of allogeneic hearts (>200 days) and secondary donor skin grafts (>100 days). DC from tolerant recipients possessed lower major histocompatibility complex class II and CD40 expression, and were poorer co-stimulators for T-cell proliferation than control DC. Furthermore, DC from tolerant mice induced Th2 differentiation, suppressed overall T-cell proliferation, and were poor presenters of T cells specific for antigen to pigeon cytochrome c 81-104.

CONCLUSIONS

The combination of LF and anti-CD45RB mAb induced stable tolerance. The synergy of these two approaches appears to be mediated through formation of tolerogenic DC and T-regulatory cells.

Soluble fibrinogen-like protein 2/fibroleukin exhibits immunosuppressive properties: suppressing T cell proliferation and inhibiting maturation of bone marrow-derived dendritic cells

Fibrinogen-like protein 2 (fgl2)/fibroleukin is a member of the fibrinogen-related protein superfamily. In addition to its established role in triggering thrombosis, it is known to be secreted by T cells. The soluble fgl2 ((s)fgl2) protein generated in a baculovirus expression system bound to both T cells and bone marrow-derived dendritic cells (DC) in a specific manner. (s)fgl2 exhibited immunomodulatory properties capable of inhibiting T cell proliferation stimulated by alloantigens, anti-CD3/anti-CD28 mAbs, and Con A in a dose-dependent manner; however, it had no inhibitory effects on CTL activity. The time- and dose-dependent inhibitory effect of (s)fgl2 on alloreactive T cell proliferation could be neutralized by a mAb against mouse fgl2. Polarization toward a Th2 cytokine profile with decreased production of IL-2 and IFN-gamma and increased production of IL-4 and IL-10 was observed in (s)fgl2-treated allogeneic cultures. Exposure of immature DC to (s)fgl2 abrogated the expression of CD80(high) and MHC class II(high) molecules and markedly inhibited NF-kappaB nuclear translocation, thus inhibiting their maturation. (s)Fgl2-treated DC had an impaired ability to stimulate allogeneic T cell proliferation. Maximal inhibition of proliferation was observed when allogeneic T cells were cultured with (s)fgl2-treated DC and (s)fgl2 protein was added in the culture. These data provide the first evidence to demonstrate that (s)fgl2 exerts immunosuppressive effects on T cell proliferation and DC maturation.

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.

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

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

Internalization of circulating apoptotic cells by splenic marginal zone dendritic cells: dependence on complement receptors and effect on cytokine production

Under steady-state conditions, internalization of self-antigens embodied in apoptotic cells by dendritic cells (DCs) resident in peripheral tissue followed by DC migration and presentation of self-peptides to T cells in secondary lymphoid organs are key steps for induction and maintenance of peripheral T-cell tolerance. We show here that, besides this traffic of apoptotic cells mediated by peripheral tissue-resident DCs, splenic marginal zone DCs rapidly ingest circulating apoptotic leukocytes, process apoptotic cell-derived peptides into major histocompatibility complex class II (MHC-II) molecules, and acquire CD8alpha during their mobilization to T-cell areas of splenic follicles. Because apoptotic cells activate complement and some complement factors are opsonins for phagocytosis and play roles in the maintenance of peripheral tolerance, we investigated the role of complement receptors (CRs) in relation to phagocytosis of apoptotic cells by DCs. Apoptotic cell uptake by marginal zone DCs was mediated in part via CR3 (CD11b/CD18) and, to a lesser extent, CR4 (CD11c/CD18) and was reduced significantly in vivo in hypocomplementemic animals. Following phagocytosis of apoptotic cells, DCs exhibited decreased levels of mRNA and secretion of the proinflammatory cytokines interleukin 1alpha (IL-1alpha), IL-1beta, IL-6, IL-12p70, and tumor necrosis factor alpha (TNF-alpha), without effect on the anti-inflammatory mediator transforming growth factor beta1 (TGF-beta1). This selective inhibitory effect was at least partially mediated through C3bi-CD11b/CD18 interaction. Characterization of apoptotic cell/DC interaction and its outcome provides insight into the mechanisms by which apoptotic cells affect DC function without disrupting peripheral tolerance.

Dendritic cells genetically engineered to express IL-4 exhibit enhanced IL-12p70 production in response to CD40 ligation and accelerate organ allograft rejection

C57BL/10 (B10; H2(b)) bone marrow-derived myeloid dendritic cells (DC) propagated in GM-CSF + IL-4 were transduced with r adenoviral (Ad) vectors encoding either control neomycin-resistance gene (Ad-Neo) or murine IL-4 (Ad-IL-4) on day 5 of culture following CD11c immunomagnetic bead purification. Both Ad-Neo- and Ad-IL-4-transduced DC displayed upregulated surface MHC class II and costimulatory molecules (CD40, CD80, CD86). Ad-IL-4 DC secreted higher levels of bioactive IL-12p70 after CD40 ligation or LPS stimulation than either Ad-Neo or unmodified DC. Only Ad-IL-4 DC produced IL-12p70 in primary MLR, in which they induced augmented proliferative responses of naïve allogeneic C3H/HeJ (C3H; H2(k)) T-cells. Compared with Ad-Neo DC, Ad-IL-4 DC were also more effective in priming naïve allogeneic recipients to exhibit specifically enhanced anti-donor T-cell proliferative and CTL responses. T-cells primed in vivo 7 days previously with Ad-IL-4 DC displayed enhanced secretion of Th2 (IL-4, IL-10) but also higher Th1 cytokine (IFNgamma) production following ex vivo challenge with donor alloAg. Moreover, pretreatment of vascularized heart graft recipients with i.v. Ad-IL-4 DC, 1 week before transplant, significantly accelerated rejection and antagonized the therapeutic effect of anti-CD40L (CD154) mAb. These data contrast markedly with recently reported inhibitory effects of autologous Ad-IL-4 DC on autoimmune inflammatory disease.

Cross-linking the B7 family molecule B7-DC directly activates immune functions of dendritic cells

B7-DC molecules are known to function as ligands on antigen-presenting cells (APCs), enhancing T cell activation. In this study, cross-linking B7-DC with the monoclonal antibody sHIgM12 directly potentiates dendritic cell (DC) function by enhancing DC presentation of major histocompatibility complex-peptide complexes, promoting DC survival; and increasing secretion of interleukin (IL)-12p70, a key T helper cell type 1 promoting cytokine. Furthermore, ex vivo treatment of DCs or systemic treatment of mice with sHIgM12 increases the number of transplanted DCs that reach draining lymph nodes and increases the ability of lymph node APCs to activate naive T cells. Systemic administration of the antibody has an equivalent effect on DCs transferred at a distant site. These findings implicate B7-DC expressed on DCs in bidirectional communication. In addition to the established costimulatory and inhibitory functions associated with B7-DC, this molecule can also function as a conduit for extracellular signals to DCs modifying DC functions.

Strategies for preclinical evaluation of dendritic cell subsets for promotion of transplant tolerance in the nonhuman primate

A role for dendritic cells (DC) as critical regulators of immune reactivity has become increasingly recognized. There is evidence in rodent models that donor-derived DC, particularly in the immature state, can prolong organ allograft survival and even induce donor-specific tolerance. To allow the potential tolerogenic properties of these cells to be evaluated more fully with a view to clinical testing, it is necessary to identify DC subsets in nonhuman primates. We have identified the putative rhesus monkey equivalents of circulating human DC subset precursors as lineage(-), HLA-DR(+), CD123(lo),CD11c(hi)(pDC1) and lineage(-), HLA-DR(+), CD123(hi),CD11c(lo)(pDC2). Testing of these DC populations both in vitro and in vivo, as well as in transplant models in combination with conventional or experimental immunosuppressive reagents, will aid the development of novel strategies for the promotion of allo-antigen specific tolerance in transplantation.

Normal donor bone marrow is superior to Flt3 ligand-mobilized bone marrow in prolonging heart allograft survival when combined with anti-CD40L (CD154)

Flt3 ligand (FL) administration markedly increases bone marrow (BM) stem cells and immature dendritic cells. We investigated the influence of CD40-CD40Ligand (CD154) pathway blockade on antidonor immunity, cytokine production, microchimerism and heart graft survival in BALB/c (H2d) recipients of fully allogeneic C57BL/10 (H2b) FL-mobilized BM (FL-BM) or normal BM. Anti-CD40L mAb strongly suppressed anti-donor T-cell proliferative responses in recipients of either normal or FL-BM, but was less efficient in inhibiting antidonor cytolytic T-cell (CTL) activity, especially in recipients of FL-BM. Interestingly, CD40L blockade was more effective in recipients of multiple compared with single donor BM infusions. Anti-donor cytokine responses revealed complete impairment of IFN-gamma, IL-4 and IL-10 production in recipients of normal BM and CD40L mAb. By contrast, and in agreement with the CTL data, mice given FL-BM retained ability to produce IFN-gamma CD40-CD40L blockade did not promote microchimerism, as evidenced by immunohistology and real time polymerase chain reaction. Nevertheless, anti-CD40L mAb enhanced heart allograft survival in recipients of FL-BM, but the effect was inferior to that achieved with normal BM. These data provide insight into the influence of growth factor-expanded donor BM and costimulation blockade on antidonor immune reactivity and transplant outcome. The comparatively poor outcome obtained using FL-BM plus anti-CD40L mAb in this model may be ascribed to the failure of effectively interdicting antidonor CTL activity.

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.