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

Regulation of human dendritic cells by a novel specific nuclear factor-kappaB inhibitor, dehydroxymethylepoxyquinomicin

Regulation of antigen-presenting cells (APC) is crucial in controlling allograft rejection. Dendritic cells (DC) are the most potent APC and must mature to present antigens to T-cell receptors. During DC maturation, nuclear factor-kappaB (NF-kappaB) is a key transcriptional factor. We synthesized dehydroxymethylepoxyquinomicin (DHMEQ), which specifically inhibits the final step of nuclear translocation of activated NF-kappaB proteins and examined its immunoregulatory effects on human monocyte-derived DC (Mo-DC). Regulatory Mo-DC were generated by pretreatment with DHMEQ before LPS stimulation, which were termed dl-DC. DHMEQ pretreatment (5 microg/ml) completely inhibited nuclear translocation of activated NF-kappaB. DHMEQ significantly inhibited DC production of proinflammatory cytokines (IL-6, TNF-alpha, and IL-12 p70) in a dose-dependent manner. IL-12 was most potently inhibited. However, IL-10 production by dl-DC was only moderately affected by DHMEQ. Although CD40 and the expression of HLA-DR (HLA-DR) expression on dl-DC was downregulated, CD80 and CD86 expression was moderately upregulated. Induction of T helper 1 cell responses was efficiently impaired by dl-DC. This confirmed that DHMEQ-treated Mo-DC exhibited immunoregulatory effects. These findings suggest that DHMEQ has potential as an immunosuppressive drug for human immune cells.

Dendritic cell tolerogenicity: a key mechanism in immunomodulation by vitamin D receptor agonists

Dendritic cells (DC) induce or tolerize T cells, and tolerogenic DCs can promote the development of regulatory T cells (Treg) with suppressive activity. Thus, the possibility of manipulating DCs and enhancing their tolerogenic properties using different pharmacologic or biologic agents could be exploited to control a variety of chronic immuno-mediated inflammatory conditions. Among agents able to promote induction of tolerogenic DCs, vitamin D receptor (VDR) agonists have attracted considerable attention, also because of their potential in clinical translation. DCs are key targets for the immunomodulatory effects of VDR agonists, which shape DC phenotype and function, enhancing their tolerogenicity in adaptive immune responses. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4+CD25+Foxp3+ Treg cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favor induction of CD4+CD25+ Treg cells, but can also enhance their recruitment at inflammatory sites. The tolerogenic properties induced by VDR agonists in DCs, leading to enhanced Treg cell development, likely contribute to the beneficial activity of these hormone-like molecules in autoimmune disease and graft rejection models, highlighting their applicability to the treatment of chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses.

[IL-2/IL-2R pathway in dendritic cell modulates both their cytokine synthesis profiles and their capacity to activate allogeneic CD4+ T lymphocytes]

The results provide new insights into the role of IL-2/IL-2R pathway in DC. We report that stimulation of human monocyte-derived DC with LPS strongly upregulated CD25 (α chain of the IL-2R) expression. In addition, by using a humanized monoclonal antibody against CD25, we demonstrated that the IL-2 signalling in DC upregulated both IL-12 and γIFN production but decreased IL-10 synthesis. Anti-CD25 treatment reduced the ability of LPS-DC to induce allogeneic CD4(+) T cell proliferation as compared to LPS-matured DC. In addition, LPS-matured DC treated with IL-2 had a higher allostimulatory capacity compared to LPS-DC. We also found that LPS-matured DC produced IL-2. Thus, IL-2 seems to contribute actively to DC activation through an autocrine pathway. Moreover, IL-2 pathway in DC is involved in T helper priming. These findings might be useful for protocols in cellular therapy and a valuable tool to understand graft rejection versus the acquisition of peripheral tolerance.

Ex vivo inhibition of NF-kappaB signaling in alloreactive T-cells prevents graft-versus-host disease

The ex vivo induction of alloantigen-specific hyporesponsiveness by costimulatory pathway blockade or exposure to immunoregulatory cytokines has been shown to inhibit proliferation, IL-2 production, and the graft-versus-host disease (GVHD) capacity of adoptively transferred T-cells. We hypothesized that inhibition of the intracellular NF-kappaB pathway in alloreactive T-cells, which is critical for T-cell activation events including IL-2 transcription, could lead to alloantigen hyporesponsiveness and loss of GVHD capacity. We demonstrate that treatment of mixed lymphocyte reaction (MLR) cultures with PS1145, a potent inhibitor of NF-kappaB activation, can induce T-cell hyporesponsiveness to alloantigen in primary and secondary responses while preserving in vitro responses to potent mitogenic stimulation. GVHD lethality in recipients of ex vivo PS1145-treated cells was profoundly inhibited. Parking of control or PS1145-treated MLR cells in syngeneic Rag(-/-) recipients resulted in intact contact hypersensitivity (CHS) responses. However, GVHD lethality capacity also was restored, suggesting that lymphopenic expansion uncoupled alloantigen hyporesponsiveness. These results indicate that the NF-kappaB pathway is a critical regulator of alloresponses and provide a novel small molecule inhibitor based approach that is effective in preventing early posttransplant GVHD lethality but that also permits donor T-cell responses to recover after a period of lymphopenic expansion.

Induction of tolerogenic dendritic cells by vitamin D receptor agonists

Dendritic cells induce and regulate T cell responses, and tolerogenic dendritic cells (DCs) can promote the development of regulatory T cells with suppressive activity. Thus, the possibility to manipulate DCs using different pharmacological or biological agents enables them to exert tolerogenic activities, could be exploited to better control a variety of chronic inflammatory conditions, from autoimmune diseases to allograft rejection. A variety of both biological and pharmacological agents can induce tolerogenic DCs, and several in vitro studies have demonstrated that human regulatory T cells can be induced by DCs manipulated to acquire and/or enhance tolerogenic properties, with in vivo data also accumulating. Within this context, we have explored the immunoregulatory activities of vitamin D receptor (VDR) agonists, secosteroid hormones able to induce tolerogenic DCs and regulatory T cells. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4(+) CD25(+) Foxp3(+) suppressor T cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favour the induction of CD4(+) CD25(+) regulatory T cells, but can also enhance their recruitment to inflammatory sites. VDR agonists have been proven effective and safe drugs in a variety of autoimmune disease and graft rejection models, highlighting their potential applicability to chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses. In addition to the topical treatment of psoriasis, a Th1-mediated autoimmune disease of the skin where VDR agonists are the most used topical drugs; these agents might eventually find a broader application in the treatment of inflammatory conditions, where their modulatory effects on DCs enhancing T cells with regulatory functions could turn out to be quite beneficial.

Modulation of nuclear factor-kappaB activity can influence the susceptibility to systemic lupus erythematosus

Autoimmune diseases, such as systemic lupus erythematosus (SLE), result from deficiencies in self-antigen tolerance processes, which require regulated dendritic cell (DC) function. In this study we evaluated the phenotype of DCs during the onset of SLE in a mouse model, in which deletion of the inhibitory receptor FcgammaRIIb leads to the production of anti-nuclear antibodies and glomerulonephritis. Splenic DCs from FcgammaRIIb-deficient mice suffering from SLE showed increased expression of co-stimulatory molecules. Furthermore, diseased mice showed an altered function of the nuclear factor-kappaB (NF-kappaB) transcription factor, which is involved in DC maturation. Compared with healthy animals, expression of the inhibitory molecule IkappaB-alpha was significantly decreased in mice suffering from SLE. Consistently, pharmacological inhibition of NF-kappaB activity in FcgammaRIIb-deficient mice led to reduced susceptibility to SLE and prevented symptoms, such as anti-nuclear antibodies and kidney damage. Our data suggest that the occurrence of SLE is significantly influenced by alterations of NF-kappaB function, which can be considered as a new therapeutic target for this disease.

The complement inhibitor low molecular weight dextran sulfate prevents TLR4-induced phenotypic and functional maturation of human dendritic cells

Low molecular weight dextran sulfate (DXS) has been reported to inhibit the classical, alternative pathway as well as the mannan-binding lectin pathway of the complement system. Furthermore, it acts as an endothelial cell protectant inhibiting complement-mediated endothelial cell damage. Endothelial cells are covered with a layer of heparan sulfate (HS), which is rapidly released under conditions of inflammation and tissue injury. Soluble HS induces maturation of dendritic cells (DC) via TLR4. In this study, we show the inhibitory effect of DXS on human DC maturation. DXS significantly prevents phenotypic maturation of monocyte-derived DC and peripheral myeloid DC by inhibiting the up-regulation of CD40, CD80, CD83, CD86, ICAM-1, and HLA-DR and down-regulates DC-SIGN in response to HS or exogenous TLR ligands. DXS also inhibits the functional maturation of DC as demonstrated by reduced T cell proliferation, and strongly impairs secretion of the proinflammatory mediators IL-1beta, IL-6, IL-12p70, and TNF-alpha. Exposure to DXS leads to a reduced production of the complement component C1q and a decreased phagocytic activity, whereas C3 secretion is increased. Moreover, DXS was found to inhibit phosphorylation of IkappaB-alpha and activation of NF-kappaB. These findings suggest that DXS prevents TLR-induced maturation of human DC and may therefore be a useful reagent to impede the link between innate and adaptive immunity.

Anti CD25 treatment of human dendritic cells modulates both their cytokine synthesis profiles and their capacity to activate allogeneic CD4 T cells: a potential tolerogenic effect

Recently, attention has been focused on the role for dendritic cells (DCs) in the promotion of peripheral tolerance. It is currently believed that the maturation/activation state of DCs might be a control point for the induction of graft tolerance through modifications of the activation state of T cells. We have observed IL-2, and IL-2 receptor gene expression by reverse transcriptase-polymerase chain reaction (RT-PCR) in human DC lysates following bacterial stimulation. It has been demonstrated in the present study IL-2R alpha (CD25) expression on human DCs upon LPS activation. DCs differentiated from monocytes were exposed to anti CD25 during maturation, anti CD25 treatment affected the abilities of human DCs to induce CD4+ T cell proliferation in response to alloantigens, maintained endocytic capacity, Anti CD25 treated DCs produce low levels of IL-12 and IFN and high level of IL-10. All these characteristics suggest that DCs may be used in cellular therapy either to induce allograft tolerance (anti CD25 treated DCs) or to restore immunity against tumors (IL-2 treated DCs).

Innate Immunity-Mediated Allograft Rejection and Strategies to Prevent It

Experimental and clinical evidence has accumulated in support of the notion that oxidative injuries to allografts induce an adaptive alloimmune response which leads to acute rejection. The link between the initial injury and subsequent rejection is the innate immune system represented by injury-activated donor-derived and recipient-derived dendritic cells which interact with naïve T cells of the recipient to induce an alloimmune T-cell response. Therefore, time is mature to consider potential therapeutic strategies that are able to suppress events of innate immunity. Such strategies refer to a "time-restricted therapeutic window" that includes treatment of the donor during organ removal and the recipient during allograft reperfusion. Major targets of such treatment include (1) mitigation of the oxidative allograft injury; (2) inhibition of injury-induced activation of complement; (3) inhibition of Toll-like receptor (TLR)-mediated and innate lymphocyte-triggered maturation of dendritic cells; and (4) blockade of innate effector functions. A considerable variety of promising experimental studies about the prevention/inhibition of innate immune events has already been performed, including the successful experimental use of gene silencing methods, eg, using RNA interference technology with the application of small interfering RNA (siRNA). In addition, a few clinical trials with antioxidants (edaravone, SOD-mimetics), complement inhibitors (pexelizumab, TP-10) in patients with acute myocardial infarction, and TLR4 antagonists (TAK-242, E-5564) in patients with sepsis have been performed or are underway. Performance of similar clinical trials in transplant patients with antioxidative drugs, complement inhibitors, and/or TLR4 antagonists is urgently warranted; siRNAs appear to be extremely attractive for investigation in experimental allogeneic transplant models.

Preventing autoimmune arthritis using antigen-specific immature dendritic cells: a novel tolerogenic vaccine

Conventional treatments for autoimmune diseases have relied heavily on nonspecific immune suppressants, which possess a variety of adverse effects without inhibiting the autoimmune process in a specific manner. In the present study we demonstrate the effectiveness of antigen-specific, maturation-resistant, tolerogenic dendritic cells (DC) in suppressing collagen-induced arthritis, a murine model of rheumatoid arthritis. Treatment of DC progenitors with the NF-κB inhibiting agent LF 15-0195 (LF) resulted in a population of tolerogenic DC that are characterized by low expression of MHC class II, CD40, and CD86 molecules, as well as by poor allostimulatory capacity in a mixed leukocyte reaction. Administering LF-treated DC pulsed with keyhole limpet hemocyanin antigen to naïve mice resulted hyporesponsiveness specific for this antigen. Furthermore, administration of LF-treated DC to mice with collagen-induced arthritis resulted in an improved clinical score, in an inhibited antigen-specific T-cell response, and in reduced antibody response to the collagen. The efficacy of LF-treated DC in preventing arthritis was substantiated by histological examination, which revealed a significant decrease in inflammatory cell infiltration in the joints. In conclusion, we demonstrate that in vitro-generated antigen-specific immature DC may have important potential as a tolerogenic vaccine for the treatment of autoimmune arthritis.

Liver X receptors regulate dendritic cell phenotype and function through blocked induction of the actin-bundling protein fascin

Liver X receptors (LXRs) are nuclear receptors regulating lipid and cholesterol metabolism. Recent data revealed a cross talk between LXR and Toll-like receptor signaling in macrophages, indicating a role in immunity. Here, we show that LXRalpha is expressed in human myeloid dendritic cells (DCs) and induced during differentiation of monocyte-derived DCs, whereas LXRbeta is expressed constitutively at a very low level. LXR activation by 2 different LXR agonists strongly interfered with lipopolysaccharide (LPS)-induced but not with CD40L-induced DC maturation by altering DC morphology and suppressing interleukin-12-but enhancing interleukin-10-secretion. LXR activation in DCs largely blocked their T-cell stimulatory ability despite essentially unaltered expression of various antigen-presenting and costimulatory molecules. Immunologic synapse formation was significantly inhibited by LXR activation along with a complete block in LPS- but not CD40L-induced expression of the actin-bundling protein fascin. Notably, overexpression of fascin in LXR agonist-treated DCs restored immunologic synapse formation and restored their ability to activate T cells. In conclusion, our data reveal LXR as a potent modulator of DC maturation and function mediated in part by blocking the expression of fascin. Due to the central position of DCs in immunity, LXRalpha could be a potential novel target for immunomodulation.

Inhibition of nuclear factor-kappa B enhances the capacity of immature dendritic cells to induce antigen-specific tolerance in experimental autoimmune encephalomyelitis

Autoimmune disorders develop as a result of deregulated immune responses that target self-antigens and cause destruction of healthy host tissues. Because dendritic cells (DCs) play an important role in the maintenance of peripheral immune tolerance, we are interested in identifying means of enhancing their therapeutic potential in autoimmune diseases. It is thought that during steady state, DCs are able to anergize potentially harmful T cells bearing T cell receptors that recognize self-peptide-major histocompatibility complexes. The tolerogenic capacity of DCs requires an immature phenotype, which is characterized by a reduced expression of costimulatory molecules. On the contrary, activation of antigen-specific naive T cells is enhanced by DC maturation, a process that involves expression of genes controlled by the transcription factor nuclear factor (NF)-kappaB. We evaluated the capacity of drugs that inhibit NF-kappaB to enhance the tolerogenic properties of immature DCs in the experimental autoimmune encephalomyelitis (EAE) model. We show that andrographolide, a bicyclic diterpenoid lactone, and rosiglitazone, a peroxisome proliferator-activated receptor gamma agonist, were able to interfere with NF-kappaB activation in murine DCs. As a result, treated DCs showed impaired maturation and a reduced capacity to activate antigen-specific T cells. Furthermore, NF-kappaB-blocked DCs had an enhanced tolerogenic capacity and were able to prevent EAE development in mice. The tolerogenic feature was specific for myelin antigens and involved the expansion of regulatory T cells. These data suggest that NF-kappaB blockade is a potential pharmacological approach that can be used to enhance the tolerogenic ability of immature DCs to prevent detrimental autoimmune responses.

Inhibition of Human Dendritic Cell Maturation and Function by the Novel Immunosuppressant FK778

BACKGROUND

FK778, a derivative of the active leflunomide-metabolite, A77 1726, has been shown to be a powerful immunosuppressant in several transplantation models, particularly efficient in the prevention of chronic allograft rejection. However, the cellular and molecular mechanisms underlying these effects of FK778 have not been investigated yet in detail. Because dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) and are essential for the initiation of immune responses including acute and chronic allograft rejection, we investigated whether FK778 affects this particular cell type.

METHODS

Allogeneic T cell stimulation by FK778-treated human monocyte-derived DCs was determined by mixed leukocyte cultures. Surface molecule expression was analyzed by flow-cytometric analysis and cytokine production by ELISA from culture supernatants. Activation of NF-kappaB in DCs was assessed by electrophoretic mobility shift assays.

RESULTS

Treatment of DCs with FK778 inhibited their potency to stimulate allogeneic T cells. In line, LPS- and CD40L-induced upregulation of DC surface activation markers and production of IL-12 was significantly inhibited, irrespective of whether cells were treated during or after the monocyte to DC differentiation period. The effects of FK778 on DCs were not reversible by exogenous uridine indicating that FK778 acts independently of its action as an inhibitor of pyrimidine synthesis. On the signaling level, activation of NF-kappaB, the essential transcription factor involved in DC maturation and function, was markedly inhibited by FK778.

CONCLUSIONS

Inhibition of activation and function of DCs as the central APCs may significantly contribute to the immunosuppressive profile of FK778 when applied after allogeneic organ transplantation.