Transduction of donor hematopoietic stem-progenitor cells with Fas ligand enhanced short-term engraftment in a murine model of allogeneic bone marrow transplantation

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

A biodegradable killer microparticle to selectively deplete antigen-specific T cells in vitro and in vivo

The specific eradication of pathogenic T cells for the treatment of allograft rejections and autoimmune disorders without impairment of overall immune function is a fundamental goal. Here, cell-sized poly(lactic-co-glycolic acid) microparticles (PLGA MPs) were prepared as a scaffold to co-display the peptide/major histocompatibility complex (pMHC, target antigen) and anti-Fas monoclonal antibody (apoptosis-inducing molecule) for the generation of biodegradable killer MPs. Ovalbumin (OVA) antigen-targeted killer MPs significantly depleted OVA-specific CD8+ T cells in an antigen-specific manner, both in vitro and in OT-1 mice. After intravenous administration, the killer MPs predominantly accumulated in the liver, lungs, and gut of OT-1 mice with a retention time of up to 48 hours. The killing effects exerted by killer MPs persisted for 4 days after two injections. Moreover, the H-2Kb alloantigen-targeted killer MPs were able to eliminate low-frequency alloreactive T cells and prolong alloskin graft survival for 41.5 days in bm1 mice. Our data indicate that PLGA-based killer MPs are capable of specifically depleting pathogenic T cells, which highlights their therapeutic potential for treating allograft rejection and autoimmune disorders.

An Antigen-Presenting and Apoptosis-Inducing Polymer Microparticle Prolongs Alloskin Graft Survival by Selectively and Markedly Depleting Alloreactive CD8+ T Cells

Selectively depleting the pathogenic T cells is a fundamental strategy for the treatment of allograft rejection and autoimmune disease since it retains the overall immune function of host. The concept of killer artificial antigen-presenting cells (KaAPCs) has been developed by co-coupling peptide–major histocompatibility complex (pMHC) multimer and anti-Fas monoclonal antibody (mAb) onto the polymeric microparticles (MPs) to induce the apoptosis of antigen-specific T cells. But little information is available about its in vivo therapeutic potential and mechanism. In this study, polyethylenimine (PEI)-coated poly lactic-co-glycolic acid microparticle (PLGA MP) was fabricated as a cell-sized scaffold to covalently co-couple H-2K^b-Ig dimer and anti-Fas mAb for the generation of alloantigen-presenting and apoptosis-inducing MPs. Intravenous infusions of the biodegradable KaAPCs prolonged the alloskin graft survival for 43 days in a single MHC-mismatched murine model, depleted the most of H-2K^b-alloreactive CD8⁺ T cells in peripheral blood, spleen, and alloskin graft in an antigen-specific manner and anti-Fas-dependent fashion. The cell-sized KaAPCs circulated throughout vasculature into liver, kidney, spleen, lymph nodes, lung, and heart, but few ones into local allograft at early stage, with a retention time up to 36 h in vivo. They colocalized with CD8⁺ T cells in secondary lymphoid organs while few ones contacted with CD4⁺ T cells, B cells, macrophage, and dendritic cells, or internalized by phagocytes. Importantly, the KaAPC treatment did not significantly impair the native T cell repertoire or non-pathogenic immune cells, did not obviously suppress the overall immune function of host, and did not lead to visible organ toxicity. Our results strongly document the high potential of PLGA MP-based KaAPCs as a novel antigen-specific immunotherapy for allograft rejection and autoimmune disorder. The in vivo mechanism of alloinhibition, tissue distribution, and biosafety were also initially characterized, which will facilitate its translational studies from bench to bedside.

High ALDHdim-expressing CD34+CD38- cells in leukapheresed peripheral blood is a reliable guide for a successful leukemic xenograft model of acute myeloid leukemia

The primary human acute myeloid leukemia (AML) cell injection xenograft mouse model is used to investigate multimodal therapies and drug screening on tumor growth. Since xenograft models using human cell lines to examine drug response are not correlated with the clinical outcomes observed in patients, a xenograft model using primary human cells has been used as a more appropriate model with which to minimize this problem. Although bone marrow (BM) cells from patients are often regarded as superior sources to establish xenograft models due to the high frequency of stem cell populations, there is a fatal drawback; only small volumes can be obtained and used for the generation of the leukemic xenograft model. Indeed, longevity of AML characteristics, as well as sufficient stem cells in the xenograft model, should be guaranteed to analyze the therapeutic response to a drug. Therefore, we examined whether leukapheresed peripheral blood (LPB) consists of reliable leukemic stem cells (LSCs) and ALDHdim‑expressing CD34+CD38- cells, and functions in grafting human AML with virulence compared to that of BM. LPB cells showed an advantage for the xenograft mouse model with AML cell homing, engraftment and a high human ALDHdim-expressing CD34+CD38- cell population, suggesting an alternative cell source to BM. Overall, this xenograft model using LPB offers the possibility of overcoming the small volume limitation of BM and prevents individual variation by using a single LPB sample. This result is noteworthy in identifying cell sources capable of generating a stable xenograft model.

Physiological functions of TNF family receptor/ligand interactions in hematopoiesis and transplantation

Secretion of ligands of the tumor necrosis factor (TNF) superfamily is a conserved response of parenchymal tissues to injury and inflammation that commonly perpetuates elimination of dysfunctional cellular components by apoptosis. The same signals of tissue injury that induce apoptosis in somatic cells activate stem cells and initiate the process of tissue regeneration as a coupling mechanism of injury and recovery. Hematopoietic stem and progenitor cells upregulate the TNF family receptors under stress conditions and are transduced with trophic signals. The progeny gradually acquires sensitivity to receptor-mediated apoptosis along the differentiation process, which becomes the major mechanism of negative regulation of mature proliferating hematopoietic lineages and immune homeostasis. Receptor/ligand interactions of the TNF family are physiological mechanisms transducing the need for repair, which may be harnessed in pathological conditions and transplantation. Because these interactions are physiological mechanisms of injury, neutralization of these pathways has to be carefully considered in disorders that do not involve intrinsic aberrations of excessive susceptibility to apoptosis.

Resistance of hematopoietic progenitors to Fas-mediated apoptosis is actively sustained by NFκB with a characteristic transcriptional signature

Umbilical cord blood (UCB) is a good source of hematopoietic progenitors with increasing implementation in the clinical transplant setting. This study evaluates the molecular mechanisms of progenitor resistance to apoptosis triggered by Fas cross-linking. CD34(+) and lineage-negative progenitors survive short-term ex vivo incubation and are not induced into apoptosis by Fas cross-linking. Furthermore, brief exposure of UCB cells to Fas-ligand for 24-48 h does not impair quantitative severe combine immune deficiency (SCID) reconstitution activity and appears to foster myelomonocyte reconstitution. The transcriptome of Fas receptor-positive CD34(+) cells that survived an apoptotic challenge showed significant transcriptional upregulation of caspase-8, mucosa-associated lymphoid tissue lymphoma translocation gene-1 (MALT1), HtrA2, and GSK3β in addition to higher levels of c-FLICE inhibitory protein (FLIP), Bcl-2, and cytosolic inhibitor of apoptosis protein (cIAP) in all Fas-positive cells. Most prominent is the transcriptional upregulation of several key components the NFκB1 pathway including the membrane receptors TGF-β, interleukin-1 (IL-1), and TCR, the associated factor TNF receptor-associated factor-6 (TRAF6), and the converting enzymes TGF-β-activated kinase-1 (TAK1), double-stranded RNA-activated protein kinase (PKR), and α-catalytic subunit of IκB kinase (IKKα), that promote activation and nuclear translocation of this transcription factor. These data indicate that hematopoietic progenitors are not insensitive to apoptosis but are actively shielded from the extrinsic and intrinsic apoptotic pathways. This may occur through inherent transcriptional upregulation of the entire NFκB pathway in the presence of competent apoptotic signaling.

Induction of the glucocorticoid-induced leucine zipper gene limits the efficacy of dendritic cell vaccines

Dendritic cell (DC) vaccines have shown great promise in generating anti-tumor immune responses but have generally fallen short of producing durable cures. Determining mechanisms by which these vaccines fail will provide one strategy towards improving their success. Several manipulations of DCs have improved their migration and longevity, but the immune inhibitory environment surrounding tumors provides a powerful suppressive influence. To determine the mechanisms by which DCs at the site of the tumor convert to a suppressive phenotype, we evaluated pathways in DCs that become expressed at the tumor site. Our results revealed that tumors lead to induction of the glucocorticoid induced leucine zipper (GILZ) gene in DCs, and that this gene is critical for the development of tumor induced tolerance of both DCs and T cells. Previous data suggested that GILZ is a pivotal gene in the balance between activation and tolerance of DCs. Our new data show that GILZ is highly upregulated in DCs in the tumor microenvironment in vivo and that blockade of this gene in DC vaccines significantly improves long term survival. These results suggest that GILZ may be an ideal candidate gene to target for novel immune-based tumor therapies.

Killer artificial antigen-presenting cells deplete alloantigen-specific T cells in a murine model of alloskin transplantation

FasL-expressing killer antigen-presenting cells (KAPCs) have the ability to delete antigen-specific T cells and, therefore, could potentially be used for the treatment of allograft rejection and autoimmunity; however, their cellular nature markedly limits their clinical use. Novel bead-based killer artificial antigen-presenting cells (KaAPCs), which are generated by coupling major histocompatibility complex (MHC) class I antigens together with the apoptosis-inducing anti-Fas monoclonal antibody (mAb) onto magnetic beads, have recently attracted more attention. KaAPCs have a number of advantages over KAPCs and are able to deplete specific T cells in cocultures. However, it remains unknown whether bead-based KaAPCs can also induce apoptosis of alloreactive or autoreactive T cells and, consequently, generate hyporesponsiveness in vivo. In this study, H-2K(b)/peptide monomers and anti-Fas mAb have been covalently coupled to latex beads and administered intravenously into BALB/c mice (H-2K(d)) that had previously been grafted with skin squares from C57BL/6 mice (H-2K(b)). Alloskin graft survival was prolonged for 6 days. A 60% decrease of H-2K(b) antigen-alloreactive T cells was demonstrated by several measures 2 days after each injection of KaAPCs, but intact immune function, including antitumor activity, was maintained. These data provide the first in vivo evidence that bead-based KaAPCs can selectively deplete antigen-specific T cells without the loss of overall immune responsiveness and, therefore, highlight the therapeutic potential of this novel strategy for the treatment of allograft rejection and autoimmune disorders.

Killer artificial antigen-presenting cells: the synthetic embodiment of a 'guided missile'

At present, the treatment of T-cell-dependent autoimmune diseases relies exclusively on strategies leading to nonspecific suppression of the immune systems causing a substantial reduced ability to control concomitant infections or malignancies. Furthermore, long-term treatment with most drugs is accompanied by several serious adverse effects and does not consequently result in cure of the primary immunological malfunction. By contrast, antigen-specific immunotherapy offers the potential to achieve the highest therapeutic efficiency in accordance with minimal adverse effects. Therefore, several studies have been performed utilizing antigen-presenting cells specifically engineered to deplete allo- or antigen-specific T cells ('guided missiles'). Many of these strategies take advantage of the Fas/Fas ligand signaling pathway to efficiently induce antigen-presenting cell-mediated apoptosis in targeted T cells. In this article, we discuss the advantages and shortcomings of a novel non-cell-based 'killer artificial antigen-presenting cell' strategy, developed to overcome obstacles related to current cell-based approaches for the treatment of T-cell-mediated autoimmunity.

Induction of pathogenic cytotoxic T lymphocyte tolerance by dendritic cells: a novel therapeutic target

IMPORTANCE OF THE FIELD

Dendritic cells (DCs) have an important role, both direct and indirect, in controlling the expansion and function of T cells. Of the different subsets of T cells, cytotoxic T lymphocytes (CTLs/CD8(+) T cells) have been implicated in the pathogenesis and development of many diseases, including various forms of autoimmunity and transplant rejection. It may therefore be of therapeutic benefit to control the function of CTL in order to modulate disease processes and to ameliorate disease symptoms. Currently, pharmacological approaches have been employed to either directly or indirectly modulate the function of T cells. However, these treatment strategies have many limitations. Many experimental data have suggested that it is possible to alter CTL activity through manipulation of DC.

AREAS COVERED IN THIS REVIEW

Novel strategies that condition DCs to influence disease outcome through manipulation of CTL activity, both directly and indirectly. This includes the modulation of co-stimulation, negative co-stimulation, as well as manipulation of the cytokine milieu during CTL generation. Furthermore, DCs may also impact CTL activity through effects on effector and regulatory cells, along with manipulation of bioenergetic regulation, apoptotic-cell mediated tolerance and through the generation of exosomes. The implications of related interventions in the clinical arena are in turn considered.

WHAT THE READER WILL GAIN

Insight into such indirect methods of controlling CTL activity allows for an understanding of how disease-specific T cells may be regulated, while also sparing other aspects of adaptive immunity for normal physiological function. Such an approach towards the treatment of disease represents an innovative therapeutic target in the clinical arena.

TAKE HOME MESSAGE

There are numerous innovative methods for using DCs to control CTL responses. Manipulation of this interaction is thus an attractive avenue for the treatment of disease, particularly those of immune dysregulation, such as seen in autoimmunity and transplantation. With the number of studies moving into clinical stages constantly increasing, further advances and successes in this area are inevitable.

Tumor necrosis factor receptors support murine hematopoietic progenitor function in the early stages of engraftment

Tumor necrosis factor (TNF) family receptors/ligands are important participants in hematopoietic homeostasis, in particular as essential negative expansion regulators of differentiated clones. As a prominent injury cytokine, TNF-alpha has been traditionally considered to suppress donor hematopoietic stem and progenitor cell function after transplantation. We monitored the involvement of TNF receptors (TNF-R) 1 and 2 in murine hematopoietic cell engraftment and their inter-relationship with Fas. Transplantation of lineage-negative (lin(-)) bone marrow cells (BMC) from TNF receptor-deficient mice into wild-type recipients showed defective early engraftment and loss of durable hematopoietic contribution upon recovery of host hematopoiesis. Consistently, cells deficient in TNF receptors had reduced competitive capacity as compared to wild-type progenitors. The TNF receptors were acutely upregulated in bone marrow (BM)-homed donor cells (wild-type) early after transplantation, being expressed in 60%-75% of the donor cells after 6 days. Both TNF receptors were detected in fast cycling, early differentiating progenitors, and were ubiquitously expressed in the most primitive progenitors with long-term reconstituting potential (lin(-)c-kit(+) stem cell antigen (SCA)-1(+)). BM-homed donor cells were insensitive to apoptosis induced by TNF-alpha and Fas-ligand and their combination, despite reciprocal inductive cross talk between the TNF and Fas receptors. The engraftment supporting effect of TNF-alpha is attributed to stimulation of progenitors through TNF-R1, which involves activation of the caspase cascade. This stimulatory effect was not observed for TNF-R2, and this receptor did not assume redundant stimulatory function in TNFR1-deficient cells. It is concluded that TNF-alpha plays a tropic role early after transplantation, which is essential to successful progenitor engraftment.

The haemopoietic stem cell: between apoptosis and self renewal

Self renewal and apoptosis of haemopoietic stem cells (HSC) represent major factors that determine the size of the haemopoietic cell mass. Changes in self renewal above or below the steady state value of 0.5 will result in either bone marrow expansion or aplasia, respectively. Despite the growing body of research that describes the potential role of HSC, there is still very little information on the mechanisms that govern HSC self renewal and apoptosis. Considerable insight into the role of HSC in many diseases has been gained in recent years. In light of their crucial importance, this article reviews recent developments in the understanding of the molecular, biological, and physiological characteristics of haemopoietic stem cells.

Induction of tolerance to cardiac allografts using donor splenocytes engineered to display on their surface an exogenous fas ligand protein

The critical role played by Fas ligand (FasL) in immune homeostasis renders this molecule an attractive target for immunomodulation to achieve tolerance to auto- and transplantation Ags. Immunomodulation with genetically modified cells expressing FasL was shown to induce tolerance to alloantigens. However, genetic modification of primary cells in a rapid, efficient, and clinically applicable manner proved challenging. Therefore, we tested the efficacy of donor splenocytes rapidly and efficiently engineered to display on their surface a chimeric form of FasL protein (SA-FasL) for tolerance induction to cardiac allografts. The i.p. injection of ACI rats with Wistar-Furth rat splenocytes displaying SA-FasL on their surface resulted in tolerance to donor, but not F344 third-party cardiac allografts. Tolerance was associated with apoptosis of donor reactive T effector cells and induction/expansion of CD4(+)CD25(+)FoxP3(+) T regulatory (Treg) cells. Treg cells played a critical role in the observed tolerance as adoptive transfer of sorted Treg cells from long-term graft recipients into naive unmanipulated ACI rats resulted in indefinite survival of secondary Wistar-Furth grafts. Immunomodulation with allogeneic cells rapidly and efficiently engineered to display on their surface SA-FasL protein provides an effective and clinically applicable means of cell-based therapy with potential application to regenerative medicine, transplantation, and autoimmunity.

Capsular polysaccharide induction of apoptosis by intrinsic and extrinsic mechanisms

A purified microbial capsular polysaccharide of Cryptococcus neoformans, glucuronoxylomannan (GXM), induces Fas ligand (FasL) upregulation on macrophages and, as a consequence, apoptosis of lymphocytes. The mechanisms that lead to lymphocyte apoptosis in both in vitro and in vivo systems were investigated by cytofluorimetric analysis and Western blotting experiments. Caspase 8 cleaves caspase 3 in two different pathways: directly as well as indirectly by activation of Bcl-2 interacting domain, which initiates caspase 9 cleavage. Therefore, the caspase 8 and caspase 9 pathways cooperate in an amplification loop for efficient cell death, and noteworthily we provide evidence that they are both activated in one single cell. Furthermore, both activation of GXM-mediated caspase 8 and apoptosis were also found in in vivo systems in an experimental model of murine candidiasis. Collectively, our data show that GXM-induced apoptosis involves, in a single cell, a cross-talk between extrinsic and intrinsic pathways. Such a finding offers opportunities for the therapeutic usage of this polysaccharide in appropriate clinical settings for taming T-cell responses.

Expression of CD95 on mature leukocytes of MRL/lpr mice after transplantation of genetically modified bone marrow stem cells

Bone marrow transplantation (BMT) is commonly used for the treatment of severe haematological and immunological diseases. For instance, the autoimmune lymphoproliferative syndrome (ALPS) caused by a complete expression defect of CD95 (Fas, APO-1) can be cured by allogeneic BMT. However, since this therapy may not generate satisfactory results when only partially compatible donors are available, we were interested in the development of a potential alternative treatment by using lentiviral gene transfer of a normal copy of CD95 cDNA in hematopoietic stem cells. Here, we show that this approach applied to MRL/lpr mice results in the expression of functional CD95 receptors on the surface of lymphocytes, monocytes, and granulocytes. This suggests that correction of CD95 deficiency can be achieved by gene therapy.

Killer artificial antigen-presenting cells: a novel strategy to delete specific T cells

Several cell-based immunotherapy strategies have been developed to specifically modulate T cell-mediated immune responses. These methods frequently rely on the utilization of tolerogenic cell-based antigen-presenting cells (APCs). However, APCs are highly sensitive to cytotoxic T-cell responses, thus limiting their therapeutic capacity. Here, we describe a novel bead-based approach to modulate T-cell responses in an antigen-specific fashion. We have generated killer artificial APCs (kappaaAPCs) by coupling an apoptosis-inducing alpha-Fas (CD95) IgM mAb together with HLA-A2 Ig molecules onto beads. These kappaaAPCs deplete targeted antigen-specific T cells in a Fas/Fas ligand (FasL)-dependent fashion. T-cell depletion in cocultures is rapidly initiated (30 minutes), dependent on the amount of kappaaAPCs and independent of activation-induced cell death (AICD). kappaaAPCs represent a novel technology that can control T cell-mediated immune responses, and therefore has potential for use in treatment of autoimmune diseases and allograft rejection.

Expression of Fas and Fas-ligand in donor hematopoietic stem and progenitor cells is dissociated from the sensitivity to apoptosis

OBJECTIVE

The interaction between the Fas receptor and its cognate ligand (FasL) has been implicated in the mutual suppression of donor and host hematopoietic cells after transplantation. Following the observation of deficient early engraftment of Fas and FasL-defective donor cells and recipients, we determined the role of the Fas-FasL interaction.

METHODS

Donor cells were recovered after syngeneic (CD45.1-->CD45.2) transplants from various organs and assessed for expression of Fas/FasL in reference to lineage markers, carboxyfluorescein succinimidyl ester dilution, Sca-1 and c-kit expression. Naïve and bone marrow-homed cells were challenged for apoptosis ex vivo.

RESULTS

The Fas receptor and ligand were markedly upregulated to 40% to 60% (p < 0.001 vs 5-10% in naïve cells) within 2 days after syngeneic transplantation, while residual host cells displayed modest and delayed upregulation of these molecules ( approximately 10%). All lin(-)Sca(+)c-kit(+) cells were Fas(+)FasL(+), including 95% of Sca-1(+) and 30% of c-kit(+) cells. Fas and FasL expression varied in donor cells that homed to bone marrow, spleen, liver and lung, and was induced by interaction with the stroma, irradiation, cell cycling, and differentiation. Bone marrow-homed donor cells challenged with supralethal doses of FasL were insensitive to apoptosis (3.2% +/- 1% vs 38% +/- 5% in naïve bone marrow cells), and engraftment was not affected by pretransplantation exposure of donor cells to an apoptotic challenge with FasL.

CONCLUSION

There was no evidence of Fas-mediated suppression of donor and host cell activity after transplantation. Resistance to Fas-mediated apoptosis evolves as a functional characteristic of hematopoietic reconstituting stem and progenitor cells, providing them competitive engraftment advantage over committed progenitors.

Fas transduces dual apoptotic and trophic signals in hematopoietic progenitors

Stem cells and progenitors are often required to realize their differentiation potential in hostile microenvironments. The Fas/Fas ligand (FasL) interaction is a major effector pathway of apoptosis, which negatively regulates the expansion of differentiated hematopoietic cells. The involvement of this molecular interaction in the function of hematopoietic stem and progenitor cells is not well understood. In the murine syngeneic transplant setting, both Fas and FasL are acutely upregulated in bone marrow-homed donor cells; however, the Fas(+) cells are largely insensitive to FasL-induced apoptosis. In heterogeneous populations of lineage-negative (lin(-)) bone marrow cells and progenitors isolated by counterflow centrifugal elutriation, trimerization of the Fas receptor enhanced the clonogenic activity. Inhibition of caspases 3 and 8 did not affect the trophic signals mediated by Fas, yet it efficiently blocked the apoptotic pathways. Fas-mediated tropism appears to be of physiological significance, as pre-exposure of donor cells to FasL improved the radioprotective qualities of hematopoietic progenitors, resulting in superior survival of myeloablated hosts. Under these conditions, the activity of long-term reconstituting cells was not affected, as determined in sequential secondary and tertiary transplants. Dual caspase-independent tropic and caspase-dependent apoptotic signaling place the Fas receptor at an important junction of activation and death. This regulatory mechanism of hematopoietic homeostasis activates progenitors to promote the recovery from aplasia and converts into a negative regulator in distal stages of cell differentiation. Disclosure of potential conflicts of interest is found at the end of this article.

Fas ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and nonimmunogenic interactions

Early after transplantation, donor lineage-negative bone marrow cells (lin(-) BMC) constitutively upregulated their expression of Fas ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a noncleavable FasL chimeric protein on the surface of donor lin(-) BMC. Despite a short life span of the protein in vivo, expression of FasL on the surface of all the donor lin(-) BMC improved the efficiency of engraftment twofold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas(-) stroma and Fas(+) BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis and thus can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of engraftment. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and nonimmunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect. Disclosure of potential conflicts of interest is found at the end of this article.

Ex Vivo Soluble Fas Ligand Treatment of Donor Cells to Selectively Reduce Murine Acute Graft Versus Host Disease

BACKGROUND

Allogeneic bone marrow transplantation (BMT) and donor lymphocyte infusion (DLI) provide valuable treatments for a range of diseases. However, the therapeutic utility of BMT and DLI is reduced by the high incidence of graft-versus-host disease (GvHD) mediated by activated donor T lymphocytes directed against recipient alloantigens.

METHODS

Using mouse models, we developed and evaluated a strategy to selectively enhance activation-induced cell death (AICD) of anti-recipient T cells within transplant donor cell populations, with the goal of reducing GvHD. Responder T lymphocytes were incubated ex vivo with irradiated allogenic stimulator cells in a mixed lymphocyte reaction (MLR) in the presence of soluble Fas ligand (sFasL) to induce AICD in alloreactive cells.

RESULTS

This ex vivo sFasL treatment reduced proliferation to the allogeneic stimulator cells in vitro and abrogated acute GvHD capacity in vivo. In contrast, the secondary immune responsiveness of the ex vivo sFasL-treated responder T cells to an unrelated model antigen was preserved. Furthermore, upon adoptive transfer in a DLI model, ex vivo sFasL-treated T cells were able to reject a model tumor. Finally, ex vivo sFasL treatment of bone marrow cells did not reduce their hematopoietic engraftment capacity.

CONCLUSIONS

Thus, ex vivo treatment with sFasL appears to have potential for translation to clinical cell processing of BMT allografts and DLI infusions.

Fas-ligand-expressing adenovirus-transfected dendritic cells decrease allergen-specific T cells and airway inflammation in a murine model of asthma

T cells expressing a type-2 T helper profile of cytokines (Th2 cells) have been demonstrated to play an important role in the initiation and progression of allergic asthma, and it is well known that Fas ligand (FasL) induces apoptosis when bound to its receptor, Fas. In the present study, we examined the possibility of modulating asthma manifestations by dendritic cells (DCs) genetically engineered to express FasL (DC-FasL), which could deliver a death signal to T cells in an antigen-specific manner. The delivery of DC-FasL into ovalbumin (OVA)-immunized allergic mice decreased the airway hyper-responsiveness (AHR). Moreover, we established a mouse model of airway inflammation by using an adoptive transfer of Th2 cells derived from ovalbumin T cell receptor transgenic mice to study the effect of DC-FasL on airway reactivity. The administration of DC-FasL in Th2-cell-induced allergic mice had significantly decreased AHR, airway inflammation, and IL-4, IL-5 and IL-13 production. Furthermore, the numbers of OVA-specific T cells were decreased in the lung of mice receiving DC-FasL. These results demonstrate that FasL-expressing dendritic cells might be applied for the modulation of allergic responses.

Induction of tolerance using Fas ligand: a double-edged immunomodulator

Apoptosis mediated by Fas ligand (FasL) interaction with Fas receptor plays a pivotal regulatory role in immune homeostasis, immune privilege, and self-tolerance. FasL, therefore, has been extensively exploited as an immunomodulatory agent to induce tolerance to both autoimmune and foreign antigens with conflicting results. Difficulties associated with the use of FasL as a tolerogenic factor may arise from (1) its complex posttranslational regulation, (2) the opposing functions of different forms of FasL, (3) different modes of expression, systemic versus localized and transient versus continuous, (4) the level and duration of expression, (5) the sensitivity of target tissues to Fas/FasL-mediated apoptosis and the efficiency of antigen presentation in these tissues, and (6) the types and levels of cytokines, chemokines, and metalloproteinases in the extracellular milieu of the target tissues. Thus, the effective use of FasL as an immunomodulator to achieve durable antigen-specific immune tolerance requires careful consideration of all of these parameters and the design of treatment regimens that maximize tolerogenic efficacy, while minimizing the non-tolerogenic and toxic functions of this molecule. This review summarizes the current status of FasL as a tolerogenic agent, problems associated with its use as an immunomodulator, and new strategies to improve its therapeutic potential.

Fas Ligand as a Tool for Immunosuppression and Generation of Immune Tolerance

The role of Fas ligand (FasL) in physiologically limiting immune responses and maintaining immune-privileged sites has led to a body of research aiming to confer protection to allogeneic grafts by expressing FasL on the allogeneic tissue or by administrating FasL-transduced donor dendritic cells. In addition, several studies have used FasL to abrogate autoimmune responses. This review presents the results of these studies and discusses the problems associated with FasL usage.

Effector mechanisms in transplant rejection

Antigens, provided by the allograft, trigger the activation and proliferation of allospecific T cells. As a consequence of this response, effector elements are generated that mediate graft injury and are responsible for the clinical manifestations of allograft rejection. Donor-specific CD8+ cytotoxic T lymphocytes play a major role in this process. Likewise, CD4+ T cells mediate delayed-type hypersensitivity responses via the production of soluble mediators that function to further activate and guide immune cells to the site of injury. In addition, these mediators may directly alter graft function by modulating vascular tone and permeability or by promoting platelet aggregation. Allospecific CD4+ T cells also promote B-cell maturation and differentiation into antibody-secreting plasma cells via CD40-CD40 ligand interactions. Alloantibodies that are produced by these B cells exert most of their detrimental effects on the graft by activating the complement cascade. Alternatively, antibodies can bind Fc receptors on natural killer cells or macrophages and cause target cell lysis via antibody-dependent cell-mediated cytotoxicity. In this review, we discuss these major effector pathways, focusing on their role in the pathogenesis of allograft rejection.

Treatment of polycythemia vera and essential thrombocythemia: the role of pipobroman

Pipobroman (PB) is a neutral amide of piperazine with a chemical structure close to that of alkylating agents, although the exact mechanism of action of PB has not been demonstrated. PB has well documented clinical activity in polycythemia vera (PV) and essential thrombocythemia (ET). Recent long-term follow-up studies on PV and ET patients receiving PB have facilitated the definition of the risk of late transformation into myelofibrosis with myeloid metaplasia (MMM) or acute leukemia (AL). This report gives an overview of the treatment with PB in patients with PV and ET focusing on clinical activity, administration dose and schedule, toxicity, impact on short- and long-term complications. From our experience and from the data reported in the literature the high clinical activity of PB in both PVand ET becomes evident. This drug allows, within 3 months, to attain a response in more than 90% of patients, without clinically relevant toxicities. The 10-years risk of thrombosis of patients treated with PB is about 15%, similar to that registered with hydroxyurea, the most widely used agent in PVand ET. The antiproliferative activity of PB on bone marrow megakaryocytes seems of particular value in lowering the occurrence of post-PV and post-ET MMM, whose risk (< 4% at 10 years) is the lowest registered with available treatments. The 10-year risk of acute leukemia with PB is 5% in PVand 3% in ET, which is only slightly higher than that expected as a natural evolution of the disease. In conclusion, the use of PB is a definite alternative to hydroxyurea in patients with PV and ET at high risk of thrombosis.

Elimination of activated but not resting primary human CD4 and CD8 T cells by Fas ligand (FasL/CD95L)-expressing Killer-dendritic cells

Dendritic cells (DC) genetically engineered to express high levels of Fas ligand (FasL/CD95L) have been demonstrated to delete T cells in an antigen specific manner in several different animal models in vivo. However, the immunomodulatory capacity of primary human FasL-expressing Killer-DC has not been determined. Therefore, human Killer-DC were generated from mature monocyte-derived DC using the inducible CRE/LoxP adenoviral vector system, and the immunoregulatory capacity of these cells was analyzed in cocultures with primary human T cells in vitro. Combined transductions of DC by AdloxPFasL and AxCANCre resulted in FasL expression in > 70% of DC without affecting the mature phenotype. Proliferation of activated primary human T cells was inhibited up to 80% in cocultures with FasL-expressing DC but not EGFP-transduced DC, which was due to induction of apoptosis in activated but not resting CD4+ and CD8+ T cells. Apoptosis induced by Killer-DC could be blocked by an anti-FasL-antibody in a dose dependent fashion. The present results demonstrate that FasL-expressing Killer-DC eliminate activated but not resting primary human CD4+ and CD8+ T cells by induction of Fas-mediated apoptosis supporting the concept to apply Killer-DC as a novel strategy for the treatment of T cell-dependent autoimmune disease and allograft rejection in humans.

Xenografts are an achievable breakthrough

The objective of this communication is to show that pig-to-human organ transplantation could be feasible through genetic engineering. By introducing into donor pigs several different tolerance promoting genetic modifications there can be a synergistic effect to produce extended tolerance for xenografted organs in human recipients. Nuclear-transfer cloning allows production of pigs with knockout mutations in the galactose-alpha-1,3-galactosyl transferase gene, in principle eliminating hyperacute rejection. Once hyperacute rejection is circumvented, long-term tolerance of xenografted organs should be possible through a combination of transgenic immunomodulating molecule, bone marrow chimerism and short to intermediate term use of immunosuppressive drugs. If immunomodulating transgenes are deleterious during pig development, inducible cre-recombinase excision of stop codons provides a means to delay expression of such transgenes until after transplantation. Zoonotic diseases can be circumvented via pathogen-free colonies and additional knockout mutations to disable porcine endogenous retrovirus and prion disease. Thus, there is now a technical and theoretical framework for serious efforts at cross-species transplantation.

Tracking death dealing by Fas and TRAIL in lymphatic neoplastic disorders: pathways, targets, and therapeutic tools

In the past decade, it was concluded from a number of investigations that death domain-containing members of the tumor necrosis factor-receptor (TNF-R) family and their ligands such as Fas/FasL and TNF-related apoptosis-inducing ligand (TRAIL)-R/TRAIL are essential for maintaining an intact immune system for surveillance against infection and cancer development and that nondeath domain-containing members such as CD30 or CD40 are involved in the fine tuning of this system during the selection process of the lymphatic system. In line with this conclusion are the observations that alterations in structure, function, and regulation of these molecules contribute to autoimmunity and cancer development of the lymphoid system. Besides controlling size and function of the lymphoid cell pool, Fas/FasL and TRAIL-R/TRAIL regulate myelopoiesis and the dendritic cell functions, and severe alterations of these lineages during the outgrowth and expansion of the lymphoid tumors have been reported. It is the aim of this review to summarize what is currently known about the complex role of these two death receptor/ligand systems in normal, disturbed, and neoplastic hemato-/lymphopoiesis and to point out how such knowledge can be used in developing novel, therapeutic options and the problems that will have to be faced along the way.

Immunotherapy of established tumors using bone marrow transplantation with antigen gene--modified hematopoietic stem cells

A major focus of cancer immunotherapy is to develop strategies to induce T-cell responses through presentation of tumor antigens by dendritic cells (DCs). Current vaccines are limited in their ability to efficiently transfer antigens to DCs in vivo. Ex vivo-generated DCs can be efficiently loaded with antigen but after reinjection, few DCs traffic to secondary lymphoid organs, the critical sites for antigen presentation. To enhance efficiency and durability of antigen presentation by DCs, we transduced hematopoietic stem-progenitor cells (HSCs) with a model tumor antigen and then transplanted the gene-modified cells into irradiated recipient mice, which resulted in efficient expression of the transgene in a large proportion of donor derived DCs in lymphoid organs. The combination of bone marrow transplantation (BMT) using transduced HSCs, systemic agents that generate and activate DCs, and mature T-cell infusion resulted in substantial expansion and activation of antigen-specific T cells. This tripartite strategy provided potent antigen-specific immunotherapy for an aggressive established tumor.

Mature but not immature Fas ligand (CD95L)-transduced human monocyte-derived dendritic cells are protected from Fas-mediated apoptosis and can be used as killer APC

Several in vitro and animal studies have been performed to modulate the interaction of APCs and T cells by Fas (CD95/Apo-1) signaling to delete activated T cells in an Ag-specific manner. However, due to the difficulties in vector generation and low transduction frequencies, similar studies with primary human APC are still lacking. To evaluate whether Fas ligand (FasL/CD95L) expressing killer APC could be generated from primary human APC, monocyte-derived dendritic cells (DC) were transduced using the inducible Cre/Loxp adenovirus vector system. Combined transduction of DC by AdLoxpFasL and AxCANCre, but not single transduction with these vectors, resulted in dose- and time-dependent expression of FasL in >70% of mature DC (mDC), whereas <20% of immature DC (iDC) expressed FasL. In addition, transduction by AdLoxpFasL and AxCANCre induced apoptosis in >80% of iDC, whereas FasL-expressing mDC were protected from FasL/Fas (CD95/Apo-1)-mediated apoptosis despite coexpression of Fas. FasL-expressing mDC eliminated Fas(+) Jurkat T cells as well as activated primary T cells by apoptosis, whereas nonactivated primary T cells were not deleted. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC and cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC, cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. The present results demonstrate that FasL-expressing killer APC can be generated from human monocyte-derived mDC using adenoviral gene transfer. Our results support the strategy to use killer APCs as immunomodulatory cells for the treatment of autoimmune disease and allograft rejection.

Lentiviral vectors with two independent internal promoters transfer high-level expression of multiple transgenes to human hematopoietic stem-progenitor cells

Lentiviral vectors (LVs) offer several advantages over traditional oncoretroviral vectors. LVs efficiently transduce slowly dividing cells, including hematopoietic stem-progenitor cells (HSCs), resulting in stable gene transfer and expression. Additionally, recently developed self-inactivating (SIN) LVs allow promoter-specific transgene expression. For many gene transfer applications, transduction of more than one gene is needed. We obtained inconsistent results in our attempts to coexpress two transgenes linked by an internal ribosomal entry site (IRES) element in a single bicistronic LV transcript. In more than six bicistronic LVs we constructed containing a gene of interest followed by an IRES and the GFP reporter gene, GFP fluorescence was undetectable in transduced cells. We therefore investigated how to achieve consistent and efficient coexpression of two transgenes by LVs. In a SIN LV containing the elongation factor 1alpha promoter, we included a second promoter from cytomegalovirus, the phosphoglycerate kinase gene, or the HLA-DRalpha gene. Using a single LV containing two constitutive promoters, we achieved strong and sustained expression of both transgenes in transduced engrafting CD34(+) HSCs and their progeny, as well as in other human cell types. Thus, such dual-promoter LVs can coexpress multiple transgenes efficiently in a single target cell and will enable many gene transfer applications.