Human CD34+ hematopoietic stem/progenitor cells express high levels of FLIP and are resistant to Fas-mediated apoptosis

Hematopoietic stem and progenitor cell signaling in the niche

Hematopoietic stem and progenitor cells (HSPCs) are responsible for lifelong maintenance of hematopoiesis through self-renewal and differentiation into mature blood cell lineages. Traditional models hold that HSPCs guard homeostatic function and adapt to regenerative demand by integrating cell-autonomous, intrinsic programs with extrinsic cues from the niche. Despite the biologic significance, little is known about the active roles HSPCs partake in reciprocally shaping the function of their microenvironment. Here, we review evidence of signals emerging from HSPCs through secreted autocrine or paracrine factors, including extracellular vesicles, and via direct contact within the niche. We also discuss the functional impact of direct cellular interactions between hematopoietic elements on niche occupancy in the context of leukemic infiltration. The aggregate data support a model whereby HSPCs are active participants in the dynamic adaptation of the stem cell niche unit during development and homeostasis, and under inflammatory stress, malignancy, or transplantation.

Brief ex vivo Fas-ligand incubation attenuates GvHD without compromising stem cell graft performance

Graft versus host disease (GvHD) remains a limiting factor for successful hematopoietic stem cell transplantation (HSCT). T cells and antigen-presenting cells (APCs) are major components of the hematopoietic G-CSF mobilized peripheral blood cell (MPBC) graft. Here we show that a short incubation (2 h) of MPBCs with hexameric Fas ligand (FasL) selectively induces apoptosis of specific donor T cell subsets and APCs but not of CD34⁺ cells. FasL treatment preferentially induces apoptosis in mature T cell subsets which express high levels of Fas (CD95), such as T stem cell memory, T central memory, and T effector memory cells, as well as T_H1 and T_H17 cells. Anti-CD3/CD28 stimulated T cells derived from FasL-treated-MPBCs express lower levels of CD25 and secrete lower levels of IFN-γ as compared to control cells not treated with FasL. FasL treatment also induces apoptosis of transitional, naïve, memory and plasmablastoid B cells leading to a reduction in their numbers in the graft and following engraftment in transplanted mice. Most importantly, ex vivo treatment of MPBCs with FasL prior to transplant in conditioned NOD-scid IL2Rγ^null (NSG) mice prevented GvHD while preserving graft versus leukemia (GvL) effects, and leading to robust stem cell engraftment.

Role of Extrinsic Apoptotic Signaling Pathway during Definitive Erythropoiesis in Normal Patients and in Patients with β-Thalassemia

Apoptosis is a process of programmed cell death which has an important role in tissue homeostasis and in the control of organism development. Here, we focus on information concerning the role of the extrinsic apoptotic pathway in the control of human erythropoiesis. We discuss the role of tumor necrosis factor α (TNFα), tumor necrosis factor ligand superfamily member 6 (FasL), tumor necrosis factor-related apoptosis-inducing (TRAIL) and caspases in normal erythroid maturation. We also attempt to initiate a discussion on the observations that mature erythrocytes contain most components of the receptor-dependent apoptotic pathway. Finally, we point to the role of the extrinsic apoptotic pathway in ineffective erythropoiesis of different types of β-thalassemia.

Protective effects of cytokine combinations against the apoptotic activity of glucocorticoids on CD34+ hematopoietic stem/progenitor cells

Haematopoietic stem cells can self-renew and produce progenitor cells, which have a high proliferation capacity. Chemotherapeutic drugs are toxic to normal cells as well as cancer cells, and glucocorticoids (GCs), which are essential drugs for many chemotherapeutic protocols, efficiently induce apoptosis not only in malignant cells but also in normal haematopoietic cells. Studies have shown that haematopoietic cytokines can prevent the apoptosis induced by chemotherapy and decrease the toxic effects of these drugs. However, the apoptosis induction mechanism of GCs in CD34⁺ haematopoietic cells and the anti-apoptotic effects of cytokines have not been well elucidated. In this study, we investigated the apoptotic effects of GCs on CD34⁺, a haematopoietic stem/progenitor cell (HSPC) population, and demonstrated the protective effects of haematopoietic cytokines. We used a cytokine cocktail containing early-acting cytokines, namely, interleukin-3 (IL-3), thrombopoietin, stem cell factor and flt3/flk2 ligand, and dexamethasone and prednisolone were used as GCs. Apoptotic mechanisms were assessed by immunohistochemical staining and quantified using H-scoring. Dexamethasone and prednisolone induced apoptosis in CD34⁺ HSPCs. GC treatment caused a significant increase in apoptotic Fas, caspase-3, cytochrome c and Bax, but a significant decrease in anti-apoptotic Bcl-2. Furthermore, as expected, cytokines caused a significant decrease in all apoptotic markers and a significant increase in Bcl-2. Thus, our findings suggest that CD34⁺ HSPCs are an extremely sensitive target for GCs and that cytokines protect these cells from GC-induced apoptosis.

Inhibition of Apoptosis by Expression of Antiapoptotic Proteins in Recombinant Human Keratinocytes

The Fas ligand/Fas interaction plays an important role in the regulation of immune responses. Allografted cells undergo Fas-mediated apoptosis induced by CD8+ T cells. Our objective was to prevent human keratinocytes from immunologically induced apoptosis. We focused on three proteins with inhibitory function on Fas-mediated apoptosis. Human keratinocytes were transfected with either Flip, Faim, or Lifeguard (LFG). The treatment proved to be practicable and efficient. The recombinant keratinocytes with expression of our target proteins were cocultured with CD8+ T cells and the apoptotic activity was then evaluated. Activation of caspase-8 was detectable in control but not in the recombinant cells. Quantitative analysis revealed significant induction of T-cell-induced apoptosis in nontransfected keratinocytes (p = 0.04, n = 12) but not in Flip (p = 0.66), Faim (p = 0.42), or LFG (p = 0.44) expressing cells. Our results suggest that heterotopic expression of antiapoptotic proteins can induce the resistance of keratinocytes to a major mechanism of rejection.

T11TS repress gliomagenic apoptosis of bone marrow hematopoietic stem cells

Combating gliomagenic global immunosuppression is one of the emerging key for improving prognosis in malignant glioma. Apoptosis plays a pivotal role within the adult hematopoietic system particularly in regulating the cells of immune system. Gliomagenic regulation of apoptotic mediators within bone marrow milieu has not been elucidated. We previously demonstrated that administration of membrane glycopeptides T11 target structure (T11TS) not only rejuvenate bone marrow hematopoietic stem cells (BMHSCs) from glioma mediated hibernation by inhibiting gliomagenic overexpression of Ang-1/Tie-2 but also stimulate glioma mediated diminution of expression CD34, c-kit, and Sca-1 markers. In the present study, we investigated the impact of glioma on apoptotic signaling cascades of BMHSCs and consequences following T11TS therapy. Bone marrow smear and Annexin V staining confirm gliomagenic acceleration of apoptotic fate of BMHSCs whereas T11TS treatment in glioma-bearing rats disrupted apoptosis of BMHSCs. Flowcytometry, immunoblotting, and immunofluorescence imagining results revealed multi potent T11TS not only significantly downregulates gliomagenic overexpression of Fas, Fas L, Bid, and caspase-8, the pro-apoptotic extrinsic mediators but also strongly inhibits cytosolic release of cytochrome-c, Apf-1, and Bax to deactivate gliomagenic caspase-9, 3 the key intrinsic apoptotic mediators followed by up modulation of anti-apoptotic Bcl-2 in glioma associated HSCs. T11TS is also able to diminish the perforin-granzyme B mediated apoptotic verdict of BMHSCs during gliomagenesis. The anti-apoptotic action of T11TS on glioma associated BMHSCs provide a crucial insight into how T11TS exerts its immunomodulatory action against glioma mediated immune devastation.

Characteristics of human CD34+ cells exposed to ionizing radiation under cytokine-free conditions

To clarify the mechanisms underlying radiation-induced hematopoietic stem cell death, we investigated the effects of excessive ionizing radiation on the clonogenic potential of CD34(+) cells obtained from human umbilical cord blood under cytokine-free conditions. The CD34(+) cells were X-ray-irradiated (up to 2 Gy) and were cultured for 0-48 h under cytokine-free conditions. At various time-points, the CD34(+) cells were investigated for survival, clonogenic potential and the generation of mitochondrial superoxide. At 12 h after X-ray irradiation, the number of viable cells had decreased to ∼70-80% compared with the 0-h non-irradiated control, whereas the clonogenic potential in the X-ray-irradiated cells had decreased to ∼50%-60% compared with the 0-h non-irradiated control. Furthermore, significant generation of mitochondrial superoxide was observed at 6 h, and reached a maximum value between 12 and 24 h after X-ray irradiation. However, no significant differences were observed between non-irradiated and X-ray-irradiated cells in terms of the generation of reactive oxygen species or in the intracellular mitochondrial contents. In addition, a cDNA microarray analysis showed that the majority of the altered genes in the CD34(+) cells at 6 h after X-ray irradiation were apoptosis-related genes. These results suggest the possibility that the elimination of the clonogenic potentials of CD34(+) cells involves the generation of mitochondrial superoxide induced by ionizing radiation.

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.

TNF-α has tropic rather than apoptotic activity in human hematopoietic progenitors: involvement of TNF receptor-1 and caspase-8

Tumor necrosis factor-α (TNF-α) has been suggested to exert detrimental effects on hematopoietic progenitor function that might limit the success of transplants. In this study, we assessed the influences of TNF-α and its two cognate receptors on the function of fresh umbilical cord blood (UCB) and cryopreserved mobilized peripheral blood (mPB). CD34(+) progenitors from both sources are less susceptible to spontaneous apoptosis than lineage-committed cells and are not induced into apoptosis by TNF-α. Consequently, the activity of UCB-derived severe combined immune deficiency (SCID) reconstituting cells and long-term culture-initiating cells is unaffected by this cytokine. On the contrary, transient exposure of cells from both sources to TNF-α stimulates the activity of myeloid progenitors, which persists in vivo in UCB cell transplants. Progenitor stimulation is selectively mediated by TNF-R1 and involves activation of caspase-8, without redundant activity of TNF-R2. Despite significant differences between fresh UCB cells and cryopreserved mPB cells in susceptibility to apoptosis and time to activation, TNF-α is primarily involved in tropic signaling in hematopoietic progenitors from both sources. Cytokine-mediated tropism cautions against TNF-α neutralization under conditions of stress hematopoiesis and may be particularly beneficial in overcoming the limitations of UCB cell transplants.

A novel potent Fas agonist for selective depletion of tumor cells in hematopoietic transplants

There remains a clear need for effective tumor cell purging in autologous stem cell transplantation (ASCT) where residual malignant cells within the autograft contribute to disease relapse. Here we propose the use of a novel Fas agonist with potent pro-apoptotic activity, termed MegaFasL, as an effective ex-vivo purging agent. MegaFasL selectively kills hematological cancer cells from lymphomas and leukemias and prevents tumor development at concentrations that do not reduce the functional capacity of human hematopoietic stem/progenitor cells both in in vitro and in in vivo transplantation models. These findings highlight the potential use of MegaFasL as an ex-vivo purging agent in ASCT.

Flice inhibitory protein is associated with the survival of neonatal neutrophils

Neonatal polymorphonuclear leukocytes (PMN) exhibit delayed apoptosis both constitutively and under inflammatory conditions, and evidence has linked PMN longevity to the presence of antiapoptotic proteins. Activation of the survival-associated transcription factor, nuclear factor kappa B (NF-κB), promotes the synthesis of several antiapoptotic proteins including Flice inhibitory protein (FLIP). Neonatal and adult PMN were compared in this study to test the hypothesis that FLIP modulates age-related apoptosis. Expression of the short isoform, FLIP-S, was prominent at baseline and persisted during spontaneous apoptosis in neonatal PMN, whereas basal expression was lower and decreased under the same conditions in adult PMN. Stable FLIP-S expression in neonatal PMN was associated with a relative resistance to apoptosis in response to the protein synthesis inhibitor, cycloheximide (CHX), or the NF-κB inhibitor, gliotoxin. In contrast, similar treatment of adult PMN promoted greater overall apoptosis accompanied by FLIP degradation. Nuclear levels of phosphorylated p65, a critical NF-κB dimer, were relatively robust in neonatal PMN under basal conditions or after stimulation with TNF-α, a cytokine that induces FLIP. In conclusion, persistent FLIP-S expression is involved in the longevity of neonatal PMN, and our data suggest a contribution of NF-κB signaling and related survival mechanisms.

MARCH-I expression in cord blood CD34+KDR+ cells

Hematopoietic stem cells transplantation has been successfully used in the treatment of patients with hematological malignances. A better knowledge of the mechanisms beyond their ability to completely repopulate the entire hematopoietic system would help in the treatment of hematological diseases. For this reason we focused our studies on a cell population that has been demonstrated to have some peculiar characteristics among the stem cells: CD34+KDR+ cells. These cells, an extremely rare population among the CD34 (0.1%-0.5%) cells, have been demonstrated from different groups to have the potential to give rise to the hematopoietic and endothelial lineage. By a subtraction library approach we found different sequences more expressed in CD34+KDR+ than their CD34+KDR- counterpart. In particular, we found an open reading frame correspondent to a newly characterized E3 ligase, MARCH-I. This gene is part of a recently described family involved in immune response modulation through the proteosomal mediated degradation. MARCH-I expression in stem cells could be important for their intrinsic immune properties.

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.

C/EBPalpha or C/EBPalpha oncoproteins regulate the intrinsic and extrinsic apoptotic pathways by direct interaction with NF-kappaB p50 bound to the bcl-2 and FLIP gene promoters

CCAAT/enhancer-binding protein alpha (C/EBPalpha) is mutated in 10% of acute myeloid leukemias, resulting in either a truncated protein or an altered leucine zipper (C/EBPalphaLZ) that prevents DNA binding. C/EBPalpha induces bcl-2 in cooperation with nuclear factor-kappaB (NF-kappaB) p50 to inhibit apoptosis. We now demonstrate that C/EBPalpha or a C/EBPalphaLZ oncoprotein binds the bcl-2 P2 promoter in chromatin immunoprecipitation assays and induces the promoter dependent on the integrity of a kappaB site. C/EBPalpha expressed as a transgene in B cells binds and activates the bcl-2 promoter, but not in nfkb1-/- mice lacking NF-kappaB p50. Bcl-2 is central to the intrinsic apoptotic pathway, whereas FLICE inhibitory protein (FLIP) modulates caspase-8, the initiator caspase of the extrinsic pathway. C/EBPalpha and C/EBPalphaLZ also bind the FLIP promoter and induce its expression dependent upon NF-kappaB p50. Moreover, induction of FLIP by C/EBPalpha protects splenocytes from Fas ligand-induced apoptosis, but only if p50 is present. We also demonstrate the direct interaction between bacterially produced C/EBPalpha and NF-kappaB p50, mediated by the C/EBPalpha basic region. These findings indicate that C/EBPalpha or its oncoproteins activate the bcl-2 and FLIP genes by tethering to their promoters through bound NF-kappaB p50. Targeting their interaction may favor apoptosis of transformed cells.

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.

Life and death in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are defined as primitive cells that are capable of both self-renewal and differentiation into any of the hematopoietic cell lineages. HSC numbers need to be precisely regulated to maintain hematopoietic homeostasis. HSCs undergo several cell fate decisions, including decisions on life and death and self-renewal and differentiation, which have crucial roles in the regulation of their numbers and lifespan. Defects in these processes have been found to contribute to hematopoietic insufficiencies and the development of hematopoietic malignancies. Recent studies have begun to elucidate how HSCs make life and death decisions and the underlying molecular mechanisms involved, highlighting the importance of a balance between survival and death in the regulation of HSCs.

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.

Insulin down-regulates TRAIL expression in vascular smooth muscle cells both in vivo and in vitro

To dissect the effect of hyperinsulinemia versus hyperglycemia on TNF-related apoptosis inducing ligand (TRAIL) expression in the macrovascular district, we measured TRAIL mRNA and protein in four groups of animals: streptozotocin (SZT)-induced diabetic rats, vehicle-treated control animals, diabetic rats treated with insulin and non-diabetic rats treated with insulin. While the aortas of diabetic rats did not show significant differences in TRAIL expression with respect to vehicle-treated control animals, the aortas of both diabetic and non-diabetic rats treated in vivo for 16 days with insulin showed a significant decrease in TRAIL expression with respect to either diabetic and control rats. Moreover, in vitro treatment of both rat and human vascular smooth muscle cells (VSMC) with insulin induced the down-regulation of TRAIL protein. While the addition of recombinant TRAIL to rat VSMC promoted the dose-dependent release of bioactive nitric oxide (NO), this effect was significantly counteracted by pre-exposure of VSMC to insulin. These findings suggest that TRAIL might act as an endogenous regulator of the vascular tone and that chronic elevation of insulin might contribute to the vascular abnormalities characterizing type-2 diabetes mellitus by down-regulating TRAIL expression and activity.

HIV-1 negatively affects the survival/maturation of cord blood CD34(+) hematopoietic progenitor cells differentiated towards megakaryocytic lineage by HIV-1 gp120/CD4 membrane interaction

To investigate the mechanisms involved in the human immunodeficiency virus type 1 (HIV-1)-related thrombocytopenia (TP), human umbilical cord blood (UCB) CD34(+) hematopoietic progenitor cells (HPCs) were challenged with HIV-1(IIIb) and then differentiated by thrombopoietin (TPO) towards megakaryocytic lineage. This study showed that HIV-1, heat-inactivated HIV-1, and HIV-1 recombinant gp120 (rgp120) activated apoptotic process of megakaryocyte (MK) progenitors/precursors and decreased higher ploidy MK cell fraction. All these inhibitory effects on MK survival/maturation and platelets formation were elicited by the interaction between gp120 and CD4 receptor on the cell membrane in the absence of HIV-1 productive infection. In fact, in our experimental conditions, HPCs were resistant to HIV-1 infection and no detectable productive infection was observed. We also evaluated whether the expression of specific cytokines, such as TGF-beta1 and APRIL, involved in the regulation of HPCs and MKs proliferation, was modulated by HIV-1. The specific protein and mRNA detection analysis, during TPO-induced differentiation, demonstrated that HIV-1 upregulates TGF-beta1 and downregulates APRIL expression through the CD4 engagement by gp120. Altogether, these data suggest that survival/differentiation of HPCs committed to MK lineage is negatively affected by HIV-1 gp120/CD4 interaction. This long-term inhibitory effect is also correlated to specific cytokines regulation and it may represent an additional mechanism to explain the TP occurring in HIV-1 patients.

Toxoplasma gondii inhibits Fas/CD95-triggered cell death by inducing aberrant processing and degradation of caspase 8

Ligation of the death receptor Fas/CD95 activates an apoptotic cascade and plays critical roles during infectious diseases. Previous work has established that infection with the intracellular parasite Toxoplasma gondii renders cells resistant to multiple inducers of apoptosis. However, the effect of T. gondii on the death receptor pathway is poorly characterized. Here we have determined the impact of the parasite on apoptosis in type I cells that transduce Fas/CD95 engagement via the death receptor pathway without the need of a mitochondrial amplification loop. The results have shown that T. gondii significantly reduced Fas/CD95-triggered apoptosis by impairing activation of the initiator caspase 8. Parasitic infection diminished the cellular amount of procaspase 8, resulting in its decreased recruitment to the death-inducing signalling complex and the impaired activation of effector caspases. Remarkably, downregulation of caspase 8 protein in T. gondii-infected cells also occurred in the absence of Fas/CD95 engagement and was associated with the appearance of non-canonical caspase 8 cleavage fragments. Distinct parasite proteins were associated with caspase 8 and its proteolytic fragments. These findings indicate that T. gondii aberrantly processes and finally degrades the initiator caspase 8, thereby, blocking Fas/CD95-mediated apoptosis which signals independently of the apoptogenic function of host cell mitochondria.

c-Flip(L) is expressed in undifferentiated mouse male germ cells

Apoptosis represents a fundamental process during fetal/post-natal testis development. Therefore pro- and anti-apoptotic proteins are essential to regulate testis physiology. c-Flip(L) is a known inhibitor of caspase 8/10 activity; in this study its perinatal expression in mouse male germ cells was investigated. In testis sections and seminiferous tubule whole mount c-Flip(L) was found to be expressed in undifferentiated spermatogonia and to co-localize with germ stem cells markers. In vivo investigations in the vitamin-A deficient mouse, lacking differentiated germ cells, confirmed c-Flip(L) expression in undifferentiated spermatogonia. Further analyses showed Fas expression but no significant caspase 8/10 activity when c-Flip(L) was highly expressed. Altogether these data suggest that c-Flip may control the survival rate of undifferentiated spermatogonia.

Apoptotic lymphocytes and CD34+ cells in cryopreserved cord blood detected by the fluorescent vital dye SYTO 16 and correlation with loss of L-selectin (CD62L) expression

Discrimination between live and apoptotic cells is important for accurate determination of viable CD34(+) cells in hematopoietic stem cell transplant products. SYTO16 is a sensitive fluorescent dye for discriminating live from apoptotic leukocytes. The incidence of apoptotic leukocytes in paired samples of fresh and cryopreserved-thawed cord blood (CB) was determined by the SYTO16/7-AAD flow cytometric assay. Cell migration and expression of the cell homing molecule L-selectin (CD62L) was determined in relation to SYTO16 staining. SYTO16 detected significant proportions of apoptotic lymphocytes and CD34(+) cells in fresh and thawed CB buffy-coat samples that were not detected by 7-AAD. Compared to fresh CB, the proportion of apoptotic lymphocytes and CD34(+) cells significantly increased following thawing. Significantly higher proportions of live SYTO16(bright) lymphocytes and CD34(+) cells were found in the migrated cell population compared to the non-migrated population. Significantly fewer lymphocytes and CD34(+) cells expressed CD62L following thawing. Absence of CD62L expression was strongly correlated with apoptotic/SYTO16(dim) lymphocytes and CD34(+) cells. Cryopreserved-thawed CB contains significant proportions of apoptotic lymphocytes and CD34(+) cells that are not detected by 7-AAD. SYTO16 offers a sensitive method for discrimination of live from apoptotic leukocytes and assists in accurate assessment of CB quality and suitability for use in clinical transplantation.

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.

Tacrolimus: in vitro effects on myelopoiesis, apoptosis, and CD11b expression

BACKGROUND

Tacrolimus is a common component of multi-drug immunosuppressive regimens that are used for the prevention of rejection in transplant recipients. Tacrolimus therapy has been associated with anemia after transplantation, and recent clinical evidence in children suggests its association with the development of neutropenia for which an alternative etiology is not apparent. Mechanisms of suspected tacrolimus-related neutropenia have not been previously elucidated. We hypothesized that this variety of neutropenia might be due to a negative effect of tacrolimus on neutrophil production and/or survival.

METHODS

We designed in vitro studies to determine the dose-dependent effect of tacrolimus on myeloid cell production and/or apoptosis. CD34+ cells and neutrophils isolated from umbilical cord blood of term gestations were cultured with tacrolimus (0-1,000 ng/ml). To evaluate apoptosis, cells cultured for 24 hours were stained with annexin V-fluorescein isothiocyanate (V-FITC) and 7-amino-actinomycin D (7-AAD) and analyzed by flow cytometry. For clonal analysis, CD34+ cells cultured in cytokine-enhanced semi-solid media were scored for their myeloid/erythroid mix colony forming units (CFU-Mix) and myeloid (CFU-GM) progenitor cell contents.

RESULTS

Tacrolimus induced a dose-dependent enhancement of clonogenesis and survival of CD34+ cells at clinically relevant doses. Conversely, tacrolimus had no effect on the survival of mature neutrophils or on the upregulation of CD11b in response to chemotactic stimulation.

CONCLUSION

In contrast to our initial hypothesis, we observed that tacrolimus at clinically relevant concentrations enhanced clonogenesis of neutrophil progenitors and promoted their survival. Our in vitro studies suggest that tacrolimus alone is unlikely to be a significant factor in the neutropenia observed during immunosuppressive therapy.

cFLIP regulation of lymphocyte activation and development

Cellular caspase-8 (FLICE)-like inhibitory protein (cFLIP) was originally identified as an inhibitor of death-receptor signalling through competition with caspase-8 for recruitment to FAS-associated via death domain (FADD). More recently, it has been determined that both cFLIP and caspase-8 are required for the survival and proliferation of T cells following T-cell-receptor stimulation. This paradoxical finding launched new investigations of how these molecules might connect with signalling pathways that link to cell survival and growth following antigen-receptor activation. As discussed in this Review, insight gained from these studies indicates that cFLIP and caspase-8 form a heterodimer that ultimately links T-cell-receptor signalling to activation of nuclear factor-kappaB through a complex that includes B-cell lymphoma 10 (BCL-10), mucosa-associated-lymphoid-tissue lymphoma-translocation gene 1 (MALT1) and receptor-interacting protein 1 (RIP1).

Pathogenesis of neutropenia in large granular lymphocyte leukemia and Felty syndrome

T-cell large granular lymphocyte leukemia (TLGL) is an atypical chronic lymphoproliferative disorder derived from cytotoxic T-cells (CTL). Unlike most forms of leukemia, the pattern of bone marrow infiltration in TLGL may be subtle and the cytopenias are often lineage specific, with neutropenia dominating. Both granulocytic survival and proliferation defects are observed and are mediated by humoral and cell-mediated mechanisms respectively. Splenic production of immune complexes induces a neutrophil survival defect, where as Fas expression by leukemic CTL results in a marrow based proliferation defect. These humoral and cell-mediated pathways induce granulocytic apoptosis through independent intracellular mechanisms which are not mutually exclusive and may be observed concurrently in individual patients with either TLGL or FS. A variety of therapeutic interventions have been utilized in the management of TLGL and Felty syndrome, including methotrexate, cyclosporine A, cyclophosphamide, glucocorticoids, myeloid colony stimulating factors and splenectomy. Their efficacy and mechanisms of action are reviewed.

A Fas agonist induces high levels of apoptosis in haematological malignancies

We developed and tested a potent hexameric Fas agonist, termed MegaFasL, for its cytotoxic effects on a panel of human haematopoietic malignant cells and healthy human haematopoietic progenitor cells (CD34+CD38low). Results demonstrated that MegaFasL induced apoptosis in cell lines and primary cells representing multiple myeloma (MM), acute myeloid leukaemia (AML), acute lymphoblastic leukaemia (ALL) and Burkitt's lymphoma. Cells from a chronic myeloid leukaemia (CML) line and from patients with chronic lymphocytic leukaemia (CLL) were resistant. Furthermore, CD34+CD38low progenitor cells were also resistant to MegaFasL. The data indicate that MegaFasL could be a highly efficient therapeutic agent ex vivo or potentially in vivo.

HIV-1 Tat protein concomitantly down-regulates apical caspase-10 and up-regulates c-FLIP in lymphoid T cells: a potential molecular mechanism to escape TRAIL cytotoxicity

In this study, we showed the existence of a positive correlation between the amount of human immunodeficiency virus-type 1 (HIV-1) RNA in HIV-1 seropositive subjects and the plasma levels of TRAIL. Since it has been previously demonstrated that HIV-1 Tat protein up-regulates the expression of TRAIL in monocytic cells whereas tat-expressing lymphoid cells are more resistant to TRAIL cytotoxicity, we next investigated the effect of Tat on the expression/activity of both apical caspase-8 and -10, which play a key role in mediating the initial phases of apoptosis by TRAIL, and c-FLIP. Jurkat lymphoblastoid human T cell lines stably transfected with a plasmid expressing wild-type (HIV-1) tat gene showed normal levels of caspase-8 but significantly decreased levels of caspase-10 at both mRNA and protein levels with respect to Jurkat transfected with the control plasmid or with a mutated (cys22) non-functional tat cDNA. A significant decrease of caspase-10 expression/activity was also observed in transient transfection experiments with plasmid carrying tat cDNA. Moreover, c-FLIP(L) and c-FLIP(S) isoforms were up-regulated in tat-expressing cells at both mRNA and protein level in comparison with control cells. Taken together, these results provide a molecular basis to explain the resistance of tat-expressing Jurkat cells to apoptosis induced by TRAIL and, possibly, to other death-inducing ligands.

Caspase-8L expression protects CD34+ hematopoietic progenitor cells and leukemic cells from CD95-mediated apoptosis

Regulation of sensitivity or resistance for apoptosis by death receptor ligand systems is a key control mechanism in the hematopoietic system. Dysfunctional or deregulated apoptosis can potentially contribute to the development of immune deficiencies, autoimmune diseases, and leukemia. Control of homeostasis starts at the level of hematopoietic stem cells (HSC). To this end, we found that CD34+ hematopoietic progenitor cells are constitutively resistant to CD95-mediated apoptosis and cannot be sensitized during short-term culture to death receptor-mediated apoptosis by cytokines. Detailed analysis of the death machinery revealed that CD34+ cells do not express caspase-8a/b, a crucial constituent of the death-inducing signaling complex (DISC) of death receptors. Instead, we found a smaller splice variant termed caspase-8L to be present in HSC. Forced expression of caspase-8L using a recombinant lentiviral vector was able to protect hematopoietic cells from death receptor-induced apoptosis even in the presence of caspase-8a/b. Furthermore, we found that caspase-8L is recruited to the DISC after CD95 triggering, thereby preventing CD95 from connecting to the caspase cascade. These results demonstrate an antiapoptotic function of caspase-8L and suggest a critical role as apoptosis regulator in HSC. Similar to CD34+ HSC, stem cell-derived leukemic blasts from AML(M0) patients only expressed caspase-8L. Additionally we found, caspase-8L expression in several AML and ALL samples. Thus, caspase-8L expression might explain constitutive resistance to CD95-mediated apoptosis in CD34+ progenitor cells and might participate in the development of stem cell-derived and other leukemias by providing protection from regulatory apoptosis.

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.

The Role of p53 and Fas in a Model of Acute Murine Graft-versus-Host Disease

Graft-vs-host disease (GVHD) is a devastating, frequently fatal, pathological condition associated with lesions in specific target organs, including the intestine, liver, lung, and skin, as well as pancytopenia and alopecia. Bone marrow (BM) atrophy is observed in acutely diseased animals, but the underlying mechanisms of hemopoietic stem cell depletion remained to be established. We used an experimental mouse model of acute GVHD in which parental cells were injected into F(1) hosts preconditioned by sublethal irradiation. The resulting graft-vs-host response was kinetically consistent, resulting in lethality within 3 wk. We observed disease pathology in the liver and small intestine, and consistent with previous observations, we found BM atrophy to be a factor in the onset of acute disease. The product of the protooncogene, p53, is known to be a key player in many physiological examples of apoptosis. We investigated the role of p53 in the apoptosis of BM cells (BMC) during the development of acute disease and found that at least one copy of the p53 gene is necessary for depletion of BM and subsequent lethality in host animals. BM depletion was preceded by induction of the death receptor, Fas, on the surface of host stem cells, and induction of Fas was coincidental with the sensitization of BMC to Fas-mediated apoptosis. Our data indicate that BM depletion in acute GVHD is mediated by p53-dependent up-regulation of Fas on BMC, which leads to Fas-dependent depletion and subsequent disease.

Apoptotic mechanisms in the control of erythropoiesis

Erythropoiesis is a complex multistep process encompassing the differentiation of hemopoietic stem cells to mature erythrocytes. The steps involved in this complex differentiation process are numerous and involve first the differentiation to early erythoid progenitors (burst-forming units-erythroid, BFU-E), then to late erythroid progenitors (colony-forming units-erythroid) and finally to morphologically recognizable erythroid precursors. A key event of late stages of erythropoiesis is nuclear condensation, followed by extrusion of the nucleus to produce enucleated reticulocytes and finally mature erythrocytes. During the differentiation process, the cells became progressively sensitive to erythropoietin that controls both the survival and proliferation of erythroid cells. A normal homeostasis of the erythropoietic system requires an appropriate balance between the rate of erythroid cell production and red blood cell destruction. Growing evidences outlined in the present review indicate that apoptotic mechanism play a relevant role in the control of erythropoiesis under physiologic and pathologic conditions. Withdrawal of erythropoietin or stimulation of death receptors such as Fas or TRAIL-Rs leads to activation of a subset of caspase-3, -7 and -8, which then cleave the transcription factors GATA-1 and TAL-1 and trigger apoptosis. In addition, there is evidence that a number of caspases are physiologically activated during erythroid differentiation and are functionally required for erythroid maturation. Several caspase substrates are cleaved in differentiating cells, including the protein acinus whose activation by cleavage is required for chromatin condensation. The studies on normal erythropoiesis have clearly indicated that immature erythroid precursors are sensitive to apoptotic triggering mediated by activation of the intrinsic and extrinsic apoptotic pathways. These apoptotic mechanisms are frequently exacerbated in some pathologic conditions, associated with the development of anemia (ie, thalassemias, multiple myeloma, myelodysplasia, aplastic anemia). The considerable progress in our understanding of the apoptotic mechanisms underlying normal and pathologic erythropoiesis may offer the way to improve the treatment of several pathologic conditions associated with the development of anemia.

Expression of FLIP(Long) and FLIP(Short) in bone marrow mononuclear and CD34+ cells in patients with myelodysplastic syndrome: correlation with apoptosis

Several apoptosis-inducing systems, including Fas/Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) and its receptors, are upregulated in myelodysplastic syndrome (MDS). FLIP (FLICE (FAS-associated death-domain-like IL-1beta-converting enzyme)-inhibitory protein)) was identified as an inhibitor of FAS and TRAIL signals. Here, we characterized FLIP(Long) (FLIP(L)) and FLIP(Short) (FLIP(S)) expression in bone marrow mononuclear cells (BMMNCs) and in CD34+ cells of 29 MDS patients, and in 17 normal volunteers. The expression was correlated with apoptotic indices. In CD34+ cells, FLIP(L) levels were higher among normal individuals than in MDS patients (P=0.04). Among total BMMNC, FLIP(L) levels also tended to be higher in normal subjects than in MDS patients, although this difference was not significant (P=0.71). FLIP(L) levels in CD34+ cells were negatively correlated with apoptosis in both normal and MDS marrows (P=0.03). FLIP(Short) RNA expression was higher in MDS patients than in normal controls in both BMMNC (P=0.03) and CD34+ cells (P=0.08). In contrast to FLIP(L), FLIP(St) levels were positively correlated with apoptosis. At the protein level FLIP was most readily detectable in patients with high blast counts. The data suggest that FLIP(L) and FLIP(S) are differentially regulated, and that the relative levels of both isoforms play a role in the regulation of apoptosis in MDS.

Inhibition of CD95 apoptotic signaling by interferon-? in human osteoarthritic chondrocytes is associated with increased expression of FLICE inhibitory protein

OBJECTIVE

Cartilage homeostasis dysregulation during osteoarthritis (OA) has been linked to an increased rate of apoptosis of chondrocytes, the only cell type resident in the cartilage. In addition, the CD95-CD95 ligand (the Fas system) has emerged as one of the major pathways of cell death in the cartilage. We undertook the present study to investigate the role of interferon-gamma (IFNgamma) in the regulation of the Fas system by analyzing the modulation of intracellular signaling molecules (FLICE inhibitory protein [FLIP] and caspases 3 and 8) in primary cultures of human OA chondrocytes.

METHODS

CD95-induced apoptotic death of human OA chondrocytes was analyzed in the presence or absence of IFNgamma using cell death immunoassay for apoptosis, real-time polymerase chain reaction for FLIP and caspase 8 expression, Western blotting for FLIP, and proteolytic activity for caspases 3 and 8.

RESULTS

CD95-induced apoptotic death of human OA chondrocytes was strongly counteracted by IFNgamma treatment, although the surface expression of CD95 was slightly up-regulated by this cytokine. The messenger RNA (mRNA) expression of FLIP and caspase 8, mediators involved in CD95 signaling, revealed that FLIP expression in human OA chondrocytes was significantly up-regulated (2-fold increase) by IFNgamma treatment. Moreover, the FLIP:caspase 8 mRNA ratio increased significantly. FLIP up-regulation by IFNgamma was confirmed at the protein level. Caspase 8 and caspase 3 proteolytic activities, both induced in these cells by stimulation with anti-CD95, were also significantly down-modulated by IFNgamma.

CONCLUSION

These findings suggest that IFNgamma impairs CD95-mediated signaling and apoptotic death in human chondrocytes. Its mechanism of action involves down-regulation of caspase 8 and caspase 3 activities and increased expression of the antiapoptotic protein FLIP, suggesting an interesting mechanism for the inhibition of chondrocyte apoptosis.

Quiescent CD34+ early erythroid progenitors are resistant to several erythropoietic 'inhibitory' cytokines; role of FLIP

In this study, quiescent bone marrow-derived CD34+ erythroid burst-forming units (BFU-E) were found to be resistant to the inhibitory effects of tumour necrosis factor (TNF)-alpha and -beta as well as interferon (IFN)-alpha, -beta and -gamma, in contrast to those stimulated by a combination of erytrhropoietin (Epo) plus kit ligand (KL). Unexpectedly, we found that TNF-alpha also inhibited the apoptosis of quiescent normal human CD34+ BFU-E cells. Accordingly, TNF-alpha added to CD34+ cells cultured for 2 d in serum-free medium protected clonogeneic BFU-E from undergoing serum deprivation-mediated apoptosis. Furthermore, the prosurvival effect of TNF-alpha in quiescent CD34+ cells was consistent with its ability to induce phosphorylation of mitogen-activated protein kinase (MAPK) p42/44. However, when added to CD34+ cells that were stimulated by Epo + KL, TNF-alpha induced apoptosis and inhibited proliferation of BFU-E. To explain this intriguing differential sensitivity between unstimulated CD34+ cells versus those stimulated by Epo + KL, we examined the expression of apoptosis-regulating genes (FLIP, BCL-2, BCL-XL, BAD and BAX) in these cells. Of all the genes tested, FLIP became rapidly downregulated in CD34+ cells 24 h after stimulation with Epo + KL, suggesting that it may protect quiescent CD34+ BFU-E progenitors residing in the bone marrow from the inhibitory effects of inflammatory cytokines. Thus, we hypothesize that cycling cells may become more sensitive to proapoptotic stimuli (e.g. chemotherapy, inhibitory cytokines) than quiescent ones because of the downregulation of protective FLIP.

Increased transmigration of G-CSF-mobilized peripheral blood CD34+ cells after overnight storage at 37°C

BACKGROUND

G-CSF-mobilized PBPCs are utilized in allogeneic and autologous PBPC transplants. Homing, adhesion, and transmigration of hematopoietic CD34+ cells are required for successful engraftment. Hematopoietic CD34+ cells undergo directional migration toward the CXCR4 receptor ligand stromal-derived factor-1 (SDF-1). Limited data are available on the effects of liquid storage and cryopreservation on PBPC CD34+ cells.

STUDY DESIGN AND METHODS

Magnetic-assisted cell sorting (MACS)-selected CD34+ cells were assayed for retention of in vitro transmigration and phenotypic changes of unit-matched liquid-stored and cryopreserved PBPC samples from healthy donors. Studies evaluated whether transmigration of CD34+ cells in Iscove's modified Dulbecco's medium plus 1 percent HSA alone or in medium supplemented with SCF or allogeneic plasma was affected by overnight incubation at 37 degrees C, relative to nonincubated conditions.

RESULTS

Transmigration was maintained during liquid storage at 1 to 6 degrees C during a 2-day period and in unit-matched cryopreserved-thawed samples that had been initially liquid stored. Overnight incubation at 37 degrees C of MACS-selected unit-matched liquid-stored or cryopreserved-thawed CD34+ cells resulted in substantially increased transmigration, in particular with noncoated filters chemoattracted with the chemokine SDF-1.

CONCLUSION

CD34+ cell transmigration was comparable between liquid-stored and cryopreserved samples, and both demonstrated similar increases after overnight incubation at 37 degrees C.

Homing defect of cultured human hematopoietic cells in the NOD/SCID mouse is mediated by Fas/CD95

OBJECTIVE

To determine the bone marrow homing efficiency (20 hours) of cultured compared to noncultured umbilical cord blood (UCB)-derived human hematopoietic cells in the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse, and to explain the difference in homing between these populations.

METHODS

Human UCB CD34+ cells were cultured for up to 5 days, reselected, and used for transplantation, phenotype analysis, and functional studies, including adhesion and trans-endothelial migration assays. Seeding of CD34+ cells was measured after labeling of cells with 111-Indium, while homing of colony-forming cells (CFC) and SCID-repopulating (SRC) cells was determined using functional assays.

RESULTS

Short-term culture was associated with a decrease in the 20-hour homing of CD34+ cells, CFC, and SRC to the BM. Although cultured compared to noncultured cells showed increased expression and function (adhesion/migration) of several cell adhesion molecules described to play a role in homing and engraftment, culture also induced expression of Fas/CD95 and rendered cells more susceptible to apoptosis. Finally, we demonstrate that the level of Fas/CD95 on cultured cells was inversely related to the ability of CFC to home to the BM, and that the homing of cultured CFC could be restored by incubating cells prior to transplantation with Fas/CD95-blocking mAb ZB4.

CONCLUSION

These data implicate Fas/CD95 in the homing defect of cultured human hematopoietic cells in the NOD/SCID transplant model and suggest that prevention of apoptosis may be an important strategy to improve engraftment of ex vivo-manipulated HSC in a clinical setting.

Apoptosis of CD34+ cells after incubation with sera of leukopenic patients with systemic lupus erythematosus

Leukopenia and anaemia are observed in about a fifth of all patients with systemic lupus erythematosus (SLE) and may be due either to the destruction of blood cells or their decreased production. The former may be humoral or cell-mediated or result from apoptosis of peripheral blood cells. Several observations suggest the occurrence of the latter reduced in vitro proliferation of pluripotent bone marrow progenitors from the bone marrow aspirates of SLE patients,reduced counts of CD34+ cells in bone marrow aspirates in SLE patients, apoptosis of lymphopoietic progenitors and apoptosis of bone marrow cells. The aim of our study was to investigate whether humoral factors may induce suppression of haematopoiesis by increased apoptosis of CD34+ cells. For this purpose, we incubated allogeneic CD34+-enriched cells with sera of 18 leukopenic SLE patients. Apoptosis was induced by four of 18 sera. This effect was independent of complement-inhibition and FAS-blockade. Although reduced proliferation of autologous pluripotent bone marrow progenitors has been attributed to an IgG serum inhibitor, removal of IgG from these four proapoptotic sera had no effect on apoptosis of allogeneic CD34+ cells. The proapoptotic effect was associated with high titres of anti-dsDNA antibodies and low haemoglobin concentrations, but not with high titres of antinuclear antibodies, TNF-alpha and IFN-alpha of the sera tested.

Chelerythrin activates caspase-8, downregulates FLIP long and short, and overcomes resistance to tumour necrosis factor-related apoptosis-inducing ligand in KG1a cells

KG1a cells (CD34+/38-) express FAS and TRAIL (tumour-necrosis-factor-related apoptosis-inducing ligand) receptors but are resistant to FAS-ligand and TRAIL/APO2-L (apoptosis antigen-2 ligand)-induced apoptosis. KG1a cells are sensitized to FAS-induced apoptosis by chelerythrin, an inhibitor of protein kinase C (PKC). As cytoplasmatic adaptor molecules of FAS, e.g. FLIP [Fas-associated death domain protein (FADD)-like interleukin 1 beta-converting enzyme [FLICE (caspase-8)-inhibitory protein]], also modulate TRAIL signals, we determined whether chelerythrin affected TRAIL-mediated apoptosis. Chelerythrin by itself induced apoptosis in KG1a cells, and apoptosis was associated with activation of caspase-8. While TRAIL alone failed to activate caspase-8 or induce apoptosis, the addition of TRAIL to chelerythrin-treated cells significantly enhanced cleavage of caspase-8 and apoptosis. Chelerythrin-pretreated KG1a cells showed decreased phosphorylation of protein kinase C (PKC)-zeta and downregulation of both FLIP long and FLIP short proteins. Downregulation of FLIP and induction of apoptosis were partially abrogated by pretreatment with the specific caspase-8 inhibitor, Z-IETD-FMK. The decrease in FLIP protein expression induced by chelerythrin was accompanied by a progressive increase in mRNA levels of both FLIP long and FLIP short. CD34+ precursors from normal human marrow were also sensitive to chelerythrin but, in contrast to KG1a cells, were not sensitized to TRAIL-mediated apoptosis. Thus, resistance to TRAIL-induced apoptosis in leukaemic KG1a cells but not in normal CD34+ precursors was overcome in the presence of chelerythrin. The mechanism appeared to involve inhibition of PKC. Central targets were FLIP long and FLIP short, and their interactions with caspase-8. Whether such a pathway can be exploited to selectively target leukaemic progenitor cells remains to be determined.

Fidelity and reproducibility of antisense RNA amplification for the study of gene expression in human CD34+ haemopoietic stem and progenitor cells

Microarrays provide a powerful tool for the study of haemopoietic stem and progenitor cells (HSC). Because of the low frequency of HSC, it is rarely feasible to obtain enough mRNA for microarray hybridizations, and amplification will be necessary. Antisense RNA (aRNA) amplification is reported to give high-fidelity amplification, but most studies have used only qualitative validation. Before applying aRNA amplification to the study of HSC, we wished to determine its fidelity and reproducibility, and whether statistically significant results can be obtained. We found that aRNA amplification introduced biases into relative RNA abundance. However, these biases were extremely consistent, and valid comparisons could be made, if amplified RNA was compared with amplified RNA. By applying this method to the effect of interferon-gamma and tumour necrosis factor-alpha on normal primary CD34+ HSC, biologically significant differences could be detected, including potential mechanisms for resistance of CD34+ cells to CD95-mediated apoptosis and evidence of the differentiating effects of the cytokines. Differences of twofold or less were detected, and most of these differences attained statistical significance after triplicate experiments. These data demonstrate that aRNA amplification can be used with microarray hybridization to study the transcriptional profiles of small numbers of primary CD34+ HSC.

Significance of endothelial cell survival programs for renal transplantation

Initial and longer term kidney transplant function is determined in part by the renal allograft microcirculation because it provides a thromboresistant surface, regulates cellular infiltration, and elaborates paracrine and autocrine growth and survival factors. Loss of endothelial-derived signaling mediators accelerates vascular injury and endothelial cell (EC) death. EC apoptosis is implicated in accelerated allograft vasculopathy and premature loss of organ function. Renal allograft EC injury and replacement by recipient-derived repair mechanisms has long been proposed to influence allograft acceptance and function. Repair of cellular injury in allografts is linked with cell-survival mechanisms, but few precise indicators exist to predict recovery and repair in organ transplants. The significance of the growth phenotype of the microvascular endothelium for acute and longer term renal allograft survival is presented.

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.

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

Fas-mediated apoptosis is a major physiologic mechanism by which activated T cells are eliminated after antigen-stimulated clonal expansion generates a specific cellular immune response. Because activated T cells are the major effectors of allograft rejection, we hypothesized that genetically modifying allogeneic bone marrow (BM) cells prior to transplantation could provide some protection from host T-cell attack, thus enhancing donor cell engraftment in bone marrow transplantation (BMT). We undertook studies to determine the outcome of lentiviral vector-mediated transduction of Fas ligand (FasL) into lineage antigen-negative (lin(-)) mouse BM cells (lin(-) BMs), in an allogeneic BMT model. FasL-modified lin(-) BMs killed Fas-expressing T cells in vitro. Mice that received transplants of allogeneic FasL(+) lin(-) BMs had enhanced short-term engraftment, after nonmyeloablative conditioning, as compared to controls. We observed no major hepatic toxicity or hematopoietic or immune impairment in recipient mice at these time points. These results suggest potential therapeutic approaches by manipulating lymphohematopoietic stem-progenitor cells to express FasL or other immune-modulating genes in the context of BMT.

Constitutive expression of the Fas receptor and its ligand in adult human bone marrow: a regulatory feedback loop for the homeostatic control of hematopoiesis

Fas ligand (FasL) mediated apoptosis in the bone marrow may contribute to suppression of hematopoiesis in myelodysplastic syndromes (MDS) and in aplastic anemia, and also to the regulation of normal erythropoiesis. To identify potential effector and target cells in this regulatory pathway, we examined the constitutive expression of Fas receptor (Fas) and FasL (total and cell-surface) in myeloid and lymphoid cells and subsets of CD34+ cells in normal healthy adult human bone marrow using multiparameter flow cytometry. A high proportion of CD34+ cells constitutively expressed cell-surface FasL. However, none of the CD34+ cells expressed Fas alone. A reciprocal gradient of expression of FasL and Fas was observed in subsets of CD34+ cells: as compared to primitive CD34+/HLA-DR(-) (DR(-)) cells, a higher proportion of committed CD34+/HLA-DR(++) (DR(++)) cells expressed FasL but fewer expressed Fas; the expression of both molecules was intermediate in CD34+/HLA-DR(dim) cells. Also, the intensity of FasL expression was higher in DR(++) than in DR(-) cells. These results suggest that the homeostatic regulation of myelopoiesis in normal bone marrow is mediated via an autoregulatory feedback loop by myeloid cells and progenitors themselves, at least partly via the Fas-FasL pathway. This notion is also consistent with our recent observation that overexpression of FasL by myeloid cells in MDS correlates directly with anemia, transfusion requirements, and shorter survival, an example of dysregulation of this pathway.