Radioprotection of mice by recombinant rat stem cell factor

Kit signaling inhibits the sphingomyelin-ceramide pathway through PLCγ1: implication in stem cell factor radioprotective effect

Previous studies demonstrated that Kit activation confers radioprotection. However, the mechanism by which Kit signaling interferes with cellular response to ionizing radiation (IR) has not been firmly established. Based on the role of the sphingomyelin (SM) cycle apoptotic pathway in IR-induced apoptosis, we hypothesized that one of the Kit signaling components might inhibit IR-induced ceramide production or ceramide-induced apoptosis. Results show that, in both Ba/F3 and 32D murine cell lines transfected with wild-type c-kit, stem cell factor (SCF) stimulation resulted in a significant reduction of IR-induced apoptosis and cytotoxicity, whereas DNA repair remained unaffected. Moreover, SCF stimulation inhibited IR-induced neutral sphingomyelinase (N-SMase) stimulation and ceramide production. The SCF inhibitory effect on SM cycle was not influenced by wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor. The SCF protective effect was maintained in 32D-KitYF719 cells in which the PI3K/Akt signaling pathway is abolished due to mutation in Kit docking site for PI3K. In contrast, phospholipase C γ (PLCγ) inhibition by U73122 totally restored IR-induced N-SMase stimulation, ceramide production, and apoptosis in Kit-activated cells. Moreover, SCF did not protect 32D-KitYF728 cells (lacking a functional docking site for PLCγ1), from IR-induced SM cycle. Finally, SCF-induced radioprotection of human CD34+ bone marrow cells was also inhibited by U73122. Altogether, these results suggest that SCF radioprotection is due to PLCγ1-dependent negative regulation of IR-induced N-SMase stimulation. Beyond the scope of Kit-expressing cells, it suggests that PLCγ1 status could greatly influence the post-DNA damage cellular response to IR, and perhaps, to other genotoxic agents.

Adult blood vessels restore host hematopoiesis following lethal irradiation

OBJECTIVE

Accumulating evidence indicates a common stem cell may be responsible for both vasculogenesis and blood cell production during early embryologic development, yet little is known about the fate of these cells during ontogeny. We sought to determine whether hematopoietic potential is associated with normal blood vessels in the adult.

MATERIALS AND METHODS

Segments of adult thoracic aorta or inferior vena cava were transplanted under the kidney capsule of lethally irradiated recipients (1100 cGy). Radioprotection, colony-forming units (CFUs), and the extent of donor-derived hematopoietic constitution were evaluated using both Ly5 congenic and ROSA26 donor mice.

RESULTS

As little as 10 mg of transplanted vascular tissue radioprotected 80% of recipients, gave rise to similar numbers of CFUs as 10(5) bone marrow cells and prevented the development of severe anemia. Bromodeoxyuridine labeling studies revealed cell proliferation within the intima of donor vascular tissue within 48 hours of transplantation. ROSA26 donor-derived vascular cells migrated to the recipient spleen; however, CFUs were of host origin, a finding confirmed using sex-mismatched transplants. Although donor-derived cells were readily detected in the peripheral blood 2 to 3 weeks after transplant, they rapidly declined in frequency to approximately 1.0% by 4 weeks and persisted at these levels for more than 1 year. Bone marrow from rescued primary recipients provided radioprotection after transplantation into secondary recipients; however, only CD3(+) donor-derived cells were detected.

CONCLUSION

These findings demonstrate the presence of a population of cells within normal adult vascular tissue that has the capacity to protect host hematopoietic stem cells from radiation-induced death.

Kit signaling and negative regulation of daunorubicin-induced apoptosis: role of phospholipase Cgamma

Previous studies have demonstrated that activation of Kit by stem cell factor (SCF), its natural ligand, or by gain-of-function point mutation in its intracellular domain, confers significant protection against apoptosis induced by growth factor deprivation or radiation. However, the effects of Kit activation on the cellular response to anti-tumor agents have not been so extensively documented. This study shows that daunorubicin-induced apoptosis and cytotoxicity were reduced in the murine Ba/F3 cells transfected with Kit (Ba/F3-Kit) in the presence of SCF and in Ba/F3 cells transfected with a constitutively active Kit variant (Ba/F3-KitDelta27), compared to either parental Ba/F3 (Ba/F3-wt) or unstimulated Ba/F3-Kit cells. In Ba/F3-wt and in Ba/F3-Kit cells, daunorubicin stimulated within 8-15 min neutral sphingomyelinase and ceramide production but not in SCF-stimulated Ba/F3-Kit or in Ba/F3-KitDelta27 whereas all Ba/F3 cells were equally sensitive to exogenous cell-permeant C6-ceramide. In Ba/F3-Kit, SCF-induced Kit activation resulted in a rapid phospholipase Cgamma (PLCgamma) tyrosine phosphorylation followed by diacylglycerol release and protein kinase C (PKC) stimulation. U-73122, a PLCgamma inhibitor, not only blocked diacylglycerol production and PKC stimulation but also restored daunorubicin-induced sphingomyelinase stimulation, ceramide production, and apoptosis. These results suggest that Kit activation results in PLCgamma-mediated PKC-dependent sphingomyelinase inhibition which contributes to drug resistance in Kit-related malignancies.

Single administration of thrombopoietin to lethally irradiated mice prevents infectious and thrombotic events leading to mortality

A sufficiently high dose of thrombopoietin to overcome initial c-mpl-mediated clearance stimulates hematopoietic reconstitution following myelosuppressive treatment. We studied the efficacy of thrombopoietin on survival after supralethal total body irradiation (9 Gy) of C57BL6/J mice and the occurrence of infectious and thrombotic complications in comparison with a bone marrow graft or prophylactic antibiotic treatment. Administration of 0.3 microg thrombopoietin, 2 hours after irradiation, protected 62% of the mice as opposed to no survival in placebo controls. A graft with a supraoptimal number of syngeneic bone marrow cells (10(6) cells) fully prevented mortality, whereas antibiotic treatment was ineffective. Blood cell recovery was observed in the thrombopoietin-treated mice but not in the placebo or antibiotic-treated group. Bone marrow and spleen cellularity as well as colony-forming unit granulocyte-macrophage and burst-forming unit erythroid were considerably increased in thrombopoietin-treated mice relative to controls. Histologic examination at day 11 revealed numerous petechiae and vascular obstructions within the brain microvasculature of placebo-treated mice, which was correlated with hypercoagulation and hypofibrinolysis. Thrombopoietin treatment prevented coagulation/fibrinolysis disorder and vascular thrombosis. High fibrinogen levels were related to bacterial infections in 67% of placebo-treated mice and predicted mortality, whereas the majority of the thrombopoietin-treated mice did not show high fibrinogen levels and endotoxin was not detectable in plasma. We conclude that thrombopoietin administration prevents mortality in mice subjected to 9-Gy total body irradiation both by interfering in the cascade leading to thrombotic complications and by amelioration of neutrophil and platelet recovery and thus protects against infections and hemorrhages.

Crystal structure of human stem cell factor: implication for stem cell factor receptor dimerization and activation

Stem cell factor (SCF) plays important roles in hematopoiesis and the survival, proliferation, and differentiation of mast cells, melanocytes, and germ cells. SCF mediates its biological effects by binding to and activating a receptor tyrosine kinase designated c-kit or SCF receptor. In this report we describe the 2.3-A crystal structure of the functional core of recombinant human SCF. SCF is a noncovalent homodimer composed of two slightly wedged protomers. Each SCF protomer exhibits an antiparallel four-helix bundle fold. Dimerization is mediated by extensive polar and nonpolar interactions between the two protomers with a large buried surface area. Finally, we have identified a hydrophobic crevice and a charged region at the tail of each protomer that functions as a potential receptor-binding site. On the basis of these observations, a model for SCF small middle dotc-kit complex formation and dimerization is proposed.

Lipopolysaccharide is radioprotective in the mouse intestine through a prostaglandin-mediated mechanism

BACKGROUND & AIMS

The bone marrow and the intestine are the major sites of radiation-induced injury. The cellular response to radiation injury in the intestine or bone marrow can be modulated by agents given before irradiation. Lipopolysaccharide is known to be radioprotective in the bone marrow, but its effect on the intestine is not known. We sought to determine if lipopolysaccharide is radioprotective in the intestine and, if so, to determine the mechanism of its radioprotective effects.

METHODS

Mice were treated with parenteral lipopolysaccharide or vehicle and then irradiated (14 Gy total body irradiation in a cesium irradiator). The number of surviving intestinal crypts was assessed 3.5 days after irradiation using a clonogenic assay.

RESULTS

Parenteral administration of lipopolysaccharide 2-24 hours before irradiation resulted in a 2-fold increase in the number of surviving crypts 3.5 days after irradiation. The radioprotective effects of lipopolysaccharide could be eliminated by coadministration of a selective inhibitor of cyclooxygenase 2. Lipopolysaccharide was radioprotective in wild-type mice but not in mice with a disrupted cyclooxygenase 2. Parenteral administration of lipopolysaccharide resulted in increased production of prostaglandins in the intestine and in the induction of cyclooxygenase 2 expression in subepithelial fibroblasts and in villous, but not crypt, epithelial cells.

CONCLUSIONS

Lipopolysaccharide is radioprotective in the mouse intestine through a prostaglandin-dependent pathway.

Enhanced antitumor effects of bone marrow transplantation in combination with fibroblast-mediated IL-2 and IL-3 gene therapy

BACKGROUND

Bone marrow transplantation (BMT) and gene therapy are potent approaches to the recovery of bone marrow depression and induction of antitumor immunity after chemotherapy for the treatment of malignancies. In the present study, enhanced antitumor effect of BMT in combination with fibroblast-mediated interleukin (IL)-2 and IL-3 gene therapy was observed in tumor-bearing mice after chemotherapy.

METHODS

BALB/c mice were inoculated s.c. with J558L plasmacytoma cells and injected i.p. with cyclophosphamide 300 mg/kg 3 days later. 24 hours after chemotherapy syngeneic bone marrow cells in combination with NIH3T3 fibroblast cells engineered to produce IL-2 (NIH3T3-IL-2) and/or NIH3T3 cells engineered to produce IL-3 (NIH3T3-IL-3) were implanted into the tumor-bearing mice.

RESULTS

BMT in combination with implantation of either NIH3T3-IL-2 or NIH3T3-IL-3 cells exerted significant inhibition on the growth of J558L tumors and prolonged the survival period of the tumor-bearing mice as compared with the treatments with Hanks solution, BMT alone, or BMT plus implantation of NIH3T3 cells transduced with Neo gene. Synergistic antitumor effect was observed in mice after combined BMT and cytokine gene therapy. The cytotoxicities of natural killer cells, cytotoxic T lymphocytes, and macrophages in mice increased markedly after the combined treatment. Recovery of CFU-GM, CFU-MK and CFU-E formation in mice after combined therapy was accelerated obviously in mice after combined therapy.

CONCLUSIONS

BMT in combination with fibroblast-mediated IL-2 and IL-3 gene therapy elicited augmented antitumor effects synergistically in tumor-bearing mice after chemotherapy mainly through induction of antitumor immune response and accelerated recovery of hematopoiesis.

Consequences of exclusive expression in vivo of Kit-ligand lacking the major proteolytic cleavage site

Membrane growth factors that are processed to produce soluble ligands may function both as soluble factors and as membrane factors. The membrane growth factor Kit-ligand (KL), the ligand of the Kit receptor tyrosine kinase, is encoded at the Sl locus, and mice carrying Sl mutations have defects in hematopoiesis, gametogenesis, and melanogenesis. Two alternatively spliced KL transcripts encode two cell-associated KL protein products, KL-1 and KL-2. The KL-2 protein lacks the major proteolytic cleavage site for the generation of soluble KL, thus representing a more stable cell-associated form of KL. We investigated the consequences of exclusive expression of KL-2 in vivo. The KL gene in embryonic stem cells was modified and KL exon 6 was replaced with a PGKneoNTRtkpA cassette by homologous recombination, and mice carrying the SlKL2 allele were obtained. SlKL2/SlKL2 mice had only slightly reduced levels of soluble KL in their serum, suggesting that in vivo KL-2 may be processed to produce soluble KL-2S. The steady-state characteristics of the hematopoietic system and progenitor numbers were normal, and the mutant animals were not anemic. However, mast cell numbers in the skin and peritoneum were reduced and the mutant animals displayed increased sensitivity to sublethal doses of gamma-irradiation. Therefore, KL-2 may substitute for KL-1 in most situations with the exception of the production of mast cells, and induced proteolytic cleavage of KL-1 to produce soluble KL may have a role in the regeneration of hematopoietic tissue after radiation injury.

FLT-3 Ligand Provides Hematopoietic Protection From Total Body Irradiation in Rabbits

Several hematopoietic cytokines have been investigated for their potential to provide protection from the lethal consequences of bone marrow aplasia after total body irradiation (TBI). Some can increase the dose of irradiation tolerated by the animals; none allow endogenous recovery after doses such as administered in clinical blood or marrow transplantation. We tested the radioprotective potential of FLT-3 ligand, an early acting hematopoietic cytokine, alone and in combination with a late acting cytokine, granulocyte-colony stimulating factor (G-CSF). Adult outbred New Zealand White rabbits were submitted to TBI of 1,200 or 1,400 cGy by a Co60 source. Recombinant human (rh) FLT-3 ligand at a dose of 500 μg/kg and/or rhG-CSF at a dose of 10 μg/kg were administered for 14 days subcutaneously daily, beginning either 2 days before or the day after TBI. All control animals given no growth factors died of aplasia at day 10 (range, 5 to 16). All 8 animals given G-CSF had severe aplasia and 7 died at day 8 (range, 5 to 10); 1 animal survived, with G-CSF being administered before TBI. In contrast, 11 of 12 animals given FLT-3 ligand, with or without G-CSF, survived. Radioprotection was best in the group given FLT-3 ligand together with G-CSF before TBI. In these animals median platelet counts were never <10 × 109/L and median white blood cell counts never <0.5 × 109/L. These data show that hematopoietic recovery can occur after 1,400 cGy TBI in rabbits, if protected by FLT-3 ligand, and suggest a radioprotective clinical potential of this cytokine.

© 1998 by The American Society of Hematology.

FLT-3 Ligand Provides Hematopoietic Protection From Total Body Irradiation in Rabbits

Several hematopoietic cytokines have been investigated for their potential to provide protection from the lethal consequences of bone marrow aplasia after total body irradiation (TBI). Some can increase the dose of irradiation tolerated by the animals; none allow endogenous recovery after doses such as administered in clinical blood or marrow transplantation. We tested the radioprotective potential of FLT-3 ligand, an early acting hematopoietic cytokine, alone and in combination with a late acting cytokine, granulocyte-colony stimulating factor (G-CSF). Adult outbred New Zealand White rabbits were submitted to TBI of 1,200 or 1,400 cGy by a Co60 source. Recombinant human (rh) FLT-3 ligand at a dose of 500 μg/kg and/or rhG-CSF at a dose of 10 μg/kg were administered for 14 days subcutaneously daily, beginning either 2 days before or the day after TBI. All control animals given no growth factors died of aplasia at day 10 (range, 5 to 16). All 8 animals given G-CSF had severe aplasia and 7 died at day 8 (range, 5 to 10); 1 animal survived, with G-CSF being administered before TBI. In contrast, 11 of 12 animals given FLT-3 ligand, with or without G-CSF, survived. Radioprotection was best in the group given FLT-3 ligand together with G-CSF before TBI. In these animals median platelet counts were never &lt;10 × 109/L and median white blood cell counts never &lt;0.5 × 109/L. These data show that hematopoietic recovery can occur after 1,400 cGy TBI in rabbits, if protected by FLT-3 ligand, and suggest a radioprotective clinical potential of this cytokine.

© 1998 by The American Society of Hematology.

Interferon-gamma (IFN-gamma) as a potential radio- and chemo-protectant

Interferons (IFN) have had increasing clinical usage in the treatment of a variety of disorders, at times being used in combination with chemo- or radiotherapy. However, interferons may have inhibitory effects on hematopoietic stem cell proliferation and the effects of this cytokine's use on long-term hematologic function have not been studied. We performed the competitive repopulation assay in the murine system, using cells exposed to irradiation or single-dose chemotherapy with or without concomitant IFN-gamma use. IFN-gamma alone had no deleterious effects on hematopoietic stem cell productivity. We measured the repopulating ability of exhaustible multilineage precursors that were present at early stages of marrow repopulation after competitive repopulation (30 days). These progenitors were minimally impacted by cyclophosphamide (CTX) with or without IFN-gamma. Irradiation (XRT) and CTX alone produced significant repopulating defects in the most primitive hematopoietic stem cell, PHSC. Addition of IFN to either treatment regimen resulted in protection of PHSC, with improved repopulating ability, although the levels of donor marrow reached control levels only when CTX and IFN were used together. The results of multiple use of IFN with chemotherapy must be studied further, but IFN may offer hematologic radio- and chemoprotection, in addition to its antitumor properties in clinical protocols for treatment of cancers.

Augmentation of hematopoiesis by fibroblast-mediated interleukin-6 gene therapy in mice with chemotherapy

Murine fibroblast NIH3T3 cells engineered to secrete interleukin-6 (NIH3T3-IL-6) were implanted intraperitoneally into mice and observed for their hematopoietic effects with or without 5-fluorouracil (5-FU) administration. In normal mice, the platelet and neutrophil counts in peripheral blood increased significantly after implantation of NIH3T3-IL-6 cells, but the total white blood cell numbers showed no obvious elevation. The granulocyte-macrophage colony-forming unit (CFU-GM) and megakaryocyte colony-forming unit (CFU-MK) numbers formed by stem cells in bone marrow and spleen were also found to be significantly increased after implantation of NIH3T3-IL-6 cells when compared with those in mice after implantation of NIH3T3 cells transduced with neomycin gene (NIH3T3-Neo). To observe the protective effects of NIH3T3-IL-6 cells on hematopoietic depression in chemotherapy-treated mice, the mice were preinjected with 5-FU at a dosage of 150 mg/kg before implantation of NIH3T3-IL-6 cells. The platelet and neutrophil counts showed accelerated recovery after implantation of NIH3T3-IL-6 cells. The numbers of CFU-GM and CFU-MK in bone marrow and spleen were also found to be markedly increased in hematopoietic-depressed mice when compared with those in mice implanted with NIH3T3-Neo cells. These data suggest that fibroblast-mediated IL-6 gene therapy, which can augment hematopoiesis in mice with or without chemotherapy, will be of great help in the recovery from hematopoietic depression after chemotherapy.

Short-term effects of early-acting and multilineage hematopoietic growth factors on the repair and proliferation of irradiated pure cord blood (CB) CD34+ hematopoietic progenitor cells

PURPOSE

Hematopoietic growth factor(s) (GF) may exert positive effects in vitro or in vivo on the survival of hematopoietic stem and progenitor cells after accidental or therapeutic total body irradiation.

METHODS AND MATERIALS

We studied the clonogenic survival and DNA repair of irradiated (0.36, 0.73, and 1.46 Gy) CD34+ cord blood (CB) cells after short-term incubation (24 h) with GFs. CD34+ cells were stimulated with basic fibroblast growth factor (bFGF), stem cell factor/c-kit ligand (SCF), interleukin-3 (IL-3), IL-6, leukemia inhibitory factor (LIF), and granulocyte-monocyte colony stimulating factor (GM-CSF) alone or in combination in short-term serum-free liquid suspension cultures (LSC) immediately after irradiation and then assayed for clonogenic progenitors. DNA repair was evaluated by analysis of DNA strand breaks using the comet assay. Survival of CFU-GM, BFU-E, and CFU-Mix was determined and dose-response curves were fitted to the data.

RESULTS

The radiobiological parameters (D[0] and n) showed significant GF(s) effects. Combination of IL-3 with IL-6, SCF or GM-CSF resulted in best survival for CFU-GM BFU-E and CFU-Mix, respectively. Combinations of three or more GFs did not increase the survival of clonogenic CD34+ cells compared to optimal two-factor combinations. The D[0] values for CFU-GM, BFU-E, and CFU-Mix ranged between 0.56-1.15, 0.41-2.24, and 0.56-1.29 Gy, respectively. As for controls, the curves remained strictly exponential, i.e., all survival curves were strictly exponential without any shoulder (extrapolation numbers n=1 for all tested GF(s). DNA repair capacity of CD34+ cells determined by comet assay, was measured before, immediately after irradiation, as well as 30 and 120 min after irradiation at 1 Gy. Notably, after irradiation the 2-h repair of cytokine-stimulated and unstimulated CD34+ cells was similar.

CONCLUSION

Our data indicate that increased survival of irradiated CB CD34+ cells after short-term GF treatment is mediated through proliferative GF effects on the surviving fraction but not through improved DNA repair capacity.

Accelerated recovery of irradiation-induced bone marrow depression by fibroblast-mediated interleukin 6 gene therapy in combination with bone marrow transplantation in mice

BACKGROUND

Both fibroblast-mediated cytokine gene therapy and bone marrow transplantation (BMT) have proven to be efficient protocols for the recovery of bone marrow depression. In this report, the effects of fibroblast-mediated interleukin (IL)-6 gene therapy, in combination with BMT, on the recovery of irradiation-induced bone marrow depression were investigated.

METHODS

NIH3T3 fibroblast cells engineered to secrete IL-6 (NIH3T3-IL-6) or NIH3T3 cells transduced with the neomycin gene (NIH3T3-Neo), in combination with 10(7), 10(6), or 10(5) syngeneic bone marrow cells, were implanted into irradiated mice.

RESULTS

The platelets and white blood cells in the peripheral blood of the irradiated mice increased greatly 12 days after implantation of NIH3T3-IL-6 cells and BMT, the white blood cell counts were restored to a normal level 32 days after the combined therapy, and the platelet number was obviously higher than that in mice implanted with NIH3T3-Neo and BMT. Twenty and 25 days after the combined therapy, the mice showed accelerated recovery of colony-forming unit (CFU)-granulocyte/macrophages and CFU-megakaryocytes when compared with the mice implanted with NIH3T3-Neo cells and BMT. Ten days after lethal irradiation with gamma rays, the spleens formed more CFU-spleen in mice implanted with NIH3T3-IL-6 cells and BMT than in mice injected with phosphate-buffered saline or NIH3T3-Neo cells. Combined therapy with NIH3T3-IL-6 cell implantation and BMT delayed the survival period of the hematopoietic-depressed mice significantly when compared with therapy with NIH3T3-Neo cell implantation and BMT.

CONCLUSIONS

These data demonstrated that the combined therapy of fibroblast-mediated IL-6 gene therapy and BMT could significantly promote the recovery of irradiation-induced hematopoietic depression.

Developmental arrest of germ cells in the pathogenesis of germ cell neoplasia

Clinical observations and epidemiological evidence suggest that important aetiopathological events that cause neoplastic transformation of the male germ cell may occur in fetal life or early infancy. The incidence of germ cell neoplasia is high in individuals with various disorders of gonadal development and sexual differentiation, such as gonadal dysgenesis or androgen insensitivity syndrome. Increased risk has also been noted in individuals with trisomy 21, idiopathic infertility and low birth weight. Infertility is sometimes associated with small aberrations of sex chromosomes (e.g. low frequency mosaicism XY/XO) which can also be found in patients with testicular cancer. The variety of conditions that predispose to testicular neoplasia and the rise in its incidence in many countries speaks for the influence of environmental factors which may affect genetically predisposed individuals. We hypothesise that if the development of the testis is disturbed or delayed, primordial germ cells or gonocytes undergo maturation delay or differentiation arrest which may render them susceptible to neoplastic transformation. Morphologically homogenous premalignant carcinoma in situ (CIS) cells have the potential to differentiate into a variety of histological forms of overt testicular tumours. Analysis of cell surface antigens expressed by CIS cells found in the vicinity of pure and mixed tumours demonstrates that CIS cells are phenotypically heterogeneous. Comparison of the phenotypes of CIS cells, primordial germ cells, human embryonal carcinoma cells and closely related primate embryonal stem cells reveals various similarities but also differences. We speculate that phenotypical heterogeneity of CIS cells may be associated with their potential to give rise to different tumour types, and may be related to the developmental stage of the early germ cell which has undergone malignant transformation.

Modulation of radiation damage by cytokines

Cytokines, hormone-like proteins that are produced by stimulated cells and tissues, serve as intercellular messengers. The production of an expanding number of recombinant cytokines in pharmacological quantities has permitted an assessment of the benefit they may provide in preserving and restoring functions of tissues compromised by irradiation. Included here are studies indicating that the cytokines interleukin 1, tumor necrosis factor, stem cell factor and interleukin 12 protect mice from radiation lethality when given prior to irradiation, and even in untreated mice these cytokines serve in innate defenses against external stimuli. In contrast, transforming growth factor beta, interleukin 6 and interferon, given before irradiation, sensitize the mice to radiation lethality. Myeloprotection against ionizing radiation and chemotherapeutic drugs by interleukin 1 depends on the regimen of treatment and may be related to the temporary patterns of induced cytokines and to the resulting changes in the cycling status of the progenitor cells. Interleukin 12, through induction and interaction with additional cytokines, has contrasting effects on different tissues, i.e., protecting the bone marrow but sensitizing the gut. Insights gained from such studies into the cellular mechanisms of regulation of radiation-induced damage by cytokines are discussed. Whether a "trade-off" of protection of some tissues and sensitization of other tissues applies to cytokine therapy in humans is unknown.

Increased Growth Promoting But Not Mast Cell Degranulation Potential of a Covalent Dimer of c-Kit Ligand

The native form of soluble c-kit ligand (KL) is a noncovalent dimer. We have isolated a soluble, disulfide-linked dimer of murine KL (KL-CD) by expressing KL in Escherichia coli and refolding the denatured protein under conditions that promote the formation of both noncovalent dimers (KL-NC) and KL-CD. KL-CD exhibits a 10- to 15-fold increase in the ability to stimulate the growth of both the human megakaryocytic cell line MO7e and murine bone marrow-derived mast cells relative to KL-NC. Colony-forming assays of murine bone marrow progenitor cells also reflected this increased potency. However, KL-CD and KL-NC are equally able to prime mast cells for enhanced IgE-dependent degranulation in vitro and activate mast cells in vivo. Improving the growth-promoting activity of KL without changing its mast cell activation potential suggests that KL-CD or a related molecule could be administered in the clinic at doses that stimulate hematopoietic recovery while avoiding significant mast cell activation.

Host Conditioning With 5-Fluorouracil and kit-Ligand to Provide for Long-Term Bone Marrow Engraftment

Administration of kit-ligand (KL) before and after doses of 5-fluorouracil (5-FU) results in marrow failure in mice, presumably because of enhanced KL-induced cycling of stem cells, which makes them more susceptible to the effects of 5-FU. In attempt to capitalize on this effect on stem cells, we studied the ability of KL and 5-FU to allow stable donor engraftment of congenically marked marrow in a C57BL/6 (B6) mouse model. KL was administered subcutaneously at 50 μg/kg, 21 hours and 9 hours before and 3 hours after each of two doses of 5-FU (125 mg/kg) given 7 days apart to B6-recipients. Animals then received three injections of 107 congenic B6-Gpi-1a-donor bone marrow cells at 24, 48, and 72 hours after the second 5-FU dose. A separate group of animals received a single dose of either 1 × 107 or 3 × 107 donor marrow cells 24 hours after the last 5-FU dose. The level of engraftment was measured from Gpi-phenotyping at 1, 3, 6, and 8 months in red blood cells (RBCs) and at 8 months by phenotyping cells from the thymus, spleen, and marrow. Percent donor engraftment in RBCs appeared stable after 6 months. The percent donor engraftment in RBCs at 8 months was significantly higher in KL + 5-FU prepared recipients (33.0 ± 2.7), compared with 5-FU alone (18.5 ± 2.6, P &lt; .0005), or saline controls (17.8 ± 1.7, P &lt; .0001). In an additional experiment, granulocyte colony-stimulating factor (100 μg/dose) was added to a reduced dose of KL (12.5 μg/dose); engraftment was similar to KL alone. At 8 months after transplantation the levels of engraftment in other tissues such as bone marrow, spleen, and thymus correlated well with erythroid engraftment to suggest that multipotent long-term repopulating stem cells had engrafted in these animals. There are concerns for the toxicity of total body irradiation (TBI)- or busulfan-based regimens in young recipients of syngeneic or transduced autologous marrow who are transplanted for correction of genetic disease. In these recipients complete donor engraftment may not be needed. The results with KL and 5-FU are encouraging for the further refinement of non-TBI, nonbusulfan techniques to achieve stable mixed chimerism.

Development of a positive method for male stem cell-mediated gene transfer in mouse and pig

Classical approaches for producing transgenic livestock require labor-intensive, time-consuming, and expensive methods with low efficiency of transgenic production. A promising approach for producing transgenic animals by using male stem cells was recently reported by Brinster and Zimmermann (1994; Proc Natl Acad Sci 91:11298-11302) and by Brinster and Avarbock (1994: Proc Natl Acad Sci USA 91:11303-11307). However, in order to apply this technique to producing transgenic animals, some difficulties have to be overcome. These include a satisfactory method for short-term in vitro culture for drug selection after transfection with exogenous DNA, and methods for the use of livestock such as pigs. We developed a new method for transferring foreign DNA into male germ cells. Mice and pigs were treated with busulfan, an alkylating agent, to destroy the developing male germ cells, and liposome/bacterial LacZ gene complexes were introduced into each seminiferous tubule by using a microinjection needle. As a control, lipofectin was dissolved in phosphate-buffered saline at a ratio of 1:1, and then injected into seminiferous tubules. In mice, 8.0-14.8% of seminiferous tubule expressed the introduced LacZ gene, and 7-13% of epididymal spermatozoa were confirmed as having foreign DNA by polymerase chain reaction. The liposome-injected testes were all negative for X-gal staining. These results indicate that some spermatozoa were successfully transformed in their early stages by liposome/DNA complexes. In pigs, foreign DNA was also incorporated efficiently into male germ cells, and 15.3-25.1% of the seminiferous tubules containing germ cells expressed the LacZ gene. The data suggest that these techniques can be used as a powerful tool for producing transgenic livestock.

Radioprotection of hematopoietic tissue by fibroblast growth factors in fractionated radiation experiments

Acidic and basic fibroblast growth factors (FGF(1/2)) myeloprotect mice in single dose total body irradiation (TBI) experiments with a dose modification factor (DMF) of approximately 1.15. CFU-C assay suggests that one of the mechanisms is augmentation of the shoulder of the radiation dose response curve, and thus protection could be greater with fractionation. Four equal fractions of TBI were delivered to C3H/He mice at times 0 h, 8 h, 24 h, and 32 h. FGF(1/2) dose was 3 microg per i.v. injection given 24 and 4 hrs before the first radiation dose. FGF2 treated mice had a significant survival advantage over saline-treated mice with a DMF of 1.22 +/- 0.07 (p < 0.01). Adding a third dose of FGF2, had no additional benefit on LD(50/30) (dose of radiation lethal to 50% of animals measured at day 30) (DMF = 1.23 +/- 0.06, p < 0.01). FGF1 was not as effective with fractionation (DMF = 1.04 +/- 0.03). Increased survival in FGF2 treated mice was due to the a more rapid recovery of bone marrow hematopoietic cells and peripheral WBC, RBC and platelets. FGF2 may prove a useful treatment response modifier in clinical fractionated irradiation.

AM218, a new polyanionic polysaccharide, induces radioprotection in mice when administered shortly before irradiation

We have shown that the polyanionic polysaccharide AM218 improves the survival rate of the potentially lethally irradiated mice. This radioprotective effect was highly dependent on the administration schedule, the most efficient protocol being that in which the drug was given shortly before irradiation. The haematopoietic implications in the pharmacological action of AM218 were confirmed by the improved recovery in the three peripheral blood lineages observed in the AM218-treated mice. However, because of a marked increase observed in the number of white blood cells during the period of highest mortality of the control irradiated mice, effects on the neutrophil lineage may account for the effects mediated by AM218 in the irradiated mice. Both in vitro and in vivo treatment with AM218 before irradiation improved the survival rate of CFU-GM progenitors, while no effects were observed on the CFU-S pool. This led us to postulate that the improved survival rate of the committed progenitors, at least the granulocyte-macrophage progenitors, accounts for the radioprotective capacity of AM218.

Radio-induced modulation of transforming growth factor beta1 sensitivity in a p53 wild-type human colorectal-cancer cell line

Unlike normal intestinal cells, colorectal-carcinoma cell lines are usually not responsive to transforming growth factor beta1. The cyclin-dependent kinase inhibitor p21 that is induced by X irradiation in cells expressing normal p53 can also be induced by TGF-beta1 by a p53-independent pathway. We have investigated possible interactions between ionizing radiation and TGF-beta1, using a panel of 8 human colorectal-cancer cell lines varying in p53 status and sensitivity to the cyto-inhibitory effect of TGF-beta1. Heterogeneity in the radiosensitivity of these cell lines was observed, with SF2 (surviving fraction after irradiation with 2 Gy) ranging from 0.19 to 0.82. Radioresistance (high SF2 values) was in general associated with abnormal expression of p53. An effect of TGF-beta1 treatment on radiosensitivity was observed with one cell line only (LS513), and manifested by enhancement of the cytotoxic effect of radiation. In an experiment with fractionated irradiation during continuous exposure to TGF-beta1, there was no change in the intrinsic radiosensitivity of LS513 cells, though irradiated cells treated with TGF-beta1 were more sensitive to the first radiation dose. Irradiated LS513 colorectal-cancer and Mv-1-Lu epithelial cells were significantly more sensitive to TGF-beta1 than were unirradiated controls, whereas no change was observed in the TGF-beta1 sensitivity of irradiated LS1034 cells. Radio-induced modulation of TGF-beta1 sensitivity was transitory and declined before the decline to baseline level of p21 mRNA expression. On the basis of these results, we postulate that radiation-induced sensitization to TGF-beta1 occurs in TGF-beta1-sensitive cells expressing wild-type p53.

Concepts of hemopoietic cell amplification. Synergy, redundancy and pleiotropy of cytokines affecting the regulation of erythropoiesis

Hemopoietic cell amplification in vivo is regulated by various mechanisms which appear to be under control of many hemopoietic growth factors. Quiescent stem cells can be activated into cell cycle, dividing progenitor cells can reduce their cycle time, the differentiation velocity (i.e. transit-time) can be manipulated, apoptosis can be prevented, and finally, at least in the murine system, migration of cells between the microenvironments in marrow and spleen may take place. Perturbations of any of the parameters by which these mechanisms are defined, will affect in vivo blood cell production. In this review the consequences of these perturbations, and the role of growth factors herein, are discussed. These fundamental aspects of the regulation of hemopoiesis are illustrated with recent data showing the synergistic, redundant and pleiotropic effects of SCF, IL-11, EPO and G-CSF on the in vivo formation of erythrocytes. Given the overwhelming number of growth factor-related studies that are now appearing, a re-evaluation of data, available in the literature, in the context of the mechanistic approach of growth factor-dependent hemopoiesis which is presented in this paper, seems to be useful and warranted.

In lethally irradiated mice interleukin-12 protects bone marrow but sensitizes intestinal tract to damage from ionizing radiation

Administration of IL-12 prior to lethal irradiation, protected a significant fraction of mice from 60Co-gamma radiation-induced lethal hematopoietic syndrome. Radioprotection was associated with an increase in the number of c-kit+ bone marrow cells (BMC) in IL-12 treated mice compared to saline-treated mice. Even after supralethal doses of radiation (1200 cGy), IL-12-treated mice had twofold greater numbers of c-kit+ BMC than controls. However the mice receiving IL-12 and 1200 cGy died of the gastrointestinal (GI) syndrome, evident by gross necroscopy and histological evaluation, within 4 to 6 days after irradiation. Induction of the GI syndrome in mice not treated with IL-12 required radiation doses of 1600 cGy. Thus, at doses of radiation at which IL-12 still protects c-kit+ hematopoietic cells, it sensitizes the intestinal tract to damage. Radioprotection with IL-12 was abrogated by anti-IL-1R or anti-SCF antibody, but not anti-IFN gamma antibody. In contrast, anti-IFN gamma antibody abrogated sensitization of the intestinal tract by IL-12.

Haematopoietic growth factors in the treatment of therapeutic and accidental irradiation-induced bone marrow aplasia

Bone marrow aplasia is one of the main syndromes following a high dose accidental exposure of ionizing radiation. Although both transfusion and bone marrow transplantation have been used with some success since the first treatments of patients, other therapeutic strategies are needed. The strategies involving haematopoietic growth factors for the treatment of radiation victims have been explored in vivo mainly in animal models and it is hoped that new therapeutic regimens will be elucidated from such approaches. The growth factors stimulate proliferation and/or differentiation of haematopoietic progenitor cells and possible stem cells. Furthermore, they act on the functions of mature cells. They now have specific uses in haematology, related to their role in the regulation of growth and differentiation of haematopoietic progenitor cells. The results of the clinical trials, performed with numerous patients and often randomized bring important clues about what to expect from growth factor therapy. Other factors are only entering the preclinical or clinical trials now. Although numerous in vitro or in vivo experiments suggest a benefit from their effects, their possible uses in therapy are still questionable. Some growth factors have already been used for the treatment of accidental radiation-induced aplasia and lessons have been learned from their medical management and follow-up.

Stem cell factor enhances the survival of irradiated human bone marrow maintained in SCID mice

The effect of recombinant human stem cell factor (SCF) on the response of human fetal bone marrow progenitor cells to irradiation was studied using immunodeficient mice with human fetal bone grafts (SCID/Hu mice). SCID/Hu mice were treated with three intraperitoneal injections of 500 micrograms/kg SCF at 20 h before, two h before, and four h after 100 cGy total body irradiation. Fourteen days following irradiation, the fetal bone grafts were harvested and studied. Most of the isolated bone marrow cells were human, as determined by flow cytometry. Colony forming assays were performed on the bone marrow to determine the survival of erythroid (BFU-E) and myeloid (CFU-GM) precursor cells. A statistically significant increase in BFU-E and CFU-GM survival after irradiation was observed for bone marrow maintained in the SCF treated mice when compared to bone marrow from mice not treated with SCF. The enhancement in colony forming unit survival after irradiation ranged from 4.3-fold for BFU-E (p = 0.05) to 13.1-fold for CFU-GM (p = 0.002). These findings suggest that SCF may be of potential clinical value for the prevention of radiation-induced myelosuppression.

Role of kit-ligand in proliferation and suppression of apoptosis in mast cells: basis for radiosensitivity of white spotting and steel mutant mice

The receptor tyrosine kinase Kit and its cognate ligand KL/steel factor are encoded at the white spotting (W) and Steel (Sl) loci of the mouse, respectively. Mutations at both the W and the Sl loci affect hematopoiesis including the stem cell hierarchy, erythropoiesis, and mast cells, as well as gametogenesis and melanogenesis. In addition, mutant mice display an increased sensitivity to lethal doses of irradiation. The role of KL/c-kit in cell proliferation and survival under conditions of growth factor-deprivation and gamma-irradiation was studied by using bone marrow-derived mast cells (BMMC) as a model. Whereas apoptosis induced by growth factor deprivation in BMMC is a stochastic process and follows zero order kinetics, gamma-irradiation-induced apoptosis is an inductive process and follows higher order kinetics. In agreement with these results, gamma-irradiation-induced apoptosis in BMMC was shown to be dependent on p53 whereas apoptosis induced by deprivation is partly dependent on p53, implying that there are other mechanisms mediating apoptosis in KL-deprived BMMC. In the presence and in the absence of serum, KL stimulated proliferation by promoting cell cycle progression. The presence of KL was required only during the early part of the G1 phase for entry into the S phase. At concentrations lower than those required for proliferation, KL suppressed apoptosis induced by both growth factor-deprivation and gamma-irradiation, and internucleosomal DNA fragmentation characteristic of apoptosis. The ability of KL to suppress apoptosis was independent of the phase of the cell cycle in which the cells were irradiated and suppression of apoptosis was a prerequisite for subsequent cell cycle progression. Moreover, addition of KL to gamma-irradiated and growth factor-deprived cells could be delayed for up to 1 h after irradiation or removal of growth factors when cells became irreversibly committed to apoptosis. KL and IL-3 induce suppression of apoptosis in mast cells by different mechanisms based on the observations of induction of bcl-2 gene expression by IL-3 but not by KL. It is proposed that the increased sensitivity of W and Sl mutant mice to lethal irradiation results from paucity of the apoptosis suppressing and proliferative effects of KL.

Mechanisms of protection of hematopoietic stem cells from irradiation

Current therapies for the treatment of malignancies are associated with significant limitations to the hematopoietic system since chemotherapy and radiation therapy do not discriminate between normal and malignant cells. Since bone marrow depression occurs at low to midlethal doses of irradiation, approaches to improving the therapeutic index of treatment must include measures to enhance the sensitivity of the tumor relative to normal hematopoietic tissue or, by reducing toxicity to normal hematopoietic tissues leaving tumor resistance unchanged. Radioprotective agents have been proposed to unravel the fundamental processes by which radiation itself damages hematopoietic tissue. In radiotherapy, the importance of these agents is derived from their potential use as selective protectors against radiation damage to normal hematopoietic tissue such that higher doses of radiation can be delivered to tumors to achieve a therapeutic advantage. A variety of agents have been and are being evaluated as possible protectants. These include aminothiols, synthetic polysaccharides, vitamins and cytokines. This review attempts to summarize the role both chemical and biological response modifiers play as hematopoietic radioprotectors. In addition, possible mechanisms of protection of hematopoietic stem cells from irradiation are discussed.

The role of stem cell factor in mobilization of peripheral blood progenitor cells: synergy with G-CSF

The use of cytokine mobilized peripheral blood progenitor cells (PBPC) in transplantation following chemotherapy has led to enhanced engraftment. Granulocyte-colony stimulating factor (G-CSF) has been shown in a number of clinical studies to be an effective mobilizer of PBPC. Preclinical data in mice and primates have demonstrated a potential role for the use of stem cell factor (SCF) in mobilization of PBPC. In the studies presented here, low doses of SCF are shown to synergize with optimal doses of G-CSF to enhance the number and quality of PBPC compared to G-CSF alone. Phase I studies using r-metHuSCF demonstrated mast cell-related dose limiting effects. The data presented here have led to Phase I/II studies to evaluate the potential use of low doses of SCF in combination with G-CSF for mobilization of PBPC.