An update on sargramostim for treatment of acute radiation syndrome.

The potential use by terrorists of an improvised nuclear device, a radiological dispersal device, or an unintended nuclear/radiological accident in heavily populated areas is a national security threat of major consequences. Although this type of security threat is considered to be low-risk, it would have a devastating impact. Health issues would be a major concern; medical care would be necessary for all those who received considerable radiation exposure (> 1 Gy) leading to hematopoietic acute radiation syndrome (ARS). In the past few years, the U.S. Food and Drug Administration (FDA) has approved for such radiation exposure contingencies recombinant human granulocyte colony-stimulating factor (rhG-CSF, filgrastim, Neupogen), PEGylated rhG-CSF (PEGylated filgrastim, Neulasta) and granulocyte-macrophage colony-stimulating factor (rhGM-CSF, sargramostim, Leukine) following the FDA's Animal Rule guidance. In this article, we have briefly reviewed the consequences of exposure to acute, potentially lethal doses of radiation and its pathologic sequelae, as well as ARS and the latest of the FDA-approved recombinant growth factors, namely sargramostim (Leukine), as a new treatment option for the subclinical, hematopoietic syndrome component of ARS. The nature of the recombinant and the preclinical and clinical research that preceded approval by the FDA are presented, as well as its use in the treatment of victims of radiation accidents.

ICRP Publication 131: Stem cell biology with respect to carcinogenesis aspects of radiological protection.

Current knowledge of stem cell characteristics, maintenance and renewal, evolution with age, location in 'niches', and radiosensitivity to acute and protracted exposures is reviewed regarding haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. The identity of the target cells for carcinogenesis continues to point to the more primitive and mostly quiescent stem cell population (able to accumulate the protracted sequence of mutations necessary to result in malignancy), and, in a few tissues, to daughter progenitor cells. Several biological processes could contribute to the protection of stem cells from mutation accumulation: (1) accurate DNA repair; (2) rapid induced death of injured stem cells; (3) retention of the intact parental strand during divisions in some tissues so that mutations are passed to the daughter differentiating cells; and (4) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the vital niche. DNA repair mainly operates within a few days of irradiation, while stem cell replications and competition require weeks or many months depending on the tissue type. This foundation is used to provide a biological insight to protection issues including the linear-non-threshold and relative risk models, differences in cancer risk between tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age.

ICRP Publication 131: Stem Cell Biology with Respect to Carcinogenesis Aspects of Radiological Protection.

This report provides a review of stem cells/progenitor cells and their responses to ionising radiation in relation to issues relevant to stochastic effects of radiation that form a major part of the International Commission on Radiological Protection's system of radiological protection. Current information on stem cell characteristics, maintenance and renewal, evolution with age, location in stem cell 'niches', and radiosensitivity to acute and protracted exposures is presented in a series of substantial reviews as annexes concerning haematopoietic tissue, mammary gland, thyroid, digestive tract, lung, skin, and bone. This foundation of knowledge of stem cells is used in the main text of the report to provide a biological insight into issues such as the linear-no-threshold (LNT) model, cancer risk among tissues, dose-rate effects, and changes in the risk of radiation carcinogenesis by age at exposure and attained age. Knowledge of the biology and associated radiation biology of stem cells and progenitor cells is more developed in tissues that renew fairly rapidly, such as haematopoietic tissue, intestinal mucosa, and epidermis, although all the tissues considered here possess stem cell populations. Important features of stem cell maintenance, renewal, and response are the microenvironmental signals operating in the niche residence, for which a well-defined spatial location has been identified in some tissues. The identity of the target cell for carcinogenesis continues to point to the more primitive stem cell population that is mostly quiescent, and hence able to accumulate the protracted sequence of mutations necessary to result in malignancy. In addition, there is some potential for daughter progenitor cells to be target cells in particular cases, such as in haematopoietic tissue and in skin. Several biological processes could contribute to protecting stem cells from mutation accumulation: (a) accurate DNA repair; (b) rapidly induced death of injured stem cells; (c) retention of the DNA parental template strand during divisions in some tissue systems, so that mutations are passed to the daughter differentiating cells and not retained in the parental cell; and (d) stem cell competition, whereby undamaged stem cells outcompete damaged stem cells for residence in the niche. DNA repair mainly occurs within a few days of irradiation, while stem cell competition requires weeks or many months depending on the tissue type. The aforementioned processes may contribute to the differences in carcinogenic radiation risk values between tissues, and may help to explain why a rapidly replicating tissue such as small intestine is less prone to such risk. The processes also provide a mechanistic insight relevant to the LNT model, and the relative and absolute risk models. The radiobiological knowledge also provides a scientific insight into discussions of the dose and dose-rate effectiveness factor currently used in radiological protection guidelines. In addition, the biological information contributes potential reasons for the age-dependent sensitivity to radiation carcinogenesis, including the effects of in-utero exposure.

N-palmitoylation of the radioprotective domain of interleukin-1 affords inhibition of LPS-induced nitric oxide generation.

Interleukin-1beta (IL-1beta), a cytokine involved in homeostatic processes such as the immune system and inflammatory reactions, is a potent inducer of nitric oxide. The nonapeptide of human IL-1beta (VQGEESNDK, position 163-171, specific radioprotective domain--SRD) has been shown to retain radioprotective, immunostimulatory, and adjuvant activities of the native molecule without any inflammatory and pyrogenic properties. Unlike the parent IL-1, SRD did not induce nitric oxide (NO) in control or irradiated RAW 264.7 cells nor did it affect inducible nitric oxide synthase (iNOS) as shown by ELISA based mRNA assay (Quantikine). A lipophillic derivative of the SRD (a palmitoyl residue at the amino terminus of the SRD) was synthesized (palmitoyl specific radioprotective domain, P-SRD) to find out if this structural derivatization would restore the NO-inducing ability of IL-1. Surprisingly, P-SRD not only did not induce NO, but significantly inhibited lipopolysaccharide (LPS) stimulated nitric oxide (NO) production. Quantikine studies indicated that P-SRD also inhibited iNOS in LPS stimulated macrophage cells, suggesting that decrease in NO production in the presence of P-SRD was the result of iNOS mRNA inhibition. These results indicate that N-palmitoylation of SRD may effectively ameliorate potentially fatal symptoms of LPS-induced endotoxemic hypotensive shock associated with IL-1 without inflammatory and pyrogenic toxic side effects.

Radioprotection, pharmacokinetic and behavioural studies in mouse implanted with biodegradable drug (amifostine) pellets.

PURPOSE: We evaluated the use of a subcutaneously (s.c.) implantable, biodegradable pellet as a drug delivery system for the radioprotector amifostine. MATERIALS AND METHODS: Mice were implanted s.c. with either the custom-made biodegradable amifostine drug pellet or the placebo pellet without amifostine, exposed to cobalt-60 gamma-radiation (bilateral, 1 Gy min(-1), 7-16 Gy), and the 30-day survival rate was monitored. The non-irradiated mouse was used for pharmacokinetic and behavioural tests. RESULTS: Significant radioprotection (85-95% survival) at 10 Gy was observed in the three-amifostine pellet implanted group 3-5 h after implantation. LD50/30 was 7.97, 8.74 and 16.64 Gy for the control, three-placebo pellet (dose reduction factor, DRF=1.10, p<0.01), and three-amifostine pellet (DRF=1.79, p<0.01) groups respectively in mouse exposed to radiation 2h after implantation. Radioprotection at 12 Gy was observed up to 4h after s.c. amifostine administration and up to 3h after implantation. Pharmacokinetic data revealed that the three-amifostine pellet group had sustained blood WR-1065 levels at 2 h after implantation, in contrast to the reported sharp peak at 30 min for s.c. administration. Although locomotor activity was significantly reduced (p<0.01) in the amifostine pellet group, the onset of the locomotor decrement was delayed as compared with groups that received 400 and 750 mg kg(-1) s.c. amifostine. CONCLUSIONS: Amifostine in biodegradable implant was effective. The radioprotection observed was comparable between conventional s.c. administration of the drug and implantation. Pharmacokinetic data and locomotor activity suggest that the implantation was beneficial though radioprotection data warrants formulation improvements in implants.

Hematopoietic responses under protracted exposures to low daily dose gamma irradiation.

In attempting to evaluate the possible health consequences of chronic ionizing radiation exposure during extended space travel (e.g., Mars Mission), ground-based experimental studies of the clinical and pathological responses of canines under low daily doses of 60Co gamma irradiation (0.3-26.3 cGy d-1) have been examined. Specific reference was given to responses of the blood forming system. Results suggest that the daily dose rate of 7.5 cGy d-1 represents a threshold below which the hematopoietic system can retain either partial or full trilineal cell-producing capacity (erythropoiesis, myelopoiesis, and megakaryopoiesis) for extended periods of exposure (>1 yr). Trilineal capacity was fully retained for several years of exposure at the lowest dose-rate tested (0.3 cGy d-1) but was completely lost within several hundred days at the highest dose-rate (26.3 cGy d-1). Retention of hematopoietic capacity under chronic exposure has been demonstrated to be mediated by hematopoietic progenitors with acquired radioresistance and repair functions, altered cytogenetics, and cell-cycle characteristics. Radiological, biological, and temporal parameters responsible for these vital acquisitions by hematopoietic progenitors have been partially characterized. These parameters, along with threshold responses, are described and discussed in relation to potential health risks of the space traveler under chronic stress of low-dose irradiation.

In vivo radioprotection by 5-androstenediol: stimulation of the innate immune system.

We showed previously that 5-androstenediol stimulates myelopoiesis, increases the numbers of circulating neutrophils and platelets, and enhances resistance to infection in gamma-irradiated mice. We have extended those studies to include monocytes, natural killer (NK) cells, eosinophils and basophils, and we have measured the activation marker CD11b using flow cytometry. Androstenediol (160 mg/kg) was administered subcutaneously to female B6D2F1 mice 24 h before whole-body gamma irradiation. Androstenediol treatments increased the blood levels of neutrophils, monocytes and NK cells in unirradiated animals; decreased the numbers of circulating eosinophils; and ameliorated radiation-induced decreases in neutrophils, monocytes, NK cells, erythrocytes and platelets. The androstenediol treatments had no significant effect on the numbers of circulating B cells or T cells. CD11b labeling intensity on monocytes was decreased slightly after androstenediol treatment. In contrast, radiation or androstenediol alone caused increases in CD11b labeling intensity on NK cells. Androstenediol and radiation combined caused a marked increase in NK cell CD11b. The results indicate that androstenediol increases the numbers of the three major cell types of the innate immune system (neutrophils, monocytes and NK cells), that androstenediol-induced changes in blood elements in irradiated animals persist for at least several weeks, and that there is a significant positive interaction between radiation and administration of androstenediol in the activation of NK cells.

Androstenediol stimulates myelopoiesis and enhances resistance to infection in gamma-irradiated mice.

The ionizing radiation-induced hemopoietic syndrome is characterized by defects in immune function and increased mortality due to infections and hemorrhage. Since the steroid 5-androstene-3beta, 17beta-diol (5-androstenediol, AED) modulates cytokine expression and increases resistance to bacterial and viral infections in rodents, we tested its ability to promote survival after whole-body ionizing radiation in mice. In unirradiated female B6D2F1 mice, sc AED elevated numbers of circulating neutrophils and platelets and induced proliferation of neutrophil progenitors in bone marrow. In mice exposed to whole-body (60)Co gamma-radiation (3 Gy), AED injected 1 h later ameliorated radiation-induced decreases in circulating neutrophils and platelets and marrow granulocyte-macrophage colony-forming cells, but had no effect on total numbers of circulating lymphocytes or erythrocytes. In mice irradiated (0, 1 or 3 Gy) and inoculated four days later with Klebsiella pneumoniae, AED injected 2 h after irradiation enhanced 30-d survival. Injecting AED 24 h before irradiation or 2 h after irradiation increased survival to approximately the same extent. In K. pneumoniae-inoculated mice (irradiated at 3-7 Gy) and uninoculated mice (irradiated at 8-12 Gy), AED (160 mg/kg) injected 24 h before irradiation significantly promoted survival with dose reduction factors (DRFs) of 1.18 and 1.26, respectively. 5-Androstene-3beta-ol-17-one (dehydroepiandrosterone, DHEA) was markedly less efficacious than AED in augmenting survival, indicating specificity. These results demonstrate for the first time that a DHEA-related steroid stimulates myelopoiesis, and ameliorates neutropenia and thrombocytopenia and enhances resistance to infection after exposure of animals to ionizing radiation.

Hematopoietic tissue repair under chronic low daily dose irradiation.

The capacity of the hematopoietic system to repair constantly accruing cellular damage under chronic, low daily dose gamma irradiation is essential for the maintenance of a functional hematopoietic system, and, in turn, long term survival. In certain individuals, however, such continuous cycles of damage and repair provide an essential inductive environment for selected types of hematopathologies, e.g., myeloid leukemia (ML). In our laboratory we have been studying temporal and causal relationships between hematopoietic capacity, associated repair functions, and propensities for hematologic disease in canines under variable levels of chronic radiation stress (0.3-26.3 cGy d-1). Results indicate that the maximum exposure rate tolerated by the hematopoietic system is highly individual-specific (three major responding subgroups identified) and is based largely on the degree to which repair capacity, and, in turn, hematopoietic restoration, is augmented under chronic exposure. In low-tolerance individuals (prone to aplastic anemia, subgroup 1), the failure to augment basic repair functions seemingly results in a progressive accumulation of genetic and cellular damage within vital progenitorial marrow compartments (particularly marked within erythroid compartments) that results in loss of reproductive capacity and ultimately in collapse of the hematopoietic system. The high-tolerance individuals (radioaccomodated and either prone- or not prone to ML, subgroup 2 & 3) appear to minimize the accumulating damage effect of daily exposures by extending repair functions, which preserves reproductive integrity and fosters regenerative hematopoietic responses. As the strength of the regenerative response manifests the extent of repair augmentation, the relatively strong response of high- tolerance individuals progressing to patent ML suggests an insufficiency of repair quality rather than repair quantity. The kinetics of these repair-mediated, regenerative hematopoietic responses within the major subgroups are under study and should provide useful insights into the nature of hematopoietic accommodation (or its failure) under greatly extended periods of chronic, low-daily-dose ionizing radiation exposure.

Response of hemopoiesis in dogs to continuous low dose rate total body irradiation.

Among the cytotoxic agents which particularly cause damage to cell renewal systems, ionizing radiation is one of the most effective ones since it leads to inactivation of all types of proliferating cells including resting stem cells. It is the aim of this paper to present the effects of continuous low dose rate total body irradiation (TBI) on hemopoiesis in dogs. The animals were exposed to gamma-rays from a 60-Co source, receiving a daily radiation dose of 0.0188 Gy for indefinite times. Sequential hematological studies performed included determinations of peripheral blood cell counts and of total cell numbers in standardized bone marrow samples, assessments of progenitor cells GM-CFC in the blood and bone marrow, and of colony-stimulating activity (CSA) in the serum. The lymphocytes, the thrombocytes and neutrophilic granulocytes uniformly showed early decreases within the first 200 to 500 days corresponding to cumulative radiation doses in the range up to 3.8 to 9 Gy, but remained stable at subnormal levels in the period up to 1,700 days of exposure. The GM-CFC numbers in bone marrow samples from the rib clearly showed a strong decrease within the first 150 days of exposure preceding the changes in the blood granulocyte concentration. A transient partial recovery of the GM-CFC was observed at later times between 700 and 1,200 days of exposure, followed by another decrease to extremely low values at cumulative doses in the range of 32 Gy.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphologic changes of apoptosis induced in human chronic myelogenous leukemia "blast" cells by SC41661A (Searle), a selective inhibitor of 5-lipoxygenase.

Several inhibitors of the arachidonic acid-metabolizing enzyme, 5-lipoxygenase reduce proliferation of hematopoietic and non-hematopoietic cells and cell lines and some cells undergo limited differentiation. Cells were cultured from patients with chronic myelogenous leukemia in "blast" crisis with the selective inhibitor of 5-lipoxygenase, SC41661A[3-(3,5-bis(1,1-dimethyl)-4-hydroxyphenyl)hiol]-N-me thyl-N-[2-(2- phridinyl-propanamide)]. Cells cultured for 3 to 5 days with 40 microM SC41661A exhibited reduced cellular numbers along with ultrastructural changes and DNA laddering characteristic of apoptosis. Similar culture conditions reduced proliferation of U937 monoblastoid cells. In U937 cells, the ultrastructural features of apoptosis were not observed at 72 hours, when DNA laddering was present and cell numbers were reduced, but was present after 144 hours of culture. Dissociation between certain morphologic and biochemical sequelae of apoptosis has been described in other systems. These observations are of interest since the induction of apoptosis in dividing chronic myelogenous leukemia (CML) cells by a non-cytotoxic agent suggests paradigmatically new sites for therapeutic intervention.

Chronic radiation-induced alteration in hematopoietic repair during preclinical phases of aplastic anemia and myeloproliferative disease: assessing unscheduled DNA synthesis responses.

Protracted, low-daily-dose gamma-ray exposure (3.8-7.5 cGy/day) segregates canines into separate survival- and pathology-based subgroups by the early elicitation of distinct, repair-mediated hemopathological response pathways. In this study, we verified the blood and marrow responses of two major subgroups prone to either aplastic anemia or myeloproliferative disease, along with two variants, and extended our analyses of hematopoietic repair to include studies of DNA repair in bone marrow blasts using an autoradiographically based unscheduled DNA synthesis (UDS) assay. The myeloproliferative disease-prone subgroup exhibited extended survival (> 200 days), related to partial, gradual restoration of blood leukocyte, platelet, and marrow progenitor levels following an initial phase of acute suppression. Marrow blasts taken during the restoration phase showed expanded and qualitatively modified UDS relative to marrow blasts of age-matched control animals. The amount of UDS per blast (signal strength) increased significantly, as did the number of UDS-positive cells and their sensitivities to high-dose UV induction and 1-beta-D-arabinofuranosylcytosine chemical inhibition. A nonevolving myeloproliferative disease-prone variant having prolonged survival (> 200 days) and restored blood cells and marrow progenitor levels also had marrow blasts with expanded UDS responses, but these were uniquely evoked by low (but not high) doses of UV inducer. The aplastic anemia-prone subgroup was characterized by short survival (< 200 days), progressive decline (without restoration) in all measured blood and marrow compartments, and largely nonsignificant changes in UDS responses of marrow blasts. A variant of this aplastic anemia-prone subgroup (with comparable short survival due to markedly ineffective hematopoiesis, but expressing select preleukemic features) exhibited reduced numbers (relative to age-matched controls) of highly responsive, UDS-positive marrow blasts (in terms of UDS signal strength and increased to sensitivity 1-beta-D-arabinofuranosylcytosine-induced UDS inhibition). From these observations we conclude that: (a) the UDS response of marrow blasts, a correlate of hematopoietic progenitorial repair, is altered differentially within selected subgroups of animals under chronic radiation exposure; and (b) the nature of altered UDS repair response patterns appears to be largely related to the preclinical status/predisposition of the individual animal and thus may provide prognostically useful information in the clinical monitoring of chronically irradiated individuals with minimal but evolving hematological disease.

5,8,11,14-eicosatetraynoic acid-induced destruction of mitochondria in human prostate cells (PC-3).

Culturing human prostate PC-3 cells for 4, 24, or 72 h in the presence of 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of arachidonic acid metabolism and cholesterol biosynthesis, markedly altered the morphology and reduced the number of mitochondria in the treated cells. Using quantitative electron microscopic morphometry, we documented changes in the number, form, area, matrix density, and integrity of the cristae and limiting membranes of mitochondria in cells cultured with ETYA. The inhibition of cholesterol synthesis or the substitution of ETYA for polyunsaturated fatty acids in the inner membrane may participate in the disruption of the mitochondria, which resembles the morphologic sequelae of oxidative stress. If sufficiently extensive, these changes could contribute to the inhibition of cellular proliferation by ETYA.

Acquired radioresistance of hematopoietic progenitors (granulocyte/monocyte colony-forming units) during chronic radiation leukemogenesis.

Protracted exposure of dogs to low daily doses of whole-body gamma-radiation (7.5 cGy/day for duration of life) elicits a high incidence of myeloid leukemia or related myeloproliferative disorders. Under such exposure, vital hematopoietic progenitors [granulocyte/monocyte colony-forming units in agar (CFU-GM)] acquire increased radioresistance along with renewed proliferative capacity at an early phase of evolving myeloid leukemia. To further characterize the expression of acquired radioresistance by CFU-GM, we evaluated the effects of various exposure rates, cumulative radiation doses, and times of exposure and postexposure in several groups of long-lived dogs under two conditions of irradiation: (a) continuous, duration-of-life exposures at dose rates of 0.3-7.5 cGy/day; and (b) discontinuous, fraction-of-life exposures at dose rates of 3.8-26.3 cGy/day, with cumulative doses of 450-3458 cGy and postexposure times of 14-4702 days. Results indicated that (a) under protracted continuous irradiation, the degree of radioresistance expressed by CFU-GM in vitro increased markedly in a biphasic pattern with rising daily rates of exposure; (b) under discontinuous, fraction-of-life exposure regimens, elevated levels of radioresistance were expressed and stably maintained by CFU-GM only following large radiation doses accumulated at high dose rates; and (c) with extended postexposure times, the magnitude of expressed radioresistance appeared to wane. These results continue to support the hypothesis that the acquisition of radioresistance and associated repair functions by vital lineage-committed progenitors, under the strong selective and mutagenic pressure of chronic irradiation, is tied temporally and causally to leukemogenic transformation elicited by radiation exposure.

Differential effects of ETYA, a PUFA-analogue, on prostate (PC3) and monoblastoid U937 ultrastructure; lack of correlation with reduced proliferation.

ETYA (5,8,11,14-eicosatetraynoic acid), a polyunsaturated fatty acid analogue, inhibits proliferation of PC3 and U937 cells and induces a limited differentiation in U937 cells. Human prostate PC3 cells cultured for 72 h with 40 microM ETYA in fetal calf serum contained putative lipofuscin bodies, myelin figures and mitochondria with damaged cristae and matrices. These changes were absent from human U937 monoblastoid cells incubated with ETYA in CPSR3, a semipurified serum replacement. U937 cells cultured with ETYA in fetal calf serum contained occasional lipofuscin bodies, while PC3 cells cultured in CPSR3 exhibited all of the changes described. ETYA reduced the oxygen consumption of both cell lines. Therefore we conclude: (a) The response to ETYA by cells of dissimilar developmental origin is not identical; (b) unidentified serum components can augment potential ETYA-induced oxidative stress-responses of cells; (c) inhibition of U937 proliferation by ETYA does not depend upon the morphologic changes seen in PC3 cells, which resemble sequelae of oxidative stress with excess free radicals; and (d) rapid ETYA-induced inhibition of oxygen consumption in both cell lines implies a reduced synthesis of ATP that could contribute to the reversible impairment of cellular proliferation.

Hematopoietic cell crisis: an early stage of evolving myeloid leukemia following radiation exposure.

Under select radiological conditions, chronic radiation exposure elicits a high incidence of myeloproliferative disease, principally myeloid leukemia (ML), in beagles. Previously we demonstrated that for full ML expression, a four-stage preclinical sequence is required, namely (I) suppression, (II) recovery, (III) accommodation, and (IV) preleukemic transition. Within this pathological sequence, a critical early event has been identified as the acquisition of radioresistance by hematopoietic progenitors that serves to mediate a newfound regenerative hematopoietic capacity. As such, this event "sets the stage" for preleukemic progression by initiating progression from preclinical phase I to II. Due to the nature of target cell suppression, the induction of crisis, and the outgrowth of progenitors with altered phenotypes, this preleukemic event resembles the "immortalization" step of the in vitro transformation sequence following induction with either physical and chemical carcinogens. The radiological, temporal, and biological dictages governing this event have been extensively evaluated and will be discussed in light of their role in the induction and progression of chronic radiation leukemia.

Probing altered hematopoietic progenitor cells of preleukemic dogs with JANUS fission neutrons.

Protracted courses of low-daily-dose gamma irradiation elicit high incidences of myeloproliferative disease, principally myeloid leukemia (ML), in beagle dogs. A four-phase preclinical sequence in the induction of ML has been described: (1) suppression, (2) recovery, (3) accommodation, and (4) preleukemic transition. Within this sequence, a critical "early"-occurring hematopoietic target cell event that promotes progression of preclinical phases I and II has been identified and characterized by an acquisition of increased radioresistance to low-LET gamma rays by granulocyte/monocyte lineage-committed progenitor cells (CFU-GM). To gain further insight into the basis of this critical event, the acquired survival response of preleukemic progenitor cells has been probed in vitro with high-LET fission neutrons. For these studies, marrow CFU-GM were isolated from chronically irradiated preleukemic dogs, as well as from nonirradiated controls, subjected to graded doses (0-300 cGy) of either JANUS fission neutrons or 60Co gamma rays, and assayed for survival by a standard cloning assay. Major observations resulting from these assays include the following. First, the acquired radioresistance of preleukemic CFU-GM to low-LET gamma rays noted previously extends to high-LET fission neutrons as well. Relative to control CFU-GM exhibited small but significant increases in radioresistance of about 10 cGy with an average D0 value of 38 (+/- 2.3) cGy for preleukemic CFU-GM, and 28 (+/- 1.3) cGy for the control levels, the CFU-GM irradiated within a marrow dose range of 10-75 cGy. Second, at higher neutron doses (150-600 cGy), fractional survival of both control and preleukemic CFU-GM declined nonexponentially, suggesting the existence of a small, radioresistant subpopulation constituting about 2% of the total marrow CFU-GM within normal nonirradiated dogs, and a 15% fraction of the progenitor cell population in preleukemic marrow (preclinical phases II-IV). The latter is most likely the result of a normally minor subpopulation gaining a growth advantage due to its inherent radioresistance and clonally expanding in the strong selective pressure of chronic marrow irradiation in vivo. We speculate that these qualitative/quantitative changes in the function of progenitor cells foster the initiation of aberrant regenerative hematopoiesis characteristic of early evolving radiation leukemogenesis.

Radon exposure system for mammalian cells in culture: design, operation, and dosimetry.

A novel system for Rn gas exposure of mammalian cells in culture has been designed, constructed, and used to directly assess both the magnitude and the nature of chronic, low-dose Rn/Rn daughter toxicity of exposed vital lung cells isolated from normal pulmonary tissue, propagated and exposed in vitro. Direct correlations between atmospheric Rn concentrations, alpha-particle fluences, and macro- and microdoses of absorbed radiation doses by lung cells provide for a heretofore unavailable assessment of critical doses to vital cells.