Chromosomal aberrations in peripheral blood lymphocytes exposed to a mixed beam of low energy neutrons and gamma radiation

Cells exposed to thermal neutrons are simultaneously damaged by radiations with high and low linear energy transfer (LET). A question relevant for the assessment of risk of exposure to a mixed beam is whether the biological effect of both radiation types is additive or synergistic. The aim of the present investigation was to calculate whether the high and low LET components of a thermal neutron field interact when damaging cells. Human peripheral blood lymphocytes were exposed to neutrons from the HB11 beam at the Institute for Energy and Transport, Petten, Netherlands, in a 37 °C water phantom at varying depths, where the mix of high and low LET beam components differs. Chromosomal aberrations were analysed and the relative biological effectiveness (RBE) values as well as the expected contributions of protons and photons to the aberration yield were calculated based on a dose response of aberrations in lymphocytes exposed to (60)Co gamma radiation. The RBE for 10 dicentrics per 100 cells was 3 for mixed beam and 7.2 for protons. For 20 dicentrics per 100 cells the respective values were 2.4 and 5.8. Within the limitations of the experimental setup the results indicate that for this endpoint there is no synergism between the high and low LET radiations.

Recovery Capacity of Glial Progenitors afterIn VivoFission-Neutron or X Irradiation: Age Dependence, Fractionation and Low-Dose-Rate Irradiations

Previous experiments on the radiosensitivity of O-2A glial progenitors determined for single-dose fission-neutron and X irradiation showed log-linear survival curves, suggesting a lack of accumulation of recovery of sublethal damage. In the present study, we addressed this question and further characterized the radiobiological properties of these glial stem cells by investigating the recovery capacity of glial stem cells using either fractionated or protracted whole-body irradiation. Irradiations were performed on newborn, 2-week-old or 12-week-old rats. Fractionated irradiations (four fractions) were performed with 24-h intervals, followed by cell isolations 16- 24 h after the last irradiation. Single-dose irradiations were followed by cell isolation 16-24 h after irradiation or delayed cell isolation (4 days after irradiation) of the O-2A progenitor cells from either spinal cord (newborns) or optic nerve (2- and 12-week-old rats). Results for neonatal progenitor cell survival show effect ratios for both fractionated fission-neutron and X irradiation of the order of 1.8 when compared with single-dose irradiation. A similar ratio was found after single-dose irradiation combined with delayed plating. Comparable results were observed for juvenile and adult optic nerve progenitors, with effect ratios of the order of 1.2. The present investigation clearly shows that fractionated irradiation regimens using X rays or fission neutrons and CNS tissue from rats of various ages results in an increase in O-2A progenitor cell survival while repair is virtually absent. This recovery of the progenitor pool after irradiation can be observed at all ages but is greatest in the neonatal spinal cord and can probably be attributed to repopulation.

Analysis of loss of heterozygosity in lymphoma and leukaemia arising in F1 hybrid mice locates a common region of chromosome 4 loss

Previous studies have identified five lymphoma-related tumour suppressor gene regions on murine chromosome 4. Using detailed allelotype analysis on a range of lympho-haematopoietic tumour types arising in F1 hybrid mice, we now show a consistent pattern of loss of heterozygosity (LOH) which identifies a common region of loss delineated by microsatellites D4Mit21 and D4Mit53 on proximal chromosome 4. This critical segment corresponds to the thymic lymphoma tumour suppressor region 5 (TLSR5) identified in an earlier study. Tumours of this type have also been reported as showing allelic loss from the Trp53 and Ikaros regions on chromosome 11. In the present study, only a small fraction of tumours showed LOH in the Ikaros region, while a minority of lymphomas, but not acute myeloid leukaemias, showed allelic loss of the chromosome 11 segment encoding Trp53. These and other data indicate strongly that the genomic regions identified as showing recurrent LOH depend on the genetic background of the mice. Overall, the results indicate a key role for a tumour suppressor gene(s) encoded in an approximately 3 cM segment on proximal chromosome 4 and provide an experimental basis for the further investigation of the functional role of candidate genes which include Pax5 and Tgfbr1.

Microdosimetry model for boron neutron capture therapy: II. Theoretical estimation of the effectiveness function and surviving fractions

A model has been developed to obtain a better understanding of the effects of boron neutron capture therapy (BNCT) on a cellular scale. This model, the microdosimetry model MICOR, has been developed to include all reactions important for BNCT. To make the model more powerful in the translation from energy deposition to biological effect, it has been designed to be capable of calculating the effectiveness function. Based on this function, the model can calculate surviving fractions, RBE values and boron concentration distributions. MICOR has been used to analyze an extensive set of biological experiments performed at the HB11 beam in Petten. For V79 Chinese hamster cells, the effectiveness function is determined and used to generate surviving fractions. These fractions are compared with measured surviving fractions, which results in a good agreement between the measured and calculated surviving fractions (within the uncertainties of the measurements).

Microdosimetry model for boron neutron capture therapy: I. Determination of microscopic quantities of heavy particles on a cellular scale

Due to the limitations of existing microdosimetry models, a new model called MICOR has been developed to analyze the spatial distribution of microscopic energy deposition for boron neutron capture therapy (BNCT). As in most existing models, the reactions independent of the incident neutron energy such as the boron and the nitrogen capture reactions can be considered. While other models do not include reactions that are dependent on the neutron energy such as the proton recoil reaction, the present model is designed so that the energy deposition resulting from these reactions is included. The model MICOR has been extended to enable the determination of the biological effects of BNCT, which cannot be done with the existing models. The present paper describes the determination of several microscopic quantities such as the number of hits, the energy deposition in the cell nucleus, and the distribution of lineal and specific energy deposition. The companion paper (Radiat. Res. 155, 000-000 2001) deals with the conversion of these microscopic quantities into biological effects. The model is used to analyze the results of a radiobiological experiment performed at the HB11 facility in the HFR in Petten. This analysis shows the value of the model in determining the dose depositions on a cellular scale and the importance of the extension to the energy deposition of the proton recoil.

Age dependence of the radiosensitivity of glial progenitors for In vivo fission-neutron and X irradiation

O-2A progenitor cells are the stem cells of the myelin-forming oligodendrocytes in the central nervous system. In the epithermal reactor beams used for boron neutron capture therapy (BNCT) for treatment of brain tumors, fission neutrons are a contaminating component. To estimate the radiosensitivity of the O-2A progenitors for fission neutrons, an in vivo-in vitro clonogenic assay was used. Radiosensitivity of progenitors obtained from the spinal cord of 1- or 5-day-old rats or the optic nerve of 2- or 12-week-old rats for 1 MeV fission neutrons was compared to that for 300 kVp X rays. Dose-survival curves were fitted according to the linear-quadratic model. The resulting beta component was very small to negligible. Progenitor cells obtained from rats of different ages show differences in radiosensitivity, characterized by different alpha values. RBE values for fission neutrons were 3.5 for 1-day-old spinal cord, 3.2 for 5-day-old spinal cord, 3.0 for 2-week-old optic nerve, and 4.3 for 12-week-old optic nerve. These high RBE values indicate the importance of minimizing the fast-neutron component in the epithermal neutron beams used for BNCT.

Postoperative treatment of glioblastoma with BNCT at the petten irradiation facility (EORTC protocol 11,961)

The boron neutron capture therapy is based on the reaction occurring between the isotope 10B and thermal neutrons. A low energy neutron is captured by the nucleus and it disintegrates into two densely ionising particles, Li nucleus and He nucleus (alpha particle), with high biological effectiveness. On the basis of comprehensive preclinical investigations in the frame of the European Collaboration with Na2B12H11SH (BSH), as boron delivery agent, the first European phase I, clinical trial was designed at the only available epithermal beam in Europe, at the High Flux Reactor, Petten, in the Netherlands. The goal of this study is to establish the safe BNCT dose for cranial tumors under defined conditions. BNCT is applied as postoperative radiotherapy in 4 fractions, after removal of the tumor for a group of patients suffering from glioblastoma, who would have no benefit from conventional treatment, but have sufficient life expectancy to detect late radiation morbidity due to BNCT. The starting dose is set at 80% of the dose where neurological effects occurred in preclinical large animal experiments following a single fraction. The radiation dose will be escalated, by constant boron concentration in blood, in 4 steps for cohorts of ten patients, after an observation period of at least 6 months after the end of BNCT of the last patient of a cohort. The adverse events on healthy tissues due to BSH and due to the radiotherapy will be analysed in order to establish the maximal tolerated dose and dose limiting toxicity. Besides of the primary aim of this study the survival will be recorded. The first patient was treated in October 1997, and further four patients have been irradiated to-date. The protocol design proved to be well applicable, establishing the basis for scientific evaluation, for performance of safe patient treatment in a very complex situation and for opening the possibility to perform further clinical research work on BNCT.

Molecular mapping of chromosome 2 deletions in murine radiation-induced AML localizes a putative tumor suppressor gene to a 1.0 cM region homologous to human chromosome segment 11p11-12

Radiation-induced acute myeloid leukemias (AMLs) in the mouse are characterized by chromosome 2 deletions. Previous studies showed that a minimal deleted region (mdr) of approximately 6.5 cM is lost from one homologue in chromosome 2-deleted AMLs. An AML tumor suppressor gene is proposed to map within this mdr. In this study, we refine the mdr to a I cM interval between markers D2Mit126 and D2Mit185 by microsatellite analysis of 21 primary radiation-induced F I AMLs. The construction of a partial yeast artificial chromosome (YAC) contig spanning the mdr and the location of six known genes indicated that the 1 cM mdr is homologous to human 11p11-12, a region implicated in some human AMLs. Screening of five cell lines derived from primary radiation-induced AMLs for homozygous loss of microsatellites and genes mapping within the mdr revealed loss of both copies of the hemopoietic tissue-specific transcription factor Sfpi1(PU.1/Spi1) in one cell line. Studies of primary and F1 AMLs failed to implicate Sfpi1 as the AML tumor suppressor gene. YAC contig construction, together with data suggesting that the critical gene flanks Sfpi1, represents significant progress toward identifying an AML tumor suppressor gene.

Interaction of mercaptoundecahydrododecaborate (BSH) with phosphatidylcholine: relevance to boron neutron capture therapy

The interaction of mercaptoundecahydrododecaborate (B12H11SH2-, BSH) with phosphatidylcholine was investigated in this study in order to illuminate possible uptake mechanisms of BSH in tumor cells. BSH has been used clinically in Japan as a boron containing agent in patients with malignant brain tumors for boron neutron capture therapy (BNCT). After infusion, BSH accumulates selectively in tumor tissue. Little is known for the mechanism of boron uptake to tumor cells. Fourier transform infrared (FTIR) spectrometry was used to quantify BSH (at wavenumber 2490 cm-1) and phosphatidylcholine (at wavenumber 2850-2970 cm-1). After extraction into carbon tetrachloride (CCl4), we could find an absorbance maximum at 2490 cm-1 as a B-H band in the mixture of BSH with phosphatidylcholine, which is attributed to a BSH-phosphatidylcholine complex, which could dissolve well in CCl4. The molar ratio of BSH to phosphatidylcholine in the CCl4 solution was at most one mole of BSH to two moles of phosphatidylcholine independent of the excess BSH. The doubly negatively charged BSH can interact with two phosphatidylcholine molecules through their singly positively charged choline residues. These ion pairs could be responsible for membrane binding and penetration, and for cell internalization.

The compound factor of the 10B(n,alpha)7Li reaction from borocaptate sodium and the relative biological effectiveness of recoil protons for induction of brain damage in boron neutron capture therapy

To make clinical trials of boron neutron capture therapy safe for patients, it is necessary to know the relative biological effectiveness (RBE) of the radiation components and the compound factor of the boron carrier to be used. Here a method is derived to determine the RBE of recoil protons and the compound factor of compounds from in vivo experiments with different concentrations of boron. The method uses a simultaneous fit of both these parameters to all experimental data. This method is applied to the studies of tolerance of healthy tissue in dogs at the High Flux Reactor in Petten, The Netherlands. The RBE for the recoil protons generated by the neutrons present in the epithermal neutron beam [together with the RBE of the protons from the 14N(n,p)14C reaction] for induction of severe neurological symptoms was found to be 3.93+/-0.43 (95% confidence limits 3.06-4.79), and 2.33+/-0.14 (2.04-2.61) for induction of changes detectable by magnetic resonance imaging. The compound factor for Na2B12H11SH in brain tissue, using severe neurological symptoms as end point, was determined to be 0.37+/-0.06 (95% confidence limits 0.24-0.50). For changes detectable by magnetic resonance imaging, the value was found to be 0.65+/-0.04 (0.58-0.73).

A review: CNS effects and normal tissue tolerance in dogs

Large animal studies have been utilized to define tolerance of normal brain to irradiation and verify treatment planning programs with two recently installed epithermal neutron beams. The normal brain tolerance studies utilized two biological endpoints, magnetic resonance visible damage only and neurologic signs progressing to death. The studies focused on defining the proton RBE for the contaminant fast neutrons, and from nitrogen capture of thermal neutrons and boron capture reaction biologic effect. The proton RBE was approximately 3.0 to 6.7, depending on whether a dose reduction factor for the low gamma dose rate was employed. The microscopic distribution of the boron compounds, coupled with the extremely short length of the fission fragments from thermal neutron capture by 10B yields an observed biologic effect much less than would be expected from such high LET irradiation. This observed biologic effect, which is a product of the microdistribution of the boron atom and the relative biologic effect of the fission fragments has been termed compound factor. The compound factor was based on the calculated physical dose from the fission fragment in blood based on measured blood 10B concentration. The approximate compound factor for BSH was studied at the two institutions and it ranged from 0.27 to 0.55, depending on the site and the endpoint chosen. The mean compound factor for BPA was only studied at one site and was found to be 1.1 for both endpoints. The increase in the compound factor for BPA is in keeping with previous calculations based on the differences in compound distribution. Results of these studies has helped the initiation of phase I and phase II clinical trials at Brook haven National Laboratory and the planned European clinical trials at Petten, The Netherlands.

Sensitivity of murine haemopoietic stem cell populations to X-rays and 1 MeV fission neutrons in vitro and in vivo under hypoxic conditions

The radiosensitivity of primitive haemopoietic stem cells that repopulate the bone marrow with precursors of granulocytes and macrophages (MRA[CFU-C]), mature stem cells capable of forming spleen colonies in lethally irradiated recipients (CFU-S-7) and colony-forming units in culture (CFU-C) were determined in vitro and under hypoxic conditions in vivo for 1 MeV fission neutrons and 300 kV X-rays. The obtained D0's were compared with previously observed D0's after irradiation in vivo under normal oxic conditions. With 1 MeV fission neutron irradiation no significant difference in radiosensitivity of the cell populations was observed between normal in vivo irradiation and in vitro irradiation. With 300 kV X-rays a lower radiosensitivity for all three cell populations was observed after in vitro compared to in vivo irradiation. In vivo irradiation with fission neutrons under hypoxic conditions led to a small decrease in radiosensitivity. The obtained oxygen enhancement ratio (OER) for fission neutrons varied from 1.2 for MRA[CFU-C] to 1.5 for CFU-C. After in vivo irradiation with 300 kV X-rays under hypoxic conditions much higher OERs were observed. An OER = 1.8 was obtained for CFU-S and for MRA[CFU-C] and for CFU-C OER 3.0 and 2.9 were observed. These results indicate that the radioresistance of primitive haemopietic stem cells (MRA[CFU-C]) compared to mature stem cells (CFU-S-7) is mainly due to intrinsic factors and not to differences in localization or oxygenation between primitive and mature stem cells.

Chromosomal abnormalities in neutron-induced acute myeloid leukemias in CBA/H mice

Acute myeloid leukemias (AMLs) induced in CBA/H mice by 1 MeV fission neutrons have been examined for chromosomal abnormalities by G-band analysis. In common with X-ray- and alpha-particle-induced AMLs in CBA/H mice, more than 90% (16/17) of the myeloid leukemias had chromosome 2 abnormalities, in this case, all interstitial deletions. Chromosome 2 breakpoints were not wholly consistent, but clustering in three specific G-band regions was observed. Very distal (H-region) breakpoints were more common in the neutron AMLs than in X-ray- or alpha-particle-induced leukemias. These data indicate that neutron-induced AMLs in CBA/H mice are not characterized by a specific chromosome deletion but that a variety of chromosome 2 deletion types are associated with the disease. Trisomy of chromosome 1(12.5% AMLs) and aneusomy of chromosomes 6 (31% AMLs) and Y (37.5% AMLs) were noted. While chromatid breakage was observed occasionally in neutron-induced AML, no clear indications of persistent chromosomal instability or high levels of stable chromosomal change were apparent.

Role of telomeric sequences in murine radiation-induced myeloid leukaemia

A previous study indicated that a highly inbred CBA/H mouse colony contained four genotypic variants for telomere-like repeat (TLR) sequence arrays and that one variant subpopulation that constituted 20% of the colony contributed the vast majority (> 90%) of radiation-induced acute myeloid leukaemias (AMLs). Through screening of a satellite CBA/H colony and rescreening of the original colony, we show that, whereas germline telomere sequence polymorphism is frequent in CBA/H mice, there is no genetic link between a specific TLR locus variant and susceptibility to AML. Studies on telomere-hybridising fragments between 200 bp and 150 kb revealed that the germline telomere mutation frequency was highest for restriction fragments > 50 kb. The hypervariability of these high-molecular-weight fragments resulted in each CBA/H mouse from the highly inbred colony having a different genotype. Although it was not possible to ascribe a specific somatic telomere mutation to AML development, telomere rearrangements were common in induced AMLs. Some terminal telomere-hybridising restriction fragments were shortened in AML samples in comparison with normal tissue, but, insofar as the reduction in size was relatively small, it seems unlikely that telomere erosion is a major contributor to the molecular pathology of murine radiation-induced AML.

Microsatellite analysis of recurrent chromosome 2 deletions in acute myeloid leukaemia induced by radiation in F1 hybrid mice

Deletions and/or rearrangements involving one copy of chromosome 2 are consistent and early events in the development of murine acute myeloid leukaemia (AML) by radiation. More than 90% of AMLs induced in the CBA strain of mice express such cytogenetic alterations, with chromosome 2 breakpoints clustering in the C and F regions of the chromosome. In inbred mouse strains, the molecular resolution of these breakpoints is problematic. However, by using x-ray-induced AMLs in FI progeny of genetically divergent CBA/H x C57BI, it has been possible to show region-specific loss of heterozygosity (LOH) in genetically linked sets of chromosome 2 microsatellite alleles from one of the two parental chromosomes. In the majority of cases, an acceptable concordance was shown for AML chromosome 2 deletion, as defined by microsatellites and as revealed by G-band cytogenetics. A degree of breakpoint clustering was found, but the identification of a number of deletion types, based on the position of proximal and distal breakpoints as defined by microsatellite analysis, strongly supports a leukaemogenic mechanism involving gene deletion. No bias towards loss of CBA or C57BI alleles was observed, and the gender of AML-presenting animals did not appear to influence the parental origin of the deletions. A molecular map of chromosome 2 breakpoints has now been established in FI AMLs as a first step towards the molecular cloning of breakpoint sequences.

A stochastic model for subcellular dosimetry in boron neutron capture therapy

The therapeutic effectiveness of boron neutron capture therapy is highly dependent on the microscopic distribution of the administered boron compound. Two boron compounds with different uptake mechanisms in the tumour cells may thus cause effects of different degrees even if the macroscopic boron concentrations in the tumour tissue are the same. This difference is normally expressed quantitatively by the so-called relative local efficiency (RLE). In this work, a stochastic model for the subcellular dosimetry has been developed. This model can be used to calculate the probability for an energy deposition above a certain threshold level in the cell nucleus due to a single neutron capture reaction. If a threshold cell-kill function is assumed, and if the dose is low enough that multiple energy depositions are rare, the model can also be applied to calculations of the survival probability for a cell population. Subcellular boron distributions in rats carrying RG 2 rat gliomas were measured by subcellular fractionation after administration of two different boron compounds: a sulphydryl boron hydride (BSH) and a boronated porphyrin (BOPP). Based on these data, the RLE factors were then calculated for these compounds using the stochastic model.

Performance of sulfhydryl boron hydride in patients with grade III and IV astrocytoma: a basis for boron neutron capture therapy

This study investigated the rationale of boron neutron capture therapy (BNCT) for the treatment of Grade III and IV astrocytoma. The European Community joint research program on BNCT plans to use sulfhydryl boron hydride (BSH) in clinical trials. The work presented here, examines the performance of BSH in eight patients with Grade III and IV astrocytoma using a measurement technique which precisely correlates the boron uptake with the histology of the tumor and the peritumoral brain. Astrocytomas are exceptionally heterogeneous and spread migrating tumor cells into the surrounding brain. The patients were infused with 50 mg BSH per kilogram of body weight at 12, 18, 24 or 48 hours before surgery. At the time of operation, specimens were obtained of the tumor, skin, muscle, dura, blood, urine, and, when surgically possible, the brain adjacent to tumor. In three patients the intracellular boron distribution was investigated by subcellular fractionation. The blood clearance was biphasic with half-lives of 0.6 and 8.2 hours. After 3 days, approximately 70% of the dose injected was excreted in the urine. The maximum boron concentration in the tumor was 20 ppm, 12 hours after the infusion. The tumor-to-blood ratios ranged between 0.2 and 1.4, with the highest values after 18 to 24 hours. In the brain specimens the boron concentration never exceeded 1 ppm. This work confirms a selective uptake of boron in the tumor compared to the surrounding brain and that boron, to some extent, is incorporated in the tumor cells.

Radiosensitivity of testicular cells in the fetal mouse

The effects of prenatal X irradiation on postnatal development of the CBA/P mouse testis was studied. At days 14, 15 and 18 post coitus pregnant female mice were exposed to single doses of X rays ranging from 0.25-1.5 Gy. Higher doses resulted in extensive loss of fetal mice. In the male offspring, at days 3 and 31 post partum, the numbers of gonocytes, type A spermatogonia and Sertoli cells per testis were determined using the disector method. Furthermore, after irradiation at day 15 post coitus, the numbers of Leydig cells, mesenchymal cells, macrophages, myoid cells, lymphatic endothelial cells, endothelial cells and perivascular cells per testis were also determined at days 3 and 31 post partum. At day 3 post partum, the number of germ cells was decreased after irradiation at days 14 and 15 post coitus. A D0 value of 0.7 Gy was determined for the radiosensitivity of the gonocytes at day 14 post coitus. A D0 value of 0.8 Gy was determined for the gonocytes at day 15 post coitus which, however, seems to be less accurate. No accurate D0 value could be determined for the gonocytes at day 18 post coitus. At day 31 post partum, the repopulation of the seminiferous epithelium as well as testis weights and tubular diameters were more affected by irradiation with increasing age of the mice at the time of irradiation. The percentage of tubular cross sections showing spermatids decreased with increasing dose after irradiation at days 15 and 18 post coitus, but not after irradiation at day 14 post coitus. Furthermore, in tubular cross sections showing spermatids, exposure of testes to 1.25 and 1.5 Gy at day 18 post coitus resulted in significantly lower numbers of spermatids per cross section when compared to those testes exposed to the same doses at day 15 post coitus. This indicates that the radiosensitivity of the gonocytes increases with fetal age. Prenatal irradiation did not cause significant changes in the numbers per testis of the Sertoli cells or the interstitial cell types. The present results indicate that, in the fetal mouse testis, the spermatogonial stem cells are more sensitive to X irradiation than in the adult testis, while Sertoli cells and interstitial cells are relatively resistant.

Monitoring of blood-10B concentration for boron neutron capture therapy using prompt gamma-ray analysis

The aim of the present study was to monitor the blood-10B concentration of laboratory dogs receiving boron neutron capture therapy, in order to obtain optimal agreement between prescribed and actual dose. A prompt gamma-ray analysis system was developed for this purpose at the High Flux Reactor in Petten. The technique was compared with inductively coupled plasma-atomic emission spectrometry and showed good agreement. A substantial variation in 10B clearance pattern after administration of borocaptate sodium was found between the different dogs. Consequently, the irradiation commencement was adjusted to the individually determined boron elimination curve. Mean blood-10B concentrations during irradiation of 25.8 +/- 2.2 micrograms/g (1 SD, n = 18) and 49.3 +/- 5.3 micrograms/g (1 SD, n = 17) were obtained for intended concentrations of 25 micrograms/g and 50 micrograms/g, respectively. These variations are a factor of two smaller than irradiations performed at a uniform post-infusion irradiation starting time. Such a careful blood-10B monitoring procedure is a prerequisite for accurately obtaining such steep dose-response curves as observed during the dog study.

Radiosensitivity of testicular cells in the prepubertal mouse

The effects of total-body X irradiation on the prepubertal testis of the CBA/P mouse have been studied. At either day 14 or day 29 post partum male mice were exposed to single doses of X rays ranging from 1.5-6.0 Gy. At 1 week after irradiation the repopulation index method was used to study the radiosensitivity of the spermatogonial stem cells. A D0 value of 1.8 Gy was determined for the stem cells at day 14 post partum as well as for the stem cells at day 29 post partum, indicating that the radiosensitivity of the spermatogonial stem cells in the prepubertal mouse testis is already comparable to that observed in the adult mouse. One, 2 or 3 weeks after irradiation total cell numbers per testis of Sertoli cells, Leydig cells, mesenchymal cells, macrophages, myoid cells, lymphatic endothelial cells, endothelium and perivascular cells were determined using the disector method. The Sertoli cells and interstitial cell types appeared to be relatively radioresistant during the prepubertal period. No significant changes in plasma testosterone levels were found, indicating that there is no Leydig cell dysfunction after exposure to doses up to 6 Gy during the prepubertal period. Taken together, the radioresponse of the prepubertal mouse testis is comparable to that of the adult mouse testis.

Pharmacokinetics in melanoma-bearing mice of 5-dihydroxyboryl-6-propyl-2-thiouracil (BPTU), a candidate compound for boron neutron capture therapy

Blood pharmacokinetics and tissue distribution of 5-dihydroxyboryl-6-propyl-2-thiouracil (BPTU), a boron carrier with postulated melanin-seeking properties for boron neutron capture therapy, were determined in C57/BL mice with subcutaneous pigmented or non-pigmented B16 melanomas. Borocaptate sodium (BSH) was used as a boron compound without melanin-seeking properties in a comparative biodistribution study in the same animal tumour models. Administration of single doses showed that BPTU was retained better in the pigmented B16 tumour than in the non-pigmented variant. BPTU was found in large concentrations in kidney and liver. Brain boron was approximately 10-fold lower than tumour boron. On a molar basis, BPTU demonstrated higher affinity for B16 tumours than BSH. Owing to solubility limits, tumour boron concentrations in this mouse study were too low for effective application of BNCT. However, the high tumour-to-blood and tumour-to-normal tissues ratios indicate that, with appropriate formulation, BPTU could be a promising candidate for clinical BNCT.

Clinical phase-I study of Na2B12H11SH (BSH) in patients with malignant glioma as precondition for boron neutron capture therapy (BNCT)

PURPOSE

Within the European collaboration on boron neutron capture therapy (BNCT), a clinical Phase I study is being carried out to establish BNCT as an alternative treatment modality for malignant glioma (WHO III/IV). Data about the pharmacokinetics, biodistribution and toxicity of the boron compound Na2B12H11SH (BSH) are of great importance to avoid radiation damage of healthy tissue and to deliver a sufficient radiation dose.

METHODS AND MATERIALS

Twenty four patients suffering from a glioblastoma multiforme entered the study to date, infused with a maximum concentration of up to 50 mg BSH/kg. Boron concentration measurements in tissues, urine, and blood were carried out, using inductively coupled plasma-atomic spectroscopy (ICP-AES) and quantitative neutron capture radiography (QNCR). A cross-calibration of these boron determination techniques was carried out.

RESULTS

In tumor tissue, confirmed by histopathology of small biopsies, we found a consistently high but heterogeneous boron uptake. Necrotic parts contain much lower amounts of boron; normal brain tissue has shown no significant uptake. In skin, bone, muscle, and dura mater only small amounts of boron were found. In blood samples, we found biphasic kinetics, but with variations of the half-lives from patient to patient. The compound is mainly excreted through the urine, but an additional entero-hepatic pathway can be demonstrated. Systematic investigations revealed no toxic side effect of the intravenously administered BSH. Comparable data were obtained by using ICP-AES and QNCR for boron concentration measurements.

CONCLUSION

Taking into account the radiobiological considerations of the neutron beam source, we found promising facts that BNCT could be a useful irradiation method for highly malignant brain tumors. Favorable amounts of the boron compound BSH were found in tumor tissue, whereas healthy brain tissue has shown no significant uptake.

A comprehensive PC-based computer model for microdosimetry of BNCT

A computer model is described that performs microdosimetric calculations of the radiation dose delivered to tumour and normal tissue in boron neutron capture therapy (BNCT) by simulating capture reactions in a predefined three-dimensional space. The role of intracellular boron distributions and cellular dimensions on the radiation dose in clinical and experimental BNCT has been studied using a PC-based computer model. In order to calculate the radiation dose to low boron uptake cells, the extent of irradiation by boron containing adjacent cells (cross fire) is also dealt with. Radiation doses from boron and nitrogen neutron capture are converted to a biological effect by means of relative individual ion track segment efficacies, based on linear energy transfer along the particle track. A good correlation was found after comparing predicted values with previously published experimental data. A number of examples is given to illustrate the program's features.

Postnatal development of testicular cell populations in mice

The postnatal development of body and testis weight and the size of the testicular cell populations were studied in CBA mice up to day 52 post partum. The body weight increased from 1.3 g at day 1 to 22.5 g at day 52. Over the same interval the testis weight showed a faster increase from about 1 mg to almost 60 mg. Spermatogenesis was found to be complete by day 35. The numbers of A spermatogonia, Sertoli cells, Leydig cells, mesenchymal cells, macrophages, myoid cells, lymphatic endothelial cells, endothelial cells and perivascular cells per testis were studied from day 3 to day 50, using the dissector method. The number of A spermatogonia increased from 0.2 x 10(5) at day 3 to 6.5 x 10(5) at day 21 and remained more or less constant thereafter. The Sertoli cell population increased during the first three weeks after birth to reach the adult level of approximately 18 x 10(5) cells per testis. In the interstitium the Leydig cells showed a sharp increase between days 11 and 31 followed by a small decrease to ultimately 9 x 10(5) cells per testis. The Leydig cells formed 8% of the total number of interstitial cells per testis at day 11, increasing to 30% at day 50. The number of mesenchymal cells did not change until day 36, decreasing thereafter from about 2.5 x 10(5) to 1 x 10(5) cells per testis at day 50. However, the percentage of the total number of interstitial cells that were mesenchymal cells decreased from 59% to 4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative neutron capture radiography for boron in biological specimens

Track-etch detectors made of cellulose nitrate (LR 115, Kodak Pathé) and polycarbonate (CR 39, Pershore Mouldings Ltd) were compared regarding sensitivity and background when used as detectors for boron determination in biological samples. Measurements were made on two kinds of sample, cryosectioned biological tissue, and liquid samples deposited directly on the detector surface as microdroplets. The CR 39 films were pretreated or washed before irradiation. When cryosectioned tissue was used, measurements were made with and without the inclusion of Mylar foils between the samples and the detectors. Foil thicknesses used were 2 microns in the case of LR 115 and 2, 4, and 6 microns in the case of CR 39. All samples were irradiated with a thermal-neutron fluence of 5 x 10(12) neutrons cm-2 at the thermal-neutron facility in Studsvik, Sweden. The use of a Mylar foil generally suppressed the background tracks relative to the tracks from the 10B disintegration. No difference in resolution between CR 39 and LR 115 was observed. Pretreatment of the CR 39 resulted in an improved sensitivity of detection but the detector became saturated at 0.25 parts per million of 10B. The background was found to be lower in the pretreated detector.

Differential effects of fractionated X irradiation on mouse spermatogonial stem cells

The response of spermatogonial stem cells to fractionated X irradiation was studied in the various stages of the spermatogenic cycle of the CBA mouse. Fractionated doses of 2 + 2, 1 + 3, and 3 + 1 Gy with a 24-h interval between the doses were compared with a single dose of 4 Gy. The numbers of undifferentiated spermatogonia present 10 days after (the second) irradiation were taken as a measure of stem cell survival. Twenty-four hours after the first irradiation a sensitization was observed that was found to be stage-dependent. The greatest sensitization occurred in that part of the spermatogonial stem cell population that was in stages X-I during the first irradiation, i.e., the part that is stimulated to proliferate or actively proliferating at that time. In stages that were quiescent during the first irradiation (VI-VII), fractionation did not influence the response. Therefore, only the spermatogonial stem cells that are initially radioresistant become sensitized 24 h after irradiation. When two unequal doses (3 + 1 Gy or 1 + 3 Gy) are given, damage correlates with the size of the second dose, indicating that priming doses of 1 and 3 Gy are both capable of inducing the sensitizing effect.

Selectivity of boron carriers for boron neutron capture therapy: pharmacological studies with borocaptate sodium, L-boronophenylalanine and boric acid in murine tumors

Borocaptate sodium (BSH) and L-boronophenylalanine (L-BPA) are two boron carriers used for boron neutron capture therapy (BNCT) in the treatment of glioblastoma and melanoma, respectively. The suitability of these two compounds was evaluated on the basis of pharmacokinetic studies aiming at characterizing their biodistribution, tumor uptake and tumor selectivity. Boric acid was also used as a reference compound since it is nonselective and relatively freely diffusible. The compounds were investigated in two tumor models, a B16 pigmented melanoma and the RIF1 sarcoma. Mice were sacrificed after different boron doses at various post-injection times and tissue and plasma levels measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The proposed minimum effective tumor boron concentration of 15 ppm was achieved in both tumor models for the three compounds tested, although only for L-BPA in the melanoma was this achieved when tumor-plasma ratios were above 1. In the RIF1 model, maximum tumor concentrations of 44 and 31 ppm B were reached after administration of 50 micrograms B/g body weight for boric acid and BSH, respectively. After administration of 12.5 micrograms B/g of L-BPA, maximum concentrations of 15 and 21 ppm were found in the RIF1 and B16 models, respectively. Tumor-plasma ratios (TPR) for BSH remained close to or below unity at all times studied in both tumors. Brain levels of BSH were very low, however, leading to tumor-brain ratios markedly greater than 1 at all times. L-BPA and boric acid showed TPR values above unity in both tumor models, reaching 3.2 in B16.(ABSTRACT TRUNCATED AT 250 WORDS)

Large animal normal tissue tolerance using an epithermal neutron beam and borocaptate sodium

Irradiation of the canine head following intravenous Na2B12H11SH (BSH) administration has provided useful information concerning the tolerance of skin and brain to the resultant complex form of irradiation. The effect of the boron capture reaction in skin and brain has provided estimates of the influence of the microscopic dosimetry involved. Dogs irradiated with the epithermal beam alone provided valuable insight into the relative biological effectiveness (RBE) of the fast neutron component (> 10 keV) of the epithermal beam. When compared with literature values for X-rays for the occurrence of skin necrosis in dogs, an RBE of 4.5 was derived. Previous pharmacokinetic data concerning the distribution of Na2B12H11SH (BSH) to blood and brain has been used to obtain input parameters for computer models of the microvasculature of the brain. Monte Carlo computer models were used to simulate the microscopic distribution of BSH in the normal brain. The term compound factor describes the product of the microscopic boron fission fragment dose hitting the nucleus and the relative biologic effectiveness divided by the macroscopic equilibrium dose of the boron reaction in the tissue of interest. The computed compound factor for Na2B12H11SH (BSH) in normal brain was 0.37. This factor agreed very well with the value of 0.32 obtained for the brain necrosis with the dog irradiations. The compound factor for the dog's skin was experimentally derived from the dog experiments and was equal to 0.5.

The effects of graded doses of 1 MeV fission neutrons or X rays on the murine hematopoietic stroma

The acute radiosensitivity in vivo of the murine hematopoietic stroma for 1 MeV fission neutrons or 300 kVp X rays was determined. Two different assays were used: (1) an in vitro clonogenic assay for fibroblast precursor cells (CFU-F) and (2) subcutaneous grafting of femora or spleens. The number of stem cells (CFU-S) or precursor cells (CFU-C), which repopulated the subcutaneous implants, was used to measure the ability of the stroma to support hemopoiesis. The CFU-F were the most radiosensitive, and the survival curves after neutron and X irradiation were characterized by D0 values of 0.75 and 2.45 Gy, respectively. For regeneration of CFU-S and CFU-C in subcutaneously implanted femora, D0 values of 0.92 and 0.84 Gy after neutron irradiation and 2.78 and 2.61 Gy after X irradiation were found. The regeneration of CFU-S and CFU-C in subcutaneously implanted spleens was highly radioresistant as evidenced by D0 values of 2.29 and 1.49 Gy for survival curves obtained after neutron irradiation, and D0 values of 6.34 and 4.85 Gy after X irradiation. The fission-neutron RBE for all the cell populations was close to 3 and varied from 2.77 to 3.28. The higher RBE values observed for stromal cells, compared to the RBE of 2.1 reported previously for hemopoietic stem cells, indicate that stromal cells are relatively more sensitive than hemopoietic cells to neutron irradiation.

The sensitivity to X rays of mouse spermatogonia that are committed to differentiate and of differentiating spermatogonia

In the CBA mouse the radiosensitivity of the undifferentiated spermatogonia that are committed to differentiate was determined by counting their more developed descendants 10 days after graded doses of X rays. Decreasing D0 values were found when these differentiating spermatogonia were derived from undifferentiated spermatogonia that were located in all likelihood in chains of increasing length. In stages IX and X of the epithelial cycle the radiosensitivity of these undifferentiated spermatogonia was characterized by a D0 of 2.2 Gy. This D0 value most likely belongs to the Asingle spermatogonia that form repopulating colonies which give rise to differentiating spermatogonia within the same epithelial cycle. In stages XII/I, where a D0 of 1.0 Gy was found, the dose-response curve is likely dominated by the Apaired spermatogonia present in these stages. In stages III to VII, the Aaligned spermatogonia transforming into A1 spermatogonia determine the radiosensitivity. During this period the D0 decreased from 0.7 to 0.4 Gy. Differentiating A1 to A3 and B spermatogonia had rather similar radiosensitivities of 0.4 to 0.5 Gy.

The sensitivity of quiescent and proliferating mouse spermatogonial stem cells to X irradiation

The radiosensitivity of spermatogonial stem cells to X rays was determined in the various stages of the cycle of the seminiferous epithelium of the CBA mouse. The numbers of undifferentiated spermatogonia present 10 days after graded doses of X rays (0.5-8.0 Gy) were taken as a measure of stem cell survival. Dose-response relationships were generated for each stage of the epithelial cycle by counting spermatogonial numbers and also by using the repopulation index method. Spermatogonial stem cells were found to be most sensitive to X rays during quiescence (stages IV-VII) and most resistant during active proliferation (stages IX-II). The D0 for X rays varied from 1.0 Gy for quiescent spermatogonial stem cells to 2.4 Gy for actively proliferating stem cells. In most epithelial stages the dose-response curves showed no shoulder in the low-dose region.

Effects of X-irradiation and adriamycin on quiescent and proliferating cells of the seminal vesicle in the castrated mouse

The sensitivity of resting and proliferating cells of the seminal vesicle to X-irradiation and adriamycin has been investigated. Stimulation with testosterone propionate (250 micrograms/day) was started 11 days after castration in BALB/c mice. X-rays (2.5-7.5 Gy total body irradiation) and intraperitoneal injections of adriamycin (4-16 mg/kg body weight) were administered at various times before or after induction of proliferation by testosterone injection. The DNA contents and the weights of the seminal vesicles were determined at 4 days after the start of stimulation. A Do for X-rays of about 10 Gy was found for the seminal vesicle epithelium. For both X-irradiation and adriamycin no significant differences in sensitivity were observed between quiescent (Go) and proliferating (G1; S) seminal vesicle cells.

Proliferative activity of gonocytes, Sertoli cells and interstitial cells during testicular development in mice

Developing mouse testis was studied from Day 14 post coitum (p.c.) until Day 35 post partum (p.p.) by [3H]thymidine autoradiography. The gonocytes proliferated actively at Day 14 p.c., the [3H]thymidine labelling index (L.I.) being 7.5%, and were quiescent from Day 16 p.c. up to the first day of life, when spermatogenesis started. The L.I. increased to 20% at Day 2 p.p. The L.I. for the Sertoli cells was approximately 20% before birth. After birth the proliferative activity decreased. After Day 11 p.p., the Sertoli cells showed their typical adult appearance. After Day 17 p.p. no labelled Sertoli cells were observed. The Leydig cells featured a very low proliferative activity up to Day 21 p.p. (L.I. of maximal 1.9%). At Day 29 p.p. there was a peak of 7.4% in L.I., followed by a sharp decrease to 0.35% at Day 35 p.p. The L.I. of mesenchymal cells decreased from 11.4% at Day 14 p.c. to 1.1% at Day 14 p.p. and remained more or less constant thereafter. The proliferative activity of myoid, endothelial and perivascular cells followed a similar course to that of mesenchymal cells, their L.I.s being high before birth (16, 12.5 and 19%, respectively, decreasing until Day 14 p.p. (0.6, 2.0 and 1.2%, respectively) and thereafter being more or less constant. There was an increase in the relative number of Leydig cells from approximately 4% of the total interstitial cell number at Day 14 p.p. to 29.5% at Day 35 p.p. At the same time, the relative number of mesenchymal cells decreased from 55 to 13%. The diameter of the seminiferous tubules showed a peak of 92 microns at Day 16 p.c., decreased to 44 microns at Day 1 p.p. and increased again to 204 microns at Day 33 p.p. These results show that, except for the Leydig cells, the proliferative activity of testicular cell types is highest during the pre- and early postnatal period. The major outgrowth of the Leydig cell population occurs around the fourth week after birth. The results are in accordance with the hypothesis that the mesenchymal cells are the progenitors of Leydig cells.

The effects of x-irradiation on hematopoietic stem cell compartments in the mouse

The sensitivity for x-irradiation of a series of hematopoietic stem cell populations has been determined. The most primitive cells identified, cells with marrow-repopulating ability (MRA), showed the highest degree of radioresistance. These MRA cells which generate many secondary day-twelve spleen colony-forming units (MRA[CFU-S-12]) or colony-forming units in culture (MRA[CFU-C]) in the marrow of primary recipients had Do values equal to 1.18 and 1.13 Gy, respectively. The more mature CFU-S-12 had intermediate radiosensitivity (Do = 0.94 Gy), whereas the less primitive CFU-S-7 were the most radiosensitive (Do = 0.71 Gy). The in vitro colony-forming precursor cells (CFU-C) showed low radiosensitivity. These data clearly show that the most primitive hematopoietic stem cell measured is less sensitive to ionizing radiation than generally has been assumed on the basis of measurements on CFU-S-7 or CFU-S-12.

Active uptake and extravesicular storage of m-iodobenzylguanidine in human neuroblastoma SK-N-SH cells

Radio-iodinated m-iodobenzylguanidine (MIBG), an analogue of the neurotransmitter norepinephrine (NE), is increasingly used in the diagnosis and treatment of neural crest tumors. Active uptake and subsequent retention of MIBG and NE was studied in human neuroblastoma SK-N-SH cells. Neuron-specific uptake of [125I]MIBG and [3H]NE saturated at extracellular concentration of 10(-6) M and exceeded by 20-30-fold that by passive diffusion alone. A minimum of 50% of accumulated MIBG remained permanently stored but the SK-N-SH cells were incapable of retaining recaptured [3H]NE. [125I]MIBG was displaced from intracellular binding sites by unlabeled MIBG with 10-fold higher potency than by unlabeled NE. MIBG stored in SK-N-SH cells was insensitive to depletion by the inhibitor of granular uptake reserpine (RSP) and was not precipitated in a granular fraction by differential centrifugation. Only few electron-dense granules were found in these cells by electron microscopy. In contrast, MIBG storage in PC-12 pheochromocytoma cells which contained many storage granules, was sensitive to RSP and part of accumulated drug was recovered in a granular fraction. Accordingly, storage of MIBG in the SK-N-SH neuroblastoma cells is predominantly extravesicular and thus essentially different from that of biogenic amines in normal adrenomedullary tissue or in pheochromocytoma tumors, while sharing with these tissues a common mechanism of active uptake.

The effect of graded doses of fission neutrons or X rays on the stromal compartment of the thymus in mice

The effect of irradiation on the supportive role of the thymic stroma in T cell differentiation was investigated in a transplantation model using athymic nude mice and transplanted irradiated thymuses. In this model, neonatal CBA/H mice were exposed to graded doses of whole-body irradiation with fast fission neutrons of 1 MeV mean energy or 300 kVp X rays. The doses used varied from 2.75 up to 6.88 Gy fission neutrons and from 6.00 up to 15.00 Gy X rays at center-line dose rates of 0.10 and 0.30 Gy/min, respectively. Subsequently, the thymus was excised and a thymus lobe was transplanted under the kidney capsule of H-2 compatible nude mice. One and two months after transplantation, the T cell composition of the thymic transplant was investigated using immunohistology with monoclonal antibodies directed to the cell surface differentiation antigens Thy-1, Lyt-1, Lyt-2, MT-4, and T-200. Furthermore, the stromal cell composition of the thymic transplant was investigated with monoclonal antibodies directed to MHC antigens and with monoclonal antibodies defining different subsets of thymic stromal cells. To investigate the reconstitution capacity of the thymic transplant, the peripheral T cell number was measured using flow cytofluorometric analysis of nude spleen cells with the monoclonal antibodies anti-Thy-1, anti-Lyt-2, and anti-MT-4. The results of this investigation show that a neonatal thymus grafted in a nude mouse has a similar stromal and T cell composition as that of a normal thymus in situ. In addition, grafting of such a thymus results in a significant increase of the peripheral T cell number. Irradiation of the graft prior to transplantation has no effects on the stromal and T cell composition but the graft size decreases. This reduction of size shows a linear dose-response curve after neutron irradiation. The X-ray curve is linear for doses in excess of 6.00 Gy. The RBE for fission neutrons for the reduction of the relative thymic graft size to 10% was equal to 2.1. Furthermore, the peripheral T cell number decreases with increasing doses of irradiation given to the graft prior to transplantation. The present data indicate that the regenerative potential of thymic stromal cells is radiosensitive and is characterized by D0 values equal to 2.45 and 3.68 Gy for neutrons and X rays, respectively. In contrast, the ability of the thymic stromal cells to support T cell maturation is highly radioresistant.

Unilateral T cell maturation arrest in the thymus of CBA/H mice as a long-term effect after neutron irradiation

Thymuses of CBA/H mice were investigated up to 570 days after whole-body irradiation with 2.5 Gy fast fission neutrons or 6.0 Gy X rays. A number of these thymuses, observed 220-270 days after neutron irradiation, have two equal sized lobes, one of which has an abnormal T cell distribution. The present paper reports on the distribution of lymphoid and stromal cell types in these thymuses. For this purpose, we employed immunohistology using the indirect immunoperoxidase method. We incubated frozen sections of these aberrant thymuses with monoclonal antibodies directed to cell surface differentiation antigens on lymphoid cells, such as Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14; monoclonal antibodies directed to major histocompatibility complex (MHC) antigens, such as I-A and H-2K; and monoclonal antibodies directed to determinants in various thymic stromal cell types. The results of this study show a T cell differentiation arrest in only one of the two thymic lobes. T cells in the aberrant lobe express Thy-1, T-200, and MEL-14 antigens but are MT-4- and Lyt-1-. In some lobes, a weak Lyt-2 expression was observed. The observed T cell maturation arrest is mainly restricted to the cortex since in the medulla, in addition to cells with an aberrant cortical phenotype, normal T cell phenotypes are observed. This indicates that cortex and medulla have independent generation kinetics in T cell maturation. The stromal cell composition in these abnormal lobes is not different from that in the normal lobe, but the size of the medulla tends to be smaller. Furthermore, the I-A expression on the cortical epithelial cells does not reveal the characteristic reticular staining pattern that is observed in the normal lobe, since the I-A determinants are not strictly confined to the epithelial cells. In addition, cortical lymphoid and stromal cells in these lobes are slightly H-2K+. These alterations in MHC expression in the cortex are discussed in relation to the observed T cell maturation arrest.

The effect of graded doses of fission neutrons or X rays on the lymphoid compartment of the thymus in mice

Young adult CBA/H mice were exposed to graded doses of whole-body irradiation with either fast fission neutrons or 300 kVp X rays at center-line dose rates of 0.1 and 0.3 Gy/min, respectively. Dose-response curves were determined at Days 2 and 5 after irradiation for the total thymic cell survival and for the survival of thymocytes defined by monoclonal anti-Thy-1, -Lyt-1, -Lyt-2, and -T-200 antibodies as measured by flow cytofluorometric analysis. Cell dose-response curves of thymocytes show, 2 days after irradiation, a two-component curve with a radiosensitive part and a part refractory to irradiation. The radiosensitive part of the dose survival curve of the Lyt-2+ cells, i.e., mainly cortical cells, has a D0 value of about 0.26 and 0.60 Gy for neutrons and X rays, respectively, whereas that of the other cell types has corresponding D0 values of about 0.30 and 0.70 Gy. The radiorefractory part of the dose-response curves cannot be detected beyond 5 days after irradiation. At that time, the Lyt-2+ cells are again most radiosensitive with a D0 value of 0.37 and 0.99 Gy for neutrons and X rays, respectively. The other measured cell types have corresponding D0 values of about 0.47 Gy. The fission neutron RBE values for the reduction in the thymocyte populations defined by either monoclonal anti-Thy-1, -Lyt-1, -Lyt-2, or -T-200 antibodies to 1.0% vary from 2.6 to 2.8. Furthermore, the estimated D0 values of the Thy-1-, T-200- intrathymic precursor cells which repopulate the thymus during the bone marrow independent phase of the biphasic thymus regeneration after whole-body irradiation are 0.64-0.79 Gy for fission neutrons and 1.32-1.55 Gy for X rays.

Repopulation of the mouse thymus after sublethal fission neutron irradiation. II. Sequential changes in the thymic microenvironment

The stromal cells of the thymus of sham-irradiated and sublethal fission neutron-irradiated CBA/H mice were analyzed with immunohistology, using monoclonal antibodies directed to I-A and H-2K antigens as well as specific determinants for cortical and medullary stromal elements. In the control thymuses, I-A expression in the thymus shows a reticular staining pattern in the cortex and a confluent staining pattern in the medulla. In contrast, H-2K expression is mainly confluently located in the medulla. Whole body irradiation with 2.5 Gy fission neutrons reduces within 24 hr the cortex to a rim of vacuolized "nurse cell-like" epithelial cells, largely depleted of lymphoid cells. The localization of I-A antigens changes in the cortex and I-A determinants are no longer associated with or localized on epithelial reticular cells. Medullary stromal cells, however, are more or less unaffected. A high rate of phagocytosis is observed during the first 3 days after irradiation. About 5 days after irradiation, the thymus becomes highly vascularized and lymphoid cells repopulate the cortex. The repopulation of the thymic cortex coincides with the appearance of a bright H-2K expression in the cortex which is associated with both stromal cells as well as lymphoid blasts. During the regeneration of the thymus, the thymic stromal architecture is restored before the expression of cell surface-associated reticular MHC staining patterns. The observed sequential changes in the thymic microenvironment are related to the lymphoid repopulation of the thymus.

Repopulation of the mouse thymus after sublethal fission neutron irradiation. I. Sequential appearance of thymocyte subpopulations

The T cell composition of the thymus of sublethal fission neutron-irradiated CBA/H mice was analyzed with cytofluorometry and immunohistology, using monoclonal antibodies directed to the cell surface antigens Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14. The results of this investigation show that whole body irradiation with 2.5 Gy fission neutrons results in a severe reduction and degeneration of the cortex, whereas the medulla is affected to a lesser extent. Irradiation selects, within 24 hr, for a population of dull Thy-1+, bright T-200+, bright Lyt-1+ cells localized in the medulla. Phenotype analysis of the regeneration of the thymus, which starts at about 5 days after irradiation, reveals the sequential appearance of: 1) "null" cells, i.e., lymphoblasts negative for all tested antigens, mainly in the subcapsular area but also in the medulla; 2) Thy-1+ "only" and T-200+ "only" cells in the subcapsular area; 3) Thy-1+, T-200+ cells; and 4) Thy-1+, T-200+, MT-4+, Lyt+ cells in the cortex. In addition, an increased MEL-14 expression is observed in correlation with the expression of Thy-1 and T-200 determinants during the regeneration of the thymus. From day 10 on up to at least 150 days after irradiation, no differences can be observed in the thymus of irradiated and age-matched sham-irradiated control mice, as measured by the expression and distribution of Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14 antigens. The observed sequence in phenotype shift in the regeneration of the thymus after irradiation is discussed in view of recently published data on the differentiation of the T cell system.

Repopulation of the mouse thymus after sublethal fission neutron irradiation. I. Sequential appearance of thymocyte subpopulations

The T cell composition of the thymus of sublethal fission neutron-irradiated CBA/H mice was analyzed with cytofluorometry and immunohistology, using monoclonal antibodies directed to the cell surface antigens Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14. The results of this investigation show that whole body irradiation with 2.5 Gy fission neutrons results in a severe reduction and degeneration of the cortex, whereas the medulla is affected to a lesser extent. Irradiation selects, within 24 hr, for a population of dull Thy-1+, bright T-200+, bright Lyt-1+ cells localized in the medulla. Phenotype analysis of the regeneration of the thymus, which starts at about 5 days after irradiation, reveals the sequential appearance of: 1) "null" cells, i.e., lymphoblasts negative for all tested antigens, mainly in the subcapsular area but also in the medulla; 2) Thy-1+ "only" and T-200+ "only" cells in the subcapsular area; 3) Thy-1+, T-200+ cells; and 4) Thy-1+, T-200+, MT-4+, Lyt+ cells in the cortex. In addition, an increased MEL-14 expression is observed in correlation with the expression of Thy-1 and T-200 determinants during the regeneration of the thymus. From day 10 on up to at least 150 days after irradiation, no differences can be observed in the thymus of irradiated and age-matched sham-irradiated control mice, as measured by the expression and distribution of Thy-1, T-200, MT-4, Lyt-1, Lyt-2, and MEL-14 antigens. The observed sequence in phenotype shift in the regeneration of the thymus after irradiation is discussed in view of recently published data on the differentiation of the T cell system.

Repopulation of the mouse thymus after sublethal fission neutron irradiation. II. Sequential changes in the thymic microenvironment

The stromal cells of the thymus of sham-irradiated and sublethal fission neutron-irradiated CBA/H mice were analyzed with immunohistology, using monoclonal antibodies directed to I-A and H-2K antigens as well as specific determinants for cortical and medullary stromal elements. In the control thymuses, I-A expression in the thymus shows a reticular staining pattern in the cortex and a confluent staining pattern in the medulla. In contrast, H-2K expression is mainly confluently located in the medulla. Whole body irradiation with 2.5 Gy fission neutrons reduces within 24 hr the cortex to a rim of vacuolized "nurse cell-like" epithelial cells, largely depleted of lymphoid cells. The localization of I-A antigens changes in the cortex and I-A determinants are no longer associated with or localized on epithelial reticular cells. Medullary stromal cells, however, are more or less unaffected. A high rate of phagocytosis is observed during the first 3 days after irradiation. About 5 days after irradiation, the thymus becomes highly vascularized and lymphoid cells repopulate the cortex. The repopulation of the thymic cortex coincides with the appearance of a bright H-2K expression in the cortex which is associated with both stromal cells as well as lymphoid blasts. During the regeneration of the thymus, the thymic stromal architecture is restored before the expression of cell surface-associated reticular MHC staining patterns. The observed sequential changes in the thymic microenvironment are related to the lymphoid repopulation of the thymus.

Short- and long-term effects of whole-body irradiation with fission neutrons or X rays on the thymus in CBA mice

Young adult (6 weeks old) female CBA mice were exposed to whole-body irradiation with either 2.5-Gy fast fission neutrons of 1 MeV mean energy or 6.0-Gy 300 kVp X rays at centerline dose rates of 0.1 and 0.3 Gy/min, respectively. The weight of spleen and animal and the weight, cellularity, and histological structure of the thymus were studied at different times after irradiation. Thymic recovery after whole-body irradiation showed a biphasic pattern with minima at 5 and 21 days after irradiation and peaks of regeneration at Days 14 and 42 after X irradiation or at Days 14 and 70 after neutron irradiation. After the second phase of recovery, a marked decrease in relative thymus weight and cellularity was observed, which lasted up to at least 250 days after irradiation. Splenic recovery showed a monophasic pattern with an overshoot on Day 21 after irradiation. After neutron irradiation a late decrease in relative spleen and animal weight was observed. The observed late effects on thymus and spleen weight and thymus cellularity are discussed in terms of a persistent defect in the bone marrow.