The effects of alpha interferon on human long-term bone marrow culture

The effect of alpha IFN on normal long term bone marrow culture (LTBMC) was assessed by measuring haemopoietic progenitor formation and stromal cell number and composition over the course of 5 weeks. When alpha IFN was added at the initiation of LTBMC, there was a marked inhibition of CFU-GEMM, BFU-E and CFU-GM formation from both adherent and non-adherent compartments of culture. There was a profound inhibition of stromal layer formation, especially the reticulo-fibroblast component, and this was not a result of TNF release from macrophages. When alpha IFN was added to established normal LTBMC, although haemopoietic progenitor formation was inhibited, there was little effect on the stromal layer in terms of number or composition. This suggests that the cytostatic effects of alpha IFN when added at commencement of LTBMC result from the anti-proliferative effects of alpha IFN on these actively dividing cells. It is concluded that in addition to the established inhibitory effects of alpha IFN on haemopoietic progenitors as previously demonstrated in semi-solid culture systems, alpha IFN has profound effects upon the marrow microenvironment. This is of particular relevance to bone marrow transplantation where such cytokines may be considered for clinical use.     

Colony promoting activity (CPA) produced in long-term culture of murine bone marrow cells

The differences between colony promoting activity(CPA) and colony stimulating activity(CSA) in the culture media of murine long-term bone marrow cultures(LTBMC) were demonstrated and the role of adherent cells and nonadherent cells in the production of CPA was studied in this culture system. Supernatant harvested from intact continuous marrow cultures showed high CPA but contained no CSA. Assayable CSA was detected in concentrated supernatant. However, there was no significant relationship between levels of CPA and CSA in the supernatant. When adherent cells and nonadherent cells from LTBMC were separately cultured, CPA was detected in the conditioned medium of adherent cells but not in that of nonadherent cells. The CPA level in LTBMC was related inversely to the number of nonadherent cells and addition of nonadherent cells to adherent cell cultures reduced the level of CPA. Conditioned medium of nonadherent cells showed no inhibitory activity to CPA. These results indicate that CPA is produced by bone marrow adherent cells and that it may be consumed by myeloid progenitor cells in nonadherent cells.     

A practical miniature long-term bone marrow culture system for investigating early myelodysplasia.

A method was devised to grow haemopoietic cells in long-term bone marrow culture (LTBMC) which requires only 1 x 10(6) cells/culture. Such miniature cultures were used to study growth patterns of marrow from patients with myelodysplastic syndromes (MDS). Consistent differences in LTBMC cellularity and cellular composition were noted between MDS and normal marrow. These differences were accentuated by rGM-CSF. The criteria which distinguished between and MDS marrows were: cell count at weeks 1 and 4, % neutrophils and % blasts. In 10 patients with unexplained macrocytosis or pancytopenia miniature LTBMC results clearly segregated into either 'normal' or 'MDS' growth patterns. Miniature LTBMC with rGM-CSF may therefore be a useful diagnostic test for early MDS.     

Differentiation induction of HL60 cells in a long-term bone marrow culture of acute myeloid leukemia

Bone marrow stromal cells are critical for the proliferation and differentiation of hemopoietic stem cells. The hemopoietic microenvironment is reproduced in long term bone marrow culture (LTBMC). Normal LTBMC versus leukemic LTBMC and their stroma conditioned medium were compared with respect to their proliferative and differentiation-inducing capacities. Myeloid leukemic cells (HL60) were layered onto LTBMCs derived from normal volunteers and patients with AML. Differentiation was measured with a comprehensive panel of maturation parameters, i.e. morphology, cytochemistry, quantitative enzyme determination, NBT test and immunophenotyping. Inhibition of proliferation occurred in all cocultures. Clear maturation in monocytic direction was obvious in one culture of HL60 cells layered onto a leukemic stroma. As stroma-derived conditioned medium has no effect, a cellular interaction seems involved. These observations support not only the concept that normal stroma influences leukemic cell growth but also that leukemic stroma can modulate cell growth and maturation.     

Generation of progenitor-cell precursors in long-term bone-marrow cultures after marrow purging with ether lipids

Alkyl-lysophospholipid derivates (ALP) are currently being tested as bone marrow (BM) purging agents prior to autologous BM transplantation in different malignancies. We evaluated the toxicity of the ALP ET-18-OCH3 (ET-18; Edelfosine, 1-0-octadecyl-2-0-methyl-rac-glycero-3-phosphocholine) towards early hematopoietic precursors by testing progenitor regeneration of non-purged and ET-18-purged BM (75 mu g and 125 mu g ET-18/ml/2x10(7) BM cells) in autologous long-term bone marrow cultures (LTBMC) from 3 different patients in complete remission. LTBMC feeder layers were irradiated with 875 rad for complete elimination of hematopoietic progenitors and recharged with cryopreserved purged and non-purged BM. In weekly intervals, adherent layer and supernatant LTBMC cells were completely removed and evaluated in colony forming unit (CFU)-assays. We have seen sufficient CFU-regeneration out of ET-18-purged BM up to 8 weeks of LTBMC (>40 CFU/flask). Total CFU-counts from LTBMC with purged BM were slightly reduced compared to non-purged control. High dose purging with 125 mu g ET-18/ml partly inhibited initial CFU-proliferation, but demonstrated elevated CFU-counts after 4 and 8 weeks of LTBMC compared to control. In conclusion, in our LTBMC series ET-18-purging yielded tolerable toxicity towards committed BM-progenitors, but no remarkable decline of early hematopoietic precursors regenerating CFU-progenitors for up to 8 weeks of culture.     

Long-term bone marrow culture-derived stromal fibroblasts as a potential target for gene therapy in acute myelogenous leukemia

As a part of our continuing efforts to develop gene therapy for acute myelogenous leukemia (AML), this study was undertaken to evaluate the possibility of using autologous bone marrow stromal fibroblasts (BMSFs) as a target cell population. Autologous BMSFs in AML were isolated from the stromal layers of long-term bone marrow culture (LTBMC) using immunomagnetic beads. BMSFs exhibited rapid proliferation even in the absence of growth factors. Cultures stimulated with bFGF produced significantly increased numbers of BMSFs than cultures without added growth factors. Using LNC/LacZ retroviral vector, the transduction efficiency of BMSFs was 13+/-4% at a 5 multiplicity of infection (MOI). LNC/interleukin-2 (IL-2)-transduced BMSFs produced between 1200 and 4800pg of IL-2/10(6) cells per 24h. Using adenoviral vector AdV/LacZ, the transduction efficiency was 84+/-10% at 100, and 92+/-8% at a MOI of 1000. Although the addition of basic fibroblast growth factor, epidermal growth factor, or platelet-derived growth factor did not affect the transduction efficiency, they increased the numbers of transduced cells significantly (P<0.01). AdV/IL-2-treated BMSFs produced high levels of IL-2 over the course of 7 days between 9820 and 22,700pg of IL-2/10(6) cells per 24h. Our finding that the genetically engineered autologous BMSFs of AML could be successfully established in vitro implies that BMSFs obtained from LTBMC might be considered as a target cell population for certain types of clinical gene therapy in AML.     

Long-term bone marrow cultures (LTBMC) from essential thrombocythemia (ET) patients with or without JAK2617V>F mutation

Approximately half of essential thrombocythemia (ET) patients and almost all with polycythemia vera (PV) bear the activating JAK2617V>F point mutation, which arises at the multipotent haemopoietic progenitor cell level. Although ET is mainly characterized by megacaryocyte proliferation, the cases that are positive for the JAK2617V>F mutation also show increased bone marrow cellularity and higher erythrocyte and granulocyte counts. After establishing short- and long-term bone marrow cultures we found that the frequency of committed haemopoietic progenitors in the bone marrow, was not increased in JAK2617V>F positive ET compared to the negative ones, whereas in long-term cultures (LTBMC) JAK2617V>F positive ET display a growth pattern more similar to that observed in LTBMC produced by PV marrow cells. Our data support the notion that JAK2617V>F positive ET and PV represents a continuum spectrum of alterations within the same disease.     

VLA-4 mediated adhesion to bone marrow stromal cells confers chemoresistance to adherent lymphoma cells.

The mechanisms of maintenance of residual lymphoma in bone marrow during chemotherapy are currently not well understood. Previous studies have shown that primary lymphoma cells obtained from histologically negative bone marrow of non-Hodgkin's lymphoma (NHL) patients grew in long-term bone marrow cultures primarily in association with bone marrow stromal cells. Furthermore, the interaction of NHL patient cells with bone marrow stromal cells inhibited their spontaneous apoptosis. The current studies were designed to characterize the components of the heterotypic interaction between lymphoma cells and bone marrow stromal cells as well as to probe the consequences of this interaction as it pertains to the potential survival of minimal numbers of lymphoma cells during chemotherapy. Cellular adhesion assays performed in the presence of either neutralizing antibodies to VCAM- or the alpha and beta subunit of VLA-4 resulted in >95%, 82% and 35% inhibition of lymphoma cell line adhesion to the bone marrow stromal line MS-5, respectively. Modulation of VLA-4 affinity by the 8A2 antibody resulted in enhanced secondary adhesion at 24 and 72 hours to either cellular fibronectin (65% and 65%) or MS-5 cells (60% and 55%), superceding levels obtained using untreated lymphoma cells (<20%). The bone marrow stromal cells induced a chemoprotective effect for adherent lymphoma cells over a 3-log dose range of vincristine, resulting in a 2-log increase in the ED50 at day 6 of culture. The failure of glutaraldehyde fixed stromal cells to induce a chemoprotective effect demonstrated that viable bone marrow stromal cells were necessary. Similarly, lymphoma/stromal cell conditioned medium also failed to provide a survival advantage. These data demonstrated that viable bone marrow stromal cells possessed the ability to actively inhibit the apoptotic pathways of intimately adherent lymphoma cells and this potentially contributes to their survival during chemotherapy.     

Mechanism of diethyldithiocarbamate modulation of murine bone marrow toxicity

Sodium diethyldithiocarbamate (DDTC) has been shown to modulate the myelosuppression that commonly occurs following treatment with anticancer drugs in mice. In order to investigate the mechanism of action of this myeloprotector, murine long-term bone marrow cultures were treated with DDTC alone or were preceded by the anticancer drug cis-diammine(cyclobutanedicarboxylato)platinum(II) (CBDCA), and the granulocyte/macrophage colony-stimulating activity of the supernatants was measured. The supernatants harvested from DDTC-treated cultures enhanced proliferation of granulocyte/macrophage progenitor cells almost 4-fold compared to cultures treated with no drug or with CBDCA alone. Pretreatment of cultures with CBDCA neither enhanced nor inhibited DDTC-induced colony-stimulating activity. Similar results were obtained by using marrow stromal cell cultures free of hematopoietic cells. Thus, DDTC may hasten bone marrow recovery by augmenting stromal cell production of a factor(s) with hematopoietic colony-stimulating activity.     

Early disappearance of murine plasmocytoma stem cells in long-term bone marrow culture.

Long-term bone marrow culture (LTBMC) was evaluated as a purging procedure in the murine plasmocytoma MOPC-315s system. MOPC-315s cells injected in Balb-c mice rapidly proliferate both in marrow and spleen, where macroscopic tumor colonies develop. A linear relationship between the number of injected cells and spleen colonies was observed, consistent with the presence of 1 clonogenic myeloma stem cell out of 1800 cells. In vitro, MOPC-315s cells are easily identifiable as rosette-forming cells (RFC+) with trinitrophenil acid (TNP) coated sheep red blood cells. When bone marrow (BM) cells containing 20-40% RFC+ were seeded in LTBMC, RFC+ rapidly decreased and were no longer detectable by day 14 of culture. Clonal Ig gene rearrangement was evident at time 0, but it was no more detectable later on. In addition, cells taken at days 14 and 21 of culture were no more tumorigenic when injected in vivo. The results suggest the efficacy of the LTBMC for the in vitro elimination of myeloma cells, including the neoplastic stem cells.     

Structural and functional changes of bone marrow stroma in chronic lymphocytic leukemia]

Marrow stroma was investigated in 20 patients with chronic lymphocytic leukemia (CLL). Such morphological and functional features as adipose tissue reduction, sinusoid pattern obliteration and increased number of reticular cells were identified as typical of marrow stroma in such patients. Foci of fibrosis, nuclear bodies in stromal cell nuclei and heightened proliferation of stromal precursors in organ cultures were recorded in the subendostal and perivascular areas in 60% of patients with diffuse lesions of bone marrow. Bone marrow stroma was shown to suffer morphological and functional changes which were particularly pronounced in diffuse lesions of the bone marrow. Such changes may be traced both to the malignant clone of lymphoid cells and stromal lesions.     

Cumulative bone marrow stromal damage caused by X-irradiation and cytosine-arabinoside in leukemic mice

A study of treated murine acute myeloid leukemia (AML) with an emphasis on the bone marrow stromal function is reported. Leukemia was induced in C57Bl mice through intraperitoneal (i.p.) inoculation of C-1498 myelogenous leukemic cells. The leukemic mice were administered: (1) total body lethal X-irradiation (t.b.i.); (2) two i.p. cytosine-arabinoside (Ara-C) injections followed by X-irradiation. Control mice received similar regimens. Bone marrow of experimental and control mice was processed for stromal cell cultures (SCC) and in vitro engraftment of hematopoietic cells onto the cultures. The results of this study indicate that the bone marrow stromal deficiency which occurs in leukemia is aggravated by Ara-C and irradiation treatments. Moreover, SCC of treated leukemic mice sustain in vitro hematopoiesis only to a limited degree. Stromal deficiency, as possible cause for graft failure in bone marrow transplanted leukemic patients, is discussed.     

The detection of Philadelphia chromosome negative metaphases in long-term bone marrow cultures of the peripheral blood from patients with chronic myeloid leukemia predicts response to interferon-alpha 2a.

The cytogenetic response of 10 patients with chronic myeloid leukaemia (CML) to human recombinant interferon-alpha 2a (rhIFN alpha 2a) was compared to the Philadelphia chromosome (Ph) status of the pre-treatment peripheral blood cells after in vitro culture under long-term bone marrow culture (LTBMC) conditions. Pre-treatment light density peripheral blood cells were cultured in LTBMC on sex-mismatched irradiated allogeneic stromal layers with weekly cytogenic examination of metaphases in the non-adherent cell fraction. This was correlated with the patients' response to rhIFN alpha. Two groups of patients, five showing a cytogenetic response (responsive) and five who failed to achieve a cytogenetic response (nonresponsive) were studied. At the initiation of the LTBMCs the Ph' was found to be present in 100% of the cells analysed for nine patients and 97% for one patient. Pretreatment peripheral blood from four responsive patients demonstrated a decline in the proportion of Ph'-positive cells (Ph+) after 1 to 2 weeks in LTBMC. In contrast, peripheral blood from all the non-responsive subjects showed persistence of the Ph+ clone in 100% of the cells analysed out to a maximum of 3 to 5 weeks in LTBMC. A significant difference was observed (Fisher exact test, p = 0.023) between the two patient groups in respect to the appearance of normal clones in the nonadherent population. The presence of Ph- metaphases in LTBMC of peripheral blood cells of CML patients may predict their cytogenetic response to rhIFN alpha 2a.     

Biologic effects of prolonged melphalan treatment of murine long-term bone marrow cultures and interleukin 3-dependent hematopoietic progenitor cell lines

The mechanism of alkylating agent-induced leukemia is unknown. For the determination of whether chronic alkylating agent treatment of hematopoietic stem cells in vitro was detectably leukemogenic, murine long-term bone marrow cultures (LTBMC) and clonal interleukin 3 (IL-3)-dependent multipotential hematopoietic progenitor cell lines [B6SUtA clone (cl) 27 and Ro cl 3-1] derived from LTBMC were chronically pulse treated in vitro with the alkylating agent melphalan [L-phenylalanine mustard (L-PAM)]. Weekly treatment of C3H/HeJ or CD-1 Swiss mouse LTBMC with 3 X 10(-6)M L-PAM significantly decreased cumulative production of nonadherent granulocytes and granulocyte-macrophage progenitor cells responsive to L-cell or WEH1-3 cell colony-stimulating factor compared to the production seen in untreated control cultures; it also significantly reduced the hematopoietic longevity (13 wk compared to greater than 20 wk for untreated control cultures). Weekly, twice weekly, or daily (3 X 10(-6)M) L-PAM treatment of IL-3-dependent cell lines induced gradual L-PAM adaptation in the absence of a detectable change in the maximum binding capacity of 125I-labeled IL-3. No leukemogenic variants of line B6SUtA cl 27 were detectably induced. However, 3 stably expressed marker chromosomes were induced after 12 months of L-PAM treatment of line B6SUtA cl 27. Thus IL-3-dependent hematopoietic progenitor cells slowly adapt to L-PAM when in suspension culture in vitro. Physiologic expression of drug toxicity in LTBMC may prevent this hematopoietic cell gradual adaptation.     

Retrovirus-mediated gene transfer of a human c-fos cDNA into mouse bone marrow stromal cells.

A cDNA encoding a complete human c-fos protein was isolated and inserted into two different murine MoMuLV-derived recombinant retroviruses allowing expression of c-fos protein in different cell types. One c-fos-expressing retrovirus, chosen for its ability to express high levels of proteins in fibroblast-like cells, was shown to potentiate long-term cultures of mouse bone marrow stromal cells in vitro and therefore constitutes a potential tool for immortalizing such cells. Moreover, when tested in an in vitro differentiation assay, stromal cells constitutively expressing c-fos favor the granulocyte differentiation of hematopoietic precursors. Interestingly, retroviruses expressing v-src and v-abl oncogenes, included as controls in our experiments, do not produce any detectable effects, whereas those expressing polyoma virus middle T antigen facilitate long-term growth in vitro of stromal cells that favor the macrophage differentiation pathway of bone marrow stem cells. Our observation supports the idea that constitutive expression of some oncogenes, including c-fos and polyoma virus middle T antigen, may influence cytokine production by bone marrow stromal cells.     

Effects of low dose rate irradiation on plateau phase bone marrow stromal cells in vitro: Demonstration of a new form of non-lethal,physiologic damage to support of hematopoietic stem cells

The clinical use of low dose rate (LDR) (5–25 rad/min) total body irradiation in bone marrow transplantation patients is well established. We have developed an in vitro system for study of the effects of LDR irradiation on bone marrow stromal cells. Purified mouse bone marrow stromal cell cultures in plateau phase with no detectable hematopoiesis were prepared and were then “engrafted” in vitro by addition of purified nonadherent hematopoietic cells from continuous bone marrow cultures. Hematopoietic cells were added in liquid medium or suspended in an overlay of semisolid 0.4% agar-containing medium. Other agar overlays contained Interleukin-3-dependent cloned multipotential hematopoietic stem cell fine B6SUtA. In parallel experiments, a cloned permanent bone marrow stromal cell fine D2XRII was used in place of purified stromal cell cultures. Stromal cultures were irradiated at 5 rad/min, 20 rad/min, or 200 rad/min, 24 hours or 3 weeks prior to “engraftment.” Two classes of irradiation damage were demonstrated following 1000 rad irradiation at 200 rad/min: 1) Decreased clonagenic survival of trypsinized replated marrow stromal cells (lethal effect), and 2) decreased production by marrow stromal cells or D2XRII cells of colony stimulating factors (CSF)s for granulocyte-macrophage progenitor cells and B6SUtA cells (physiologic effect). Holding the cultures in plateau phase for 3 weeks after irradiation was associated with significantly more repair of the lethal effect compared to the physiologic effect. Cultures irradiated at 5 rad/min or 20 rad/min to doses producing significantly less lethal effect showed a complex alteration of production of growth factors. Cumulative cell production by hemopoietic stem cells added in liquid culture was comparably decreased for all three dose rates. These data demonstrate a distinct physiologic expression of irradiation damage to bone marrow stromal cells that affects cell to cell interaction, responds differently to changes in dose rate, and is repaired with kinetics different from those of the lethal effect of irradiation. The present system should prove valuable for investigation of cellular interactions in hematopoietic stem cell engraftment that are altered by total body irradiation.     

甲异靛对白血病骨髓基质细胞调节白血病细胞增殖的影响

 【摘要】　目的　探讨甲异靛对白血病骨髓基质细胞干预白血病细胞增殖的影响。方法 　利用白血病骨髓单个核细胞培养骨髓基质细胞,并建立白血病细胞和骨髓基质的共培养体系。用锥虫蓝拒染实验测定甲异靛对共培养体系中白血病细胞增殖的影响;流式细胞术检测白血病细胞膜上CXCR4的表达。结果　白血病骨髓基质细胞可抑制白血病细胞的增殖。低浓度的甲异靛（5 μmol／L）可促进白血病和骨髓基质细胞共培养体系中白血病细胞的增殖。白血病细胞膜异常高表达CXCR4,甲异靛可明显抑制HL-60细胞和原代白血病细胞膜上CXCR4的表达,骨髓基质细胞可以促进白血病细胞膜上CXCR4的表达。结论　甲异靛可能通过下调白血病细胞膜上CXCR4的表达起抗白血病作用。     

A hematopoietic stromal lesion associated with fractionated radiotherapy (FxRT): time- and dose-effects

Earlier, we reported that the local exposure of femoral bone marrow to a clinically-relevant course of fractionated radiotherapy [FxRT; 2.0 Gy (q24h x 5) --> 74 Gy] resulted in the delayed appearance of a stromal lesion which temporally appeared after exposures to a threshold dose of 20 Gy FxRT. To further define this threshold dose, the temporal recovery of blood-forming elements ("cobblestone area" forming colonies; CAFC(7-28d)) and the microenvironmental stroma (long-term bone marrow cultures; LTBMC) of the marrow was examined over a 17-week period following 10 and 30 Gy FxRT. After a subthreshold dose of 10 Gy, each of the 4 CAFC subpopulations were significantly dampened, with recovery initiated within a 6-week interval of 10 Gy of FxRT. Above the threshold dose (30 Gy FxRT), the CAFC subpopulations were again reduced to a level similar to that observed with 10 Gy FxRT. However, after exposures to 30 Gy FxRT, CAFC recovery was significantly well beyond the 6-week interval observed with a 10 Gy Dose of FxRT. Similarly, cell production in LTBMC prepared from marrow exposed to either 10 or 30 Gy FxRT was significantly dampened for at least 3 weeks following the FxRT. Moreover, while cell production in LTBMC derived from marrow exposed to 10 Gy was eventually restored to normal, the dampened cell production observed in LTBMC prepared after 30 Gy FxRT persisted for a period in excess of 17 weeks. Collectively, these observations provide additional support to our earlier observation suggesting that FxRT generates two forms of dose-dependent damage in the marrow: the first an early lesion arising in the blood-forming CAFC subpopulations; the second form, a delayed lesion that involves the persistent expression of a dysfunctional microenvironmental phenotype, appearing to disrupt the normal regulation of hematopoietic stem cell (HSC) proliferation and differentiation of the HSC during the recovery process.     

In vitro quantitation of lethal and physiologic effects of total body irradiation on stromal and hematopoietic stem cells in continuous bone marrow cultures from Rf mice

The role of stromal-supportive cells in hematopoietic stem cell responses to irradiation is poorly understood. The effects of in vivo total body irradiation (TBI) and interval from TBI to explant of marrow on: stromal cell proliferation in vitro; stromal cell support of hematopoiesis in continuous bone marrow culture; and generation of WEHI-3 growth factor (GF)-dependent lines of hematopoietic progenitor cells were evaluated. Continuous marrow cultures from non-irradiated control RfM/UN, C57BL/6J, C3H/HeJ, and N:NIH (Swiss) mice generated pluripotential hematopoietic stem cells (CFUs) and committed granulocyte-macrophage progenitor cells (GM-CFUc) for over 20 weeks. Explant of marrow at 2, 4, 5, or 6 months after single fraction TBI (300-800 rad) was associated with decreased longevity of hemopoiesis (2-12 weeks), and a decrease in the proliferative capacity of fibroblastic adherent-stromal colony forming cells (CFUf) as measured by colony size at 14 days and number of colonies per 10(6) cells plated. In contrast, explant of marrow 8 to 24 months after TBI produced cultures with longevity that was indistinguishable from age-matched control cultures (19-24 weeks). Marrow from irradiated first and second generation recipients of serially transferred marrow demonstrated a similar 7-month in vivo recovery period; however, the plateau maximum duration of hemopoiesis did not return to control levels. Purified stromal cell cultures were prepared by corticosteroid-deprivation of explanted marrow for 28 days and were then engrafted in vitro with marrow from C57BL/6J or RfM/UN mice that had been irradiated 1 month previously. Hemopoiesis in these cultures was restored, and they produced GM-CFUc and granulocytes for 15-24 weeks. Thus, healthy stroma supported growth of recently irradiated hemopoietic cells in vitro. Nonadherent cells removed from the above continuous marrow cultures generated clonal non-leukemogenic WEHI-3 GF-dependent hemopoietic progenitor cell lines with a frequency concordant with radiation effects on culture longevity, and this was increased by the presence of purified healthy stromal cultures. Indirect effects of x-irradiation on hemopoietic stem cells through damage and repair in the stromal cell compartment can be effectively studied with the present bone marrow culture system.