MOBILIZATION OF HAEMOPOIETIC STEM CELLS (CFU) INTO THE PERIPHERAL BLOOD OF THE MOUSE; EFFECTS OF ENDOTOXIN AND OTHER COMPOUNDS

Factors affecting the circulation of haemopoietic stem cells (CFU) in the peripheral blood of mice were investigated. I.v. injection of sublethal doses of endotoxin, trypsin and proteinase appeared to raise the number of CFU per ml blood from about 30–40 to about 300–400 or more within 10 min. The effect was smaller when smaller doses of the substances were injected. After this initial rise the number of circulating cells returned to normal in a few hours. Following endotoxin there was a second rise which started 2–3 days after injection and attained a peak on the 6th–7th day. The first rise is explained as a mobilization of stem cells from their normal microenvironments into the blood stream; the second rise is considered to reflect proliferation of CFUs in the haemopoietic tissues. The spleen seems to be acting as an organ capturing CFUs from the blood and not as a source adding stem cells to the blood.
The early mobilization of CFU after endotoxin injection did not coincide with a mobilization of neutrophils. The number of circulating band cells was increased during the first hours.
The importance of 'open sites'in the haemopoietic tissue for capturing CFUs was studied by emptying these sites through a lethal X-irradiation and injecting normal bone marrow cells. When a greater number of syngeneic bone marrow cells was injected intravenously, the level of circulating CFU in irradiated mice was slightly lower than the level in unirradiated mice during the first hours.     

A COMPARATIVE STUDY OF THE REPOPULATING POTENTIAL OF GRAFTS FROM VARIOUS HAEMOPOIETIC SOURCES: CFU REPOPULATION

The growth rate of the CFU populations in spleens and femora has been studied in irradiated mice injected with cell suspensions, containing equivalent number of CFU, from various sources. The doubling times are shown to be dependent upon the source of the cells. Grafts of bone marrow, spleen and foetal liver cells produced doubling times in the spleen of approximately 25, 19 and 16 hr respectively. Grafts of marrow-derived and spleen-derived spleen colony cells both gave rise to CFU doubling times lower than those of the corresponding primary grafts (approx. 33 and 26 hr respectively in the spleen). In the case of both bone marrow and spleen grafts the CFU population growth was shown to be independent of the size of the graft. A hypothesis is advanced which invokes at least a dual population of CFU, having different doubling times, different seeding capacities in the haemopoietic tissues following i.v. injection and present in different proportions in the various haemopoietic tissues.     

THE f NUMBER OF PRIMARY TRANSPLANTED SPLENIC COLONY-FORMING CELLS

The proportion of murine haemopoietic stem cells that settled in the spleen, after transplanting spleen cells into lethally-irradiated recipient mice, was found by comparing the number of spleen colonies obtained by transplanting a whole spleen with an estimate of the total number of colony-forming units (CFU) present in the intact spleen. the latter number was estimated assuming that endogenous spleen colonies were produced from surviving spleen-derived CFU which exhibited the same survival parameters as transplanted CFU.
Account was taken of the post-irradiation loss of CFU from the spleen in the endogenous assay, which was found to be a reasonably constant factor for doses higher than about 100 rad X-rays.
The total measured number of CFU/spleen from transplantation was less than the number calculated in the intact spleen by a factor, the primary f number, of 0.03 ± 0.02.     

GRAFT SIZE CONSIDERATIONS IN THE KINETICS OF SPLEEN COLONY DEVELOPMENT

The kinetics of spleen colony development has been studied after the injection of 10⁶, 10⁵ and 3 × 10⁴ bone marrow cells. The results indicate that:

• 1 The CFU population growth rate is independent of cell dose until the logarithmic growth phase is passed. Slowing of growth was seen by day 12 after the highest dose, by day 15 after the median dose, but was not observed during the period of observation after the low dose.
• 2 The growth rate of CFU per colony is independent of cell dose, but the curves are not identical. The differences between the curves leads to the conclusion that there is a dose-dependent delay in the commencement of CFU proliferation. The delay is roughly equal to one cell cycle time between the medium and high inoculum groups and also between the medium and low inoculum groups.
• 3 The number of cells per colony is graft size dependent, the doubling times, where these can be roughly assessed, being inversely related to the graft size. From the average number of cells per colony on day 6 it is calculated that the mean doubling time in the early stages of colony development is less than 7 hr.
• 4 The proportion CFU:colony cells is dose dependent with the highest inoculum having the highest proportion and the low inoculum group having the lowest proportion.

     

THE PERSISTENCE OF HEMOPOIETIC STEM CELLS IN VITRO

Cells capable of forming colonies in spleens of irradiated mice (CFU) are lost temporarily when bone marrow cells from rats or mice are maintained in culture. Rat marrow CFU go through a minimum at about 3 days after which there is a slow increase in the number of CFU in culture, reaching a maximum at 9 days. Mouse marrow CFU reach a minimum at 3 days and a maximum at 7 days. Some rat marrow CFU persist in culture for as long as 28 days.     

THE RELATIONSHIP BETWEEN SPLEEN COLONY PRODUCTION AND SPLEEN CELLULARITY

The fraction, f, of injected spleen colony-forming cells which can be recovered from the spleen of a radiated mouse has been determined at various times up to 24 hr following the initial cell injection. the cellularity of the spleen at the time of assay was also measured and compared with the calculated f number. the linear relationship between these two parameters indicated that over the period of 2-24 hr the number of CFU/10⁶ spleen cells was constant, both falling in a parallel fashion. A further experiment using non-irradiated W/W^v mice in which the spleen size did not change showed the same f number at the end of this period as at 2 hr. It is concluded that CFU are expelled from the spleen during its post-irradiation contraction thus leading to the apparent fall in f number. It is also concluded that a more realistic f number is obtained by assaying the splenic CFU content 24 hr after the primary cell transfer rather than 2 hr.     

KINETICS OF HAEMOPOIETIC RECOVERY IN ENDOTOXIN-TREATED MICE

Kinetics of mouse spleen colony forming units were studied after intra-peritoneal injection of 1 μ/g body weight bacterial endotoxin S. typhosa. When these mice were used as unirradiated and sublethally irradiated donors, it was possible to study the effect of the endotoxin injection upon the cells. Use of the treated mice as irradiated recipients of normal cells gave information about the host effect. In treated unirradiated mice, the total nucleated cell and the CFU counts were disturbed, and 2 days later a large fraction of the CFU were found in the DNA synthesis (S) phase. This meant that injection of endotoxin generated factors affecting the kinetics of the CFU and triggering the resting CFU into the proliferative cycle. If then the mice were given supralethal irradiation and used as recipients of normal bone marrow cells, more CFU seeded to the spleen as compared to normal recipients; but the dip and the growth rate of the CFU were not changed. Hence the endotoxin-generated factors had been eliminated in 2 days. A total body sublethal irradiation by 400 rad X-ray 2 days after endotoxin injection reduced the post-irradiation dip in the recovery curve of the CFU, indicating that though the factors affecting the cell kinetics had been eliminated, the cycling CFU behaved like a growing population. During the first week, the growth rate of the CFU remained the same as in control irradiated mice. The growth rate of the spleen CFU of the endotoxin-treated mice slowed down during the second week, and their self-replicating ability was low. Fluctuations in the DNA synthesizing fraction of the spleen CFU suggested a variability in the ratio of the length of the S phase and the cell generation time.     

THE ROLE OF THE SPLEEN IN THE REPOPULATION OF THE HAEMOPOIETIC SYSTEM OF HEAVILY-IRRADIATED MICE

The respective role of the spleen or of the bone marrow in the regeneration of the haemopoietic progenitor compartment of heavily-irradiated mice has been investigated. Splenectomy was used to this end in animals injected with exogenous isogenic cells or regenerating from endogenous spleen or marrow cells. Analysis of the data as a function of time shows that the presence of the spleen affects marrow CFU repopulation only at the early post-irradiation stages. The expansion of the marrow progenitor pool proceeds, however, rather independently of the spleen and marrow CFU remain eventually as the main source of haemopoietic cells in the surviving mice. Thus the reaction of the spleen may be envisaged as a fast, important but transient contribution to the overall haemopoietic function of heavily-irradiated animals.     

Factors influencing hematopoietic spleen colony formation in irradiated mice. V. Effect of foreign plasma upon colony forming cell kinetics

Foreign plasma injection induces a profound and somewhat complex change in the size and location of the colony forming unit (CFU) cell compartment. Injection of foreign plasma before irradiation induces an increase in CFU cells as judged by endogenous colonies as well as by a modification of the endogenous method which excludes spleen colony formation from in situ spleen cells. However, the enlargement does not take place in the most populous CFU cell areas, the spleen and marrow. The concentration and/or total number of CFU cells in spleen and marrow was not increased by plasma injection whether judged by the number of transplantable cells or by the number of migrating endogenous cells. These studies emphasize the complexity of this cellular system and suggest that the use of but one type of stem cell assay may yield results which do not reflect changes within the total compartment. Evidence for cell damage in vitro as a factor influencing results in studies involving transplantation was searched for but was not forthcoming.     

EFFECT OF CYCLOPHOSPHAMIDE ON THE HEMATOPOIETIC MICROENVIRONMENTAL FACTORS WHICH INFLUENCE HEMATOPOIETIC STEM CELL PROLIFERATION

Transplanted hematopoietic stem cells (HSC) regenerate more rapidly in the femoral marrow of lethally irradiated hosts pretreated with cyclophosphamide (CY) 4 days prior to X-irradiation than they do in that of uninjected irradiated hosts (control). On the other hand, regeneration of HSC transplanted into irradiated hosts given CY 7 days before X-irradiation is slower than in controls.
The microenvironment in the femoral marrow was studied at various times after giving CY. Four days after injecting CY, the number of colony forming units (CFU), total nucleated hematopoietic cells, and mature myeloid and erythroid cells in the femoral marrow is markedly reduced. Seven days after injecting CY, the number of CFU in the femoral marrow is still reduced, the total nucleated cell count is back to normal, but the number of mature myeloid elements in the marrow are significantly increased. These observations suggest the conclusion that the rate of proliferation of HSC is modulated by the number of mature myeloid cells in the microenvironment.     

EFFECT OF ERYTHROPOIETIN ON PROLIFERATION OF ERYTHROPOIETIN-RESPONSIVE CELLS

A single dose of Myleran suppressed CFU in polycythemic mice to around 1% of normal for a period of 2 weeks and permitted the study of effects of erythropoietin on unipotential, erythroid stem cells (erythropoietin-responsive cells, ERC) in the absence of cell inflow from the CFU compartment. Without erythropoietin no ERC were detectable for 12 days after Myleran. Injections of erythropoietin had no effect on CFU but restored ERC populations in proportion to the dose of erythropoietin. Hydroxyurea given after erythropoietin markedly inhibited ERC repopulation and the latter is attributed to a stimulation of ERC proliferation by erythropoietin. Evidence in support of an age structure in the ERC population is presented. Daily erythropoietin injection resulted in stable ERC populations, indicating that ERC loss through differentiation and ERC self-replication were in balance.     

RESPONSE OF MOUSE BONE MARROW COLONY FORMING UNITS IN DIFFERENT STAGES OF THE CELL CYCLE TO IN VITRO INCUBATION WITH MITOMYCIN-C

A short-term in vitro method was employed to study the Mitomycin-C sensitivity of normal mouse bone marrow CFU without triggering the G₀-phase cells into the proliferative cycle. Comparison was made of the toxicities of the drug against cells in different phases of the cell cycle including G₀. Mitomycin-c killed CFU both in and out of the S-phase. No significant difference could be found between its toxicities against normal and proliferating CFU; along the exponential part of the survival curve 1·6 μg/ml concentration of the drug reduced survival to 10%. Although in the normal bone marrow only a few CFU are in the S-phase and are killed by the agent, presence of the sensitive G₀ cells produce a significant amount of non-S-phase mortality. Among the proliferating CFU population the non-S-phase lethality is less due to the absence of G₀ cells. About 75% of the S-phase cells are killed after incubation with 1 μg/ml drug; outside the S-phase, the lethality is about 40–50%. The studies indicate that the G₀ cells which are situated near the G₁-S boundary are almost as sensitive to the drug as other non-S-phase cells like G₁ cells. The clinical significance of the findings is discussed.     

Kinetics of haemopoietic recovery in endotoxin-treated mice.

Kinetics of mouse spleen colony forming units were studied after intra-peritoneal injection of 1 mug/blody weight bacterial endotoxin S. typhosa. When these mice were used as unirradiated and sublethally irradiated donors, it was possible to study the effect of the endotoxin injection upon the cells.Use of the treated mice as irradiated recipients of normal cells gave information about the host effect. In treated unirradiated mice, the total nucleated cell and the CFU counts were disturbed, and 2 days later a large fraction of the CFU were found in the DNA synthesis (S) phase. This meant that injection of endotoxin generated factors affecting the kinetics of the CFU and triggering the resting CFU into the proliferative cycle. If then the mice were given supralethal irradiation and used as recipients of normal bone marrow cells, more CFU seeded to the spleen as compared to normal recipients; but the dip and the growth rate of the CFU were not changed. Hence the endotoxin-generated factors had been eliminated in 2 days. A total body sublethal irradiation by 400 rad X-ray 2 days after endotoxin injection reduced the post-irradiation dip in the recovery curve of the CFU, indicating that though the factors affecting the cell kinetics had been eliminated, the cycling CFU behaved like a growing population. During the first week, the growth rate of the CFU remained the same as in control irradiated mice. The growth rate of the spleen CFU of the endotoxin-treated mice slowed down during the second week, and their self-replicating ability was low. Fluctuations in the DNA synthesizing fraction of the spleen CFU suggested a variability in the ratio of the length of the S phase and the cell generation time.     

雌百灵端脑新生神经前体细胞的产生和分布并与白腰文鸟的比较

用BrdU标记DNA的ABC免疫细胞化学方法,观察雌性蒙古百灵端脑神经前体细胞的产生和分布特点,并与白腰文鸟作比较。结果如下:1.在百灵和白腰文鸟胸肌注射BrdU短时程组(存活1天),在端脑室带区外侧壁(LVZ)有大量的标记细胞,新生神经细胞起源于端脑室带区(VZ)中的增殖细胞层,并在纹状体腹侧的VZ形成标记细胞增殖热点,如在百灵和白腰文鸟靠近中缝线处的外侧纹状体(LSt)与内侧纹状体(MSt)腹侧的LVZ形成标记最多的‘第1增殖热点’区;在靠近中缝线处LVZ的头端形成密集的新生标记细胞,形成‘第2增殖热点’区;在百灵LSt尾端的LVZ标记细胞形成‘第3增殖热点’,但白腰文鸟此脑区的标记细胞较少。2.在百灵胸肌注射BrdU长时程组中5天起,大量的LVZ的标记细胞开始迁移,存活5-30天期间在高级发声中枢(HVc)和高位发声运动中枢-古纹状体栎核(RA)有新生标记细胞,在端脑靠近LVZ的区域有较多的标记细胞。但在雌性白腰文鸟胸肌注射BrdU存活30天期间,在HVC、RA内未见到标记细胞。结果提示雌百灵端脑HVc和RA不断地产生新生神经细胞,这可能与雌性需要不断地感知、识别雄百灵鸣唱的新语句有关,而白腰文鸟不需要这种功能。     

UPTAKE AND TRANSFER OF PARTICULATE MATTER FROM THE PERITONEAL CAVITY OF THE RAT

1. Colloidal mercuric sulfide or thorium dioxide injected intraperitoneally passes into the cytoplasm of the mesothelium of the mesentery and of the diaphragm as early as 15 to 30 minutes after the injection. 2. Between 15 minutes and 12½ hours the number of particles within the mesothelial cells increases as the time between injection and termination of the experiment is lengthened. 3. The particulate matter is usually localized in the cytoplasm within clear vacuoles or bodies having a relatively dense matrix. 4. A greater quantity of the absorbed material is commonly observed within the cytoplasm of the diaphragmatic than of the mesenteric mesothelium.     

THE PROLIFERATIVE CAPACITY OF HAEMOPOIETIC COLONY FORMING UNITS IN THE RAT

After transplantation into rats lethally treated with cytotoxic chemicals both bone marrow and spleen CFU in the spleen and spleen derived CFU in the bone marrow expand with doubling times ( T _d) of approximately 18 hr. However, bone marrow derived CFU in the bone marrow have a T _d of 36 hr. Evidence obtained using tritiated thymidine in vitro and methotrexate in vivo show that the proliferation rate of bone marrow derived CFU is similar in both the bone marrow and spleen and calculations suggest that the different T _d between these two sites is due to the higher loss of CFU through differentiation in the bone marrow compared to the spleen. These findings further support the hypothesis of an environment in the spleen which favours CFU self-maintenance over differentiation with the opposite situation occurring in the bone marrow.     

THE ROLE OF BONE MARROW OF X-IRRADIATED MICE IN THYMIC RECOVERY

The influence of the bone marrow on the repopulation of the thymus in X-irradiated mice has been investigated.
It was observed that the thymus and a certain population of bone marrow lymphocytic cells were repopulated in parallel in a cyclic fashion. This occurred either after a single exposure of mice to 400 R or after serial weekly X-ray treatments with 170 R. Lethally irradiated recipients which were grafted with bone marrow cells obtained 12-24 days after four weekly irradiations of donor mice with 170 R also exhibited a cyclic repopulation of both the thymus and the bone marrow lymphocytic population. In contrast, mice which were transplanted with bone marrow cells from unirradiated donors, containing an equal number of stem cells (CFU), exhibited a continuous rather than a cyclic recovery of both cell populations. the bone marrow stem cells of mice recovering from X-irradiation were found to have a decreased proliferative activity, since they produced significantly smaller spleen colonies in lethally irradiated recipients than marrow cells from unirradiated mice.
The results were interpreted as indicating that the bone marrow lymphocytic cells may act as thymic precursor cells and that thymic lymphopoiesis is dependent on the presence of such cells. Evidently, the production of lymphocytic cells will decrease when the stimulus for granulocyte production increases due to the limited proliferative activity of the surviving bone marrow stem cells after irradiation. This may result in a cyclic variation of the production of bone marrow lymphocytic cells and it follows that thymic lymphopoiesis will run parallel.     

THE EFFICIENCY OF THE ASSAY FOR HAEMOPIETIC COLONY FORMING CELLS

The quantitative efficiency of the spleen colony assay in mice is discussed in the light of recent findings on the kinetics of colony forming cells. Arguments are presented showing that the f factor, the 2 hr CFU recovery fraction in the spleen, markedly over-estimates the assay efficiency which is the ratio of the numbers of colony forming units and colony forming cells.     

EFFECT OF METHOTREXATE ON THE CELL CYCLE OF L1210 LEUKEMIA

The influence of methotrexate (MTX) on the proliferative activity of cells in different phases of cell cycle has been studied. MTX (5 mg/kg) was injected i.p. 3 days after the inoculation of 5 × 10⁶ leukemia cells into F₁ (DBA × C57 BL) mice. It was shown that MTX causes degeneration of cells, being in G₁- as well as in S-phase at the time of drug injection. Incorporation of ³H-TdR was suppressed for a period ranging from 2 to 12 hr after MTX administration, which is demonstrated by the decrease in the number of grains per cell. The number of cells labeled after ³H-TdR injection was also sharply decreased during this period. For a period of 3 until 15 hr after MTX administration the mitotic index decreased significantly as a result of inhibition of DNA synthesis. The blocking of the G₁-S transition was evident during 4 hr after MTX. Thereafter the G₁-S transition proceeds at a rate which is practically equal to that for nontreated controls. MTX did not inhibit transition to mitosis of cells being in G₂-phase and in a very late S-phase at the time of drug injection. The sensitivity of G₁-cells to the cytocidal effect of MTX shows that for L1210 leukemia cells MTX can be classified as a cycle-specific drug killing both G₁ and S-cells rather than S-phase specific agent with self-limitation.     

THE COMPARATIVE EFFECT OF BUSULPHAN ('MYLERAN') AND AMINOCHLORAMBUCIL ON HAEMOPOIETIC COLONY FORMING UNITS IN THE RAT

Using the spleen colony assay technique, it has been shown that busulphan (‘Myleran’) in a dose of 10^-2 g/kg (1/2 LD₅₀), causes a marked and prolonged depression (over 90%) in the number of colony forming units per femur (CFU/femur). This depression is apparent before there is any marked reduction of the total cell count per femur and is maximal 2–4 days after an intraperitoneal (i.p.) injection of the drug. It is then followed by a steady recovery, normal values being reached after about 20 days. In contrast, aminochlorambucil (2·5 x 10^-3) g/kg = 1/2 LD⁵⁰) although producing a rapid fall in the marrow cellularity has no discernable effect on the CFU/femur. If, however, a depression of CFUs is first induced by busulphan and, after allowing time for 50% recovery (13 days), aminochlorambucil is now given, a further severe depression of the CFUs/femur occurs resulting in a considerable prolongation of the neutropenia observed in the blood. The possible implications of this in the mode of action of these two drugs, and in the chemotherapy of leukaemia, are discussed.