Growth factor-dependent inhibition of normal hematopoiesis by N-ras antisense oligodeoxynucleotides

To determine whether N-ras expression is required at specific stages of the process of in vitro normal human hematopoiesis, adherent- and T lymphocyte-depleted mononuclear marrow cells (A-T-MNC) or highly purified progenitors (CD34+ cells) were cultured in semisolid medium, under conditions that favor the growth of specific progenitor cell types, after exposure to N-ras sense and antisense oligodeoxynucleotides. N-ras antisense, but not sense, oligodeoxynucleotide treatment of A-T-MNC and CD34+ cells resulted in a significantly decreased number of granulocyte/macrophage colony-forming units (CFU-GM) induced by interleukin 3 (IL-3) or granulocyte/macrophage colony-stimulating factor (GM-CSF) and of macrophage colonies (CFU-M) induced by M-CSF, but not of granulocytic colonies induced with G-CSF or IL-5. However, the same treatment significantly inhibited colony formation induced by each of the above factors in combination with IL-3. Megakaryocytic colony (CFU-Meg) formation from A-T-MNC or CD34+ cells in the presence of IL-6 + IL-3 + erythropoietin (Epo) was also markedly decreased after antisense oligodeoxynucleotide treatment. Erythroid colonies derived from A-T-MNC in the presence of Epo (CFU-E) were not inhibited upon antisense treatment, whereas those arising from A-T-MNC or CD34+ cells in the presence of IL-3 + Epo (BFU-E) were markedly affected. These results are consistent with the hypothesis that distinct signal transduction pathways, involving N-ras or not, are activated by different growth factors in different hematopoietic progenitor cells.

[Intra-systemic inhibitors of the hematopoietic system]

Physiological, experimental and theoretical aspects concerning haemopoietic inhibitors are reviewed. In the paper there are also discussed future research directions of this branch of haematology.

[Effect of intraperitoneal administration of sandoglobulin on peritoneal fluid IgG level in patients treated by standard peritoneal dialysis (SPD)]

In 6 patients (PTS) treated with SPD for 369 patient weeks (ptw) we evaluated the influence of three intraperitoneal SI on the IgG concentration in the PE. The concentration of IgG was checked before, on 1-st, 3-rd day 2-nd and 3-rd week after SI. In the course of the following 229 ptw, we observed three episodes of peritonitis (PN) in two pts, while in the control period of 140 ptw no PN occurred. The concentrations in PE of opsonins (IgG, C3 complement, fibronectin) at the start of SPD shoved enormous individual differences and did not correlate with occurrence of PN in the follow-up. Intraperitoneal SI caused significant increase of IgG in PE, but only to the third day after infusion. Our preliminary results do not confirm the protective effect of intraperitoneal SI on occurrence of PN in SPD patients.

[Changes of certain parameters of renal function in patients with allogenic bone marrow transplantation]

Serum creatinine and urea changes as well as protein, erythrocytes, and leucocytes excretion with the urine in allogenic bone marrow recipients are discussed. An increase in serum creatinine and urea together with proteinuria and hematuria were noted in 1/3 of patients treated with busulphan and cyclophosphamide prior to bone marrow transplantation. In cases of bone marrow graft rejection or post-transplantation complications no abnormalities in the value of investigated parameters were noted, especially immediately before death. Possible causes of such renal function disorders are also discussed.

Selective inhibition of leukemia cell proliferation by BCR-ABL antisense oligodeoxynucleotides

To determine the role of the BCR-ABL gene in the proliferation of blast cells of patients with chronic myelogenous leukemia, leukemia blast cells were exposed to synthetic 18-mer oligodeoxynucleotides complementary to two identified BCR-ABL junctions. Leukemia colony formation was suppressed, whereas granulocyte-macrophage colony formation from normal marrow progenitors was unaffected. When equal proportions of normal marrow progenitors and blast cells were mixed, exposed to the oligodeoxynucleotides, and assayed for residual colony formation, the majority of residual cells were normal. These findings demonstrate the requirement for a functional BCR-ABL gene in maintaining the leukemic phenotype and the feasibility of gene-targeted selective killing of neoplastic cells.

Normal and leukemic hematopoietic cells manifest differential sensitivity to inhibitory effects of c-myb antisense oligodeoxynucleotides: an in vitro study relevant to bone marrow purging

The c-myb protooncogene is preferentially expressed in hematopoietic cells, and its encoded protein, Myb, is required for hematopoietic cell proliferation. To analyze the relative Myb dependence of normal and leukemic human hematopoietic progenitor cells, normal bone marrow cells, several types of leukemic blast cells, and 1:1 mixtures of normal and leukemic cells were cultured in the presence of c-myb sense or antisense oligodeoxynucleotides; cell viability and cloning efficiency were then assessed. c-myb sense oligomers had negligible effects on normal and leukemic cells. In contrast, c-myb antisense oligomers strongly inhibited or completely abolished clonogenic growth of a T-cell leukemia line, 78% (18 of 23) of primary acute myelogenous leukemia cases examined, and 4 of 5 primary chronic myelogenous leukemia (CML) cases in blast crisis. In three of the latter patients, polymerase chain reaction analysis of a 1:1 mixture of c-myb antisense-treated normal and CML cells revealed a complete absence of bcr-abl expression, suggesting that the CML clonogenic units had been completely eliminated from the cultures. At antisense doses that inhibited leukemic cell growth, normal hematopoietic progenitor cells survived. Thus, normal and leukemic hematopoietic cells show differential sensitivity to the toxic effects of c-myb antisense DNA. Perturbation of c-myb function with antisense oligodeoxynucleotides might eventually form the basis for a molecular approach to leukemia therapy, perhaps most immediately as ex vivo bone marrow purging agents.

Down-regulated c-myb expression inhibits DNA synthesis of T-leukemia cells in most patients

We have investigated the functional relevance of c-myb expression for DNA synthesis in patients' T-leukemia cells. [3H]Thymidine incorporation assays of 32 patients' leukemia cells exposed in vitro to c-myb sense or antisense oligodeoxynucleotides served to define two groups of patients: a responder group whose leukemia cells showed 2- to 16-fold lower levels of [3H]thymidine incorporation in c-myb antisense-treated cultures than in c-myb sense-treated cultures (20 patients) and a nonresponder group whose cells showed comparable [3H]thymidine incorporation levels in either c-myb sense- or antisense-treated cultures (12 patients). Down-regulation of c-myb mRNA levels in cells exposed to c-myb antisense oligodeoxynucleotides was comparable in both groups of patients, indicating that differential sensitivity to c-myb antisense oligodeoxynucleotides was not due to differential uptake of these oligodeoxynucleotides. DNA polymerase alpha mRNA levels were down-regulated in cells from the responders but were unaffected in the nonresponder group. These results suggest that c-myb is required for DNA synthesis in cells of many but not all T-leukemia patients and that leukemia cells in which DNA synthesis is not inhibited despite down-regulation of c-myb expression may have undergone some genetic change(s) that obviate(s) the requirement for myb protein.

Return of immunohematopoietic impairment a long time after murine syngeneic bone marrow transplantation

We have tested the immunologic status and hematologic parameters of mice 2 months (short-term survivors) or 18 months (long-term survivors) after lethal total body irradiation and syngeneic bone marrow transplantation (SBMT), and of normal mice of corresponding age. Long-term SBMT survivors showed significantly lowered bone marrow and spleen cellularities, decreased numbers of CFU-S in hemopoietic organs and severe impairment in the formation of CFU-F colonies compared with short-term SBMT survivors and normal mice. The peripheral blood parameters (hematocrit, erythrocytes, reticulocytes, platelets, white blood cells and granulocyte counts), however, remained unaltered. In long-term SBMT survivors we also observed a relative increase of Lyt-2+ lymphocytes (CD8+, cytotoxic/suppressor) and Mac-1+ cells among splenocytes. At the same time the L3T4+/Lyt-2+ ratio (CD4+/CD8+) was decreased. Relative contents of Ig+, Thy-1+ and L3T4+ cells were unchanged. The ability of splenocytes to generate IL-2R+ cells after in vitro stimulation with concanavalin A was greatly diminished. In summary, the immunohematopoietic status after initial normalization is again impaired in long-term SBMT survivors.

[Interleukin-2 in the treatment of malignant melanoma. A case report]

Interleukin-2 is a lymphokine with documented antineoplastic influence, with not completely understood mechanism of action. The case of 46-years old patient with relapsed metastatic malignant melanoma treated with constant-infusion of rIL-2 is described. 9-month remission was achieved. During the course of treatment a lot of side effects including flu-like symptoms, hypotonia, anemia and thrombocytopenia, and also many biochemical disturbances were observed.

[Effect of hemodialysis fluids with different pH values and buffer types used in peritoneal dialysis on the phagocyte system in the peritoneal cavity. The experimental part]

In a four-hour exchange of dialysing fluid in 120 healthy mice it was checked whether and in what degree changes of the pH of the dialysing fluid in the range from 5.09 to 7.02 and change of the buffer in the dialysing fluid (acetate, lactate) have an effect on the phagocytic and bactericidal ability of the phagocytic cells in the peritoneal dialysate. The study of the count of phagocytic cells, their bactericidal ability (NBT test) and phagocytosis ability (latex test) showed that neither the type of the used buffer nor pH changes of the dialysing fluid in the range 5.09-6.18 had no effect on the defensive properties of the phagocytes in healthy peritoneal cavity. The observation of a statistically significant increase in the proportion of cells of low phagocytic ability in the group of mice receiving acetate buffer of pH 7.02 requires further studies.

Multiple lineage colony growth from human marrow in plasma clot diffusion chambers

Marrow cells from ten healthy adult donors were cultured in plasma clot diffusion chambers implanted intraperitoneally into mice. Host animals were conditioned by two injections of phenylhydrazine and 600 cGy of x-rays. Cultures (5 X 10(4) cells/chamber) were continued for between 2 and 40 days and the chambers were retransplanted into new host animals every 5 days. Following termination of cultures, plasma clots were stained with benzidine-hematoxylin and analyzed microscopically. Erythroid, neutrophil, monocyte, eosinophil, megakaryocyte, mixed, undifferentiated, and fibroblastoid colonies were grown with neutrophil, erythroid, monocyte, and eosinophil colonies being the most frequent. A total of between 25 and 60 colonies was observed per chamber at any time point.

Success of bone marrow transplantation in congenital Diamond-Blackfan anaemia: a case report

A 6-yr-old girl with congenital corticosteroid-resistant pure red cell aplasia was treated with bone marrow transplant from her HLA-identical, MLC-unreactive sister in November 1984 following conditioning with busulfan and cyclophosphamide. Full engraftment was obtained and the patient at 21 months post-transplant is in excellent clinical condition maintaining normal red cell counts. We conclude that BMT should be considered as a therapy for at least the most severe cases of Diamond-Blackfan anaemia resistant to corticosteroids. Successful outcome of this therapy provides an argument for the stem cell origin of this disorder.

Congenital murine osteopetrosis inherited with osteosclerotic (oc) gene: hematological characterization

Two- to three-week-old mice homozygous for the recessive oc gene had negligible numbers of marrow cells but possessed no significant spleno- and hepatomegaly. They also maintained normal numbers of blood cells except for monocytes, which were significantly lower. Additionally, they had reduced numbers of total cells and resident macrophages in the peritoneum, as determined by cell counts in the peritoneal lavage fluid. The frequency of spleen colony-forming units (CFU-S) in the spleens of oc/oc mice was the same as that in the spleens of normal littermate control mice. These oc/oc CFU-S showed essentially similar differentiation patterns as CFU-S of control mice. Also, a few CFU-S could be detected in livers of oc/oc mice. On the other hand, the frequency of cells that formed macrophage colonies in a four-day liquid-culture system in the presence of colony-stimulating activity was significantly reduced in oc/oc mice and abnormalities were observed in the formation of the adherent (stromal) layers by oc/oc spleen cells in liquid cultures. Numbers of fibroblastoid cell colonies in these layers were reduced and, moreover, cultures demonstrated a marked decrease in the number of macrophages both within and outside the fibroblastoid cell colonies. Transplants of spleen and thymus cells of oc/oc mice into lethally irradiated +/? recipients induced oc/oc-like lesions. They included peritoneal macrophage deficiency, marrow deficiency, as well as hepatosplenomegaly. This suggests a hemopoietic stem cell and not microenvironmental defect in this particular type of osteopetrosis. The murine mutant characterized in this study may be useful in studies of cellular interactions during blood and bone formation and in studies of the mononuclear phagocyte system.

Hemopoiesis in mouse heterozygous for the W trait. Difference in postirradiation recovery related to the defect in formation of transient endogenous spleen colonies

The hemopoietic regeneration following midlethal irradiation in Wv/+ mice had similarly biphasic kinetics as in their hematologically normal +/+ littermates. The first abortive phase of regeneration was either severely reduced (formation of transient endogenous spleen colonies, reticulocyte count, granulocyte count) or absent (spleen and femur cellularity, platelet count, PCV) in Wv/+ mice, when compared to +/+ mice. The second phase leading to permanent recovery of hemopoiesis was in Wv/+ mice delayed in time. Moreover, although to a lesser extent the values of spleen and femur cellularity, PCV and platelet count were decreased in Wv/+ mice. Postirradiation bleeding, which stimulated particularly the 1st phase of regeneration both in Wv/+ and +/+ mice did not lead to the minimization of differences between above two genotypes. It is suggested that the observed differences in the abortive regeneration between Wv/+ and +/+ mice are primarily dependent on the presence in Wv/+ mice of selective defect of transient endogenous colony forming units (TE-CFUs). Moreover, it is possible that the differences in the second phase of regeneration leading to permanent recovery are secondarily dependent on the TE-CFUs defect, as most probably the TE-CFU is the step in stem cell differentiation to mature cells.

The effect of thymus cells on bone marrow transplants into sublethally irradiated mice. Lack of correlation between spleen colony formation and competitive repopulation of granulocytic system

Bone marrow cells formed similar numbers of 10-day spleen colonies in sublethally (6 Gy) irradiated C57B1/6 mice in lethally (7.5 Gy) irradiated mice i.e. approximately 20 per 10(5) cells. Numbers of 10 day endogenous spleen colonies in sublethally irradiated mice (0.2 to 0.6 per spleen) did not differ significantly from the numbers in lethally irradiated mice. Yet, transplants of 10(7) coisogenic marrow cells into sublethally irradiated mice resulted in predominantly endogenous recovery of granulocyte system as evidenced by utilization of "beige" marker for transplanted cells. Nevertheless, transplanted cells engrafted into sublethally irradiated mice were present in their hemopoietic tissues throughout the observation period of 2 months never exceeding 5 to 10% of all cells. Thymus cells stimulated endogenous and exogenous spleen colony formation as well as endogenous granulopoietic recovery. Additionally, they increased both the frequency and absolute numbers of graft-derived granulocytic cells in hemopoietic organs of transplanted mice. They failed, however, to essentially change the quantitative relationships between endogenous and exogenous hemopoietic recovery. These results may suggest that spleen colony studies are not suitable for prediction of events following bone marrow transplant into sublethally irradiated mice. Simultaneously, they have strengthened the necessity for appropriate conditioning of recipients of marrow transplants.

Reaction to intradermally applied phytohaemagglutinin in asthma patients in relation to corticosteroid therapy

Phytohaemagglutinin (PHA) skin test (diameter of induration 24 h following intradermal injection of 1.0 microgram purified PHA) was carried out on 23 patients with exacerbated atopic asthma and 28 patients with exacerbated non-atopic asthma. Preselected adult patients had either not previously been treated with systemic corticosteroids or steroid therapy had been suspended for at least 3 months. Nearly all non-atopic asthma patients and patients with atopic asthma previously treated with corticosteroids showed increased reactions to PHA. Patients with atopic asthma not earlier treated with corticosteroids demonstrated normal responses. None of the asthmatics showed a negative PHA reaction. Administration of single depot doses of corticosteroids produced decreased reactivity to PHA in nearly all patients. These results suggest that neither atopic nor non-atopic asthma is in itself associated with impaired PHA skin reactivity but that changes in this reactivity are largely due to the corticosteroid therapy administered to these patients. In relation to PHA reactivity certain effects of this therapy may persist for as long as 3 months after its cessation.

Adherent cell growth from murine bone marrow in liquid cultures. Inbred strain variability and effects of mutations affecting hemopoiesis

The marrow cells of mice from seven hematologically normal strains (AKR/J, AKR/Cum, CBA/J, C3H/HeJ, C57B1/6J, DBA/2J, and RF/J) and of mice with five mutations in loci affecting hemopoiesis (W, Sl, nu, xid, NZB) were tested. The following parameters of adherent cell growth in 14 day liquid culture were analyzed: number and diameter of macroscopic adherent colonies, cellular composition with particular reference to the number of macrophage-like cells attached to a single fibroblast-like cell on peripheries of adherent colonies and the number of macrophage-like cells per eyefield of intercolony spaces. No qualitative differences were observed between tested genotypes in the cellular composition of adherent layers. These layers were uniformly made of colonies of fibroblast-like cells that were overgrown by macrophage-like cells. Macrophage-like cells grew also in intercolony spaces but this never occurred to fibroblast-like cells. At least using light microscopy and Wright's staining no morphological differences could be observed between either fibroblast-like cells or macrophage-like cells derived from marrow of various murine strains. This is of particular significance for Sl/Sld mice that suffer from the functional defect of hemopoietic microenvironment. On the other hand, large strain variability was observed in the number (5-16/10(6) marrow cells, 10.6 on the average) and diameter (1.9-3.7 mm, 2.9 mm on the average) of macroscopic adherent colonies. Among mutant genotypes, only NZB mice and animals with xid mutation formed significantly increased numbers of macroscopic colonies, while values for other mutant genotypes, Sl/Sld included, did not differ from normal littermate controls.

Hematological characterization of congenital osteopetrosis in op/op mouse. Possible mechanism for abnormal macrophage differentiation

Compared with normal littermates, the op/op mice had very few macrophages in the peritoneal cavity and severely reduced numbers of monocytes in the peripheral blood. Moreover, osteopetrotic animals demonstrated an altered distribution of hemopoietic tissue with a 10-fold decrease in the number of marrow cells. Liver hemopoiesis persisted in 4-wk-old mice as evidenced by the presence of hemopoietic stem cells (HSC). Moreover, the concentration of HSC was decreased in marrow and increased in the spleen of op/op mice. In spite of the paucity of cells of monocyte-macrophage lineage in vivo, progenitor cells from hemopoietic tissues of op/op mice formed increased numbers of monocyte-macrophage colonies in vitro in the presence of exogenous colony-stimulating activity (CSA). The source of this critical CSA was a medium conditioned by stromal fibroblastoid colonies formed in vitro by normal marrow cells. Therefore, these data suggest that op/op mice possess normal monocyte-macrophage-osteoclast progenitor cells but these cells are unable to fully differentiate in the op/op mouse microenvironment. In support of this, in cultures of stromal fibroblastoid colonies from op/op marrow or spleen, the concomitant growth of macrophages, normally very dense, was drastically reduced. Moreover, transplantation of op/op spleen cells into lethally irradiated littermate recipients resulted in their hemopoietic reconstitution without signs of macrophage defect. Thus, the op/op splenic cells do not transfer the disease and are capable of normal differentiation in normal in vivo environment. These observations support the hypothesis that the defect in op/op mice is a result of the failure of hemopoietic stromal fibroblastoid cells to release sufficient amounts of CSA necessary for normal differentiation of cells of the monocyte-macrophage lineage.

Aplastic anaemia with marrow defective in formation of fibroblastoid cell colonies in vitro

The formation of fibroblastoid colonies by marrow cells in vitro has been used as a putative assay for a stem cell of haemopoietic stroma. Bone marrow from one patient with aplastic anaemic did not form any of these colonies, while its growth in diffusion chambers as an indirect measure of a haemopoietic stem cell was even better than normal. On the other hand, marrow from the other aplastic patient showed only quantitative decrease in the formation of fibroblastoid colonies and simultaneously grew very poorly in diffusion chambers. These patients were indistinguishable by the cytological examination of their marrow, however, the peripheral blood abnormalities were expressed less severely in the first patient. These results suggest the contribution of the defect in marrow cells, which form fibroblastoid colonies in vitro to the development of aplastic anaemia in these patients.

Reaction of the human granulocyte system in successfully treated bacterial bronchopneumonia

The behaviour of peripheral blood granulocyte pools, nitroblue tetrazolium reduction by granulocytes, migration of granulocytes out of skin windows, serum muramidase activity and bone marrow reserve of granulocytes during uncomplicated bronchopneumonia were studied. The time course of changes in these parameters during infection was established, and suggests maximal changes on day 2 of infection with gradual regression until 30 days after the disease onset, when they disappear.

Conditions of adherent cell growth from murine bone marrow in liquid cultures, and partial characterization of function

The number of adherent colonies grown and the type of their growth were influenced by both the serum and the medium used for the culture. The test results were obtained using either Dulbecco MEM, TC-199 or NCTC-109 together with 10% fetal calf serum and 10% horse serum (FCS/HS). Murine marrow cultures made in these conditions gave the linear relationship between the number of cells explanted (in the range 5 X 10(3) cells/ml to 5 X 10(6) cells/ml) nd the number of macroscopic adherent colonies grown. The number of colonies reached plateau by day 9 of culture and did not increase for up to 90 days of culture. Adherent colony forming cells disappeared from the liquid phases of such cultures within 3 days. Adherent layer formed had the capacity to support maintenance of spleen colony forming units and produced colony stimulating factor (CSF). This CSF supported 4-day granulocyte and macrophage colony formation in liquid cultures but not 7-day granulocyte-macrophage colony formation in semisolid medium. Therefore, the described culture system allowed to quantitatively grow cells responsible for some properties attributed to hemopoietic microenvironment (HM), possessed defined growth characteristics and may be utilized for further in vitro studies of HM.

Haematopoiesis in mice heterozygous for the W trait: defective formation of transient endogenous spleen colonies

It has been determined that W/+ and Wv/+ heterozygous mice, as compared with normal +/+ homozygous littermates, form significantly lower numbers of transient 5-day endogenous spleen colonies in response to X-irradiation. This defect was evident for doses of irradiation between 2-6 Gy (200-600 rad) and was associated with a slightly increased radiosensitivity of the assayed precursor cells (TE-CFU) in W heterozygotic mice. Moreover, the defect was transplantable, i.e., intrinsic to the marrow cells and not to the microenvironment, and was not associated with a similar decrease in cells which form erythropoietic bursts in vitro (BFUe). This study provides a cellular basis for increased radiosensitivity of W/+ and Wv/+ mice and suggests that the 'W' mutation is semi-dominant, both with respect to the white spotting and TE-CFU formation.

The effects of x-irradiation in vitro on subpopulations of human lymphocytes

Human peripheral blood lymphocytes isolated using a Ficoll-Uropolinum gradient were exposed to between 1 and 25 Gy X-rays and were cultured for period of up to 72 hours. Before irradiation and after 2, 24, 48 and 72 hours total numbers of surviving cells per ml of culture were determined as well as the frequencies of cells spontaneously forming rosettes (E) and active rosettes (AE) with sheep red blood cells, cells bearing receptor for activated complement (EAC) and cells spontaneously forming rosettes with mouse red blood cells (ME). Irrespective of the duration of in vitro culture, the radiosensitivity of the subpopulations of human lymphocytes increased in the order E less than EAC = ME less than AE rosette forming cells. Calculation of the frequencies of surviving cells, null with respect to the surface markers (E and EAC), showed that disappearence of rosette-forming cells under the influence of radiation was largely due to the shedding of their receptors and not exclusively to cell death. It is suggested that the extreme radiosensitivity of the subpopulation of T lymphocytes capable of AE rosette formation, if confirmed by in vivo studies, may possess diagnostic significance in persons accidentally exposed to radiation.

Antitheta immunization affects hemopoietic colony growth in a theta-incompatible mouse system

Seeking a system with which to study the antitheta-sensitive regulatory cell (TSRC), other than the W/Wv anemic mouse, we tested a model which utilizes mouse strains differing almost exclusively in theta allotype. AKR/FuRd Thy 1.1 mice were immunized against CBA Thy 1.2 mouse thymocytes to produce antibody. The immunized mice were lethally irradiated and grafted with marrow cells from AKR/Cum Thy 1.2 mice. It was reasoned that in this situation antitheta-sensitive cells present in or produced by the graft will be destroyed, while theta-incompatible host cells may not be as capable of collaboration with the graft stem cells as are the graft's theta-compatible cells. Anti-Thy 1.2 immunization did not influence the number of spleen colonies formed by marrow from Thy 1.2 mice; but the colonies formed, especially the erythropoietic colonies, were reduced in size. Also, both the relative and absolute frequencies of erythropoietic colonies were decreased, while those of granulopoietic colonies were increased. Therefore, the destruction of antitheta-sensitive cells in this system resulted in decreased effectiveness of spleen colony growth and altered the differentiation pattern of colony-forming cells. These effects were not observed in control groups immunized with CBA tissues not bearing the Thy 1.2 antigen. This study further stresses the importance of the theta antigen in hemopoietic differentiation and may provide an alternative in vivo model for studying the TSRC.

Different marrow cell number requirements for the haemopoietic colony formation and the curve of the W/Wv anemia

The lowest cell number in the normal marrow transplant, which allows the cure of W/Wv anemia was found to be between 10(4) and 10(5). This exceeds by several times the lowest cell number necessary for the haemopoietic colony formation. Therefore, either the colony forming cell is not the haemopoietic stem cell but rather its progeny, or this cell requires an aid from some other cells to exert is activity.