Cells capable of bone production engraft from whole bone marrow transplants in nonablated mice.

Allogeneic and autologous marrow transplants are routinely used to correct a wide variety of diseases. In addition, autologous marrow transplants potentially provide opportune means of delivering genes in transfected, engrafting stem cells. However, relatively little is known about the mechanisms of engraftment in transplant recipients, especially in the nonablated setting and with regard to cells not of hemopoietic origin. In particular, this includes stromal cells and progenitors of the osteoblastic lineage. We have demonstrated for the first time that a whole bone marrow transplant contains cells that engraft and become competent osteoblasts capable of producing bone matrix. This was done at the individual cell level in situ, with significant numbers of donor cells being detected by fluorescence in situ hybridization in whole femoral sections. Engrafted cells were functionally active as osteoblasts producing bone before being encapsulated within the bone lacunae and terminally differentiating into osteocytes. Transplanted cells were also detected as flattened bone lining cells on the periosteal bone surface.

The fluctuating phenotype of the lymphohematopoietic stem cell with cell cycle transit.

The most primitive engrafting hematopoietic stem cell has been assumed to have a fixed phenotype, with changes in engraftment and renewal potential occurring in a stepwise irreversible fashion linked with differentiation. Recent work shows that in vitro cytokine stimulation of murine marrow cells induces cell cycle transit of primitive stem cells, taking 40 h for progression from G0 to mitosis and 12 h for subsequent doublings. At 48 h of culture, progenitors are expanded, but stem cell engraftment is markedly diminished. We have investigated whether this effect on engraftment was an irreversible step or a reversible plastic feature correlated with cell cycle progression. Long-term engraftment (2 and 6 mo) of male BALB/c marrow cells exposed in vitro to interleukin (IL)-3, IL-6, IL-11, and steel factor was assessed at 2-4-h intervals of culture over 24-48 h using irradiated female hosts; the engraftment phenotype showed marked fluctuations over 2-4-h intervals, with engraftment nadirs occurring in late S and early G2. These data show that early stem cell regulation is cell cycle based, and have critical implications for strategies for stem cell expansion and engraftment or gene therapy, since position in cell cycle will determine whether effective engraftment occurs in either setting.

Recombinant murine granulocyte macrophage colony-stimulating factor has megakaryocyte colony-stimulating activity and augments megakaryocyte colony stimulation by interleukin 3.

Recombinant murine granulocyte macrophage colony-stimulating factor (rGM-CSF) has been produced in Escherichia coli and purified to homogeneity. GM-CSF has an established role as an in vitro regulator of granulocyte and macrophage colony formation. We have determined that rGM-CSF also has intrinsic activity as a megakaryocyte colony-stimulating factor and that rGM-CSF augments the effect of interleukin 3 (IL-3) on megakaryocyte colony formation. The dose-response curve for megakaryocyte colony induction with rGM-CSF showed plateau megakaryocyte stimulation at 9 ng/ml. When IL-3 (at a plateau dose for megakaryocyte colony induction) was added to rGM-CSF over a 0-22-ng/ml dose range, the resultant megakaryocyte colony stimulation approximated the sum of the levels of stimulation produced by either factor alone. These results establish GM-CSF as a multilineage growth factor with definite megakaryocyte colony-stimulating activity and indicate that both GM-CSF and IL-3 are important in the regulation of megakaryocytopoiesis.

Regulation of macrophage alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein by lipopolysaccharide and interferon-gamma.

alpha 2-Macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2M-R/LRP) is a broad specificity receptor that may function in lipoprotein metabolism, proteinase regulation, and growth factor regulation. In this study, we demonstrated that alpha 2M-R/LRP expression in macrophages can be markedly decreased by LPS and by IFN-gamma. Regulation of alpha 2M-R/LRP in RAW 264.7 cells was demonstrated at the mRNA, antigen, and receptor-function levels. In receptor-function studies, the decrease in alpha 2M-R/LRP expression was detected as a 90% decrease in the Bmax or maximum receptor binding capacity for activated alpha 2M after treatment with LPS or IFN-gamma. Western blot analysis of whole cell lysates demonstrated significant loss of alpha 2M-R/LRP heavy-chain. Northern blot analysis of poly(A)+ RNA revealed a marked decrease in alpha 2M-R/LRP mRNA after treatment with LPS (79% decrease) or IFN-gamma (70% decrease). Other cytokines, including tumor necrosis factor-alpha, transforming growth factor-beta-1, and interleukin-6 did not regulate alpha 2M-R/LRP. The ability of LPS and IFN-gamma to regulate alpha 2M-R/LRP was confirmed in experiments with primary cultures of murine bone marrow macrophages. These studies demonstrate that macrophage alpha 2M-R/LRP is subject to significant downregulation by physiologically significant cytokines and signaling macromolecules.

The stem cell continuum: a new model of stem cell regulation.

Most models of hematopoiesis have been hierarchical in nature. This is based on a large volume of correlative data. Recent work has indicated that, at least at the stem/progenitor level, hematopoiesis may, in fact, be a continuum of transcriptional opportunity. The most primitive hematopoietic stem cells are either continually cycling at a slow rate or entering and exiting cell cycle. Associated with this cycle passage are changes in functional phenotype including reversible alterations in engraftment, adhesion protein expression, cytokine receptor expression, homing to marrow, and progenitor cell numbers. Global gene expression, as measured in one point in cycle, is also markedly altered. The differentiation potential of the marrow as it transits cell cycle in response to a set differentiation stimulus also shows marked variations. This cycle-related plasticity has been clearly established for hematopoiesis. It also holds for the ability of murine marrow stem cells to home to lung and to convert to pulmonary cells. These data indicate that bone marrow stem cells can probably not be defined as discrete entities but must rather be studied on a population basis. They also indicate that mathematical modeling will become progressively more important in this field.

TrkA expression decreases the in vivo aggressiveness of C6 glioma cells.

We stably expressed the nerve growth factor receptor trkA or a truncated trkA lacking the kinase domain (trkA delta) in a highly tumorigenic rat glioma cell line, C6. Survival of rats with large intrastriatal inocula of C6trkA cells was significantly longer than for rats bearing C6 or C6trkA delta cells. Histological studies revealed that C6trkA cells were much less invasive than C6 or C6trkA delta cells and had a greater rate of apoptosis. There was no apparent induction of differentiation of C6 cells by trkA. Therefore, unlike what is observed in neuroblastomas, trkA decreases tumorigenicity by modulating invasiveness and tumor cell death independent of inducing differentiation. This novel mechanism suggests a new therapeutic strategy for malignant gliomas.

Expression of the LMP1 oncoprotein in the EBV negative Hodgkin's disease cell line L-428 is associated with Reed-Sternberg cell morphology.

The latent membrane protein 1 (LMP1) oncogene is one of the major proteins synthesized by the Epstein-Barr virus (EBV) and is expressed in Reed-Sternberg (RS) cells of EBV-associated Hodgkin's disease (HD). We have studied the effect of this oncoprotein on the formation of RS cells in the EBV-negative HD cell lines L-428 and KM-H2 as well as on the formation of multinucleated cells in the mononuclear human embryonic kidney cell line 293. LMP1 prototype (B95-8) and its naturally occurring carboxy terminal 30 base pair deletion variant LMP1-del were transfected into the cell lines and cytocentrifuge preparations were analysed after 24, 48, 72, 144, 216, and 240 h. While no oncoprotein expression was seen in the KM-H2 cell lines, expression of LMP1 and LMP1-del was observed in the L-428 and 293 cell lines. In the HD cell line L-428 oncoprotein expression was infrequent but when observed was very strong and preferentially associated with multinucleated RS cell morphology (71% of LMP1 positive cells). This is in contrast with the untransfected or transfected but not expressing cells where intermediate mononuclear elements predominated over multinucleated RS cells (< 3%). Frequent oncoprotein expression was observed in the 293 cells and again was associated with multinuclearity. These LMP1 expressing 293 giant cells showed strong expression of ICAM-1(CD54), not detectable in the untransfected cells. In the LMP1-del transfectants giant cells with more than four nuclei were frequently observed. However, giant cells were much less frequent in 293 cells transfected with the amino terminal deletion variant of LMP1 or the lytic form of LMP1, known to induce low NF-kappa B activation compared to the LMP1 prototype. Therefore, LMP1 mediated NF-kappa B activation appears to be involved in polycaria formation. The strong association of LMP1 expression with multinuclearity in a genetically unstable condition -the L-428 and 293 cells show multiple chromosomal abnormalities-suggests that this oncoprotein including its naturally occurring carboxy terminal deletion variant promote the formation of multinuclear cells, in particular of RS cells in EBV-associated HD.

Latent membrane protein 1 associated signaling pathways are important in tumor cells of Epstein-Barr virus negative Hodgkin's disease.

The latent membrane protein 1 (LMP1) oncogene of Epstein-Barr virus (EBV) is selectively expressed in the Reed-Sternberg (RS) cells of EBV-associated Hodgkin's disease (HD). However, no differences in clinical presentation and course are found between EBV positive and EBV negative forms of HD suggesting a common pathogenetic mechanism. We have studied the LMP1 associated signaling pathways and their dominant negative inhibition in the myelomonocytic HD-MyZ and the B-lymphoid L-428 HD cell lines. In both EBV negative cell lines expression of LMP1 is associated with the formation of multinuclear RS cells. Dominant negative inhibition of NF-kappa B mediated signaling at the step of I kappa B-alpha phosphorylation results in increased cell death with only a few typical RS cells resistant to overexpression of the dominant negative inhibitor I kappa B-alpha-N delta 54. However, dominant negative inhibition of NF-kappa B mediated signaling at the early step of TRAF2 interaction results in the formation of multinuclear cells in both cell lines and, in addition, in clusters of small mononuclear cells in the HD-MyZ cell line. In HD-MyZ cells overexpression of the powerful JBD-inhibitor of the JNK signal transduction pathway is restricted to small cells and never observed in RS cells. These small cells undergo apoptosis as shown by the TUNEL technique. Apoptosis of small cells is still observed after co-transfection of JBD and LMP1 but in addition a few apoptotic HD-MyZ cells with large fused nuclear masses are identified suggesting that specific inhibition of JNK leads also to apoptosis of LMP1 induced RS cells. Thus, activation of the JNK signaling pathway is also important in the formation of Reed-Sternberg cells. Our findings are consistent with a model where all three LMP1 associated functions, i.e. NF-kappa B mediated transcription, TRAF2 dependent signaling, and c-Jun activation act as a common pathogenetic denominator of both EBV negative and EBV positive HD.

The adult hemopoietic stem cell plasticity debate: idols vs new paradigms.

The debate surrounding adult stem cell plasticity derives from a confusion regarding definitions of important terms and the identification of key questions. After defining plasticity, lineage, differentiation and transdifferentiation, the authors put forth a framework for future dialogue as well as their perspective on the stem cell plasticity debate.

Murine marrow cellularity and the concept of stem cell competition: geographic and quantitative determinants in stem cell biology.

In unperturbed mice, the marrow cell numbers correlate with the stem cell numbers. High levels of long-term marrow engraftment are obtained with infusion of high levels of marrow cells in untreated mice. To address the issue of stem cell competition vs 'opening space', knowledge of total murine marrow cellularity and distribution of stem and progenitor cells are necessary. We determined these parameters in different mouse strains. Total cellularity in BALB/c mice was 530+/-20 million cells; stable from 8 weeks to 1 year of age. C57BL/6J mice had 466+/-48 million marrow cells. Using these data, theoretical models of infused marrow (40 million cells) replacing or adding to host marrow give chimerism values of 7.5 and 7.0%, respectively; the observed 8-week engraftment of 40 million male BALB/c marrow cells into female hosts (72 mice) gave a value of 6.91+/-0.4%. This indicates that syngeneic engraftment is determined by stem cell competition. Our studies demonstrate that most marrow cells, progenitors and engraftable stem cells are in the spine. There was increased concentration of progenitors in the spine. Total marrow harvest for stem cell purification and other experimental purposes was both mouse and cost efficient with over a four-fold decrease in animal use and a financial saving.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) priming with successive concomitant low-dose Ara-C for elderly patients with secondary/refractory acute myeloid leukemia or advanced myelodysplastic syndrome.

Patients with advanced MDS and secondary AML respond poorly to chemotherapy. Granulocyte-macrophage colony-stimulating factor (GM-CSF) can stimulate proliferation of leukemic blasts and sensitize these cells to the cytotoxic effects of S-phase-specific drugs. This is the first report of safety and efficacy of GM-CSF prior to and during cytarabine in a low-dose, intermittent regimen for elderly patients with poor risk acute myelogenous leukemia or myelodysplastic syndrome. Twenty patients, age 68 to 86 years, each received 250 microg/m2 of GM-CSF (Sargramostatin; Immunex, Seattle, WA, USA) subcutaneously (s.c.) or intravenously (i.v.) for 3 days followed by GM-CSF at the same dose and cytarabine 100 mg/m2 i.v. for 3 days. GM-CSF and cytarabine were both administered for 3 days during weeks 2 and 3 followed by a 3-week rest period. Rates of CR and PR were 20% and 40%, respectively. These included clinically significant resolution of cytopenias and transfusion requirements. Many of the responding patients had been heavily pretreated prior to enrollment. One- and 2-year survival estimates are 44% and 19%, respectively. Myelosuppression was the most significant toxicity. Our findings suggest that this novel combination of GM-CSF with sequential and concomitant low-dose cytarabine can benefit patients with poor risk myeloid malignancies.

Tolerance induction by costimulator blockade in 100 cGy treated hosts with varying degrees of genetic disparity.

Long-term multilineage allochimerism can be obtained in H2-mismatched B6.SJL to BALB/c transplants with host irradiation of 100 cGy, donor spleen cell pre-exposure and costimulator blockade with anti-CD40 ligand (CD40L) antibody. We evaluated this allochimerism approach in murine marrow transplants with different degrees of major histocompatibility complexe (MHC) mismatching; these include: (1) H2-mismatched transplant H2Kk to H2Kb, (2) full haplo-identical transplant H2Kbd to H2Kbk, (3) a partial haplo-identical transplant H2Kd to H2Kbd and (4) an MHC class II mismatch. Levels of chimerism increased up to 12 weeks and then stayed relatively stable up to 1 year after transplant. At 18 weeks post-transplant, the H2-mismatched, haplo-identical, partial haplo-identical and class II-mismatch transplants evidenced 17.9+/-4.4, 40.7+/-0.9, 25.1+/-4.19 and 33.7+/-3.5% donor chimerism, respectively. Dropping the anti-CD40 antibody treatment and spleen cells or changing the schedule of antibody to one injection, in haplo-identical or full-mismatched transplants resulted in no donor-derived chimerism. On the other hand, these still resulted in minor chimerism in class II-mismatched transplants. Lineage analysis of peripheral blood at 6 and 12 months post-transplant demonstrated a significant shift toward increased chimeric lymphocytes and decreased chimeric granulocytes in the full H2 as compared with haplo-identical or class II transplants. Transplantation with anti-CD40L antibody eliminated both graft-versus-leukemia and graft-versus-host disease (GVHD) and delayed lymphocyte infusion did not rescue animals from fatal leukemia. In conclusion, under the conditions of our tolerization regimen, a haplo transplant gives higher engraftment levels than a full H2 mismatch, and despite lower engraftment levels, a class II-mismatched transplant can be successfully accomplished with only 100 cGy and no CD40L blockade.

Cyclic hematopoiesis: disorders of primitive hematopoietic stem cells.

Utilizing both in vivo and in vitro techniques, a great deal of information has been obtained on the structure and regulation of the hematopoietic cell lineages. A number of hematopoietic stem cells and regulators have now been well characterized and their possible physiologic relevance at least in part established. More recently, new "alternative" or primitive stem cells have been described which may provide important insights into the nature of hematopoietic regeneration and regulation. These include late CFUs, high proliferative potential colony-forming cells, colony-forming unit diffusion chamber and both bipotent and blast colonies assayed in in vitro culture systems. Regulators active at these stem cell levels are also under study and in large part appear to be produced by monocytes or lymphocytes. Cyclical hematopoiesis can be viewed as a genetic abnormality at the multipotent stem cell level with defective cell production. At present however, details of the defect await further investigation possibly including an evaluation of the role of primitive stem cells and their regulators.

Radioresistant murine marrow stromal cells: a morphologic and functional characterization.

Two types of murine marrow adherent cells derived from Dexter cultures have been characterized. Exposure of C57B1/6J, ICR, or BDF1 mice to 1000 R x-ray 24 h prior to killing and establishment of liquid marrow cultures resulted in the growth of two types of adherent cells. A macrophage-like cell was phagocytic, nonspecific esterase, and acid phosphatase, positive and alkaline phosphatase, myeloperoxidase, and factor-VIII negative. The second cell was large and epithelioid in appearance, had a subpopulation of giant fat cells, was nonphagocytic, alkaline phosphatase positive, and negative for acid phosphatase, nonspecific esterase, myeloperoxidase, and factor VIII. At low inoculum levels these cells formed three types of colonies within 1-3 weeks--macrophage, epithelioid, and mixed--while at higher inoculum levels they formed confluent monolayers. These radioresistant cells supported myeloid pluripotent stem cells (CFU-S) and granulocyte-macrophage stem cells (GM-CFU-C) in liquid culture of long term nonadherent marrow cells and stimulated GM-CFU-C in agar over-lays. Refeeding liquid cultures with nonadherent cells from long-term Dexter cultures revealed that myeloperoxidase-positive cells adhered predominantly to the colonies containing epithelioid cells.

A 17-kd polypeptide, sensitive to bacterial collagenase, is synthesized by bone marrow macrophages.

A novel polypeptide with a molecular weight of 17 kd (17k protein) was identified in bone marrow cell cultures. The synthesis of 17k protein is elevated in cell cultures maintained under Dexter conditions, which support myelopoiesis. The predominance of macrophages in the stromal layer of these cultures and the observation that a mouse myelomonocytic cell line P388D1 is capable of synthesizing large amounts of 17k protein led us to the study of its synthesis by bone marrow macrophages. Metabolic labeling with [14C]proline and partial amino acid analysis of 17k protein demonstrated that the polypeptide contains relatively high amounts of proline and is also sensitive to degradation with bacterial collagenase. However, no hydroxyproline is detectable in 17k protein, and it is extensively degraded with bacterial collagenase. However, no hydroxyproline is detectable in 17k protein, and it is extensively degraded by proteolysis with pepsin, using conditions under which collagen triple helices are resistant to degradation, suggesting that collagen-like structures are not contained in 17k protein. This polypeptide is found predominantly in the cellular layers of bone marrow macrophage cultures. Incorporation of [14C]proline into 17k protein is diminished by increasing concentrations of colony-stimulating factor 1 (CSF-1). The 17k protein may be involved in macrophage proliferation because its synthesis is inhibited by CSF-1, which is required for the maintenance of bone marrow macrophages in vitro.

Effect of lithium on stem cell and stromal cell proliferation in vitro.

Lithium is recognized as a potent stimulator of hematopoiesis both in vivo and in vitro. Previous work has suggested that this stimulation is mediated as an indirect, humoral effect by the action of lithium upon the stromal cell population. In the present study, the effects of lithium on the stromal population were investigated using a long-term liquid marrow culture model. These findings indicate that exposure of in vitro cultures to lithium results in an increase in the total cellularity and in the number of various hematopoietic progenitor cells residing within the stromal layer. A distinct morphologically recognizable cell has not been identified as the target cell responsible for the indirect stimulation of hematopoiesis by lithium. However, two candidate radioresistant stromal cells believed to be active in the production of humoral mediators of hematopoiesis did proliferate in response to lithium exposure.

Granulocyte colony-stimulating factor augments in vitro megakaryocyte colony formation by interleukin-3.

Granulocyte colony-stimulating factor (G-CSF) has been purified to homogeneity and the cDNA isolated. The reported properties of G-CSF have suggested that it is specific for the granulocytic lineage and only forms pure granulocyte colonies in in vitro cultures of murine bone marrow. We have demonstrated in this report that G-CSF augments the effect of interleukin 3 (IL3) on megakaryocyte formation. G-CSF alone had no stimulatory effect on megakaryocyte colony formation, however, the addition of G-CSF to IL3 in cultures of normal murine bone marrow increased the number of megakaryocyte colonies to 176% compared to cultures containing IL3 alone. Also, the combination of G-CSF plus IL3 stimulated the formation of larger megakaryocyte colonies than those formed in cultures of IL3 alone. In contrast, G-CSF had no effect on the number or size of megakaryocyte colonies stimulated by granulocyte-macrophage CSF. These results demonstrate that G-CSF augments the megakaryocyte colony formation of IL3, but not GM-CSF, and expands the lineage potential of G-CSF.

Relationship between cells forming colonies in diffusion chambers in vivo (CFU-D) and cells with high proliferative potential in vitro (HPP-CFC-1 and -2).

In vivo diffusion chambers implanted in normal mice after 5 days of bone marrow cell culture contained precursor cells that in the presence of recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF), interleukin 3 (IL-3), or colony-stimulating factor 1 (CSF-1), alone or in combination, formed both small and large (high proliferative potential colony-forming cells, HPP-CFC) macrophage-containing colonies in vitro. Synergistic factor from serum-free 5637 cell-conditioned medium (SF5637) enhanced HPP-CFC colony growth only in cultures containing CSF-1. Higher numbers of CSF-1- plus IL-3-responsive colony-forming cells (HPP-CFC-2) were detected in diffusion chamber colony-forming unit (CFU-D) colonies than in intercolony areas, suggesting that they were derived from cells that give rise to the diffusion chamber colony. Further study demonstrated that CFU-D colonies contained cells that formed large macrophage-containing colonies (HPP-CFC-1) in CSF-1- plus SF5637-containing cultures. These findings suggest that single cells (CFU-D) forming colonies in diffusion chambers in mice can give rise to both HPP-CFC-1 and to cells probably representing their progeny, HPP-CFC-2.

Characterization of engraftable hematopoietic stem cells in murine long-term bone marrow cultures.

OBJECTIVE: Long-term bone marrow cultures (LTBMC) are a potential source of hematopoietic stem cells (HSC) for transplantation. Previous reports indicate that feeding LTBMCs induces hematopoietic progenitor cycling, and other studies link HSC cycle phase with engraftability. Our study was initiated to further characterize LTBMC engraftability and determine if a cycle phase-related engraftment defect affects HSC from LTBMCs. MATERIALS AND METHODS: Competitive repopulation of lethally irradiated BALB/c females was used to examine engraftability of LTBMCs under "fed" or "unfed" conditions at 3 to 5 weeks culture. Tritiated thymidine suicide was used to determine the cycle status of HPP-CFC and CFU-S from LTBMCs. RESULTS: Total cell number in LTBMCs decreases from input. Quantitatively, both fed and unfed 3-, 4-, or 5-week cultures compete strongly with fresh marrow for 2 and 8 weeks, but not 6 months, after transplantation. Short-term engraftable HSCs expand between 3 and 5 weeks of culture. Clonal assays indicate no peak in S-phase of CFU-S at 24 and 48 hours after feeding, and fluctuation in both content and cycle status of HPP-CFC after feeding. CONCLUSIONS: Our LTBMCs engraft in all conditions, and the level of engraftment capability does not correlate with cell-cycle phase of CFU-S or HPP-CFC, or with time from feeding. Although the total cell number decreases from input, the proportion of short- and intermediate-term engrafting HSC in whole LTBMCs approximates that of fresh marrow and expands from 3 to 5 weeks in culture, whereas long-term engraftable HSCs are decreased in culture.

Effect of adenine nucleotides on granulopoiesis and lithium-induced granulocytosis in long-term bone marrow cultures.

Studies were undertaken to evaluate the role of adenine nucleotides in regulating hematopoiesis using a long-term liquid culture system. In contrast to early investigations using clonogenic stem cell assays, where inhibitory effects were observed, adenosine and adenosine-5'-monophosphate (AMP) were found to stimulate myelopoiesis whereas the dibutyryl derivative of cyclic adenosine-3',5'-monophosphate (dcAMP) had either a modest inhibitory effect or no effect on long-term hematopoiesis. Dose effects for AMP enhancement of hematopoiesis were relatively narrow. When cultures were exposed to a broad range of concentrations (10 mM-10 nM), stimulation was only seen at a molar concentration of 1 X 10(-4) M. Stem cell assays revealed stimulation of multipotent stem cells (CFU-S), as well as committed progenitor cells (CFU-C). Lithium chloride has been shown to cause granulocytosis both in vivo and in vitro. Reductions in intracellular cAMP levels resulting from adenylate cyclase inhibition is a proposed mechanism for this stimulatory effect. However, lithium-induced granulocytosis in long-term cultures could not be blocked by the addition of dcAMP. Measurement of nucleotide levels on spent medium revealed rapid utilization and/or degradation of these reagents. This suggests that failure to abrogate the lithium effect with dcAMP may have been related to the inability to maintain constant intracellular concentrations. The varied observations regarding adenine nucleotide effects on hematopoiesis, as well as the reproducible stimulation by lithium, may be explained by our current appreciation of the complex adenylate cyclase system, which contains both inhibitory and stimulatory subunits for nucleotides and monovalent cations.