Cell interactions in the bone marrow microenvironment: role of endogenous colony-stimulating activity

Stress as a fundamental theme in cell plasticity

Over a decade of intensive investigation of the possible plasticity of mammalian cells has eventually substantiated that mammalian species are endowed with a remarkable capacity to change mature cell fates. We review below the evidence for the occurrence of processes such as dedifferentiation and transdifferentiation within mammalian tissues in vivo, and in cells removed from their protective microenvironment and seeded in culture under conditions poorly resembling their physiological state in situ. Overall, these studies point to one major conclusion: stressful conditions, whether due to in vivo tissue damage or otherwise to isolation of cells from their in vivo restrictive niches, lead to extreme fate changes. Some examples of dedifferentiation are discussed in detail showing that rare cells within the population tend to turn back into less mature ones due to severe cell damage. It is proposed that cell stress, mechanistically sensed by isolation from neighboring cells, leads to dedifferentiation, in an attempt to build a new stem cell reservoir for subsequent regeneration of the damaged tissue. This article is part of a Special Issue entitled: Stress as a fundamental theme in cell plasticity.

The hemopoietic stem cell niche versus the microenvironment of the multiple myeloma-tumor initiating cell

Multiple myeloma cells are reminiscent of hemopoietic stem cells in their strict dependence upon the bone marrow microenvironment. However, from all other points of view, multiple myeloma cells differ markedly from stem cells. The cells possess a mature phenotype and secrete antibodies, and have thus made the whole journey to maturity, while maintaining a tumor phenotype. Not much credence was given to the possibility that the bulk of plasma-like multiple myeloma tumor cells is generated from tumor-initiating cells. Although interleukin-6 is a major contributor to the formation of the tumor’s microenvironment in multiple myeloma, it is not a major factor within hemopoietic stem cell niches. The bone marrow niche for myeloma cells includes the activity of inflammatory cytokines released through osteoclastogenesis. These permit maintenance of myeloma cells within the bone marrow. In contrast, osteoclastogenesis constitutes a signal that drives hemopoietic stem cells away from their bone marrow niches. The properties of the bone marrow microenvironment, which supports myeloma cell maintenance and proliferation, is therefore markedly different from the characteristics of the hemopoietic stem cell niche. Thus, multiple myeloma presents an example of a hemopoietic tumor microenvironment that does not resemble the corresponding stem cell renewal niche.

The myelopoietic supportive capacity of mesenchymal stromal cells is uncoupled from multipotency and is influenced by lineage determination and interference with glycosylation

Cultured bone marrow stromal cells create an in vitro milieu supportive of long-term hemopoiesis and serve as a source for multipotent mesenchymal progenitor cells defined by their ability to differentiate into a variety of mesodermal derivatives. This study aims to examine whether the capacity to support myelopoiesis is coupled with the multipotency. Our results show that the myelopoietic supportive ability of stromal cells, whether from the bone marrow or from embryo origin, is not linked with multipotency; cell populations that possess multipotent capacity may or may not support myelopoiesis, whereas others, lacking multipotency, may possess full myelopoietic supportive ability. However, upon differentiation, the ability of multipotent mesenchymal progenitors to support myelopoiesis is varied. Osteogenic differentiation did not affect myelopoietic supportive capacity, whereas adipogenesis resulted in reduced ability to support the maintenance of myeloid progenitor cells. These differences were accompanied by a divergence in glycosylation patterns, as measured by binding to lectin microarrays; osteogenic differentiation was associated with an increased level of antennarity of N-linked glycans, whereas adipogenic differentiation caused a decrease in antennarity. Inhibition of glycosylation prior to seeding the stroma with bone marrow cells resulted in reduced capacity of the stromal cells to support the formation of cobblestone areas. Our data show that myelopoietic support is unrelated to the multipotent phenotype of cultured mesenchymal progenitors but is dependent on the choice of differentiation pathway and upon correct glycosylation of the stromal cells.

The Stem State: Plasticity Is Essential, Whereas Self-Renewal and Hierarchy Are Optional

The prevailing stem cell concept is derived from the large body of evidence available on the structure of the blood-generating system. Hemopoiesis is organized such that a multipotent stem cell, endowed with self-renewal capacity, is viewed as being positioned at the origin of a hierarchical tree of branching specificities, increasing maturity and decreasing self-renewal ability. Data accumulated in recent years on various stem cell systems often contradict this traditional view of stem cells and are reviewed herein. It is suggested that other options should be considered and put to experimental scrutiny; it is argued that the organization of the hemopoietic system may not represent a general structure of stem cell systems. The basic trait of the stem state is proposed to be plasticity. Self-renewal is not a specific stem cell trait; rather, it is exhibited by some mature cell types, whereas other particular stem cells are endowed with relatively poor renewal ability. Hierarchical structuring is also proposed to be an optional stem cell trait and may exist only in specific tissues where it serves the need for rapid expansion. The stem state is therefore defined by the highest degree of plasticity of a cell, within the repertoire of cell types present in the organism.

Environmental guidance of normal and tumor cell plasticity: epithelial mesenchymal transitions as a paradigm

Epithelial mesenchymal transitions are a remarkable example of cellular plasticity. These transitions are the hallmark of embryo development, are pivotal in cancer progression, and seem to occur infrequently in adult organisms. The reduced incidence of transitions in the adult could result from restrictive functions of the microenvironment that stabilizes adult cell phenotypes and prevents plastic behavior. Multipotential progenitor cells exhibiting a mesenchymal phenotype have been derived from various adult tissues. The ability of these cells to differentiate into all germ layer cell types, raises the question as to whether mesenchymal epithelial transitions occur in the adult organism more frequently than presently appreciated. A series of cytokines are known to promote the transitions between epithelium and mesenchyme. Moreover, several transcription factors and other intracellular regulator molecules have been conclusively shown to mediate these transitions. However, the exact molecular basis of these transitions is yet to be resolved. The identification of the restrictive mechanisms that prevent cellular transitions in adult organisms, which seem to be unleashed in cancerous tissues, may lead to the development of tools for therapeutic tissue repair and effective tumor suppression.

Astrocytes play a major role in the control of neuronal proliferation in vitro

The elements that control neuronal proliferation are largely unknown. Proliferating neurons in cultures of goldfish brain were studied in an attempt to identify the cell types involved. Neuronal proliferation was found to occur only when the neuronal stem cells were in direct contact with astrocytes, and never directly on the substrate. The regulation of neuronal proliferation thus appears to be mediated, at least in part, by contact with astrocytes. In addition, neurite extension was inhibited by medium conditioned by fish astrocytes. Since neurite extension and neuronal proliferation are mutually exclusive processes, inhibition of neurite extension by soluble substances derived from the astrocytes is probably one of the mechanisms controlling neuronal proliferation. The complex reciprocal relationship between neurons and astrocytes is also demonstrated by an observed inhibition of astrocytic proliferation by medium conditioned by differentiating fish neurons. This inhibition of astrocytic proliferation might be part of a mechanism through which interference with neuronal differentiation by astrocytes is avoided. The results of this study thus suggest that astrocytes, in addition to their known roles in controlling neuronal migration, neuronal differentiation and neurite elongation, may also play a role in the control of neuronal proliferation.

Dynamic changes in cytokine secretion by stromal cells during prolonged maintenance under protein-free conditions

Stromal cells of bone marrow origin produce a variety of known cytokines and some factors exhibiting apparently new biological activities. Several of these were identified by the study of cell to cell interactions and were not found in detectable amounts in media conditioned by the cells. We describe here a culture system that enables the release of stromal cytokines into medium free of any added proteins and supplemented with peptides from casein hydrolysate (0.1%). The absence of serum proteins allows extensive concentration and monitoring of activities that are otherwise undetectable. Stromal cells of the MBA-2.1 clonal cell line were seeded in a stationary bed reactor packed with a carrier of non-woven fabric matrix. After a proliferation phase with serum containing medium, the cells were maintained for over 10 months in protein-free medium. Throughout this extended incubation in the absence of serum or serum replacing proteins, stromal cells retained their viability and continuously released transforming growth factor-beta (TGF-beta), macrophage-colony stimulating factor (M-CSF) and restrictin-P, a cytotoxic factor that specifically arrested the growth of plasmacytoma cells. In addition, interleukin-6 (IL-6) was first undetectable, and later in culture its titer reached a maximum of 180,000 international units (IU)/ml. Concomitantly, the production of restrictin-P diminished and reached its lowest levels at the end of 10 months. The results may imply a possible causal relationship between the expression of IL-6 and restrictin-P, since no similarly significant changes were observed in the titers of M-CSF and TGF-beta. This novel bioreactor system may be adaptable for efficient production of different cytokines under absolute serum-free conditions.

Diffusion chamber colony-forming unit (CFU-d): a primitive stem cell

Assay of hematopoietic precursor cells in diffusion chambers (DCs) implanted intraperitoneally in experimental animals provides a powerful tool for studying stem cell kinetics in vivo. In this system, the effect of cell migration (which complicates whole animal studies) is eliminated because the membranes utilized in the construction of the chambers are impermeable for cells, while permitting free passage of molecules present in the humoral phase of the host. As judged by light microscopy, conditions in the DC cultures primarily favor macrophage and granulocyte growth. However, the use of in vitro and in vivo subculture to further analyze chamber contents has demonstrated that the system supports proliferation of early hematopoietic progenitors. Additionally, cells capable of rescuing lethally irradiated mice proliferate in DC cultures. Development of the plasma clot DC technique has revealed that most of the growth occurs in colonies which are derived from single cells (CFU-d). Characterization of these cells indicates that they are at least as primitive as other colony-forming cells and, also based on subculture studies, can differentiate along several hematopoietic lineages. In addition to normal CFU-d, both embryonal and leukemic cells can give rise to granulocytes, macrophages, megakaryocytes and erythroid cells in the DC cultures. Evaluation of the effects of humoral factors on hematopoietic cell proliferation and differentiation in the system has led to the identification of both stimulators and inhibitors that may be different from the well-characterized cytokines. Thus, the system seems to be useful for detecting molecules controlling the most primitive stages of hematopoiesis. We believe that the DC culture technique holds enormous potential in the study of stem cell proliferation and differentiation in vivo.

Stromal cells of hemopoietic origin

Hemopoiesis is a multistep process involving stem cell renewal, commitment, differentiation, maturation and consequent positioning of the cells within the tissue. Stromal cells are a major component of the hemopoietic microenvironment. The in vitro culture of cloned stromal cells has enabled detailed analysis of their functions and has provided answers relating to the contribution of stromal cells to the control of hemopoiesis. Cultured stromal cells were found to support the renewal of stem cells through a mechanism that did not seem to involve already known cytokines. Cloned stromal cells from both marrow and thymus supported the in vitro accumulation of myeloid as well as T and B lymphoid cells. Thus, cloned stromal cells had the ability to induce multilineage hemopoiesis, irrespective of the organ from which they were derived. Invariably, stromal cells tended to select in culture for hemopoietic cells at early differentiation stages and restricted the accumulation of mature cells. These functions may be part of the mechanism that protects the stem cell pool from excess differentiation.

Response of simian virus 40 (SV40)-transformed, cultured human marrow stromal cells to hematopoietic growth factors

The response of marrow stromal cells transformed with wild-type simian virus 40 to recombinant growth factors was examined. When transformed stromal cells were plated in semisolid medium without the addition of growth factors, only 0.4% of cells formed colonies while with the addition of recombinant factors such as interleukin 1 (IL-1) or tumor necrosis factor (TNF), up to 10% of the cells formed colonies. Colonies were individually plucked and cell lines were developed that could be analyzed for expression of growth factors. The data show that unstimulated marrow stromal cells lines produced no detectable colony-stimulating activity. However, cell lines derived from "autonomously growing colonies" and from colonies grown with T cell-conditioned medium, with IL-1 alpha or beta, or with TNF alpha produced colony-stimulating activity and transcripts for granulocyte/macrophage-colony-stimulating factor (CSF), granulocyte-CSF, and IL-1 beta. A novel feature of the cell lines derived from colonies was that the production of growth factors was constitutive and persisted in excess of 4 m.

Myelopoietic effect of bone marrow fibroblasts cultured from patients with myelofibrosis

Bone marrow fibroblasts (BMF) have been shown to be able to support granulopoiesis. The present studies were designed to see whether BMF in patients with agnogenic myeloid metaplasia (AMM) are also able to support granulopoiesis. Myeloid colony-stimulating activity (CSA) was assayed in BMF derived from the three groups of patients. 1) Group I: Patients with hip fracture but without underlying hematological disease; patients with solid tumor but without bone marrow metastasis; and patients with iron deficiency anemia. 2) Group II: Patients with myeloproliferative disorders other than AMM. 3) Group III: Patients with AMM or myelofibrosis with prior history of polycythemia vera. CSA was determined in a bilayer agar culture system in which BMF served as a feeder layer and either mouse or human marrow cells were employed as target cells. There was no difference of CSA production by BMF among these three patient groups. These studies suggest that BMF cultured from patients with myelofibrosis exhibit similar myeloid stimulating activity as do BMF from other patients. A significant correlation was also found between CSA production by BMF and white blood cell counts in patients with myelofibrosis (group III). This suggests that BMF may have a role in either supporting or producing CSA for granulopoiesis in myelofibrosis.

Stromal cell effects on clonal growth of tumors

Clonal growth of tumor cell lines originating from a variety of solid tumors was studied. The seeding efficiency of these tumors in methylcellulose medium was in the range of 0.036 to 0.177. Stromal cell lines from mouse bone marrow as well as primary stromal cells from human bone marrow stimulated the growth of HCT and oat human carcinoma cells 32-fold and 25-fold, respectively. In contrast, these stromal cells inhibited the in vitro cloning of human and mouse sarcoma cell lines. Both activities of the stromal cells diffused through agar layers and operated across species barriers. Despite the diffusable nature of the factors involved, no biologic activity was observed in concentrated conditioned media prepared in the presence or absence of serum. Human foreskin fibroblasts tested under identical conditions, could neither stimulate nor inhibit the clonal growth of tumors. This preferential growth of tumor cells in the presence of tissue specific stroma may be used as an in vitro model for the study of the role of stromal cells in tumor cell spread.

Suppression of bone marrow stromal cell function by benzene and hydroquinone is ameliorated by indomethacin

Administration of benzene to mice will inhibit bone marrow stromal cell-supported hemopoiesis in culture. Hydroquinone, a major metabolite of benzene, will cause a similar inhibition of stromal cell function in vitro. Stromal cells produce both an inducer (colony-stimulating factor) and an inhibitor (prostaglandin E2; PGE2) of hemopoiesis. This research was conducted to determine if prostaglandin synthesis is involved in the suppression of stromal cell function by benzene and hydroquinone. Male B6C3F1 mice were administered benzene (100 mg/kg), indomethacin (1 mg/kg), or benzene plus indomethacin twice a day for 4 consecutive days. On Day 5 bone marrow cells were removed to determine the effect of treatment. In a second series of experiments mouse bone marrow stromal cells in culture were treated with hydroquinone (10(-7) to 10(-4) M), indomethacin (10(-6) M), or a combination of hydroquinone plus indomethacin. Stromal cell function was based on the ability of the treated stromal cells to support granulocyte/monocyte colony development in coculture. The results demonstrated that preadministration of indomethacin in vivo ameliorated benzene-induced inhibition of bone marrow stromal cell function. In vitro, indomethacin ameliorated hydroquinone toxicity to stromal cell function. Benzene administration in vivo induced elevated PGE2 in bone marrow samples which were prevented by preadministration of indomethacin. However, hydroquinone in vitro did not induce a consistent increase in PGE2 levels. These results suggested that toxicity to stromal cells was not due solely to increased prostaglandin synthetase activity.

Differentiation stage and lineage-specific inhibitor from the stroma of mouse bone marrow that restricts lymphoma cell growth

We have examined the mechanism by which stromal cells from the microenvironment of the bone marrow restricted the in vitro growth of certain hemopoietic tumors. A series of leukemia cell lines was used to monitor biological activities of stromal cell lines representing five distinct subtypes. Only an endothelial-like clone derived from mouse stroma (MBA-2.1) was consistently found to produce a cell-surface-associated glycoprotein that selectively inhibited the growth of plasmacytomas. The factor, designated leukemia cell inhibitory activity (LCIA), was not detected in anchorage-dependent cells of nonhemopoietic origin. Tumors of the lymphoid lineage and plasmacytomas in particular were the most sensitive to LCIA. Myeloid, macrophage, and erythroleukemia tumors were resistant to the factor, as were normal hemopoietic target cells including pluripotent stem cells, myeloid progenitor cells, and mitogen-stimulated spleen cells. Fractionation of trypsin-released proteins from MBA-2.1 cells by gel filtration and affinity binding to concanavalin A-Sepharose revealed two types of inhibitors; one was the specific leukemia cell inhibitor (i.e., LCIA); the other, present at a lower titer, was non-target-cell specific. The high sensitivity of plasmacytomas to LCIA versus the resistance of normal stem cells may be utilized for selective elimination of plasma cell tumors from bone marrow inocula.

Pre-CFU-f: young-type stromal stem cells in murine bone marrow following administration of DNA inhibitors

Occurrence of young-type stromal stem cells (defined here as "pre-CFU-f") in murine bone marrow is reported in this study. Two consecutive intraperitoneal (i.p.) cytosine arabinoside (ara-C) injections were administered to C57B1 mice (2 X 200 mg/kg at 6-h intervals). Two days later the bone marrow was collected and assayed for colony-forming units-fibroblastoid (defined here as "CFU-f"). In additional experiments, ara-C-treated marrow was exposed in vitro to hydroxyurea (HU; "hydroxyurea killing test"), prior to plating, to establish the cycling state of stromal stem cells. In separate cultures of ara-C-treated marrow, replating of adherent cells was carried out up to quaternary sub-cultures. The results indicate ara-C-treated marrow produces approximately 20% "huge" fibroblastoid colonies (approximately 5 mm diameter versus 0.5-2 mm normal size); most stromal stem cells producing huge colonies are cycling cells; and adherent cells from primary ara-C-treated marrow cultures replated to secondary cultures produce adherent layers with double the number of cells than in the control secondary cultures. We conclude that the ara-C-treated murine bone marrow contains certain young-type cycling stromal stem cells which we refer to as pre-CFU-f. These stem cells produce huge fibroblastoid colonies in culture, indicating that they probably go through more cell cycles than CFU-f during the culture period. Alternatively, pre-CFU-f may have a higher self-replicative capacity than CFU-f.

Long-term culture of infant leukemia cells: dependence upon stromal cells from the bone marrow and bilineage differentiation

Infant leukemia cells with 46XY,t(11; 17)(q23; p13) karyotype and a hybrid pre B myeloid phenotype (HLA-DR, (Ia), B4 and My7-positive and CALLA and T11-negative) and immunoglobulin heavy chain gene rearrangement were maintained in long-term culture for over 10 months. The in-vitro survival and growth of the leukemia cells were strictly dependent upon the presence of their autologous marrow stromal cells. The latter could be replaced by the 14F1.1 clone of preadipocytes derived from mouse bone marrow. Neither heterologous human marrow or foreskin fibroblasts nor fibroblast or endothelial like cell lines from mouse stroma could mimic the effect of autologous stroma or 14F1.1 adipocytes. The leukemia cells maintained their original phenotype throughout the 10-month culture period with either their autologous stroma or the 14F1.1 adipocytes. They could be induced to differentiate in two distinct directions. Phorbol myristate acetate induced adherence of the leukemia cells and development of macrophage properties. In contrast, conditioned medium from a hybridoma producing B-cell growth factor caused aggregation of the leukemia cells and expression of CALLA antigen and surface IgM. This bipotency of the leukemia cells and their dependence upon marrow stroma are properties in common with stem cells.

The stem cell concept revisited: self-renewal capacity is a dynamic property of hemopoietic cells

A rigid developmental program of stem cell division and progressive maturation into blood cells is challenged. It is proposed that the capacity for self-renewal is not limited to pluripotent stem cells but is shared by committed progenitors and even cells of later compartments. The relative probability of self-replication vs maturation in mitotic cells is controlled by extracellular influences. At the cell population level, the balance between proliferation and maturation and between compartments is regulated by feedback interactions. Inducibility of maturation in response to regulatory signals is smaller at earlier stages; consequently, at steady state primitive cells self-renew while their more differentiated progeny are forced to be transitory. The proposed dynamic linkage between compartments can be destabilized in a number of ways, resulting in defective hemopoiesis or leukemia. At all stages hemopoietic cells are able to change their patterns of gene expression, in an inheritable manner, in response to changes in their microenvironment. In particular, the capacity for self-renewal itself can vary even within a conventionally-defined compartment. On this basis of adaptive differentiation and self-renewal it is possible to account for the progression of chronic myelocytic leukemia and its "blastic conversion"; to analyse the hemopoietic system's response to various physiological and experimental perturbations; and to re-interpret the excessive phenotypic plasticity and apparent "lineage infidelity" manifested by leukemic cells and cell lines.

Cultured mouse marrow stromal cell lines. II. Distinct subtypes differing in morphology, collagen types, myelopoietic factors, and leukemic cell growth modulating activities

A series of stromal cell lines were studied for their growth properties, electron microscopic morphology, cytochemical profile, collagen types, production of myelopoietic factors, and modulation of leukemic cell growth. Three cell types were identified in addition to the previously described macrophages (14M and 14M1) and preadipocytes (14F). MBA-1 cells were found to be fibroblasts by their ability to synthesize collagen types I and III, while the cell line MBA-13 shared properties in common with both fibroblasts and endothelial cells (collagen types I, III, IV, V). The third cell type, represented by the stromal cell line MBA-2, produced mainly collagen types IV and V and exhibited junctional complexes between adjacent cells. All of the cell lines tested produced and secreted a macrophage-colony-stimulating factor, CSF-1. MBA-2 and to a lesser extent, MBA-13, produced an additional activity resistant to anti-CSF-1 antiserum. Trypsin extraction of outer surface components from two clones of the MBA-2 cell line (MBA-2.1 and MBA-2.4) yielded high molecular weight factor(s) that specifically inhibited the growth of a plasmacytoma cell line (MPC-11). Such inhibitory activity was not detected in other stromal cell lines. It is possible that this variability in the nature of stromal cell lines represents corresponding diversity of cell types comprising the hematopoietic microenvironment in vivo.

In vitro effects of benzene metabolites on mouse bone marrow stromal cells

Benzene exposure can result in bone marrow myelotoxicity. We examined the effects of benzene metabolites on bone marrow stromal cells of the hemopoietic microenvironment. Male B6C3F1 mouse bone marrow adherent stromal cells were plated at 4 X 10(6) cells per 2 ml of DMEM medium in 35-mm tissue culture dishes. The growing stromal cell cultures were exposed to log 2 doses of five benzene metabolites: hydroquinone, benzoquinone, phenol, catechol, or benzenetriol for 7 days. The dose which caused a 50% decrease in colony formation (TD50) was 2.5 X 10(-6) M for hydroquinone, 17.8 X 10(-6) M for benzoquinone, 60 X 10(-6) M for benzenetriol, 125 X 10(-6) M for catechol, and 190 X 10(-6) M for phenol. We next examined the effect of benzene metabolites on the ability of stromal cells to influence granulocyte/monocyte colony growth (G/M-CFU-C) in a coculture system. Adherent stromal cells were plated and incubated for 14 days and then exposed to a benzene metabolite. After 3 days the medium and metabolite were removed and an agar:RPMI layer containing 10(6) fresh bone marrow cells was placed over the stromal layer. After incubation for 7 days the cultures were scored for G/M colony formation. Hydroquinone and benzoquinone were most toxic, while catechol and benzenetriol inhibited colony growth only at high doses. These results indicate that injured bone marrow stromal cells may be a significant factor in benzene-induced hemotoxicity.

Cultured mouse marrow cell lines: interactions between fibroblastoid cells and monocytes

Continuous cell lines were derived from primary cultures of adherent bone marrow cells from SJL/J, BALB/c, C3H/eb, RF, and nude-ICR mice. All these lines readily assumed a pure fibroblastoid appearance with the exception of the BALB/c line (MBA-14), which retained both fibroblastoid and monocytoid cells. This particular line could promote the proliferation of myeloid progenitors (CFU-C) in short-term bone-marrow cultures. The two cell types that composed the MBA-14 cell line were successfully isolated and grown separately; the monocytes as the 14M and 14M1 cell lines and the fibroblastoid cells as the 14F clones. The latter were found to be preadipocytes and accumulated fat in the absence of added hydrocortisone, in medium supplemented with fetal calf serum. Growth of the monocyte lines (14M and 14M1) was dependent upon the mononuclear phagocyte stimulator CSF-1. In the parent MBA-14 cell line the growth of monocytes seemed to depend upon stimulating factor(s) produced by the fibroblastoid cells. The 14M1 monocytes were able to process and degrade antigen as efficiently as primary macrophages. Furthermore, processed antigen produced by 14M1 cells evoked proliferative response by antigen-primed lymph-node cells. In addition to these immunological functions the 14M1 cells were capable of modulating the colony-stimulating activity and degree of adipogenesis exhibited by the fibroblastoid cells. These interactions between monocytes and fibroblastoid cells may constitute part of the mechanism controlling the activity of the hematopoietic microenvironment.

Myelopoiesis following phorbol ester exposure in human long-term bone marrow cell culture

Phorbol esters have two opposing effects on bone marrow granulocyte/macrophage colony forming cells (CFC-GM): they reduce the number of CFC-GM which respond to colony-stimulating activity (CSA), and they also induce the release of CSA from accessory marrow cells. We questioned whether the direct inhibitory effect of phorbol esters on CFC-GM was reversible and whether phorbol ester-treated accessory bone marrow cells could increase the in vitro recovery of cultured CFC-GM. Normal human bone marrow cells were exposed to phorbol-12,13-dibutyrate (PDB) for 5 days in liquid cultures. Total nucleated cell counts decreased in a dose-dependent fashion to 42 +/- 6% of control at 5 X 10(-8) M PDB, and CFC-GM decreased to 20 +/- 8% of control. The cultures were then washed to remove PDB and continued for up to 8 weeks. Total cell counts and CFC-GM/ml remained reduced in PDB-pretreated cultures compared to control. To determine whether this was a direct effect of PDB on nonadherent, proliferating cells or whether PDB acted indirectly by affecting adherent, accessory cells, nonadherent bone marrow cells from control and PDB-preincubated cultures were co-cultured for 8 weeks with either control or PDB-preincubated adherent bone marrow cells. Nonadherent cells preincubated with PDB had a similar growth pattern whether or not the co-cultured adherent cells had been preincubated with PDB. In contrast, nonadherent cells preincubated in control medium and co-cultured with PDB-preincubated adherent cells displayed increased numbers of nucleated cells, CFC-GM and adherent hematopoietic islands compared to control. This was associated with increased amounts of CSA present in the culture supernatants. Thus PDB causes an irreversible decrease in CFC-GM in long term marrow cultures but enhances the ability of adherent stromal cells to support normal CFC-GM growth in culture.

The influence of fibroblast-like cells derived from canine fetal hematopoietic tissues on the regulation of lymphohematopoiesis

Media conditioned by fibroblast-like cells derived from organs active in fetal lymphohematopoiesis were studied for their effects on adult granulocyte/macrophage colony-forming units (CFU-GM). Fibroblasts from fetal liver produced a factor stimulatory for CFU-GM, whereas fibroblasts from fetal marrow produced a factor inhibitory for CFU-GM which was not completely relieved by adding indomethacin to the assay. Our studies indicated that neither fetal marrow nor fetal liver produced factors affecting lymphocyte colony-forming units (CFU-L). Cell-cell interactions between fibroblast-like cells derived from fetal liver or marrow and normal adult CFU-GM were also studied. We observed that fibroblasts derived from both fetal and adult marrow inhibited colony formation, whereas inhibition in the presence of fetal liver fibroblasts was minimal. Loss of inhibitory activity by a liver fibroblast cell line over repeated passages was seen. Differential analysis of colonies formed above an adherent layer of fetal marrow fibroblasts suggested that these fibroblasts suppress myeloid/macrophage differentiation to a far greater degree than did adult marrow fibroblasts. A role in the regulation of fetal lymphohematopoiesis may be played by stromal fibroblasts.

Incidence and characteristics of colony-forming units fibroblasts (CFU-F) in the bone marrow of weanling mice treated with cytosine arabinoside and phenylhydrazine: correlation with CFU-S, progenitors of diffusion-chamber colonies (CFU-D), and CFU-C

A study of murine adherent marrow cells (AMC) under conditions of high and low concentrations of hematopoietic stem cells and progenitors was carried out. In one group of weanling mice, decreased marrow cellularity and increased concentrations of CFU-S, CFU-D, and CFU-C were observed two days after administration of two consecutive intraperitoneal (i.p.) cytosine arabinoside (Ara-C) injections (2 x 200 ng/kg, at 6 h interval). Fibroblast colony-forming units (CFU-F) of this marrow were studied. In a second group of mice, which were given three i.p. phenylhydrazine (PHZ) injections (3 x 60 mg/kg on days 0, 1, and 3), CFU-S and CFU-C levels were unchanged or lowered 6 days after the start of PHZ administration. In these animals, however, the number of CFU-D was four times higher than in controls. The study of CFU-F in experiments of groups 1 and 2 indicated a high concentration of these progenitors in group 1 and lower concentration in group 2. Furthermore, fibroblastoid colonies produced in vitro by CFU-F of Ara-C-treated marrow were significantly larger than colonies from control marrow, and they markedly inhibited G/M colonies in split-phase agar cultures. By contrast, fibroblastoid colonies produced by PHZ-treated marrow were of regular size and did not inhibit G/M colonies from test bone marrow.