Support versus inhibition of hematopoiesis by two characterized stromal cell types

In vitro crosstalk between fibroblasts and native human acute myelogenous leukemia (AML) blasts via local cytokine networks results in increased proliferation and decreased apoptosis of AML cells as well as increased levels of proangiogenic Interleukin 8

Interactions between native human acute myelogenous leukemia (AML) blasts and nonleukemic cells in the bone marrow microenvironment seem important both for disease development chemosensitivity. Native human AML blasts from consecutive patients were cultured with normal human bone marrow stromal cells and two fibroblast lines (HFL1 and Hs27) separated by a semipermeable membrane. This bidirectional crosstalk via the cytokine network between AML blasts and fibroblasts caused (i) increased proliferation, (ii) mediated antiapoptotic signalling and (iii) increased local levels of proangiogenic IL8.

Umbilical Cord Stromal Cells (UCSC)

The identification of appropriate cell types is necessary to establish cell-based therapies in regenerative medicine. These cell types must (1) be available in an appropriate amount, (2) be easy to obtain, (3) be sufficiently expandable in vitro, and (4) fit to or at least be able to differentiate into the required cell type. Since the umbilical cord is available without any intervention and represents a notable amount of tissue, we consider it to be a promising source for isolating cells for cell-based therapies. This study demonstrates that umbilical cord stromal cells (UCSC), the connective tissue cells of the umbilical cord, can be isolated in sufficient quantities and be well expanded. UCSC feature phenotypic plasticity and thus are functionally similar to stem cells. UCSC can be differentiated into cells with osteoblastic properties (expression of alkaline phosphatase, formation of bone nodules). It is concluded that the umbilical cord should no longer be regarded as valueless tissue and be unthinkingly discarded. Instead, it should be considered a valuable resource for the isolation of potent cells for cell-based therapies, especially for treatment of bone defects.

Intraclonal plasticity for bone, smooth muscle, and adipocyte lineages in bone marrow stroma fibroblastoid cells

Bone marrow stroma fibroblastoid cells (BMSFC) develop from a single clone of cells within each of the in vitro fibroblastoid colonies (CFU-F) derived from either murine or human bone marrow. All of the clones represented by these colonies displayed antigenic and product markers for osteoblast, smooth muscle, and adipocyte lineages when tested separately for each marker. Separate sets of fibroblastoid colonies derived from the same individual donor's culture tested positive with antibodies specific for smooth muscle-specific heavy chain myosin (SMMHC), smooth muscle alpha actin-1, bone sialoprotein, osteocalcin, or alkaline phosphatase, and developed von Kossa-positive deposits shown by X-ray microanalysis and electron diffraction to be hydroxyapatite. Individual cells were positive for both SMMHC and osteocalcin. All cells in the multiple clones tested were capable of metabolizing a fatty acid to form intracellular lipid droplets. PCR transcripts obtained from the human cell cultures that provided these BMSFC clones were consistent with the immunocytochemical findings. Transcripts for PPAR (gamma)-2 and Cbfa-1 were dependent upon the culture medium content, suggesting an osteoblast/adipocyte differentiation switch point. Cell lineage specificity for markers and RNA transcripts was determined by comparison to skin fibroblast controls. These findings demonstrate a high degree of interlineage plasticity in vitro for BMSFC.

Cell fate determination from stem cells

In the adult, tissue-specific stem cells are thought to be responsible for the replacement of differentiated cells within continuously regenerating tissues, such as the liver, skin, and blood system. In this review, we will consider the factors that influence stem cell fate, taking as a primary example the cell fate determination of hematopoietic stem cells.

Stromal support augments extended long-term ex vivo expansion of hemopoietic progenitor cells

Current technology to numerically expand hemopoietic stem/progenitor cells (HSPC) ex vivo within 1 to 2 weeks is insufficient to warrant significant gain in reconstitution time following their transplantation. In order to more stringently test the parameters affecting HSPC expansion, we followed ex vivo cultures of CD34+-selected umbilical cord blood (UCB) HSPC for up to 10 weeks and investigated the effects of stromal support and cytokine addition. The cytokine combinations included FL + TPO, FL + TPO plus SCF and/or IL6, or SCF + IL6. To identify the HSPC in uncultured and cultured material, we determined the number of colony-forming cells (CFC), cobblestone area forming cells (CAFC), the NOD/SCID repopulating ability (SRA), and CD34+ subsets by phenotyping. The highest fold-increase obtained for CD34+ and CD34+ CD38- cell numbers was, respectively, 1197 and 30,937 for stroma-free and 4066 and 117,235 for stroma-supported cultures. In general, CFC generation increased weekly in FL + TPO containing groups up to week 5 with a 28- to 195-fold expansion whereafter the weekly CFC output stabilized. Stroma support enhanced the expansion of CAFC week 6 maximally 11-fold to 89-fold with FL + TPO + IL6. Cultures stimulated with at least FL + TPO gave an estimated 10- to 14-fold expansion of the ability of CD34+ UCB cells to multilineage engraft the BM of sublethally irradiated NOD/SCID mice at 2 weeks of stroma-free and stroma-supported cultures, while at week 5 and later the estimated SRA decreased to low or undetectable levels in all groups. Our results show that stroma and FL + TPO but also inclusion of bovine serum albumin, greatly increase the long-term generation of HSPC as measured by in vitro assays and is indispensable for long-term expansion of CD34+ CD38- CXCR4+ cells. However, the different surrogate methods to quantify the HSPC (CD34+ CD38-, CFC, CAFC week 6 and SRA) show increasing incongruency with increasing culture time, while especially the phenotypic analysis and the CFC generation greatly overestimate the CAFC and SRA expansion in 10-week cultures.

The Hematopoietic Microenvironment: Stromal Cell Types: Characterization and Function In Situ and In Vitro

The murine hematopoietic stroma is essential for the homing and the continued replication of hematopoietic stem cells following transplantation and appears to control their lineage proliferation. In our studies it has been found to consist of 3 cell types with differing antigenic displays, growth factor responses and interactions when placed in culture. These characteristics are in keeping with their classifications as myofibroblasts, endothelial like cells and macrophages, respectively. Cell adhesion molecules (CAMs) and early acting cytokines have been found to be associated with these cell types. Only the myofibroblast type has been found to support proliferation of the descendants of long term repopulating stem cells (LTRC) plated over their confluent layer. The endothelial like cell type, and possibly the macrophage, were found to suppress not only the growth of LTRC but, also, that of the myofibroblasts obtained from the same marrow samples. Culture medium obtained from near-confluent growth of the endothelial like cells contained a growth inhibiting molecule of less than 3000 da. capable of inhibiting cells of several types, including the stromal myofibroblasts. The endothelial like cells were shown to unilaterally express VCAM-1 in culture, as well as vWf and endothelial cell specific antigens. Tissue sections of femurs from normal irradiated recipients taken 1 hour post-transplantation revealed Lac-Z marked donor stem cells lodged just external to the endothelial lining of thin walled vessels and in contact with myofibroblasts. Previous studies had shown that some stem cells enter replication at these sites within 48 hours of lodgment. It is suggested that a balance of growth stimulation versus suppression may be maintained between the myofibroblasts and the endothelial like cell types, respectively, in the functioning marrow stroma. Both the myofibroblastoid and the endothelial like stromal cells of old mice displayed a reduced replicative rate in vivo and a reduced replicative capacity in vitro. This suggests that treatments that induce damage to the marrow microenvironment may be especially destructive in aged individuals. In recent experiments it has become clear that the stromal cell population that includes cells identifiable as alkaline phosphatase positive myofibroblasts also contains cells that produce osteocalcin, osteonectin and bone sialoprotein, and deposit von Kossa positive calcium. Whether this results from a single cell lineage or two lineages of similar morphology and alkaline phosphatase expression is under investigation.

Regulated on activation, normal T-cell-expressed and -secreted mRNA expression in normal endometrium and endometriotic implants: assessment of autocrine/paracrine regulation by in situ hybridization

Chemoattraction of macrophages and T cells into the normal endometrium and inflammatory sites within endometriotic foci is mediated by chemokine gene expression. mRNA transcripts encoding regulated on activation, normal T-cell-expressed and -secreted (RANTES), a monocyte and T-cell chemokine, were demonstrated in the stroma of normal endometrium and endometriotic implants using in situ mRNA hybridization. Epithelial glands failed to express RANTES mRNA. In histological serial sections, we observed CD68-positive macrophages in the stroma of endometriotic implants adjacent to regions with prominent RANTES mRNA hybridization. In adjacent sections, monoclonal antibodies against tumor necrosis factor (TNF)-alpha showed this cytokine to be localized to stromal and epithelial compartments of the endometriotic implant with weak staining in unaffected ovarian tissue. Subconfluent monolayers of endometriotic stromal cells were tested for RANTES gene expression in situ, but we could only detect RANTES mRNA in isolated stromal cells after treatment with TNF-alpha. No RANTES mRNA was observed in unstimulated stromal cells or TNF-alpha stimulated or unstimulated epithelial cells. The data are consistent with a model in which proinflammatory cytokines (eg, TNF-alpha) induce RANTES gene expression limited to specific cells within endometrial and endometriotic stroma. Production of this chemokine, in turn, stimulates recruitment of CD68-positive macrophages into these tissues.

Effect of cytokines on the proliferation/differentiation of stroma-initiating cells

A culture system that identifies the precursor of murine bone marrow fibroblastic stromal cells (stroma-initiating cells, SIC) has been developed. In this system, mature fibroblasts are depleted by adherence to plastic dishes and the nonadherent cells are seeded at a low density, which results in the formation of colonies composed of fibroblastic cells. Macrophage colony-stimulating factor (M-CSF) has been shown to accelerate the colony formation in the system. In this study, we examined the stroma-inducing activity of a number of cytokines. Neither granulocyte-CSF, stem cell factor, interleukin (IL)-1, IL-6, transforming growth factor, epidermal growth factor, insulin-like growth factor, platelet-derived growth factor, nor fibroblast growth factor showed the activity. Similarly, tumor necrosis factor (TNF) did not show any stroma-inducing activity, but the factor inhibited the stromal colony formation induced by M-CSF. In this study, we found that granulocyte/macrophage-CSF (GM-CSF) and IL-3, as well as M-CSF had the stroma-inducing activity. Neither an additive nor synergistic effect was observed when the three factors were assayed in various combinations. The stroma-inducing activity of M-CSF, GM-CSF and IL-3 was observed even if lineage-negative bone marrow cells were used as target cells, suggesting that mature hematopoietic cells such as macrophages and granulocytes were not involved in the induction of stromal colony formation by these factors. Our results raise the possibility that GM-CSF and IL-3 as well as M-CSF stimulate the proliferation or differentiation of the precursor of bone marrow fibroblastic stromal cells.

Vascular smooth muscle differentiation of murine stroma: a sequential model

Previous studies by our group showed that stromal cells from human long-term marrow cultures were mesenchymal cells following a vascular smooth muscle pathway. The present study using 58 immortalized stromal lines from different hematopoietic sites was conducted to verify whether this hypothesis also held true for murine stroma. Principal components analysis performed using cytoskeletal and extracellular matrix proteins allowed the segregation of five factors explaining more than 70% of the variance. Factor I, including osteopontin and vimentin, and factor II, laminins and fibronectins, were representative of the mesenchyme. The remaining three factors were representative of vascular smooth muscle: factor III, including alphaSM actin, SM alpha actinin, SM22alpha, EDa+ fibronectin, and thrombospondin-1; factor IV, metavinculin and h-caldesmon; and factor V, smooth muscle myosin SM1 and desmin. All lines expressed factors I and II; 53 lines expressed factor III, 35 lines expressed factor IV; and 11 lines expressed factor V. A second principal components analysis including membrane antigens indicated the cosegregration of vascular cell adhesion molecule-1 with osteopontin and that of Ly6A/E with vimentin, whereas CD34 and Thy-1 appeared to be independent factors. The heterogeneity of vascular smooth muscle markers expression suggests that harmonious maintenance of hematopoiesis depends on the cooperation between different stromal cell clones.

The hematopoietic stroma

All blood cells are derived from a small common pool of totipotent cells, called hematopoietic stem cells. The process is strictly regulated by the hematopoietic microenvironment, which includes stromal cells, extracellular matrix molecules and soluble regulatory factors. Several experimental in vitro assays have been developed for the study of hematopoietic differentiation, and have provided valuable information on the stroma, which includes, among other cell types, macrophages, fibroblasts, adipocytes, and endothelial cells. The composition, ontogeny, and function in physiological as well as pathological conditions of stroma are discussed.