Haemopoietic stem cell concentration and CFUs in DNA synthesis in bone marrow from different bone regions

Combined Analysis of Endothelial, Hematopoietic, and Mesenchymal Stem Cell Compartments Shows Simultaneous but Independent Effects of Age and Heart Disease

Clinical trials using stem cell therapy for heart diseases have not reproduced the initial positive results obtained with animal models. This might be explained by a decreased regenerative capacity of stem cells collected from the patients. This work aimed at the simultaneous investigation of endothelial stem/progenitor cells (EPCs), mesenchymal stem/progenitor cells (MSCs), and hematopoietic stem/progenitor cells (HSCs) in sternal bone marrow samples of patients with ischemic or valvular heart disease, using flow cytometry and colony assays. The study included 36 patients referred for coronary artery bypass grafting or valve replacement surgery. A decreased frequency of stem cells was observed in both groups of patients. Left ventricular dysfunction, diabetes, and intermediate risk in EuroSCORE and SYNTAX score were associated with lower EPCs frequency, and the use of aspirin and β-blockers correlated with a higher frequency of HSCs and EPCs, respectively. Most importantly, the distribution of frequencies in the three stem cell compartments showed independent patterns. The combined investigation of the three stem cell compartments in patients with cardiovascular diseases showed that they are independently affected by the disease, suggesting the investigation of prognostic factors that may be used to determine when autologous stem cells may be used in cell therapy.

The location and cellular composition of the hemopoietic stem cell niche

While it is accepted that hemopoietic stem cells (HSC) are located in a three-dimensional microenvironment, termed a niche, the cellular and extracellular composition, as well as the multifaceted effects the components of the niche have on HSC regulation, remains undefined. Over the past four decades numerous advances in the field have led to the identification of roles for some cell types and propositions of potentially a number of HSC niches. We present evidence supporting the roles of multiple cell types and extracellular matrix molecules in the HSC niche, as well as discuss the potential significant overlap and intertwining of previously proposed distinct HSC niches.

Hematopoietic stem cell lodgment in the adult bone marrow stem cell niche

Treatment of malignant blood disorders, such as leukemia, that can provide a better chance of long-term remission involves myeloablation followed by transplantation of matched donor hematopoietic stem cells (HSCs). For successful engraftment and re-establishment of hematopoiesis to occur in the recipient, the transplanted HSCs must first migrate from the blood circulation to the bone marrow (BM), a process known as homing, then localize and anchor in suitable microenvironments within the BM, a process known as lodgment. After lodgment, the specific fate of the transplanted HSCs is determined through complex, bidirectional interactions with various stromal cell components in the niche. Ultimately, these interactions dictate the clinical outcome of the transplantation. Through the use of transgenic mouse models, considerable evidence has been accumulated in an attempt to unveil the possible underlying mechanisms that govern these processes. Here, we will emphasize the major factors that are involved in the regulation of lodgment of transplanted HSCs. Specifically, we will first introduce early observations on the spatial distribution of hematopoietic progenitors within the BM, then we will discuss the soluble factors, chemokines, cell-cell interactions, and cell-matrix interactions that have been studied and known to influence the site of HSC lodgment within the BM following transplantation.

The tale of early hematopoietic cell seeding in the bone marrow niche

Since introduction of the notion of a "niche" that hosts engraftment and activity of hematopoietic cells, there is a massive effort to discover its structure and decipher its function. Our understanding of the niche is continuously changing with reinterpretation of traditional concepts and apprehension of new insights into the biology of hematopoietic cell homing, seeding, and engraftment. Here we discuss some of the early events in hematopoietic stem cell seeding and engraftment and propose a perspective based on visualization of labeled bone marrow cells in real time in vivo. Primary seeding of hematopoietic cells in the bone marrow niches evolves as a complex and dynamic process; however, it follows discrete topological and chronological patterns. Initial seeding occurs on the endosteal surface of the marrow, which includes heterogeneous niches for primary seeding. Several days after transplantation the endosteal niches become more restrictive, hosting primarily mitotically quiescent cells, and gradual centripetal migration is accompanied by engagement in proliferation and differentiation. The hematopoietic niches evolve as heterogeneous three-dimensional microenvironments that are continuously changing over time.

Perspective: fundamental and clinical concepts on stem cell homing and engraftment: a journey to niches and beyond

In many ways, the homing of hematopoietic stem cells to bone marrow and other tissues defines these cells and their immediate and long-term fates Once homed, an inevitable series of proliferative and differentiative events presumptively follows. These comments, of course, hold for both homing to marrow, or alternatively, to other nonmarrow tissues. In this review, we will specifically focus on homing and engraftment to bone marrow because this is the best-studied and clinically applicable system.

The topologic and chronologic patterns of hematopoietic cell seeding in host femoral bone marrow after transplantation

The early stages of homing, seeding, and engraftment of hematopoietic stem and progenitor cells are poorly characterized. We have developed an optical technique that allows in vivo tracking of transplanted, fluorescent-tagged cells in the host femurs. In this study we used fluorescence microscopy to monitor the topologic and chronologic patterns of hematopoietic cell seeding in the femoral bone marrow (BM) of mice. PKH-labeled cells homed to the femur within minutes after injection into a peripheral vein. Most cells drifted within the marrow space and gradually seeded in clusters close to the endosteal surface of the epiphyseal cortex. Three days after transplantation 85% to 94% (14%) of PKH-labeled cells in the femoral marrow were located within 100 microm of the epiphyseal bone surface (P <.001 versus the more central cells), whereas labeled cells were absent in the femoral diaphysis. Primary seeding of juxtaendosteal, epiphyseal marrow occurred independently of recipient conditioning (myeloablated and nonconditioned hosts), donor-recipient antigen disparity, or the phenotype of the injected cells (whole BM and lineage-negative cells) and was consistently observed in secondary recipients of BM-homed cells. Seeding in regions close to the epiphyseal bone was also observed in freshly excised femurs perfused ex vivo and in femurs assessed without prior placement of optical windows, indicating that the site of primary seeding was not affected by surgical placement of optical windows. Four to 5 days after transplantation, cellular clusters appeared in the more central regions of the epiphyses and in the diaphyses. Centrally located cells showed decreased PKH fluorescence, suggesting that they were progeny of the seeding cells, and brightly fluorescent cells (quiescent first-generation seeding cells) were observed close to the bone surface for as long as 24 days after transplantation. These data indicate that the periphery of the femoral marrow hosts primary seeding and that quiescent cells continue to reside in the periphery for weeks and do not divide. The site of proliferation of transplanted cells is the center of the marrow space.

Transplanted hematopoietic cells seed in clusters in recipient bone marrow in vivo

The process of hematopoietic stem and progenitor cell (HSPC) seeding in recipient bone marrow (BM) early after transplantation is not fully characterized. In vivo tracking of HSPCs, labeled with PKH dyes, through an optical window surgically implanted on the mouse femur revealed that transplanted cells cluster in the recipient BM. Within the first day after intravenous injection, 86 +/- 6% of the cells seeded in clusters (p < 0.001 versus scattered cells) in the endosteal surfaces of the epiphyses. The primary clusters were formed by concomitant seeding of 6-10 cells over an area of approximately 70 microm, and secondarily injected cells did not join the already existing clusters but formed new clusters. Major antigen-disparate HSPCs participated in formation of the primary clusters, and T lymphocytes were also incorporated. After 4 to 5 days, some cellular clusters were observed in the more central regions of the BM, where the brightness of PKH fluorescence decreased, indicating cellular division. These later clusters were classified as secondary, assuming that the mechanisms of migration in the BM might be different from those of primary seeding. Some clusters remained in the periphery of the BM and retained bright fluorescence, indicating cellular quiescence. The number of brightly fluorescent cells in the clusters decreased exponentially to two to three cells after 24 days (p < 0.001). The data suggest that the hematopoietic niche is a functional unit of the BM stromal microenvironment that hosts seeding of a number of transplanted cells, which form a cluster. This may be the site where auxiliary non-HSPC cells, such as T lymphocytes, act in support of HSPC engraftment.

Spatial localization of transplanted hemopoietic stem cells: inferences for the localization of stem cell niches

The spatial distribution of subpopulations of hemopoietic progenitor cells following syngeneic transplantation was investigated at the single-cell level. The location of infused hemopoietic progenitor cells within the femoral bone marrow of nonablated recipients was determined by 5-(and-6)-carboxyfluorescein diacetate succinimidyl ester labeling of cells and in situ fixation by perfusion. Analysis performed over 15 hours after infusion demonstrated that the spatial distribution of transplanted marrow cells is not a random process. Although the majority of cells enter the bone marrow from the central marrow vessels, the subsequent localization within the bone marrow varied according to their phenotype. Candidate "stem cells" demonstrated selective redistribution and were significantly enriched within the endosteal region, whereas mature terminally differentiated and lineage-committed cells selectively redistributed away from the endosteal region and were predominantly in the central marrow region. Together, these data strongly support historical evidence of the presence of endosteal hemopoietic stem cell niches.

Lymphohematopoietic Engraftment in Minimally Myeloablated Hosts

The concept that myeloablation to open space was a prerequisite for marrow stem cell engraftment has been challenged by studies showing high rates of engraftment in nonmyeloablated mice (Stewart et al,Blood 81:2566, 1993; Quesenberry et al, Blood Cells20:97, 1994; Brecher et al, Blood Cells 5:237, 1979; Saxe et al, Exp Hematol 12:277, 1984; and Wu et al, Exp Hematol21:251, 1993). However, relatively large numbers of marrow cells were necessary to achieve high long-term donor percentages. We have demonstrated, using a BALB/c male/female marrow transplant model and detecting male DNA in host tissues by Southern blot or fluorescent in situ hybridization, that exposure to doses of irradiation that cause minimal myeloablation (50 to 100 cGy) leads to very high levels of donor chimerism, such that relatively small numbers of marrow cells (10 to 40 million) can give donor chimerism in the 40% to 100% range. Studies of radiation sensitivity of long-term engrafting cells have shown that 100 cGy, although not myelotoxic, is stem cell toxic, and indicate that the final host:donor ratios are determined by competition between host and donor stem cells. These data indicate that low levels of irradiation should be an effective approach to nontoxic marrow transplantation in gene therapy or in attempts to create allochimerism to treat such diseases as cancer, sickle cell anemia, or thalassemia.

Lymphohematopoietic Engraftment in Minimally Myeloablated Hosts

The concept that myeloablation to open space was a prerequisite for marrow stem cell engraftment has been challenged by studies showing high rates of engraftment in nonmyeloablated mice (Stewart et al,Blood 81:2566, 1993; Quesenberry et al, Blood Cells20:97, 1994; Brecher et al, Blood Cells 5:237, 1979; Saxe et al, Exp Hematol 12:277, 1984; and Wu et al, Exp Hematol21:251, 1993). However, relatively large numbers of marrow cells were necessary to achieve high long-term donor percentages. We have demonstrated, using a BALB/c male/female marrow transplant model and detecting male DNA in host tissues by Southern blot or fluorescent in situ hybridization, that exposure to doses of irradiation that cause minimal myeloablation (50 to 100 cGy) leads to very high levels of donor chimerism, such that relatively small numbers of marrow cells (10 to 40 million) can give donor chimerism in the 40% to 100% range. Studies of radiation sensitivity of long-term engrafting cells have shown that 100 cGy, although not myelotoxic, is stem cell toxic, and indicate that the final host:donor ratios are determined by competition between host and donor stem cells. These data indicate that low levels of irradiation should be an effective approach to nontoxic marrow transplantation in gene therapy or in attempts to create allochimerism to treat such diseases as cancer, sickle cell anemia, or thalassemia.

Potential and Distribution of Transplanted Hematopoietic Stem Cells in a Nonablated Mouse Model

Increasingly, allogeneic and even more often autologous bone marrow transplants are being done to correct a wide variety of diseases. In addition, autologous marrow transplants potentially provide an opportune means of delivering genes in transfected, engrafting stem cells. However, despite its widespread clinical use and promising gene therapy applications, relatively little is known about the mechanisms of engraftment in marrow transplant recipients. This is especially so in the nonablated recipient setting. Our data show that purified lineage negative rhodamine 123/Hoechst 33342 dull transplanted hematopoietic stem cells engraft into the marrow of nonablated syngeneic recipients. These cells have multilineage potential, and maintain a distinct subpopulation with “stem cell” characteristics. The data also suggests a spatial localization of stem cell “niches” to the endosteal surface, with all donor cells having a high spatial affinity to this area. However, the level of stem cell engraftment observed following a transplant of “stem cells” was significantly lower than that expected following a transplant of the same number of unseparated marrow cells from which the purified cells were derived, suggesting the existence of a “nonstem cell facilitator population,” which is required in a nonablated syngeneic transplant setting.

The development of spatial distributions of CFU-S and in-vitro CFC in femora of mice of different ages

The radial distributions of spleen colony forming units (CFU-S) and in-vitro colony forming cells (in-vitro-CFC) were measured in the diaphyseal marrow cavity of femora removed from 3-, 5- and 11-week-old mice. The distributions observed in 11-week-old mice confirm earlier findings that the highest concentrations of CFU-S exist near bone surfaces whereas the concentration of in-vitro-CFC increases to a peak value approximately 300 microns from the femoral axis with a low value at the bone surface. The gradients of the distributions in all three age groups are very similar suggesting that spatial organization in marrow is established by 3 weeks at the latest and, as the marrow cavities grow, so the distributions extend into the new space following their respective gradients. The peak of CFU-S concentration at the bone surfaces in all age groups coincides with increased rates of DNA synthesis and a low self-renewal capacity. Conversely, CFU-S nearer the centre of the cavity maintain a low turnover but have a high self-renewal capacity. Measurements made on 1-week-old mice show that the marrow contains a lower average concentration of CFU-S in the femoral cavity compared to older mice. However, these CFU-S have both a high rate of turnover and a high self-renewal capacity. It appears that these better quality CFU-S remain in a central location while the rest of the population ages and expands in association with growing bone regions.

Cell stimulation by haemopoietic lymphokines

Haemopoietic cells appear to respond to two distinct classes of growth factors, cell-bound molecules and lymphokines. Stimulation by lymphokines can be modelled by IL-3-dependent cell lines, and evidence is presented that such stimulation may represent a novel system of cell signalling unrelated to those where stimulation is a progression from a resting cell state into DNA synthesis and mitosis. The concept of a "rolling" cycle is introduced, and discussed in relationship to recent results suggesting that phosphorylation of a specific 33 kDa protein may be part of the control mechanism. Based on responses to different IL-3s, it is suggested that one function of lymphokines is to modulate responses to others by mechanisms other than regulation of receptor expression.

Stromal stem cells (CFU-f) in yolk sac, liver, spleen and bone marrow of pre- and postnatal mice

Our results demonstrate that in yolk sac, liver, spleen and femoral bone marrow of mice at ages ranging between 11 d of gestation and adult life, important changes in the stromal stem cell population (CFU-f assay) occur which are correlated with haemopoiesis. In the liver, spleen and bone marrow, high numbers of CFU-f precede high haemopoietic stem cell values. As haemopoiesis starts in the spleen, CFU-f numbers in fetal liver are low. Similarly, CFU-f numbers decrease in the spleen as bone marrow haemopoiesis starts. This suggests the existence of a migration stream of stromal stem cells. In spleen and bone marrow, CFU-f numbers decrease towards adult life as these organs maintain a stable haemopoietic activity.