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

Biomedical Applications of Translational Optical Imaging: From Molecules to Humans

Light is a powerful investigational tool in biomedicine, at all levels of structural organization. Its multitude of features (intensity, wavelength, polarization, interference, coherence, timing, non-linear absorption, and even interactions with itself) able to create contrast, and thus images that detail the makeup and functioning of the living state can and should be combined for maximum effect, especially if one seeks simultaneously high spatiotemporal resolution and discrimination ability within a living organism. The resulting high relevance should be directed towards a better understanding, detection of abnormalities, and ultimately cogent, precise, and effective intervention. The new optical methods and their combinations needed to address modern surgery in the operating room of the future, and major diseases such as cancer and neurodegeneration are reviewed here, with emphasis on our own work and highlighting selected applications focusing on quantitation, early detection, treatment assessment, and clinical relevance, and more generally matching the quality of the optical detection approach to the complexity of the disease. This should provide guidance for future advanced theranostics, emphasizing a tighter coupling-spatially and temporally-between detection, diagnosis, and treatment, in the hope that technologic sophistication such as that of a Mars rover can be translationally deployed in the clinic, for saving and improving lives.

Interferon and tumor necrosis factor as humoral mechanisms coupling hematopoietic activity to inflammation and injury

Enhanced hematopoiesis accompanies systemic responses to injury and infection. Tumor necrosis factor (TNF) produced by injured cells and interferons (IFNs) secreted by inflammatory cells is a co-product of the process of clearance of debris and removal of still viable but dysfunctional cells. Concomitantly, these cytokines induce hematopoietic stem and progenitor cell (HSPC) activity as an intrinsic component of the systemic response. The proposed scenario includes induction of HSPC activity by type I (IFNα/β) and II (IFNγ) receptors within the quiescent bone marrow niches rendering progenitors responsive to additional signals. TNFα converges as a non-selective stimulant of HSPC activity and both cytokines synergize with other growth factors in promoting differentiation. These physiological signaling pathways of stress hematopoiesis occur quite frequent and do not cause HSPC extinction. The proposed role of IFNs and TNFs in stress hematopoiesis commends revision of their alleged involvement in bone marrow failure syndromes.

Differential expression of matrix metalloproteinase-2 expression in disseminated tumor cells and micrometastasis in bone marrow of patients with nonmetastatic and metastatic prostate cancer: theoretical considerations and clinical implications-an immunocytochemical study

Matrix metalloproteinase-2 (MMP-2) is important in the dissemination and invasion of tumor cells and activates angiogenesis. We present an immunocytochemical study of MMP-2 expression in circulating prostate cells (CPCs), disseminated tumor cells (DTCs), and micrometastasis (mM) in bone marrow of men with prostate cancer. Methods and Patients. Tumor cells were identified with anti-PSA immunocytochemistry. Positive samples underwent processing with anti-MMP-2, its expression was compared with Gleason score, concordance of expression, and metastatic and nonmetastatic disease. Results. 215 men participated, CPCs were detected in 62.7%, DTCs in 62.2%, and mM in 71.4% in nonmetastatic cancer; in metastatic cancer all had CPCs, DTCs, and mM detected. All CPCs and DTCs expressed MMP-2; in mM MMP-2 expression was positively associated with increasing Gleason score. MMP-2 expression in CPCs and DTCs showed concordance. In low grade tumors, mM and surrounding stromal cells were MMP-2 negative, with variable expression in high grade tumors; in metastatic disease, both mM and stromal cells were MMP-2 positive. Conclusions. CPCs and DTCs are different from mM, with inhibition of MMP-2 expression in mM of low grade tumors. With disease progression, MMP-2 expression increases in both mM and surrounding stromal cells, with implications for the use of bisphosphonates or MMP-2 inhibitors.

Primitive stem cells derived from bone marrow express glial and neuronal markers and support revascularization in injured retina exposed to ischemic and mechanical damage

Ischemic or mechanical injury to the optic nerve is an irreversible cause of vision loss, associated with limited regeneration and poor response to neuroprotective agents. The aim of this study was to assess the capacity of adult bone marrow cells to participate in retinal regeneration following the induction of anterior ischemic optic neuropathy (AION) and optic nerve crush (ONC) in a rodent model. The small-sized subset of cells isolated by elutriation and lineage depletion (Fr25lin(-)) was found to be negative for the neuroglial markers nestin and glial fibrillary acidic protein (GFAP). Syngeneic donor cells, identified by genomic marker in sex-mismatched transplants and green fluorescent protein, incorporated into the injured retina (AION and ONC) at a frequency of 0.35%-0.45% after intravenous infusion and 1.8%-2% after intravitreous implantation. Perivascular cells with astrocytic morphology expressing GFAP and vimentin were of the predominant lineage that engrafted after AION injury; 10%-18% of the donor cells incorporated in the retinal ganglion cell layer and expressed NeuN, Thy-1, neurofilament, and beta-tubulin III. The Fr25lin(-) cells displayed an excellent capacity to migrate to sites of tissue disruption and developed coordinated site-specific morphological and phenotypic neural and glial markers. In addition to cellular reconstitution of the injured retinal layers, these cells contributed to endothelial revascularization and apparently supported remodeling by secretion of insulin-like growth factor-1. These results suggest that elutriated autologous adult bone marrow-derived stem cells may serve as an accessible source for cellular reconstitution of the retina following injury.

Splenectomy influences homing of transplanted stem cells in bone marrow-ablated mice

Cell mobilization, a process that influences circulation, margination, and finally, homing play key roles in the regeneration processes mediated by stem cells. Recent studies as well as prior studies from our group indicate an important role of the spleen in hematopoietic reconstitution, but to date the role of the spleen in hematopoietic reconstitution has been unclear and it has not been precisely documented in ablated animals. Therefore, we undertook the present study to define more closely the role of the spleen in hematopoietic reconstitution in lethally irradiated mice. After transplantation of irradiated mice with lacZ+ -marked lin- / CD117+ bone marrow cells, we compared splenectomized mice (T(S), splenectomy performed prior to irradiation) to nonsplenectomized, irradiated mice (T(N)) as well as to normal (unirradiated) mice. Impaired hematopoietic reconstitution was observed in T(S) mice. Splenectomy markedly altered the distribution of hematopoietic stem cells, as demonstrated by fluorescence-activated cell sorting analysis of endogenous CD117+ cells in the thymus and bone marrow of recipients. Cell engraftment was demonstrated by histochemical and polymerase chain reaction analyses of recipient tissues. These experiments demonstrated that in T(S) animals, transplanted hematopoietic stem cells mobilized to extravascular tissues, particularly the gastrointestinal tract. The number of donor cells in recipient tissues continued to increase for 30 days after transplantation with the highest numbers observed in the T(S) group. DNA marking analysis led to the conclusion that engrafted cells were not only integrated into recipient tissues but were also capable of performing complex cellular processes, including proliferation and repair. Our results are consistent with the novel possibility that cellular repair markedly affects stem cell regenerative functions and that repair is markedly influenced by the integrity and presence of organs not directly involved in specific tissue regeneration processes, particularly the spleen.

Tumor necrosis factor receptors support murine hematopoietic progenitor function in the early stages of engraftment

Tumor necrosis factor (TNF) family receptors/ligands are important participants in hematopoietic homeostasis, in particular as essential negative expansion regulators of differentiated clones. As a prominent injury cytokine, TNF-alpha has been traditionally considered to suppress donor hematopoietic stem and progenitor cell function after transplantation. We monitored the involvement of TNF receptors (TNF-R) 1 and 2 in murine hematopoietic cell engraftment and their inter-relationship with Fas. Transplantation of lineage-negative (lin(-)) bone marrow cells (BMC) from TNF receptor-deficient mice into wild-type recipients showed defective early engraftment and loss of durable hematopoietic contribution upon recovery of host hematopoiesis. Consistently, cells deficient in TNF receptors had reduced competitive capacity as compared to wild-type progenitors. The TNF receptors were acutely upregulated in bone marrow (BM)-homed donor cells (wild-type) early after transplantation, being expressed in 60%-75% of the donor cells after 6 days. Both TNF receptors were detected in fast cycling, early differentiating progenitors, and were ubiquitously expressed in the most primitive progenitors with long-term reconstituting potential (lin(-)c-kit(+) stem cell antigen (SCA)-1(+)). BM-homed donor cells were insensitive to apoptosis induced by TNF-alpha and Fas-ligand and their combination, despite reciprocal inductive cross talk between the TNF and Fas receptors. The engraftment supporting effect of TNF-alpha is attributed to stimulation of progenitors through TNF-R1, which involves activation of the caspase cascade. This stimulatory effect was not observed for TNF-R2, and this receptor did not assume redundant stimulatory function in TNFR1-deficient cells. It is concluded that TNF-alpha plays a tropic role early after transplantation, which is essential to successful progenitor engraftment.

Placenta as a source of hematopoietic stem cells

The placenta is a large, highly vascularised hematopoietic tissue that functions during the embryonic and foetal development of eutherian mammals. Although recognised as the interface tissue important in the exchange of oxygen, nutrients and waste products between the foetus and mother, the placenta has increasingly become a focus of research concerning the ontogeny of the blood system. Here, we describe recent data showing the intrinsic hematopoietic potential and appearance of hematopoietic cells in the mouse and human placenta and probe the biological rationale behind its hematopoietic function. As a rest tissue that contains potent hematopoietic stem cells (HSCs), the human placenta could represent (in addition to umbilical cord blood cells) an accessible supplemental source of cells for therapeutic strategies.

Stem cell niche in the Drosophila ovary and testis; a valuable model of the intercellular signalling relationships

One of the key factors determining the function of all types of stem cells is their location in a specific microenvironment called a niche which is understood as a system of adjacent cells directly influencing their ability to carry out self-renewal divisions. The cells which compose the niche influence cytophysiological processes of stem cells both directly via the intercellular junction system and via the synthesis and release of many protein regulatory substances which are ligands of specific receptors in a particular stem cell. These proteins are often the products of distinct genes whose expression tends to be specific for niche-composing cells. The niches formed of a few cells only observed in Drosophila gonads may become a valuable functional model in the studies of mammal stem cells since their analysis proves that the preservation of the stem cells' unique features does not require a large number of cells to be present in its vicinity.

In vivo imaging of hematopoietic stem cells and their microenvironment

In this review we provide a description of the basic concepts and paradigms currently constituting the foundations of adult stem cell biology, and discuss the role that live imaging techniques have in the development of the field. We focus on live imaging of hematopoietic stem cells (HSCs) as the basic biology and clinical applications of HSCs have historically been at the forefront of the stem cell field, and HSC are the first mammalian tissue stem cells to be visualized in vivo using advanced light microscopy techniques. We outline the current technical challenges that remain to be overcome before stem cells and their niche can be more fully characterized using the live imaging technology.

Hematopoietic recovery after transplantation CD117+B220- (LACZ*) bone marrow cells in lethally irradiated mice

Experiments presented here were aimed at the description of hematopoiesis repair and in vivo homing of transplanted separated CD117+B220- bone marrow cells after whole-body lethal irradiation at LD 9Gy. ROSA 26 mice were used as donors of marrow cells for transplantation [B6;129S/Gt (ROSA)26Sor] and were tagged with lacZ gene, and F2 hybrid mice [B6129SF2/J] were used as recipients of bone marrow transplanted cells. Hematopoiesis repair was provided by transplantation, both suspension of whole bone marrow cells (5x106) and isolated CD117+B220- cells (5x10(4)). Mice survived up to thirty days after irradiation. We demonstrated that transplantation of suspension of whole bone marrow cells led to faster recovery of CFU-GM (Granulocyte-macrophage colony forming units) in bone marrow and in the spleen too. It is not clear what the share of residential and transplanted cells is in the repair process. Our results demonstrate that sufficient hematopoietic repair occurs after transplantation of CD117+B220- (lacZ+) in lethally irradiated mice, and the difference in CFU-GM numbers in the bone marrow and spleen found on day 8 posttransplant has no influence on the survival of lethally irradiated mice (30 days follow-up).

Bone marrow microenvironment and tumor progression

The bone marrow constitutes an unique microenvironment for cancer cells in three specific aspects. First, the bone marrow actively recruits circulating tumor cells where they find a sanctuary rich in growth factors and cytokines that promote their proliferation and survival. When in the bone marrow, tumor cells profoundly affect the homeostasis of the bone and the balance between osteogenesis and osteolysis. As a consequence, growth and survival factors normally sequestered into the bone matrix are released, further fueling cancer progression. Second, tumor cells actively recruit bone marrow-derived precursor cells into their own microenvironment. When in the tumors, these bone marrow-derived cells contribute to an inflammatory reaction and to the formation of the tumor vasculature. Third, bone marrow-derived cells can home in distant organs, where they form niches that attract circulating tumor cells. Our understanding of the contribution of the bone marrow microenvironment to cancer progression has therefore dramatically improved over the last few years. The importance of this new knowledge cannot be underestimated considering that the vast majority of cancer treatments such as cytotoxic and myeloablative chemotherapy, bone marrow transplantation and radiation therapy inflict a trauma to the bone marrow microenvironment. How such trauma affects the influence that the bone marrow microenvironment exerts on cancer is still poorly understood. In this article, the reciprocal relationship between the bone marrow microenvironment and tumor cells is reviewed, and its potential impact on cancer therapy is discussed.

Expression of Fas and Fas-ligand in donor hematopoietic stem and progenitor cells is dissociated from the sensitivity to apoptosis

OBJECTIVE

The interaction between the Fas receptor and its cognate ligand (FasL) has been implicated in the mutual suppression of donor and host hematopoietic cells after transplantation. Following the observation of deficient early engraftment of Fas and FasL-defective donor cells and recipients, we determined the role of the Fas-FasL interaction.

METHODS

Donor cells were recovered after syngeneic (CD45.1-->CD45.2) transplants from various organs and assessed for expression of Fas/FasL in reference to lineage markers, carboxyfluorescein succinimidyl ester dilution, Sca-1 and c-kit expression. Naïve and bone marrow-homed cells were challenged for apoptosis ex vivo.

RESULTS

The Fas receptor and ligand were markedly upregulated to 40% to 60% (p < 0.001 vs 5-10% in naïve cells) within 2 days after syngeneic transplantation, while residual host cells displayed modest and delayed upregulation of these molecules ( approximately 10%). All lin(-)Sca(+)c-kit(+) cells were Fas(+)FasL(+), including 95% of Sca-1(+) and 30% of c-kit(+) cells. Fas and FasL expression varied in donor cells that homed to bone marrow, spleen, liver and lung, and was induced by interaction with the stroma, irradiation, cell cycling, and differentiation. Bone marrow-homed donor cells challenged with supralethal doses of FasL were insensitive to apoptosis (3.2% +/- 1% vs 38% +/- 5% in naïve bone marrow cells), and engraftment was not affected by pretransplantation exposure of donor cells to an apoptotic challenge with FasL.

CONCLUSION

There was no evidence of Fas-mediated suppression of donor and host cell activity after transplantation. Resistance to Fas-mediated apoptosis evolves as a functional characteristic of hematopoietic reconstituting stem and progenitor cells, providing them competitive engraftment advantage over committed progenitors.

Fas ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and nonimmunogenic interactions

Early after transplantation, donor lineage-negative bone marrow cells (lin(-) BMC) constitutively upregulated their expression of Fas ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a noncleavable FasL chimeric protein on the surface of donor lin(-) BMC. Despite a short life span of the protein in vivo, expression of FasL on the surface of all the donor lin(-) BMC improved the efficiency of engraftment twofold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas(-) stroma and Fas(+) BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis and thus can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of engraftment. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and nonimmunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect. Disclosure of potential conflicts of interest is found at the end of this article.