In vivo imaging of specialized bone marrow endothelial microdomains for tumour engraftment

Abstract

The organization of cellular niches has been shown to play a key role in regulating normal stem cell differentiation and regeneration, yet relatively little is known about the architecture of microenvironments that support malignant metastasis. 1,2 Using dynamic in vivo confocal imaging, we show that the murine bone marrow (BM) contains unique anatomic regions defined by specialized endothelium. This vasculature expresses the adhesion molecule E-selectin and the chemoattractant SDF-1 in discrete, discontinuous areas that localize the homing of a variety of tumor cell lines. Disruption of SDF-1/CXCR4 interactions inhibits Nalm-6 cell (acute lymphoblastic leukaemia) homing to these vessels. Further studies revealed that circulating leukemic cells engraft surrounding these vessels, suggesting that this molecularly distinct vasculature denotes a microenvironment for early metastatic tumor spread in BM. Finally, purified hematopoietic stem/progenitor cells and lymphocytes also localize to the same microdomains, indicating that this vasculature may function in benign states to demarcate specific portals for entry of cells into the marrow space. Specialized vascular structures therefore appear to delineate a microenvironment with unique physiology that is exploited by circulating malignant cells.It has been thought that tumor cells derive their ability to transit to specific organs by co-opting the same tissue-homing mechanisms used by benign leukocytes. 3 Substantial in vitro and more limited in vivo data provide evidence that tumors depend on selectin-, integrin-, and chemokinemediated vascular cell adhesion events in order to identify and bind to vascular beds at sites of tissue entry. 4,5 These molecular mechanisms are thought to enable the efficient spread of malignancies to target organs. Differential expression of these endothelial signals among tissues is known to control the destination of cellular traffic, but the contributions of the vascular molecular framework to the regulation of complex cellular microenvironments remain to be fully elucidated. Competing interests statementThe authors declare that they have no competing financial interests. The bone marrow (BM) is a frequent site for solid tumor spread. It can also be considered the most ubiquitous site for leukemic cell metastasis, as disease is seen to migrate from the initial birthplace of the leukemic clone to marrow spaces in distant sites throughout the body. These observations suggest that BM provides an avid environment for circulating tumor lodgement and growth. Moreover, the BM is commonly the source of latent or "minimal residual disease" following treatment, raising the possibility that specific anti-apoptotic "niches" for metastatic growth may exist. Understanding the biologic architecture of this host microenvironment therefore has significant implications for our approach to tumor treatment. NIH Public AccessWhile a variety of in vitro and in vivo techniques exist to study cell transit through BM, these are limited in their ability...

CXCR4 antagonists: targeting the microenvironment in leukemia and other cancers

“…15 Constitutive high-level CXCL12 secretion by reticular stromal cells in the marrow is essential for homing 16 and maintenance of hematopoietic stem cells (HSCs) in distinct vascular and endosteal niches for their development and growth. 17 Through CXCL12, these stromal cells also attract circulating hematopoietic progenitor cells 16 or leukemia cells 18 for homing to the marrow ( Figure 1). Actually, CXCR4 is the only functional chemokine receptor on hematopoietic progenitor cells, 19 emphasizing the predominant role of this chemokine receptor for homing and maintenance of HSC in the marrow niches.…”

“…[25][26][27] This maturation-dependent CXCL12 responsiveness appears to be retained in malignancies that correspond to respective maturation stages of their normal counterparts; that is, pre-and pro-B-cell acute lymphoblastic leukemia (ALL) cells and multiple myeloma cells utilize the CXCR4/CXCL12 axis for bone marrow homing. 18,28 There is also growing evidence suggesting that leukemia progression is driven by a sub-population of cells referred to as leukemia stem cells (LSCs) that are more leukemogenic than other cells of the same clone. LSCs share phenotypic and functional characteristics with their normal counterparts, and the hierarchical organization of the neoplastic clone mimics differentiation and cell turnover as part of homeostasis or tissue repair, nurtured by infrequent stem cells.…”

In vivo photothermal flow cytometry: Imaging and detection of individual cells in blood and lymph flow

“…However, to date, images of single cells in fast flow ($1-5 mm/s) are obtained during one short exposure only (i.e., no successive high-speed framing), while continuous imaging (i.e., successive framing) can be realized only in relatively slow flow (<0.1 mm/s). For example, very interesting data on the accumulation of fluorescently-labeled cancer cells on blood vessel endothelium has been obtained using the relatively slow frame rate (15-30 fps) of confocal imaging [Sipkins et al, 2005]. It is still a challenge to realize the successive high-resolution imaging of cells in relatively fast (>0.5 mm/s) blood and lymph flow, including flow in lymph nodes, which is crucial for studying cellular dynamics (e.g., cell deformability), including time-resolved monitoring of the interaction of metastatic cells with normal cells in flow and in endothelium.…”

Potential of tumour cells for delivering oncolytic viruses

“…For example, carriers could be modified to express chemokine receptors (for example CXCR4) causing the cells to follow chemokine gradients to the metastatic niche. 22 ALTC vehicles may also represent the standard packaging cell lines for some defective vectors, and serve as local (recombinant) vector factories. Further possibilities might be to build in antisense systems rendering the ALTCs insensitive to anti-viral defence mechanisms or to improve their adhesion to endothelial matrices or their predilection for hypoxic conditions.…”