Human hematopoietic microenvironments.

Dormancy of hematopoietic stem cells and formation of progenitors are directed by signals that come from the bone marrow microenvironment. Considerable knowledge has been gained on the murine hematopoietic stem cell microenvironment, while less so on the murine progenitor microenvironment and even less so on these microenvironments in humans. Characterization of these microenvironments is decisive for understanding hematopoiesis and finding new treatment modalities against bone marrow malignancies in the clinic. However, it is equally challenging, because hematopoietic stem cells are difficult to detect in the complex bone marrow landscape. In the present study we are characterizing the human hematopoietic stem cell and progenitor microenvironment. We obtained three adjacent bone marrow sections from ten healthy volunteers. One was used to identify a population of CD34+/CD38- "hematopoietic stem cells and multipotent progenitors" and a population of CD34+/CD38+ "progenitors" based on immunofluorescence pattern/intensity and cellular morphology. The other two were immunostained respectively for CD34/CD56 and for CD34/SMA. Using the combined information we performed a non-computer-assisted quantification of nine bone marrow components (adipocytes, megakaryocytes, bone surfaces, four different vessel types (arteries, capillaries, sinusoids and collecting sinuses), other "hematopoietic stem cells and multipotent progenitors" and other "progenitors") within 30 µm of "hematopoietic stem cells and multipotent progenitors", "progenitors", and "random cell profiles". We show that the microenvironment of the "hematopoietic stem cells and multipotent progenitors" is significantly enriched in sinusoids and megakaryocytes, while the microenvironment of the "progenitors" is significantly enriched in capillaries, other "progenitors", bone surfaces and arteries.

Re-thinking the bone remodeling cycle mechanism and the origin of bone loss.

Proper bone remodeling necessarily requires that osteoblasts reconstruct the bone that osteoclasts have resorbed. However, the cellular events connecting resorption to reconstruction have remained poorly known. The consequence is a fragmentary understanding of the remodeling cycle where only the resorption and formation steps are taken into account. New tools have recently made possible to elucidate how resorption shifts to formation, thereby allowing to comprehend the remodeling cycle as a whole. This new knowledge is reviewed herein. It shows how teams of osteoclasts and osteoblast lineage cells are progressively established and how they are subjected therein to reciprocal interactions. Contrary to the common view, osteoclasts and osteoprogenitors are intermingled on the eroded surfaces. The analysis of the resorption and cell population dynamics shows that osteoprogenitor cell expansion and resorption proceed as an integrated mechanism; that a threshold cell density of osteoprogenitors on the eroded surface is mandatory for onset of bone formation; that the cell initiating osteoprogenitor cell expansion is the osteoclast; and that the osteoclast therefore triggers putative osteoprogenitor reservoirs positioned at proximity of the eroded bone surface (bone lining cells, canopy cells, pericytes). The interplay between magnitude of resorption and rate of cell expansion governs how soon bone reconstruction is initiated and may determine uncoupling and permanent bone loss if a threshold cell density is not reached. The clinical perspectives opened by these findings are discussed.

Treatment strategies and outcomes in diffuse large B-cell lymphoma among 1011 patients aged 75 years or older: A Danish population-based cohort study.

BACKGROUND: Optimal treatment strategy for the oldest patients with diffuse large B-cell lymphoma (DLBCL) remains controversial, as this group often is precluded from clinical trials, and population-based studies are limited. METHODS: All Danish DLBCL-patients ≥75 years diagnosed from 2003 to 2012 were identified, using the Danish National Lymphoma Registry (LYFO). Information regarding baseline characteristics, treatment, comorbidities and outcomes was retrieved from LYFO, the Danish National health registries and medical records. Patients were stratified by age (75-79; 80-84 and 85 + years), comorbidity score and treatment modality (standard treatment [R-CHOP/CHOP-like], less intensive regimens or palliative treatment). FINDINGS: A total of 1011 patients were included. Standard treatment was initiated in 64%, ranging from 83% among patients aged 75-79 years to 32% among patient aged 85 + years. With standard treatment, median overall survival (OS) estimates were 4·6, 2·6, and 1·9 years for the age groups 75-79, 80-84 and 85+ years. Among patient aged 75-79 and 80-84 years, OS was superior with standard treatment, although high comorbidity scores attenuated this association. Among patients aged 85+ years, survival was not influenced by treatment intensity. Patients ≥80 years had similar OS regardless of intended (R-)CHOP dosing, whereas patients of 75-79 years scheduled for full dose had higher OS. Standard treatment was not associated with increased hospitalisation. INTERPRETATION: Standard treatment is feasible with good outcomes in a large proportion of elderly DLBCL-patients. Planned dose reduction in patients aged ≥80 years had no negative impact on OS.

Increased presence of capillaries next to remodeling sites in adult human cancellous bone.

Vascularization is a prerequisite for osteogenesis in a number of situations, including bone development, fracture healing, and cortical bone remodeling. It is unknown whether a similar link exists between cancellous bone remodeling and vascularization. Here, we show an association between remodeling sites, capillaries, proliferative cells, and putative osteoblast progenitors. Iliac crest biopsies from normal human individuals were subjected to histomorphometry and immunohistochemistry to identify the respective positions of bone remodeling sites, CD34-positive capillaries, smooth muscle actin (SMA)-positive putative osteoblast progenitors, including pericytes, Ki67-positive proliferative cells, and bone remodeling compartment (BRC) canopies. The BRC canopy is a recently described structure separating remodeling sites from the bone marrow, consisting of CD56-positive osteoblasts at an early differentiation stage. We found that bone remodeling sites were associated with a significantly increased presence of capillaries, putative osteoblast progenitors, and proliferative cells in a region within 50 µm of the bone or the canopy surface. The increases were the highest above eroded surfaces and at the level of the light-microscopically assessed contact of these three entities with the bone or canopy surfaces. Between 51 and 100 µm, their densities leveled to that found above quiescent surfaces. Electron microscopy asserted the close proximity between BRC canopies and capillaries lined by pericytes. Furthermore, the BRC canopy cells were found to express SMA. These ordered distributions support the existence of an osteogenic-vascular interface in adult human cancellous bone. The organization of this interface fits the current knowledge on the mode of action of vasculature on osteogenesis, and points to the BRC canopy as a central player in this mechanism. We propose a model where initiation of bone remodeling coincides with the induction of proximity of the vasculature to endosteal surfaces, thereby allowing capillary-BRC canopy interactions that activate marrow events, including recruitment of osteoblast progenitors to bone remodeling sites.