Transplantation of human hematopoietic repopulating cells: mechanisms of regeneration and differentiation using human???mouse xenografts:

Regional localization within the bone marrow influences the functional capacity of human HSCs

Numerous studies have shown that the bone marrow (BM) niche plays a key role in mouse hematopoietic stem cell (HSC) function and involves contributions from a broad array of cell types. However, the composition and role of the human BM HSC niche have not been investigated. Here, using human bone biopsy specimens, we provide evidence of HSC propensity to localize to endosteal regions of the trabecular bone area (TBA). Through functional xenograft transplantation, we found that human HSCs localizing to the TBA have superior regenerative and self-renewal capacity and are molecularly distinct from those localizing to the long bone area (LBA). In addition, osteoblasts in the TBA possess unique characteristics and express a key network of factors that regulate TBA- versus LBA-localized human HSCs in vivo. Our study reveals that BM localization and architecture play a critical role in defining the functional and molecular properties of human HSCs.

Activin A promotes hematopoietic fated mesoderm development through upregulation of brachyury in human embryonic stem cells

The development of the hematopoietic system involves multiple cellular steps beginning with the formation of the mesoderm from the primitive streak, followed by emergence of precursor populations that become committed to either the endothelial or hematopoietic lineages. A number of growth factors such as activins and fibroblast growth factors (FGFs) are known to regulate the early specification of hematopoietic fated mesoderm, notably in amphibians. However, the potential roles of these factors in the development of mesoderm and subsequent hematopoiesis in the human have yet to be delineated. Defining the cellular and molecular mechanisms by which combinations of mesoderm-inducing factors regulate this stepwise process in human cells in vitro is central to effectively directing human embryonic stem cell (hESC) hematopoietic differentiation. Herein, using hESC-derived embryoid bodies (EBs), we show that Activin A, but not basic FGF/FGF2 (bFGF), promotes hematopoietic fated mesodermal specification from pluripotent human cells. The effect of Activin A treatment relies on the presence of bone morphogenetic protein 4 (BMP4) and both of the hematopoietic cytokines stem cell factor and fms-like tyrosine kinase receptor-3 ligand, and is the consequence of 2 separate mechanisms occurring at 2 different stages of human EB development from mesoderm to blood. While Activin A promotes the induction of mesoderm, as indicated by the upregulation of Brachyury expression, which represents the mesodermal precursor required for hematopoietic development, it also contributes to the expansion of cells already committed to a hematopoietic fate. As hematopoietic development requires the transition through a Brachyury+ intermediate, we demonstrate that hematopoiesis in hESCs is impaired by the downregulation of Brachyury, but is unaffected by its overexpression. These results demonstrate, for the first time, the functional significance of Brachyury in the developmental program of hematopoietic differentiation from hESCs and provide an in-depth understanding of the molecular cues that orchestrate stepwise development of hematopoiesis in a human system.

Bone marrow mesenchymal stem cells for improving hematopoietic function: an in vitro and in vivo model. Part 2: Effect on bone marrow microenvironment

The aim of the present study was to determine how mesenchymal stem cells (MSC) could improve bone marrow (BM) stroma function after damage, both in vitro and in vivo. Human MSC from 20 healthy donors were isolated and expanded. Mobilized selected CD34⁺ progenitor cells were obtained from 20 HSCT donors. For in vitro study, long-term bone marrow cultures (LTBMC) were performed using a etoposide damaged stromal model to test MSC effect in stromal confluence, capability of MSC to lodge in stromal layer as well as some molecules (SDF1, osteopontin,) involved in hematopoietic niche maintenance were analyzed. For the in vivo model, 64 NOD/SCID recipients were transplanted with CD34+ cells administered either by intravenous (IV) or intrabone (IB) route, with or without BM derived MSC. MSC lodgement within the BM niche was assessed by FISH analysis and the expression of SDF1 and osteopontin by immunohistochemistry. In vivo study showed that when the stromal damage was severe, TP-MSC could lodge in the etoposide-treated BM stroma, as shown by FISH analysis. Osteopontin and SDF1 were differently expressed in damaged stroma and their expression restored after TP-MSC addition. Human in vivo MSC lodgement was observed within BM niche by FISH, but MSC only were detected and not in the contralateral femurs. Human MSC were located around blood vessels in the subendoestal region of femurs and expressed SDF1 and osteopontin. In summary, our data show that MSC can restore BM stromal function and also engraft when a higher stromal damage was done. Interestingly, MSC were detected locally where they were administered but not in the contralateral femur.

Novel approaches to vaccination against the feline immunodeficiency virus

Inadequate antigen presentation and/or suboptimal immunogenicity are considered major causes in the failure of human immunodeficiency vaccine to adequately protect against wild-type virus. Several approaches have been attempted to circumvent these hurdles. Here we reviewed some recent vaccinal strategies tested against the feline immunodeficiency virus and focused on: (i) improving antigen presentation by taking advantage of the exquisite ability of dendritic cells to process and present immunogens to the immune system; (ii) boosting immune responses with vaccinal antigens presented in a truly native conformation by the natural target cells of infection. Significance of the studies, possible correlates of protection involved, and implications for developing anti-human immunodeficiency virus vaccines are discussed.