In vitro requirement for periostin in B lymphopoiesis

The extracellular matrix of hematopoietic stem cell niches

•

Comprehensive overview of different classes of ECM molecules in the HSC niche.

•

Overview of current knowledge on role of biophysics of the HSC niche.

•

Description of approaches to create artificial stem cell niches for several application.

•

Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Niche-Mediated Integrin Signaling Supports Steady-State Hematopoiesis in the Spleen

Outside-in integrin signaling regulates cell fate decisions in a variety of cell types, including hematopoietic stem cells (HSCs). Our earlier published studies showed that interruption of periostin (POSTN) and integrin-αv (ITGAV) interaction induces faster proliferation in HSCs with developmental stage-dependent functional effects. In this study, we examined the role of POSTN-ITGAV axis in lymphohematopoietic activity in spleen that hosts a rare population of HSCs, the functional regulation of which is not clearly known. Vav-iCre-mediated deletion of Itgav in the hematopoietic system led to higher proliferation rates, resulting in increased frequency of primitive HSCs in the adult spleen. However, in vitro CFU-C assays demonstrated a poorer differentiation potential following Itgav deletion. This also led to a decrease in the white pulp area with a significant decline in the B cell numbers. Systemic deletion of its ligand, POSTN, phenocopied the effects noted in Vav-Itgav^-/- mice. Histological examination of Postn-deficient spleen also showed an increase in the spleen trabecular areas. Importantly, these are the myofibroblasts of the trabecular and capsular areas that expressed high levels of POSTN within the spleen tissue. In addition, vascular smooth muscle cells also expressed POSTN. Through CFU-S₁₂ assays, we showed that hematopoietic support potential of stroma in Postn-deficient splenic hematopoietic niche was defective. Overall, we demonstrate that POSTN-ITGAV interaction plays an important role in spleen lymphohematopoiesis.

Fibronectin and Its Receptors in Hematopoiesis

Fibronectin is a ubiquitous extracellular matrix protein that is produced by many cell types in the bone marrow and distributed throughout it. Cells of the stem cell niche produce the various isoforms of this protein. Fibronectin not only provides the cells a scaffold to bind to, but it also modulates their behavior by binding to receptors on the adjacent hematopoietic stem cells and stromal cells. These receptors, which include integrins such as α4β1, α9β1, α4β7, α5β1, αvβ3, Toll-like receptor-4 (TLR-4), and CD44, are found on the hematopoietic stem cell. Because the knockout of fibronectin is lethal during embryonal development and because fibronectin is produced by almost all cell types in mammals, the study of its role in hematopoiesis is difficult. Nevertheless, strong and direct evidence exists for its stimulation of myelopoiesis and thrombopoiesis using in vivo models. Other reviewed effects can be deduced from the study of fibronectin receptors, which showed their activation modifies the behavior of hematopoietic stem cells. Erythropoiesis was only stimulated under hemolytic stress, and mostly late stages of lymphocytic differentiation were modulated. Because fibronectin is ubiquitously expressed, these interactions in health and disease need to be taken into account whenever any molecule is evaluated in hematopoiesis.

Periostin supports hematopoietic progenitor cells and niche-dependent myeloblastoma cells in vitro

The expression of extracellular matrix protein periostin (POSTN) was attenuated in Med1(-/-) mouse embryonic fibroblasts (MEFs), which exhibited a decreased capability to support hematopoietic progenitor cells (HPCs) in vitro. When bone marrow (BM) cells were cocultured with mitomycin C-treated Med1(+/+) MEFs, or OP-9 or MS-5 BM stromal cells, in the presence of anti-POSTN antibody, the growth of BM cells and number of long-term culture-initiating cells (LTC-ICs) were attenuated. When BM cells were cocultured with Med1(-/-) MEFs in the presence of recombinant POSTN, the growth of BM cells and the number of LTC-ICs were restored. Moreover, antibody-mediated blockage of stromal cells-derived POSTN markedly reduced the growth and cobblestone formation, a leukemic stem cell feature, of stromal cell-dependent MB-1 myeloblastoma cells. POSTN was expressed both in BM cells and variably in different BM stromal cells. Expression in the latter cells was increased by physical interaction with hematopoietic cells. The receptor for POSTN, integrin αvβ3, was expressed abundantly in BM stromal cells. The addition of recombinant POSTN to BM stromal cells induced intracellular signaling downstream of integrin αvβ3. These results suggest that stromal cell POSTN supports both normal HPCs and leukemia-initiating cells in vitro, at least in part, indirectly by acting on stromal cells in an autocrine or paracrine manner.

Bone marrow fat and the decline of B lymphopoiesis in rabbits

B lymphopoiesis is necessary to generate a diverse pool of naïve B cells that are able to respond to a broad spectrum of antigens during immune responses to pathogens and to vaccination. Rabbits have been utilized for many years to generate high affinity monoclonal and polyclonal antibodies. Specific antibodies generated in rabbits have greatly advanced scientific discoveries, but the unique qualities of rabbit B cell development have been underappreciated. Unlike in humans and mice, where B lymphopoiesis declines in mid to late life, B lymphopoiesis in rabbits arrests early in life, between 2 and 4 months of age. This review focuses on the early loss of B cell development in rabbits and the contribution of the bone marrow microenvironment to this process. We also propose directions for future research in this area, and discuss how the rabbit can be used as a model to understand the decline of B lymphopoiesis that occurs in humans late in life. Such studies will be important for developing therapeutics targeted to prevent and/or reverse declining B lymphopoiesis in the elderly, as well as boosting immunity and antibody responses after infection or vaccination.

Tenascin C promotes hematoendothelial development and T lymphoid commitment from human pluripotent stem cells in chemically defined conditions

The recent identification of hemogenic endothelium (HE) in human pluripotent stem cell (hPSC) cultures presents opportunities to investigate signaling pathways that are essential for blood development from endothelium and provides an exploratory platform for de novo generation of hematopoietic stem cells (HSCs). However, the use of poorly defined human or animal components limits the utility of the current differentiation systems for studying specific growth factors required for HE induction and manufacturing clinical-grade therapeutic blood cells. Here, we identified chemically defined conditions required to produce HE from hPSCs growing in Essential 8 (E8) medium and showed that Tenascin C (TenC), an extracellular matrix protein associated with HSC niches, strongly promotes HE and definitive hematopoiesis in this system. hPSCs differentiated in chemically defined conditions undergo stages of development similar to those previously described in hPSCs cocultured on OP9 feeders, including the formation of VE-Cadherin⁺CD73⁻CD235a/CD43⁻ HE and hematopoietic progenitors with myeloid and T lymphoid potential.

•

Hemogenic endothelium is generated in a completely defined xenogen-free system

•

The system reproduces all stages of hematopoietic development

•

Tenascin C enhances hematoendothelial development from pluripotent stem cells

•

Tenesacin C uniquely supports T cell specification

Non-cell-autonomous hedgehog signaling promotes murine B lymphopoiesis from hematopoietic progenitors

The role of hedgehog (Hh) signaling in B lymphopoiesis has remained unclear. We observed that the proliferation of pro-B cells in stromal cocultures was impaired by interruption of Hh signaling, prompting us to investigate whether the target of Hh antagonism was intrinsic or extrinsic to the B-lymphoid compartment. In the present study, using conditional deletion of the pathway activator gene Smo, we found that cell-autonomous Hh signaling is dispensable for B-cell development, B-lymphoid repopulation of the BM, and humoral immune function. In contrast, depletion of the Smo protein from stromal cells was associated with impaired generation of B-lymphoid cells from hematopoietic stem progenitor cells, whereas reciprocal removal of Smo from these cells had no effect on the production of B-cell progenitors. Depletion of Smo from stromal cells was associated with coordinate down-regulation of genes for which expression is associated with osteoblastoid identity and B-lymphopoietic activity. The results of the present study suggest that activity of the Hh pathway within stromal cells promotes B lymphopoiesis in a non-cell-autonomous fashion.

Natural killer cell differentiation from hematopoietic stem cells: a comparative analysis of heparin- and stromal cell-supported methods

Natural killer (NK) cells differentiated from hematopoietic stem cells (HSCs) may have significant clinical benefits over NK cells from adult donors, including the ability to choose alloreactive donors and potentially more robust in vivo expansion. Stromal-based methods have been used to study the differentiation of NK cells from HSCs. Stroma and cytokines support NK cell differentiation, but may face considerable regulatory hurdles. A recently reported clinical-grade, heparin-based method could serve as an alternative. How the stromal-based and heparin-based approaches compare in terms of NK cell generating efficiency or function is unknown. We show that compared with heparin-based cultures, stroma significantly increases the yield of HSC-derived NK cells by differentiating less-committed progenitors into the NK lineage. NK cells generated by both approaches were similar for most NK-activating and -inhibiting receptors. Although both approaches resulted in a phenotype consistent with CD56(bright) stage IV NK cells, heparin-based cultures favored the development of CD56(+)CD16(+) cells, whereas stroma produced more NK cell immunoglobulin-like receptor-expressing NK cells, both of which are markers of terminal maturation. At day 21, stromal-based cultures demonstrated significantly more IL-22 production, and both methods yielded similar amounts of IFN-γ production and cytotoxicity by day 35. These findings suggest that heparin-based cultures are an effective replacement for stroma and may facilitate clinical trials testing HSC-derived NK cells.