Temporal changes in monocyte and macrophage subsets and microglial macrophages following spinal cord injury in the Lys-Egfp-ki mouse model.

The role of hematogenous (hMΦ) and microglial (mMΦ) macrophages following spinal cord injury (SCI) remains unclear as they are not distinguished easily from each other in the lesion area. We have recently described the temporal and spatial response to SCI of each MΦ population using the lys-EGFP-ki mouse that enables EGFP(+) hMΦ to be distinguished from EGFP(-) mMΦ at the lesion site. In the present study, we characterized the response of monocyte and hMΦ subsets and mMΦ to SCI. We describe, for the first time, the responses of circulating classical (pro-inflammatory) and non-classical monocyte subsets to SCI. Additionally, we show the presence of classical and non-classical hMΦ at the SCI lesion. Importantly, we demonstrate that the 'classical pro-inflammatory' hMΦ respond in the acute (1d, 3d) stages of SCI while the 'non-classical' hMΦ respond in the sub-acute (7d, 14d) phase of SCI. At later time points (6weeks post injury) classical hMΦ return to the injury site. Our study offers new insight into the cellular inflammatory response that occurs after SCI and suggests that the timing and targets of anti-inflammatory therapies may be crucial to maximize neuroprotection at the acute and more chronic stages of SCI.

Clonal isolation of hESCs reveals heterogeneity within the pluripotent stem cell compartment.

Human embryonic stem cell (hESC) lines are known to be morphologically and phenotypically heterogeneous. The functional nature and relationship of cells residing within hESC cultures, however, has not been evaluated because isolation of single hESCs is limited to drug or manual selection. Here we provide a quantitative method using flow cytometry to isolate and clonally expand hESCs based on undifferentiated markers, alone or in combination with a fluorescent reporter. This method allowed for isolation of stage-specific embryonic antigen-3-positive (SSEA-3+) and SSEA-3- cells from hESC cultures. Although both SSEA-3+ and SSEA-3- cells could initiate pluripotent hESC cultures, we show that they possess distinct cell-cycle properties, clonogenic capacity and expression of ESC transcription factors. Our study provides formal evidence for heterogeneity among self-renewing pluripotent hESCs, illustrating that this isolation technique will be instrumental in further dissecting the biology of hESC lines.

Smad7 alters cell fate decisions of human hematopoietic repopulating cells.

Intracellular Smad proteins mediate signal transduction of the transforming growth factor-beta (TGF-beta) superfamily that play pleiotropic roles in hematopoietic development, suggesting that intracellular Smad proteins may play key roles in hematopoietic regulation. Although inhibitory Smad7, which negatively regulates TGF-beta signaling, has been implicated in the development of mature hematopoietic cells, a role for Smad7 in regulating more primitive hematopoietic cells has yet to be examined. Here, Smad7 was overexpressed in primary human severe combined immunodeficient (SCID) repopulating cells (SRCs), representing a common myeloid/lymphoid precursor cell with the functional capacity to repopulate the bone marrow of nonobese diabetic (NOD)/SCID recipient mice. Retroviral transduction of Smad7 into human umbilical cord blood (CB)-SRCs caused a shift from lymphoid dominant engraftment toward increased myeloid contribution, and increased the myeloid-committed clonogenic progenitor frequency in reconstituted mice. Neither myeloid nor B-lymphoid lineage developmental stages were compromised by Smad7 overexpression, suggesting Smad7 regulates cell fate commitment decisions of myeloid/lymphoid precursors by augmenting myeloid differentiation at the expense of lymphoid commitment. In addition, global gene expression analysis using microarray was used to identify potential target genes regulated by Smad7 in primitive hematopoietic cells that may control this process. Our study demonstrates a novel and unexpected role for Smad7 in modulating the cell fate decisions of primary multipotent human repopulating cells and establishes a role for Smad7 in the development of primitive human hematopoietic cells.

Retroviral transduction of hematopoietic cells differentiated from human embryonic stem cell-derived CD45(neg)PFV hemogenic precursors.

Human embryonic stem cells (hESCs) provide a unique opportunity to study molecular mechanisms that regulate specification of the hematopoietic lineage in the human. Exploitation of this model using transgenic strategies depends on the ability to target cells of the hematopoietic lineage effectively and establish stable transgene expression. Here, a recently defined subpopulation of endothelial-like precursors derived from hESCs that is exclusively responsible for hematopoietic cell fate (CD45(neg)PFV) is shown to express GALVR-1 receptor and be efficiently transduced with GALV-pseudotyped retrovirus. Retroviral transduction, measured by enhanced green fluorescent protein, of hESC-derived CD45(+) cells differentiated from isolated CD45(neg)PFV precursors was 26.5 +/- 13% with 5.6 +/- 4% of these cells coexpressing CD34. An average of 17.5% of clonogenic hematopoietic progenitors derived from CD45(neg)PFV precursors expressed the retroviral transgene. Addition of serum to cultures after retroviral exposure supported transgene expression in resulting hematopoietic cells derived from hemogenic CD45(neg)PFV precursors. Our study represents the first report to demonstrate that retroviral transduction systems, similar to those used currently in clinical gene therapy protocols, are capable of efficient transduction of hematopoietic progenitors derived from hESCs.

Human embryonic stem cells possess immune-privileged properties.

Human embryonic stem cells (hESCs) are envisioned to be a major source for cell-based therapies. Efforts to overcome rejection of hESCs include nuclear transfer and collection of hESC banks representing the broadest diversity of major histocompatability complex (MHC) polymorphorisms. Surprisingly, immune responses to hESCs have yet to be experimentally evaluated. Here, injection of hESCs into immune-competent mice was unable to induce an immune response. Undifferentiated and differentiated hESCs failed to stimulate proliferation of alloreactive primary human T cells and inhibited third-party allogeneic dendritic cell-mediated T-cell proliferation via cellular mechanisms independent of secreted factors. Upon secondary rechallenge, T cells cocultured with hESCs were still responsive to allogeneic stimulators but failed to proliferate upon re-exposure to hESCs. Our study demonstrates that hESCs possess unique immune-privileged characteristics and provides an unprecedented opportunity to further investigate the mechanisms of immune response to transplantation of hESCs that may avoid immune-mediated rejection.

Cytokines and BMP-4 promote hematopoietic differentiation of human embryonic stem cells.

Human embryonic stem cells (hESCs) randomly differentiate into multiple cell types during embryoid body (EB) development. To date, characterization of specific factors capable of influencing hematopoietic cell fate from hESCs remains elusive. Here, we report that the treatment of hESCs during EB development with a combination of cytokines and bone morphogenetic protein-4 (BMP-4), a ventral mesoderm inducer, strongly promotes hematopoietic differentiation. Hematopoietic progenitors of multiple lineages were generated from EBs and were found to be restricted to the population of progeny expressing cell surface CD45. Addition of BMP-4 had no statistically significant effect on hematopoietic differentiation but enabled significant enhancement in progenitor self-renewal, independent of cytokine treatment. Hematopoietic commitment was characterized as the temporal emergence of single CD45+ cells first detectable after day 10 of culture and was accompanied by expression of hematopoietic transcription factors. Despite the removal of cytokines at day 10, hematopoietic differentiation of hESCs continued, suggesting that cytokines act on hematopoietic precursors as opposed to differentiated hematopoietic cells. Our study establishes the first evidence for the role of cytokines and BMP-4 in promoting hematopoietic differentiation of hESC lines and provides an unprecedented system to study early developmental events that govern the initiation of hematopoiesis in the human.

Wnt-5A augments repopulating capacity and primitive hematopoietic development of human blood stem cells in vivo.

Human hematopoietic stem cells are defined by their ability to repopulate multiple hematopoietic lineages in the bone marrow of transplanted recipients and therefore are functionally distinct from hematopoietic progenitors detected in vitro. Although factors capable of regulating progenitors are well established, in vivo regulators of hematopoietic repopulating function are unknown. By using a member of the vertebrate Wnt family, Wnt-5A, the proliferation and differentiation of progenitors cocultured on stromal cells transduced with Wnt-5A or treated with Wnt-5A conditioned medium (CM) was unaffected. However, i.p. injection of Wnt-5A CM into mice engrafted with human repopulating cells increased multilineage reconstitution by >3-fold compared with controls. Furthermore, in vivo treatment of human repopulating cells with Wnt-5A CM produced a greater proportion of phenotypically primitive hematopoietic progeny that could be isolated and shown to possess enhanced progenitor function independent of continued Wnt-5A treatment. Our study demonstrates that Wnt-5A augments primitive hematopoietic development in vivo and represents an in vivo regulator of hematopoietic stem cell function in the human. Based on these findings, we suggest a potential role for activation of Wnt signaling in managing patients exhibiting poor hematopoietic recovery shortly after stem cell transplantation.

Human embryonic-derived hematopoietic repopulating cells require distinct factors to sustain in vivo repopulating function.

OBJECTIVE: We have previously identified a novel circulating embryonic blood cell capable of pluripotent hematopoietic reconstitution, which may serve as a target for in utero stem cell therapy. Based on its unique biological properties and ontogenic origin, we aim to examine the ability to maintain and retrovirally transduce fetal blood (FB) reconstituting cells in ex vivo culture conditions previously optimized for pluripotent hematopoietic repopulating cells derived from later stages of human ontogeny. METHODS: FB cells were evaluated for proliferative potential, progenitor composition, and SCID-repopulating cell (SRC) capacity before and after 3 days of serum free (SF) ex vivo culture using the previously optimized growth factor conditions of SCF, Flt-3L, IL-3, IL-6, and G-CSF (GF Mix), in comparison to cultures using GF Mix + oncostatin M (OSM), or SCF + Flt-3L. We further examined the ability to retrovirally transduce FB-SRC maintained in culture using SCF + Flt-3L alone. RESULTS: Circulating FB-SRC could not be maintained under GF Mix conditions previously shown to sustain CB (cord blood)-SRC. Ex vivo culture with SCF + Flt-3L reduced the proliferation of primitive FB cells lacking lineage commitment markers (Lin(-)), but expanded FB progenitors and sustained FB-SRC compared to culture with GF Mix with and without OSM. Using SCF + Flt-3L, FB-SRC capable of multilineage reconstitution were successfully transduced, suggesting that SCF and Flt-3L are necessary and sufficient for the survival and transduction of human hematopoietic repopulating cells of embryonic origin. CONCLUSION: Our study provides novel insights into the requirements of primitive FB reconstituting cells that are essential for developing in utero stem cell gene therapy protocols, and further illustrates the biological distinctiveness of FB-SRC compared to hematopoietic repopulating cells from other stages of human ontogeny.

Characterization of retroviral gene transfer into highly purified human CD34(-) cells with primitive hematopoietic capacity.

Primitive human hematopoietic cells have recently been identified within a rare subfraction of CD34(-) lineage-depleted (Lin(-)) cells and further characterized by their restriction to a rarer subset expressing AC133. Here we show that CD34(-)AC133(+)Lin(-) cells can be transduced by retrovirus at a comparatively higher efficiency than either CD34(-)AC133(-)Lin(-) or CD34(+)CD38(-)Lin(-) cells. Subpopulations were transduced by enhanced green fluorescent protein (eGFP)-containing retrovirus in serum-free conditions. During the culture period, both CD34(-)AC133(+)Lin(-) and CD34(+)CD38(-)Lin(-) subfractions expanded, whereas CD34(-)AC133(-)Lin(-) cells could not be sustained. Fluorescent microscopic examination of progenitors assayed by colony-forming units (CFU) derived from CD34(-)AC133(+)Lin(-) cells revealed expression of eGFP, with the presence of provirus confirmed by clonal PCR analysis. Flow cytometry detecting eGFP revealed that cultures seeded with CD34(-)AC133(+)Lin(-) cells had a greater than threefold higher frequency of eGFP(+) cells compared with transduced cultures of CD34(+)CD38(-)Lin(-) cells. Our results demonstrate that retroviral transduction efficiency and level of transgene expression into CD34(-)AC133(+)Lin(-) cells is distinct to either CD34(-)AC133(-)Lin(-) or CD34(+)CD38(-)Lin(-) cells. This study represents the first evaluation of retroviral transduction into this population of primitive CD34(-) cells, and therefore provides the basis for optimization of gene transfer protocols to examine the role of gene-marked CD34(-) stem cells in a clinical setting.