Umbilical cord blood (CB) has been used as a source of haemopoietic stem cells in both related and unrelated bone marrow transplantation and in both settings there appears to be a reduced incidence of graft-versus-host disease (GVHD) when compared to bone marrow transplantation. The aim of this study was to perform a phenotypic and cellular analysis of both cord and adult mononuclear cells <I>in vitro</I> in order to identify any immunological characteristic that could account for the apparent reduced incidence of GVHD observed, <I>in vivo</I>. For this, the cell surface phenotype, allogeneic cellular responses, cytokine secretion and expression of co-stimulatory molecule CD40L was studied. The phenotypic analysis was carried out by determining the proportion of cells expressing the CD4, CD8, CD16, CD19, CD45RA and CD45RO markers using 3 colour flow cytometric analysis. The results showed that in cord blood mononuclear cells, there was an increased number of cells expressing CD19⁺ whereas the number of cells expressing CD8⁺ was decreased compared to adult blood mononuclear cells (ABMNCs). In contrast the number of cells expressing CD4⁺ and CD16⁺ was similar in both cord blood mononuclear cells (CBMNCs) and ABMNCs. Further phenotypic analysis confirmed that cord blood contained primarily 'unprimed' T cells expressing CD45RA, while adult peripheral blood lymphocytes express mainly the CD45R 'memory' phenotype. The primary and secondary allogeneic responses of cord and adult mononuclear cells were assessed using the standard mixed lymphocyte reaction (MLR) and the primed lymphocyte test (PLT), respectively. These results showed that there was no significant difference in primary MLR responses of CBMNCs and ABMNCs. In contrast, CBMNCs had a decreased ability to mount a secondary proliferative response compared to ABMNCs. However this impaired ability to respond could be overcome by the addition of exogenous IL-2 to the cultures. The secretion of cytokines such as IL-2, IL-4, γIFN and TNF-α and the expression of CD40L on PMA- and ionomycin-activated CBMNCs and ABMNCs was also analysed by 3 colour flow cytometry. These results showed that upon activation CBMNCs produced reduced levels of cytokines and had reduced expression of CD40L. It is likely that the reduced cytokine production and CD40L expression observed is due to the predominance of CD45RA⁺ cells in CB compared to AB. In summary, this study has shown that CB and AB have different phenotypic and functional characteristics in vitro which may well explain the reduced incidence of GVHD observed in cord blood transplantation.

Attempts for the in vitro differentiation of reconstituting human HSCs from ESCs have been unsuccessful as key factors of HSC specification remain unclear. Enforced HOXB4 expression can enhance haemopoietic differentiation of mouse and human ESCs and generate reconstituting HSCs from mouse ESCs. We have previously shown that HOXB4 can enhance haemopoietic differentiation of mouse ESCs in a paracrine manner. Microarray analysis identified a number of secreted factors upregulated by HOXB4 potentially mediating this paracrine effect. The aim of this study was to assess whether these factors alone and/or in combination can enhance the in vitro haemopoietic differentiation of mouse and human ESCs. We first developed a defined, serum and feeder-free protocol to test the effects of these secreted factors on haemopoietic differentiation. This defined protocol allowed us to compare the haemopoietic potential of mouse ESCs with the recently derived epiblast stem cells (EpiSCs), thought to be comparable to hESCs. Haemopoietic colony forming assay and flow cytometry analysis showed that serum-free conditions generated 8- 10 fold more CD144+CD41+ and c-Kit+CD41+ haemopoietic progenitor cells (HPCs) compared to serum conditions from both ESCs and EpiSCs, with ESCs giving the most significant increase. We then validated the panel of HOXB4 target genes by QRT-PCR and selected those increased in expression by at least 2.5-fold when HOXB4 was activated. We used this defined, serum-free protocol to assess the effects of our panel of secreted factors, FGF17, RSPO3 and APLN, on the haemopoietic differentiation of mouse ESCs. We demonstrated that FGF17 can mediate HOXB4 haemopoietic activity by enhancing the generation of c-Kit+ HPCs. On the other hand increasing concentrations of RSPO3 inhibited haemopoietic development by reducing the numbers of CD41+ and CD41+CD45+ HPCs, while, APLN did not have any effects on the haemopoietic activity of the cells. We finally used the secreted factors in human ESC and iPSC differentiation cultures. We observed differences in the activity of the tested factors not only between species but also between human cell lines. These results suggest that HOXB4 haemopoietic activity is partly mediated by paracrine signalling but more complex cell interactions are probably required for it to fully exert its effects. More importantly, HOXB4 regulatory pathways differ between mouse and human cells stressing the need for careful translation of data between the two species and more detailed analysis of key human haemopoietic factors for the successful generation of reconstituting HSCs.

The engraftment of haemopoietic stem/progenitor cells (HSPCs) from umbilical cord blood (UCB) into adult recipients, although advantageous in terms of sourcing units, the decreased need to match donor and recipient and reduced risk of graft versus host disease (GvHD), is delayed compared to grafts using HSPCs from mobilised peripheral blood (MPB) or bone marrow (BM). One reason for this is the limited number of HSPCs (CD34+/CD133+ cells) in a unit of UCB compared to MPB or BM. The CXCR4-CXCL12 axis is widely recognised as a key player in the bone marrow homing, retention, and engraftment of HSPCs. The aim of this thesis was to investigate whether the engraftment of HSPCs from UCB into the bone marrow could be improved. Firstly, a novel in vitro 3D time-lapse chemotaxis assay to assess the homing capacity of human UCB CD133+ HSPCs, towards the chemokine CXCL12 was developed. One advantage of this assay was that it distinguished cell chemotaxis from chemokinesis and allowed these parameters to be quantified. Human UCB CD133+ HSPC chemotaxis towards CXCL12 was inhibited by the CXCR4 antagonist, AMD3100. Importantly, the presence of CXCL12 or AMD3100 had no affect on cell chemokinesis. To complement the in vitro chemotaxis assay, a short term in vivo homing assay in NSG mice was successfully established. The effect of siRNA silencing of the CXCR4 co-receptor, CD164, which is also expressed on CD133+ HSPCs, on cell migratory and homing ability was investigated. CD164 knock-down using siRNA in human UCB CD133+ HSPCs did not demonstrate an effect on homing to NSG bone marrow in vivo or chemotaxis to CXCL12 in vitro. However, homing to NSG mouse spleen was significantly reduced in cells silenced for CD164. Following this, an 8 day HSPC expansion system using nanofibre scaffolds (Nanex) and differing cytokines was investigated. These serum and feeder free conditions yielded a significant expansion of cells that retained CD133+CD34+ expression and their in vitro chemotactic ability to CXCL12. Time constraints did not permit the engrafting ability of these cells to be analysed in an in vivo HSC reconstitution assay that was initiated. However these studies will provide the basis to support future related research in this laboratory.

Improving the repopulation potential of human umbilical cord blood (UCB) haemopoietic stem cells (HSCs) remains a paramount goal in HSC transplantation (HSCT) therapy. This implies enhancing the homing and engraftment potential of UCB-CD34+CD133+ cells to the bone marrow (BM). Although an array of molecules continues to be identified as ‘key’ homing molecules, the molecular mechanisms controlling HSC homing are still not fully understood. The regulatory implications of hypoxia in the BM, with the concomitant stabilisation of hypoxia inducible transcription factor-1α (HIF-1α), are becoming more apparent, yet at the commencement of this thesis no study had explored whether hypoxia induced signalling can be adopted to regulate the homing and engraftment of transplanted HSCs. The aim of this DPhil project was thus to investigate whether hypoxic conditions as detected in the BM influence the adhesion of UBC-CD133+ cells to osteoblasts, BM stromal cells and BM endothelial cells-60 (BMEC-60), as well as their transmigration towards chemokine SDF-1α across BMEC-60. Increasing the exposure of UCB-CD133+ cells to 1.5% O2 doubled the percentage of transmigrating cells (p<0.05), and while hypoxia stimulated UCB-CD133+ cells preferentially adhered to IL-1β stimulated BMEC-60, their adhesion to non-stimulated (BMEC-60) was significantly improved (p<0.001). To help unravel the underlying molecular mechanisms, we attempted to examine the potential involvement of hypoxia regulated scaffolding protein HEF-1/NEDD9/Cas-L (HEF-1) in the increased percentage of migrating UCB-CD133+ cells after hypoxia pre-conditioning. The role of HEF-1 in HSCs is unexplored, and its multifunctional contribution in a variety of processes including cell migration, attachment and invasion make HEF-1 a prime candidate as a contributing homing molecule. After identifying a suitable short-hairpin RNA (shRNA) sequence to knockdown HEF-1, generating lentiviral (LV)-particles in house and optimising transduction protocols, HEF-1 knockdown was achieved in haemopoietic model cell lines KG-1 and KG-1A (KG-1/KG-1A–HEF1). Significantly decreased KG-1A–HEF1 cell adhesion to non-stimulated BMEC-60 was detected. Together, these studies provide a promising platform to further explore the role of HEF-1 in hypoxia induced UCB-CD133+ cell transmigration towards the key homing molecule SDF-1α.