Role of Integrin Linked Kinase in Expansion and Chemotaxis of CD34+ Cord Blood Cells.

Abstract 3616

Poster Board III-552

Despite being an excellent alternative to bone marrow or mobilized peripheral blood as a source of hematopoietic stem progenitor cells (HSPCs), the limiting factor to wider use of cord blood (CB) in transplantation is the 10-fold lower cell dose in a typical CB unit compared to harvested bone marrow or peripheral blood stem cells. Successful ex-vivo expansion of HSPCs as well as increasing transplantation efficiency by adopting protocols that enhance homing and engraftment of transplanted HSPC provides hope of making the applicability of a single unit of CB wider in the adult population. Interaction and adhesion of HSPCs with extracellular matrix (ECM) is an important event that regulates cell differentiation, proliferation, survival/ apoptosis as well as migration of HSPCs. Based on evidence present in the literature it appears that in addition to cooperative action between adhesion and growth factors, integrin mediated cell adhesion also provides distinct growth regulatory cues. Although it is known that in hematopoietic system, β1 integrin plays an important role in the interaction of HSPCs with integrin ligands, the underlying molecular mechanisms regulating βl integrin activity in hematopoietic cells remains largely unknown. Based on studies in other systems- Integrin linked kinase (ILK) appears to be an important molecule. ILK not only modulates β1 integrin activity, but also functions as an adapter protein, physically coupling downstream signals from both integrins and growth factors. ILK is preferentially expressed in murine stem / early progenitor cells (based on I. Lemiscka's Stem cell database) and we have found that ILK is expressed in both primitive (CD38^-/loCD34⁺) and committed progenitor (CD38⁺ CD34⁺) cells from CB by western blot. Moreover, using a co-immunoprecipitation approach, we found that in freshly isolated CD34+CB cells ILK interacts with β1 integrin. To investigate the role of ILK in adhesion-dependent and -independent growth of CB CD34+ cells both in the presence and absence of growth factors we expressed constitutive active ILK (CAILK) or dominant negative ILK (DNILK) in CD34+CB cells. In addition, we have also evaluated the effect of modulating ILK activity on chemotaxis of CD34+ CB cells towards stromal derived factor-1 (SDF-1)/CXCL12. We found that expression of CAILK enhances expansion of total CD34+ cells as well as colony forming cells stimulated ex-vivo by growth factors (stem cell factor- SCF; Flt-3 ligand and thrombopoietin-TPO), compared to cells transfected with vector alone. In contrast, expression of DNILK inhibited expansion of CD34+CB cells; this effect was more pronounced when cells were cultured in the absence of fibronectin, the ECM. Expression of CAILK increases cell-cycling since a greater proportion of cells were in ‘S’-phase compared to cells expressing DNILK or vector alone, both when the cells were expanded in the presence or absence of fibronectin for 20h. Expression of CAILK also leads to improved survival of CD34+ cells in the absence of serum and growth factors. Interestingly, β-catenin could be detected in CD34+ cells expressing CAILK, but not in cells transfected with DNILK or vector alone. Furthermore, in response to growth factor stimulation, Akt is phosphorylated in cells expressing either CAILK or DNILK; however, the amount of Akt phosphorylation was lower in cells expressing DNILK. Expression of CAILK improved modestly but not significantly the chemotaxis of CD34+ cells towards SDF-1 compared to cells expressing vector alone. However, DNILK significantly impaired chemotaxis of CD34+CB cells towards SDF-1. This impairment of chemotaxis is associated with defective actin polymerization in response to SDF-1, both at the ‘leading-edge’ and ‘tail’ of a polarized DNILK expressing CD34+ cell. Our findings implicate a role for ILK in both growth factor stimulated ex-vivo expansion of HSPCs as well as SDF-1 mediated chemotaxis. This may have potential implications in the therapeutic use of CB cells.