Enhanced In Vivo Homing of Uncultured and Selectively Amplified Umbilical Cord Blood CD34+ Cells by Co-Transplantation with Cord Blood Derived Unrestricted Somatic Stem Cells.

Mesenchymal stem cells have been implicated as playing an important role in stem cell engraftment. A new pluripotent population of umbilical cord blood (UCB)-derived Unrestricted Somatic Stem Cells (USSCs), with intrinsic and directable potential to develop into mesodermal, endodermal and ectodermal fates, has recently been identified and characterized (1). In this study we evaluated the capacity of USSCs to influence the homing of UCB -derived CD34+ cells into the marrow and spleen of NOD/SCID mice. Cultured USSCs were co-transplanted with CFDA-labeled cord blood CD34+ cells into sublethally irradiated NOD/SCID mice. Femurs and spleens were harvested 16 hrs thereafter and the percentage of CD34+ cells determined by flow cytometry.

USSCs induced a significant enhancement of CD34+ cell homing to both bone marrow and spleen (2.2 ± 0.3 and 2.4 ± 0.6 -fold, respectively; p<0.05). Similar findings were obtained with frozen USSC samples that had been thawed prior to transplantation. The effect of USSCs was specific, as no homing enhancement could be observed by co-transplantation of CD34+ cells with lineage-positive UCB cells, which contain T, B and mature myelo-erythroid cells. Enhanced marrow homing by USSCs was unaltered by extensive culture passaging of the cells, with a similar degree of enhancement observed for both early (p5) and late (p10) passage USSCs (1.7 ± 0.1 and 1.9 ± 0.1 -fold, respectively). Enhanced homing was dose-dependent, detectable at USSC/CD34+ ratios of 1:1 and above.

USSCs were also found to enhance the homing of day 14 cells harvested from cultures of selectively amplified^TM, ex-vivo expanded UCB lineage-negative (lin-) cells. The relative proportion of homing CD34+ cells within the culture-expanded cell population, was unaffected by USSC co-transplantation. Our findings thus demonstrate that USSCs enhance the homing of UCB hematopoietic stem cells and suggest a clinical potential for these cells in facilitating engraftment under conditions of limiting cord blood stem cell numbers.