Long-Term in Vitro Cytokine-Free and Serum-Free Culture of Human Cord Blood Mononuclear Cells in a Three-Dimensional Scaffold.

Abstract 503

The ability to expand cord blood (CB) cells ex vivo overcomes an important limitation to its wider clinical application in cellular therapies. The current practice of hematopoietic cell culture is based on two-dimensional (2D) tissue culture flasks or well plates which require either co-culture with allogeneic or xenogeneic stromal cells and the exogenous provision of several cytokines. This 2D culture environment is artificial and lacks the 3D cellular niches that characterise the in vivo hematopoietic inductive microenvironment. Specifically, the cultured cells are exposed to abnormally high cytokine concentrations, which may result in differentiation and loss of pluripotency.

We have previously developed a 3D bone marrow biomimicry through the use of synthetic scaffolds made of poly (D,L-lactide-co-glycolide) (PLGA) and polyurethane (PU) coated with collagen type I. Our previous work has shown that these scaffolds, which were seeded with cord blood (CB) mononuclear cells (MNCs) at a cell density of 3-6×10⁶cells per scaffold (5×5×5mm³), could successfully support long-term culture in the absence of exogenous growth factors for over 4 weeks. Specifically, the 3D biomimicry facilitated a 53-fold total MNC expansion, with an increase in the BFU-E and CFU-GM progenitor cell population. However, these cultures, although cytokine-free, contained 20-30% (v/v) fetal calf serum which can have both conducive and inhibitory effects on hematopoietic cell cultures due to the unknown composition and concentration of humoral factors contained within. Inclusion of serum in expansion-type cultures can limit the clinical application of the derived product.

The serum-free and cytokine-free culture and expansion of hematopoietic cells has not been achieved until now. Herein, we report that for at least 4 weeks the polyurethane (PU) scaffolds coated with collagen type I were able to maintain and expand human CB MNCs. Furthermore the progenitor population, as determined by the colony forming unit assay, was also maintained and preferentially directed towards the granulocytic lineage, even though the CFU-GEMMs declined. Immunophenotypic analysis of the extracted cells confirmed the presence of erythroid precursors (CD71⁺CD45^-) as well as early maturing myeloid cells. In contrast, the 2D cytokine- and serum-free cultures collapsed within 3-4 days. We hypothesized that the 3D biomimicry was able to facilitate serum- and cytokine-free conditions because it can recapitulate the three-dimensional architecture of the human bone marrow. This hypothesis was supported by scanning electron microscopy of the central sections of the scaffolds that showed the migration of cells within the pores and establishment of “niche-like” structures. In conclusion, this novel 3D culture system is capable of long-term, cytokine- and serum-free expansion of haematopoietic cells from cord blood, enabling the study of haematopoiesis as well as facilitating the expansion of cells for future clinical applications.