Challenges of Stem Cells for Clinical and Therapeutic Applications
The clinical application and therapeutic uses of stem cells has become a hot research topic for the scientific community as researchers investigate novel treatment modalities for cancer and other debilitating diseases. In particular, the use of hematopoietic stem cells (HSCs) derived from umbilical cord blood holds great promise mainly because it is believed to be a source of high quality stem cells that is readily accessible, has minimal risk for the donor, and has relatively no ethical issues associated with isolating stem cells from this source. However, the pitfall of umbilical cord blood is the limited stem cell dose available for bone marrow transplantation. Ex vivo expansion of HSCs is typically utilized to overcome this issue with the use of a specific cell surface marker, CD34, which is considered the ‘gold standard’ marker for HSCS. Despite using CD34 isolation method, there still remain challenges with improving the quality and quantity of HSCs. Recent approaches have included co-culturing HSCs with mesenchymal stem cells (MSCs), which may represent an integral component of the HSCs microenvironment and may impact self-renewal, proliferation, and differentiation capacity of these stem cells. Although co-culturing has shown improvement in ex vivo expansion, the cell number of MSCs in cord blood is also limited and often requires isolation and expansion from an alternative source before the additions of HSCs.
A plausible approach to alleviate these issues could include isolating only CB mononuclear cells (no cell surface markers), which would minimize loss of HSCs and prevent exclusion of MSCs. This approach would also include a tissue culture substrate (e.g. extracellular matrix protein) that promotes cell attachment of MSCs and would deliver an in vitro niche for HSCs. This substrate coupled with serum free media containing the appropriate growth factors and additional supplements/nutrients that promote and supports proliferation of HSCs and MSCs synergistically would be the ideal combination. With the right media and extracellular matrix substrate, it is possible that HSCs and MSCs could be expanded from the same source, which would mean faster expansion and implantation of HSCs for stem cell transplantation. Ultimately, it is the quality and quantity of HSCs available for transplantation that greatly impact the success of engraftment and the applicability of this treatment modality for cancer.
Due to the therapeutic application of these stem cells, it is important that stem cells are propagated with a defined cell culture system that utilizes animal-free media, growth factors, and supplements. If the use of undefined biological substances derived from animals were not an issue, then Fetal Bovine Serum (FBS) could be used to supplement media and promote cell growth and expansion. Due to the risk of viral and prion contamination, the use of animal products in therapeutic stem cell applications is discouraged and alternatives that achieve similar results to FBS, like animal-free supplements, are ideally utilized instead. Supplements like animal-free albumin, animal-free transferrin, and animal-free insulin-transferrin supplements are key components to include when creating an animal-free substitute for FBS and ultimately, the ideal media to expand HSCs and MSCs from umbilical cord blood.