Methods to expand hematopoietic stem cells have continued to be examined extensively because stem cell numbers in the graft are important for clinical outcomes following transplantation. These numbers are particularly relevant in umbilical cord blood (UCB) transplantation, where low numbers of stem cells are directly related to delayed hematopoietic and immune reconstitution. Improved HSC expansion strategies may significantly impact transplantation outcome, enabling broader applications beyond UCB transplantation. Furthermore, these strategies are also needed to realize the full therapeutic potential of genome editing technologies to correct hematopoietic stem cells derived from patients with hematologic disorders. Since efforts to expand HSCs in cytokine-supported liquid cultures have been largely unsuccessful, efficient expansion will require an appropriate context that is provided by the hematopoietic stem cell niche. Future studies must also evaluate how niche signals regulate stem cell function to optimize cell expansion, and proper humanized mouse models must be developed to help predict stem cell function and regulation by the niche.

We now can generate large quantities of critical cell types whose deficiencies underlie many chronic diseases like heart failure. This breakthrough brings us to the next-level impediment: the immune system. While induced pluripotent stem cells have the potential to obviate rejection, in practical terms this is cost-prohibitive: It will cost huge amounts of money to produce and qualify a single patient's cell dose. Moreover, human cardiomyocytes are potent when given to infarcted hearts in the acute or sub-acute phase of infarction, but they have no benefit with chronic heart failure. The 6 months required to produce iPSC-cardiomyocytes precludes their autologous use for myocardial infarction.

We need an off the shelf cell therapy product for myocardial infarction that can be mass produced and qualified for large numbers of patients. This means an allogeneic product is necessary. Identifying the immune response to cardiomyocytes or other cell products will teach us how to precisely immunosuppress the patient, thereby minimizing complications, or alternatively, how to engineer the cells so as to avoid immunogenicity in the first place.

Lessons from the study of cardiomyocyte transplantation could extend to dopamine neurons, pancreatic beta-cells, retinal cells, myelinating cells and many other areas that cause common chronic disease.

We know a great deal of transplant immunology from hematopoietic stem cell transplantation (graft versus host) and from solid organ transplantation (host versus graft). There are good mouse and large animal (including non-human primate) models of stem cell differentiation and organ transplantation. This offers low hanging fruit where, in perhaps 5 years, we could discern the critical similarities and differences between transplanting stem cell derivatives and organ or marrow transplantation. These studies will inform clinical trials of allogeneic human stem cell derivatives that will be underway by then.

Success in this area will require bringing together researchers interested in stem cell biology and transplant immunology. A properly resourced RFA from NIH could be just the thing needed to promote this interaction.

Physiologically relevant in vitro environments could potentially impact research on every tissue type, disease, and intervention, including transfusion-based treatments. Basic research, drug-testing, and translational medicine would all be fundamentally altered. Individualized medicine, cellular therapies, and regenerative medicine could all benefit from in vitro conditions that best support the care of the patient's cells and guide those cells in the direction needed for effective treatment.

Research and Industry are in the early stages of developing the techniques and know-how needed to address the technical challenges in establishing human-relevant in vitro environments. We already have the technology to control in vitro oxygen and other critical gas components. Mimicking cell-cell interactions and variable cell states such as states of differentiation or stress are areas under active research. Computational and analytical techniques are being developed that can gain insight from large data sets. More of a challenge may be assessing distant effects like metabolism of drugs by the liver or potential drug and cellular interactions with the external environment. However, making a human-predictive in vitro environment affordable is the challenge that could define success or failure of any particular approach. It is feasible within the next ten years to have truly predictive in vitro environments for drug cellular therapy development.