Peter Zandstra, PhD, chief scientific officer, Centre for Commercia-lization of Regenerative Medicine, Toronto.
BioPharm: Why is an understanding of stem cells' interaction with their microenvironment crucial to development of stem-cell-based
Zandstra: One of the big challenges with stem cells is that often they are rare populations, and often they are heterogeneous populations.
The environment that we try to control in our bioreactors isn't always the same environment that the cells see in situ. We're trying very hard to isolate and identify the types of environments that one might want to dial in to your bioprocess
in order to get stem-cell growth while taking into account the other signals that are endogenously produced either by off-target
cells or cells put into the bioreactor that are not part of the population you want to grow. That local microenvironment has
an in vivo correlate, called the stem cell niche. The stem cells we want to grow typically are in association with other cell types,
or in unique places in our bodies that allow them to be maintained throughout our lives. So the question is, how can we take
advantage of the knowledge of where stem cells are in vivo to design better technologies to grow them, ideally at purity, at manufacturing-scale.
BioPharm: Cell-culture condtions are obviously a concern for all cell types. What, in particular, is different about stem cells?
Zandstra: We have to worry about all the same things that one would worry about when growing CHO cells—oxygen, glucose, lactate production.
In addition, we have to worry about at least two other things. One factor is cytokine concentrations that are supplemented
to the bioprocess, because these cells are often dependent on specific growth factors for their growth and differentiation.
These growth factors have their own half lives, their own activity levels, and the dose responses are often quite tight, in
that if you added a dose at a certain concentration you'll get mainly stem-cell growth, but if you add it too high or too
low you'll lose a population due to differentiation. The other aspect that's different is the heterogeneity. CHO cells, for
instance, are a relatively homogeneous population, in that the cells have generally the same function and the same behavior
within a bioreactor. In a stem-cell culture, you might start a culture where only one in 10 cells that you're trying to grow,
for example in blood stem cells, is a stem cell and you need to deal with this endogenous heterogeneity that is due to secreted
factors and interactions with other cell types and control that in order to get better growth.
BioPharm: From a practical perspective, what do you see as the biggest hurdles in being able to produce stem cells at large enough
scale and cost-effectiveness for commercial use?
Zandstra: I think we're making great progress in this. I think it's important to clarify that there are different types of stem cells,
and so the answer to the question a little bit depends on what type of stem cell we're talking about. The first problem is
to develop conditions that maintain high enough purity of the stem cells while still allowing for good growth. What often
happens is that stem cells either differentiate when you try to grow them or they don't grow fast enough to get enough cells
produced. The other challenge is that often the formulation of how stem cells like to grow is not ideal for scale up. Maybe
they grow on surfaces at low density. One has to learn to grow them in suspension, or using technologies that mimic suspension,
and intensify the process to get much higher yields and productivity. That's been a big challenge. The third challenge we
have to overcome is the cost. Many of the stem-cell culture processes and technologies that are in place are high-cost, small-scale
systems that use poorly defined reagents. Moving production technologies to, ideally, chemically defined media, where we can
use small molecules or defined components that can be manufactured at scale themselves will make a big difference in reducing
the cost of cell production.