How?
As a cell product derived from pluripotent stem cells, the source and quality of the starting cell bank are major factors affecting the cell manufacturing process. Clinical grade hESCs that have been established under GMP conditions are available such as from the WiCell Institute or the UK Stem Cell Bank, though the use of hESCs face ethical concerns in certain regulatory environments. As such, clinical grade hiPSCs are gaining more traction as the starting cell source. Services to generate such cells appear to be available through several commercial players such as Lonza, Pluristyx, RoslinCT, REPROCELL, among others. There is a high level of complexity involved due to considerations of the donor somatic cell source, method of reprogramming (viral or non-viral), and usage of a xeno-free culture under GMP conditions. There are currently no regulatory specifications on the required standards for hiPSCs designated for clinical applications. However, as more hiPSC-derived cell products have now advanced into clinical testing, there is some consensus on the key release criteria expected for hiPSC stocks or cell banks. These can cover cell identity, genomic stability or integrity, absence of residual vectors, markers of pluripotency, and sterility testing (29).
hPSCs are conventionally cultured in adherent cultures on attachment matrices (Figure 1A). To facilitate large scale expansion, the cells may be adapted to suspension cultures as 3D cell aggregates (Figure 1B) or microcarrier-based aggregates. For the differentiation of hPSCs to pancreatic islet cells, several research labs begin with adherent cultures during differentiation to pancreatic progenitors before transition to suspension cultures as 3D spheroids for the remainder of the differentiation to islet cells (17, 28, 30). Adherent cultures will require use of multiple flasks or cell factories, which may be challenging to manage in large scale cultures. Other labs utilize suspension spheroid cultures throughout the differentiation process (25, 31, 32), which are more amenable for scale up such as in shaking flasks, spinner flasks or stirred tank bioreactors. Adherent culture systems will present a limitation especially when large scale cell production is needed. Additionally, academic research does not require xeno-free, Good Manufacturing Practice (GMP)-compliant methods of cell production, and therefore substantial changes in culture protocol and re-optimization will be required to generate these cells for clinical use.
Another noteworthy challenge relates to managing the immunogenicity of cells in the case of allogeneic cell therapy. hPSCs provide a cell-based platform that is highly tractable for innovation, such as modification of starting hiPSCs by gene-editing to knock out or express immune genes and/or immune modulatory genes (12, 33), or establishing selective donor-derived hiPSC banks (such as that derived from HLA-homozygous donors) that enable HLA-matching of hiPSC lines for use in cell therapies (34, 35). Successful development in these areas will open up a significant pathway for hiPSC-based cell therapy products to reach larger patient populations.
Did you know that…
Primary human islets are known to contain 40 – 60% beta cells, 20 – 40% alpha cells, 5 – 15% delta cells and a small abundance of other minor cell types (with cell composition differing across different individuals). Based on prevailing differentiation protocols, hPSC-islet cells largely capture the different cell types within similar ranges of gold standard human islets (based on marker expression), but also contain poly-hormonal cells and non-endocrine cells that may justify further protocol refinement. Much of the spotlight had been thrown on specifically deriving functional insulin-secreting beta cells or enriching for such populations, while far fewer focus on deriving other cell types such as alpha cells, which are also important for maintaining blood glucose levels (36). With the versatility of the hPSC differentiation platform, perhaps it is possible in future to specifically design and manufacture off-the-shelf islet cells with different desired compositions that can recapitulate the function of other islet cell types, providing a system in which different cell types work in concert to achieve precise glucose homeostasis, much like primary human islets.
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