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iPSCs Pass the Test in Non-human Primate model

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Review of “Path to the Clinic: Assessment of iPSC-Based Cell Therapies In Vivo in a Nonhuman Primate Model” from Cell Reports by Stuart P. Atkinson.

Induced pluripotent stem cell (iPSC) technology has provided us with a wealth of therapeutically relevant cell types, although their feasibility and safety for human clinical applications still remains uncertain. Murine models, useful as they are, do not fully recapitulate human physiology [1], suggesting that a more relevant large animal preclinical model is required. The rhesus macaque is physiologically similar to humans and, furthermore, rhesus iPSCs (rhiPSCs) greatly resemble human iPSCs (hiPSCs) [2]. Now in a study published in Cell Reports, researchers from the laboratory of Cynthia E. Dunbar have now assessed the utility of a r rhesus macaque model through rhiPSC-based autologous cell transplantation studies [3].

The group generated rhiPSCs via transduction of the STEMCCA excisable polycistronic lentiviral vector (POU5F1 (OCT4), SOX2, MYC, and KLF4) into bone marrow stem cells, skin fibroblasts and CD34+ cells. rhiPSCs were then adapted to a feeder free defined growth system (completely animal-free on the synthetic surface Synthemax II-SC, which had been previously determined not to elicit systemic toxicity nor irritation in immune-competent animals), and assessed using a novel rhesus macaque plasma clot scaffold as a substitute for Matrigel in teratoma assays. While low cell doses (500,000 cells) did not lead to tumor growth, high cell doses (10 million cells) led to the development of masses within 7 days, which slowly increased in size and then plateaued within 10 weeks, displaying mature teratoma structures with cells derived from all three germ cell layers. Overall statistics suggested that, compared to the same cells xenotransplanted into immunodeficient mice, teratoma formation is less efficient following autologous transplantation in the rhesus macaque. 

Hong et al then moved on to derive mesenchymal stem cells (MSCs) from rhiPSCs and assess if they could safely generate viable bone tissue using this autologous transplantation model. Differentiation capacity was comparable from clones generated from different sources (fibroblasts, BMSCs or CD34+ cells), and enrichment of cultures with the cell surface MSC marker CD73 yielded a phenotypically and morphologically homogenous population of cells similar to primary rhBMSCs, which could be further differentiated toward the osteogenic lineage in vitro. rhiPSC-MSCs were next mixed with hydroxyl apatite/tricalcium phosphate (HA/TCP) ceramic particles and implanted subcutaneously at multiple sites in recipient macaques. By 8 weeks, the researchers observed bone formation in all rhiPSC-MSCs transplantation sites, with samples being histologically similar to structures derived from primary rhBMSC implantation, and no teratoma formation. Encouragingly, assessment at later time points also demonstrated bone growth without teratoma formation. 

Finally, at the immunological level, autologous implantation of undifferentiated rhiPSCs led to a large infiltration of inflammatory cells into subsequent teratomas, although this did not occur for autologous BMSC and rhiPSC-MSC-grafts, or for autologous plasma clots with no cells or RhiPSC-SCs. However, the group found no link between residual endogenous pluripotency gene expression and immunogenicity and, while inflammatory cell presence was observed at up to 30 weeks in teratomas, no significant tissue damage was observed.

In summary; testing the therapeutic relevance of iPSCs in the rhesus macaque, a model that is closer to human, has demonstrated that iPSCs are clinically relevant. iPSC-derived cells safely allowed generation of bone and, importantly, the authors observed that the risk that iPSCs carry in terms of generating tumors after transplantation may be considerably less than studies in mice indicate. Even so, improved differentiation methods and methodologies to remove pluripotent cells from differentiated cultures will hopefully further reduce any risk that may exist. So, where to now for the rhesus macaque model? The team report they are moving their research forward with multiple collaborators by testing the differentiation of rhiPSCs into liver, heart and white blood cells and testing their applicability.

References

  1. Seok, J., et al., Genomic responses in mouse models poorly mimic human inflammatory diseases. Proc Natl Acad Sci U S A, 2013. 110(9): p. 3507-12.
  2. Liu, H., et al., Generation of induced pluripotent stem cells from adult rhesus monkey fibroblasts. Cell Stem Cell, 2008. 3(6): p. 587-90.
  3. Hong, S.G., et al., Path to the Clinic: Assessment of iPSC-Based Cell Therapies In Vivo in a Nonhuman Primate Model. Cell Rep, 2014. 7(4): p. 1298-309.