Stem Cells: Simpler to Make, Easier on the Immune System

Converting somatic cells directly into neurons might have just gotten easier, while induced pluripotent stem cells (iPSCs) may be more immune friendly than previously thought, according to two papers. Xiang-Dong Fu, University of California, San Diego, and colleagues found that suppression of a single gene converts a variety of cells, including fibroblasts, directly into neurons. Published in the January 17 Cell, this procedure represents one of the simplest methods of generating neurons to date. Since it requires no foreign DNA, it may move these cells one step closer to the clinic. On that front, scientists led by Masumi Abe, National Institute of Radiological Sciences, Chiba, Japan, reported in the January 9 Nature that iPSC-derived cells did not irk the immune system when transplanted into genetically identical mice. Contrary to a previous report, the result hints that tissue made from iPSCs may be suitable for therapeutic transplantation. However, other scientists remain unconvinced.

Fu and colleagues found a simple way to convert differentiated cells directly into neural lineages. Researchers did this previously by adding a cocktail of reprogramming factors to fibroblasts (see ARF related news story on Pang et al., 2011; ARF related news story on Caiazzo et al., 2011). The new method requires taking away just one component.

While studying the basic function of the pyrimidine-tract-binding (PTB) protein, a known RNA splicing regulator, Fu and colleagues found that silencing it caused non-neuronal cell types to sprout neurites and express neuronal transcription factors such as Ascl1, Brn2, Myt1l, Zic1, and Olig2. This phenotype pointed to a previously unknown function for PTB in repressing neuronal differentiation. The repression is lifted during neuronal development, when microRNA-124 represses PTB, unleashing a host of neuronal genes. The upshot is that silencing PTB drives non-neuronal cells to a neuronal fate. “We show for the first time that you can subtract something from the cell, mimic a natural developmental program, and rewire the cell into a neuronal lineage,” Fu told Alzforum. However, he added, more cofactors are required to commit the cell to its final cell type—for example, dopaminergic or striatal neurons, he said.

Fu is now working to figure out if this method might be applied in vivo. One idea is to convert endogenous brain support cells into neurons and thereby replace damaged tissue.

“This is an exciting paper that represents an important advance in the trans-differentiation field,” Zhiping Pang, Child Health Institute of New Jersey in New Brunswick, told Alzforum in an e-mail. “It offers an alternative and simpler method in generating neuronal cells from other cell lineages.”

Cell reprogramming technology, including the generation of induced pluripotent stem cells, had raised hopes that scientists could one day replace dying cells with new ones derived from a patient’s own healthy tissue. Such grafts would be recognized by the immune system as “self” and escape rejection. Avoiding the use of embryonic tissue or embryonic stem (ES) cells brought an added bonus. However, iPSC technology suffered a setback in 2011, when researchers led by Yang Xu, University of California, San Diego, reported that immunogenicity emerges during the reprogramming that creates iPSCs (see ARF related news story). After injecting iPSCs into mice, teratomas formed that attracted far more T cells than cancers growing from transplanted embryonic stem cells, suggesting that iPSCs were more immunogenic. Now, Abe and colleagues have explored the issue further by assessing immunogenicity of fully differentiated cells (rather than iPSCs), which they said would be a more clinically relevant paradigm.

To generate fully differentiated cells, first author Ryoko Araki and colleagues slipped iPSCs or ES cells into mouse embryos at the eight-cell stage and allowed them to develop into chimeric mice. Once the chimeras were fully grown, the researchers removed skin or bone marrow cells and grafted them onto or injected them into genetically identical mice. All transplants survived for more than five months, and the researchers observed no abnormal T cell infiltration into them.

“An important finding of this study is that the immunogenicity of iPSC-derived tissues is indistinguishable from that of ES cell-derived tissues,” wrote the paper authors. However, further research is needed to say whether immunogenicity still plagues iPSCs differentiated in vitro, which would be more clinically relevant, they added. Other researchers agreed. Xu, who was not involved in this research, noted that since these cells were differentiated in vivo from early development, the mouse immune system could have selectively eliminated the most immunogenic cells, leaving only the immune-compatible ones. In fact, when Araki and colleagues did differentiate iPSCs into cardiomyocytes in vitro and transplant them, T cells infiltrated the injection sites, Xu pointed out. “These iPSCs could be immunogenic, especially when they are differentiated in vitro, so their use for cell therapy is still an open question,” he told Alzforum. He added that some cells derived from iPSCs might be less immunogenic than others. “We just have to find the right lineage of cells that develop normally and cause less immunogenicity,” he said.

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