Stem Cell Research: Mice with MS-like Condition Regain Mobility After Transplantation with Human Stem Cells
Researchers in California experienced a series of “happy accidents” while studying the common problem of stem cell rejection. To conduct their experiments, scientists injected human neural precursor cells (hNPCs) derived from human embryonic stem cells (hESCs) into the spinal cords of mice with an MS-like disease caused by immunization with a virus 1. Mice infected with the neurotropic JHM strain of mouse hepatitis virus (JHMV) experience a condition similar to multiple sclerosis that is characterized by demyelination, neuroinflammation, and hind-limb paralysis. There are several models of MS-like disease that can be induced by injecting mice with viruses or antigens that trigger inflammatory demyelinating disease or by exposing them to specific neurotoxins such as the copper-chelator cuprizone 2.
Stem cell rejection following transplantation has been a particular challenge to researchers when using subjects with fully intact immune systems. In MS research, the pursuit of a solution has led to the use of powerful, high-dose chemotherapeutic drugs to drastically lower the patient’s immune system before stem cell transplantation as in the ongoing HALT-MS (High-Dose Immunosuppression and Autologous Transplantation for Multiple Sclerosis) Study conducted by NIH 3. But drastic immunosuppression carries serious risks and is not practical for all patients.
One of the happy accidents was the result of a graduate student at The Scripps Research Institute (TSRI), Ron Coleman, diverting from a common technique used to coax human stem cells into neural precursor cells (NPCs). Neural precursor cells can differentiate into neurons and other nervous system cells, such as oligodendrocytes, which encourage neural repair and regeneration. Partway through the coaxing process, Coleman transferred the developing cells to another Petri dish.
“I wanted the cells to all have similar properties, and they looked really different when I didn’t transfer them,” said Coleman, who was motivated to study MS after his mother died from the disease. This step, called “passaging,” proved key. “It turns out that passaging alters the types of proteins that the cells express,” he said 4.
After injection into the spines of JHMV-infected mice, the stems cells were completely rejected by the immune system within 8 days, as expected. However, unexpectedly, the hNPC-transplanted mice experienced improved motor skills and a reduction in neuroinflammation and demyelination about two weeks after the injections.
Tom Lane, a University of California, Irvine immunologist who now works at the University of Utah recalls receiving a call from his postdoctoral fellow, Lu Chen, who said, “These mice are walking.” Lane tells The Salt Lake Tribune that he had doubts, but went to the lab to see for himself. "Sure enough, there were groups of mice that had gone from being paralyzed to walking around the cage," Lane said 5.
In a follow-up study co-funded by the National MS Society and the California Institute for Regenerative Medicine, scientists at TSRI injected 96 mice into the spine with human embryonic stem cells in immunocompetent mice (with fully functioning immune systems) who had virally-induced MS. The presence of the injected stem cells diminished over days 0-8 post-transplantation, but during that short time period they apparently triggered certain cells in the immune system to reduce inflammation, decrease demyelination, and enhance remyelination over a longer, sustained period of time. Results are reported online ahead of print in the journal Stem Cell Reports 6.
Within ten to 14 days, symptoms were partially reversed in 66 (73%) of the mice injected with hESC-derived hNPCs, but only 9 of the 63 mice (14%) transplanted with the control (human fibroblasts) improved. Improvements persisted for up to 6 months post-transplantation. The use of living cells and intraspinal injection were necessary for clinical recovery as researchers found that transplantation with dead cells or delivery of cells via intravenous or intraperitoneal injection did not result in improved clinical outcome.
The reduced neuroinflammation seen here correlated with an increased number of CD4+CD25+FOXP3+ regulatory T cells (Tregs), a type of white blood cell, within the spinal cords, suggesting that the transplanted hNPCs may have promoted recovery through Treg-mediated mechanisms. When hNPC-transplanted mice were treated with an anti-CD25 monoclonal antibody, which efficiently blocks Treg activity, functional improvement was inhibited and neuroinflammation increased. When hNPCs were cocultured with activated T cells, the proliferation of T cells was reduced and Treg numbers were increased. Researchers also discovered that hNPCs increased Treg frequency, in part, through the secretion of TGF-β1 and TGF-β2 (transforming growth factor-β), a class of proteins known to have neuroprotective qualities 7.
Tom Lane says that he wants to test the therapy in another mouse model available at the University of Utah, where he recently took a faculty position, hoping to take advantage of the university’s system for translating research into treatments. If the team can pinpoint which proteins released by the neural precursor cells are responsible for the animals’ recovery, it may be possible to devise MS treatments that don’t involve the use of human stem cells.
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