The Promise of Immortalized Neural Stem Cells in CNS Cell-Based Therapies
Cell Therapy Candidates for Neurological Disease
Cell replacement therapy (CRT) and cell-based therapy (CBT) are promising strategies for treating several neurological diseases, including Parkinson’s disease (PD), Huntington’s disease (HD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Alzheimer’s disease (AD), and malignant gliomas (GBM). Four cell types are most studied as candidates for CRT and CBT in these diseases:
- Embryonic stem cells (ESCs)
- Induced pluripotent stem cells (iPSCs)
- Mesenchymal stem cells (MSCs)
- Neural stem cells (NSCs)
All of these pluripotent cell types can make different neurons and glial cells in vitro. But several obstacles stand in the way of using them in the clinic:
- Earlier studies raise concerns about whether stem cell-derived neurons or glial cells survive long-term and keep a stable identity in vivo after transplant.
- Even a highly purified population of one neuronal cell type — from ESCs, iPSCs, MSCs, or NSCs — risks containing other cell types. These could interact badly with other grafted cells or with the host central nervous system (CNS).
- Any leftover cells that didn’t fully differentiate — even a small number — raise a real risk of tumor formation in the host CNS after transplant.
- There are also practical hurdles, like producing these cells at a large, clinically approved, industrial scale.
HB1.F3: An Immortalized Neural Stem Cell Line
In a review in the Journal of Neuropathology, Seung U. Kim’s group proposed that immortalized human NSC lines are the best cell source for CBT in neurological disease. Kim’s group had already created several clonally derived immortalized human NSC lines. One of these — well characterized and now in phase II clinical trials as a glioma therapy — came from a fetal human telencephalon at 15 weeks gestation.
Researchers immortalized it using an amphotropic, replication-incompetent retroviral vector, pLCN.v-myc, which carries the v-myc oncogene. This method is safe and also avoids spontaneous differentiation, producing a non-tumorigenic, uniform NSC line called HB1.F3.
HB1.F3 cells have a normal human karyotype (46XX). They self-renew and are multipotent — they can turn into neurons, astrocytes, and oligodendrocytes, both in vivo and in vitro. They also express genes for several neurotrophic factors:
- Nerve growth factor (NGF)
- Brain-derived neurotrophic factor (BDNF)
- Neurotrophin-3 (NT-3)
- Glial-derived neurotrophic factor (GDNF)
- Ciliary neurotrophic factor (CNTF)
- Hepatocyte growth factor (HGF)
- Insulin-like growth factor (IGF)-1
- Basic fibroblast growth factor (bFGF)
- Vascular endothelial growth factor (VEGF)
This mix of factors could make HB1.F3 a strong therapeutic candidate for protecting neurons damaged by injury or disease.
HB1.F3 in Animal Models of Disease
Kim’s group reported functional improvement in a rat model of PD after transplanting HB1.F3 cells into the striatum. In another study, they saw functional recovery in an HD rat model after giving HB1.F3 cells intravascularly (IV). Their data suggests this improvement comes from BDNF secreted by HB1.F3 cells, since BDNF has already been shown to block neuronal injury in HD animal models. Another notable finding: after IV administration, HB1.F3 cells moved into the striatum and homed to the site of neuronal injury — showing they can cross the BBB freely.
Restoring Function in Alzheimer’s and ALS Models
In AD patients, low levels of acetylcholine (ACh) contribute to cognitive impairment. This drop is caused by reduced activity of choline acetyltransferase (ChAT), the enzyme that makes ACh. Kim’s group engineered HB1.F3 cells to over-express the ChAT gene (F3.ChAT) and transplanted these NSCs into the brains of AD animal models. The result: the presynaptic cholinergic system recovered function, and learning and memory were fully restored.
The group also made motor neurons from HB1.F3 cells carrying the Olig2 basic helix loop helix (bHLH) transcription factor gene, combined with sonic hedgehog (Shh) protein (F3.Olig2-Shh). They transplanted these cells into the L5 spinal cord segment of an ALS animal model, which significantly delayed disease onset and extended average survival.
Clinical Use and What’s Next
Using HB1.F3 in human trials was one of the first FDA-permitted trials in the US to use genetically modified human stem cells for malignant brain tumor CBT. These results show immortalized human NSCs work well as a source for genetic manipulation and gene transfer into the CNS, for treating several neurological disorders.
That said, autologous iPSC-derived CNS cells may end up being a more promising overall strategy for CRT — mainly because immortalized cells carry risks, and may not survive long-term after transplant. Still, each stem cell source has strengths that suit it to different disorders.
