Cell Type Used in Clinical Trial: Rationale

The available pharmacological therapies in Parkinson’s disease (PD) do not rescue the nigrostriatal system, do not deliver dopaminergic (DA) specifically to the brain regions needed, do not mimic the normal release of DA, lose efficacy over time, and are often limited by distressing side effects, including hallucinations and troublesome dyskinesia. Cell replacement therapies are a promising avenue for treatment of PD and other neurodegenerative disorders. The first studies investigating the effects of human fetal ventral mesencephalic (hfVM) tissue in PD patients in the late 1980s to early 1990s yielded promising results [1–3]. However this approach demonstrated some limitations. For instance, a number of subjects were found to suffer from graft-induced dyskinesias (GID) [4,5].

Following the early encouraging reports of fetal transplants, other potential avenues have been explored. These include the differentiated derivatives from human pluripotent stem cell sources such as human embryonic stem cells (hESCs), induced pluripotent stem cells (iPSCs), and human parthenogenetic stem cells (hpSCs). Human pluripotent stem cells have the ability to proliferate indefinitely and differentiate into all three lineages, providing potential advantages and limitations.

We are transplanting ISC-hpNSC, which are neural stem cells (NSCs) derived from hpSCs. hpSCs are derived from the chemical activation of unfertilized oocytes [6,7]. Differentiated derivatives from parthenogenetic stem cells have been found to be safe and effective in various disease models by multiple laboratories around the world [8–13]. In 2014 the US Food and Drug Administration (FDA) cleared the hpSC line used to derive ISC-hpNSC for investigational clinical use. hpSCs have a number of potential advantages compared to other sources of cellular replacement. hpSCs bypass the ethical concerns associated with hfVM tissue or hESCs because no fetus or viable embryo is used in their derivation [6,7].

The hpSCs used in this study were derived from a consenting young donor’s oocyte. Derivation of iPSCs from older donors may pose the additional risk of acquiring cancer causing mutations [14,15]. HLA-matched iPSCs do not necessarily confer improvements in DA neuron graft survival (see Fig. 8n–p in [16]). Parthenogenetically derived stem cells may have other advantages: a recent study published in Cell Stem Cell showed that hpSCs have lower number of de novo coding mutations than iPSCs [17], a potential risk factor for tumorigenicity. Overall, hpSCs have comparable therapeutic potential and risks to other pluripotent stem cells. In this study we explain why we chose to differentiate hpSCs into NSCs to treat PD.

NSCs are self-renewing multipotent cells that generate the neurons and glia of the nervous system. NSCs were shown to be effective in treating PD models over a decade ago by Snyder, Redmond and colleagues [18–22]. Neural stem/progenitor cells have also been investigated in PD by many other laboratories [23–35]. In contrast to DA neurons, ISC-hpNSC have multiple mechanisms of action, including neurotrophic support, neuroregeneration, and immunomodulation [36]. ISC-hpNSC secrete various neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF), which have been shown to increase the survival of DA neurons [37–40]. ISC-hpNSC also increase the concentration of these neurotrophic factors in vivo [39].

ISC-hpNSC have been shown to efficiently differentiate in vitro into DA neurons that release dopamine, fire spontaneous action potentials, and express DA markers [41]. Together with our scientific collaborators, we have been able to demonstrate that ISC-hpNSC engraft and differentiate into tyrosine hydroxylase⁺ (TH⁺) DA neurons in rodent and nonhuman primate models of PD [13,39]. Although between 1% and 2% of the engrafted ISC-hpNSC differentiate into TH⁺ DA neurons in vivo, sufficient numbers can be implanted to allow improvements in motor symptoms. Assuming a 10% survival of the implanted cells, patients receiving 30–70 million ISC-hpNSC would have approximately 30,000–140,000 grafted TH⁺ DA neurons. Postmortem brain analyses from PD patients who received hfVM tissue have estimated that only 30,000–100,000 grafted TH⁺ DA neurons are necessary for long-term symptomatic relief [42–45].

Most of the recovery observed in striatal DA neuron innervation is host derived and is due to the neuroprotection and neurotrophic support provided by ISC-hpNSC, where the majority of the engrafted cells remain as NSCs that undergo growth arrest and become quiescent and a small percentage differentiates into glia and neurons [13,39]. There were no signs of uncontrolled proliferation in any of the ISC-hpNSC transplanted animals.

The potential immunogenicity of the engrafted ISC-hpNSC may be low. Human parthenogenetic derived NSCs express HLA-G and show unique resistance to NK cell-mediated killing [46]. The binding of HLA-G to its receptors leads to the destruction of T and NK cells [47]. In general, graft failure in stem cell transplantation increases with increasing numbers of mismatches in the Host versus Graft direction. However, autologous or HLA-matched transplantation may not be strictly necessary based on observations that hfVM grafts, derived from multiple tissue sources, can show stable integration and long-term persistence—confirmed on functional imaging, as well as postmortem analyses—without use of concomitant immunosuppression [5,48].

The long-term survival of the grafts might be attributed to the fact that hfVM cells express HLA-G for immune tolerance during pregnancy [49]. Placental HLA-G proteins facilitate semiallogeneic pregnancy by inhibiting maternal immune responses to foreign antigens [50]. NSCs can further decrease neuroinflammation by reducing the expression of tumor necrosis factor-α⁺ (TNF-α⁺) and major histocompatibility complex II⁺ (MHC II⁺) activated inflammatory cells, which have been shown to be effective in multiple neurological disease models [51–54].

Other potential mechanisms of action for NSCs include the observation that RNAseq analysis of brain tissues showed that transplantation of ISC-hpNSC in nonhuman primates induces the expression of genes and pathways downregulated in PD [13]. A recent study also showed that human neural progenitor cells reduce α-synuclein oligomers and rescue cognitive and motor deficits in a Dementia with Lewy bodies mouse model [55]. Normalization of α-synuclein aggregation was also observed after human NSC transplantation in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned nonhuman primates [18].

Transplantation of neural precursor cells induces the proliferation, neurogenesis, and migration to the graft site of host subventricular zone neural precursors that lead to the significant preservation of striatal TH expression and substantia nigra TH cell number in 6-hydroxydopamine (6-OHDA)-lesioned rats [56]. NSCs harness endogenous repair mechanisms to promote tissue regeneration and have the intrinsic capacity to rescue dysfunctional DA neurons in the brains of aged mice [22,57]. The combined actions of neurotrophic support, neuroregeneration, immunomodulation, downregulation of PD associated genes, and recruitment of host neural precursors promote behavioral recovery and increase striatal DA concentration, striatal DA neuron fiber innervation, and nigral DA neuron number after ISC-hpNSC transplantation in MPTP-lesioned nonhuman primates [13].