Multiple Sclerosis (MS) is a chronic, autoimmune disease of the central nervous system (CNS), for which there is no cure. Deficits in neurological function occur as a result of widespread demyelination and axonal loss caused by infiltrating immune cells, mainly lymphocytes (T cells) and myeloid cells. Regulatory T cells (Tregs), characterized by expression of CD4 and FoxP3, are a specialized subset of T cells that are essential for the maintenance of T cell homeostasis and prevention of autoimmunity. Thus, dysregulation of Tregs has been linked to MS disease pathogenesis. Current therapies for MS are only focused on immunosuppression and fail to address the need for sustained remyelination and axonal protection. Transplantation of neural stem cells (NSCs) or neural precursor/ progenitor cells (NPCs) is a promising therapeutic strategy to treat conditions and diseases affecting white matter integrity, such as MS. NSCs and NPCs are a mixed population of self-renewing stem cells and cells that are poised to differentiate into the three types of neural cells found in the CNS; neurons, astrocytes, and myelin-producing oligodendrocytes. Early studies employing animal models of demyelination have demonstrated that surgical implantation of NSCs into the CNS reduces neuroinflammation and enhances remyelination. However, the mechanism by which these cells promote tissue repair remains poorly understood.
The focus of this dissertation is to elucidate the mechanisms by which NSCs promote remyelination following transplantation in murine models of MS. Viral infection has long been considered a potential triggering mechanism involved in immune-mediated demyelination, therefore viral models of demyelination are relevant for modeling human demyelinating diseases, such as MS. Intracranial inoculation of susceptible mice with the neurotropic JHM variant of mouse hepatitis virus (JHMV) leads to immune-mediated demyelination and axonopathy. We have demonstrated that intraspinal injection of syngeneic mouse NSCs into the CNS of JHMV-infected mice results in remyelination associated with axonal sparing due to cell replacement. In addition, JHMV-infected mice that received transplants of xenogeneic human NPCs also displayed sustained remyelination and decreased neuroinflammation, which was associated with an increase in CD4+CD25+FoxP3+Tregs. Importantly, recovery was not a result of cell replacement, hNPCs underwent xenograft rejection, rather due to immune modulation (Chapter 1). Chapter 1 is a re-print of a review article published in Developmental Dynamics that summarizes our recent findings transplanting mNPCs and hNPCs into the spinal cord of JHMV infected mice, and serves as an introduction to the dissertation. Immunization of mice with myelin oligodendrocyte glycoprotein is the prototypic murine model of MS, experimental autoimmune encephalomyelitis (EAE), which is characterized by ascending immune-mediated inflammation and demyelination of axonal tracks, as well as neuronal death resulting in motor deficits. In support of our previous findings, engraftment of syngeneic mNSCs facilitated remyelination in EAE mice with no effect upon the immune system. Transplantation of xenogeneic hNSCs into EAE mice resulted in dampened neuroinflammation and remyelination due to expansion of neural antigen-specific Tregs. Ablation of Tregs abrogated remyelination. Additionally, hNSCs promoted expansion of neural antigen-specific Tregs (hNSC-Tregs) in vitro, from the ‘exTreg’ pool, a population of thymically derived Tregs that lost FoxP3 expression in the periphery. hNSCs reinvigorated FoxP3 expression in ‘exTregs’ following co-culture with hNSCs, supporting the hypothesis that self-peptide/MHC is important for maintenance of FoxP3 expression and Treg function. hNSC-Tregs display a unique gene expression signature, upregulating molecules known to be involved in suppression of neuroinflammation, Dickkopf 3, and Transglutaminase 2 which facilitates oligodendrocyte progenitor cell differentiation to mature, myelin producing oligodendrocytes (Chapter 2). Utilizing two-photon microscopy of spinal cord explants, we discovered that Tregs localize to the sites of hNSC transplantion and interact with cells, most likely oligodendrocytes, that produce a component of the myelin sheath, proteolipid protein (Chapter 3). These findings support the concept that Tregs not only function in suppressing inflammatory responses, but also support their role as potentiators of tissue repair. Therefore, there has been an increasing interest utilizing Tregs as immunotherapies to treat autoimmune diseases. Therapies to promote expansion of Tregs and adoptive T cell therapy are currently under investigation in clinical trials for autoimmune diseases including, graft versus host disease and type I diabetes myelitis, although the ability of Tregs to modulate tissue repair is not well studied in the clinic (Chapter 4). Thus, hNSC expanded Tregs may provide new therapeutic strategies for treatment of autoimmune, neurodegenerative diseases, such as MS.