Investigating interplay between human regulatory T cells and oligodendrocyte lineage cells

Abstract

In multiple sclerosis (MS), loss of myelin or demyelination, can be initially restored by oligodendrocyte progenitor cells (OPCs) that migrate to, and proliferate in the site of injury, differentiate into mature oligodendrocytes (OLs) and generate new myelin sheaths, a process termed remyelination. The signals that influence this process in the central nervous system (CNS) of MS patients are still not fully understood, however, recent studies have uncovered an important role for the immune system in promoting remyelination in mice via an enhancement of OPC differentiation. These studies now warrant translational research in humans to investigate the full regenerative potential of immune cells in CNS repair.

Based on findings from the mouse that depletion of regulatory T cells (Treg) impairs CNS remyelination and that soluble factors secreted from murine Treg promote OPC differentiation, this PhD Thesis hypothesized that human Treg recapitulate the effect of murine Treg in promoting the differentiation of human OPCs in vitro. To this end, human Treg subsets, including in vitro-induced (iTreg) and naturally-occurring Treg (nTreg) cells were activated in parallel with three non-Treg control populations, total CD4+ T cells that contained a low frequency of nTreg, FOXP3+ T cells that exhibited no suppressive function and whole peripheral blood mononuclear cells (PBMCs) that consisted both of total CD4+ and CD8+ T cells. Conditioned media (CM) from these activated cultures were next harvested and added on human induced pluripotent stem cell (iPSC)-derived mixed neuroglial cells to examine changes in OPC differentiation and proliferation by immunocytochemistry. Soluble factors secreted from activated T cell subsets, all had a trophic influence on OPCs resulting in reduced numbers of O4+ and MBP+ oligodendrocyte lineage cells. This trophic effect was greatly lessened by nTreg, which had no impact on OPC differentiation, but could be clustered into two subgroups of donors, one that induced a high number of mature MBP+ OLs, and one that had no influence on the process. When in culture with glia, activated nTreg cells were permissive to OPC differentiation. When in culture with naive T cells, activated nTreg cells suppressed the production of factors that inhibited OPC differentiation as shown by the treatment of glia with suppression assay-CM. Multivariate analyses of T cell secretion data confirmed the relatedness of iTreg and nTreg, and differentiated these cells from other non-Treg populations. In addition, iTreg cells were found to be enriched for proteins that hinder OPC differentiation, whereas nTreg cells exhibited low secretion of such proteins, and variable secretion of proteins with the capacity to promote this process. These findings establish the phenotypic and functional integrity of regulatory features as both being critical determinants of OPC differentiation.

The in vitro studies on the interaction of human T cell subsets with oligodendrocytes presented in this Thesis, advance our knowledge about the influence of human Treg on OPC proliferation and differentiation. These are critical processes required for the efficient regeneration of the CNS myelin sheath, which occurs at significantly slower rates in patients with MS. The results of this Thesis demonstrate that human Treg are permissive to OPC differentiation and protective to this process via the suppression of effector T cell responses. This new knowledge could prove valuable for therapies that aim to enhance Treg function in MS.

Date of Award	Jul 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Queen's University Belfast
Supervisor	Siddharthan Chandran (Supervisor), Denise Fitzgerald (Supervisor) & Andriana Margariti (Supervisor)