Prevention of Relapses in Mice With Ongoing EAE via Combined Allogeneic Bone Marrow and Neural Stem Cell Transplantation

Abstract

Multilineage mixed hematopoietic chimerism induces transplantation tolerance to allogeneic solid organ transplants in both animal models and humans. Yet we still have an imperfect understanding of the underlying mechanism and potential applications. I hypothesized that mixed chimerism could restore self-tolerance in the context of experimental autoimmune encephalomyelitis (EAE). To test this tolerizing strategy I designed a model whereby mixed chimerism could be induced in mice with ongoing EAE and which was amenable to downstream perturbations. I divided my investigation into two main studies; (1) an examination of induction of mixed chimerism with a subsequent neural stem cell transplant (Chapter 2), and (2) an examination of extracellular vesicles (EVs) replacing bone marrow in the aforementioned model (Chapter 3). In the first study, I found that mixed chimerism alone was insufficient to prevent, and could only delay, further EAE relapses. However, the addition of a donor-matched neural stem cell transplant could induce self-tolerance. Specifically, I found that lymphocytes from mice receiving this treatment were not responsive to restimulation with the relevant self-antigen in vitro. However, the mice themselves still possessed memory CD4 T cells, which could demonstrate a memory response upon reimmunization. In this manner I showed that deletion of autoreactive T cells alone cannot be responsible for the restoration of self-tolerance and it likely relies upon a regulatory effect. In the second study, I examined whether EVs derived from allogeneic bone marrow could replace bone marrow, in order to delay relapses in mice with ongoing EAE. I found that these EVs could in fact delay relapses in a manner consistent with results obtained using bone marrow. Additionally, while this effect was found to be dose-independent above a certain threshold, the effect could be prolonged by repeated injections at lower doses. Examining tolerance in this model has enabled the discovery of a novel mechanism of regulation by mixed chimerism as well as described a logical pathway by which these results could be translated into the clinic. This extension of our scientific understanding should advance further explorations of the mechanism of mixed chimerism and its application to human autoimmunity.