Regulatory T cell plasticity and its role in the rejection of pancreatic islet allograft tissue
Aker, Jonathan Edward
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The healthy immune system is a delicate and precisely orchestrated balance between activation and suppression. It is well established that regulatory T cells (Tregs) have substantial immunosuppressive properties and play a pivotal role in maintaining this balance. Many autoimmune states have been characterized by disproportionately high numbers of T effector cells, and comparatively low numbers of regulatory T cells (Hori et al., 2003; Sakaguchi et al., 1995; Choileain et al., 2006). Furthermore, mouse models in which regulatory T cells are removed or rendered ineffective show rapid development of autoimmunity. It is therefore hypothesized that regulatory T cells are essential to the acquisition and maintenance of self-tolerance. Type 1 diabetes is an increasingly common autoimmune condition, with 30,000 new diagnoses each year (JDRF Fact Sheet). Pancreatic islet transplantation holds great promise as a potential cure for this difficult disease; however human trials have had limited success. Attempts to promote self-tolerance or maintain a physical barrier to the transplanted islets have largely failed (Groot et al., 2004). Because of this, insulin dependence normally resumes fiver years post-operation. The deleterious effects of long-term immunosuppression to promote extended islet survival are considered too great to justify this treatment. Because of their important role in promoting self-tolerance, many immunologists believe regulatory T cells are the key to developing tolerance of islet allograft tissue. Rapamycin and anti-CD154 are immunoregulatory treatments that specifically inhibit the activation of T effector cells and promote the growth of regulatory T cell populations. As regulatory T cell numbers increase, self-tolerance is established and the need for immunosuppressant drugs is eliminated. Unfortunately, treatments such as anti-CD154 and rapamycin have had limited success due to the ability of toll-like receptor (TLR) pathways to bypass such activation blockades. TLR stimulation results in a potent and direct activation that acts to bolster the immune response. This TLR activation results in the release of inflammatory cytokines, which render regulatory T cells unstable. Regulatory T cells have been shown to adopt effector phenotypes in such environments and may have pathogenic potential. This study aims to elucidate aspects of Treg plasticity that result from TLR activation. In vitro models were used to demonstrate how TLR agonists change Treg phenotypic expression. Our findings indicate that the presence of lipopolysaccharides (LPS) has a relatively significant effect on regulatory T cell phenotypes. Specifically, our findings indicate that LPS causes increased GATA3 expression in Tregs, promoting differentiation to a TH2 phenotype (p= 0.0543). Regulatory T cells were also examined for the expression of RORγt and Tbet transcription factors. Neither transcription factor was significantly expressed, indicating the absence of TH17 and TH1 phenotypes, respectively. It is also worth noting that stability of the foxp3 transcript appeared to be greater in cells treated with LPS, than in those without (p= 0.0009). In addition, this study utilized an in vivo model for tracking regulatory T cell changes after pancreatic islet transplantation. Diabetic reporter mice received pancreatic islet transplants, as well as TLR agonist to induce allograft rejection. Mice were treated with rapamycin, anti-CD154 and TLR agonist. After 12 days, regulatory and ex-regulatory T cells were harvested from the transplanted area and analyzed. This experiment is still in progress and results have yet to be determined. This study establishes proof of concept of an effective system for the study of regulatory T cell plasticity. Additional investigation must be done in order to more thoroughly understand these important cells. This study is not complete, but our progress thus far is a strong foundation for further experimentation.
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