Despite the actions of central tolerance during thymic selection, it is clear that the peripheral T cell repertoire contains significant numbers of self-reactive T cells. The immune system needs to curtail the risk of autoimmune disease by controlling the activity of these self-reactive T cells. Various mechanisms are in place to achieve this control (peripheral tolerance). Activation of CD4+ T cells requires two signals; engagement of the T cell receptor (TCR) with an appropriate peptide:MHC complex (signal 1), and the aggregate effect of multiple signals generated following ligation of costimulatory and coinhibitory molecules (signal 2). Both signals are required for the generation of a productive T cell response and both are provided by the professional antigen presenting cell, the dendritic cell (DC). T cells are fully activated upon receiving both signal 1 and 2, but are rendered tolerant when they receive only signal 1. This can be exploited therapeutically through the administration of peptides to induce tolerance in peptidereactive T cells. Administration of peptide with an adjuvant provides both signal 1 and 2, and leads to a sustained T cell response against the administered peptide (immunity). However, if the same peptide is administered in soluble form, only signal 1 is provided, leading to the establishment of T cell tolerance. The studies in this thesis explore the role of both signal 1 and signal 2 in peptide-induced T cell tolerance. Previous data from our laboratory have highlighted PD-1 and RANKL as costimulatory molecules which could play a role in peptide-induced T cell tolerance. Here we show that PD-1, an important coinhibitory molecule, plays a vital role in restraining peripheral T cell expansion under conditions leading to T cell immunity. However, in contrast to data from other studies, we demonstrate that PD-1 plays no role in the induction, establishment or maintenance of peptide-induced T cell tolerance. We show that the costimulatory receptor ligand pair RANK:RANKL plays a role in the balance between T cell tolerance and immunity; as administration of anti-RANKL was seen to potentiate both tolerance and immunity. We also explored the effect of altering the affinity of a peptide for MHC on the induction of peptide tolerance. We demonstrate that use of a peptide with a high-affinity for MHC induces tolerance via a novel, non-deletional mechanism of peptide-tolerance induction. Importantly, we show that the high-affinity peptide can form peptide- MHC complexes which persist in a biologically relevant form for fourteen days following peptide administration. We suggest that this leads to chronic stimulation of peptide-reactive T cells which promotes acquisition of a novel tolerant phenotype. Collectively the work described in this thesis demonstrates the important roles both signal 1 and 2 play in therapeutic-tolerance induction and how the qualitative and quantitative alteration of these signals can alter T cell fate and/or responsiveness.