The ability to therapeutically manipulate regulatory T (Treg) cell survival/function would have far reaching implications; with the potential to limit immune pathology in autoimmune disease, allergy and transplantation; and to reduce regulation of anti-tumour responses in cancer. This study has established a method to study the survival and function of antigen specific Treg cells in vivo, adapting an existing approach in which an endogenous naïve T cell population is expanded and tracked. Multiple immunisation of antigen, and an agonistic anti-DR3 antibody were used to ensure a sufficient number and proportion of Treg cells could be expanded. Further to this, an assay for investigating Treg cell function in vivo was applied to this system. This approach revealed that the tumour necrosis factor receptors OX40 and CD30 may play a role in Treg function, as well as expansion. Unexpectedly, these data also revealed that in the absence of OX40 there is a gross defect in the function of CD4 T cells. A regulatory role of group 3 Innate Lymphoid cells is emerging in the literature, and in accordance with this, this study demonstrates that Treg cell expansion is grossly impaired in mice which lack RORγt, their lineage defining transcription factor.