The T helper 17-regulatory T cell axis in transplant rejection and tolerance

Purpose of review Regulatory T cells (Tregs) are potential therapeutic tools in averting transplant rejection and promoting lifelong tolerance. However, Tregs can be subverted by inflammatory conditions, resulting in a T helper 17 (Th17) cell response. This review looks at the relationship between Tregs and Th17 cells. Recent findings Both naturally occurring and transforming growth factor-beta-induced Tregs can be converted into Th17 cells in the presence of inflammatory cytokines. Transforming growth factor-beta upregulates the Treg transcription factor, forkhead box P3, as well as the Th17 transcription factor, retinoic acid receptor-related orphan receptor-gamma. However, forkhead box P3 binds to retinoic acid receptor-related orphan receptor gamma, inhibiting its promotion of IL-17 gene transcription. Inflammatory cytokines can disrupt this interaction through the inhibition of forkhead box P3 expression by signal transducer and activator of transcription 3. Summary The plasticity of the Treg population during inflammation presents a challenge to the use of Tregs as a therapeutic tool in solid organ transplantation. Investigations into the Th17-Treg axis have identified a number of potential pharmacological targets to avoid the risk of conversion to Th17 cells, but further work must be done before we can separate the benefits of Treg therapy from the hazards of the Th17 response.