The role of T helper 17 (Th17) and regulatory T cells (Treg) in human organ transplantation and autoimmune disease

Abstract

Uncommitted (naive) murine CD4+ T helper cells (Thp) can be induced to differentiate towards T helper 1 (Th1), Th2, Th17 and regulatory (Treg) phenotypes according to the local cytokine milieu. This can be demonstrated most readily both in vitro and in vivo in murine CD4+ T cells. The presence of interleukin (IL)-12 [signalling through signal transduction and activator of transcription (STAT)-4] skews towards Th1, IL-4 (signalling through STAT-6) towards Th2, transforming growth factor (TGF)-beta towards Treg and IL-6 and TGF-beta towards Th17. The committed cells are characterized by expression of specific transcription factors, T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for Tregs and RORgammat for Th17 cells. Recently, it has been demonstrated that the skewing of murine Thp towards Th17 and Treg is mutually exclusive. Although human Thp can also be skewed towards Th1 and Th2 phenotypes there is as yet no direct evidence for the existence of discrete Th17 cells in humans nor of mutually antagonistic development of Th17 cells and Tregs. There is considerable evidence, however, both in humans and in mice for the importance of interferon (IFN)-gamma and IL-17 in the development and progression of inflammatory and autoimmune diseases (AD). Unexpectedly, some models of autoimmunity thought traditionally to be solely Th1-dependent have been demonstrated subsequently to have a non-redundant requirement for Th17 cells, notably experimental allergic encephalomyelitis and collagen-induced arthritis. In contrast, Tregs have anti-inflammatory properties and can cause quiescence of autoimmune diseases and prolongation of transplant function. As a result, it can be proposed that skewing of responses towards Th17 or Th1 and away from Treg may be responsible for the development and/or progression of AD or acute transplant rejection in humans. Blocking critical cytokines in vivo, notably IL-6, may result in a shift from a Th17 towards a regulatory phenotype and induce quiescence of AD or prevent transplant rejection. In this paper we review Th17/IL-17 and Treg biology and expand on this hypothesis.

Fig. 1

Proinflammatory effects of interleukin-17.

Fig. 2

Model of mouse helper T cell (Th) commitment to Th1, Th17 and T regulatory cell (T_reg) phenotypes following encounter with antigen. Production of transforming growth factor (TGF)-β by naturally occurring T_regs leads to lineage commitment of precursor helper T cells (Thp) towards T_reg phenotypes. Stimulation of dendritic cells (DC) by microbial antigens causes production of interleukin (IL)-6, IL-23 and/or IL-12. Predominant production of IL-12 promotes commitment of Thp to a Th1 phenotype while IL-6, in combination with T_reg-derived TGF-β promotes skewing of Thp towards a Th17 phenotype. IL-23 produced by DCs causes proliferation and cytokine production by Th17 cells.

Fig. 3

T helper cell commitment towards specific lineages in mice. T helper cell precursors (Thp) can be skewed towards mutually exclusive Th1, Th2, Th17 and T regulatory cell (T_reg) phenotypes on the basis of the cytokine environment. Presence of interleukin (IL)-12 promotes skewing towards Th1 commitment by signalling through signal transduction and activator of transcription (STAT)-4. Th1 cells are characterized by expression of T-bet and produce interferon-γ and tumour necrosis factor-α. Th2 cell commitment is promoted by IL-4 via STAT-6 signalling. Th2 committed cells are characterized by expression of GATA-3. Development of T_reg and Th17 phenotypes both require the presence of transforming growth factor-β but the presence of IL-6 preferentially skews the response towards a Th17 phenotype. T_regs are characterized in mice by expression of Foxp3.