Role of human leukocyte antigen-G in the induction of adaptive type 1 regulatory T cells.

Adaptive type 1 regulatory T (Tr1) cells are suppressor cells characterized by the production of interleukin (IL)-10 in the absence of IL-4. IL-10 is essential not only for suppression of effector cells by Tr1 cells, but also for their differentiation in vitro and in vivo. However, little is known on the molecular mechanisms underneath the IL-10-mediated induction of Tr1 cells. Human Leukocyte Antigen (HLA)-G, a non-classical HLA class I molecule, has both direct inhibitory effects on natural killer cells, dendritic cells (DC), and T cells and long-term tolerogenic indirect effects by inducing regulatory T (Tr) cells. In the present review, we discuss current findings on Tr-cell induction by the different isoforms of HLA-G, focusing on the relationship among HLA-G, its ligands, and IL-10. We recently described a subset of human DC, termed DC-10, that express high levels of HLA-G and ILT4, secrete high amounts of IL-10, and induce allospecific Tr1 cells in vitro via an IL-10-dependent ILT4/HLA-G pathway. IL-10, HLA-G, and ILT4 may also be involved in Tr1-cell induction in vivo. Overall, these data demonstrate that cross-regulation between IL-10 and HLA-G may be instrumental for Tr1-cell induction and tolerance.