Glucocorticoid-induced TNF receptor family-related protein ligand is requisite for optimal functioning of regulatory CD4+ T cells

Glucocorticoid-induced tumor necrosis factor receptor family-related protein (TNFRSF18, CD357) is constitutively expressed on regulatory T cells (Tregs) and is inducible on effector T cells. In this report, we examine the role of glucocorticoid-induced TNF receptor family related protein ligand (GITR-L), which is expressed by antigen presenting cells, on the development and expansion of Tregs. We found that GITR-L is dispensable for the development of naturally occurring FoxP3(+) Treg cells in the thymus. However, the expansion of Treg in GITR-L-/- mice is impaired after injection of the dendritic cells (DCs) inducing factor Flt3 ligand. Furthermore, DCs from the liver of GITR-L-/- mice were less efficient in inducing proliferation of antigen specific Treg cells in vitro than the same cells from WT littermates. Upon gene transfer of ovalbumin into hepatocytes of GITR-L(-/-)FoxP3 (GFP) reporter mice using adeno-associated virus (AAV8-OVA) the number of antigen-specific Treg in liver and spleen is reduced. The reduced number of Tregs resulted in an increase in the number of ovalbumin specific CD8(+) T effector cells. This is highly significant because proliferation of antigen-specific CD8(+) cells itself is dependent on the presence of GITR-L, as shown by in vitro experiments and by adoptive transfers into GITR-L / Rag / and Rag / mice that had received AAV8-OVA. Surprisingly, administering alpha CD3 significantly reduced the numbers of FoxP3(+) Treg cells in the liver and spleen of GITR-L-/- but not WT mice. Because soluble Fc-GITR-L partially rescues alpha CD3 induced in vitro depletion of the CD103(+) subset of FoxP3(+)CD4(+) Treg cells, we conclude that expression of GITR-L by antigen presenting cells is requisite for optimal Treg mediated regulation of immune responses including those in response during gene transfer.