Tolerance induction by dendritic cells (DCs) is, in part, mediated by the activation of regulatory T cells (Tregs). We have previously shown in vitro that human DCs treated with glucocorticoids (GCs), IL-10, or TGF-β upregulate the GC-Induced Leucine Zipper protein (GILZ). GILZ overexpression promotes DC differentiation into regulatory cells that generate IL-10–producing Ag-specific Tregs. To investigate whether these observations extend in vivo, we have generated CD11c-GILZhi transgenic mice. DCs from these mice constitutively overexpress GILZ to levels observed in GC-treated wild-type DCs. In this article, we establish that GILZhi DCs display an accumulation of Foxp3+ Tregs in the spleens of young CD11c-GILZhi mice. In addition, we show that GILZhi DCs strongly increase the Treg pool in central and peripheral lymphoid organs of aged animals. Upon adoptive transfer to wild-type recipient mice, OVA-loaded GILZhi bone marrow–derived DCs induce a reduced activation and proliferation of OVA-specific T cells as compared with control bone marrow–derived DCs, associated with an expansion of thymus-derived CD25+Foxp3+ CD4 T cells. Transferred OVA-loaded GILZhi DCs produce significantly higher levels of IL-10 and express reduced levels of MHC class II molecules as compared with OVA-loaded control DCs, emphasizing the regulatory phenotype of GILZhi DCs in vivo. Thus, our work demonstrates in vivo that the GILZ overexpression alone is sufficient to promote a tolerogenic mode of function in DCs.

Ag sampling is a key process in dendritic cell (DC) biology. DCs use constitutive macropinocytosis, receptor-mediated endocytosis, and phagocytosis to capture exogenous Ags for presentation to T cells. We investigated the mechanisms that regulate Ag uptake by DCs in the steady-state and after a short-term LPS exposure in vitro and in vivo. We show that the glucocorticoid-induced leucine zipper protein (GILZ), already known to regulate effector versus regulatory T cell activation by DCs, selectively limits macropinocytosis, but not receptor-mediated phagocytosis, in immature and recently activated DCs. In vivo, the GILZ-mediated inhibition of Ag uptake is restricted to the CD8α DC subset, which expresses the highest GILZ level among splenic DC subsets. In recently activated DCs, we further establish that GILZ limits p38 MAPK phosphorylation, providing a possible mechanism for GILZ-mediated macropinocytosis control. Finally, our results demonstrate that the modulation of Ag uptake by GILZ does not result in altered Ag presentation to CD4 T cells but impacts the efficiency of cross-presentation to CD8 T cells. Altogether, our results identify GILZ as an endogenous inhibitor of macropinocytosis in DCs, the action of which contributes to the fine-tuning of Ag cross-presentation.