CTLA-4-mediated transendocytosis of costimulatory molecules primarily targets migratory dendritic cells

CTLA-4 is a critical negative regulator of the immune system and a major target for immunotherapy. However, precisely how it functions in vivo to maintain immune homeostasis is not clear. As a highly endocytic molecule, CTLA-4 can capture costimulatory ligands from opposing cells by a process of transendocytosis (TE). By restricting costimulatory ligand expression in this manner, CTLA-4 controls the CD28-dependent activation of T cells. Regulatory T cells (T_regs) constitutively express CTLA-4 at high levels and, in its absence, show defects in TE and suppressive function. Activated conventional T cells (T_conv) are also capable of CTLA-4-dependent TE; however, the relative use of this mechanism by T_regs and T_conv in vivo remains unclear. Here, we set out to characterize both the perpetrators and cellular targets of CTLA-4 TE in vivo. We found that T_regs showed constitutive cell surface recruitment of CTLA-4 ex vivo and performed TE rapidly after TCR stimulation. T_regs outperformed activated T_conv at TE in vivo, and expression of ICOS marked T_regs with this capability. Using TCR transgenic T_regs that recognize a protein expressed in the pancreas, we showed that the presentation of tissue-derived self-antigen could trigger T_regs to capture costimulatory ligands in vivo. Last, we identified migratory dendritic cells (DCs) as the major target for T_reg-based CTLA-4-dependent regulation in the steady state. These data support a model in which CTLA-4 expressed on T_regs dynamically regulates the phenotype of DCs trafficking to lymph nodes from peripheral tissues in an antigen-dependent manner.