Cytotoxic T lymphocyte antigen 4 (CTLA-4) is an essential negative regulator of T cell immune responses whose mechanism of action is the subject of debate. CTLA-4 shares two ligands (CD80 and CD86) with a stimulatory receptor, CD28. Here, we show that CTLA-4 can capture its ligands from opposing cells by a process of trans-endocytosis. After removal, these costimulatory ligands are degraded inside CTLA-4-expressing cells, resulting in impaired costimulation via CD28. Acquisition of CD86 from antigen-presenting cells is stimulated by T cell receptor engagement and observed in vitro and in vivo. These data reveal a mechanism of immune regulation in which CTLA-4 acts as an effector molecule to inhibit CD28 costimulation by the cell-extrinsic depletion of ligands, accounting for many of the known features of the CD28-CTLA-4 system.

1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the active form of vitamin D, exerts potent effects on several tissues including cells of the immune system, where it affects T cell activation, differentiation and migration. The circulating, inactive form of vitamin D, 25(OH)D3, is generally used as an indication of “vitamin D status”. However, utilization of this precursor depends on its uptake by cells and subsequent conversion by the enzyme 25(OH)D3-1α-hydroxylase (CYP27B1) into active 1,25(OH)2D3. Using human T cells, we now show that addition of inactive 25(OH)D3 is sufficient to alter T cell responses only when dendritic cells (DCs) are present. Mechanistically, CYP27B1 is induced in DCs upon maturation with LPS or upon T cell contact resulting in the generation and release of 1,25(OH)2D3 which subsequently affects T cell responses. In most tissues, vitamin D binding protein (DBP) acts as a carrier to enhance the utilization of vitamin D. However, we show that DBP modulates T cell responses by restricting the availability of inactive 25(OH)D3 to DC. These data indicate that the level of “free” 25(OH)D3 available to DCs determines the inflammatory/regulatory balance of ensuing T cell responses.