Cytotoxic T lymphocyte antigen 4 (CTLA-4) is expressed by activated and regulatory T cells and helps to prevent T cell-mediated immune responses from getting out of control. The costimulatory pair of ligands bind CTLA-4 and are expressed on antigen-presenting cells. CD80 and CD86 also bind to CD28 expressed on T cells, which is critical for T cell activation. Qureshi et al. (p. 600, published online 7 April; see the Perspective by Sakaguchi and Wing) find that CTLA-4, expressed by both human and mouse T cells, plucks CD80 and CD86 from the surface of antigen-presenting cells and internalizes them through trans-endocytosis.
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