Role of CTLA-4 in the regulation of effector T-cell and regulatory T-cell function. CTLA-4 serves to inhibit T-cell activation in effector T cells after CD28 has interacted with its ligands B7-1 (CD80) and B7-2 (CD86). This inhibition by CTLA-4 prevents unbridled T-cell activation. CTLA-4 expressed on regulatory T cells can transendocytose the ligand CD86 by removing it from the antigen-presenting cell and internalizing it, after which the ligand is degraded and CTLA-4 is recycled to the cell membrane. This transendocytosis process is exploited in the assessment of CTLA-4 variants. This figure was created with BioRender.com.