Abstract
Regulatory T Cells (Tregs) constitutively express the inhibitory receptor CTLA-4, which is fundamental to their role in immune suppression. Mechanistically, CTLA-4 on Tregs can attenuate T cell activation by physically removing and internalizing costimulatory ligands CD80 and CD86 from the surface of antigen-presenting cells by transendocytosis. Therefore, the process of transendocytosis can be harnessed as a tool to study the molecular basis of CTLA-4 biology and a key aspect of Treg suppressive function. In this chapter, we describe a method of human Treg isolation and expansion resulting in high CTLA-4 expression. We then detail a transendocytosis assay using artificial antigen-presenting cells (DG-75 B Cell lines) expressing fluorescently tagged ligands mixed with the expanded Tregs. This methodology can be applied to testing of patients carrying CTLA-4 mutations, providing a robust model to assess the degree of functional disruption.