Treg-expressed CTLA-4 depletes CD80/CD86 by trogocytosis, releasing free PD-L1 on antigen-presenting cells

Foxp3-expressing CD4⁺CD25⁺ regulatory T cells (Tregs) constitutively and highly express the immune checkpoint receptor cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), whose Treg-specific deficiency causes severe systemic autoimmunity. As a key mechanism of Treg-mediated suppression, Treg-expressed CTLA-4 down-regulates the expression of CD80/CD86 costimulatory molecules on antigen-presenting cells (APCs). Here, we show that Treg-expressed CTLA-4 facilitated Treg-APC conjugation and immune synapse formation. The immune synapses thus formed provided a stable platform whereby Tregs were able to deplete CD80/CD86 molecules on APCs by extracting them via CTLA-4-dependent trogocytosis. The depletion occurred even with Tregs solely expressing a mutant CTLA-4 form lacking the cytoplasmic portion required for its endocytosis. The CTLA-4-dependent trogocytosis of CD80/CD86 also accelerated in vitro and in vivo passive transfer of other membrane proteins and lipid molecules from APCs to Tregs without their significant reduction on the APC surface. Furthermore, CD80 down-regulation or blockade by Treg-expressed membrane CTLA-4 or soluble CTLA-4-immunoglobulin (CTLA-4-Ig), respectively, disrupted cis-CD80/programmed death ligand-1 (PD-L1) heterodimers and increased free PD-L1 on dendritic cells (DCs), expanding a phenotypically distinct population of CD80^lo free PD-L1^hi DCs. Thus, Tregs are able to inhibit the T cell stimulatory activity of APCs by reducing their CD80/CD86 expression via CTLA-4-dependent trogocytosis. This CD80/CD86 reduction on APCs is able to exert dual suppressive effects on T cell immune responses by limiting CD80/CD86 costimulation to naïve T cells and by increasing free PD-L1 available for the inhibition of programmed death-1 (PD-1)-expressing effector T cells. Blockade of CTLA-4 and PD-1/PD-L1 in combination may therefore synergistically hinder Treg-mediated immune suppression, thereby effectively enhancing immune responses, including tumor immunity.