CD86 and CD80 Differentially Modulate the Suppressive Function of Human Regulatory T Cells

Regulatory T cells (Treg) are important in maintaining tolerance to self tissues. As both CD28 and CTLA-4 molecules are implicated in the function of Treg, we investigated the ability of their two natural ligands, CD80 and CD86, to influence the Treg-suppressive capacity. During T cell responses to alloantigens expressed on dendritic cells, we observed that Abs against CD86 potently enhanced suppression by CD4(+)CD25(+) Treg. In contrast, blocking CD80 enhanced proliferative responses by impairing Treg suppression. Intriguingly, the relative expression levels of CD80 and CD86 on dendritic cells are modulated during progression from an immature to a mature state, and this correlates with the ability of Treg to suppress responses. Our data show that CD80 and CD86 have opposing functions through CD28 and CTLA-4 on Treg, an observation that has significant implications for manipulation of immune responses and tolerance in vivo

Acquisition of suppressive function by activated human CD4+ CD25- T cells is associated with the expression of CTLA-4 not FoxP3.

The role of CTLA-4 in Regulatory T cell (Treg) function is not well understood. We have examined the role of CTLA-4 and its relationship with the transcription factor FoxP3 using a model of Treg induction in human peripheral blood. Activation of human CD4(+)CD25(-) T cells resulted in the appearance of a de novo population of FoxP3-expressing cells within 48h. These cells expressed high levels of CTLA-4 and cell sorting on expression of CTLA-4 strongly enriched for FoxP3(+) expressing cells with suppressive function. Culture in IL-2 alone also generated cells with suppressive capacity which also correlated with the appearance of CTLA-4. To directly test the role of CTLA-4, we transfected resting human T cells with CTLA-4, and found that this conferred suppression, similar to that of natural Treg, even though these did not express FoxP3. Furthermore, transfection of FoxP3 did not induce CTLA-4 and these cells were not suppressive. By separating the expression of CTLA-4 and FoxP3, our data show that FoxP3 expression alone is insufficient to upregulate CTLA-4, however, activation of CD4(+) CD25(-) T cells can induce both FoxP3 and CTLA-4 in a sub-population of T cells which are capable of suppression. These data suggest that the acquisition of suppressive behaviour by activated CD4(+) CD25(-) T cells requires the expression of CTLA-4, a feature which is appears to be facilitated by, but is not dependent on, expression of FoxP3

Exocytosis of CTLA-4 Is Dependent on Phospholipase D and ADP Ribosylation Factor-1 and Stimulated during Activation of Regulatory T Cells

CTLA-4 is an essential protein in the regulation of T cell responses that interacts with two ligands found on the surface of APCs (CD80 and CD86). CTLA-4 is itself poorly expressed on the T cell surface and is predominantly localized to intracellular compartments. We have studied the mechanisms involved in the delivery of CTLA-4 to the cell surface using a model Chinese hamster ovary cell system and compared this with activated and regulatory human T cells. We have shown that expression of CTLA-4 at the plasma membrane (PM) is controlled by exocytosis of CTLA-4-containing vesicles and followed by rapid endocytosis. Using selective inhibitors and dominant negative mutants, we have shown that exocytosis of CTLA-4 is dependent on the activity of the GTPase ADP ribosylation factor-1 and on phospholipase D activity. CTLA-4 was identified in a perinuclear compartment overlapping with the cis-Golgi marker GM-130 but did not colocalize strongly with lysosomal markers such as CD63 and lysosome-associated membrane protein. In regulatory T cells, activation of phospholipase D was sufficient to trigger release of CTLA-4 to the PM but did not inhibit endocytosis. Taken together, these data suggest that CTLA-4 may be stored in a specialized compartment in regulatory T cells that can be triggered rapidly for deployment to the PM in a phospholipase D- and ADP ribosylation factor-1-dependent manner