Double negative regulatory T cells in transplantation and autoimmunity: recent progress and future directions

Abstract

T lymphocytes bearing the αβ T cell receptor (TCR) but lacking CD4, CD8, and markers of natural killer (NK) cell differentiation are known as 'double-negative' (DN) T cells and have been described in both humans and rodent models. We and others have shown that DN T cells can act as regulatory T cells (Tregs) that are able to prevent allograft rejection, graft-versus-host disease, and autoimmune diabetes. In the last few years, new data have revealed evidence of DN Treg function in vivo in rodents and humans. Moreover, significant advances have been made in the mechanisms by which DN Tregs target antigen-specific T cells. One major limitation of the field is the lack of a specific marker that can be used to distinguish truly regulatory DN T cells (DN Tregs) from non-regulatory ones, and this is the central challenge in the coming years. Here, we review recent progress on the role of DN Tregs in transplantation and autoimmunity, and their mechanisms of action. We also provide some perspectives on how DN Tregs compare with Foxp3(+) Tregs.

Figure 1

DN Tregs employ multiple mechanisms to exert antigen-specific control over immune responses. This figure summarizes the major mechanisms discussed in the text. Note that the different mechanisms have been demonstrated to be operative in different models and contexts. (1) Immature/tolerogenic DCs secreting EBI3 can elicit IFNγ from DN Tregs, resulting in allograft tolerance by an undetermined mechanism (Hill et al., 2011). (2) Mature or immature DCs presenting alloantigen to DN Tregs are susceptible to apoptosis via, at least partially, the Fas pathway (Gao et al., 2011). (3) DN Treg-expressed CTLA4 also interacts with CD80/CD86 on mature DCs causing downregulation of these molecules on the DC surface, making the DCs less capable of priming allogeneic T cell responses (Gao et al., 2011). (4) DN Tregs can acquire alloantigen from DCs via trogocytosis and present them to T cells bearing the same TCR specificity as the DN Tregs. The T cell is then susceptible to Fas-mediated apoptosis (Ford McIntyre et al., 2008). This mechanism has only been demonstrated for CD8⁺ T cells, but CD4⁺ T cells are also susceptible to killing by DN Tregs. (5) Similarly, B cells presenting specific antigen in the context of MHC class II are susceptible to killing by DN Tregs (Hillhouse et al., 2010), and this appears to be via a perforin/granzyme-mediated process (Ford McIntyre et al., 2010). NK cells can also be killed by DN Treg perforin/granzyme (He et al., 2007). (6) The ability of DN T cells from lpr mice to kill syngeneic Fas⁺ T cells is under the control of autocrine IFNγ, secreted upon TCR stimulation (our unpublished data). (7) IL-10 secreted by DN Tregs can inhibit autoimmune diabetes development by an as-yet unknown mechanism (indicated by the question mark) while also limiting DN Treg expansion (Dugas et al., 2010).