αβ-TCR+CD3+CD4-CD8-NK1.1- double negative (DN) T cells comprise 1-5% of T lymphocytes in mice and humans. Previous studies have demonstrated that DN T cells can suppress auto-, allo- and xeno-immune responses in an antigen-specific fashion. However, the mechanisms by which DN T cells regulate immune responses remain elusive. Whether DN T cells can regulate antigen presenting cells has not been investigated previously. The focus of this thesis is to determine the consequences of DN T cells interaction with antigen presenting cells (APCs) and the underlying mechanisms. In this thesis, using a murine skin transplantation model, we found that donor B cells, but not dendritic cells (DCs), are the major surviving donor APCs in recipients following donor lymphocyte infusion. Infusing donor B, but not non-B, cells resulted in significantly enhanced donor-specific skin allograft survival. Mice that had received donor B cells showed higher expression of activation markers on antigen-specific DN T cells. B cells could present alloantigen to DN T cells and prime DN T cell proliferation in an antigen-specific fashion. Activated DN T cells were not able to down regulate the expression of CD80 or CD86 on LPS-activated B cells, but they could kill activated allogeneic as well as syngeneic B cells via a perforin-dependent pathway in vitro. In addition, DN T cells expressed high levels of CTLA4 and were capable of down regulating CD80 and CD86 expressed on antigen-expressing mature DCs through CTLA4. DN T cells killed both immature and mature allogeneic DCs, as well as antigen-loaded syngeneic DCs, in an antigen-specific manner in vitro and in vivo, mainly through the Fas-FasL pathway. Taken together, the data presented in this thesis demonstrate, for the first time, that DN T cells are potent regulators of APCs and further clarify the mechanisms of DN T cell-mediated immune suppression. These findings provide novel insights for DN T cells to be developed as a potent immune suppression treatment for a variety of diseases.

αβ-TCR+CD3+CD4-CD8-NK1.1- double negative (DN) T cells comprise 1-5% of T lymphocytes in mice and humans. Previous studies have demonstrated that DN T cells can suppress auto-, allo- and xeno-immune responses in an antigen-specific fashion. However, the mechanisms by which DN T cells regulate immune responses remain elusive. Whether DN T cells can regulate antigen presenting cells has not been investigated previously. The focus of this thesis is to determine the consequences of DN T cells interaction with antigen presenting cells (APCs) and the underlying mechanisms. In this thesis, using a murine skin transplantation model, we found that donor B cells, but not dendritic cells (DCs), are the major surviving donor APCs in recipients following donor lymphocyte infusion. Infusing donor B, but not non-B, cells resulted in significantly enhanced donor-specific skin allograft survival. Mice that had received donor B cells showed higher expression of activation markers on antigen-specific DN T cells. B cells could present alloantigen to DN T cells and prime DN T cell proliferation in an antigen-specific fashion. Activated DN T cells were not able to down regulate the expression of CD80 or CD86 on LPS-activated B cells, but they could kill activated allogeneic as well as syngeneic B cells via a perforin-dependent pathway in vitro. In addition, DN T cells expressed high levels of CTLA4 and were capable of down regulating CD80 and CD86 expressed on antigen-expressing mature DCs through CTLA4. DN T cells killed both immature and mature allogeneic DCs, as well as antigen-loaded syngeneic DCs, in an antigen-specific manner in vitro and in vivo, mainly through the Fas-FasL pathway. Taken together, the data presented in this thesis demonstrate, for the first time, that DN T cells are potent regulators of APCs and further clarify the mechanisms of DN T cell-mediated immune suppression. These findings provide novel insights for DN T cells to be developed as a potent immune suppression treatment for a variety of diseases.