The ability to control the response of B cells is of particular interest in xenotransplantation as Ab-mediated hyperacute and acute xenograft rejection are major obstacles in achieving long-term graft survival. Regulatory T cells have been proven to play a very important role in the regulation of immune responses to self or non-self Ags. Previous studies have shown that TCRαβ+CD3+CD4−CD8− (double-negative (DN)) T cells possess an immune regulatory function, capable of controlling antidonor T cell responses in allo- and xenotransplantation through Fas-Fas ligand interaction. In this study, we investigated the possibility that xenoreactive DNT cells suppress B cells. We found that DNT cells generated from wild-type C57BL/6 mice expressed B220 and CD25 after rat Ag stimulation. These xenoreactive B220+CD25+ DNT cells lysed activated, but not naive, B and T cells. This killing, which took place through cell-cell contact, required participation of adhesion molecules. Our results indicate that Fas ligand, TGF-β, TNF-α, and TCR-MHC recognition was not involved in DNT cell-mediated syngenic cell killing, but instead this killing was mediated by perforin and granzymes. The xenoreactive DNT cells expressed high levels of granzymes in comparison to allo- or xenoreactive CD8+ T cells. Adoptive transfer of DNT cells in combination with early immune suppression by immunosuppressive analog of 15-deoxyspergualin, LF15-0195, significantly prolonged rat heart graft survival to 62.1 ± 13.9 days in mice recipients. In conclusion, this study suggests that xenoreactive DNT cells can control B and T cell responses in perforin/granzyme-dependent mechanisms. DNT cells may be valuable in controlling B and T cell responses in xenotransplantation.

This study suggests that adoptive transfer of xenoreactive DN-Treg cells can inhibit B-cell responses in vivo. DN-Treg cells may be valuable in controlling B-cell responses in xenotransplantation.

Bone marrow (BM) transplantation is an efficient approach to develop donor-specific tolerance and prevent chronic rejection. Allogeneic BM transplantation is limited by donor T cell-mediated graft-versus-host disease, requirement of cytoreduction and high numbers of BM cells. In addition of these drawbacks, recent studies demonstrate that not only T cells, but also NK cells can mediate BM rejection, and long-term mixed chimerism depends on NK cell tolerance. Thus, NK cell is another potential barrier against engraftment of BM and an important target in efforts to induce transplant tolerance. We have previously identified a novel type of Treg with the phenotype TCRab + CD3 + CD4 -CD8 -(double-negative, DN). We and others have demonstrated that DN-Treg can effectively suppress anti-donor T cell responses. In this study, we found that donor-derived DN-Treg can suppress NK cell-mediated allogeneic BM graft rejection in both parent-to-F1 and fully MHC-mismatched BM transplantation models. Perforin and FasL in DN-Treg play important roles in the suppression of NK cells. Furthermore, adoptive transfer of DN-Treg can promote a stable mixed chimerism and donor-specific tolerance without inducing graft-versus-host disease. These results demonstrate a potential approach to control innate immune responses and promote allogeneic BM engraftment. IntroductionEstablishing donor-specific transplant tolerance is beneficial for several reasons: it can prevent chronic rejection and avoid side effects related to long-term immunosuppressive drug treatment. It also can help patients to preserve their own natural immunity against infections and tumors. Achieving mixed chimerism via BM transplantation is an efficient approach that directs the immune system to regard the donor as 'self'. By generating specific immunologic tolerance to donor transplants, chronic rejection as well as the need for ongoing immunosuppression can be avoided [1][2][3].Rejection of MHC-mismatched allografts is largely mediated by a recipient's CD4 + and CD8 + T cells. Recent studies, however, have demonstrated that NK cells are another potential barrier in allograft rejection and tolerance [4][5][6][7]. Furthermore, studies have shown that NK cells can mediate BM rejection [8,9]. This notion has been well addressed in the hybrid resistance model, in which parental BM cells, not solid organs, are vigorously rejected by an F1 hybrid [10,11]. This rejection is mediated solely by a recipient's NK cells, not by T cells, NKT cells or B cells [12][13][14] [20,21].In recent studies, we found that TCRab + CD3 + CD4 -CD8 -(double-negative, DN) T cells possess immuneregulatory functions and play an important role in the development of tolerance post-transplantation [22][23][24][25]. We have demonstrated that mouse DN-Treg specifically can eliminate activated syngeneic anti-donor CD4 + T cells, CD8 + T cells, and B cells [22,23,25,26]. Furthermore, adoptive transfer of DN-Treg leads to significant prolongation of donor-specific skin and heart allografts in a single MHC-mis...