New insights into mechanisms of spontaneous liver transplant tolerance: the role of Foxp3-expressing CD25+CD4+ regulatory T cells

Liver allografts in mice are accepted across MHC barriers without requirement for immunosuppressive therapy. The mechanisms underlying this phenomenon remain largely undefined. In this study, we investigated the role of Foxp3-expressing CD25(+)CD4(+) regulatory T cells (Treg) in the induction of murine liver transplant tolerance. Foxp3(+)CD25(+)CD4(+) T cells were increased in liver grafts and recipient spleens from day 5 to day 100 posttransplantation, associated with enhanced CTLA4 and TGF-beta expression and IL-4 production. Depletion of recipient CD25(+)CD4(+) T cells using anti-CD25 mAb (250 microg/day) induced acute liver allograft rejection. This was associated with a decreased ratio of Foxp3(+) Treg: T effector cells, decreased IL-4 and elevated IL-10 and IL-2 production by graft-infiltrating T cells, and reduced apoptotic activity of graft-infiltrating CD4(+) and CD8(+) T cells in anti-CD25-mAb-treated recipients. Thus, the data suggest that Foxp3(+)CD25(+)CD4(+)Treg are involved in spontaneous acceptance of liver allografts in mice. The ratio of Treg to T effector cells appears to determine liver transplant outcome. CTLA4, IL-4, TGF-beta and apoptosis of graft-infiltrating T cells are also associated with liver transplant tolerance and may contribute, at least in part, to the mechanisms of Treg-mediated immune regulation in this model.

The role of Foxp3+ regulatory T cells in liver transplant tolerance

The liver has long been considered a tolerogenic organ that favors the induction of peripheral tolerance. The mechanisms underlying liver tolerogenicity remain largely undefined. In this study, we characterized Foxp3-expressing CD4+ CD25+ regulatory T cells (Treg) in liver allograft recipients and examined the role of Treg in inherent liver tolerogenicity by employing the mouse spontaneous liver transplant tolerance model. Orthotopic liver transplantation was performed from C57BL/10 (H2b) to C3H/HeJ (H2k) mice. The percentage of CD4+ CD25+ Treg was expanded in the liver grafts and recipient spleens from day 5 up to day 100 posttransplantation, associated with high intracellular Foxp3 and CTLA4 expression. Immunohistochemistry further demonstrated significant numbers of Foxp3+ cells in the liver grafts and recipient spleens and increased transforming growth factor beta expression in the recipient spleens throughout the time courses. Adoptive transfer of spleen cells from the long-term liver allograft survivors significantly prolonged donor heart graft survival. Depletion of recipient CD4+ CD25+ Treg using anti-CD25 monoclonal antibody (250 microg/d) induced acute liver allograft rejection, associated with elevated anti-donor T-cell proliferative responses, CTL and natural killer activities, enhanced interleukin (IL)-2, interferon-gamma, IL-10, and decreased IL-4 production, and decreased T-cell apoptotic activity in anti-CD25-treated recipients. Moreover, CTLA4 blockade by anti-CTLA4 monoclonal antibody administration exacerbated liver graft rejection when combined with anti-CD25 monoclonal antibody. Thus, Foxp3+ CD4+ CD25+ Treg appear to underpin spontaneous acceptance of major histocompatability complex- mismatched liver allografts in mice. CTLA4, IL-4, and apoptosis of alloreactive T cells appear to contribute to the function of Treg and regulation of graft outcome.

Anti-CD25 mAb Administration Prevents Spontaneous Liver Transplant Tolerance

Liver allografts are accepted spontaneously in all mouse strain combinations without immunosuppressive therapy. The mechanisms underlying this phenomenon remain largely undefined. In this study, we examined the effect of CD4+ CD25+ T regulatory cells (Treg) on the induction of mouse liver transplant tolerance. Orthotopic liver transplantation was performed from B10 (H2b) to C3H (H2k) mice. Depleting rat anti-mouse CD25 mAb (PC61) was given to the donors or recipients (250 microg/d IP) pretransplant or to the recipients postoperatively. At day 5 posttransplantation, both effector T cells (mainly CD8) and CD4+ CD25+ Treg were increased in the liver allografts and host spleens compared to naïve mice. Anti-CD25 mAb administration, either pretransplantation or posttransplantation, reduced the ratio of CD4+ CD25+ Treg to the CD3 T cells of liver grafts and recipient spleens and induced liver allograft acute rejection compared to IgG treatment. Anti-CD25 mAb administration elevated anti-donor T-cell proliferative responses and CTL and NK activities of graft infiltrates and host splenocytes; reduced CTLA4, Foxp3, and IDO mRNA levels; increased IL-10 and IFN-gamma; and decreased IL-4 mRNA levels in the livers or host spleens. The number of apoptotic T cells was reduced significantly in the liver grafts and treated host spleens. Therefore, anti-CD25 mAb administration changed the balance of CD4+ CD25+ Treg to activated T cells of liver graft recipients, preventing liver transplant tolerance. This was associated with enhanced anti-donor immune reactivity, downregulated Treg gene expression, and reduced T cell apoptosis in the grafts and host spleens.