BLOCKING OF 4-1BBL PROMOTES LONG-TERM SURVIVAL IN SKIN ALLOGRAFT TREATED WITH ANTI-CD45RB AND ANTI-CD40L MONOCLONAL ANTIBODIES.

Abstract

P943 Aims: 4-1BB (CD137) is a T cell costimulatory molecule that promotes T cell activation. Using a skin transplantation model, we have observed that simultaneous administration of monoclonal antibodies (mAb) targeting CD45RB and CD40L prolonged skin allograft in costimulation blockade (CTLA4-Ig and anti-CD40L mAb)-resistant mice because of reducing CD8+ T cells and CD4+CD45RBhigh T cells. Anti-CD45RB mAb (45RB) block the activation of Th1 cells and generates regulatory T cells (Treg). In this study, we highlight the involvement of 4-1BB/4-1BBL in the development of T cell responses. Methods: C57BL/6 recipients of BALB/c skin grafts were treated with 45RB, anti-CD40L mAb (MR1) and antagonistic anti-4-1BBL mAb (4-1BBL) on days 0, 2, 4, 6 and 8 posttransplant as the following. Group 1: control (No treatment), Group 2: 45RB+MR1, Group 3: 45RB+MR1+4-1BBL, Group 4: CD4 depletion+45RB+MR1+4-1BBL, Group 5: CD8 depletion+45RB+MR1+4-1BBL. The surface molecules of T cells were observed by using flow cytometry and cytokines were measured by ELISA. Results: Additional 4-1BBL (Group 3) more prolonged skin graft survival than Group 2. Even though the percentage of splenocyte-derived CD8+ T cells was reduced similarly in both groups (Group 2 and 3), the Group 3 augmented the proportion of Treg cells such as CD4+CD45RBlow and CD4+CD25+ T cells in comparison to the Group 2. Interestingly, Group 4, but not Group 5, prevented prolongation in allograft survival with high proportion of CD8+ T cells and CD25 expression on activated CD8+ T cells. The Group 5 enjoyed long-term graft survival with higher level of IL-4 than other groups. Conclusions: Use of 4-1BBL might have additive effects on Treg cells which play a major role in the maintenance of tolerance. Even after triple immunosuppressive therapy in combination with the depletion of CD4+ T cells, we could not achieve prolonged graft survival. This rejection might result from absent Treg cells and require CD4-independent CD8+ T cells based on the observation of increasing proportion of CD8+ T cells in similar degree as the control group.