Pretransplant infusion of donor B cells enhances donor-specific skin allograft survival.

Pretransplant donor lymphocyte infusion (DLI) has been shown to enhance donor-specific allograft survival in rodents, primates and humans. However, the cell subset that is critical for the DLI effect and the mechanisms involved remain elusive. In this study, we monitored donor cell subsets after DLI in a murine MHC class I L(d)-mismatched skin transplantation model. We found that donor B cells, but not DCs, are the major surviving donor APCs in recipients following DLI. Infusing donor B, but not non-B, cells resulted in significantly enhanced donor-specific skin allograft survival. Furthermore, mice that had received donor B cells showed higher expression of Ly6A and CD62L on antigen-specific TCRαβ(+)CD3(+)CD4(-)CD8(-)NK1.1(-) double negative (DN) regulatory T cells (Tregs). B cells presented alloantigen to DN Tregs and primed their proliferation in an antigen-specific fashion. Importantly, DN Tregs, activated by donor B cells, showed increased cytotoxicity toward anti-donor CD8(+) T cells. These data demonstrate that donor B cells can enhance skin allograft survival, at least partially, by increasing recipient DN Treg-mediated killing of anti-donor CD8(+) T cells. These findings provide novel insights into the mechanisms underlying DLI-induced transplant tolerance and suggest that DN Tregs have great potential as an antigen-specific immune therapy to enhance allograft survival.