Early regulation of CD8 T cell alloreactivity by CD4+CD25- T cells in recipients of anti-CD154 antibody and allogeneic BMT is followed by rapid peripheral deletion of donor-reactive CD8+ T cells, precluding a role for sustained regulation

While acquisition of regulatory function by CD4(+)CD25(-) T cells has been reported following antigenic stimulation, naturally occurring regulatory CD4(+) T cells (Treg) are believed to express CD25. We examined the mechanisms involved in peripheral CD8 T cell tolerance by induction of mixed chimerism using non-myeloablative conditioning with low-dose (3 Gy) total body irradiation and anti-CD154 antibody. Recipient CD4(+) T cells were initially required for the induction of CD8 cell tolerance, but were not needed beyond 2 weeks. Depletion of CD25(+) Treg prior to bone marrow transplantation and blockade of IL-2 with neutralizing antibody did not impede tolerance induction. Tolerance was dependent on CTLA4, but not on IFN-gamma. In C57BL/6 mice containing a fraction of 2C TCR transgenic CD8(+) T cells, which recognize the MHC class I alloantigen L-d, induction of chimerism with L, but not L, bone marrow cells led to deletion of peripheral 2C(+) CD8(+) cells within 1 week in peripheral blood and spleen. Complete deletion required the presence of recipient CD4(+) T cells. Thus, a novel, rapid form of regulation by CD4(+)CD25(-) T cells permits initial CD8 T cell tolerance in this model. Rapid peripheral deletion of donor-specific CD8 T cells precludes an ongoing requirement for CD4 T cell-mediated regulation.