CD4+CD25+ regulatory T cells (Tregs) play a critical role in the prevention of
autoimmune diseases as well as in the induction and maintenance of dominant tolerance
in transplantation models. While their suppressive function has been extensively studied
in vitro, their homeostasis and mechanisms of immunoregulation still remain to be
clarifi ed in vivo. Using a murine adoptive transfer and skin allograft model, we analysed
the expansion, effector function and traffi cking of effector T cells in the presence or
absence of donor-specifi c Tregs. Although hyporesponsive to allogeneic and polyclonal
stimulation in vitro, transferred Tregs survived and expanded, in response to an allograft
in vivo. When co-transferred with naive CD4+CD25- effector T cells, they specifi cally
prevented donor but not 3rd party allograft rejection by inhibiting the production
of effector cytokines rather than the proliferation of effector T cells in response to
alloantigens. The co-transfer of donor-specifi c Tregs did not affect the homing of
effector T cells towards the graft draining lymph nodes, but it markedly reduced the
infi ltration of the allograft by these pathogenic cells. Furthermore, in recipients where
donor-specifi c transplantation tolerance was induced, Tregs preferentially accumulated
in the allograft draining lymph nodes and within the grafted skin itself. Taken together,
our results suggest that the suppression of graft rejection is an active process that involves
the persistent presence of Tregs at the site of antigenic challenge.