ANTIGEN-PROCESSING AND PRESENTATION IN TRANSPLANTATION

For many years the direct stimulation of T cells in response to donor MHC antigens expressed on donor antigen-presenting cells has been the focus of transplantation immunology. Indirect recognition in response to peptides of donor antigens presented by self MHC molecules on recip ient antigen-presenting cells has not generally been considered an imp ortant feature of graft rejection. Recent evidence suggests that indir ect responses may be more important than previously considered and the new emphasis on indirect pathways in allograft rejection has raised n ew issues, many of which are unresolved.