The putative mechanism of the immunomodulating effect of HLA-DR shared allogeneic blood transfusions on the alloimmune response.

Pretransplant blood transfusions have shown to improve organ allograft survival. However, the immunologic mechanism leading to this beneficial effect in clinical transplantation is still not clear. The observation that blood transfusions sharing an HLA-DR antigen with the recipient are more effective than HLA mismatched transfusions has led to the hypothesis that CD4(+) regulatory T (Treg) cells that recognize a foreign peptide in the context of the shared HLA-DR molecule play an important role in down-regulation of the immune response toward the graft. Available experimental evidence supports this hypothesis. Furthermore, these CD4(+) Treg cells are able to modulate antigen-presenting cells, which in their turn will induce Treg cells. As long as clinical transplantation tolerance by blood transfusions is not a reality, further studies on the mechanisms of the beneficial effect of pretransplant allogeneic blood transfusions are needed to obtain an effective protocol for the induction of clinically relevant Treg cells.