Cell migration and chimerism--a unifying concept in transplantation--with particular reference to HLA matching and tolerance induction.

WE recently reported that the acceptance of wholeorgan transplants connotes (and requires) a special kind of mixed chimerism involving an exchange between organ and recipient oflymphodendritic leukocytes (Fig 1 ).1 The consequence in five of five patients studied 27 to 29V2 years after kidney transplantation from HLA-mismatched donors was the diffuse presence of donor dendritic cells in the lymph nodes and skin of the recipient. 2 Chronically surviving liver recipients have even more prominent evidence of systemic and graft chimerism,I.3 which can be shown with immunocytochemical techniques and confirmed with polymerase chain reaction (PCR) technology. Chimerism has been particularly easy to study after intestinal transplantation.4.5 With the thesis that this cell migration, repopulation, and consequent chimerism is the basis of graft acceptance, no matter what the organ transplanted, I we are able to reexamine some controversies in transplantation immunology, including why HLA tissue matching to govern the distribution of cadaveric organs has been so nonpredictive of outcome. To understand these controversies, we must turn back the pages 50 years, when Peter Medawar planted the seed of our specialty.