Is chimerism necessary for tolerance and how?

This article reviews recent reports that have addressed the role of multilineage donor hematopoietic cell microchimerism in the induction and maintenance of organ transplant tolerance. A major focus of the search for a mechanistic explanation for tolerance induction by microchimerism has been donor-derived dendritic cell progenitors that have been shown to have tolerogenic potential. Greater understanding of the bidirectional donor-host leukocyte interactions that occur after organ transplantation may allow development of improved therapeutic strategies based on tolerance-promoting manipulation of these interactions.