Summary Microchimerism after allogeneic organ transplantation has been widely documented using DNA identification techniques. However, the question as to whether the detected donor DNA is present in the surviving donor passenger cells, recipient macrophages phagocytizing rejected donor cells, or dendritic cells (DC) internalizing donor apoptotic bodies or cell fragments has not been answered. We provide evidence that allogeneic organ transplantation is followed not only by cellular microchimerism caused by release of graft passenger cells but also dissemination of donor DNA from the ischemic rejecting graft cells and its internalization in recipient DC. The high levels of donor DNA at the time of heart rejection were inversely proportional to the concentration of donor passenger cells detected with use of flow cytometry. Depending on the type of graft, the kinetics of DNA distribution in recipient tissues were different. Immunosuppressive drugs attenuated the rejection reaction and release of DNA from grafts. Allogeneic but not syngeneic donor DNA fragments were found in recipient splenic DC‐enriched population. Interestingly, that donor DNA fragments could be detected in recipient tissue at high levels on day 30. This challenges the notion that fragments of DNA are immediately cleaved by cell plasmatic enzymes. The biologic significance of our findings is not clear. We speculate that donor DNA fragments in recipient DC may play a, so far unknown, role in the immunization/tolerance process to allogeneic antigens.

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