Fate of Donor Cells in Vascularized Bone Grafts: Identification of Systemic Chimerism by the Polymerase Chain Reaction

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Abstract
Systemic chimerism, or the movement of cells from a transplanted tissue into host organs, is a phenomenon known to occur in association with development of immunological tolerance in allotransplantation. However, little is known about the fate and movement of cells into or out of autogenous free tissue transfers, including vascularized bone grafts. The purpose of this study was to identify systemic chimerism in vascularized bone grafts by transplantation of a vascularized tibiofibular graft from isogenous (inbred) male Lewis rats to female recipients. Donor (male) cells could be identified in the recipient (female) tissues by semiquantitative polymerase chain reaction analysis for a Y chromosome-specific DNA sequence. Chimerism was assessed at 1, 12, 18, and 24 weeks after transplantation. Competitive polymerase chain reaction study using the specific primers for a Y-chromosome marker (Sry gene) and an autosomal gene (GAPDH) allowed detection of small amounts of male cells in a large pool of female cells and measurement of their relative proportions as a function of time. Of 19 nonimmunosuppressed recipients, nine animals (47 percent) showed lowlevel chimerism (1 percent) in the spleen, liver, and thymus, respectively, at final assessment. Donor cells were detected in all bone grafts and in six contralateral tibial bones (i.e., 67 percent of sampled contralateral tibial bones) at 18 and 24 weeks after transplantation. Twenty-four recipients were immunosuppressed with FK506 (tacrolimus) to suppress reaction to a minor histocompatibility barrier present on the Y chromosome. In this group, 14 animals (58 percent) showed low-level chimerism in peripheral blood and 12 (50 percent), eight (33 percent), and one (4 percent) recipients showed high-level chimerism in the spleen, thymus, and liver, respectively. Transplanted cells were detected in nine contralateral tibial bones (i.e., 60 percent of sampled contralateral tibial bones) at 12 and 18 weeks after surgery. The results indicate that polymerase chain reaction for the Y chromosome is a useful tool for differentiating between donor and recipient cell populations experimentally using sex-mismatched tissues in a rat model. This study demonstrated that systemic chimerism occurs after successful vascularized bone transplantation. Transplanted cells not only survive in the graft but also gradually migrate into the recipient's body. (Plast. Reconstr. Surg. 111: 763, 2003.)