PURPOSE: In animal models, the level of chimerism and tissue distribution of donor-origin cells is variable and depends on the type of transplanted tissue. Donor-specific chimerism and maintenance is known to facilitate allograft acceptance. This may be accomplished by specific tolerance inducing protocols and engraftment of donor-origin cells. Various components of transplanted facial allograft may contain skin, which is abundant with different type of immunocompetent cells such as dendritic cells and dermal T-lymphocytes and bone with bone marrow (BM) cells. The skin is considered as the most immunogenic tissue, and BM as a permissive tissue for allograft acceptance.
We present the functional outcome and long-term survival based on chimerism induction in different facial allograft models under low-dose of CsA monotherapy.
METHODS: A total of 158 face transplants were performed. The models differed in the content of tissue components from vascularized facial skin to vascularized composite facial skin/bone allografts.  Sixty-seven isografts served as controls for all tested models. Ninety-one face transplants were performed between LBN(RT1^l+n) donors and LEW(RT1^l) recipients in 5 experimental models: face/scalp (n=20), hemiface (n=36), hemiface/calvaria (n=7), hemiface/mandible (n=18) and maxilla (n=10), under low maintenance dose of CsA monotherapy.
Flow cytometry assessed donor-specific chimerism for T-cells (RT1ⁿ/CD4 and RT1ⁿ/CD8) and B-cells (RT1ⁿ/CD45RA) in the blood and BM, and immunocytochemistry tested engraftment of donor-origin cells into lymphoid organs of recipients. MLR assay assessed responsiveness to donor antigens.
RESULTS: All isografts survived indefinitely. Facial allografts survival was: for face/scalp up to 225 days, hemiface to 440 days, hemiface/calvaria to 220 days, hemiface/mandible to 378 days and for maxilla up to 105 days. At 100 days post-transplant chimerism (RT1ⁿ) was represented predominantly by T-cell populations’ at 3.04% in full face versus 10.7% in hemiface recipients.  In contrast, in models containing vascularized BM component chimerism was maintained by B-cell population RT1ⁿ/CD45RA and was determined at: 6.7% in hemiface/calvaria, 4.6% in hemiface/mandible and at 4.7% in maxilla, when compared to full face (0.6%) and hemiface (0.87%) allografts without bone components.
Donor-origin cells detected in the BM compartment of face allografts ranged from 2.5% to 3.1% of RT1ⁿ.
Immunocytochemistry confirmed engraftment of donor-derived cells into lymphoid organs of long-term surviving face transplants recipients. MLR assay confirmed hyporesponsiveness to donor antigens.

CONCLUSIONS: Presence of active BM cells within vascularized BM compartment in the hemiface/calvaria, hemiface/mandible and maxilla allograft models facilitated migration of passenger leukocytes from the transplanted  facial flaps into donor cmpartments.  In these models the presence of oral mucosa, submandibular and submaxillary lymph nodes, and salivary glands, rich in hematopoietic cells could contribute to donor chimerism induction and maintenance. This study confirmed induction of chimerism in all face allograft models, however facial flaps without bone component were chracterized predominantely by T-cell population whereas in facial/bone allograft models chimerism was maintained by B-cells. The stable level of donor-origin cells (RT1ⁿ) in the BM compartment of recipients was attributed to chimerism maintenance in the peripheral blood and lymphoid organs of recipients.