Toward Development of the Delayed Tolerance Induction Protocol for Vascularized Composite Allografts in Nonhuman...

Transplantation of vascularized composite allografts is limited mainly by the need for life-long immunosuppression. The consequent side effects and looming specter of chronic rejection portend eventual allograft loss. Development of tolerogenic protocols is thus of utmost importance to the field of vascularized composite allograft transplantation.

With a modified delayed tolerance induction protocol, 10 cynomolgus macaques received hand (n = 2) or face vascularized composite allografts across both full and haploidentical major histocompatibility complex barriers before donor bone marrow transplantation at a later date. Protocol and for-cause allograft skin biopsies were performed for immunohistochemical analysis and analysis of donor-recipient leukocyte contribution; mixed chimerism in peripheral blood and in vitro immune responses were assessed serially.

Before bone marrow transplantation, maintenance immunosuppression for 4 months led to lethal complications, including posttransplant lymphoproliferative disorder (in two of four recipients), which necessitated early study termination. Shortening the maintenance period to 2 months was clinically relevant and allowed all subsequent subjects (n = 6) to complete the delayed tolerance induction protocol. Acute rejection developed within the first 2 to 4 weeks after transplantation, with corresponding near-complete turnover of allograft leukocytes from donor to recipient origin, but donor-specific antibodies remained negative. After bone marrow transplantation, mixed chimerism failed to develop, although carboxyfluorescein succinimidyl ester mixed lymphocyte reaction demonstrated generalized unresponsiveness. However, the accrual of subsequent rejection episodes eventually culminated in graft vasculopathy and irreversible allograft loss.

Despite the various advantages of the delayed tolerance induction protocol, it failed to reliably induce mixed chimerism and thus immunologic tolerance to vascularized composite allografts, given currently available immunosuppression treatment options. Ongoing work shows promise in overcoming these limitations.