Hindawi Publishing Corporation Journal of Transplantation Volume 2011, Article ID 928759, 9 pages doi:10.1155/2011/928759 Research Article Attempted Depletion of Passenger Leukocytes by Irradiation in Pigs Hao-Chih Tai, 1, 2 Xiaocheng Zhu, 1 Yih Jyh Lin, 1 Hidetaka Hara, 1 Mohamed Ezzelarab, 1 Michael Epperley, 3 Mubina A. Quader, 3 and David K. C. Cooper 1 1 Thomas E. Starzl Transplantation Institute, University of Pittsburgh Medical Center, Pittsburgh, PA 15261, USA 2 Department of Surgery, National Taiwan University Hospital and College of Medicine, Taipei 100, Taiwan 3 Department of Radiation Oncology, University of Pittsburgh Medical Center, Pittsburgh, PA 15261, USA Correspondence should be addressed to David K. C. Cooper, cooperdk@upmc.edu Received 21 July 2011; Revised 1 September 2011; Accepted 18 September 2011 Academic Editor: Kazuhiko Yamada Copyright © 2011 Hao-Chih Tai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Allograft/xenograft rejection is associated with “passenger leukocyte” migration from the organ into recipient lymph nodes. In Study 1, we attempted to deplete leukocytes from potential kidney “donor” pigs, using two regimens of total body irradiation. A dose of 700 cGy was administered, followed by either 800 cGy (“low-dose”) or 1,300 cGy (“high dose”) with the kidneys shielded. Neither regimen was entirely successful in depleting all leukocytes, although remaining T and 8 cell numbers were negligible. Study 2 was aimed at providing an indication of whether near-complete depletion of leukocytes had any major impact on kidney allograft survival. In non-immunosuppressed recipient pigs, survival of a kidney from a donor that received high-dose irradiation was compared with that of a kidney taken from a non-irradiated donor. Kidney graft survival was 9 and 7 days, respectively, suggesting that depletion had little impact on graft survival. The lack of effect may have been related to (i) inadequate depletion of passenger leukocytes, thus not preventing a direct T cell response, (ii) the presence of dead or dying leukocytes (antigens), thus not preventing an indirect T cell response, or (iii) constitutive expression of MHC class II and B7 molecules on the porcine vascular endothelium, activating recipient T cells. 1. Introduction Passenger leukocytes (contained within a transplanted organ and which migrate from the graft into the lymphoid tissues of the recipient) have complex dual roles in determining the outcome after transplantation (Tx). Donor-derived passen- ger leukocytes can initiate graft rejection in some settings, whereas in others they contribute to graft acceptance [1–3]. In a small animal model, donor pretreatment with sublethal total body irradiation (TBI) or antilymphocyte globulin re- sulted in significant prolongation of heart allograft survival [4]. In animal models of intestinal Tx, passenger leukocytes in the intestine have a lineage profile that predisposes to graft-versus-host disease, which can be eliminated by ex vivo irradiation (10 Gy) of mature lymphoid elements from the bowel allografts [5]. In contrast, lethal TBI of the donor re- sulted in loss of ability to achieve tolerance in a pig kidney alloTx model [6]. The absolute number of MHC class II + dendritic cells present in a donor organ is a poor predictor of graft outcome, and survival of solid organ allografts is more closely related to the density of the donor dendritic cell network within the graft [7]. Prior irradiation of the potential organ donor leads to killing of the passenger leukocytes. Complete depletion of passenger leukocytes would likely prevent the direct T-cell response, though not the indirect, but might impact subse- quent kidney graft survival. The primary aim of the present study, therefore, was to determine whether a sufficient dose of irradiation could be administered to the “donor” pig to kill the leukocytes without seriously damaging the kidney (to be transplanted). The sec- ondary aim was to determine whether complete or near- complete depletion of passenger leukocytes delayed rejection after kidney Tx. Total body irradiation of the donor of 700 cGy with a fur- ther 800 cGy or 1,300 cGy with the kidneys shielded proved