0041-1337/04/7703-386/0 TRANSPLANTATION Vol. 77, 386–391, No. 3, February 15, 2004 Copyright © 2004 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. PROLONGED SURVIVAL OF RAT ISLET XENOGRAFTS IN MICE AFTER CD45RB MONOTHERAPY LYDIA VISSER,SIBRAND POPPEMA, 1 BART DE HAAN,PIETER KLOK,JUDITH VAN DER LEIJ, ANKE VAN DEN BERG, AND PAUL DE VOS Background. Pancreatic islet transplantation can correct the disordered glucose metabolism of type 1 diabetes, but the number of successful transplants has been low because of the need for long-term immuno- suppression and the limited availability of human is- lets. New approaches, such as the use of tolerance- inducing treatment modalities and the use of islets of nonhuman sources, can possibly improve the success of islet transplantation. In the present study, the au- thors investigated the effect of anti-CD45RB treat- ment on the survival of islet xenografts. Methods. Chemically induced diabetic mice under- went xenografting with rat islets and were treated with CD45RB antibodies on days 1, 0, and 5. Immu- nohistology and real-time polymerase chain reaction were used to study the effect of the treatment in the xenografts. The effect of anti-CD45RB treatment in peripheral blood of normal mice was measured with flow cytometry. Results. In the treated mice, survival of the grafts was prolonged substantially. In the treated mice with functioning grafts, no lymphocytes were found infil- trating the transplanted islets on day 6; whereas in the untreated animals with functioning grafts, signs of rejection were evident. In the grafts of the treated animals, significantly less mRNA for interleukin (IL)-2, interferon-, and IL-4 was found compared with the untreated mice. After CD45RB treatment, there was depletion or decrease of CD45RB bright cells from the peripheral blood. Conclusions. Our results show that a short course of anti-CD45RB monotherapy prolongs the survival of rat islet xenografts in C57BL/6 mice. The major challenge in the treatment of type 1 diabetes mellitus is to prevent or delay the onset of diabetic com- plications and thereby improve the quality of life of the patient. The onset of diabetic complications can be delayed by tight control of blood glucose levels (1, 2). However, tight control of glucose levels by insulin therapy is fre- quently associated with episodes of hypoglycemia, and daily glucose profiles approach that of healthy humans but are still not normal. It has been shown by several groups that adequate meta- bolic control can be achieved by successful transplantation of a pancreatic islet graft (3, 4). Unfortunately, however, the large-scale application of islet transplantation in type 1 dia- betes has been hampered by the low success rates in humans, which are usually not more than 11%. Major factors have been responsible for these low success rates, including quan- titative insufficiencies in the number of islets available for transplantation and the destruction of the islet graft as a consequence of ineffective immunosuppression (5). Only re- cently were successful islet transplants reported by the Ed- monton group (6). However, the requirement for long-term immunosuppression limits the application of this protocol to a small subgroup of patients. In another recent study, three of seven patients retained their grafts longer than 52 weeks. All three patients had a history of antithymocyte globulin (ATG) therapy at the time of prior kidney transplantation. In these patients, an absence of auto- and alloreactivity to islets was found. ATG is hypoth- esized to deplete autoreactive memory T cells (7). It has been shown that antibodies against CD45 are a main ingredient of polyclonal antisera such as ATG (8 –10). Early studies showed that rabbit antisera to pure CD45 could completely suppress renal allograft rejection in rats, whereas large doses of a mouse monoclonal antibody to a common determinant of rat CD45 was not immunosuppressive (11). The effective use of antibodies against CD45RB has been previously reported in transplantation and autoimmune disease. CD45RB antibody MB23G2 protects against rejec- tion and reverses rejection in a mouse model for kidney transplantation. Two injections of the monoclonal antibody (mAb) on days 0 and 1 induced indefinite allograft accep- tance and subsequent tolerance to skin transplants (12). Similar results were found in islet allografts in mice (13, 14) and in an experimental allergic encephalomyelitis model (15). The mechanism by which CD45RB antibodies induce tol- erance is still unclear, but the depletion of a subset of CD45RB bright T cells from the peripheral blood appears to play an important role. As a consequence, the composition of the infiltrates and the cytokine profile in the grafts is also changed. In the islet allografts, IL-4 and IL-10 mRNA were found to be increased (13), indicating that the T-helper (Th)1- Th2 balance had shifted toward a Th2 immune response by the anti-CD45RB treatment. In the present study, we tested the efficacy of CD45RB antibody MB23G2 in a xenograft model for islet transplan- tation. To this end, we compared the success rates of CD45RB-treated mice and untreated controls. In function- ing and nonfunctioning grafts, we studied and compared the type and quantity of infiltrating cells and the ex- pression of mRNA for cytokines. In a separate experiment, we investigated the kinetics of the depletion of CD45RB bright cells from peripheral blood after treatment with CD45RB. Department of Pathology and Laboratory Medicine, University of Groningen, Groningen, The Netherlands. 1 Address correspondence to: Sibrand Poppema, M.D., Ph.D., De- partment of Pathology and Laboratory Medicine, University Hospi- tal Groningen, P.O. Box 30.001, 9700 RB Groningen, The Nether- lands. E-mail: s.poppema@med.rug.nl. Received 17 July 2003. Revision requested 22 August 2003. Ac- cepted 2 September 2003. 386 DOI: 10.1097/01.TP.0000111741.85249.EC