Levels of Anti-A/B Antibodies After ABO-Incompatible Hematopoietic Stem Cell Transplantation G. Stussi, K. Huggel, U. Schanz, J.R. Passweg, and J.D. Seebach ABSTRACT In contrast to solid organ transplantation, ABO incompatibility is generally not associated with survival differences in hematopoietic stem cell transplantation. Therefore, patients receiving ABO-incompatible stem cell transplantation can be analyzed to study the mechanism of tolerance induction after antigen-mismatched transplantation. The goal of this study was to analyze the levels of anti-A/B antibodies after ABO-incompatible transplantation. Host-derived antidonor antibodies disappeared rapidly after transplanta- tion and did not reappear in the further posttransplant course. Donor-derived antihost antibodies did not significantly increase and compatible anti-A/B antibodies remained positive after hematopoietic stem cell transplantation. Thus, there is no evidence for stimulation of donor B lymphocytes to produce antirecipient antibodies suggesting a potential B cell tolerance. T HE CURRENT organ shortage stimulates the explo- ration of new strategies to expand the donor pool in transplantation medicine, including ABO blood group– incompatible grafts. Anti-A/B antibodies (Ab) present in the serum of the recipient mediate hyperacute rejection of ABO-incompatible grafts and, therefore, represent a major hurdle to the employment of such strategies. In contrast to solid organ transplantation, ABO incompatibility is gener- ally not associated with a worse outcome in allogeneic hematopoietic stem cell transplantation (HSCT). 1–3 Conse- quently, ABO-incompatible HSCT is routinely performed in approximately one-third of the patients. 4 Three groups of ABO incompatibility are distinguished in HSCT, as out- lined in Table 1: minor ABO incompatibility, characterized by the ability of donor B lymphocytes to produce antirecipi- ent Ab; major ABO incompatibility, characterized by the presence of preformed antidonor Ab; and bidirectional ABO incompatibility, a combination of major and minor incompatibility. The occurrence of graft-vs-host (GvH) Ab in minor and host-vs-graft (HvG) Ab in major ABO incompatibility may lead to several immunohematological complications. 5,6 To avoid hemolysis in this context, several protocols to remove Ab or red blood cells (RBC) prior to transplantation as well as specific transfusion policies have been estab- lished. The long-term goal of this project is to explore the regulation of Ab production in humans undergoing alloge- neic ABO-incompatible HSCT as an in vivo model for ABO-incompatible solid organ transplantation. It is well recognized that the hematopoietic system of HSCT recipi- ents, including the ABO blood group antigen of the recon- stituted RBC, completely changes to donor type, but there is only limited data regarding the effect of this change on the levels of anti-A/B Ab. Thus, the levels of anti-A/B Ab were analyzed in the present study before and after ABO- incompatible HSCT using a new flow cytometry– based method. PATIENTS AND METHODS Serum samples of patients undergoing allogeneic HSCT were collected during hospitalization and at regular outpatient visits upon informed consent. The method used to quantify anti-A/B Ab will be reported elsewere. Briefly, human blood groups A and B RBC were isolated from heparinized venous blood and treated with Karnovsky buffer containing formaldehyde and glutaralde- hyde to avoid agglutination. Undiluted sera were incubated with a 1% solution of fixed RBC and binding of anti-A/B Ab was measured by indirect immunofluorescence and flow cytometry using isotype-specific secondary fluorescence-labeled Ab (goat From the Laboratory for Transplantation Immunology, Univer- sity Hospital of Zürich, Zürich, and Department of Hematology, University Hospital of Basel, Basel, Switzerland. Supported by a Swiss National Science Foundation grant (4046-058668) and the Julius Müller Stiftung. Address reprint requests to Jörg D. Seebach, MD, Laboratory for Transplantation Immunology, Department of Internal Medi- cine, University Hospital of Zürich, Rämistrasse 100, C HOER 31, CH-8091 Zürich, Switzerland. E-mail: klinseeb@usz.unizh.ch © 2005 by Elsevier Inc. All rights reserved. 0041-1345/05/$–see front matter 360 Park Avenue South, New York, NY 10010-1710 doi:10.1016/j.transproceed.2004.12.281 Transplantation Proceedings, 37, 1385–1387 (2005) 1385