Donor-reactive HLA antibodies in renal allograft recipients: Considerations, complications, and conundrums Howard M. Gebel a, *, Omar Moussa b , David D. Eckels c , Robert A. Bray a a Department of Pathology, Emory University, Atlanta, Georgia, USA b Department of Pathology, Medical University of South Carolina, Charleston, South Carolina, USA c Department of Pathology, University of Utah, Salt Lake City, Utah, USA ARTICLE INFO Article history: Received 8 March 2009 Accepted 9 April 2009 Available online 16 April 2009 Keywords: HLA antibodies HLA antigens HLA alleles Donor-directed antibodies Solid phase assays ABSTRACT Whether sensitized patients wait for a compatible crossmatch with a deceased donor, enter a paired exchange program with the hope of finding a compatible living donor, or go through a desensitization protocol depends on a number of factors, not the least of which is the overall philosophy of the transplant center. Centers such as ours take the position that donor-directed antibodies detected by solid phase assays (even those that are “weak”) present an unacceptable risk factor to the patient. This philosophy is predicated on the biologic role of the immune system, specifically that antibodies were generated in response to a non-self (allo) antigen and that a successful immune response eliminates that which caused its stimulation. Although obviously an oversimplification, this philosophy mandates a comprehensive evaluation of HLA antibodies in sensitized recipients. This article addresses the challenges and conundrums associated with human leukocyte antigen antibody identification.  2009 Published by Elsevier Inc. on behalf of American Society for Histocompatibility and Immunogenetics. 1. Introduction Following the landmark report of Patel and Terasaki [1], a posi- tive cytotoxic crossmatch between donor cells and recipient serum was considered a contraindication to renal transplantation. The high rate of immediate graft loss among patients undergoing trans- plantation across this barrier was unacceptable. It soon became apparent that the clinically relevant antibodies in a lymphocyte crossmatch were those directed against antigens encoded by the human major histocompatibility complex (MHC), henceforth re- ferred to as the human leukocyte antigen (HLA) complex. The paradigm to not cross the positive crossmatch barrier was modified when it was recognized that cytotoxic crossmatches resulting from non-HLA antibodies (e.g., autoantibodies) had no impact on allo- graft survival and could be safely ignored (reviewed in [2]). How- ever, there was a growing appreciation that complement-fixing, donor-directed HLA antibodies undetectable by standard cytotoxicity assays were clinically relevant (reviewed in [3]). Such observations led to the development of more sensitive antibody detection tests including the antiglobulin-enhanced cytotoxicity (AHG-CDC) and flow-cytometric crossmatch assays [4 – 8]. Although these tests were more sensitive, their shortcoming was their relative lack of specificity [9,10]. Subsequently, the development and implementation of solid phase antibody detection systems (SPADS) that specifically identified HLA antibodies represented a major advancement [3,11,12]. These sensitive and specific assays provided the transplant community with analytical tools not previously available and, in many situations, actually changed how crossmatch data were interpreted. For example, in a single-center study by Kerman et al., no differences were initially reported in the number of rejection episodes and/or graft losses among recipients undergoing transplantation with renal allografts from donors whose flow crossmatches were positive or negative as long as the AHG-CDC crossmatches were negative [13]. Subsequently, retesting of sera from these patients by solid phase technology revealed that several of the “positive” flow-cytometric crossmatches were actually caused by non-HLA antibodies [14]. Im- portantly, the above patients experienced no rejection episodes or graft loss during the study period. Thus, it should not be surprising that SPADS have supplanted cell-based assays and have become the gold- standard for identifying HLA antibodies. Nonetheless, how information from solid phase assays is trans- lated into clinical practice is quite controversial. For example, in terms of risk, should “weak” (low titered, low fluorescence inten- sity) donor-directed antibodies be given the same clinical signif- icance as “strong” (high titered, strong fluorescence intensity) donor-directed antibodies [15–18]? Can non– complement-fixing donor-directed HLA antibodies be considered less problematic than complement-fixing antibodies, as suggested by Bohmig et al. [19], or should they be considered to confer the same degree of risk to long- term graft survival as proposed by Cai and Terasaki [20]? Such dispar- ate data beg the question of how antibodies are classified as “strong” or “weak” or as complement fixing/non– complement fixing. Indeed, * Corresponding author. E-mail address: hgebel@emory.edu (H.M. Gebel). Human Immunology 70 (2009) 610 – 617 Contents lists available at ScienceDirect 0198-8859/09/$32.00 - see front matter  2009 Published by Elsevier Inc. on behalf of American Society for Histocompatibility and Immunogenetics. doi:10.1016/j.humimm.2009.04.012