How Much Donor Human Leukocyte Antigen-Specific Antibody Is Too Much for a Renal Transplant?

The communication in this issue from Gupta et al. (1) is timely and raises important practical questions which the Histocompatibility and Immunogenetics community needs to find coherent answers too. Most centers regard the presence of donor human leukocyte antigen (HLA)-specific antibody (DHSA) at the time of transplant as a contraindication to renal, pancreas, and heart transplantation. The authors have retrospectively analyzed 121 patients who received a renal transplant between 1999 and 2001. The recipients had negative complement-dependent lymphocytotoxic and flow cytometric crossmatches with their donor and received maintenance immunosuppression of Neoral, azothioprine, and prednisolone without induction therapy. Day-of-transplant sera were recently tested for HLA-specific antibody using the modern sensitive Luminex-based Labscreen (One Lambda Inc.) assay. Sixteen patients were found with detectable DHSA, and their allografts performed remarkably well when compared with the antibody-negative and antibody-positive but non-DHSA cohorts. There were no cases of hyperacute rejection, and no significant difference in delayed graft function, biopsy-proven acute rejection, and 1-year graft survival between the three groups. Multivariate analysis did indicate that the DHSA group had a worse long-term graft outcome. Registration of a UK patient for a deceased donor renal transplant requires that patients are screened for antibodies directed at HLA antigens and these antigens are listed as unacceptable. If the antigen is present on a deceased donor the recipient will not be offered the kidney. Patients are slightly discriminated against within the national sharing scheme if their antibodies are not fully characterized. The advent of methods for locus-specific HLA purification and recombinant technology has allowed single HLA alleles to be attached to microbeads suitable for flow cytometric analysis. Luminex-based kits are one of the most popular and sensitive of these methods. This has considerably increased the sensitivity and discriminatory powers of HLA-specific antibody detection. This, in turn, has led to an increasing list of unacceptable specificities being defined in sensitized patients. This article would suggest that many patients having their antibodies defined by these methods may not be receiving offers of renal transplantation that would provide them with good renal function for many years. The complement-dependent lymphocytotoxic crossmatching and antibody screening test has long been established as the gold standard methodology to which all emerging technologies are compared. The introduction of new and more sensitive techniques raises the question of whether we need more sensitivity? The authors present data from their center, which would indicate, in the short term at least, that by using these more sensitive techniques we may be disadvantaging our patients by refusing them potentially successful transplants. An alternative view would be to suggest that this highlights the need to understand the context of the test, and instead of analyzing antibody specificities as positive or negative, we should in fact be interpreting our results and defining antibodies by clinical significance, in the context of a patient's sensitization history and using epitope analysis. For example an antibody raised directly to an HLA specificity present on a previous graft will potentially be more clinically significant than a low-level antibody to a crossreactive epitope. Most centers use these single antigen beads in addition to other antibody identification methods to allow a picture of clinically relevant specificities to develop. These specificities alone should be registered as unacceptable in matching algorithms, while accepting that other specificities may be present at low levels and may give negative crossmatches. This comprehensive analysis allows the laboratory to give as much information as possible to clinicians on the relative risk of a proposed transplant. The data presented in this article add to the argument to remove current positive and negative reporting systems and replace them with risk reporting, categorizing results into low, medium, and high risk. Many centers are now removing DHSA to enable a successful renal transplant and the reported results are generally very good. This is giving some insights with regard to acceptable thresholds for the anti-donor crossmatch and indicates the importance of the process known as accommodation. It is becoming clear that a low level of antibody may promote an antiapoptotic response from the endothelium and be beneficial to long-term engraftment (2). Collation and analysis of the laboratory tests and results of transplantation both before and after DHSA removal should provide a framework for the biological cutoff to a crossmatch. At the heart of this lies the question of the clinical significance of DHSAs in solid organ transplantation. Liver is accepted as being something of an exception, apparently capable of tolerating high levels of direct HLA antibody with hyperacute rejection being recorded but only rarely (3). Are there acceptable HLA antibody levels with other solid organ transplants? It is accepted that low to moderate levels of antibodies to some carbohydrate antigens may not be deleterious (4). We really need a theoretical rationale for the decision to transplant a sensitized patient and whether to regard the transplant as low, medium, or high risk. A few publications (5, 6) have outlined a practical rationale based on a number of factors, including sensitization event, class, and temporal presence of antibody, but with a rather tenuous evidence base, rooted in older methods that were comparatively nonspecific and insensitive. There is interesting work to be done to take HLA-specific antibody definition to the molecular level, the level at which we can now easily define HLA alleles. Even more important will be refining the decisions we make as to the correct clinical interpretation of these antibodies.