Anti-HLA Antibody: The Role of Epitopes in Organ Transplantation

Recent developments have been achieved for better matching in solid-organ transplantation by epitope matching. HLA matching remains a standard immunologic strategy to determine organ compatibility for recipients. Advancements in tissue typing have introduced HLA matching at the epitope level. For this, B cells are able to recognize polymorphic amino acid configurations, which are named epitopes. However, antibody production against these epitopes may be a leading cause of rejection. Although data to support the added clinical benefit of epitope matching over traditional antigen matching are still lacking, there are at least 2 theoretical reasons for epitope matching: (1) to avoid future allosensitization with the development of anti-HLA antibodies and (2) to allow selection of a suitable allograft for highly sensitized patients through virtual crossmatch. Sensitive antibody tests with a comprehensive panel of single alleles have yielded informative, donor-specific antibody reactivity patterns that can be analyzed with a computer algorithm. This program (HLAMatchmaker) uses structurally defined eplets to describe epitopes that can react with specific antibodies. Although low mean fluorescence intensity levels by Luminex (Austin, TX, USA) assay are usually considered low risk for solid organ transplant, it could be important in posttransplant humoral rejection. Thus, specific shared eplets should always be investigated with respect to previous transplant mismatches. Epitopes are present on a single HLA (private epitope) or shared by multiple antigens (public epitope). The phenomenon of crossreactivity in HLA testing, often explained as crossreactive groups of antigens with antibody, can be clearly explained now by public epitopes, an antibody targeting an epitope that shows positive reaction with all antigens sharing the epitope. A better understanding of the immunogenicity and structural characteristics of HLA epitopes will guide us to consider epitope matching as an important parameter for donor selection in kidney transplant in the near future.