Practical value of identifying antibodies to cryptic HLA epitopes in cardiac transplantation

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Identification of antibodies to human leukocyte antigens (HLA) by single antigen bead arrays has led to the common practice of virtual crossmatching. However, inappropriate assignment of anti-HLA specificities can lead to false-positive virtual crossmatching, resulting in the decline of potentially crossmatch-negative organ offers. In this study we describe identification of antibodies to cryptic HLA present on denatured forms of HLA on single antigen bead array and provide a reassessment of calculated panel-reactive antibody (CPRA) based on elimination of false-positive reactions due to antibodies to cryptic HLA epitopes. Sera from 96 patients with positive HLA antibodies detected on a standard single antigen bead platform were tested under denaturing conditions and with a new single antigen bead product (iBeads; One Lambda, Inc., Canoga Park, CA) to identify antibodies to cryptic HLA vs. native HLA. Flow cytometry crossmatching and complement-fixation assays were performed to assess clinical relevance. Antibodies to cryptic HLA were present in approximately 21% of patients on our waiting list for cardiac transplantation. These antibody responses were not associated with factors commonly thought to be associated with antibody responses to HLA such as age, gender, transfusions or presence of circulatory support. Antibodies to cryptic HLA can be reliably identified by iBeads technology, and usually do not fix complement nor produce positive flow cytometry crossmatches. Identification and removal of antibodies to cryptic HLA from the panel of unacceptable antigens may have dramatic and meaningful effects on CPRA and virtual crossmatch strategies.