Epitope Analysis Aids in Transplant Decision Making by Determining the Clinical Relevance of Apparent Pre-Transplant Donor Specific Antibodies (DSA)

Abstract

Pre-transplantation work-up on a patient with end stage renal disease using Single Antigen Bead (SAB) testing showed significant anti-HLA-B*44:02 (>5,000 MFI) and anti-HLA-B*44:03 (>1,000 MFI) antibodies, with persistence on quarterly testing. No significant Class II anti-HLA antibodies were present. The patient received a potential offer from a living unrelated-donor expressing HLA-B*44:02. Based on the presence of anti-HLA-B*44:02 antibody, the crossmatch (XM) was predicted to be positive. However, the actual fluorescence cytometry crossmatch (FCXM) was negative. FCXM and Complement Dependent Cytotoxicity-XM (CDC-XM) studies with three surrogate donors who expressed HLA-B*44:02 (and no other potential confounding HLA types) were also negative. Additional assays were performed for detecting anti-HLA antibodies. Immucor® LSA® SAB analyses also revealed presence of anti-HLA-B*44:02 and anti-HLA-B*44:03 antibodies. However, One Lambda® Antigen Trays, C1q analysis, and iBeads®, did not detect elevated anti-HLA-B*44:02 and/or anti-HLA-B*44:03 antibodies. An extensive evaluation of all exposed and non-exposed epitopes expressed by the patient and the donor was performed to identify the non-shared epitopes between them. The donor specific antibody (DSA) pattern detected would be expected to conform to non-shared epitopes; however, non-shared "exposed" epitopes were not present in the DSA antibody pattern. Whereas, the apparent DSA antibody pattern consisted of antibodies to "non-exposed" epitopes. Altogether, it was concluded that the anti-HLA-B*44:02 antibody detected by SAB testing was directed against some denatured component(s) (non-exposed) of the HLA antigen attached to the SAB, and would not be clinically significant. The patient received the transplant and the post-transplant course has been uneventful for greater than 5 years. This case emphasizes: (1) A significant number of SAB may have denatured HLA molecules attached to them (2) The DSA and non-DSA anti-HLA antibody patterns should be evaluated for the expected epitope components to determine the clinical relevance. Additional testing should be considered to help with these analyses (3) The FCXM remains the "gold standard" for making the final decision to transplant, since the "stringent" use of a "predicted" positive XM using apparent DSA detected by SAB analysis may exclude XM-compatible donors.

Keywords: Allele-specific antibodies; DSA; Epitope analysis; Kidney transplantation.