P195 Single antigen dilution reveals interference and falsely low MFI for donor specific antibodies

Abstract

A 48 year old man presented for second kidney transplant with a potential living donor, his brother. His first transplant was in 2000. At evaluation, the patient had HLA single antigen class I and II testing on neat serum treated with DTT, flow crossmatch (FCXM, ±Pronase) and cytotoxic crossmatch (CDC, ±DTT). Neat serum showed antibodies to HLA-A29 and B51, both repeat mismatches from his prior transplant, and an odd pattern of DQA1 antibodies (Table 1). The initial FCXM with his brother was positive on T (187MCS) and B cells (408MCS). The B CDC was also positive. The patient displayed DSA to HLA-A29 (8277MFI), B51 (1588MFI), and DQA1∗02 (8467MFI) in the same serum. However, the crossmatch was unexpectedly strong given the MFI. Importantly, the patient’s first transplant was from another brother, who was HLA identical to this potential sibling donor. We suspected that the patient in fact had strong antibodies to HLA-DQ2 and DQ7, which were repeat mismatches. It was curious that the beads with DQA1∗05 had the lowest MFI as this combination was exactly that present in the first donor. In addition, there were apparently allele-specific DQA1∗03:01 antibodies, which is unusual. Thus, we performed dilution of the serum at 1:10 and 1:100 to determine whether there was interference with very strong antibodies. Dilution of the patient’s serum resulted in a significant increase in MFI for DQ beads. For example, DQA1∗03:02/DQB1∗03:02 went from 944MFI to 10,533 MFI at 1:10. Dilution also showed that the patient’s DSA was to HLA-DQ2 and DQ7 rather than to DQA1 alleles. The donor was declined and paired exchange was recommended. We also noted on the single antigen reports that the patient exhibits “prozone” and the antibodies to DQ7 and DQ2 are much stronger than represented by the MFI in neat serum. This case demonstrates the utility of serum dilution to identify strong antibodies with falsely low MFI, and illustrates a pattern of antibody reactivity that should appear “suspicious” for interference: for example, apparent DQA1 allele-specific antibodies.Download : Download high-res image (208KB)Download : Download full-size image