P144 Impact of low-level (MFI) HLA-antibodies on living donor kidney transplant rejection risk

Abstract

Histocompatibility testing is used to predict the risk of hyperacute and accelerated-acute transplant rejection. The presence of HLA-specific antibodies, as detected by single antigen bead testing (SAB), are used in organ allocation through the calculated panel reactive antibody (cPRA) and by assessing the presence of donor HLA-specific antibodies (DSA). The presence of DSA and cPRA are considered in the algorithm for assigning patients to immunosuppression protocols. However, the interpretation of low level antibodies in SAB and DSA testing is challenging. Our aim was to look at antibodies below our current threshold of 2000 MFI but above our analytical threshold of 1000 MFI to determine if there is residual risk related to these antibodies. Patient antibody screens were evaluated with 2000 MFI and 1000 MFI cutoff values. 181 patients were assessed for pre-transplant patient risk category and donor compatibility by determining: the change in cPRA, number of identified HLA specific antibodies between the thresholds, the presence of DSA identified between thresholds and the predicted change in immunosuppression protocols. Living donor kidney transplants with an average of 2 years of follow-up were reviewed. 81 recipients were scored for the presence of DSA and examined for the development of de novo DSA (dnDSA) post-transplant. Biopsy data was also reviewed on 40 living donor kidney transplant recipients to determine the incidence of biopsy-proven rejection. The reduction in antibody threshold was associated with an average increase in cPRA of 15% for HLA-Class I and 18% for HLA-Class II which corresponded to detection of 10 additional HLA specificities. We determined that using the 1000 MFI threshold, 15% of patients would have been considered higher risk based on transplant center protocols and would have necessitated increased immunosuppression. Changes in risk category were weakly associated with higher rates of dnDSA post-transplant and biopsy-proven rejection. In this study, the development of biopsy-proven rejection was not associated with pre-transplant DSA or post-transplant dnDSA that were between 1000 and 2000 MFI.