OR49: ASSESSMENT OF THE LUMINEX® SINGLE ANTIGEN AND C1q ASSAYS’ ABILITY TO CORRELATE DONOR SPECIFIC ANTIBODIES WITH KIDNEY TRANSPLANT REJECTION

Abstract

Aim The association between donor specific antibodies (DSA) and renal transplant rejection has been generally established, but there are cases when a DSA is present without rejection. The Luminex® Single Antigen (SA) assay is the current standard for identifying Human Leukocyte Antigen (HLA) IgG antibody specificities and determining if they are donor specific. To improve the ability to discriminate between harmful and benign antibodies, the SA C1q assay was developed to detect antibodies that are capable of fixing complement. Our objective was to explore the detection capacity of the two assays and their correlation with kidney transplant rejection. Methods The study included 23 renal transplant recipients with transplant dysfunction. Patients in the study also had a biopsy with complement fixation, as exhibited by C4d deposition on >50% of the peritubular capillaries. Using sera from the time of the C4d + biopsy, the SA IgG assay and SA C1q assay were performed. Results Of the 23 study patients, all had DSA identified by the SA IgG assay: six with only Class I antibodies, three with only Class II antibodies, and 13 with both. Only 14 of the 23 patients had DSA detected by the C1q assay: six Class I only, seven Class II only, and one with both. The mean total of the DSAs was 12,353 by SA IgG and 18,843 by SA C1q. All but one of the patients with C1q + DSA had at least one IgG + DSA with MFI > 5000, whereas only two of the nine patients without a C1q + DSA had an IgG + DSA with MFI > 5000. Therefore, the C1q assay is relatively insensitive at lower MFI. Conclusion Complement fixation is dependent not only on the subclass of the antibody but also on the crosslinking of complement molecules. On a cell, multiple different antibodies together can additively reach the threshold for complement fixation. As shown in the data, the sensitivity of the C1q assay appears to be limited when renal damage is likely to have been caused by multiple weak antibodies in concert. Given that all of the patients studied had transplant dysfunction and signs of complement activation, these data suggest that the results of the SA C1q assay correlate poorly with clinical signs of rejection, whereas the standard SA IgG assay correlated in all 23 patients.