OR35 IgG subclass analysis in C4d+ DSA positive patients

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 HLA IgG antibody specificities and determining if they are donor specific. As a means 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. We also have adapted the standard SA IgG assay by utilizing IgG-subclass specific monoclonal antibodies. We sought to explore the detection capacity of the three SA assays and their correlation with renal transplant rejection. Methods 23 patients were selected who had received biopsies due to graft dysfunction and all had C4d deposition on the peritubular capillaries. Out of the 23 patients who presented with graft dysfunction, 5 proceeded to graft loss. Serum samples collected within a week of the biopsy were used for Luminex SA IgG, C1q and IgG subclass analysis. Results All 23 C4d+ patients had SA IgG DSAs with an average of 12,500 MFI (cumulative DSA MFI). 25 control patients with C4d− biopsies had average DSAs less than 500 MFI. The C1q assay showed 16 positive results out of the 23 DSA and C4d+ patients. The C1q assay appeared to be less sensitive particularly when multiple low level IgG DSAs occurred. There was no significant correlation to graft loss and C1q positivity. The IgG subclass assay showed complement fixing IgG1 in 20 of 23 patients. IgG2 DSAs were found in six patients and IgG3 DSAs were found in two patients, all in conjunction with IgG1 DSAs. Interestingly, IgG4 was seen in 10 of the 23 recipients’ sera, but always along with complement fixing IgG1. The detection of IgG4 subclass did not correspond to any enhanced graft survival outcome. Conclusions Cumulative DSA’s above 10,000 MFI were associated with C4d deposition and complement fixation. In this study of patients with C4d+ biopsies, 100% had IgG DSAs, 70% had C1q+ DSAs, and 83% had complement fixing IgG subclass antibodies. We could not demonstrate any enhanced benefit of IgG4. However, in this cohort of patients, IgG4 appeared coincidently to IgG1, and we have previously seen excellent function in patients when IgG4 DSA exists alone.