Persistent strong anti-HLA antibody at high titer is complement binding and associated with increased risk of antibody-mediated rejection in heart transplant recipients

Abstract

BACKGROUND Sensitized heart transplant candidates are evaluated for donor-specific anti-HLA IgG antibody (DSA) by Luminex single-antigen bead (SAB) testing (SAB-IgG) to determine donor suitability and help predict a positive complement-dependent cytotoxicity crossmatch (CDC-XM) by virtual crossmatching (VXM). However, SAB testing used for VXM does not correlate perfectly with CDC-XM results and individual transplant programs have center-specific permissible thresholds to predict crossmatch positivity. A novel Luminex SAB-based assay detecting C1q-binding HLA antibodies (SAB-C1q) contributes functional information to SAB testing, but the relationship between SAB strength and complement-binding ability is unclear. METHODS In this retrospective study, we identified 15 pediatric and adult heart allograft candidates with calculated panel-reactive antibody (cPRA) >50% by SAB-IgG and compared conventional SAB-IgG results with SAB-C1q testing. RESULTS Pre- and post-transplant DSA by SAB-C1q correlated with DSA by SAB-IgG and also with CDC-XM results and early post-transplant endomyocardial biopsy findings. Individual HLA antibodies by SAB-IgG in undiluted sera correlated poorly with SAB-C1q; however, when sera were diluted 1:16, SAB-IgG results were well correlated with SAB-C1q. In some sera, HLA antibodies with low mean fluorescent intensity (MFI) by SAB-IgG exhibited high SAB-C1q MFIs for the same HLA antigens. Diluting or heat-treating these sera increased SAB-IgG MFI, consistent with SAB-C1q results. In 13 recipients, SAB-C1q–positive DSA was associated with positive CDC-XM and with early clinical post-transplant antibody-mediated rejection (cAMR). CONCLUSIONS Risk assessment for positive CDC-XM and early cAMR in sensitized heart allograft recipients are correlated with SAB-C1q reactivity.

Utility of HLA Antibody Testing in Kidney Transplantation

“…47 However, another study found C1q+DSA more likely to be of DQ specificity and also, associated with a 30% decrement in 5-year survival. 84 To bind complement, antibodies must be IgG1/3 isotope and be of sufficiently high titer; closer examination of these studies and others indicates that antibody amount (estimated albeit imperfectly by MFI) is the major predictor of C1q positivity and outcomes, 48,[84][85][86][87]117 and the role of this new assay in addition routine SAB testing remains to be fully determined.…”

Impact of IgG3 Subclass and C1q-Fixing Donor-Specific HLA Alloantibodies on Rejection and Survival in Liver Transplantation

“…This is based on our previous findings that IgG3 subclass DSA was an independent risk factor for allograft loss (26) and recent studies in kidney and heart transplantation that showed C1q testing, detected using a sensitive bead assay, was able to identify DSAs associated with the lowest survival and highest rejection rate (30,31). However, IgG3 subclass and C1q testing do not necessarily document the in vivo pathologic potential of IgG3 subclass DSA and complement fixing DSA, and instead may simply represent either more evolved alloimmune responses or simply more specific tests.…”

Antibody-mediated rejection across solid organ transplants: manifestations, mechanisms, and therapies

“…In vitro assays measure binding of C1q (45) or deposition of C3d (46) or C4d (47,48) to single-antigen beads, enabling identification of potentially complement-activating antibodies as well as their HLA specificity. The prognostic value of these assays remains controversial (5, 7, 20, 49-52), in part because in vitro complement activity appears to be tightly tied to antibody titer (20,44,53,54).…”