Clinical Relevance of C1q-Fixing HLA Antibodies in Kidney Transplantation

Introduction: De novo production of donor-specific HLA antibodies (DSA) represents the major risk factor of graft failure inkidney transplantation. However, some patients show persistent presence of circulating DSA without occurrence of graftdysfunction/loss.The complement-dependent citotoxicity (CDC) assay has demonstrated low sensitivity in detecting DSA. Newer solid phaseassays, such as Luminex Single Antigen (LSA) beads (microbeads coated with single HLA class I or class II molecules), arehighly sensitive but less predictive of transplant outcome because of detection of both complement-fixing and less clinicallyrelevant no complement-fixing HLA antibodies.Methods: Using a modified LSA assay that identifies antibodies able to fix C1q, we investigate the clinical relevance of denovo HLA-DSA in relation to their capability to fix complement.In serum samples from 40 kidney transplanted patients who had developed IgG-LSA DSA after transplantation, we measuredC1q-fixing ability of detected DSA by Class I and Class II LSA beads (One Lambda, CA). As for transplant outcome, 22 (55%)patients suffered graft failure (within 10±1 months from DSA appearance) and 18 (45%) had good graft function (GF) during allthe follow up (mean follow up from DSA appearance: 54±34 months).Results: Twenty-three of the 40 patients showed production of C1q-positive DSA; the remaining 17 patients produced C1qnegativeDSA. Correlating graft outcome and capability of DSA to fix C1q, we evidenced that graft failure due to antibodymediatedrejection occurred in 20 of the 23 patients showing C1q-positive DSA; only 2 of the 17 patients showing C1qnegativeDSA suffered graft failure (87% vs. 12% respectively, P< 0.0001; RR = 7.39; Sensitivity=0.91; Specificity=0.83;PPV=0.87; NPV=0.88). It is to underlay that both C1q-positive and C1q-negative DSA were mainly specific for DQmismatched molecules of the donor (74% and 53% respectively). Analyzing the target molecules of DQ-specific DSA inrelation to the ability to fix complement, we evidenced that 53% (9/17) of anti-DQ C1q-positive DSA were specific for DQ1molecules while 75% (6/8) of anti-DQ C1q-negative DSA were specific for DQ2 molecules.Conclusion: Our findings demonstrate that C1q-LSA assay shows the capability to identify the subset of IgG-LSA DSAstrongly associated to antibody-mediated rejection and graft loss in kidney transplantation. Moreover the ability of C1q assayin distinguishing less harmful no complement-fixing DSA from clinically relevant C1q-fixing DSA represent a non-invasive toolfor identifying patients that need specific immunosuppressive-therapy strategy to prolong graft survival.All abstracts are available in a quotable version on the Transplantation Journal Website www.transplantjournal.com from beginning of September 2012