Clinical Relevance of C1q-Fixing HLA Antibodies in Kidney Transplantation

Abstract

Introduction: De novo production of donor-specific HLA antibodies (DSA) represents the major risk factor of graft failure in kidney transplantation. However, some patients show persistent presence of circulating DSA without occurrence of graft dysfunction/loss. The complement-dependent citotoxicity (CDC) assay has demonstrated low sensitivity in detecting DSA. Newer solid phase assays, such as Luminex Single Antigen (LSA) beads (microbeads coated with single HLA class I or class II molecules), are highly sensitive but less predictive of transplant outcome because of detection of both complement-fixing and less clinically relevant no complement-fixing HLA antibodies. Methods: Using a modified LSA assay that identifies antibodies able to fix C1q, we investigate the clinical relevance of de novo 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 measured C1q-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 all the 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 C1q-negative DSA. Correlating graft outcome and capability of DSA to fix C1q, we evidenced that graft failure due to antibody-mediated rejection occurred in 20 of the 23 patients showing C1q-positive DSA; only 2 of the 17 patients showing C1q-negative DSA 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 DQ mismatched molecules of the donor (74% and 53% respectively). Analyzing the target molecules of DQ-specific DSA in relation to the ability to fix complement, we evidenced that 53% (9/17) of anti-DQ C1q-positive DSA were specific for DQ1 molecules 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 DSA strongly associated to antibody-mediated rejection and graft loss in kidney transplantation. Moreover the ability of C1q assay in distinguishing less harmful no complement-fixing DSA from clinically relevant C1q-fixing DSA represent a non-invasive tool for identifying patients that need specific immunosuppressive-therapy strategy to prolong graft survival.