C1Q-FIXING HLA ANTIBODIES AND KIDNEY GRAFT OUTCOME

Development of de novo donor-specific HLA antibodies (DSA) represents the major risk factor for 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, are highly sensitive but less predictive of transplant outcome because of detection of both complement-fixing and no complementfixing HLA antibodies. It has been suggested that discrimination between potentially harmful complement-fixing DSA and presumably less harmful no complement-fixing DSA could be clinically relevant in kidney transplantation. Using a modified LSA assay that identifies antibodies able to fix C1q, we investigated the clinical relevance of de novo complement-fixing DSA on kidney graft outcome. In serum samples from 40 kidney transplanted patients who developed IgG-LSA DSA after transplantation, we measured C1q-DSA using Class I and Class II LSA beads. Twenty-three of the 40 patients showed production of C1q-positive DSA; the remaining 17 patients had 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). Both C1q-positive and C1q-negative DSA were mainly specific for the DQ mismatched molecules of the donor (74% and 53% respectively). Analyzing the target molecules of DQ-specific DSA, we evidenced that 9 of the 17 (53%) anti-DQ C1q-positive DSA were specific for DQ1 molecules while 6 of the 8 (75%) anti- DQ C1q-negative DSA were specific for DQ2 molecules. In conclusion our data demonstrate that C1q-LSA assay has the capability to identify the subset of IgG-LSA DSA strongly associated with graft failure (RR = 7.39) in kidney transplantation. Moreover the ability of C1q assay in distinguishing less harmful no complement-fixing DSA from clinically relevant C1qfixing DSA represent a non-invasive tool for identifying patients that need specific immunosuppressive therapy to prolong graft survival.