The criteria for reporting unacceptable HLA antigens of renal transplant candidates differ greatly between transplant centers. In the attempt to refine these criteria, the experience of each center is valuable. In this study, we investigated whether pre-transplant screening of complement binding HLA antibodies using the C1q assay may improve the identification of unacceptable HLA antigens and the predictive value of the virtual crossmatch.

From Jan 1^st 2011 to Sept 1^st 2013, 296 patients received renal transplantation at our center. Anti-HLA antibodies were tested prior to transplantation using conventional (IgG) Mixed and Single Antigen Bead assays (One Lambda). Targets of HLA-A, -B, and -DR (MFI>5,000) and HLA-C and -DQ (MFI>10,000) antibodies were listed as unacceptable antigens in UNET. Thus, donor offers to patients with strong DSA were precluded. In patients who received offers, we accurately predicted negative/weak positive flow crossmatch (XM) and negative complement dependent cytotoxicity (CDC) XM results (p<0.0001). C1q binding capacity of pre-transplant DSA was tested retrospectively. Fifty-nine out of 60 patients with pre-formed DSA had no C1q binding DSA (98%). In only one patient, a DSA directed to DQ9 (DQB1*03:03/DQA1*02:01; MFI<3000) showed strong C1q reactivity (MFI>10,000). However, this antibody did not cause a positive crossmatch and its reactivity after DTT treatment was consistent with that of an IgM antibody.

These results indicate that testing the C1q binding capacity of pre-transplant DSA did not significantly change the stratification of immunological risk in our patients. Thus, the use of the C1q assay is dispensable in the assignment of unacceptable HLA antigens. Using well calibrated MFI cutoff values ensures a high predictive value of the virtual crossmatch in deceased donor renal transplant candidates.

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