LAUSR

Aim DSA contribute to hyperacute, acute or chronic transplant rejection. Therefore it is of major interest to define their ‘pathogenicity’, which could help to guide therapeutic interventions. As complement fixation is a primary mechanism for the induction of inflammation and organ damage during AMR, the CDC test has been used to detect DSA but its sensitivity has often been questioned. Most laboratories are now using more sensitive IgG-Luminex assays. They can’t however not distinguish complement-fixing antibodies. C3d-Luminex detects complement bound to antibody/antigen complex and the aim of our study is to evaluate this assay. Methods 63 sera were tested with CDC and C3d-Luminex. The specificities of two EPT CDC samples have also been compared as the CDC results can vary significantly between laboratories. Sera from 39 HLA-immunized patients waiting for a renal transplant were tested with IgG-Luminex with and without EDTA treatment (EDTA) and compared with C3d-Luminex. Results The % co-positivity between C3d-Luminex class I and CDC is 100% and % co-negativity is 90%. The amount of specificities of CDC was lower than those identified by C3d-Luminex. With regard to the EPT samples, one sample was not C3d positive for a consensus CDC specificity and a second sample was also not C3d positive for a CDC specificity detected by 29 of 39 participating labs. IgG-Luminex showed higher MFI values with EDTA than without. The prozone effect also disappeared after EDTA treatment and with C3d-Luminex. C3d positivity of one of the immunized patients was variable in time, DSA initially only positive with IgG-Luminex, 3 months later also positive with C3d and weakly positive with CDC and finally all the 3 assays were positive. Our pre-transplant data suggest that the C3d-binding capacity of antibodies is correlated with a higher MFI. C3d-Luminex was always negative/positive under/above a certain IgG-Luminex cutoff. Conclusions Antibodies directed against the graft can cause injury via different mechanisms, also in the absence of complement (ADCC and/or direct activation of endothelial cells). Technique-dependent differences between antibody reactivity could be associated with antibody titer (prozone), other DSA characteristics (avidity, IgM, IgG subclass) and the expression level of HLA (CDC).