C1q Donor-Specific Antibody Associates with Post-transplant Biopsy Findings in Highly- Sensitized Kidney Transplant Recipients.

Donor-specific anti-human leukocyte antigen antibodies (DSA) are associated with antibody-mediated rejection (AMR) in kidney transplantation, but the spectrum of graft injury seen in patients with DSA ranges from no damage to florid rejection. Since immunoglobulin G (IgG) antibodies with cytotoxic potential can be distinguished by their binding complement fraction C1q, the level of C1q-binding IgG (C1q+) DSA may be useful for stratifying risk or diagnosing AMR. We therefore investigated the value of IgG and C1q+ DSA in predicting pathologic features of AMR on kidney biopsies. We tested the associations between DSA at different cut-off levels and pathologic features of AMR on biopsy in a cohort of consecutive, highly-sensitized patients transplanted after December 2014 who had 1-, 3-, and 6-month protocol kidney biopsies and monitoring for IgG and C1q+ DSA. Eight patients with cPRA >90% and negative flow crossmatch underwent kidney transplant and completed six months of follow-up contributing 23 pairs of biopsy/ serum samples for analysis. C1q+ DSA was significantly associated with C4d finding on biopsy at mean fluorescence intensity (MFI) cut-offs of >100 (p=0.046) and >300 (p=0.008) and showed superior positive and negative predictive value in comparison to conventional IgG DSA. C1q+ DSA also showed significant association and good predictive value for any AMR feature on biopsy (p=0.003, for >100 MFI; p=0.005 for >300 MFI), while IgG DSA showed no association. In a small cohort of high cPRA transplant recipients, C1q+ DSA outperformed IgG DSA as an indicator of AMR biopsy findings. Including C1q+ DSA testing in post-transplant DSA monitoring of highly-sensitized patients may aid the timely diagnosis of AMR.