RE: Imlifidase-generated Single-cleaved IgG: Implications for Transplantation

With great interest, we read the article by Bockermann et al¹ on crossmatch assays after the administration of the novel desensitization drug Imlifidase. Proteolysis by Imlifidase generates single-cleaved immunoglobulin G (IgG) in a rapid first reaction and subsequently 2 F(ab’)₂ fragments and a fully separated Fc fragment in a longer second reaction.² Bockermann et al¹ demonstrated that Luminex single antigen bead (SAB) testing, and crossmatching with both flowcytometry (FCXM) and the anti-human IgG-amplified complement-dependent cytotoxicity crossmatch (CDCXM) do not fully distinguish single-cleaved IgG from intact IgG. Bockermann et al¹ suggest that the nonamplified CDCXM is best capable of demonstrating relevant residual intact IgG.

Our center performed the first Imlifidase reimbursed HLA-incompatible kidney transplantation in March 2022. Contrary to Bockermann et al,¹ we had difficulties in the interpretation of nonamplified CDCXM after Imlifidase treatment.

A 29-y-old female with 6-y dialysis vintage after 2 failed kidney transplants was running out of vascular access. Despite high-urgency status and priority allocation via the Eurotransplant Acceptable Mismatch program, she remained without kidney offers. This was caused by her virtual panel-reactive antibody status of 100%. Her historical SAB mean fluorescence intensity (MFI) levels were high with broad reactivity in B-cell memory, with current MFI levels in the lower ranges. To enable for a timely HLA-incompatible kidney transplant, we delisted all unacceptable antigens with corresponding MFI levels of <3000, provided that CDC screening was negative. By this delisting, we increased her donor frequency of 0.000%–3.95%. Eight days later, this resulted in an HLA 1-2-2 mismatched kidney offer, with 2 “delisted DR-unacceptables.” Imlifidase was administered based on a virtual crossmatch. Serum was sampled 2 and 4 h after Imlifidase administration. Nonamplified CDCXMs with donor spleen cells were performed with preblood simultaneously tested with post 2-h and post 4-h sampling. Pre-CDCXM without dithiothreitol (DTT) treatment (which dissolves the disulfide bridges of IgM tetramers) was positive, indicating autoreactivity. DTT treatment revealed a negative pre-Imlifidase CDCXM. To our surprise, both the 2- and 4-h Imlifidase CDCXMs with DTT were positive (value 35 of 100). We had to decide whether or not to proceed with transplantation based on the nonamplified CDCXMs alone, as additional FCXM and SAB testing were scheduled for the next day. Because our patient had not recently received blood transfusions, was without infectious episodes, and had a negative pre-Imlifidase CDCXM, we expected assay interference by Imlifidase and decided to proceed with transplantation. The retrospectively performed pre-Imlifidase FCXM was negative for T cells and was 1.94 MCS for B cells. Luminex SAB was positive on 2 donor DR loci with 2500 and 2200, and became negative after Imlifidase. We investigated CDCXM on frozen donor spleen cells retrospectively and noticed CDC-positivity 2 and 4 h after Imlifidase, with negative CDCXMs on day 3. The patient did experience a vascular rejection after 1 wk, without signs of AMR. SAB testing revealed HLA antibody rebound from day 4 onward; however, DSA remained negative until current follow-up of 4 mo.

This case demonstrates that even in nonamplified CDCXM, an interference by Imlifidase may occur. We advise to combine different assays when interpreting desensitization results after Imlifidase treatment.