Biological Effects of F(ab′)2 Fragments Generated by Imlifidase From Anti‐ <scp>HLA IgG</scp> Antibodies From Transplant Patients

Imlifidase cleaves intact IgG molecules, generating F(ab')2 fragments that still bind to the target antigen but are devoid of the Fc-dependent effector functions, notably complement activation. Imlifidase (Idefirix) has been conditionally approved for desensitisation to enable kidney transplantation in highly sensitised adult patients. We hypothesised that the F(ab')2 fragments can prevent the attachment of the full IgG at the time of anti-HLA antibodies rebound when imlifidase action fades away, lowering the deleterious effect of the antibody burden on the transplant. This competition was explored in vitro using the Luminex Mixed or Single Antigen bead assays, and with an antibody detecting the specific imlifidase-cleaved IgG. Functional assays for complement activation were also performed, which were the C1q bead assay and complement-dependent cytotoxicity crossmatches. We selected sera from patients with a high amount of class I or class II anti-HLA antibodies. Competition by imlifidase-digested serum was observed in Luminex antibody-binding assays and cellular complement activation assays. F(ab')2 fragments generated from anti-HLA antibodies after imlifidase cleavage had a biological impact on intact IgG binding, leading to reduced complement activation. Additional studies are needed to demonstrate whether these in vitro features have beneficial clinical consequences in vivo, that is, protecting the graft from potentially deleterious effects of DSA rebound.