Anti-HLA antibodies with complementary and synergistic interaction geometries promote classical complement activation on platelets.

High titers of HLA antibodies are associated with platelet refractoriness, causing poor platelet increments after transfusions in a subset of patients with HLA antibodies. Currently, we do not know the biological mechanisms that explain the variability in clinical responses in HLA alloimmunized patients receiving platelet transfusions. Previously we showed that a subset of anti-HLA IgG-antibodies induces FcγRIIa-dependent platelet activation and enhanced phagocytosis. Here, we investigated whether anti-HLA IgG can induce complement activation on platelets. We found that a subset of anti-HLA IgG induced complement activation via the classical pathway, causing C4b and C3b deposition and formation of the membrane-attack complex. This resulted in permeabilization of platelet membranes and increased calcium influx. Complement activation also caused enhanced α-granule release, as measured by CD62P surface exposure. Blocking studies revealed that platelet activation was caused by FcγRIIa-dependent signaling as well as HLA antibody induced complement activation. Synergistic complement activation employing combinations of monoclonal IgGs suggested that assembly of oligomeric IgG complexes strongly promoted complement activation through binding of IgGs to different antigenic determinants on HLA. In agreement with this, we observed that preventing anti-HLA-IgG hexamer formation using an IgG-Fc:Fc blocking peptide, completely inhibited C3b and C4b deposition. Our results show that HLA antibodies can induce complement activation on platelets including membrane attack complex formation, pore formation and calcium influx. We propose that these events can contribute to fast platelet clearance in vivo in patients refractory to platelet transfusions with HLA alloantibodies, who may benefit from functional-platelet matching and treatment with complement inhibitors.

The 25th anniversary of the Eurotransplant Acceptable Mismatch program for highly sensitized patients.

In 2014, the Eurotransplant Acceptable Mismatch (AM) program celebrated its 25th anniversary. The AM program was initiated to enhance transplantation of highly sensitized patients awaiting a renal transplant within the Eurotransplant region. Unlike the regular renal transplant allocation, in which the histocompatibility parameters consist of the degree of compatibility with the patient's human leucocyte antigen (HLA) type and the absence of unacceptable antigens, the AM program is based on compatibility of the possible donor with the combination of the patient's HLA type and the acceptable antigens. These acceptable antigens are defined as HLA antigens to which the patient has never made antibodies. This strategy aims at the prediction of a negative cross match. Since the start of the program almost 2000 patients participated and more than 1000 patients were transplanted with excellent transplant outcome, comparable to that of non-immunized transplant recipients within Eurotransplant. Progressive insights have led to fine-tuning of the AM program through the years, as well as to novel initiatives, including a recent consortium study to determine the feasibility of a Europe-wide AM program. The current review will tell the story of the AM program in a historical perspective, but will also provide an open-minded look into the future of transplanting highly sensitized patients.