title: Detection of alloantibody-mediated complement activation: A diagnostic advance in monitoring kidney transplant rejection?
abstract: OBJECTIVE
  Antibody-mediated rejection (ABMR) is an important cause of kidney allograft injury. In the last two decades, detection of complement split product C4d along transplant capillaries, a footprint of antibody-mediated classical complement activation, has evolved as a useful diagnostic marker of ABMR. While it was recognized that ABMR may occur also in the absence of C4d, numerous studies have shown that C4d deposition may indicate a more severe rejection phenotype associated with poor graft survival. Such studies suggest a possible diagnostic benefit of ex vivo monitoring the complement-activating capability of circulating alloantibodies.
  
  
  METHODS
  We reviewed the literature between 1993 and 2015, focusing on in vivo (biopsy work-up) and in vitro detection (modified bead array technology) of HLA antibody-triggered classical complement activation in kidney transplantation.
  
  
  RESULTS
  Precise HLA antibody detection methods, in particular Luminex-based single antigen bead (SAB) assays, have provided a valuable basis for the design of techniques for in vitro detection of HLA antibody-triggered complement activation reflected by C1q, C4 or C3 split product deposition to the bead surface. Establishing such assays it was recognized that deposition of complement products to SAB, which critically depends on antibody binding strength, may be a cardinal trigger of the prozone effect, a troublesome in vitro artifact caused by a steric interference with IgG detection reagents. False-low IgG results, especially on SAB with extensive antibody binding, have to be considered when interpreting studies analyzing the diagnostic value of complement in relation to standard IgG detection. Levels of complement-fixing donor-specific antibodies (DSA) were shown to correlate with the results of standard crossmatch tests, suggesting potential application for crossmatch prediction. Moreover, while the utility of pre-transplant complement detection, at least in crossmatch-negative transplant recipients, is controversially discussed, a series of studies have shown that the appearance of post-transplant complement-fixing DSA may be associated with C4d deposition in transplant capillaries and a particular risk of graft failure.
  
  
  CONCLUSIONS
  The independent value of modified single antigen bead assays, as compared to a careful analysis of standard IgG detection, which may be affected considerably by complement dependent artifacts, needs to be clarified. Whether they have the potential to improve the predictive accuracy of our current diagnostic repertoire warrants further study.
authors.affiliation: Division of Nephrology and Dialysis, Department of Medicine III, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria. Electronic address: georg.boehmig@meduniwien.ac.at.
authors.forename: Georg A
authors.initials: GA
authors.lastname: Böhmig
authors.affiliation: Division of Nephrology and Dialysis, Department of Medicine III, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Zeljko
authors.initials: Z
authors.lastname: Kikic
authors.affiliation: Division of Nephrology and Dialysis, Department of Medicine III, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Markus
authors.initials: M
authors.lastname: Wahrmann
authors.affiliation: Division of Nephrology and Dialysis, Department of Medicine III, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Farsad
authors.initials: F
authors.lastname: Eskandary
authors.affiliation: Department of Cardiac Surgery, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Arezu Z
authors.initials: AZ
authors.lastname: Aliabadi
authors.affiliation: Institute of Immunology, Medical University Vienna, Lazarettgasse 19, A-1090 Vienna, Austria.
authors.forename: Gerhard J
authors.initials: GJ
authors.lastname: Zlabinger
authors.affiliation: Clinical Institute of Pathology, Medical University Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria.
authors.forename: Heinz
authors.initials: H
authors.lastname: Regele
authors.affiliation: Department of Organ Transplantation/Nephrology, Fachklinik Bad Heilbrunn, Wörnerweg 30, 83670 Bad Heilbrunn, Germany.
authors.forename: Helmut E
authors.initials: HE
authors.lastname: Feucht
mesh_terms: D003167:Complement Activation / Q000276:immunology*; D006084:Graft Rejection / Q000175:diagnosis* / Q000276:immunology; D006801:Humans; D007518:Isoantibodies / Q000276:immunology*; D016030:Kidney Transplantation
chemical_list: D007518:Isoantibodies
keywords: Antibody-mediated rejection; C4d; Complement; HLA antibodies; Kidney transplantation; Single antigen beads