Human leukocyte antigen antibodies and human complement activation: role of IgG subclass, specificity, and cytotoxic potential.

Background: Human leukocyte antigen (HLA) antibodies are defined as complement (C) fixing and clinically relevant based upon the complement-dependent cytotoxicity (CDC) test. However, the sub-lytic activation of individual C components is of critical biologic significance. The requirements of HLA antibodies to activate human C are not known.
Methods: IgG, IgM, IgG subclasses, and human C3b deposition upon T cells were evaluated by flow cytometry with sera from HLA-sensitized patients and human monoclonal HLA antibodies.
Results: Comparative studies showed that there was poor correlation between the amount of IgG on target cells and their ability to produce CDC. Human C3b deposition was influenced more by the particular serum/cell combination under study than by the amount of IgG, with some combinations showing high IgG and low C3b and others showing low IgG and high C3b. IgG1 was the predominant IgG subclass in all patients. The other subclasses were low or undetectable and did not correlate with C3b deposition. Human monoclonal HLA antibodies, mostly IgG1, did not activate human C efficiently despite high IgG binding. However, combinations of two monoclonal antibodies to different epitopes of the same antigen did produce significant C3b deposition.
Conclusions: Contrary to common assumptions, CDC, IgG binding, and IgG subclass are poor predictors of human C activation by HLA antibodies. The mix of specificities in a given serum and the antigens of a particular target cell appear to determine the efficiency of C activation. Measuring both antibody and C3b deposition (or other C component) may improve the assessment of donor-recipient compatibility.