1.2000
#50-OR
CRITICAL IMPORTANCE OF HLA ANTIBODY (Ab) EPITOPE SPECIFICITY TO ACTIVATE HUMAN COMPLEMENT (C).
Fumiki Kushihata, Jota Watanabe, Arend Mulder, Frans Claas and Juan Scornik. Gainesville FL, USA, University of Florida College of Medicine, 32610, Pathology and Leiden Netherlands, Leiden University Medical Center.
It is now known that the activation of some C components, even when there is no cell lysis, is crucial for producing graft injury. Since most previous studies have focused in C-dependent cytotoxicity (CDC), little is known about the ability of HLA Abs to activate individual C components. Here we evaluated total IgG, IgG subclasses (sc) and Ab specificity in the activation of C from fresh human normal serum. IgG, IgM, IgG sc and C3b on T cells were measured by flow cytometry using sera from HLA sensitized patients (negative for IgM Abs) and human monoclonal HLA Abs. There was a poor correlation of CDC with rabbit C and the amount of IgG on T cells. Activation of C3 was dependent on the serum/cell combination: in some cases there was high IgG/low C3b whereas in other cases there was low IgG/high C3b. IgG sc did not explain these results as IgG1 was the predominant Ab in all patients, with little or no contribution from IgG2, IgG3 or IgG4. Of 8 IgG1 human monoclonal Abs specific for various class I antigens, 6 did not activate C3, 1 activated C3 efficiently, and 1 activated C3 at high Ab levels. Remarkably, combinations of 2 non-activating monoclonal Abs to different epitopes of the same antigen, but not to different antigens on the same cell, did produce significant C3b deposition. These results strongly suggest that the mix of HLA specificities of a given patient reacting with a given target cell is what determines if Abs will activate human C or not. This behavior can be established only by measuring C3b deposition (or other C component) as it is not predicted by the standard CDC, amount of IgG deposition or measurement of IgG sc.