IgG subclass distribution and complement activation ability of autoantibodies to neutrophil cytoplasmic antigens...

To study the IgG subclass distribution and complement activation ability of ANCA, 24 sera containing C-ANCA (cytoplasmatic) and 7 sera containing P-ANCA (perinuclear), as determined by a routine immunofluorescence test, were examined. The subclass distribution was tested by the use of immunofluorescence and ELISA technique and monoclonal antibodies to IgG subclasses. The complement activating activity was studied by the use of immunofluorescence technique and antibodies to C3c and the terminal complement complex (TCC, C5b-9), directed against a neoepitope on C9. For C-ANCA, IgG1 and IgG4 were the dominating subclasses. The subclass distribution differed from that of other autoantibodies tested and that of the total IgG subclass concentrations. For P-ANCA, the results were inconclusive, but the lack of IgG3 was striking. All of the C-ANCA-containing sera caused deposition of C3c, and 50% of the sera gave formation of TCC when reacting with ethanol-fixed granulocytes. P-ANCA-containing sera caused some C3c deposition, but not TCC formation. The unusual IgG subclass distribution for C-ANCA is possibly due to repeated antigenic stimulations and/or to T cell factors influencing the antibody isotype switching. Despite the high IgG4 activity, sera containing C-ANCA are often able to activate complement and are therefore potentially harmful.