Differences in the competitive capacity of monoclonal antibody sensitized human A and B erythrocytes in complement mediated cytotoxicity.

Lysis of group A and B erythrocytes by human complement was studied using 9 anti-A (IgM) and 10 anti-B (IgM) monoclonal antibodies in direct and cold target competition 51Cr-release assays. The relative concentration of membrane-bound immunoglobulins was detected by indirect immunofluorescence in flow cytometric analysis. The number of binding sites/cell and affinity of the monoclonal antibodies was measured in ELISA assays. Complete haemolysis was obtained at 2 micrograms.ml-1 antibody concentrations with the majority of reagents when only a small proportion of the antibody binding sites were occupied. The influence of membrane-bound antibody on complement binding was examined by the complement consumption capacity of sensitized erythrocytes at supra-haemolytic antibody concentration. We found that complement consumption correlated with the amount of membrane-bound immunoglobulins. To obtain equal level of target and competitor cell lysis group B erythrocytes had to be equipped with higher amounts of antibodies than group A erythrocytes. The different lytic activity of IgM anti-A and anti-B monoclonal antibodies could not be accounted for merely by differences in their surface density.