Monoclonal antibodies to human plasma low-density lipoproteins. I. Enhanced binding of ¹²⁵I-labeled low-density lipoproteins by combined use of two monoclonal antibodies

Four monoclonal antibodies ((IgG2b) to human plasma low-density lipoproteins (LDL) have been characterized. The binding affinities of each monoclonal antibody to ¹²⁵I-labeled LDL were moderately high, ranging from 10⁸ to 10¹⁰ L/mol at 4°C, but were reduced by at least 50-70% at 37°C. The maximum binding of each monoclonal antibody was unique, ranging from 20 to 95% of total ¹²⁵I-labeled LDL, suggesting that LDL particles were immunochemically heterogeneous. One antibody, LP-34, had both high and low binding affinities to LDL. Another, LP-47, exhibited high affinity for isolated LDL, yet reacted poorly with native LDL in plasma, indicating that the confirmation of isolated LDL differs from that of native LDL in plasma. Unlike polyclonal serum antibodies, a mixture of four monoclonal antibodies failed to precipitate LDL, but did show a drastic increase in binding to LDL. We found that only two of our monoclonal antibodies were necessary for such synergistic enhancement. We propose that one of the monoclonal antibodies may serve as a catalytic reagent, and discuss the clinical significance of this finding.