Journal of Lipid Research, Vol 30, 587-596, Copyright © 1989 by Lipid Research, Inc.
A potential complication in the use of monoclonal antibodies: inhibition of apoB-mediated receptor binding by an anti-apoE antibody
R Maurice, YL Marcel, TL Innerarity and RW Milne
Laboratoire du Metabolisme des Lipoproteines, Institut de Recherches Cliniques de Montreal, Quebec, Canada.
Monoclonal antibody (Mab) 1D7 is specific for human apolipoprotein (apo) E
and blocks binding of lipid-associated apoE to the low density lipoprotein
(LDL) receptor. We report here that 1D7 can also block the binding of
apoE-free LDL to the LDL receptor. The inhibition of LDL- receptor binding
is not due to immunological cross-reactivity between the anti-apoE Mab and
apoB, the ligand responsible for the interaction of LDL with the LDL
receptor: 1) Mab 1D7 did not react with apoE- depleted LDL; 2) the LDL
receptor binding inhibitory activity of 1D7 immunoglobulin G (IgG)
preparations could be dissociated from the anti- apoE activity; 3) the
inhibition was maintained when the fibroblasts were preincubated with the
1D7 IgG, extensively washed, and only then exposed to 125I-labeled LDL.
Rather, it appears that 1D7 recognizes mouse apoE, that mouse apoE-1D7
immune complexes contaminate 1D7 IgG preparations and that the
contaminating mouse apoE can compete with 125I-labeled LDL for the LDL
receptor. We have demonstrated mouse apoE in IgG preparations of 1D7 but
not in those of other anti-apoE Mabs that do not influence LDL-receptor
binding. Precipitation of 1D7 IgG with NH4SO4 eliminates both apoE and the
capacity of 1D7 to block LDL receptor binding. Finally, mouse apoE can be
isolated by immunoaffinity chromatography of mouse serum on immobilized 1D7
Mab. As this is probably not a unique case, the observation has important
implications for the use of Mabs as structural probes.
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