Polyclonal antibodies against formaldehyde-modified apolipoprotein A-I. An approach to circumventing fixation-induced loss of antigenicity in immunocytochemistry.

A central requirement for the subcellular localization of synthesized and/or endocytozed lipoproteins is the availability of antibodies against the antigen to be localized. The preparation of cells for electron microscopy, in particular the fixation and embedding routine, influences the antigenicity, often resulting in a markedly reduced labeling intensity. To ameliorate fixation-induced changes in antigenicity, we produced antibodies against pre-fixed human apolipoprotein (apo) A-I. Purified apo A-I was fixed with 4% formaldehyde and was used to raise polyclonal antibodies in rabbits. The antiserum was purified by protein A-Sepharose followed by affinity chromatography with the fixed antigen coupled to vinylsulfone-activated agarose. The specificity of the antibodies was ascertained by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis against different fixed and unfixed lipoproteins. Nonspecific binding to unfixed or to fixed apolipoproteins was not observed. Thus, the antibodies reacted specifically with apo A-I and recognized the fixed as well as the unfixed protein. In ELISA, the reaction of the antibodies was markedly enhanced with the fixed antigen, indicating that the antibodies were directed against epitopes characteristically modified by the fixation. The efficacy of the antibodies for light and electron microscopy was tested on HepG2 cells and on human liver cells which are known to synthesize apo A-I. When HepG2 cells were exposed to anti-apo A-I antibodies followed by a secondary fluorescein isothiocyante-labeled antibody, fluorescence was found intracellularly in distinct regions. Electron microscopy revealed that the endoplasmic reticulum, and in particular the trans elements of the Golgi complexes, were the main compartments stained for apo A-I both in HepG2 cells, as shown by the immunoperoxidase technique, and in human hepatocytes, as shown by the protein A-gold technique on ultrathin cryosections. Furthermore, we were able to localize apo A-I in foam cells of the human aortic plaque by postembedding immunolabeling techniques. These findings demonstrate the potential of antibodies to fixed lipoproteins in impaired localization at the light microscopic and electron microscopic levels.