Using Glycosyltransferases for Conjugation of Single-Chain Antibodies and Lipids

Division Of Basic Sciences - Nci

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Abstract

<P><i><b>Glycoconjugation of single chain antibodies (scFv) with various molecules for cancer diagnosis and treatment:</i></b>To extend our bioconjugation work towards the immunoliposome formulation, we have taken up single chain antibodies (scFv) against CD22 and HER-2 proteins. The cDNAs of scFv against CD22 and HER-2 proteins were obtained from Dr. Dimitrovs group. These cDNAs were manipulated such that the single chain antibodies expressed in E. coli have a C-terminal fusion polypeptide containing 1, 3, or 17 threonine (Thr) residues. These scFv antibodies were expressed in E. coli mainly in inclusion bodies and very poorly as a soluble protein, and only micro gram quantities were obtained from the soluble fraction. Therefore an in vitro folding method has been developed that produced nearly 20 mgs of each from a one liter bacterial culture. Competitive ELISA assay indicated that the in vitro folded anti HER-2 scFv is correctly folded active protein. Furthermore, the C-terminal extended fusion polypeptides of these recombinant scFv fusion proteins are used as the acceptor substrate for human polypeptide-R-Ν-acetylgalactosaminyltransferase II (h-ppGalNAc- T2) that transfers either GalNAc or 2-keto-Gal, a modified galactose with a chemical handle, from their respective UDP-sugars to the side-chain hydroxyl group of the Thr residue(s). Upon protease cleavage, the MALDI-TOF spectra of the glycosylated C-terminal fusion polypeptides showed that the glycosylated scFv fusion protein with a single Thr residue is fully glycosylated with a single 2-keto-Gal, whereas the glycosylated scFv fusion protein with 3 and 17 Thr residues is found as an equal mixture of 2-3 and 5-8 2-keto-Gal glycosylated fusion proteins, respectively. These fusion scFv proteins with the modified galactose are then conjugated with a fluorescence probe, Alexa488, that carries an orthogonal reactive group. The fluorescence labeled scFv proteins bind specifically to a human breast cancer cell line (SK-BR-3) that over expresses the HER2 receptor, indicating that the in Vitro folded scFv fusion proteins are biologically active and the presence of conjugated multiple Alexa488 probes in their C-terminal end does not interfere with their binding to the antigen.</p><p>A large mucin protein like, muc6, has been expressed as a soluble protein in E. coli and has been in Vitro glycosylated with more than 50 sugars with GalNAc, using ppGalNAc-T1. Therefore, using our present site-specific and multiple site conjugating method, scFv proteins with a C-terminal muc6 fusion protein can be glycosylated with modified sugars and conjugated with bioactive molecules. Such complexes are expected to carry not just a few but several tens of bioactive molecules conjugated to scFv molecules. The methodology described here can generate site-specific and multiple site conjugated antibody-bioactive molecules that are in great need for the development of targeted MRI image contrast agents and a targeted drug delivery system.</p><P><i><b>Synthesis of lipids carrying aminooxy or alkyne group for linking with a glycoprotein that has a sugar moiety linked with an orthogonal reactive group:</i></b>The lipid molecule 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) was converted into either DPPE-aminooxy or DPPE-alkyne derivatives for conjugation with scFV that carry sugar moiety with an orthogonal reactive group. The scFV molecules carrying lipid molecules will next used (in a collaborative project with Dr. Blumenthals and Dr. Dimitrovs groups), for the formulation of liposomes for the targeted drug delivery.</p>