In this report we describe a novel gene delivery system using malaria circumsporozoite (CS) protein as a specific ligand. The CS protein covers the entire surface of sporozoites of malaria parasites. Previous studies have demonstrated that intravenously injected CS protein binds specifically to the basolateral surface of hepatocytes within minutes, indicating the high hepatocyte specificity of CS protein. This characteristic of CS protein prompted us to explore the possibility of using this protein as a liver-specific ligand for hepatic gene delivery vehicle. As an initial step, we investigated the efficacy of CS protein-mediated gene transfer into primary hepatocytes as well as established cell lines. Recombinant CS proteins were chemically conjugated to poly(L-lysine). The CS conjugates were complexed with recombinant plasmid DNA carrying a reporter gene. When the DNA complex was used to transfect primary hepatocytes, a very low level of expression of the reporter gene was observed. The level of expression was greatly enhanced when the cells were cotransfected with adenovirus, which presumably releases the internalized DNA from endosomal entrapment. The CS-mediated gene transfer into the cells required region II+, an evolutionarily conserved amino acid sequence conferring the binding of CS protein to its receptor. CS protein also efficiently mediated gene transfer into a number of cell lines, i.e. HepG, HeLa, NIH3T3, and K562, but not HL-60, which contains low levels of receptor. Thus, the CS conjugate can be used to deliver DNA into many different cultured cells. Most importantly, the CS conjugate has a potential to be further developed into a liver-specific gene delivery vehicle in vivo.

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