Enhancing the immunogenicity of liposomal hepatitis B surface antigen (HBsAg) by controlling its delivery from polymeric microspheres

Microencapsulated liposome systems (MELs) were investigated as a potential immunization carrier for a recombinant 22-nm hepatitis B surface antigen (H BsAg) particle. MELs were prepared by first entrapping the HBsAg particles within liposomes composed of phosphatidylcholine:cholesterol (1:1 molar rat io), which were then encapsulated within alginate-poly(L-lysine) (PLL) hydr ogel microspheres. The entrapped HBsAg particles retained immunoreactivity, as judged by an enzyme-linked immunosorbent assay (ELISA). Direct imaging of HBsAg particles and HBsAg incorporated into liposomes by cryo-transmissi on electron microscopy (cryo-TEM) indicated that HBsAg is embedded in the l iposomal membrane. The antigenic particles were released from MELs mainly w ithin the context of liposomes. The release rates in vitro and in vivo depe nded on the molecular weight of PLL used for MEL coating; MELs-214, coated with 214 kDa PLL, released the liposomal HBsAg at much higher rates than ME Ls-25, which was coated with 25 kDa PLL. Concomitantly, the specific anti-H BsAg titers in mice receiving HBsAg in MELs-214 were higher than those indu ced by MELs-25. MELs-214 were more efficient than conventional liposomes or alum in eliciting higher and prolonged antibody levels in mice. The abilit y of MELs to provide an HBsAg depot as well as a sustained release of lipos omal HBsAg suggests that these carriers may be an ideal immunoadjuvant. (C) 2000 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89:1550-1557, 2000.