HYBRID HEPATITIS-B VIRUS CORE ANTIGEN AS A VACCINE CARRIER MOIETY .1.PRESENTATION OF FOREIGN EPITOPES

Hepatitis B virus (HBV) core antigen (HBcAg) is a highly immunogenic s ubviral particle. Here, we review recent progress in the use of HBcAg as a carrier moiety for heterologous epitopes. To define surface expos ed and immunogenic insertion sites for foreign epitopes in HBcAg, pept idic epitopes representing binding sites for virus neutralizing antibo dies on the HBV surface antigens were inserted at different positions within HBcAg using genetic engineering in an Escherichia coli expressi on system (Schodel et al, (1992) J. Virol. 66, 106-114). While fusion to the N-terminus required a linker to become surface accessible, both fusion to the N-terminus and to the C-terminus was compatible with pa rticle assembly and preserved the native antigenicity and immunogenici ty of HBcAg. Fusion to an immunodominant internal site of HBcAg reduce d the HBcAg immunogenicity and antigenicity and most drastically enhan ced the immunogenicity of the inserted foreign epitope. This internal site of HBcAg was used to express circumsporozoite antigen (CS) repeat epitopes of two rodent malaria parasites and of Plasmodium falciparum (Schodel et al. (1994b) J. Exp. Med. 180, 1037-1046 and Schodel et al . (1995a) 95th ASM General Meeting, Washington DC, Abstr. E61). When p urified from recombinant Salmonella typhimurium, the hybrid HBcAg-CS p roteins were particulate and displayed CS antigenicity as well as redu ced HBc antigenicity, as compared to native HBcAg. Immunization of sev eral mouse strains with HBcAg-CS hybrid particles resulted in high tit ered serum anti-CS antibodies representing all murine IgG isotypes. Im munization of mice with HBcAg or HBcAg-CS particles formulated on alum , complete Freunds or incomplete Freunds adjuvant resulted in equivale nt anti-CS and anti-HBc serum antibody titres. The possible influence of carrier-specific immunosuppression was examined and pre-existing im munity to HBcAg did not significantly alter the immunogenicity of hybr id HBcAg particles suggesting that they would be useful carrier moieti es for repeated immunizations against multiple haptens or in immune su bjects after HBV infection. Examination of T cell recognition of HBcAg -CS particles revealed that HBcAg-specific T cells were universally pr imed and CS-specific T cells were primed if the insert contained a CS- specific T cell recognition site. This indicates that the internal ami no acid position in HBcAg is permissive for the inclusion of heterolog ous functional T helper as well as B cell epitopes. BALB/c mice immuni zed with HBcAg-CS1 were protected against P. berghei challenge to 90% and 100%, respectively, in two independent experiments.