IMMUNITY TO MALARIA ELICITED BY HYBRID HEPATITIS-B VIRUS CORE PARTICLES CARRYING CIRCUMSPOROZOITE PROTEIN EPITOPES

The hepatitis B virus (HBV) nucleocapsid antigen (HBcAg) was investiga ted as a carrier moiety for the immunodominant circumsporozoite (CS) p rotein repeat epitopes of Plasmodium falciparum and the rodent malaria agent P. berghei. For this purpose hybrid genes coding for [NANP](4) (C75CS2) or [DP4NPN](2) (C75CS1) as internal inserts in HBcAg (between amino acids 75 and 81) were constructed and expressed in recombinant Salmonella typhimurium. The resulting hybrid HBcAg-CS polypeptides pur ified from S. typhimurium were particulate and displayed CS and HBc an tigenicity, however, the HBc antigenicity was reduced compared to nati ve recombinant HBcAg. Immunization of several mouse strains with HBcAg -CS1 and HBcAg-CS2 particles resulted in high titer, P. berghei- or P. falciparum-specific anti-CS antibodies representing all murine immuno globulin G isotypes. The possible influence of carrier-specific immuno suppression was examined, and preexisting immunity to HBcAg did not si gnificantly affect the immunogenicity of the CS epitopes within HBcAg- CS1 particles. Similarly, the choice of adjuvant did not significantly alter the immunogenicity of HBcAg-CS hybrid particles. Immunization i n complete or incomplete Freund's adjuvant or alum resulted in equival ent anti-HBc and anti-CS humoral responses. Examination of T cell reco gnition of HBcAg-CS particles revealed that HBcAg-specific T cells wer e universally primed and CS-specific T cells were primed if the insert contained a CS-specific T cell recognition site. This indicates that the internal site in HBcAg is permissive for the inclusion of heterolo gous pathogen-specific T as well as B cell epitopes. Most importantly, 90 and 100% of BALB/c mice immunized with HBcAg-CS1 particles were pr otected against a P. berghei challenge infection in two independent ex periments. Therefore, hybrid HBcAg-CS particles may represent a useful approach for future malaria vaccine development.