Immunogenicity of liposomal malaria sporozoite antigen in monkeys: adjuvant effects of aluminium hydroxide and non-pyrogenic liposomal lipid A

Abstract

The immunogenicity of a recombinant protein (R32tet32) containing sequences from the tetrapeptide repeat region of the circumsporozoite protein of Plasmodium falciparum was enhanced by encapsulation in liposomes containing lipid A and adsorption of the liposomes with alum. The toxicities and efficacies of preparations containing different types and doses of lipid A were assessed by studying pyrogenicity in rabbits and adjuvanticity in monkeys. In each case liposomal lipid A was 25-fold to 200-fold less pyrogenic than free lipid A. Monophosphoryl lipid A, whether free or in liposomes, was the least pyrogenic of the three lipid A preparations tested. High antibody levels were obtained after immunization of rhesus monkeys with a formulation consisting of alum-adsorbed liposomes in which the liposomes contained R32tet32 and a strongly pyrogenic dose of native lipid A. Excellent antibody levels were also observed in monkeys immunized with a combination of R32tet32 encapsulated in alum-adsorbed liposomes containing non-pyrogenic doses of monophosphoryl lipid A and alum. The adjuvant effect was related to the dose of the lipid A in the liposomes, and the adjuvant effect was still strongly expressed despite suppression of the pyrogenic effect of lipid A. Antibody levels were considerably lower in monkeys immunized with liposomes lacking lipid A. It was concluded that a non-pyrogenic formulation of alum-adsorbed liposomes, in which the liposomes contained both lipid A and an encapsulated synthetic sporozoite antigen, shows considerable promise for inducing high titres of antibodies to sporozoites.