Adjuvant-dependent immune response to malarial transmission-blocking vaccine candidate antigens

Immune responses in major histocompatibility complex (MHC)-disparate congenic mouse strains immunized with sexual stage malaria parasites or purified recombinant protein were adjuvant dependent. Whereas mice exhibited a limited antibody response to immunization with newly emerged Plasmodium falciparum gametes in Freund's adjuvant, all five congenic mouse strains responded to several transmission-blocking vaccine candidate antigens, when parasites were emulsified in a monophosphoryl lipid A (MPL) and trehalose dimycolate (TDM) adjuvant. The humoral response in those animals immunized with the antigen in a MPL/TDM adjuvant was helper T cell dependent, as evident by boosting of the antibody response after a second immunization. If the immunogen consisted of purified recombinant protein, then the immune response was not MHC class II limited in mice immunized with either complete Freund's adjuvant or TDM/MPL. The potential role of adjuvants in overcoming apparent immune nonresponsiveness and the implications for development of a malaria transmission-blocking vaccine are discussed.