A New Rodent Model to Assess Blood Stage Immunity to the Plasmodium falciparum Antigen Merozoite Surface Protein 1(19) Reveals a Protective Role for Invasion Inhibitory Antibodies

Antibodies capable of inhibiting the invasion of Plasmodium merozoites into erythrocytes are present in individuals that are clinically immune to the malaria parasite. Those targeting the 19-kD COOH-terminal domain of the major merozoite surface protein (MSP)-1(19) are a major component of this inhibitory activity. However, it has been difficult to assess the overall relevance of such antibodies to antiparasite immunity. Here we use an allelic replacement approach to generate a rodent malaria parasite (Plasmodium berghei) that expresses a human malaria (Plasmodium falciparum) form of MSP-1(19). We show that mice made semi-immune to this parasite line generate high levels of merozoite inhibitory antibodies that are specific for P. falciparum MSP-1(19). Importantly, protection from homologous blood stage challenge in these mice correlated with levels of P. falciparum MSP-1(19)–specific inhibitory antibodies, but not with titres of total MSP-1(19)–specific immunoglobulins. We conclude that merozoite inhibitory antibodies generated in response to infection can play a significant role in suppressing parasitemia in vivo. This study provides a strong impetus for the development of blood stage vaccines designed to generate invasion inhibitory antibodies and offers a new animal model to trial P. falciparum MSP-1(19) vaccines.