Active immunisation with RAMA does not provide protective immunity against Plasmodium yoelii challenge despite its association with protective responses in endemic populations

Abstract

The rhoptry associated membrane antigen (RAMA) of Plasmodium falciparum has been proposed as a potential candidate for inclusion in a multivalent subunit vaccine against malaria. Previous studies have found that the RAMA gene is refractory to genetic deletion in vitro and is conserved in a range of clinical isolates. Importantly, two independent studies demonstrated that antibodies against the C-terminal region of RAMA are associated with immunity in endemic populations of both Asia and Africa. However, there is presently no direct evidence that anti-RAMA immune responses have a demonstrable anti-parasitic effect either in vitro or in vivo. In this study we used an in vitro invasion inhibition assay and the Plasmodium yoelii mouse model of infection to evaluate the potential of RAMA as a vaccine candidate. Our results demonstrate that anti-PfRAMA antibodies have only a weak inhibitory effect on P. falciparum invasion in vitro. Immunisation with recombinant PyRAMA protein did not protect mice against a lethal P. yoelii infection and did not boost the level of protection induced by a known protective antigen, merozoite surface protein 4/5. Taken together, these data do not support RAMA as a priority vaccine candidate.