The reason for the lack of anti-PfRH5 antibody in naturally exposed Kenyans is unclear. The facts that anti-PfRH5FL IgG induced in animals by these viral vectored vaccines binds both the ELISA antigen and native parasites by IFA (), and that our ELISA coating antigen has been confirmed to bind its host RBC receptor36
, indicate that incorrect conformation of our PfRH5 ELISA coating antigen is highly unlikely to account for poor detection of anti-PfRH5 IgG in humans. PfRH5 mRNA has also been shown to be expressed in field isolates cultured ex-vivo
both from The Gambia in West Africa37
, and Kenya in East Africa38
, suggesting this inherent lack of immune recognition is not due to the absence of PfRH5 expression by parasites circulating in endemic areas. Selective down-regulation of PfRH5 in parasites infecting malaria-experienced individuals who control parasitemia (as suggested by Gomez-Escobar et al
) might lead anti-PfRH5 IgG titers to be higher in children than in adults. However, additional ELISA assays conducted using serum from Kenyan children () yielded results comparable to those observed in adults (). The mechanism by which this crucial antigen escapes immune recognition in both adults and children thus requires further investigation.
The density of known polymorphisms in PfRH5 (10 out of 526 amino acids) is 3.3-fold lower than in the well-characterized 19kDa C-terminus of PfMSP1 (PfMSP119
) (6 out of 96 amino acids)39
, which in itself is widely regarded as a highly conserved vaccine target. Data from non-human primate models demonstrate that the amino acid polymorphism in PfMSP1 can result in strain-specific efficacy of vaccine-induced anti-PfMSP1 antibody responses5
. There may thus be a contrast in the immunological effect of the polymorphism in PfMSP1 versus that in PfRH5. It is possible that non-immune mechanisms such as adaptation to host erythrocyte variability could drive the apparent excess of non-synonymous relative to synonymous mutations in PfRH533,40,41
Attempts to delete the PfRH5 gene have failed repeatedly, suggesting it is essential for parasite viability24,33
, and immunization against rodent malaria homologues of the PfRH family provides protection in a mouse challenge model42
– a finding first reported in 1980, but not translated into a human-compatible vaccine formulation against P. falciparum
until now. Complementing our data obtained using PfRH5FL as a vaccine target, we have recently identified an erythrocyte receptor for PfRH5 and demonstrated a critical role for this receptor-ligand interaction36
. In contrast to the prevailing views that erythrocyte invasion by P. falciparum
is a degenerate process mediated by multiple ligand-receptor pairs and that any merozoite antigen susceptible to antibody is highly polymorphic, PfRH5-mediated invasion appears to be both essential and vulnerable to blockade.
Multiple strands of evidence now suggest that PfRH5FL is the best candidate for blood-stage malaria vaccine development to emerge from the P. falciparum
genome to date. The traditional immuno-epidemiological approach to antigen identification may have focussed vaccine-development efforts more upon immunogenic but polymorphic ‘decoy antigens’ than ‘Achilles’ heels’43,44
. Assaying parasite growth inhibition by vaccine-induced antibodies has, instead, directly demonstrated strain-transcending antibody effects obtained with an antigen which would not have appeared to be an important target of naturally-acquired responses. In this study, anti-PfRH5FL IgG out-performed anti-PfAMA1 IgG against homologous 3D7 strain parasites when induced by the same vaccine delivery platform, and importantly for clinical significance, also showed strain-transcending neutralization against other strains. Recently a 3D7 strain AMA1 protein-in-adjuvant vaccine has demonstrated significant strain-specific efficacy in a pre-specified secondary analysis of a Phase IIb study in Malian children8
. This result further encourages the development of blood-stage vaccines, but highlights that antigenic polymorphism is a central problem for the field and one currently shared by all leading blood-stage vaccine candidate antigens. Our findings reported here suggest that the PfRH5FL antigen offers a solution to this problem.
possesses over 5000 genes. Although the genome sequence of P. falciparum
was published in 200245
, PfMSP3 (first described in 199446
) is the most recently identified antigen to have entered clinical trials as a candidate blood-stage vaccine2
. While genome sequence data has substantially advanced basic scientific knowledge, vaccinologists working on complex human pathogens such as P. falciparum
have yet to fully exploit pathogen genome data to identify, comparatively assess and validate much-needed novel antigens47
. This study has highlighted PfRH5FL from a small candidate panel: continued and similar efforts with a larger panel of antigens should thus yield further promising pre-erythrocytic, blood-stage and sexual-stage malaria vaccine candidates as well as permit truly effective exploitation of the genomes of other challenging vaccine targets.