Protection induced by Plasmodium falciparum MSP1(42) is strain-specific, antigen and adjuvant dependent, and correlates with antibody responses

Abstract

Vaccination with Plasmodium falciparum MSP1(42)/complete Freund's adjuvant (FA) followed by MSP1(42)/incomplete FA is the only known regimen that protects Aotus nancymaae monkeys against infection by erythrocytic stage malaria parasites. The role of adjuvant is not defined; however complete FA cannot be used in humans. In rodent models, immunity is strain-specific. We vaccinated Aotus monkeys with the FVO or 3D7 alleles of MSP1(42) expressed in Escherichia coli or with the FVO allele expressed in baculovirus (bv) combined with complete and incomplete FA, Montanide ISA-720 (ISA-720) or AS02A. Challenge with FVO strain P. falciparum showed that suppression of cumulative day 11 parasitemia was strain-specific and could be induced by E. coli expressed MSP1(42) in combination with FA or ISA-720 but not with AS02A. The coli42-FVO antigen induced a stronger protective effect than the bv42-FVO antigen, and FA induced a stronger protective effect than ISA-720. ELISA antibody (Ab) responses at day of challenge (DOC) were strain-specific and correlated inversely with c-day 11 parasitemia (r = -0.843). ELISA Ab levels at DOC meeting a titer of at least 115,000 ELISA Ab units identified the vaccinees not requiring treatment (noTx) with a true positive rate of 83.3% and false positive rate of 14.3 %. Correlation between functional growth inhibitory Ab levels (GIA) and cumulative day 11 parasitemia was weaker (r = -0.511), and was not as predictive for a response of noTx. The lowest false positive rate for GIA was 30% when requiring a true positive rate of 83.3%. These inhibition results along with those showing that antigen/FA combinations induced a stronger protective immunity than antigen/ISA-720 or antigen/AS02 combinations are consistent with protection as ascribed to MSP1-specific cytophilic antibodies. Development of an effective MSP1(42) vaccine against erythrocytic stage P. falciparum infection will depend not only on antigen quality, but also upon the selection of an optimal adjuvant component.

Figure 1. Individual value plots showing strain-specific Ab responses by ELISA for vaccine recipients.

Responses are grouped by vaccine strain received and then by the Ag strain use for ELISA. Panel A includes all 3D7 strain vaccine recipients in the trial. Panel B includes all FVO strain vaccine recipients in the trial. Connect lines join each individual animal's ELISA Ab response measured with the FVO Ag to that measured with the 3D7 Ag. Both Ag were expressed in baculovirus.

Figure 2. C-day 11 parasitemias for regimens 3, 7, 8, and 9 grouped by adjuvant and antigen.

Open circles, individual responses; filled circles, means; bar is the 95% confidence interval for the mean. Panel A compares grouped log(c-day 11 parasitemia) levels. Panel B shows the estimated marginal means from the GLM used to analyze the responses.

Figure 3. Growth/invasion inhibition for regimens 3, 7, 8, and 9 grouped by adjuvant and antigen.

Open circles, individual responses; filled circles, means; bar is the 95% confidence interval for the mean. Panel A compares grouped %Inhibition levels. Panel B shows the estimated marginal means from the GLM used to analyze the responses.

Figure 4. ELISA Ab responses for regimens 3, 7, 8, and 9 grouped by adjuvant and antigen.

Capture Ags were expressed in baculovirus. Open circles, individual responses; filled circles, means; bar is the 95% confidence interval for the mean. Panel A compares grouped ELISA Ab unit levels. Panel B shows the estimated marginal means from the GLM used to analyze the responses.

Figure 5. Correlation between log(c-day 11 parasitemia) and serologic response in animals vaccinated with FVO strain Ag.

Animals were vaccinated with either E. coli or baculovirus expressed MSP1₄₂ in combination with FA, ISA-720 or AS02A. Growth/Invasion inhibition is against FVO (Panel A) or 3D7 (Panel B) strain targets. ELISA was performed with recombinant FVO (Panel C) or 3D7 (Panel D) strain MSP1₄₂ expressed in baculovirus. Correlations were determined with Pearson's test after testing data distributions for normality.

Figure 6. Responses in animals vaccinated with coli42-FVO/ISA-720 or control.

Shaded Open circles indicate individual responses, the boxed area shows defines the median and the second and third quartile responses, and whiskers define the first and fourth quartile responses.