Priming with an Adenovirus 35-Circumsporozoite Protein (CS) Vaccine followed by RTS,S/AS01B Boosting Significantly Improves Immunogenicity toPlasmodium falciparumCS Compared to That with Either Malaria Vaccine Alone

Abstract

The RTS,S/AS02A protein-based vaccine consistently demonstrates significant protection against infection with Plasmodium falciparum malaria and also against clinical malaria and severe disease in children in areas of endemicity. Here we demonstrate with rhesus macaques that priming with a replication-defective human adenovirus serotype 35 (Ad35) vector encoding circumsporozoite protein (CS) (Ad35.CS), followed by boosting with RTS,S in an improved MPL-and QS21-based adjuvant formulation, AS01B, maintains antibody responses and dramatically increases levels of T cells producing gamma interferon and other Th1 cytokines in response to CS peptides. The increased T-cell responses induced by the combination of Ad35.CS and RTS,S/ AS01B are sustained for at least 6 months postvaccination and may translate to improved and more durable protection against P. falciparum infection in humans.Plasmodium falciparum malaria affects millions of people throughout the world annually, and very young children are particularly vulnerable to anemia, cerebral malaria, and death. An effective P. falciparum malaria vaccine could have a profound impact on the lives of the estimated 2 billion at risk (34). The feasibility of development of an effective subunit vaccine against P. falciparum malaria has been convincingly demonstrated by a protein-based antigen (RTS,S), comprising part of the preerythrocytic circumsporozoite (CS) protein, in the AS02A adjuvant system (RTS,S/AS02A; GlaxoSmithKline Biologicals). The RTS,S antigen incorporates part of the CS central tetrapeptide repeat region and C-terminal flanking region, known to contain both B-and T-cell epitopes, into a chimeric gene expressed in Saccharomyces cerevisiae. This construct was named RTS,S to indicate the presence of the CS repeat region (R), T-cell epitopes (T), and hepatitis B virus surface antigen (S) in a mixture of the RTS fusion protein and the S protein that assembles into virus like particles (14, 16).RTS,S formulated in AS02A, a proprietary adjuvant system containing an oil-in-water emulsion and the immune stimulants MPL and QS21, protects approximately 41% of malarianaïve humans against challenge with Plasmodium falciparum sporozoites (18). RTS,S/AS02A efficacy in a field trial was 35% (95% confidence interval [95% CI], 22 to 47%; P Ͻ 0.0001) for protection against first clinical episodes and 49% (95% CI, 12 to 71%; P ϭ 0.02) for protection against severe malaria during an 18-month period for 1-to 4-year-old African children (1, 2). While the unprecedented protection conferred by RTS,S/ AS02A remains partial, several approaches to increasing the efficacy of the vaccine are being studied (16), including new adjuvant formulations and new vaccination strategies.The immune correlates of RTS,S-induced protection are not well defined. However, protection induced by the RTS,S/ AS02A vaccine has been associated with high anti-CS antibody titers, perhaps via inhibition of binding (7) or paralysis of sporozoites (13), or by their opsonization and destruction by phagocytes (32). ...

Enhancing Blood-Stage Malaria Subunit Vaccine Immunogenicity in Rhesus Macaques by Combining Adenovirus, Poxvirus, and Protein-in-Adjuvant Vaccines

“…This represents an important test of the adenovirus-protein and adenovirus-MVA-protein regimens in macaques, prior to evaluation of these promising regimens in clinical trials. Further studies will be needed to compare the titers induced by vector-protein regimens with protein-only regimens in macaques, although other murine and macaque vaccine studies with different constructs have suggested that they may perform comparably (34,37). Interestingly, boosting with protein formulated in CoVaccine HT only led to moderately higher-peak and long-term IgG titers in comparison with formulation in Alhydrogel.…”

“…These murine studies also described further possible advantages of viral vectors in achieving consistent Ab priming, enhanced Ab avidity, and Th1-type IgG isotype skew. Data from the HIV-1 (32-36) and liver-stage malaria vaccine fields (37,38) also encourage efforts to combine adenoviral-vectored vaccines with protein vaccines.…”

Impact of Recombinant Adenovirus Serotype 35 Priming versus Boosting of aPlasmodium falciparumProtein: Characterization of T- and B-Cell Responses to Liver-Stage Antigen 1

“…This observation was corroborated by the results obtained in the ELISPOT assay and with pentamer staining. A difference in the effect of Ad35 when used as a priming or a boosting vaccine has been indicated in an earlier study in which prime-boost immunizations of nonhuman primates with rAd35.CS and adjuvanted RTS,S protein were performed (56). Although the complete sequences of the antigen expressed by the rAd35 vector and the adjuvanted protein were not completely matching (N-terminal truncation in the protein), the study showed that rAd35 priming/protein boosting was a superior regimen for inducing IFN-␥-producing CD4 ϩ T cells.…”

“…We demonstrated recently in a nonhuman primate study that a prime-boost immunization regimen combining the rAd35 vector coding for the Plasmodium falciparum circumsporozoite (CS) protein and adjuvanted RTS,S was superior to either modality alone in the induction of T-cell immunity (56). In the current study, we investigated whether this effect can be extended to other Plasmodium falciparum antigens, in particular to liver-stage antigen 1 (LSA-1).…”

Protective immunity to pre-erythrocytic stage malaria

“…Combining RTS,S with a viral vector expressing either CS protein or other appropriate sporozoite or liver-stage Ags in a prime and boost regimen could induce both strong Ab and CD4 and CD8 T cell responses leading to more durable protection [71]. In support, a study in macaques demonstrated that two doses of RTS,S following an adenovirus 35 CS-protein priming immunization retained the RTS,S-induced Ab response while promoting a markedly higher CD4 T-cell response that remained elevated for about 2 months [72].…”