A Plasmodium Promiscuous T Cell Epitope Delivered within the Ad5 Hexon Protein Enhances the Protective Efficacy of a Protein Based Malaria Vaccine

Abstract

A malaria vaccine is a public health priority. In order to produce an effective vaccine, a multistage approach targeting both the blood and the liver stage infection is desirable. The vaccine candidates also need to induce balanced immune responses including antibodies, CD4+ and CD8+ T cells. Protein-based subunit vaccines like RTS,S are able to induce strong antibody response but poor cellular reactivity. Adenoviral vectors have been effective inducing protective CD8+ T cell responses in several models including malaria; nonetheless this vaccine platform exhibits a limited induction of humoral immune responses. Two approaches have been used to improve the humoral immunogenicity of recombinant adenovirus vectors, the use of heterologous prime-boost regimens with recombinant proteins or the genetic modification of the hypervariable regions (HVR) of the capsid protein hexon to express B cell epitopes of interest. In this study, we describe the development of capsid modified Ad5 vectors that express a promiscuous Plasmodium yoelii T helper epitope denominated PyT53 within the hexon HVR2 region. Several regimens were tested in mice to determine the relevance of the hexon modification in enhancing protective immune responses induced by the previously described protein-based multi-stage experimental vaccine PyCMP. A heterologous prime-boost immunization regime that combines a hexon modified vector with transgenic expression of PyCMP followed by protein immunizations resulted in the induction of robust antibody and cellular immune responses in comparison to a similar regimen that includes a vector with unmodified hexon. These differences in immunogenicity translated into a better protective efficacy against both the hepatic and red blood cell stages of P. yoelii. To our knowledge, this is the first time that a hexon modification is used to deliver a promiscuous T cell epitope. Our data support the use of such modification to enhance the immunogenicity and protective efficacy of adenoviral based malaria vaccines.

A prime-boost immunization regimen based on a simian adenovirus 36 vectored multi-stage malaria vaccine induces protective immunity in mice

“…We previously determined that heterologous adenoviral prime-protein boost regimens are the most efficacious where the combination of adenovirus and protein is used [15, 30]. We, therefore, used one of three doses of the recombinant simian adenoviral vector at either 10 6 , 10 7 , or 10 10 v.p for priming to determine the optimal dose to induce protective efficacy.…”

“…(High dose), or the previously protective recombinant Ad5 10 7 v.p. dose [15, 30]. All mice received two boosting immunizations with 20 μg of PyCMP emulsified in Montanide ISA 51 (Seppic, Fairfield, NJ) at days 30 and 60.…”

“…The avidity of antibodies was assessed by a thiocyanate elution ELISA as described previously [30, 32, 33] and calculated as described by Perciani et al [34]. …”

“…Cellular immune responses in the spleen were measured by ICS to simultaneously analyze IL-2, IFN-γ, and TNF-α at the single-cell level in T cells 5 days after the final boosting immunization as described [30]. Cells were stimulated with either PyCMP or one of three peptide pools of 15-mer synthetic peptides overlapped by 10 amino acids peptides representing PyCMP.…”

A potent malaria vaccine based on adenovirus with dual modifications at Hexon and pVII

“…More recently, other groups have constructed a recombinant Ad5 that has an insertion of either CSP-specific B-cell epitopes [22, 23] or a CSP-specific CD4+ T-cell epitope [24] into its Hexon and shown an increased protective efficacy not only in mice [22, 24] but also an increased immunogenicity in non-human primates [23]. …”