Modulation of the humoral response to repeat and non-repeat sequences of the circumsporozoite protein of Plasmodium vivax using novel adjuvant and delivery systems

In the use of sub-unit vaccines, it is important to identify the protective epitopes and to generate the optimal immune response by using appropriate immuno-modulatory adjuvants and/or delivery systems. The main aim of the present study was to generate an MHC-non-restricted immune response against one promising vaccine candidate, the circumsporozoite protein (CSP) of Plasmodium vivax. Four synthetic peptides were chosen: three repeat-region sequences (AA, DA and ANG) and a putative T-cell epitope extended from a conserved region (region II) containing a hepatocyte-binding region (HBP). The humoral response against each peptide was studied in outbred mice and three strains of inbred mice (with different genetic backgrounds). Delivery of each peptide in microspheres or inclusion of a bio-active casein-fragment analogue as adjuvant with alum/liposome delivery considerably enhanced the humoral response against the peptide (when compared with the response to the peptide delivered in alum alone). The maximal immune response was observed when the peptide was delivered in microspheres, with no booster doses required, the antibodies raised against peptide delivered with adjuvant or in modulatory delivery vehicles had two- to five-fold lower binding affinities. The predominant IgG isotypes elicited using microspheres or adjuvant with alum/liposome delivery were IgG_2a/IgG_2b and/or IgG₁. Importantly, conjugation of HBP to the B-cell repeat peptides increased the titres of peptide-specific antibodies, especially of antibodies against the supposedly cryptic HBP. Delivery of a mix of all four peptides in microspheres elicited an intense immune response in outbred mice, indicating that such a delivery system efficiently presents the peptides to the immune effector cells. That antibodies in the anti-peptide sera bound strongly to air-dried sporozoites of P. vivax was confirmed by immunofluorescence. The present results, based on the use of individual peptides or a conjugate or cocktail of the peptides, highlight the utility of the casein-fragment analogue as an adjuvant, when used with alum/liposome delivery, and also demonstrate the potential of microspheres as a single-shot delivery system for sub-unit peptides.