Extended protection capabilities of an immature dendritic-cell targeting malaria sporozoite vaccine

Highlights

• A novel vaccine platform greatly enhances antibody responses to a malaria antigen.

• Protection against transgenic sporozoite challenge in a mouse model persists for at least 23 weeks.

• The observed protection requires only two immunizations.

Abstract

Mouse studies evaluating candidate malaria vaccines have typically examined protective efficacy over the relatively short time frames of several weeks after the final of multiple immunizations. The current study examines the protective ability in a mouse model system of a novel protein vaccine construct in which the adjuvant polyinosinic polycytidilic acid (poly(I:C)) is used in combination with a vaccine in which the immature dendritic cell targeting chemokine, macrophage inflammatory protein 3 alpha (MIP3α), is fused to the circumsporozoite protein (CSP) of Plasmodium falciparum (P. falciparum). Two vaccinations, three weeks apart, elicited extraordinarily high, MIP3α-dependent antibody responses. MIP3α was able to target the vaccine to the CCR6 receptor found predominantly on immature dendritic cells and significantly enhanced the cellular influx at the vaccination site. At three and 23 weeks after the final of two immunizations, mice were challenged by intravenous injection of 5 × 10³ transgenic Plasmodium berghei sporozoites expressing P. falciparum CSP, a challenge dose approximately one order of magnitude greater than that which is encountered after mosquito bite in the clinical setting. A ninety-seven percent reduction in liver sporozoite load was observed at both time points, 23 weeks being the last time point tested.

Introduction

Major problems encountered during the effort to develop an effective pre-erythrocytic malaria vaccine include (1) the need for three or more immunizations to obtain even short term protection in settings in which health care delivery may be problematic and (2) the inability of the vaccine to provide sustained protection over an extended period. These problems were evident in results from the large-scale clinical trial of the RTS,S/AS01 (RTS,S) malaria vaccine. In the first year following completion of the vaccination protocols, vaccine efficacy was at best 50% and this level fell to 16% over the ensuing three years [1], [2], [3].

There is clear evidence that antibody specific for the circumsporozoite protein of the pre-erythrocytic malaria sporozoite stage, at sufficient concentrations, is capable of providing sterilizing immunity against malaria infection. Protection has been demonstrated with passive administration of both monoclonal and polyclonal antibodies, as well as with immunization with adjuvanted peptide vaccines [4], [5], [6], [7], [8]. Studies in humanized mice of passively administered monoclonal antibodies developed from an individual immunized with the RTS,S vaccine concluded that the poor performance of the RTS,S vaccine in clinical trials was attributable to the 95% decline in antibody concentrations over the course of one year [8]. Of note, that rapid decline was presaged by earlier studies in mice using similar vaccine formulations [9].

One line of vaccine development efforts has focused on the use chemokines or cytokines to enhance the inflammatory infiltrate, promote binding of antigen to specific receptors or stimulate differentiation of specific subsets of immune cells [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. In many instances these chemokines or cytokines were used with the goal of eliminating the need for more traditional adjuvants.

We have previously reported on a DNA vaccine in which DNA encoding the chemokine macrophage inflammatory protein 3α, (MIP3α), also known as Chemokine (C-C motif) ligand 20 (CCL20), was fused to DNA encoding a portion of the circumsporozoite protein (CSP) of Plasmodium falciparum (P. falciparum) [21]. This vaccine construct was combined with use of the adjuvant Vaxfectin® (Vical, Inc., San Diego, CA). In a mouse challenge model, this combination yielded complete, antibody-mediated protection against bloodstream infection with transgenic sporozoites three weeks after the last of three immunizations. The enhanced protection was dependent on the presence of the MIP3α component in the vaccine construct. However, unreported follow up studies showed loss of protection three months after the last immunization.

In the current studies we examined the immunogenicity and extended protection capabilities of the protein formulation of this vaccine used with the adjuvant polyinosinic:polycytidylic acid (poly (I:C)). This vaccine formulation yielded significantly higher antibody responses after just two immunizations and protection against bloodstream challenge 23 weeks after the final immunization, the last time point evaluated.