Functional immunogenicity of baculovirus expressing Pfs25, a human malaria transmission-blocking vaccine candidate antigen

Abstract

We have focused on development of a novel vaccine vector based on “Baculophage”, a baculovirus display system for expression of proteins on the surface of the viral envelope, as a non-pathogenic and non-vertebrate insect virus. In the present study, recombinant baculovirus (AcNPV–Pfs25surf) were generated, which displayed Pfs25, a potent Plasmodium falciparum transmission-blocking vaccine candidate. Both intranasal and intramuscular immunizations of mice with AcNPV–Pfs25surf induced high levels of Pfs25-specific antibodies, which strongly reacted with ookinetes of transgenic Plasmodium berghei expressing Pfs25 (TrPfs25Pb). Importantly, sera obtained from immunized rabbits exhibited a significant transmission-blocking effect (>90% reduction in infection intensity) in standard membrane feeding assay using P. falciparum gametocytes. Additionally, active immunization (both intranasal and intramuscular routes) of mice followed by challenge using TrPfs25Pb demonstrated an effective transmission-blocking response, with an 83% (intranasal) and ∼95% (intramuscular) reduction in oocyst intensity, respectively. Thus, the baculovirus-based vaccines offer a promising new alternative to current human vaccine delivery platforms for the development of malaria multi-stage vaccines.

Introduction

Development of the malaria parasite, Plasmodium spp. in the mosquito midgut can be arrested by antibodies directed at parasite surface antigens and this has led to the concept of a transmission-blocking vaccine (TBV) [1]. Sexual development of Plasmodium parasites begins with the formation of male and female gametocytes in the vertebrate host. Among the well-characterized surface antigens expressed by sexual stages of Plasmodium falciparum, Pfs25 is a leading TBV candidate antigen. Antibodies directed against the antigen are effective in preventing parasite development in the mosquito.

Various approaches considered for formulation and delivery of transmission-blocking vaccine candidates include proteins emulsified with adjuvants and naked plasmid DNA encoding Pfs25. Although a couple of vaccine formulations based on Pfs25 and Pvs25 have undergone limited phase I clinical trials [2], [3], improvement of their immunogenicity is needed. In addition, due to safety concerns in various parts of the developing world and the general discomfort associated with needle vaccine delivery, intranasal (i.n.) or oral immunization are being explored even for diseases that are not transmitted through the mucosal/nasal routes.

We have recently developed an Autographa californica nucleopolyhedrosis virus (AcNPV)-based vaccine expressing the 19 kDa carboxyl terminal fragment of Plasmodium yoelii merozoite surface protein 1 (PyMSP1₁₉) on the surface of the viral envelope [4]. Adjuvant-free i.n. immunization with this vaccine induced not only strong systemic humoral immune responses with high titers of PyMSP1₁₉-specific antibody but also primed for natural boosting of PyMSP1₁₉-specific antibody responses within a short time following challenge, and conferred complete protection. AcNPV has been successfully engineered for expression of complex eukaryotic proteins on the surface of the viral envelope, termed ‘Baculophage’ [5], [6], [7], [8], [9] and has emerged as a new vaccine vector with several attractive attributes, i.e., (i) low cytotoxicity, (ii) inability to replicate in mammalian cells, and (iii) absence of pre-existing antibodies. AcNPV also possesses strong adjuvant properties which can activate dendritic cell-mediated innate immunity through MyD88/TLR9-dependent and -independent pathways [10], and i.n. immunization with AcNPV protects mice from a lethal challenge of influenza [11]. Therefore, nasal mucosal tissues, which contain dendritic cells and macrophages, may be attractive sites for immunization with AcNPV-based vaccines to induce TLR9-mediated immune responses.

In the present study, we describe results from i.n. immunization in mice with AcNPV-based Pfs25 vaccine (AcNPV–Pfs25surf) and functional effectiveness using transgenic P. berghei parasites expressing Pfs25 (TrPfs25Pb) [12]. Needle-free nasal drop immunization with this vaccine induced high titers of Pfs25-specific antibodies, capable of inhibiting sporogonic stage development of TrPfs25Pb. These results suggest that recombinant baculovirus AcNPV expressing a human malaria transmission-blocking target antigen, Pfs25 administered through the i.n. route can elicit functional antibodies and can provide a needle-free approach to immunize against malaria transmission.