Plasmodium vivax is the leading cause of malaria outside of Africa, with over 100 million clinical cases annually. Increasing reports of clinical severity of vivax malaria due to emergence of more virulent and drug resistance forms of the parasite, the ability of the parasite to undergo relapse in the liver and the strain specific natur of the immunity to P. vivax infections has contributed in making control more difficult. There is therefore an urgent need for a vaccine to control the disease. Merozoite invasion of human erythrocytes is essential for blood stage development of malaria parasites. P. vivax has a selective preference for infecting reticulocytes, a process mediated by the reticulocyte binding proteins (RBPs), which target reticulocytes for invasion. This essential role in the invasion process makes RBPs ideal targets for vaccine development induced against blood stage P. vivax. We hypothesize that an effective RBP based vaccine will require targeting the functionally conserved receptor-binding domains of multiple RBPs. The large size of the RBPs makes it difficult to produce correctly refolded recombinant proteins using the traditional expression systems. Refolded proteins may surfer from poor immunogenicity due to ineffective presentation of protective epitopes. Thus, it is difficult to induce a strong, high-titer response to all potental epitopes. Usually, immunodominant epitopes, which in most part are polymorphic, do not necessarily correspond to the most effective neutralizing epitopes. This project will define the immunological properties of P. vivax reticulocyte protein 1 (PvRBP1), and identify functionally conserved epitopes that are correlates of protective immunity. The native conformation of RBP1 epitopes from the P. vivax Sal1 strain will be recreated in vitro as recombinant proteins expressed by virus-like particles (VLPs) of the RNA bacteriophage MS2. Broadly neutralizing anti-RBP inhibitory immune sera and/or monoclonal antibodies raised against recombinant fragments of RBP1 will be used to screen MS2 phage libraries displaying fragments of RBP1 or random peptides on VLPs to define target epitopes of these neutralizing antibodies. The immunogenicity of the selected VLP targets will be evaluated by immunization of laboratory animals and the vaccine potential of these targets will be evaluated by the ability of epitope-specific antibodies to inhibit RBP-reticulocyte binding of allelic RBP variants, recognition of the native protein on the parasite by IFA and immunoblot and anti-parasite activity in short-term in vitro cultures. It is expected that a sub-unit vaccine based on defined protective epitopes expressed on the VLP surface will induce a stronger immune response to potential neutralizing epitopes on RBP than recombinant antigens. The development of an effective malaria vaccine will be an important tool to complement the existing malaria prevention strategies including reduction in parasitemia, prevention of clinical symptoms and the overall disease burden.
Malaria is a deadly disease prevalent in tropical and subtropical countries of the world and caused by the parasite Plasmodium, yet there is no vaccine to control transmission. Plasmodium vivax has a unique characteristic of selectively targeting reticulocytes for invasion, a process mediated by the reticulocyte binding proteins (RBPs). A vaccine targeting the RBPs will inhibit the invasion process and blood stage development, thus preventing the development of clinical symptoms, transmission and the overall burden of disease caused by Plasmodium vivax.